Cl assi calEl ect r odynami cs Pa r tI I b y Co n t e n t s Pr e f a c e 0 . 1Th eI n t e r p l a yofPh y s i c sa n d Ma t h e ma t i c s. . . . . . . . . . . . . L i n k s 0 . 1Pe r s on a l Con t a c tI n f or ma t i on. . . . . . . . . . . . . . . . . . . . 0 . 2Us e f u l Te x t sa n dWe bRe f e r e n c e s. . . . . . . . . . . . . . . . . . v i i x i 3 3 3 I Ma t h e ma t i c a l Ph y s i c s 5 1 Ma t h e ma t i c a l Pr e l u d e 7 2 Nu mb e r s 2 . 1 Re a l Nu mb e r s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . 2 Comp l e xNu mb e r s. . . . . . . . . . . . . . . . . . . . . . . . . . 9 9 1 0 3 Ve c t o r sa n dVe c t orPr od u c t s 3 . 1 Sc a l a r sa n dVe c t or s. . . . . . . . . . . . . . . . . . . . . . . . . . 3 . 2 Th eSc a l a r , orDotPr od u c t. . . . . . . . . . . . . . . . . . . . . 3 . 2 . 1 Th eL a wofCos i n e s. . . . . . . . . . . . . . . . . . . . . 3 . 3 Th eVe c t or , orCr os sPr od u c t. . . . . . . . . . . . . . . . . . . . 3 . 4 Tr i p l ePr odu c t sofVe c t or s. . . . . . . . . . . . . . . . . . . . . . 15 1 6 1 6 1 8 1 8 2 0 a n dǫ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . 5 δ 2 1 i j i j k. 3 . 5 . 1 Th eKr on e c k e rDe l t aF u n c t i ona n dt h eEi n s t e i nSu mma t i onCon v e n t i on2 1 3 . 5 . 2 Th eL e v i Ci v i t aTe n s or. . . . . . . . . . . . . . . . . . . 2 2 3 . 5 . 3 Th eEp s i l on De l t aI d e n t i t y. . . . . . . . . . . . . . . . . 2 2 4 Te n s o r s 4 . 1 Th eDy a da n dNa d i cF or ms. . . . . . . . . . . . . . . . . . . . 4 . 2 Coor d i n a t eTr a n s f or ma t i on s. . . . . . . . . . . . . . . . . . . . . 25 2 5 2 8 5 Gr ou pTh e or y 5 . 0 . 1 Su b g r ou p s. . . . . . . . . . . . . . . . . . . . . . . . . . . 5 . 0 . 2 Ab e l i a n( Commu t a t i v e )Gr ou p s. . . . . . . . . . . . . . . 5 . 0 . 3 L i e( Con t i n u ou s )Gr ou p s. . . . . . . . . . . . . . . . . . i 33 3 4 3 4 3 5 5 . 1 5 . 1 . 1 5 . 1 . 2 5 . 1 . 3 Coor d i n a t eTr a n s f or ma t i on Gr ou p s. . . . . . . . . . . . . . . . . 3 5 Th eTr a n s l a t i onGr ou p . . . . . . . . . . . . . . . . . . . 3 6 Th eRot a t i onGr ou p. . . . . . . . . . . . . . . . . . . . . 3 6 Th eI n v e r s i onGr ou p. . . . . . . . . . . . . . . . . . . . . 3 7 6Sc a l a ra n dVe c t orCa l c u l u s 6 . 1 Sc a l a rDi ffe r e n t i a t i on. . . . . . . . . . . . . . . . . . . . . . . . . 6 . 2 Ve c t orDi ffe r e n t i a t i on. . . . . . . . . . . . . . . . . . . . . . . . 6 . 2 . 1 Th ePa r t i a l De r i v a t i v e. . . . . . . . . . . . . . . . . . . . 6 . 3 Th eGr a di e n t. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . 4 Ve c t orDe r i v a t i v e s. . . . . . . . . . . . . . . . . . . . . . . . . . 6 . 4 . 1 Th eSu mRu l e s. . . . . . . . . . . . . . . . . . . . . . . . 6 . 4 . 2 Th ePr od u c tRu l e s. . . . . . . . . . . . . . . . . . . . . . 6 . 5 Se c on dDe r i v a t i v e s. . . . . . . . . . . . . . . . . . . . . . . . . . 6 . 6 Sc a l a rI n t e g r a t i on. . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . 6 . 1 Th eF u n d a me n t a l Th e or e mofCa l c u l u s. . . . . . . . . . 6 . 7 Ve c t orI n t e g r a t i on. . . . . . . . . . . . . . . . . . . . . . . . . . 6 . 8 Th eF u n d a me n t a l Th e or e m( s )ofVe c t orCa l c u l u s. . . . . . . . . 6 . 8 . 1 ASc a l a rF u n c t i onofVe c t orCoor d i n a t e s. . . . . . . . . . 6 . 8 . 2 Th eDi v e r g e n c eTh e or e m. . . . . . . . . . . . . . . . . . 6 . 8 . 3 St ok e s ’ Th e or e m. . . . . . . . . . . . . . . . . . . . . . . 6 . 9 I n t e g r a t i onb yPa r t s. . . . . . . . . . . . . . . . . . . . . . . . . 6 . 9 . 1 Sc a l a rI n t e g r a t i onb yPa r t s. . . . . . . . . . . . . . . . . 6 . 9 . 2 Ve c t orI n t e g r a t i onb yPa r t s. . . . . . . . . . . . . . . . . 6. 10 I n t e g r a t i onByPa r t si nE l e c t r od y n a mi c s. . . . . . . . . . . . . . 39 4 0 4 1 4 1 4 2 4 2 4 3 4 3 4 4 4 5 4 5 4 5 4 6 4 6 4 7 4 8 4 9 4 9 4 9 5 1 7Coo r d i n a t eSy s t e ms 7 . 1Ca r t e s i a n. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 . 2Sp h e r i c a l Pol a r. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 . 3Cy l i n dr i c a l. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5 7 5 8 6 0 8 Th eDi r a cδF u n c t i o n 63 9 Ma t hRe f e r e n c e s 67 I INo n Re l a t i v i s t i cE l e c t r od y n a mi c s 69 10Ma x we l l ’ sE qu a t i on s 1 0 . 1Th eMa x we l l Di s p l a c e me n tCu r r e n t. . . . . . . . . . . . . . . . . 1 0 . 2Pot e n t i a l s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 7 1 7 5 10. 2. 1 Ga u g eTr a n s f or ma t i on s. . . . . . . . . . . . . . . . . . . 10. 2. 2 Th eL or e n t zGa u g e. . . . . . . . . . . . . . . . . . . . . . 10. 2. 3 Th eCo u l omborTr a n s v e r s eGa u g e. . . . . . . . . . . . . 1 0 . 3Poy n t i n g ’ sTh e or e m, Wor ka n dEn e r g y. . . . . . . . . . . . . . . 1 0 . 4Ma g n e t i cMon o p ol e s. . . . . . . . . . . . . . . . . . . . . . . . . 7 7 7 9 8 1 8 4 8 8 1 0 . 4 . 1 Di r a cMon op ol e s. . . . . . . . . . . . . . . . . . . . . . . 8 9 11Pl a n eWa v e s 1 1 . 1Th eF r e eSp a c eWa v eE qu a t i on. . . . . . . . . . . . . . . . 1 1 . 1 . 1 Ma x we l l ’ sEqu a t i on s. . . . . . . . . . . . . . . . . . 1 1 . 1 . 2 Th eWa v eE qu a t i on. . . . . . . . . . . . . . . . . . 1 1 . 1 . 3 Pl a n eWa v e s. . . . . . . . . . . . . . . . . . . . . . 1 1 . 1 . 4 Pol a r i z a t i onofPl a n eWa v e s. . . . . . . . . . . . . . 1 1 . 2Re f l e c t i ona n dRe f r a c t i ona ta Pl a n eI n t e r f a c e. . . . . . . . 1 1 . 2 . 1 Ki n e ma t i c sa n dSn e l l ’ sL a w. . . . . . . . . . . . . . 1 1 . 2 . 2 Dy n a mi c sa n dRe f l e c t i on / Re f r a c t i on. . . . . . . . . 1 1 . 3Di s p e r s i on. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 . 3 . 1 St a t i cCa s e. . . . . . . . . . . . . . . . . . . . . . . 1 1 . 3 . 2 Dy n a mi cCa s e. . . . . . . . . . . . . . . . . . . . . 1 1 . 3 . 3 Th i n g st oNot e. . . . . . . . . . . . . . . . . . . . . 1 1 . 3 . 4 An oma l ou sDi s p e r s i on , a n dRe s on a n tAb s or p t i on. . 1 1 . 3 . 5 At t e n u a t i onb yac omp l e xǫ. . . . . . . . . . . . . . 1 1 . 3 . 6L owF r e qu e n c yBe h a v i or. . . . . . . . . . . . . . . . 1 1 . 3 . 7 Hi g hF r e qu e n c yL i mi t ; Pl a s maF r e qu e n c y. . . . . . 1 1 . 4Pe n e t r a t i onofWa v e sI n t oaCon d u c t or–Sk i nDe p t h. . . . 1 1 . 4 . 1 Wa v eAt t e n u a t i oni nTwoL i mi t s. . . . . . . . . . . 1 1 . 5Kr a me r s Kr on i gRe l a t i on s. . . . . . . . . . . . . . . . . . . 1 1 . 6Pl a n eWa v e sAs s i g n me n t. . . . . . . . . . . . . . . . . . . . 93 . . . 9 3 . . . 9 3 . . . 9 5 . . . 9 6 . . . 9 9 . . . 1 0 2 . . . 1 0 3 . . . 1 0 4 . . . 1 1 1 . . . 1 1 1 . . . 1 1 3 . . . 1 1 4 . . . 1 1 5 . . . 1 1 6 . . . 1 1 7 . . . 1 1 8 . . . 1 2 0 . . . 1 2 0 . . . 1 2 2 . . . 1 2 4 12Wa v eGu i d e s 1 2 . 1Bou n d a r yCon d i t i on sa taCo n d u c t i n gSu r f a c e : Sk i nDe p t h 1 2 . 2Mu t i l a t e dMa x we l l ’ sE qu a t i on s( MME s ). . . . . . . . . . . 1 2 . 3TEMWa v e s. . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 . 4TEa n dTMWa v e s. . . . . . . . . . . . . . . . . . . . . . . 1 2 . 4 . 1 TMWa v e s. . . . . . . . . . . . . . . . . . . . . . . 1 2 . 4 . 2 Su mma r yofTE/ TMwa v e s. . . . . . . . . . . . . . 1 2 . 5Re c t a n g u l a rWa v e g u i d e s. . . . . . . . . . . . . . . . . . . . 1 2 . 6Re s on a n tCa v i t i e s. . . . . . . . . . . . . . . . . . . . . . . 1 2 . 7Wa v eGu i d e sAs s i g n me n t. . . . . . . . . . . . . . . . . . . 127 . . . 1 2 7 . . . 1 3 3 . . . 1 3 6 . . . 1 3 7 . . . 1 3 9 . . . 1 4 0 . . . 1 4 1 . . . 1 4 2 . . . 1 4 3 13Ra d i a t i on 1 3 . 1Ma x we l l ’ sEqu a t i on s , Ye tAg a i n. . . . . . . . . . . . . . . . 1 3 . 1 . 1 Qu i c k i eRe v i e wofCh a p t e r6. . . . . . . . . . . . . 1 3 . 2Gr e e n ’ sF u n c t i on sf ort h eWa v eE qu a t i on. . . . . . . . . . 1 3 . 2 . 1 Poi s s onE qu a t i on. . . . . . . . . . . . . . . . . . . . 1 3 . 2 . 2 Gr e e n ’ sF u n c t i onf ort h eHe l mh ol t zEqu a t i on. . . . 1 3 . 2 . 3 Gr e e n ’ sF u n c t i onf ort h eWa v eE qu a t i on. . . . . . . 1 3 . 3Si mp l eRa di a t i n gSy s t e ms. . . . . . . . . . . . . . . . . . . 145 . . . 1 4 5 . . . 1 4 5 . . . 1 4 7 . . . 1 4 9 . . . 1 4 9 . . . 1 5 1 . . . 1 5 4 1 3 . 3 . 1 Th eZon e s. . . . . . . . . . . . . . . . . . . . . . . . 1 3 . 3 . 2 Th eNe a rZon e. . . . . . . . . . . . . . . . . . . . . 1 3 . 3 . 3 Th eF a rZon e. . . . . . . . . . . . . . . . . . . . . . . . . 1 5 5 . . . 1 5 6 . . . 1 5 7 1 3 . 4Th eHomog e n e ou sHe l mh ol t zEqu a t i on. . . . . . . . . . . . 13. 4. 1 Pr op e r t i e sofSp h e r i c a l Be s s e l F u n c t i on s. . . . . . . . . . 1 5 8 . . . 1 5 9 ( r ) , NL( r ) , a n dHL( r ). . . . . . . . . . . . . . . 13. 4. 2J L 13. 4. 3 Ge n e r a l Sol u t i on st ot h eHHE. . . . . . . . . . . . . 13. 4. 4 Gr e e n ’ sF u n c t i on sa n dF r e eSp h e r i c a l Wa v e s. . . . . 1 3 . 5E l e c t r i cDi p ol eRa d i a t i on. . . . . . . . . . . . . . . . . . . 13. 5. 1 Ra d i a t i onou t s i d et h es ou r c e. . . . . . . . . . . . . 13. 5. 2 Di po l eRa d i a t i on. . . . . . . . . . . . . . . . . . 1 3 . 6Ma g n e t i cDi p ol ea n dEl e c t r i cQu a d r u p ol eRa d i a t i onF i e l d s 13. 6. 1 Ma g n e t i cDi p ol eRa d i a t i on. . . . . . . . . . . . . . 13. 6. 2E l e c t r i cQu a d r u p ol eRa d i a t i on. . . . . . . . . . . . 1 3 . 7Ra d i a t i onAs s i g n me n t. . . . . . . . . . . . . . . . . . . . . . . . 1 6 1 . . . 1 6 1 . . . 1 6 2 . . . 1 6 3 . . . 1 6 4 . . . 1 6 4 . . . 1 6 8 . . . 1 6 9 . . . 1 7 0 . . . 1 7 3 ± 14Ve c t orMu l t i p o l e s 1 4 . 1An g u l a rmome n t u ma n ds p h e r i c a l h a r mon i c s. . . . . . . . 1 4 . 2Ma g n e t i ca n dEl e c t r i cMu l t i p ol e sRe v i s i t e d. . . . . . . . . 1 4 . 3Ve c t orSp h e r i c a l Ha r mon i c sa n dMu l t i p ol e s. . . . . . . . . 177 . . . 1 7 7 . . . 1 7 9 . . . 1 8 1 15Th eHa n s e nMu l t i p ol e s 1 5 . 1Th eHa n s e nMu l t i p ol e s. . . . . . . . . . . . . . . . . . . . 15. 1. 1 Th eBa s i cSol u t i on s. . . . . . . . . . . . . . . . . . 15. 1. 2 Th e i rSi g n i f i c a n tPr op e r t i e s. . . . . . . . . . . . . . 15. 1. 3E x p l i c i tF or ms. . . . . . . . . . . . . . . . . . . . . 1 5 . 2Gr e e n ’ sF u n c t i on sf ort h eVe c t orHe l mh ol t zEqu a t i on. . . . 1 5 . 3Mu l t i p ol a rRa di a t i on , r e v i s i t e d. . . . . . . . . . . . . . . . 1 5 . 4AL i n e a rCe n t e r F e dHa l f Wa v eAn t e n n a. . . . . . . . . . . 1 5 . 5Con n e c t i ont oOl d( Ap p r ox i ma t e )Mu l t i p ol eMome n t s. . . 1 5 . 6An g u l a rMome n t u mF l u x. . . . . . . . . . . . . . . . . . . 1 5 . 7Con c l u d i n gRe ma r k sAb ou tMu l t i p ol e s. . . . . . . . . . . . 1 5 . 8Ta b l eofPr op e r t i e sofVe c t orHa r mon i c s. . . . . . . . . . . 189 . . . 1 8 9 . . . 1 8 9 . . . 1 9 0 . . . 1 9 0 . . . 1 9 1 . . . 1 9 2 . . . 1 9 9 . . . 2 0 2 . . . 2 0 4 . . . 2 0 7 . . . 2 0 8 16Op t i c a l Sc a t t e r i n g 1 6 . 1Ra d i a t i onRe a c t i onofaPol a r i z a b l eMe d i u m. . . . . . . . . 1 6 . 2Sc a t t e r i n gf r omaSma l l Di e l e c t r i cSp h e r e. . . . . . . . . . 1 6 . 3Sc a t t e r i n gf r omaSma l l Con d u c t i n gSp h e r e. . . . . . . . . 1 6 . 4Ma n ySc a t t e r e r s. . . . . . . . . . . . . . . . . . . . . . . . 211 . . . 2 1 1 . . . 2 1 4 . . . 2 1 9 . . . 2 2 2 I I IRe l a t i v i s t i cE l e c t r o d y n a mi c s 227 17Sp e c i a l Re l a t i v i t y 1 7 . 1E i n s t e i n ’ sPos t u l a t e s. . . . . . . . . . . . . . . . . . . . . . . . . 1 7 . 2Th eEl e me n t a r yL or e n t zTr a n s f or ma t i on. . . . . . . . . . . . . . 1 7 . 34 Ve c t or s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 2 2 9 2 3 0 2 3 4 1 7 . 4Pr op e rTi mea n dTi meDi l a t i on. . . . . . . . . . . . . . . . . . . 1 7 . 5Add i t i onofVe l oc i t i e s. . . . . . . . . . . . . . . . . . . . . . . . 2 3 9 2 4 0 1 7 . 6Re l a t i v i s t i cE n e r g ya n dMome n t u m. . . . . . . . . . . . . . . . 2 4 2 18Th eL o r e n t zGr ou p 1 8 . 1Th eGe ome t r yofSp a c e –Ti me. . . . . . . . . . . . . . . . 1 8 . 2Te n s or si n4Di me n s i on s. . . . . . . . . . . . . . . . . . . 1 8 . 3Th eMe t r i cTe n s or. . . . . . . . . . . . . . . . . . . . . . 1 8 . 4Ge n e r a t or soft h eL or e n t zGr ou p. . . . . . . . . . . . . . 18. 4. 1I n f i n i t e s i ma l Tr a n s f or ma t i on s. . . . . . . . . . . . 1 8 . 5Th oma sPr e c e s s i on. . . . . . . . . . . . . . . . . . . . . . 1 8 . 6Cov a r i a n tF or mu l a t i onofEl e c t r od y n a mi c s. . . . . . . . . 1 8 . 7Th eTr a n s f or ma t i onofEl e c t r oma g n e t i cF i e l d s. . . . . . . 247 . . . . 2 4 7 . . . . 2 4 9 . . . . 2 5 1 . . . . 2 5 4 . . . . 2 5 6 . . . . 2 6 4 . . . . 2 7 0 . . . . 2 7 4 19Re l a t i v i s t i cDy n a mi c s 1 9 . 1Cov a r i a n tF i e l dTh e or y. . . . . . . . . . . . . . . . . . . 19. 1. 1 Th eBr u t eF or c eWa y. . . . . . . . . . . . . . . . 19. 1. 2 Th eEl e g a n tWa y. . . . . . . . . . . . . . . . . . . 1 9 . 2Mot i onofaPoi n tCh a r g ei naSt a t i cMa g n e t i cF i e l d. . . 1 9 . 3Bu i l d i n gaRe l a t i v i s t i cF i e l dTh e or y. . . . . . . . . . . . 1 9 . 4Th eSy mme t r i cSt r e s sTe n s or. . . . . . . . . . . . . . . . 1 9 . 5Cov a r i a n tGr e e n ’ sF u n c t i on s. . . . . . . . . . . . . . . . . 277 . . . . 2 7 7 . . . . 2 7 7 . . . . 2 8 1 . . . . 2 8 6 . . . . 2 8 7 . . . . 2 9 0 . . . . 2 9 2 20Ra d i a t i onf r o mPo i n tCh a r ge s 2 0 . 1L a r mor ’ sF or mu l a. . . . . . . . . . . . . . . . . . . . . . . 2 0 . 2Th oms onSc a t t e r i n gofRa d i a t i on. . . . . . . . . . . . . . 299 . . . . 3 0 3 . . . . 3 0 7 21Ra d i a t i onRe a c t i o n 2 1 . 1Th eDe a t hofCl a s s i c a l Ph y s i c s. . . . . . . . . . . . . . . 2 1 . 2Ra d i a t i onRe a c t i ona n dE n e r g yCon s e r v a t i on. . . . . . . 2 1 . 3I n t e g r od i ffe r e n t i a l Equ a t i on sofMot i on. . . . . . . . . . . 2 1 . 4Ra d i a t i onDa mp i n gofa nOs c i l l a t i n gCh a r g e. . . . . . . 2 1 . 5Di r a c ’ sDe r i v a t i onofRa di a t i onRe a c t i on. . . . . . . . . . 2 1 . 6Wh e e l e ra n dF e y n ma n ’ sDe r i v a t i onofRa d i a t i onRe a c t i on 2 1 . 7MyOwnF i e l d F r e eDe r i v a t i onofRa d i a t i onRe a c t i on. . . 311 . . . . 3 1 1 . . . . 3 1 3 . . . . 3 1 7 . . . . 3 1 9 . . . . 3 2 2 . . . . 3 2 2 . . . . 3 2 2 Pr e f a c e Cl a s s i c a l El e c t r ody n a mi c si son eoft h emos tb e a u t i f u l t h i n g si nt h ewor l d .F ou rs i mp l e v e c t ore qu a t i on s( oron et e n s ore qu a t i ona n da na s s s oc i a t e dd u a l )de s c r i b et h eu n i f i e d e l e c t r oma g n e t i cf i e l da n dmor eorl e s sd i r e c t l yi mp l yt h et h e or yofr e l a t i v i t y .Th e di s c ov e r ya n dp r ooft h a tl i g h ti sa ne l e c t r oma g n e t i cwa v ea n du n i f i e st wof i e l d ss t a n d s t ot h i sd a ya son eoft h eg r e a t e s tmome n t si nt h eh i s t or yofs c i e n c e . Th e s ef ou re qu a t i on se v e nc on t a i nwi t h i nt h e mt h es e e d soft h e i rownde s t r u c t i ona sa c l a s s i c a lt h e or y .On c eMa x we l l ’ se qu a t i on swe r ek n owni nt h e i re n t i r e t y ,i tr a p i dl yb e c a me c l e a rt h a tt h e i rp r e d i c t i on s–h owe v e rb e a u t i f u l l yv e r i f i e dt h e ywe r ef orf r e e l yp r op a g a t i n g f i e l d sa n dt h ec on n e c t i onoft h os ef i e l dswi t hma c r os c op i cc h a r g e / c u r r e n td i s t r i b u t i on s– we r ei n c on s i s t e n twi t hv i r t u a l l ya l lob s e r v a t i on sa tt h ea t omi corn u c l e a rl e v e l .Th i sf or c e d t h ec l a s s i c i s t soft h ed a y ,ma n yoft h e m me t a p h or i c a l l yk i c k i n gors c r e a mi n g ,t oi n v e n t q u a n t u mme c h a n i c sa n dqu a n t u me l e c t r ody n a mi c st oe x p l a i np h y s i c sa tt h i ss c a l e . I n d e e d ,on c et h es i n g l ef a c tt h a ta na c c e l e r a t e dc h a r g e dp a r t i c l en e c e s s a r i l y r a d i a t e se l e c t r oma g n e t i ce n e r g ywa sk n own ,i tb e c a me v i r t u a l l yi mp os s i b l et o c on c e p t u a l l ye x p l a i nt h ep e r s i s t e n c eofs t r u c t u r ea tt h emi c r os c op i cl e v e l( s i n c et h e f or c e sa s s oc i a t e dwi t hb i n d i n gob j e c t st og e t h e rou tofdi s c r e t ec h a r g e dp a r t si n e v i t a b l y p r od u c ea nos c i l l a t i onofc h a r g ed u et os ma l lp e r t u r b a t i on sofp os i t i on ,wi t ha n a s s oc i a t e da c c e l e r a t i on ) .Th ef e wh y p ot h e s e st h a twe r ea dv a n c e dt oa c c ou n tf ori t “ wi t h ou t ”a nov e r t l yos c i l l a t or ymod e lwe r er a p i d l ya n dd e c i s i v e l ys h otd ownb y( n ow f a mou s )e x p e r i me n t sb yRu t h e r f or d, Mi l l i k a n , a n dot h e r s . E v e nt h ou g ht h eUn i v e r s ep r ov e st ob equ a n t u mme c h a n i c a la tt h emi c r os c op i cl e v e l , c l a s s i c a le l e c t r ody n a mi c si sn e v e r t h e l e s se x t r e me l yr e l e v a n ta n du s e f u li nt h er e a lwor l d t od a ya tt h ema c r os c o p i cl e v e l .I td e s c r i b e se x t r e me l yp r e c i s e l yn e a r l ya l lt h emu n d a n e a s p e c t sofor d i n a r ye l e c t r i c a le n g i n e e r i n ga n de l e c t r oma g n e t i cr a d i a t i onf r om t h es t a t i c l i mi tt h r ou g hop t i c a lf r e qu e n c i e s .E v e na tt h emol e c u l a rl e v e lorp h ot on i cl e v e lwh e r ei t b r e a k sdowna n daqu a n t u mt h e or ymu s tb eu s e di ti sf i r s tn e c e s s a r yt ou n d e r s t a n dt h e c l a s s i c a l t h e or yb e f or ee x p l or i n gt h equ a n t u mt h e or y , a st h equ a n t u mt h e or yi sb u i l tont op oft h ee n t i r er e l a t i v i s t i ce l e c t r od y n a mi cc on c e p t u a l f r a me wor ka l r e a d ye s t a b l i s h e d . Th i ss e tofl e c t u r en ot e si sde s i g n e dt ob eu s e dt ot e a c hg r a d u a t es t u de n t s( a n d p os s i b l ya d v a n c e da n dmot i v a t e du n d e r g r a d u a t e s )c l a s s i c a l e l e c t r od y n a mi c s .I np a r t i c u l a r , i ts u p p or t st h es e c on d( mor ed i ffic u l t )s e me s t e rofat wo v i i s e me s t e rc ou r s ei ne l e c t r od y n a mi c st h a tc ov e r sp r e t t ymu c h“ a l l ”oft h et h e or yi t s e l f ( omi t t i n g , ofc ou r s e , ma n yt op i c sors p e c i f i ca r e a swh e r ei tc a nb ea p p l i e d )ou tt ot h e p oi n t swh e r et h et h e o r yi t s e l fb r e a k sd owna sn ot e da b ov e .Att h a tp oi n t ,t oma k e f u r t h e rp r o g r e s sas t u d e n tn e e dst ol e a r na b ou tmor ef i e l d s ,qu a n t u m( f i e l d )t h e or y , a dv a n c e d( g e n e r a l )r e l a t i v i t y–t op i c sg e n e r a l l yb e y on dt h es c op eoft h e s en ot e s . Th er e qu i r e me n t sf ort h i sc ou r s ei n c l u d eat h or ou g hu n de r s t a n di n gofe l e c t r i c i t ya n d ma g n e t i s ma tt h el e v e lofa tl e a s ton e ,i d e a l l yt wo,u n d e r g r a d u a t ec ou r s e s .AtDu k e ,f or e x a mp l e ,p h y s i c sma j or sa r ef i r s te x p os e df i r s tt oa ni n t r odu c t or yc ou r s et h a tc ov e r st h e i n t e g r a lf or mu l a t i onofMa x we l l ’ se qu a t i on sa n dl i g h tt h a tu s e sn omu l t i v a r i a t ed i ffe r e n t i a l c a l c u l u s ,t h e nas e c on dc ou r s et h a td e v e l op st h ev e c t ordi ffe r e n t i a lf or mu l a t i onof Ma x we l l ’ se qu a t i on sa n dt h e i rc on s e qu e n c e s )a sd oe st h i sc ou r s e )b u twi t hc on s i d e r a b l y l e s sma t h e ma t i c a lr i g ora n dc omp l e t e n e s soft h et r e a t me n ta ss t u d e n t st a k i n gi th a v e l i k e l ys t i l l n oth a dac ou r s ei ne . g .c on t ou ri n t e g r a t i on .St u d e n t su s i n gt h e s en ot e swi l lf i n d i tu s e f u lt ob ea tl e a s ts ome wh a tc omf or t a b l ewi t hv e c t ord i ffe r e n t i a l a n di n t e g r a lc a l c u l u s , t oh a v eh a de x p os u r et ot h et h e or ya n ds ol u t i onme t h od ol og yofor d i n a r ya n dp a r t i a l d i ffe r e n t i a le qu a t i on s ,t ob ef a mi l i a rwi t ht h ema t h e ma t i c sofc omp l e xv a r i a b l e sa n d a n a l y t i cf u n c t i on s ,c on t o u ri n t e g r a t i on ,a n di twou l db es i mp l yl ov e l yi ft h e ya tl e a s tk n e w wh a ta“ t e n s or ”wa s . Howe v e r ,e v e nmor es ot h a ni st h ec a s ef ormos tp h y s i c st e x t s ,t h i sb ookwi l l e n d e a v ort op r ov i d ei n t e r n a l s u p p or tf ors t u d e n t st h a ta r ewe a ki non eormor eoft h e s e r e qu i r e dma t h e ma t i c a ls k i l l s .Th i ss u p p or twi l lc omei non eofs e v e r a lf or ms .Att h e v e r yl e a s t ,c on s i d e r a b l ee ffor th a sb e e nma det oh u n td ownonb e h a l foft h es t u d e n t a n de x p l i c i t l yr e c omme n du s e f u lt e x t b ook sa n d on l i n er e s ou r c e s on v a r i ou s ma t h e ma t i c a l a n dp h y s i c a l t op i c st h a tma yb eofu s et ot h e m. Ma n yoft h e s er e s ou r c e s a r ef r e e l ya v a i l a b l e on t h e we b .Some ma t h e ma t i c a lme t h od sa r ec omp l e t e l y d e v e l op e di nt h ec on t e x toft h ed i s c u s s i on , e i t h e rb e c a u s ei tma k e ss e n s et od os oor b e c a u s et h e r es i mp l ya r en or e f e r e n c e sas t u d e n ti sl i k e l yt ob ea b l et of i n d .F i n a l l y , s e l e c t e dt op i c swi l lb ec ov e r e di ne . g .a p p e n d i c e sora si n s e r t i on si nt h et e x twh e r e t h e ya r es h or te n ou g ht ob ec ov e r a b l ei nt h i swa ya n di mp or t a n te n ou g ht h a ts t u d e n t s a r el i k e l yt ob eh i g h l yc on f u s e dwi t h ou tt h i ss or tofs u p p or t . Av e r yb r i e fr e v i e wo ft h ee l e c t r o d y n a mi c st op i c sc ov e r e di n c l u d e s :Ma x we l l ’ se qu a t i on s t h e ms e l v e s( s k i p p i n gt h eu s u a l c ov e r a g eofe l e c t r os t a t i c sa n dma g n e t os t a t i c st h a tof t e nma k e s u pt h ef i r s ts e me s t e rofat wos e me s t e rc ou r s e ) ,t h e np l a n ewa v e s ,d i s p e r s i on ,p e n e t r a t i onof wa v e sa tab ou n d a r y( s k i nd e p t h ) ,wa v eg u i d e sa n dc a v i t i e sa n dt h ev a r i ou s( TE,TM,TE M) mod e sa s s oc i a t e dwi t ht h e m, a n dr a di a t i oni nt h emor eg e n e r a l c a s eb e g i n n i n gwi t hs ou r c e s . I nt h ec ou r s eofs t u dy i n gr a d i a t i onf r om s ou r c e swede v e l opmu l t i p ol a rr a di a t i oni n de t a i l .Th i st e x ti n c l u d e saf a i r l yt h or ou g he x p os i t i onoft h eu n de r l y i n gPDE s , t h ep r op e r t i e s oft h eGr e e n ’ sf u n c t i on su s e dt og e n e r a t emu l t i p ol e sb ot ha p p r ox i ma t ea n de x a c t ,a n d f or ma l l yp r e c i s es ol u t i on st h a te x t e n di n s i d et h es ou r c ec h a r g e c u r r e n td e n s i t y( a si n d e e d t h e ymu s tf ort h i sf or ma l i s mt ob eofu s ei ne . g .s e l f c on s i s t e n tf i e l dt h e or i e st r e a t i n g e x t e n d e dc h a r g ed e n s i t yd i s t r i b u t i on s ) .I na d d i t i ont ot h ev e c t ors p h e r i c a lh a r mon i c s ,i t d e f i n e sa n dd e r i v e s t h ep r op e r t i e soft h eHa n s e nmu l t i p ol e s( wh i c ha r eot h e r wi s ev e r yn e a r l yal os ta r t ) de mon s t r a t i n gt h e i rp r a c t i c a lu t i l i t ywi t he x a mp l ep r ob l e msi n v ol v i n ga n t e n n a e .I t c on c l u d e st h i sp a r toft h ee x p os i t i onwi t has h or td e s c r i p t i onofop t i c a ls c a t t e r i n ga s wa v e si n t e r a c twi t h“ me di a ” , e . g . s ma l l s p h e r e si n t e n d e dt omod e l a t omsormol e c u l e s . Th et e x tt h e np r oc e d e st od e v e l opr e l a t i v i t yt h e or y ,f i r s tr e v i e wi n gt h ee l e me n t a r y t h e or yp r e s u ma b l ya l r e a d yf a mi l i a rt os t u de n t s , t h e nde v e l op i n gt h ef u l l L or e n t zGr ou p . Ass t u de n t st e n dt on o tb ef a mi l i a rwi t ht e n s or s , t h en o t e sc on t a i nas p e c i a la p p e n d i x ont e n s or sa n dt e n s orn ot a t i ona sas u p p l e me n t .I ta l s oc on t a i n sab i tofs u p p l e me n t a l s u p p or tona tl e a s tt h os ea s p e c t sofc on t ou ri n t e g r a t i onr e l e v a n tt ot h ec ou r s ef or s i mi l a rr e a s on s .Wi t hr e l a t i v i t yi nh a n d ,r e l a t i v i s t i ce l e c t r od y n a mi c si sd e v e l op e d , i n c l u d i n gt h ep r op e r t i e sofr a d i a t i one mi t t e df r omap oi n tc h a r g ea si ti sa c c e l e r a t e d. F i n a l l y , t h et e x tc on c l u d e swi t han i c eov e r v i e wofr a d i a t i onr e a c t i on( e x p l or i n gt h e wor kofL or e n t z ,Di r a c ,a n dWh e e l e ra n dF e y n ma n )a n dt h ep u z z l e st h e r e i n–s e l f i n t e r a c t i onv e r s u sa c t i ona tadi s t a n c e ,t h en e e df orac l a s s i c a lr e n or ma l i z a t i oni na t h e or yb a s e dons e l f i n t e r a c t i on .Th i sma k e st h et e x tj u s tab i tt ool on gt op r e s e n ti na s i n g l es e me s t e r( a tl e a s tt omyowne x p e r i e n c e ) ; i n s t r u c t or st h a tb e g i nwi t hMa x we l l ’ s e qu a t i on si nd e t a i l ( i n c l u di n gt h et r e a t me n tofmon op ol e s )ma yn oth a v et i met og e tt o r a d i a t i onr e a c t i on , b u ti n s t r u c t or swh ob e g i nwi t hp l a n ewa v e sorwa v e g u i de sl i k e l ywi l l . On en ot e wor t h yf e a t u r eoft h i st e x ti ni t son l i n ef o r m( s or r y , b u tIdol i k ep u n sa n d y ou ’ l lj u s th a v et og e tu s e dt ot h e m: )i st h a tt h ee l e c t r on i c / on l i n ev e r s i onoft h e m 1 i n c l u d e ss e v e r a li n v e n t i on sofmyowns u c ha sawi k i n ot e, ar e f e r e n c et os u p p or t i n g wi k i p e d i aa r t i c l e st h a ta p p e a r sa saURLa n df oot n ot ei nt h et e x tc op yb u twh i c hi sa n a c t i v el i n ki naPDForHTML( on l i n e )c op y .Si mi l a r l y ,t h e r ea r eg oog l el i n k sa n d or di n a r ywe bl i n k sp r e s e n t e di nt h es a mewa y . Th i st e x tas e tofr e a ll e c t u r en ot e sa n di st h e r e f or el i k e l yt oc h a n g ea st h e ya r e u s e d ,s e me s t e rb ys e me s t e r .I ns omec a s e st h ec h a n g e sa r equ i t ei mp or t a n t ,f or e x a mp l ewh e nak i n dr e a de rg e n t l yp oi n t sou tab on e h e a d e dmi s t a k eIma d et h a t ma k e ss omea s p e c toft h ep h y s i c sorp r e s e n t a t i onqu i t ei n c or r e c t .I not h e r st h e ya r e s ma l l e ri mp r ov e me n t s :an e wl i n k ,as l i g h t l yi mp r ov e ddi s c u s s i on ,f i x i n gc l u ms y l a n g u a g e ,an e wf i g u r e( orp u t t i n gi non eoft h emi s s i n gol don e s ) ,mor eorb e t t e r p r ob l e ms . F ora l loft h e s er e a s o n s , s t u d e n t swh oa r eu s i n gt h i st e x t b ookma ywi s ht oh a v eb ot ha b ou n dp a p e rc op y( h ome ma d eorp u r c h a s e df oraf a i r l yn omi n a l s u m 1Wi k i p e d i a : h t t p : / / www. wi k i p e di a . or g / wi k i / wi k i p e di a .Awi k i n ot ei sb a s i c a l l yaf oot n ot et h a tdi r e c t sas t u de n tt o au s e f u la r t i c l ei nt h eWi k i p e d i a .Th e r ei ss ome( f r a n k l ys i l l y )c on t r ov e r s yonj u s th owa c c u r a t ea n du s e f u lt h e Wi k i p e d i ai sf ors c h ol a r l ywor k ,b u tf ort e a c h i n gorl e a r n i n gs c i e n c ea n dma t h e ma t i c sony ou rowni ti sr a p i d l y b e c omi n gi n di s p e n s i b l ea ss omee x c e l l e n ta r t i c l e sa r ec on s t a n t l yb e i n ga d d e da n di mp r ov e dt h a tc ov e r ,b a s i c a l l y , a l lofe l e c t r ody n a mi c sa n dt h er e qu i s i t es u p p or t i n gma t h e ma t i c s .Pe r s on a l l y , It h i n kt h eob j e c t i on st oi ta r el a r g e l y e c on omi c–i naf e wmor ey e a r st h i ss u p e r bf r e er e s ou r c ewi l le s s e n t i a l l yde s t r oyt h el u c r a t i v et e x t b ookma r k e t a l t og e t h e r , wh i c hh on e s t l yi sp r o b a b l yag oodt h i n g .Att h ev e r yl e a s t , at e x t b ookwi l l h a v et oa d ds i g n i f i c a n tv a l u et o s u r v i v e , a n dma y b ewi l l b eab i tl e s se x p e n s i v et h a nt h e$ 1 0 0 a b ookc u r r e n ts t a n d a r d . t h r ou g hL u l uorAma z on )–t h a twi l li n e v i t a b l yc on t a i nomi s s i on sa n dmi s t a k e sor ma t e r i a lId on ’ ta c t u a l l yc ov e ri nt h i sy e a r ’ sc l a s s–a n dt h ec u r r e n te l e c t r on i cc op y .I g e n e r a l l yma i n t a i nt h ec u r r e n ts n a p s h otoft h ee l e c t r on i cc op yt h a tI ’ ma c t u a l l yu s i n g t ot e a c hf r omwh e r ei ti sa v a i l a b l e , f orf r e et oa l l c ome r s , onmyp e r s on a l / c l a s swe b s i t e a t : h t t p : / / www. p h y . d u k e . e d u / ∼r g b / Cl a s s / E l e c t r ody n a mi c s . p h p ( wh i c hc l e v e r l ya n ds e l f c on s i s t e n t l yde mon s t r a t e sa na c t i v el i n ki na c t i on ,a sdi dt h e wi k i l i n ka b ov e ) .I nt h i swa yas t u d e n tori n s t r u c t orc a nh a v et h ec on v e n i e n c eofa s l i g h t l y ou t of d a t ep a p e rc op yt ob r ows eo rs t u d yorf ol l owa n dma r ku pdu r i n gl e c t u r e a swe l l a sa ne l e c t r on i cc op yt h a ti su pt oda t ea n dwh i c hc on t a i n su s e f u l a c t i v el i n k s . L e ti tb en ot e dt h a tI ’ ma sg r e e dya n dn e e d ya st h en e x th u ma n , a n dc a na l wa y su s e e x t r amon e y .AsI ’ v ewo r k e dqu i t eh a r dont h i st e x t( a n df r omob s e r v a t i ont h e yg oqu i t e b e y on dwh a te . g .mos tofmyc ol l e a g u e si nt h ep h y s i c swor l dma k ea v a i l a b l ea son l i n e n ot e sf ort h e i rownc ou r s e s )a n dIh a v ed on et h ewor kr e qu i r e dt ot r a n s f or mt h e mi n t o a na c t u a lb ou n db ookt h a ts t u d e n t sc a ne l e c tt op u r c h a s ea l la ton c ei n s t e a dof d own l oa d i n gt h ef r e ePDF , p r i n t i n gi tou ta st wos i d e dp a g e s , p u n c h i n gi t , a n di n s e r t i n g i ti n t oat h r e er i n gb i n d e rt h a ta n on y mou s l yj oi n st h er e s toft h e i rn ot e sa n du l t i ma t e l y i st h r owna wa yorl os t . Th i sp r i n t e db ooki sr e ma r k a b l yi n e x p e n s i v eb yt h es t a n da r dsofmod e r nt e x t b ook s ( wh e r ee . gWy l d, wh i c hIo n c ep u r c h a s e dn owa t$ 1 6ac op y , i sn ota v a i l a b l en e wf or$ 70a c op y ) .Att h es a mes i t e , s t u d e n t sc a nf i n dt h ea c t u a l PDFf r omwh i c ht h eb ooki sg e n e r a t e d a v a i l a b l ef orav e r yl owc os ta n da r ea tl i b e r t yt op u r c h a s ea n dk e e pt h a tont h e i rp e r s on a l l a p t op sorPDF c a p a b l ee b ookr e a d e r s , orf ort h a tma t t e rt oh a v ei tp r i n t e da n db ou n db ya l oc a l p r i n t e r .I nb ot hc a s e sI ma k eas ma l l r oy a l t y( ont h eor d e rof$ 5 )f r omt h e i rs a l e , wh i c h i sb ot hf a i ra n dh e l p ss u p p or tmes ot h a tI c a nwr i t emor et e x t ss u c ha st h i s . Howe v e r ,s t u d e n t sa r ou n dt h ewor l dh a v ev e r yd i ffe r e n tme a n s .Pu r c h a s i n ga$ 7 . 5 0 d own l oa di nt h eUn i t e dSt a t e sme a n s( f ormo s ts t u d e n t s )t h a tas t u d e n th a st og i v eu pa f e wL a t t eE n or me sf r om St a r b u c k s .Pu r c h a s i n gt h a ts a med own l oa dc ou l db ear e a l h a r d s h i pf ors t u d e n t sf r om ma n yc ou n t r i e sa r ou n dt h ewor l di n c l u d i n gs omef r om t h e Un i t e dSt a t e s .F ort h i sr e a s ons t u d e n t swi l la l wa y sh a v et h eop t i onofu s i n gt h eon l i n e n ot e sd i r e c t l yf r omt h ec l a s swe b s i t ef orf r e eorp r i n t i n gt h e i rownc op yonp a p e ra tc os t . Al lt h a tIa s kofs t u de n t swh oe l e c tt ou s et h e mf orf r e ei st h a tt h e y“ p a yi tf or wa r d ”–t h a t on ed a yt h e yh e l pot h e r swh oa r el e s sf or t u n a t ei ns omewa yf orf r e es ot h a twec a na l l k e e pt h ewor l dmov i n ga l on gi nap os i t i v ed i r e c t i on . Th eon er e s t r i c t i onIh a v e , a n dIt h i n ki ti se n t i r e l yf a i r , i st h a ti n s t r u c t or swh oe l e c tt o u s et h e s en ot e st oh e l ps u p p or tt h et e a c h i n goft h e i rownc l a s s e s( e i t h e rb u i l d i n gt h e mwi t h orwi t h ou tmod i f i c a t i on sf r omt h es ou r c e soru s i n ga n yoft h ef r e ep r e b u i l ti ma g e s )ma yn ot r e s e l lt h e s en ot e st ot h e i rowns t u d e n t sf orap r of i torot h e r wi s ewi t h ou tmye x p l i c i t p e r mi s s i on , n orma yt h e ya l t e rt h i sp r e f a c e , t h ea u t h or s h i porc op y r i g h tn ot i c e( b a s i c a l l ya l l t h ef r on t ma t t e r )ort h el i c e n s e .I n s t r u c t or sa r ef r e et oa d dt oore d i tt h ec on t e n tt os u p p or t t h e i rownc l a s s , h owe v e r , a n dt h en ot e ss h ou l de a s i l yb u i l dona n ye . g . l i n u xs y s t e m. An y wa y ,g oodl u c ka n dr e me mb e rt h a tId oc h e r i s hf e e d b a c kofa l ls or t s ,c or r e c t i on s , a d d i t i on s( e s p e c i a l l yi nr e a d y t ob u i l dl a t e xwi t hE PSf i g u r e s : ) , s u g g e s t i on s , c r i t i c i s ms , a n d orc ou r s emon e y . Youc a na l wa y ss e n dmemon e y . . . 0. 1 Th eI n t e r p l a yo fPh y s i c sa n d Ma t h e ma t i c s Be f or eweb e g i n , i ti swor t hma k i n gon ev e r yi mp or t a n tr e ma r kt h a tc a ng u i d eas t u de n t a st h e yt r yt oma k es e n s eoft h ema n y , ma n yt h i n g sde v e l op e di nt h i swor k .Asy oug o t h r ou g ht h i sma t e r i a l , t h e r ewi l l b eas t r o n gt e n de n c yt ov i e wi ta l l a sb e i n gn ot h i n gb u t ma t h e ma t i c s .F ore x a mp l e ,we ’ l ls p e n dal otoft i mes t u dy i n gt h ewa v e( p a r t i a l d i ffe r e n t i a l )e qu a t i on , Gr e e n ’ sf u n c t i on s , a n dt h el i k e .Th i swi l l“ f e e ll i k e ”ma t h e ma t i c s . Th i si nt u r ni n s p i r e ss t u de n t st oa tl e a s ti n i t i a l l yv i e we v e r yh ome wor kp r ob l e m,e v e r y c l a s sd e r i v a t i on , a sb e i n gj u s ta n ot h e rp i e c eofa l g e b r a . Th i si sab a dwa yt ov i e wi t .Don ’ tdot h i s .Th i si sap h y s i c sc ou r s e ,a n dt h e d i ffe r e n c eb e t we e np h y s i c sa n da b s t r a c tma t h e ma t i c si st h a tp h y s i c s me a n s s ome t h i n g ,a n dt h ema t h e ma t i c su s e di np h y s i c si sa l wa y sgr o u n d e di np h y s i c a ll a w. Th i sme a n st h a ts ol v i n gt h ev e r yd i ffic u l tp r ob l e msa s s i g n e dt h r ou g h ou tt h es e me s t e r , u n d e r s t a n d i n gt h el e c t u r e sa n dn ot e s ,de v e l op i n gac on c e p t u a lu n de r s t a n di n goft h e p h y s i c si n v ol v e san u mb e rofme n t a la c t i on s ,n otj u s ton e ,a n dr e qu i r e sy ou rwh ol e b r a i n , n otj u s tt h es y mb ol i cs e qu e n t i a l r e a s on i n gp or t i on sofy ou rl e f tb r a i n . Tode v e l opi n s i gh ta swe l l a sp r ob l e ms ol v i n gs k i l l s , y oun e e dt ob ea b l et o: •Vi s u a l i z e wh a t ’ sg oi n g on .E l e c t r od y n a mi c si si n c r e d i b l yg e ome t r i c .Vi s u a l i z a t i ona n ds p a t i ot e mp or a lr e l a t i on s h i p sa r ea l lr i gh tb r a i nf u n c t i on sa n d t r a n s c e n da n dg u i d et h ep a r s e dl og i coft h el e f tb r a i n . •Ca r ea b ou twh a t ’ sg oi n gon .Youa r e( p r e s u ma b l y )g r a du a t es t u d e n t si n t e r e s t e di n p h y s i c s ,a n dt h i si ss omeoft h ec ool e s tp h y s i c se v e rd i s c ov e r e d .Ev e nb e t t e r ,i ti s c oola n da c c e s s i b l e ; y ouc a nma s t e ri tc omp l e t e l yi fy ouc a r et oa n dwor kh a r doni t t h i ss e me s t e r .Bee n ga ge di nc l a s s ,pa r t i c i p a t ei nc l a s s r oom d i s c u s s i on s ,s h ow i n t i a t i v ei ny ou rg r ou ps t u d i e sou t s i d eoft h ec l a s s r oom.Ma y b eIs u c ka sa n i n s t r u c t or–f i n e ,s owh a t ?Youa r ei nc h a r g eofy ou rownl e a r n i n ga tt h i sp oi n t ,I ’ m j u s tt h e‘ f a c i l i t a t or ’ ofap r oc e s sy ouc ou l dp u r s u eony ou rown . •Re c o gn i z et h edi v i s i onb e t we e np h y s i c sa n dma t h e ma t i c sa n dge ome t r yi nt h e p r ob l e my ou ’ r ewor k i n gon !Th i si st h emos td i ffic u l ts t e pf ormos ts t u d e n t st o a c h i e v e . Mos ts t u de n t s ,a l a s ,wi l lt r yt os ol v ep r ob l e msa si ft h e ywe r ema t hp r ob l e msa n dn ot u s ea n yp h y s i c a li n t u i t i on ,g e ome t r i cv i s u a l i z a t i on ,or( mos ti mp or t a n t )t h ef u n d a me n t a l p h y s i c a lr e l a t i o n s h i psu p onwh i c ht h es ol u t i oni sf ou n de d .Con s e qu e n t l yt h e y ’ l l of t e ns t a r t i tu s i n gs omep h y s i c s , a n dt h e nt r yt ob u l l t h e i r wa yt h r ou g ht h ea l g e b r a ,n otr e a l i z i n gt h a ta tt h e yn e e dt oa d dmo r ep h y s i c sf r om d i ffe r e n tr e l a t i on sa tv a r i ou sp oi n t so nt h ewa yt h r ou g ht h a ta l g e b r a .Th i sh a p p e n s , i n f a c t ,s t a r t i n gwi t has t u de n t ’ sf i r s ti n t r odu c t or yp h y s i c sc l a s swh e nt h e yt r yt os ol v ea l oop t h e l oopp r ob l e mu s i n gon l ya ne x p r e s s i onf orc e n t r i p e t a lf or c e ,p e r h a p swi t h Ne wt on ’ sl a ws ,b u ti g n or et h ef a c tt h a te n e r g yi sc on s e r v e dt oo.I ne l e c t r od y n a mi c si t mor eof t e nc ome sf r ome . g .s t a r t i n gwi t ht h ewa v ee qu a t i on( c or r e c t l y )b u tf a i l i n gt or e i n s e r ti n di v i d u a lMa x we l le qu a t i on si n t ot h er e a s on i n gp r oc e s s ,f a i l i n gt ou s ee . g . c h a r g ec on s e r v a t i on , f a i l i n gt or e c og n i z eap h y s i c a l c on s t r a i n t . Af t e ral on gt i mea n dma n yt r i e s( e s p e c i a l l ywi t hJ a c k s onp r ob l e ms ,wh i c ha r e n ot or i ou sf ort h i s )as t u d e n twi l lof t e nr e a c ht h ep e r f e c tl e v e lofu t t e rf r u s t r a t i ona n d s t op , s c r a t c ht h e i rh e a dab i t , a n dd e c i d et os t opj u s td oi n gma t ha n dt r yu s i n gab i tof p h y s i c s ,a n dh a l fap a g el a t e rt h ep r ob l e mi ss ol v e d .Th i si sav a l u a b l el e a r n i n g e x p e r i e n c e , b u ti ti si ns omes e n s ema x i ma l l yp a i n f u l .Th i ss h or ts e c t i oni sd e s i g n e dt o h e l py oua tmi n i mi z et h a tp a i nt oa tl e a s ts omee x t e n t . I nt h ef ol l owi n gt e x ts omes ma l l e ffor twi l l b ema d eonoc c a s i ont od i ffe r e n t i a t et h e “ ma t h y ”p a r t sofad e mon s t r a t i onorde r i v a t i onf r omt h e“ p h y s i c s y ”p a r t s ,s oy ouc a n s e ewh e r ep h y s i c si sb e i n gi n j e c t e di n t oama t hr e s u l tt oob t a i nan e wu n d e r s t a n di n g , a n e wc on s t r a i n torc on d i t i onona not h e r wi s eg e n e r a ls ol u t i on , t h en e x tc r i t i c a ls t e pon t h et r u ep a t ht oad e s i r e ds ol u t i ont oap r ob l e m.St u d e n t smi g h twe l lb e n e f i tf r om ma r k i n gu pt h e i rt e x t sorn ot e sa st h e yg oa l on gi nt h es a mewa y . Wh a tp a r tofwh a ty oua r ewr i t i n gd owni s“ j u s tma t h ”( a n dh e n c es ome t h i n gy ou c a nr e a s on a b l ye x p e c ty ou rma t hs k i l l st oc a r r yy out h r ou g hl a t e ri fn e e db e )a n dwh a t p a r ti sp h y s i c sa n dr e l i e sony ou rk n owl e d g eofp h y s i c a ll a ws ,v i s u a l i z a b l ep h y s i c a l r e l a t i on s h i p s , a n di n t u i t i on ?Th i n ka b ou tt h a ta sy oup r oc e e dt h r ou g ht h i st e x t . Us e f u l L i n k s 0. 1 Pe r s on a l Co n t a c tI n f or ma t i o n Rob e r tG. Br own E ma i l : r g ba tp h yd otd u k et oe d u Not e sURL : h t t p : / / www. p h y . du k e . e d u / ∼r g b / Cl a s s / El e c t r od y n a mi c s . p h p 0. 2 Us e f u l Te x t sa n dWe bRe f e r e n c e s •Ab ou n dc op yoft h i sb ook( i nc a s ey oua r er e a d i n gi ton l i n ea n dwa n tap a p e r c op yy ouc a nc a r r ywi t hy ou )c a nb ep u r c h a s e dh e r e : h t t p : / / www. l u l u . c om/ c on t e n t / 1 1 4 4 1 8 4 •An ot h e re x c e l l e n ton l i n et e x t b ooki sOr f a n i d i ’ sE l e c t r oma gn e t i cWa v e sa n d An t e n n a s : h t t p : / / www. e c e . r u t g e r s . e d u / ∼or f a n i di / e wa / •Th e“ c l a s s i c ”t e x t b ook ofE l e c t r od y n a mi c si sJ .D.J a c k s on ’ s ,Cl a s s i c a l E l e c t r od y n a mi c s ,3 r de d .I ti sf a i r l ye n c y c l op e d i c ,b u tt h ema t e r i a li tp r e s e n t s f oc u s e sont h i n g st h a ta r el e s si mp or t a n t , s u c ha sb ou n da r yv a l u ep r ob l e mswi t h ob s c u r eGr e e n ’ sf u n c t i on s ,a tt h ee x p e n s eofmu l t i p ol a rme t h od sa n dot h e r a p p r oa c h e st h a tt r e a ts y s t e ms ofc h a r g e c u r r e n td e n s i t ywi t hn oa c t u a l b ou n d a r i e s( s u c ha sa t oms ) .I ta l s oh a sat e n d e n c yt op r e s e n taf or mu l a ,t h e n s a ys ome t h i n gl i k e“ a n df r omt h i si tc a nb es h ownt h a t ”a n dp r e s e n tas e c on d f or mu l a ,omi t t i n gt h ef o u rp a ge so fd i ffic u l ta l ge b r ac on n e c t i n gt h et wo.Th i s c a nb eh a r dons t u d e n t s( a n di n s t r u c t or ) ,a l t h ou g ht h e r ei sn od e n y i n gt h a ta n y s t u de n twh oc a nf i l l i nt h ef ou rp a g e swi l l h a v el e a r n e ds ome t h i n g . •H.Wy l d,Me t h od so fMa t h e ma t i c a lPh y s i c s ,I SBN9 7 8 0 7 3 8 2 0 1 2 5 2 ,a v a i l a b l e f r ome . g .h t t p : / / a ma z on . c om.Ot h e rma t h e ma t i c a lp h y s i c st e x t ss u c ha sAr f k e n orMor s ea n dF e s h b a c ka r ee qu i v a l e n t l yu s e f u l . •Don a l dH.Me n z e l ’ sMa t h e ma t i c a lPh y s i c s ,Dov e rp r e s s ,I SBN0 4 8 6 6 0 0 5 6 4 .Th i s r e f e r e n c eh a sav e r yn i c ed i s c u s s i onofd y a d sa n dh ow t oe x p r e s sc l a s s i c a l me c h a n i c si nt e n s orf or m, wh i c hi sa c t u a l l yqu i t el ov e l y . 3 •Th e r ei saf a b u l ou sc omp l e xv a r i a b l e / c on t ou ri n t e g r a t i onr e f e r e n c eb yMa r k Tr od d e na tSy r a c u s eh e r e : h t t p : / / p h y s i c s . s y r . e d u / ∼t r od d e n / c ou r s e s / ma t h me t h od s / Th i s on l i n el e c t u r en ot e / b ook a c t u a l l ywor k st h r ou g ht h e Eu l e r L a g r a n g e e qu a t i ona swe l l ,b u ts t op su n f or t u n a t e l ys h or tofdoi n gEVE RYTHI NGt h a twe n e e di nt h i sc ou r s e . I ti son l y7 0p a g e s , t h ou g h–p r ob a b l yu n f i n i s h e d . •I n t r odu c t i ont ot e n s or sb yJ os e p hC. Kol e c k i a tNASA: www. g r c . n a s a . g ov / WWW/ K1 2 / Nu mb e r s / Ma t h / d oc u me n t s / Te n s or sTM2 0 0 2 2 1 1 7 1 6 . p d f •Sh or tr e v i e woft e n s or sf oraBe r k e l e yc os mol og yc ou r s e : h t t p : / / g r u s . b e r k e l e y . e d u / ∼j r g / a y 2 0 2 / n od e 1 8 3 . h t ml •Sh or tr e v i e woft e n s or sf oraWi n n i p e gUn i v e r s i t yc os mol og yc ou r s e : h t t p : / / i o. u wi n n i p e g . c a / ∼v i n c e n t / 4 5 0 0 . 6 0 0 1 / Cos mol og y / Te n s or s . h t m •Wi k i p e d i a : h t t p : / / www. wi k i p e d i a . or gWi k i p e d i an owc on t a i n ss omee x c e l l e n ta r t i c l e sonr e a l g r a du a t e l e v e le l e c t r ody n a mi c s ,r e l a t i v i t yt h e or y ,a n dmor e .Th ema t ha n ds c i e n c e c ommu n i t ya r ed e t e r mi n e dt oma k ei taon es t ops h opf ors u p p or t i n ga l ls or t sof c ou r s e wor k .Is t r o n gl yr e c o mme n dt h a ts t u d e n t su s ei tt os e a r c hf ors u p p or t i n g ma t e r i a lf ort h i sc ou r s e ,ort of i n dan e a ta n du s u a l l ywe l l wr i t t e ne x p l a n a t i onof t h i n g swh e nt h i st e x tf a i l st h e m. •Ma t h wor l d : h t t p : / / ma t h wor l d . wol f r a m. c omTh i ss i t e , t oo, i sv e r yg ooda l t h ou g hs omeoft h e a r t i c l e st e n dt ob ee i t h e rt h i norov e r l yt e c h n i c a l a tt h ee x p e n s eofc l a r i t y . •GI YF( Goog l eI sYou rF r i e n d ) .Wh e nl ook i n gf orh e l pona n yt op i c , g i v eg oog l ea t r y . I d o. I ti squ i t ea ma z i n gwh a tp e op l ep u tu pont h ewe b , f orf r e e . Pa r tI Ma t h e ma t i c a l Ph y s i c s 5 Ch a p t e r1 Ma t h e ma t i c a l Pr e l u d e Wh e nIf i r s ts t a r t e dt e a c h i n gc l a s s i c a le l e c t r od y n a mi c s ,i tr a p i d l yb e c a mea p p a r e n tt ome t h a tIwa ss p e n d i n ga smu c ht i met e a c h i n gwh a ta mou n t e dt or e me d i a lma t h e ma t i c sa sI wa st e a c h i n gp h y s i c s .Af t e ra l l , t oe v e nwr i t eMa x we l l ’ se qu a t i on sd owni ne i t h e ri n t e g r a l or di ffe r e n t i a l f or mr e qu i r e smu l t i v a r i a t ec a l c u l u s–p a t hi n t e g r a l s , s u r f a c ei n t e g r a l s , g r a di e n t s , di v e r g e n c e s ,c u r l s .Th e s ee qu a t i on sa r er a p i d l yc on v e r t e di n t oi n h omog e n e ou sp a r t i a l d i ffe r e n t i a le qu a t i on sa n dt h e i rs t a t i ca n ddy n a mi cs ol u t i on sa r ee x p a n d e di n( mu l t i p ol a r ) r e p r e s e n t a t i on s , r e qu i r i n gak n owl e d g eofs p h e r i c a l h a r mon i c sa n dv a r i ou sh y p e r g e ome t r i c s ol u t i on s .Th es ol u t i on sa r ei nma n yc a s e sn a t u r a l l ye x p r e s s e di nt e r msofc omp l e x e x p on e n t i a l s , a n don er e qu i r e sac e r t a i nf a c i l i t yi nd oi n ge . g .c on t ou ri n t e g r a l st ob ea b l et o ( f ore x a mp l e )u n d e r s t a n dd i s p e r s i onore s t a b l i s hr e p r e s e n t a t i on sb e t we e nv a r i ou sf or msof t h eGr e e n ’ sf u n c t i on .Gr e e n ’ sf u n c t i on st h e ms e l v e sa n dGr e e n ’ st h e or e me me r g e ,wh i c hi n t u r nr e qu i r e sas t u d e n tt ol e a r nt oi n t e g r a t eb yp a r t si nv e c t orc a l c u l u s .Th i sc u l mi n a t e s wi t ht h ede v e l op me n tofv e c t o rs p h e r i c a l h a r mon i c s , Ha n s e nf u n c t i on s , a n dd y a d i ct e n s or s i nt h ei n t e g r a l e qu a t i on st h a ta l l owon et oe v a l u a t emu l t i p ol a rf i e l d sd i r e c t l y . Th e non eh i t st h e or yofs p e c i a lr e l a t i v i t ya n dd oe si ta l la ga i n ,b u tn owe x p r e s s i n g e v e r y t h i n gi nt e r msoft e n s o r sa n dt h et h e o r yofc o n t i n u o u sgr o u p s .I tt u r n sou tt h a ta l l t h e e l e c t r od y n a mi c swewor k e ds oh a r doni smu c h ,mu c he a s i e rt ou n d e r s t a n di fi ti s 1 e x p r e s s e di nt e r msoft e n s or sofv a r i ou sr a n k. Wed i s c ov e rt h a ti ti se s s e n t i a lt ou n de r s t a n dt e n s or sa n dt e n s orop e r a t i on sa n d n ot a t i on i n or d e rt of ol l ow t h ef o r mu l a t i on ofr e l a t i v i t yt h e or ya n dr e l a t i v i s t i c e l e c t r od y n a mi c si nac omp a c t ,wor k a b l ef or m.Th i si si np a r tb e c a u s es omeoft h e d i ffic u l t i e sweh a v ee n c ou n t e r e di nd e s c r i b i n gt h ee l e c t r i ca n dma g n e t i cf i e l d ss e p a r a t e l y r e s u l tf r omt h ef a c tt h a tt h e ya r en o t , i nf a c t , v e c t or 1 Somep a r t sa r es i mp l e rs t i l li fe x p r e s s e di nt e r msoft h eg e ome t r i ce x t e n s i onoft h eg r a d e dd i v i s i on a l g e b r aa s s oc i a t e dwi t hc omp l e xn u mb e r s : “ g e ome t r i ca l g e b r a ” .Th i si st h ea l g e b r aofac l a s sofob j e c t st h a t i n c l u d e st h er e a l s , t h ec omp l e xn u mb e r s , a n dt h equ a t e r n i on s–a swe l l a sg e n e r a l i z e dob j e c t sofwh a tu s e d t ob ec a l l e d“ Cl i ffor da l g e b r a ” .Iu r g ei n t e r e s t e ds t u d e n t st oc h e c kou tL a s e n b y ’ sl ov e l yb ookonGe ome t r i c Al g e b r a , e s p e c i a l l yt h ep a r t st h a td e s c r i b et h eq u a t e r n i on i cf or mu l a t i onofMa x we l l ’ se qu a t i on s . 7 f i e l ds !Th e ya r ec omp on e n t sofas e c on dr a n kf i e l ds t r e n g t ht e n s ora n dh e n c emi x wh e non ec h a n g e sr e l a t i v i s t i cf r a me s . Te n s or sa r ei n de e dt h en a t u r a l l a n g u a g eoff i e l d t h e or i e s( a n dmu c he l s e )i np h y s i c s ,on et h a ti su n f or t u n a t e l yn ote ffe c t i v e l yt a u g h t wh e r et h e ya r et a u g h ta ta l l . Th es a mei st r u eofg r ou pt h e or y .Re l a t i v i t yi sb e s ta n dmos tg e n e r a l l yd e r i v e db y l ook i n gf ort h eg r ou pofa l l( c oor di n a t e )t r a n s f or ma t i on st h a tp r e s e r v eas c a l a rf or m f orc e r t a i np h y s i c a lqu a n t i t i e s ,t h a tl e a v ee . g .e qu a t i on sofmot i ons u c ha st h ewa v e e qu a t i on f or m i n v a r i a n t . Th e r ea r es t r on gc on n e c t i on sb e t we e ng r ou p s of t r a n s f or ma t i on st h a tc on s e r v eap r op e r t y , t h eu n de r l y i n gs y mme t r yoft h es y s t e mt h a t r e qu i r e st h a tp r op e r t yt ob ec on s e r v e d,a n dt h el a b e l sa n dc oor d i n a t i z a t i onoft h e p h y s i c a ld e s c r i p t i onoft h es y s t e m.Bye ffe c t i v e l ye x p l oi t i n gt h i ss y mme t r y ,wec a n of t e nt r e me n d ou s l ys i mp l i f you rma t h e ma t i c a ld e s c r i p t i onofap h y s i c a ls y s t e me v e n a swede du c ep h y s i c a l l a wsa s s oc i a t e dwi t ht h es y mme t r y . Un f or t u n a t e l y , i ti st h er a r eg r a du a t es t u d e n tt h a ta l r e a d yk n owsc omp l e xv a r i a b l e s a n di ss k i l l e da td oi n gc on t ou ri n t e g r a l s ,i sv e r yc omf or t a b l ewi t hmu l t i v a r i a t e / v e c t or c a l c u l u s ,i sf a mi l i a rwi t ht h er e l e v a n tp a r t i a ld i ffe r e n t i a le qu a t i on sa n dt h e i rb a s i c s ol u t i on s ,h a sa n yi d e awh a ty ou ’ r et a l k i n ga b ou twh e ny oui n t r odu c et h en ot i onof t e n s or sa n dma n i f ol ds ,h a swor k e dt h r ou g ht h eg e n e r a lt h e or yoft h eg e n e r a t or sof g r ou p sofc on t i n u ou st r a n s f or ma t i on st h a tp r e s e r v es c a l a rf or ms ,orh a v ee v e nh e a r d ofe i t h e rg e ome t r i ca l g e b r aorHa n s e nmu l t i p ol e s .Sor a r ea st ob ep r a c t i c a l l yn on e x i s t e n t . Id on ’ tb l a met h es t u d e n t s , ofc ou r s e .Id i dn ’ tk n owi t , e i t h e r , wh e nIwa sas t u d e n t ( i fi tc a nh on e s t l yb es a i dt h a tIk n o wa l loft h i sn ow,f ora l lt h a tIt r yt ot e a c hi t ) . Ne v e r t h e l e s sf i l l i n gi na l loft h emi s s i n gp i e c e s ,on es t u de n ta tat i me ,v e r yd e f i n i t e l y d e t r a c t sf r omt h ef l owoft e a c h i n ge l e c t r od y n a mi c s , wh i l ei fon ed oe s n ’ tb ot h e rt of i l l t h e mi n , on emi g h ta swe l l n otb ot h e rt r y i n gt ot e a c ht h ec ou r s ea ta l l . Ov e rt h ey e a r si nb e t we e nI ’ v et r i e dma n ya p p r oa c h e st od e a l i n gwi t ht h emi s s i n g ma t h .Th emos ts u c c e s s f u lon eh a sb e e nt oi n s e r tl i t t l emi n i l e c t u r e st h a tf oc u sont h e ma t ha ta p p r op r i a t ep oi n t sdu r i n gt h es e me s t e r ,wh i c hs e r v et ob ot hp r e p a r et h e s t u d e n ta n dt og i v et h e m as ma t t e r i n goft h eb a s i cf a c t st h a tag oodb ookon ma t h e ma t i c a l p h y s i c swou l dg i v et h e m, a n dt oa l s or e qu i r et h a tt h es t u d e n t sp u r c h a s e ad e c e n tb ookonma t h e ma t i c a lp h y s i c se v e nt h ou g ht h eon e sa v a i l a b l et e n dt ob e e n c y c l op e d i a ca n ds a yf a rt oomu c horomi twh ol ec r u c i a lt op i c sa n dt h e r e b ys a yf a r t ool i t t l e( ore v e nb ot h ) . I ’ mn ow t r y i n gou tan e w,s e mi i n t e g r a t e da p p r oa c h .Th i sp a r toft h eb ooki s d e v ot e dt oal i g h t n i n gf a s t ,l e c t u r en ot e l e v e lr e v i e wofma t h e ma t i c a lp h y s i c s .F a s tor n ot , i twi l le n d e a v ort ob equ i t ec omp l e t e , a tl e a s ti nt e r msofwh a ti sdi r e c t l yr e qu i r e d f ort h i sc ou r s e .Howe v e r , t h i si sv e r ymu c hawor ki np r og r e s sa n dI we l c omef e e d b a c k ont h ei de ai t s e l fa swe l la smi s t a k e sofomi s s i ona n dc ommi s s i ona sa l wa y s .Att h e e n dofIl i s ts e v e r a l r e a d i l ya v a i l a b l es ou r c e sa n dr e f e r e n c e st h a tI ’ mu s i n gmy s e l fa sI wr i t ei ta n dt h a ty oumi g h tu s ei n d e p e n d e n t l yb ot ht ol e a r nt h i sma t e r i a lmor e c omp l e t e l ya n dt oc h e c kt h a twh a tI ’ v ewr i t t e ni si nf a c tc or r e c ta n dc omp r e h e n s i b l e . Ch a p t e r2 Nu mb e r s I tma ys e e ms i l l yt od e v ot es p a c et on u mb e r sa sp h y s i c i s t sb yh y p ot h e s i sl ov en u mb e r s , b u tt h es t a n d a r du n de r g r a d u a t et r a i n i n gofp h y s i c i s t sd oe sn oti n c l u deac ou r s ei n n u mb e rt h e or yp e rs e , s omos tofwh a tt h e ya r el i k e l yt ok n owi sg l e a n e df r oma tmos t on ec ou r s ei nc omp l e xn u mb e r s( ma t hd ou b l ema j or sa n dma t hmi n or se x c e p t e d ) .Th i s c h a p t e rma k e sn oa t t e mp tt op r e s e n ta ne x h a u s t i v er e v i e wofn u mb e rt h e or y( h owe v e r c oola n dwor t h yofade e p e rt r e a t me n ti tmi g h tb e )b u ti n s t e a dc on f i n e si t s e l ft oj u s ta f e wp oi n t sofd i r e c tr e l e v a n c et oe l e c t r od y n a mi c s . 2. 1 Re a l Nu mb e r s Re a ln u mb e r sa r eofob v i ou si mp or t a n c ei np h y s i c s ,a n de l e c t r od y n a mi c si sn o e x c e p t i on .Me a s u r e dorc ou n t e dqu a n t i t i e sa r ea l mos ti n v a r i a b l yd e s c r i b e di nt e r msof r e a ln u mb e r sort h e i re mb e d d e dc ou s i n s ,t h ei n t e g e r s .Th e i rv i r t u ei np h y s i c sc ome s 1 f r om f r om t h ef a c tt h a tt h e yf or m a( ma t h e ma t i c a l )f i e l dt h a ti s ,t h e ys u p p or tt h e ma t h e ma t i c a lop e r a t i on sofa dd i t i on ,s u b t r a c t i on ,mu l t i p l i c a t i ona n dd i v i s i on ,a n di t e mp i r i c a l l yt u r n sou tt h a tp h y s i c a l l a wst u r nou tt ob ed e s c r i b a b l ei nt e r msofa l g e b r a i c f or msb a s e don( a tl e a s t )r e a ln u mb e r s .Re a ln u mb e r sf or m agr o u pu n de ror d i n a r y mu l t i p l i c a t i ona n d , b e c a u s emu l t i p l i c a t i oni sa s s oc i a t i v ea n de a c he l e me n tp os s e s s e s 2 au n i qu ei n v e r s e , t h e yf or mad i v i s i ona l ge b r a Ad i v i s i ona l g e b r ai son ewh e r ea n ye l e me n tot h e rt h a nz e r oc a nb ed i v i d e di n t oa n y ot h e re l e me n tt op r odu c eau n i qu ee l e me n t .Th i sp r op e r t yofr e a ln u mb e r si se x t r e me l y i mp or t a n t–i n d e e di ti st h ep r op e r t yt h a tma k e si tp os s i b l et ou s ea l g e b r ap e rs et os ol v e f orma n yp h y s i c a l qu a n t i t i e sf r omr e l a t i on se x p r e s s e d 1Wi k i p e d i a : h t t p : / / www. wi k i p e d i a . or g / wi k i / F i e l dma t h e ma t i c s . ; 2Wi k i p e di a : h t t p : / / www. wi k i pe di a . or g / wi k i / Di v i s i ona l ge b r a .. 9 i nt e r msofp r od u c t sa n ds u ms . Th eop e r a t i on a l s t e p s : b·c = a −1 ( b·c )·c −1 −1 =a·c − 1 b·( c·c )=a·c −1 b=b·1 = a·c ( 2 . 1 ) a r es op e r v a s i v e l yi mp l i c i ti nou ra l g e b r a i cop e r a t i on sb e c a u s et h e ya r ea l ll e a r n e di n t e r msofr e a ln u mb e r st h a twen ol on g e re v e nt h i n ka b ou tt h e mu n t i lwer u ni n t ot h e m i not h e rc on t e x t s ,f ore x a mp l ewh e na ,b ,ca r ema t r i c e s ,wi t ha tl e a s tcb e i n ga n i n v e r t i b l ema t r i x . I na n ye v e n tr e a ln u mb e r sa r ei d e a l l ys u i t e df ora l g e b r ab e c a u s et h e yf or maf i e l d ,i n s omes e n s et h ea r c h e t y p i c a lf i e l d,wh e r e b yp h y s i c a ll a wc a nb ewr i t t e ndowni nt e r msof s u msa n dp r od u c t swi t hme a s u r a b l equ a n t i t i e sa n dp h y s i c a lp a r a me t e r sr e p r e s e n t e db y r e a l n u mb e r s .Ot h e rf i e l ds( orr i n g s )a r eof t e nde f i n e di nt e r msofe i t h e rs u b s e t soft h er e a l n u mb e r sore x t e n s i on soft h er e a ln u mb e r s , i fon l yb e c a u s ewh e nwewr i t eas y mb olf ora r e a ln u mb e ri na na l g e b r a i cc omp u t a t i onwek n owe x a c t l ywh a twec a na n dc a n n otd owi t h i t . Re a l n u mb e r sa r et h eb a s i sofr e a l “ s p a c e ”a n d“ t i me ”i np h y s i c s–t h e ya r eu s e dt o f or ma na l g e b r a i cge ome t r ywh e r e i nr e a ln u mb e r sa r es p a t i ot e mp or a lc oo r d i n a t e s . Th i su s ei ss ome wh a tp r e s u mp t i v e–s p a c e t i mec a n n otb ep r ob e da td i s t a n c e ss h or t e r − 3 5 t h a nt h ePl a n c kl e n gt h( 1 . 6 1 6×1 0 me t e r s )–a n dma yb equ a n t i z e da n dg r a n u l a ra t t h a ts c a l e .Wh a t e v e rt h i sma yorma yn otme a n( c l os et on ot h i n g ,l a c k i n gac omp l e t e qu a n t u mt h e or yofg r a v i t y )i tma k e sn ome a n i n g f u l di ffe r e n c ea sf a ra st h ea p p l i c a b i l i t y ofe . g .c a l c u l u sd ownt ot h a ta p p r ox i ma t el e n g t hs c a l e ,a n ds o ou rc l a s s i c a l a s s u mp t i onofs moot hs p a c e t i mewi l l b equ i t er e a s on a b l e . Ar er e a l n u mb e r ss u ffic i e n tt od e s c r i b ep h y s i c s , i np a r t i c u l a rc l a s s i c a l e l e c t r od y n a mi c s ? Th ea n s we rma yi ns omes e n s eb ey e s( b e c a u s ec l a s s i c a lme a s u r a b l equ a n t i t i e sa r e i n v a r i a b l yr e a l , a sa r ec o mp on e n t sofe . g . c omp l e xn u mb e r s )b u ta swewi l l s e e , i twi l l b ef a r e a s i e rt owor kov e rad i ffe r e n tf i e l d :c omp l e xn u mb e r s ,wh e r ewewi l lof t e nv i e wr e a l n u mb e r sa sj u s tt h er e a lp a r tofamor eg e n e r a lc omp l e xn u mb e r ,t h er e a ll i n ea sj u s ton e l i n ei namor eg e n e r a lc omp l e xp l a n e .Aswewi l ls e e , t h e r ei sac l os er e l a t i on s h i pb e t we e n c omp l e xn u mb e r sa n dat wod i me n s i on a lEu c l i d e a np l a n et h a tp e r mi t su st ov i e wc e r t a i n a s p e c t soft h edy n a mi c soft h er e a ln u mb e rv a l u e dme a s u r a b l equ a n t i t i e sofp h y s i c sa st h e r e a lp r oj e c t i onofd y n a mi c st a k i n gp l a c eont h ec omp l e xp l a n e .Os c i l l a t or yp h e n ome n ai n g e n e r a l a r eof t e nv i e we di nt h i swa y . 2. 2 Co mp l e xNu mb e r s Th eop e r a t i onoft a k i n gt h es qu a r er oot( ora n yot h e rr oot s )ofar e a ln u mb e rh a sa n i n t e r e s t i n gh i s t or ywh i c hwewi l ln otr e v i e wh e r e .Twoa s p e c t sofn u mb e rt h e or yt h a th a v e g r ownd i r e c t l you tofe x p l or i n gs qu a r er oot sa r e ,h owe v e r ,i r r a t i on a ln u mb e r s( s i n c et h e s qu a r er ootofmos ti n t e g e r sc a nb es h ownt ob e i r r a t i on a l )a n di ma g i n a r yn u mb e r s .Th ef or me rwi l ln oti n t e r e s tu sa swea l r e a dywor k ov e ra tl e a s tt h er e a ln u mb e r swh i c hi n c l u d ea l lr a t i on a l sa n di r r a t i on a l s ,p os i t i v ea n d n e g a t i v e . I ma g i n a r yn u mb e r s , h owe v e r , a r eat r u ee x t e n s i onoft h er e a l s . Si n c et h ep r od u c tofa n yt won on n e g a t i v en u mb e r si sn on n e g a t i v e , a n dt h ep r od u c tof a n yt won e g a t i v en u mb e r si ss i mi l a r l yn on n e g a t i v e ,wec a n n otf i n da n yr e a ln u mb e rt h a t , wh e ns qu a r e d , i san e g a t i v en u mb e r .Th i sp e r mi t su st o“ i ma g i n e ”af i e l dofn u mb e r swh e r e t h es qu a r er ootofan on z e r on e g a t i v en u mb e re x i s t s . Su c haf i e l dc a n n otb ei d e n t i c a l t ot h e r e a l sa l r e a d ydi s c u s s e da b ov e .I tmu s tc o n t a i nt h er e a ln u mb e r s ,t h ou g h ,i nor d e rt ob e c l os e du n d e rmu l t i p l i c a t i on( a st h es qu a r eofa n“ i ma g i n a r y ”n u mb e ri san e g a t i v er e a l n u mb e r , a n dt h es qu a r eoft h a tr e a l n u mb e ri sap os i t i v er e a l n u mb e r ) . I fwed e f i n et h eu n i ti ma g i n a r yn u mb e rt ob e : √ i =+− 1 ( 2 . 2 ) s u c ht h a t 2 ± i=− 1 ( 2 . 3 ) wec a nt h e nf or mt h er e s toft h ef i e l db ys c a l i n gt h i si ma g i n a r yu n i tt h r ou g hmu l t i p l i c a t i on b yar e a ln u mb e r( t of or mt h ei ma gi n a r ya x i s )a n dt h e ng e n e r a t i n gt h ef i e l dofc omp l e x n u mb e r sb ys u mmi n ga l lp os s i b l ec omb i n a t i on sofr e a la n di ma g i n a r yn u mb e r s .Not et h a t t h ei ma g i n a r ya x i sa l on ed oe sn o tf or maf i e l dore v e namu l t i p l i c a t i v eg r ou pa st h ep r od u c t ofa n yt woi ma g i n a r yn u mb e r si sa l wa y sr e a l , j u s ta si st h ep r odu c tofa n yt wor e a l n u mb e r s . Howe v e r , t h ep r odu c tofa n yr e a l n u mb e ra n da ni ma g i n a r yn u mb e ri sa l wa y si ma g i n a r y , a n d c l os u r e , i de n t i t y , i n v e r s ea n da s s oc i a t i v i t yc a ne a s i l yb ed e mon s t r a t e d . Th ee a s i e s twa yt ov i s u a l i z ec omp l e xn u mb e r si sb yor i e n t i n gt h er e a la x i sa tr i g h t a n g l e st ot h ei ma g i n a r ya x i sa n ds u mmi n gr e a la n di ma g i n a r y“ c omp on e n t s ”t of or m a l l oft h ec omp l e xn u mb e r s .Th e r ei saon e t oon ema p p i n gb e t we e nc omp l e xn u mb e r s a n daE u c l i d e a nt wodi me n s i on a lp l a n ea sac on s e qu e n c et h a ti sv e r yu s e f u lt ou sa s wes e e kt ou n de r s t a n dh owt h i s“ i ma g i n a r y ”g e n e r a l i z a t i onwor k s . Wec a nwr i t ea na r b i t r a r yc omp l e xn u mb e ra sz=x+i yf orr e a l n u mb e r sxa n dy .As y ouc a ne a s i l ys e e ,t h i sn u mb e ra p p e a r st ob eap oi n ti na( c omp l e x )p l a n e .Ad di t i on a n ds u b t r a c t i onofc omp l e xn u mb e r sa r et r i v i a l –a d dors u b t r a c tt h er e a l a n di ma g i n a r y c omp on e n t ss e p a r a t e l y( i nama n n e rd i r e c t l ya n a l og ou st ov e c t ora dd i t i on ) . Mu l t i p l i c a t i on ,h owe v e r ,i sab i todd .Gi v e nt woc o mp l e xn u mb e r sz n dz ,we 1a 2 h a v e : z=z z x ( x y x )−y y 1· 2=x 1 2+i 1 2+y 1 2 1 2 ( 2 . 4 ) s ot h a tt h er e a l a n di ma g i n a r yp a r t sa r e ℜz x y 1 2−y 1 2 =x ( 2 . 5 ) ℑz y x 1 2+y 1 2 =x ( 2 . 6 ) Th i si squ i t ed i ffe r e n tf r oma n yoft h er u l e swemi g h tu s et of or mt h ep r od u c tof t wov e c t or s .I ta l s op e r mi t su st of or mt h es oc a l l e dc omp l e xc on j u ga t eofa n y i ma g i n a r yn u mb e r ,t h en u mb e rt h a ton ec a nmu l t i p l yi tb yt oob t a i nap u r e l yr e a l n u mb e rt h a ta p p e a r st ob et h es qu a r eoft h eEu c l i de a nl e n g t hoft h er e a l a n di ma g i n a r y c omp on e n t s z =x+i y ∗ z 2 ∗ ∗ | z |=zz=z z =x−i y 2 2 =x +y ( 2 . 7 ) ( 2 . 8 ) ( 2 . 9 ) Aqu i t ep r of ou n di n s i g h ti n t ot h ei mp or t a n c eofc omp l e xn u mb e r sc a nb eg a i n e db y r e p r e s e n t i n gac omp l e xn u mb e ri nt e r msoft h ep l a n ep ol a rc o or d i n a t e soft h e u n d e r l y i n gEu c l i d i a n∗ c oor d i n a t ef r a me .Wec a nu s et h e2 p r od u c tofan u mb e rza n di t s c omp l e xc on j u g a t ez t od e f i n et h ea mp l i t u d e| z |=+| z ||t h a ti st h ep ol a rd i s t a n c eof t h ec omp l e xn u mb e rf r om t h ec omp l e xor i g i n .Th eu s u a lp ol a ra n g l eθc a nt h e nb e s we p tou tf r omt h ep os i t i v er e a la x i st oi d e n t i f yt h ec omp l e xn u mb e ront h ec i r c l eof r a d i u s| z | . Th i sr e p r e s e n t a t i onc a nt h e nb ee x p r e s s e di nt r i g on ome t r i cf or msa s : z=x+i y=| z | c os ( θ)+i | z | s i n ( θ ) | z | ( c o s ( θ ) + i s i n ( θ ) ) = i θ z | e =| ( 2 . 1 0 ) ( 2 . 1 1 ) ( 2 . 1 2 ) wh e r et h ef i n a l r e s u l tc a nb eob s e r v e da n yn u mb e rofwa y s , f ore x a mp l eb ywr i t i n gou t u 2 t h ep owe rs e r i e sofe =1+u+u/ 2 ! +. . .f orc omp l e xu=i θa n dma t c h i n gt h er e a l a n d i ma g i n a r ys u b s e r i e swi t ht h os ef ort h ec os i n ea n ds i n er e s p e c t i v e l y . I nt h i se x p r e s s i on −1 y θ=t a n x ( 2 . 1 3 ) d e t e r mi n e st h ea n g l eθi nt e r msoft h eor i g i n a l “ c a r t e s i a n ”c omp l e xc oor d i n a t e s . Tr i g on ome t r i cf u n c t i on sa r et h u ss e e nt ob equ i t en a t u r a l l ye x p r e s s i b l ei nt e r msoft h e e x p on e n t i a l sofi ma g i n a r yn u mb e r s .Th e r ei sap r i c et op a yf ort h i s , h owe v e r . Th e r e p r e s e n t a t i oni sn ol on g e rs i n g l ev a l u e di nθ. I nf a c t , i ti s qu i t ec l e a rt h a t : i θ± 2 n π z=| z | e f ora n yi n t e g e rv a l u eofn . Weu s u a l l ya v oi dt h i sp r ob l e mi n i t i a l l yb yr e qu i r i n g θ∈( − π, π] ( t h e“ f i r s tl e a f ” )b u ta swes h a l l s e e , t h i sl e a d st op r ob l e mswh e n c on s i d e r i n gp r od u c t sa n dr oot s . I ti squ i t ee a s yt omu l t i p l yt woc omp l e xn u mb e r si nt h i sr e p r e s e n t a t i on : i θ z 1 = | z | e1 1 i θ z 2 = | z | e2 2 i ( θ+ θ) z=z z 1 2 = | z | | z | e1 2 1 2 ( 2 . 1 4 ) ( 2 . 1 5 ) ( 2 . 1 6 ) ( 2 . 1 7 ) ort h ea mp l i t u deoft h er e s u l ti st h ep r o d u c toft h ea mp l i t u de sa n dt h ep h a s eoft h er e s u l ti s t h es u moft h et wop h a s e s . Si n c eθ1+θ ywe l l b el a r g e rt h a n 2ma πe v e ni ft h et woa n g l e si n d i v i d u a l l ya r en ot ,t os t i c kt oou rr e s ol u t i ont ok e e pt h e r e s u l t a n tp h a s ei nt h er a n g e( π,π]wewi l lh a v et of or mas u i t a b l emod u l u st op u ti t b a c ki nr a n g e . Di v i s i onc a ne a s i l yb er e p r e s e n t e da smu l t i p l i c a t i onb yt h ei n v e r s eofac omp l e x n u mb e r : 1 − 1 −i θ z = e ( 2 . 1 8 ) | z | a n di ti se a s yt os e et h a tc omp l e xn u mb e r sa r eamu l t i p l i c a t i v eg r ou pa n dd i v i s i on a l g e b r aa n dwec a na l s os e et h a ti t smu l t i p l i c a t i oni sc ommu t a t i v e . On el a s top e r a t i ono fs omei mp or t a n c ei nt h i st e x ti st h ef or ma t i onofr oot sofa c omp l e xn u mb e r .I ti se a s yt os e et h a tt h es qu a r er ootofac omp l e xn u mb e rc a nb e wr i t t e na s : √ z=± i θ/ 2 | z | e = i ( θ/ 2 ± n π) | z | e ( 2 . 1 9 ) f ora n yi n t e g e rn .Weu s u a l l yi n s i s tonf i n d i n gr oot son l ywi t h i nt h ef i r s t“ b r a n c hc u t ” ,a n d r e t u r na na n s we ron l ywi t haf i n a l p h a s ei nt h er a n g e( − π, π] . Th e r ei sac on n e c t i onh e r eb e t we e nt h eb r a n c h e s ,l e a v e s ,a n dt op ol og y–t h e r ei s r e a l l yon l yo n ea c t u a lp oi n ti nt h ec omp l e xp l a n et h a tc or r e s p on d st oz ; t h er e s toft h e wa y st or e a c ht h a tp oi n ta r ea s s oc i a t e dwi t hawi n d i n gn u mb e rmt h a tt e l l son eh ow ma n yt i me son emu s tc i r c l et h eor i g i n( a n di nwh i c hd i r e c t i on )t or e a c hi tf r om t h e p os i t i v er e a l a x i s . Th u st h e r ea r et wou n i qu ep oi n t sont h ec omp l e xp l a n e( ont h ep r i n c i p l eb r a n c h ) t h a ta r es qu a r er oot s( p l u smu l t i p l ec op i e swi t hd i ffe r e n twi n d i n gn u mb e r sonot h e r b r a n c h e s ) .I np r ob l e mswh e r et h ec h o i c ed oe s n ’ tma t t e rweof t e nc h oos et h ef i r s ton e r e a c h e dt r a v e r s i n gt h ec i r c l ei nac ou n t e r c l oc k wi s ed i r e c t i on( s ot h a ti th a sap os i t i v e a mp l i t u de ) .I np h y s i c sc h oi c eof t e nma t t e r sf oras p e c i f i cp r ob l e m –wewi l lof t e n c h oos et h er ootb a s e done . g .t h ed i r e c t i onwewi s has ol u t i ont op r op a g a t ea si t e v ol v e si nt i me . 1 Pu r s u i n gt h i sg e n e r a li d e ai ti se a s yt os e et h a tzn wh e r eni sa ni n t e g e ra r et h e p oi n t s 1 i ( θ/ n ± 2mπ/ n ) | z |n e ( 2 . 2 0 ) wh e r em =0 ,1 ,2 . . .a sb e f or e .Nowwewi l lg e n e r a l l yh a v enr oot si nt h ep r i n c i p l e b r a n c hofza n dwi l lh a v et op e r f or mac u tt os e l e c tt h eon ed e s i r e dwh i l ea c c e p t i n g t h a ta l l oft h e mc a nwor ke qu a l l ywe l l . Ch a p t e r3 Ve c t o r sa n dVe c t o r Pr o d u c t s Av e c t ori saqu a n t i t ywi t hd i me n s i on s ,ama gn i t u de ,a n dad i r e c t i o nr e l a t i v et oas p e c i f i c c oo r d i n a t ef r a me . Not et h a ti ti s n ’ ts u ffic i e n tt oh a v eal i s tof( s a y )t h r e en u mb e r sl a b e l l e dx , y ,a n dz–t h ec omp on e n t sh a v et ot r a n s f or m wh e nt h eu n d e r l y i n gc oor d i n a t ef r a mei s t r a n s f or me d“ l i k eav e c t or ” .Al t h ou g ht h e r ea r emu l t i p l ec oor d i n a t es y s t e msi nwh i c h v e c t or sc a nb ee x p r e s s e d ,t h e“ s i mp l e s t ”on ei sCa r t e s i a n ,wh e r eav e c t orc a nt y p i c a l l yb e wr i t t e n : A =Axx ˆ+Ayy ˆ+Azz ˆ i nt e r msofc omp on e n ts c a l a ra mp l i t u d e s( Ax, Ay, Az)a n du n i tv e c t or si nt h e or t h og on a l d i r e c t i on s( x ˆ , y ˆ , z ˆ ) . Toa ddv e c t or s( i nCa r t e s i a nc oor d i n a t e s )wea d dc omp on e n t s : C =A+B=( Ax+Bx) x ˆ+( Ay+By) y ˆ+( Az+Bz) z ˆ Th er e s u l t a n ti sa l s ot h er e s u l tofag e ome t r i ct r i a n g l eo rp a r a l l e l og r a mr u l e : ( B) A C= A+B B ( A) Su b t r a c t i oni sj u s ta dd i t i onofan e g a t i v e : C=A−B=( Ax−Bx) x ˆ+( Ay−By) y ˆ+( Az−Bz) z ˆ 1 5 I tc a na l s ob ev i s u a l i z e db yme a n sofag e ome t r i ct r i a n g l es ot h a t( A−B)+ B =A. −B C= A−B = A+( −B) A B 3. 1 Sc a l a r sa n dVe c t or s Anor d i n a r yn u mb e rt h a td oe sn otc h a n g ewh e nt h ec oor d i n a t ef r a mec h a n g e si sc a l l e d as c a l a r .Mu l t i p l i c a t i onofav e c t orb yas c a l a rr e s c a l e st h ev e c t orb ymu l t i p l y i n ge a c h ofi t sc omp on e n t sa sas p e c i a l c a s eoft h i sr u l e : a A=a ( Axx ˆ+Ayy ˆ+Azz ˆ )=( a Ax) x ˆ+( a Ay) y ˆ+( a Az) z ˆ Not ewe l lt h a tt h ev e c t orc omp on e n t sAx,Ay,Aza r et h e ms e l v e ss c a l a r s .I n d e e d , web u i l dav e c t ori nt h ef i r s tp l a c eb yt a k i n gau n i tv e c t or( ofl e n g t hon e ,“ p u r e d i r e c t i on ” )a n ds c a l i n gi tb yi t sc omp on e n tl e n g t h ,e . g .Axx ˆ ,a n dt h e ns u mmi n gt h e v e c t or st h a tma k eu pi t sc omp on e n t s ! Th emu l t i p l i c a t i onofav e c t orb yas c a l a ri sc ommu t a t i v e : a A=Aa a n dd i s t r i b u t i v e . a ( A+B)=a A+a B 3. 2 Th eSc a l a r , o rDo tPr o d u c t I ti sa l s op os s i b l ef or ms e v e r a l“ mu l t i p l i c a t i on l i k e ”p r od u c t soft wo( ormor e )v e c t or s . Wec a nt a k et wov e c t or sa n dma k eas c a l a r ,a n ot h e rv e c t or ,ora“ b i v e c t or ”( t e n s or ) . Someoft h e s emi g h tb er e g u l a rv e r s i onoft h eob j e c t s ,s omemi g h tb e“ p s e u d o” v e r s i on st h a twewi l lc omet ou n de r s t a n d.Howe v e r ,weh a v et ob ec a r e f u ln ott og e t s we p toffofou rf e e tb yt h ed a z z l i n ga r r a yofp os s i b i l i t i e sr i g h ta tt h eb e g i n n i n g . Wewi l l t h e r e f or es t a r twi t ht h ea r g u a b l ys i mp l e s tf or mofv e c t ormu l t i p l i c a t i on : t h e s c a l a rord otp r od u c t : C=A·B. Not et h a tt h edotp r od u c tt u r n s t wov e c t or si n t oas c a l a r .I ti sa l s oof t e nc a l l e da ni n n e rp r od u c t , a l t h ou g ht h el a t t e ri s s ome wh a tmor eg e n e r a l t h a nt h ed otp r od u c ti naEu c l i d e a n( e . g . Ca r t e s i a n )s p a c e . Th edotp r od u c ti sc ommu t a t i v e : A· B= B· A I ti sdi s t r i b u t i v e : A· ( B+ C) = A· B+ A· C I tc a nb ee v a l u a t e dt wo( i mp or t a n t )wa y s : C=A·B=ABc os ( θ )=AxBx+AyBy+AzBz 1 wh e r eAa n dBa r et h es c a l a rma gn i t u de soft h ev e c t or sAa n dBr e s p e c t i v e l ya n dθi s t h ea n g l ei nb e t we e nt h e m: B B θ B A F r omt h ef i r s toft h e s ef or ms , wes e et h a tt h ed otp r odu c tc a nb et h ou g h tofa st h e ma g n i t u deoft h ev e c t orAt i me st h ema g n i t u d eoft h ec o mp on e n toft h ev e c t orBi nt h e s a med i r e c t i ona sA, i n d i c a t e da sBi nt h ef i g u r ea b ov e . I n d e e d : A· B= AB= AB ( Th el a t t e rt h ema g n i t u d eofBt i me st h ec omp on e n tofAp a r a l l e lt oB. )Th es e c on d f ol l owsf r omt h ef ol l owi n gmu l t i p l i c a t i ont a b l eofu n i tv e c t or s ,wh i c hc a nb et h ou g h tofa sd e f i n i n gt h edotp r od u c ta n dt h eu n i tv e c t or sof “ or t h on or ma l c oor d i n a t e s ”s i mu l t a n e ou s l y : x ˆ·x ˆ=y ˆ·y ˆ=z ˆ·z ˆ=1 x ˆ·y ˆ=y ˆ·z ˆ=z ˆ·x ˆ=0 ( p l u st h ec ommu t a t e df o r msoft h el a s tr ow, e . g . y ˆ·x ˆ=0a swe l l ) . Twov e c t or st h a ta r ep e r p e n d i c u l a r( or t h og on a l )h a v ead otp r odu c tofz e r oa n d v i c e v e r s a .I fa n don l yi f( wr i t t e nh e n c e f or t ha s“ i ff” )A·B=0t h e nA⊥B. Wemi g h ts a y t h a tAi sn or ma l t o, p e r p e n d i c u l a rt o, a tr i g h ta n g l e st o, oror t h og on a l t oB.Al l oft h e s e me a nt h es a met h i n g . 1 Not et h a twed e f i n et h ema g n i t u deoft h ev e c t orA( wr i t t e ne i t h e rAor| A| )i nt e r msoft h ei n n e rp r od u c t : 2 2 2 1 A=| A| =+ A· A=( A x+A y+A z)2 p 3. 2. 1 Th eL a wo fCo s i n e s Th el a wo fc o s i n e si se a s i l yd e r i v e d( on eofs e v e r a lwa y s )b yf i n di n gt h es c a l a rl e n g t h oft h ed i ffe r e n c ev e c t orA−B. A B B θ A 2 | A− B|= ( A− B) · ( A− B) = A· A− A· B− B· A+ B· B or( c ol l e c t i n gt e r msa n du s i n gr u l e sf r oma b ov e ) : | A−B| = 2 2 A +B −2 ABc osθ Not et h a tt h ePy t h a g or e a nTh e or e mi sas p e c i a l c a s eoft h i sr u l ewi t hθ=π/ 2 . 3. 3 Th eVe c t o r , o rCr o s sPr od u c t Th e r ei sas e c on dwa yt omu l t i p l yt wov e c t or s .Th i sp r od u c toft wov e c t or sp r od u c e sa t h i r dv e c t or ,wh i c hi swh yi ti sof t e nr e f e r r e dt oa s“ t h e ”v e c t orp r od u c t( e v e nt h ou g h t h e r ea r ean u mb e rofp r odu c t si n v ol v i n gv e c t or s ) .I ti ss y mb ol i c a l l yd i ffe r e n t i a t e db y t h emu l t i p l i c a t i ons y mb olu s e d , wh i c hi sal a r g e×s i g n , h e n c ei ti sof t e nr e f e r r e dt oa s t h ec r os sp r od u c tb ot hf ort h e( c r os s l i k e )s h a p eoft h i ss i g na n db e c a u s eoft h e p a t t e r nofmu l t i p l i c a t i onofc omp on e n t s .Wewr i t et h ec r os sp r od u c toft wov e c t or sa s e . g . C=A×B. Th ec r os sp r od u c ta n t i c ommu t e s : A× B= − B× A I ti sdi s t r i b u t i v e : A× ( B+C) = A× B+ A× C ( a l t h ou g ht h eor de roft h ep r odu c tmu s tb ema i n t a i n e d ! ) I ta sn ot e da b ov ep r od u c e sav e c t o r( r e a l l yap s e u d ov e c t or ,e x p l a i n e dl a t e r )f r om t wov e c t or s . Th ema g n i t u d eoft h ec r os sp r odu c toft wov e c t or si sd e f i n e db y : | A×B| =ABs i nθ=AB⊥=A⊥B u s i n gt e r mss i mi l a rt ot h os eu s e da b ov ei nou rd i s c u s s i onofdotp r od u c t s . Not ewe l l ! I f t h ev e c t or sb ot hh a v ed i me n s i on sofl e n g t h , t h ec r os sp r od u c t i st h ea r e aoft h ep a r a l l e l og r a mf or me db yt h ev e c t or sa si l l u s t r a t e di nf i g u r e3 . 1 .I ti s s ome t i me sc a l l e dt h ea r e a l p r od u c tf ort h i sr e a s on , a l t h ou g hon e B Ar e a=| AxB| B θ B A AxBdi r e c t i oni sou tofpa ge F i g u r e3 . 1 :Th ea r e ab e t we e nt wov e c t or si nap l a n ei st h ema g n i t u deoft h ec r os s p r od u c toft h os ev e c t or s . wou l dt h i n kt won a me si se n ou g h( a n di nma n yc on t e x t s ,a r e a lp r od u c tma k e sn o s e n s e ) . Th edi r e c t i onofA×Bi sg i v e nb yt h er i g h t h a n dr u l e .Th ed i r e c t i oni sa l wa y s p e r p e n di c u l a rorn or ma lt ot h ep l a n ed e f i n e db yt h et won on c ol i n e a rv e c t or si nt h e c r os sp r od u c t .Th a tl e a v e st wop os s i b i l i t i e s . I fy oul e tt h ef i n g e r sofy ou rr i g h th a n dl i n e u pwi t hA( t h ef i r s t )s ot h a tt h e yc a nc u r lt h r ou g ht h es ma l la n g l e( t h eon el e s st h a nπ t h a twi l ln oth u r ty ou rwr i s t )i n t oBt h e nt h et h u mbofy ou rr i g h th a n dwi l lp i c kou tt h e p e r p e n d i c u l a rdi r e c t i onoft h ec r os sp r od u c t . I nt h ef i g u r ea b ov e , i ti sou toft h ep a g e . F i n a l l y : A× A= − ( A× A) = 0 Tog e t h e rwi t ht h er u l ef orr e s c a l i n gv e c t or st h i sp r ov e st h a tt h ec r os sp r od u c tofa n y v e c t orwi t hi t s e l fora n yv e c t orp a r a l l e l o ra n t i p a r a l l e l t oi t s e l fi sz e r o.Th i sa l s of ol l ows f r omt h ee x p r e s s i onf ort h ema g n i t u d eABs i nθwi t hθ=0or π. L e tu sf or mt h eCa r t e s i a nr e p r e s e n t a t i onofac r os sp r od u c toft wov e c t or s .We b e g i nb yn ot i n gt h a tar i g h th a n d e dc oor d i n a t es y s t e mi sd e f i n e db yt h er e qu i r e me n t t h a tt h eu n i tv e c t or ss a t i s f y : x ˆ×y ˆ=z ˆ Th i si si l l u s t r a t e dh e r e : y y z x x z Youc a ne a s i l yc h e c kt h a ti ti sa l s ot r u et h a t : x ˆ×y ˆ=z ˆ y ˆ×z ˆ=z ˆ z ˆ×x ˆ=y ˆ Weu s et h ea n t i c ommu t i onr u l eont h e s et h r e ee qu a t i on s : y ˆ×x ˆ=− z ˆ z ˆ×y ˆ=− z ˆ x ˆ×z ˆ=− y ˆ An dn ot et h a t : x ˆ×x ˆ=y ˆ×y ˆ=z ˆ×z ˆ=0 Th i sf or mst h ef u l lmu l t i p l i c a t i ont a b l eoft h eor t h on or ma lu n i tv e c t or sofas t a n d a r d r i g h t h a n d e dCa r t e s i a nc oor d i n a t es y s t e m,a n dt h eCa r t e s i a n( a n dv a r i ou sot h e r or t h on or ma l )c oor di n a t ec r os sp r od u c tn owf ol l ows . Ap p l y i n gt h ed i s t r i b u t i v er u l ea n dt h es c a l a rmu l t i p l i c a t i onr u l e ,mu l t i p l you ta l lof t h et e r msi nA×B: ( Axx ˆ+Ayy ˆ+Azz ˆ )× ( Bxx ˆ+Byy ˆ+Bzz ˆ )= AxBxx ˆ×x ˆ+AxByx ˆ×y ˆ+AxBzx ˆ×z ˆ + AyBxy ˆ×x ˆ+AyByy ˆ×y ˆ+AyBzy ˆ×z ˆ + AzBxz ˆ×x ˆ+AzByz ˆ×y ˆ+AzBzz ˆ×z ˆ Th edi a g n on a l t e r msv a n i s h . Th eot h e rt e r msc a na l l b es i mp l i f i e dwi t ht h eu n i t v e c t orr u l e sa b ov e . Th er e s u l ti s : A×B=( AyBz−AzBy) x ˆ+( AzBx−AxBz) y ˆ+( AxBy−AyBx) z ˆ Th i sf or mi se a s yt or e me mb e ri fy oun ot et h a te a c hl e a d i n gt e r mi s ac y c l i cp e r mu t a t i onofx y z .Th a ti s , AyBzx ˆ , AzBxy ˆa n dAxByz ˆa r ey z x , z x y , a n dx y z . Th es e c on dt e r mi ne a c hp a r e n t h e s e si st h es a mea st h ef i r s tb u ti nt h eop p os i t eor d e r , wi t ht h ea t t e n da n tmi n u ss i g n , f r omt h ec y c l i cp e r mu t a t i on sofz y x . 3. 4 Tr i p l ePr o d u c t sofVe c t o r s Th e r ea r et wot r i p l ep r odu c t sofv e c t or s . Th ef i r s ti st h es c a l a rt r i p l ep r od u c t : A· ( B× C) I fA,Ba n dCa r ea l ll e n gt hv e c t or s ,t h i sr e p r e s e n t st h ev ol u meofp a r a l l e l op i p e d f or me db yt h ev e c t or s . Th es e c on di st h ev e c t ort r i p l ep r odu c t : A× ( B× C) = B( A· C) − C( A· B) Th i sl a s ti d e n t i t yi sc a l l e dt h eBACCAB( p a l i n d r omi c )r u l e .I ti st e di ou sb u t s t r a i g h t f or wa r dt op r ov ei tf orCa r t e s i a nv e c t orc omp on e n t s .F i r s t ,h owe v e r , wewou l d l i k et oi n t r odu c et wos p e c i a lt e n s orf or mst h a tg r e a t l ys i mp l i f yt h ea l g e b r aofb ot hd ot a n dc r os sp r odu c t sa n de n a b l eu st op r ov ev a r i ou sv e c t ori d e n t i t i e su s i n ga l g e b r a i n s t e a dofat e d i ou se n u me r a t i onoft e r ms . 3. 5 δ n dǫ i ja i j k Asn ot e da b ov e , wewou l dl i k et ob ea b l et os i mp l i f yv e c t ora l g e b r ai nor d e rt op r ov et h e t r i p l ep r od u c tr u l ea n dv a r i ou sot h e rv e c t ori d e n t i t i e swi t h ou th a v i n gt oe n u me r a t ewh a t ma yt u r nou tt ob eal a r g en u mb e roft e r ms .Ag r e a td e a lofs i mp l i f i c a t i oni sp os s i b l e u s i n gt wo“ s p e c i a l ”t e n s or st h a ta p p e a ri nt h ema n ys u mma t i on st h a toc c u ri nt h e e x p r e s s i on sa b ov e , a swe l la sas p e c i a lr u l et h a ta l l owsu st o“ c omp r e s s ”t h ea l g e b r a b ye l i mi n a t i n gar e d u n d a n ts u mma t i o ns y mb ol . 3. 5. 1 Th eKr on e c k e rDe l t aF u n c t i o na n dt h eE i n s t e i nSu mma t i on Co n v e n t i o n Th eKr on e c k e rd e l t af u n c t i oni sd e f i n e db yt h er u l e s : 1i fi =j δi j = 0i fi =j Us i n gt h i swec a nr e d u c et h ed otp r od u c tt ot h ef ol l owi n gt e n s orc on t r a c t i on , u s i n gt h e Ei n s t e i ns u mma t i onc on v e n t i on : 3 A·B= Ai Bi=Ai δ Bi i jB j=Ai i = 1 wh e r ewes u mr e p e a t e di n d i c e sov e ra l l oft h eor t h og on a l c a r t e s i a n 3 c oor d i n a t ei n d i c e swi t h ou th a v i n gt owr i t ea ne x p l i c i ti .Wewi l lh e n c e f or t hu s et h i s = 1 c on v e n t i ona l mos ta l lt h et i met os t r e a ml i n et h en ot a t i onofc e r t a i nk i n d sof( v e c t or a n dt e n s or )a l g e b r a . Th eKr on e c k e rd e l t af u n c t i oni sob v i ou s l yu s e f u lf orr e p r e s e n t i n gt h edotp r odu c ti na c omp a c twa y .Wec a ns i mi l a r l yi n v e n tas y mb olt h a ti n c or p or a t e sa l loft h ed e t a i l soft h e wa y st h eu n i tv e c t or smu l t i p l yi nt h ec r os sp r od u c t , n e x t . 3. 5. 2 Th eL e v i Ci v i t aTe n s o r Th eL e v i Ci v i t at e n s ori sa l s ok n owa st h et h i r dr a n kf u l l ya n t i s y mme t r i cu n i tt e n s or a n di sd e f i n e db y : 1i fi j ka r ea n yc y c l i cp e r mu t a t i onof1 2 3ǫi j k= − 1i fi j ka r ea n yc y c l i cp e r mu t a t i onof3 2 1 0 ot h e r wi s e( i fa n yp a i rofi n d i c e sa r er e p e a t e d ) . Us i n gt h i swec a nr e d u c et h ec r os sp r od u c tt ot h ef ol l owi n gt e n s orc on t r a c t i on , u s i n gt h eEi n s t e i ns u mma t i onc on v e n t i on : 3 ( A×B) k= 3 Ai Bjǫi Bjǫi j k=Ai j k i = 1j = 1 wh e r e( a sb e f or e )wes u mr e p e a t e di n di c e sov e ra l loft h eor t h og on a lc a r t e s i a n c oor d i n a t ei n d i c e s .Not ewe l lt h a ti ti su n d e r s t oodt h a ta n yl e f t ov e ri n d e xi na c on t r a c t i onoft h i ss or tr e p r e s e n t sac omp o n e n ti nav e c t ora n s we r . 3. 5. 3 Th eE p s i l o n De l t aI d e n t i t y Ac ommon l yoc c u r r i n gr e l a t i oni nma n yoft h ei d e n t i t i e sofi n t e r e s t–i np a r t i c u l a rt h e A×( B×C)t r i p l ep r od u c t–i st h es oc a l l e de p s i l on d e l t ai d e n t i t y: ǫijkǫimn=δ j mδ k n−δ j n δkm 2 Not ewe l l t h a tt h i si st h ec on t r a c t i o n oft wot h i r dr a n kt e n s or s . ! Th er e s u l th a st h e r e ma i n i n gf ou ri n d i c e s .Al s on ot ewe l lt h a ton ec a nu s et h i si d e n t i t ywh e ns u mmi n g ov e rt woi n d i c e st h a td on ot“ l i n eu p ”a c c o r d i n gt ot h i sb a s i ci d e n t i t yb yp e r mu t i n gt h e i n d i c e si nac y c l i cora n t i c y c l i c( wi t ha ne x t r ami n u ss i g n )wa yu n t i l t h e yd o.Soon ec a n e v a l u a t e : ǫjikǫmni=− ( δjmδkn−δjnδ k m) b yu s i n gǫmni=ǫi n dǫj ǫi . mna i k=− j k Ane x a mp l eofh owt ou s et h i sf ol l ows . Su p p os ewewi s ht op r ov et h a t : A· ( B× C) = B· ( C× A) = C· ( A× B) L e t ’ swr i t et h ef i r s tt e r mu s i n gou rn e wn ot a t i on A·( B×C)=Ai δi ǫmnjBmCn) j( wh e r eI l e f ti np a r e n t h e s e st oma k ei tc omp a r a t i v e l ye a s yt ot r a c kt h ec on v e r s i on . Wec a nn owu s et h ed e l t af u n c t i ont oe l i mi n a t et h ej i nf a v oroft h ei : A·( B×C)=ǫ Ai BmCn=Bmǫmni CnAi=Bmǫni Ai mn i mCn 2 Se et h ec h a p t e rc omi n gu pont e n s or st ol e a r nwh a tac on t r a c t i on( a n dat e n s or )i s , b u ti nt h eme a n t i me , t h i sj u s tme a n st h a tt h ei t hi n d e xi ss u mme d“ ou t ”oft h ee x p r e s s i on , s ot h a tt h er e s u l th a sf e we ri n d i c e son t h el e f tt h a ni th a sont h er i g h t . wh e r ewec a nn owr e or d e rt e r msa n di n di c e si nt h ep r od u c tf r e e l ya sl on ga swef ol l ow t h ec y c l i cp e r mu t a t i onr u l ea b ov ei nt h eǫt e n s orwh e nwea l t e rt h et e n s orc on n e c t i n g t h e m. F i n a l l y , wer e i n s e r ta( r e d u n da n t )δf u n c t i ona n dp a r e n t h e s e s : A ·( B×C)=Bmδmj( ǫni Ai )=B·( C×A) jCn Ob v i ou s l yt h et h i r df or mf ol l owsj u s tf r om a p p l y i n gt h i sr u l ea n dr e n a mi n gt h e v e c t or s . Th i ss a mea p p r oa c hc a nb eu s e dt op r ov et h eBACCABr u l e .Th e r ea r ean u mb e rof e qu i v a l e n tp a t h st h r ou g ht h ea l g e b r a .Wewi l ll e a v et h ep r ooft ot h es t u d e n t ,a f t e r g i v i n gt h e mas ma l l p u s hs t a r t . F i r s t : A× ( B× C) h a sc o mp on e n t s , s owee x p e c tt oh a v ep r e c i s e l yon e“ l e f t ov e r ”i n d e xa f t e rc on t r a c t i on ofs u i t a b l ee x p r e s s i on su s i n gt h er u l e sa n dt e n s or sd e v e l op e da b ov e . He n c e : A×( B×C)=Ai ( BmCnǫmnj) ǫi j kk wh e r et h et e r mi np a r e n t h e s e si st h ej t hc omp on e n tofB×C. Wei g n or et h e p a r e n t h e s e sa n dp e r mu t et h er e p e a t e di n d e xt ot h ef i r s ts l ot : A×( B×C) =Ai BmCnǫj ǫj mn k i k Ap p l yt h ei d e n t i t ya b ov e : A×( B×C) =Ai BmCn( δmkδni−δ δnk) mi k Wea p p l yt h ed e l t af u n c t i onr u l e st oe l i mi n a t ea l l oft h ema n dnc omb i n a t i on si n f a v orofi a n dk : A ×( B×C)=Ai BkCi−Ai Bi Ck=Bk( A·B)−Ck( A·B) k wh i c hi st r u ef ora l l t h r e ec omp on e n t soft h ev e c t or sr e p r e s e n t e donb ot hs i de s , Q. E . D. I nc a s et h i sl a s ts t e pi sob s c u r e , n ot et h a ton ewa yt or i n gau n i tv e c t ori n t oEi n s t e i n n ot a t i oni st ou s eag e n e r a l s y mb ol f o ru n i tv e c t or s . Ac o mmon ˆ ˆ ˆ he r eonec ans e e on ei se ˆ, wh e r ee ˆ =x ˆ=i , e ˆ =y ˆ=k , e ˆ =z ˆ=kw i j k i 1 2 3 ˆˆˆ i mme di a t e l yt hepr obl e mwi t hus i ng ,, i nan yc a r t e s i a nt en s ort h e or ywh e r e on ep l a n st ou s eEi n s t e i ns u mma t i on–on eofs e v e r a l r e a s on sI d on otc a r ef or √ t h e m( t h e ya l s oc a nc on f l i c twi t he . g .i=− 1orkt h ewa v en u mb e r ,wh e r ex ˆi s u n a mb i g u ou s l ya s s oc i a t e dwi t hxorAx) . Th el a s ts t e pc a nn owb es u mme da s : A× ( B× C)= A×( B×C) e ˆ ( A·C)−Ck( A·B) k= B k k e ˆ A· B) − C( A· B) k=B( Th i sg e n e r a la p p r oa c hwi l lp r ov ev e r yu s e f u lwh e non en e e d st op r ov et h er e l a t e d v e c t ord i ffe r e n t i a li d e n t i t i e sl a t e ron .Wi t h ou ti t , t r a c k i n ga n dr e or d e r i n gi n d i c e si sv e r y t e d i ou si n de e d . Weh a v ea tt h i sp oi n tc ov e r e ds e v e r a lk i n dsof“ v e c t or ”p r od u c t s , b u th a v eomi t t e d wh a ti ns o mewa y si st h emos tob v i ou son e .Th eou t e rp r od u c twh e r et h ep r od u c tofA a n dBi sj u s tABt h es a mewa yt h es c a l a rp r od u c tofaa n dbi sa b .Howe v e r , t h i sf or m i sdi ffic u l tt oi n t e r p r e t .Wh a tk i n dofob j e c t , e x a c t l y , i st h equ a n t i t yAB, t wov e c t or sj u s t wr i t t e nn e x tt oe a c hot h e r ? I ti sat e n s or , a n di ti st i met ol e a r nj u s twh a tat e n s ori s( wh i l el e a r n i n gab u n c hof n e wa n dv e r yi n t e r e s t i n gt h i n g sa l on gt h ewa y ) . Ch a p t e r4 Te n s o r s 4. 1 Th eDy a da n dNa d i cF o r ms Th e r ea r et wov e r yd i ffe r e n twa y st oi n t r od u c et h en ot i onofat e n s or .On ei si nt e r msof d i ffe r e n t i a lf or ms ,e s p e c i a l l yt h ed e f i n i t i onoft h et ot a ld i ffe r e n t i a l .Th i sf or mi s u l t i ma t e l yt h emos tu s e f u l ( a n dwewi l l d we l l u p oni tb e l owf ort h i sr e a s on )b u ti ti sa l s o a l g e b r a i c a l l ya n di n t u i t i v e l yt h emos tc omp l i c a t e d .Th eot h e rwa yi sb yc on t e mp l a t i n g t h eo u t e rp r od u c toft wov e c t or s , ot h e r wi s ek n owna sad y a d . Wewi l li n t r od u c et h ed y a di nat wod i me n s i on a lE u c l i d e a ns p a c ewi t hCa r t e s i a nu n i t v e c t or s ,b u ti ti sac omp l e t e l yg e n e r a li d e aa n dc a nb eu s e di na na r b i t r a r yn ma n i f ol d wi t h i nal oc a l l yE u c l i d e a np a t c h .Su p p os eon eh a sav e c t orA=Axx ˆ+Ayy ˆa n da n ot h e r v e c t orB=Bxx ˆ+Byy ˆ .I fon es i mp l ymu l t i p l i e st h e s et wov e c t or st og e t h e ra sa no u t e r p r od u c t( or d i n a r ymu l t i p l i c a t i onwi t ht h edi s t r i b u t i onoft h et e r ms )on eob t a i n st h ef ol l owi n g r e s u l t : AB=AxBxx ˆ x ˆ+AxByx ˆ y ˆ+AyBxy ˆ x ˆ+AyByy ˆ y ˆ ( 4 . 1 ) Th i sp r od u c tofv e c t or si sc a l l e dad y a d i c , a n de a c hp a i rofu n i tv e c t or swi t h i ni sc a l l e d ad y a d . Ady a di sa ni n t e r e s t i n gob j e c t .E a c ht e r ma p p e a r st ob ef or me dou toft h eor d i n a r y mu l t i p l i c a t i v ep r od u c toft won u mb e r s( wh i c hwec a ne a s i l ya n df u l l yc omp u t ea n d u n de r s t a n d)f ol l owe db yap a i rofu n i tv e c t or st h a ta r ej u x t a p os e d.Wh a t , e x a c t l yd oe s t h i sj u x t a p os i t i onofu n i tv e c t or sme a n ?Wec a nv i s u a l i z e( s or tof )wh a tx ˆb yi t s e l fi s– i ti sau n i tv e c t ori nt h exd i r e c t i ont h a twec a ns c a l et ot u r ni n t oa l lp os s i b l ev e c t or s t h a ta r ea l i g n e dwi t ht h ex a x i s( ori n t oc omp on e n t sofg e n e r a lv e c t or si nt h et wo d i me n s i on a l s p a c e ) . I ti sn ots os i mp l et ov i s u a l i z ewh a tady a dx ˆ x ˆi si nt h i swa y . Th ef u n c t i onofs u c hap r od u c tb e c ome smor ea p p a r e n twh e nwed e f i n eh owi twor k s . Su p p os ewi t ht a k et h ei n n e rp r od u c t( ors c a l a rp r od u c t , orc on t r a c t i on )ofou rv e c t orAwi t h t h ee l e me n t a r yd y a dx ˆ h x . Wec a ndot h i si ne i t h e ror d e r 2 5 ( f r ome i t h e rs i d e ) : A·( x ˆ x ˆ )=( A·x ˆ ) x ˆ=Axx ˆ ( 4 . 2 ) or ( x ˆ x ˆ )·A=x ˆ ( x ˆ·A)=Axx ˆ ( 4 . 3 ) Wes e et h a tt h ei n n e rp r odu c tofau n i td y a dx ˆ x ˆwi t hav e c t ors e r v e st op r oj e c tou t t h ev e c t ort h a ti st h ex c omp on e n tofA( n ott h es c a l a rma g n i t u d eoft h a tv e c t orAx) . Th ei n n e rp r od u c tofad y a dwi t hav e c t ori sav e c t or . Wh a ta b ou tt h ep r od u c tofot h e rd y a dswi t hA? ( x ˆ y ˆ )·A=x ˆ ( y ˆ·A) ˆ =Ayx ( 4 . 4 ) A·( x ˆ y ˆ )=( A·x ˆ ) y ˆ ˆ =Axy ( 4 . 5 ) wh i c ha r en ote qu a l .I nf a c t , t h e s et e r mss e e mt oc r e a t et h en e wv e c t orc omp on e n t s t h a tmi g h tr e s u l tf r omt h ei n t e r c h a n g eoft h exa n dyc omp on e n t soft h ev e c t orA, a sd o ( y ˆ x ˆ )·A=Axy ˆe t c . Not ewe l l !Someoft h ed y a d sc ommu t ewi t hr e s p e c tt oa ni n n e rp r od u c toft h e d y a dwi t hav e c t or ,ot h e r s( e . g .x ˆ y ˆ )d on ot !Ou rg e n e r a l i z e dd y a d i cmu l t i p l i c a t i on p r od u c e swh a ta p p e a rt ob e“ i n t r i n s i c a l l y ”n on c ommu t a t i v er e s u l t swh e nc on t r a c t e d wi t hv e c t o r sont h el e f tort h er i g h tr e s p e c t i v e l y . Th i si si nf a c tab r e a kp oi n t–i fwep u r s u et h i sp r odu c ti non ed i r e c t i onwec ou l d e a s i l ymo t i v a t ea n di n t r od u c eGe o me t r i cAl ge b r a ,i nt e r msofwh i c hMa x we l l ’ s e qu a t i on sc a nb ewr i t t e ni nac omp a c ta n dc omp e l l i n gf or m.Howe v e r ,e v e nwi t h ou t doi n gt h i s , wec a na r r i v ea tt h a tac omp e l l i n gf or m( t h a ti s , i nf a c t , qu a t e r n i on i c ) , s owe wi l lr e s t r a i nou r s e l v e sa n don l yl e a r ne n ou g ha b ou tt e n s or st ob ea b l et op u r s u et h e u s u a lt e n s orf or mwi t h o u twor r y i n ga b ou twh e t h e rorh owi tc a nb ed e c omp os e di na di v i s i ona l g e b r a . Th et h i n gt ot a k eou toft h edi s c u s s i ons of a ri st h a ti ng e n e r a lt h ei n n e rp r od u c tofa d y a dwi t hav e c t ors e r v e st op r oj e c tou tt h es c a l a ra mp l i t u d eoft h ev e c t oront h el e f tort h e r i g h ta n dr e c on s t r u c tap os s i b l yn e wv e c t orou toft h er e ma i n i n gu n i tv e c t or .Ve r ys h or t l y wea r eg oi n gt os t a r twr i t i n gr e l a t i on st h a ts u mov e rb a s i sv e c t or swh e r et h eb a s i si sn ot n e c e s s a r i l yor t h on or ma l( a st h i si s n ’ tr e a l l yn e c e s s a r yord e s i r e a b l ewh e ndi s c u s s i n g c u r v i l i n e a rc oor d i n a t es y s t e ms ) .Tod ot h i s ,Iwi l li n t r od u c ea tt h i sp oi n tt h eE i n s t e i n s u mma t i onc o n v e n t i onwh e r ewr i t i n gap r od u c twi t hr e p e a t e di n d i c e si mp l i e ss u mma t i on ov e rt h os ei n d i c e s : A= Ai x ˆ x ˆ i=Ai i ( 4 . 6 ) i Youc a ns e eh owt h es u mma t i ons y mb oli si ns omes e n s er e d u n d a n tu n l e s sf ors ome r e a s onwewi s ht of oc u sonas i n g l et e r mi nt h es u m.I nt e n s ora n a l y s i st h i si sa l mos tn e v e r t h ec a s e , s oi ti se a s i e rt oj u s ts p e c i f yt h ee x c e p t i on s . Not et h a twec a nf o r mg e n e r a ld y a d i cf or msd i r e c t l yf r omt h eu n i td y a d swi t h ou tt h e i n t e r me di a t es t e poft a k i n gt h eou t e rp r od u c tofp a r t i c u l a rv e c t or s ,p r od u c i n gt e r msl i k e { x ˆ x ˆ , x ˆ y ˆ , y ˆ x ˆ , y ˆ y ˆ } .Wec a na l s ot a k ea n ot h e rou t e rp r od u c tf r omt h el e f torr i g h twi t h a l l oft h e s ef or msp r od u c i n gt r y a ds , t e r msl i k e { x ˆ x ˆ x ˆ , x ˆ y ˆ x ˆ , . . . y ˆ x ˆ y ˆ , y ˆ y ˆ y ˆ }( e i g h tt e r mst ot a l ) . F u r t h e r mor ewec a nr e p e a ta l l oft h e a r g u me n t sa b ov ei nh i g h e rd i me n s i on a l s p a c e s , e . g . { x ˆ x ˆ , x ˆ y ˆ , x ˆ z ˆ , . . . , z ˆ z ˆ } . Th e r ei sac l e a ron e t oon ec or r e s p on d a n c eoft h e s emon a du n i tv e c t or st os p e c i f i c c ol u mnv e c t or s , e . g . : ( 4 . 7 ) 1 0 0 x ˆ= ( 4 . 8 ) 0 1 0 y ˆ= ( 4 . 9 ) 0 0 1 z ˆ= Th i sc or r e s p on d a n c ec on t i n u e st h r o u g ht h ev a r i ou su n i td y a d s , t r y a d s : x x ˆ ˆ= x y 100 0 0 0 0 0 0 010 0 0 ( 4 . 1 0 ) 0 0 0 0 ( 4 . 1 1 ) ˆ ˆ= a n ds oon . Wewi l l c a l l a l l oft h e s eu n i tmon a d s , d y a ds , t r y a d s , a n ds oon , a swe l l a st h equ a n t i t i e s f or me db ymu l t i p l y i n gt h e mb yor d i n a r yn u mb e r sa n ds u mmi n gt h e ma c c or d i n gt os i mi l a ra d i ct y p e ,t e n s or s .Aswec a ns e e ,t h e r ea r es e v e r a lwa y sofr e p r e s e n t i n gt e n s or st h a ta l l l e a dt oi d e n t i c a la l g e b r a i cr e s u l t s ,wh e r eon eoft h emos tc omp e l l i n gi st h ema t r i x r e p r e s e n t a t i oni l l u s t r a t e da b ov e .Not ewe l lt h a tt h ef e a t u r et h a td i ffe r e n t i a t e st e n s or sf r om “ or d i n a r y ”ma t r i c e si st h a tt h ec omp on e n t sc or r e s p on dt op a r t i c u l a ra di cc omb i n a t i on sof c oo r di n a t ed i r e c t i o n si ns omel i n e a rv e c t ors p a c e ;t e n s or swi l lc h a n ge ,a sag e n e r a lr u l e , wh e nt h eu n d e r l y i n gc oor di n a t ed e s c r i p t i oni sc h a n g e d.L e tu sd e f i n es omeoft h et e r mswe wi l l c ommon l yu s ewh e nwor k i n gwi t ht e n s or s . Th edi me n s i onoft h ema t r i xi nama t r i xr e p r e s e n t a t i onofat e n s orqu a n t i t ywec a l l i t sr a n k . Weh a v es p e c i a l ( a n dy e tf a mi l i a r )n a me sf ort h ef i r s tf e wt e n s orr a n k s : 0t hr a n kt e n s orors c a l a r .Th i si sa n“ or d i n a r yn u mb e r ” , wh i c hma ya tt h ev e r yl e a s tb e r e a lorc omp l e x ,a n dp os s i b l yc ou l db en u mb e r sa s s oc i a t e dwi t hg e ome t r i c a l g e b r a sofh i g h e rg r a d e .I t ’ sc h a r a c t e r i s t i cd e f i n i n gf e a t u r ei st h a ti si si n v a r i a n t u n d e rt r a n s f or ma t i on soft h eu n d e r l y i n gc oor di n a t es y s t e m.Al loft h ef ol l owi n g a r ea l g e b r a i ce x a mp l e sofs c a l a rqu a n t i t i e s : x , 1 . 1 8 2 , π, Ax, A·B. . . 1s tr a n kt e n s o rorv e c t o r .Th i si sas e tofs c a l a rn u mb e r s ,e a c ha na mp l i t u de c or r e s p on d i n gt oap a r t i c u l a ru n i tv e c t or or mon a d ,a n di n h e r i t si t s t r a n s f or ma t i on a l p r op e r t i e sf r omt h os eoft h eu n de r l y i n gu n i tv e c t or s .E x a mp l e s : i A=Axx ˆ+Ayy ˆ , { x } , { x} , i Ax z ˆ= Ay Az i wh e r et h eii ne . g .xd oe sn otc o r r e s p o n dt oap o we rb u ti sr a t h e rac oor d i n a t e i n de xc or r e s p on d i n gt oac o n t r a v a r i a n t( or d i n a r y )v e c t orwh e r ex i mi l a r l y is c or r e s p on d st oac ov a r i a n tv e c t or , a n dwh e r ec ov a r i a n c ea n dc on t r a v a r i a n c ewi l l b ed e f i n e db e l ow. 2n dr a n kt e n s o rorD×Dma t r i x( wh e r eDi st h ed i me n s i onoft h es p a c e , s o ⇔ 2 j i j t h ema t r i xh a sD c omp on e n t s ) . E x a mp l e s : Cxyx ˆ y ˆ , AB, C, Ai Ai, A, j, x x ⇔A A= Axy Axz Ayx Azx Ayy Azy Ayz Azz wh e r ea g a i ni nma t r i xc on t e x tt h ei n d i c e sma yb er a i s e dorl owe r e dt oi n d i c a t e c ov a r i a n c eorc on t r a v a r i a n c ei nt h ep a r t i c u l a rr oworc ol u mn . 3r da n dh i gh e rr a n kt e n s or sa r et h eD×D×D. . .ma t r i c e swi t har a n kc or r e s p on d i n gt ot h e n u mb e rofi n d i c e sr e qu i r e dt od e s c r i b ei t .I np h y s i c swewi l lh a v eoc c a s s i ont ou s e t e n s or st h r ou g ht h ef ou r t hr a n koc c a s i on a l l y , t h r ou g ht h et h i r dr a n kf a i r l yc ommon l y , a l t h ou g hmos toft h ep h y s i c a lqu a n t i t i e sofi n t e r e s twi l lb et e n s or sofr a n k0 2 .F or e x a mp l e swewi l l s i m⇔ p l yg e n e r a l i z et h a toft h ee x a mp l e sa b ov e , u s i n gTa sag e n e r i ct e n s orf or mor( mor e of t e n )e x p l i c i t l yi n d i c a t i n gi t si n d i c i a l f or ma si nT111, T112, . . . or ǫijk. Us i n ga ni n di c i a l f o r mwi t ht h eE i n s t e i ns u mma t i onc on v e n t i oni sv e r yp owe r f u l , a s wes h a l ls e e ,a n dp e r mi t su st of a i r l ys i mp l yr e p r e s e n tf or mst h a twou l dot h e r wi s e i n v ol v eal a r g en u mb e rofn e s t e ds u mma t i on sov e ra l lc oor di n a t ei n d i c e s .To u n d e r s t a n dp r e c i s e l yh owt og oa b ou ti t , h owe v e r , weh a v et of i r s te x a mi n ec oor d i n a t e t r a n s f or ma t i on s . 4. 2 Co or d i n a t eTr a n s f o r ma t i o n s Su p p os eweh a v eac oor d i n a t ef r a meKi nDd i me n s i on s ,wh e r eDwi l lt y p i c a l l yb e4f or r e l a t i v i s t i cs p a c e t i me( wi t ht h e0 t hc oor d i n a t ee qu a l t oc ta su s u a l )or3f orj u s tt h es p a t i a l p a r t .Tos i mp l i f you rn ot a t i on ,wewi l lu s er oma nc h a r a c t e r ss u c ha si ,j ,kf ort h et h r e e v e c t ors p a t i a l on l yp a r tofaf ou r v e c t or , a n du s eg r e e kc h a r a c t e r ss u c ha sµ, ν , γ , δf ort h e e n t i r ef ou r v e c t or( wh e r er e c a l l , r e p e a t e di n d i c e si mp l ys u mma t i onov e re . g .i=1 ,2 ,3orµ=0 ,1 ,2 ,3 ,h e n c et h e d i s t i n c t i ona si tc a nb eu s e dt od e f a c t or e s t r i c tt h es u mma t i onr a n g e ) . ′ Nows u p p os et h a twewi s ht ot r a n s f or mt oan e wc oor d i n a t ef r a meK.Att h i st i me wep l a c ev e r yf e wr e s t r i c t i on sont h i st r a n s f or ma t i on .Th et r a n s f or ma t i onmi g h t , t h e r e f or e ,t r a n s l a t e ,r ot a t e ,r e s c a l e orot h e r wi s ea l t e rt h e or i g i n a lc oor d i n a t e d e s c r i p t i on .Aswedot h i s , ou rd e s c r i p t i o nofp h y s i c a lqu a n t i t i e se x p r e s s e di nt h eol d c oor d i n a t e smu s ts y s t e ma t i c a l l yc h a n g et oad e s c r i p t i oni nt h en e wc oor di n a t e s , s i n c e t h ea c t u a l p h y s i c a l s i t u a t i onb e i n gd e s c r i b e di sn ota l t e r e db yt h ec h a n g ei nc oor d i n a t e f r a me s . Al l t h a ti sa l t e r e di sou rp oi n tofv i e w. Ou rf i r s tob s e r v a t i onmi g h tb et h a ti tma yn otb ep os s i bl et od e s c r i b eou rp h y s i c a l qu a n t i t i e si nt h en e wf r a mei ft h et r a n s f or ma t i onwe r ec omp l e t e l yg e n e r a l .F ore x a mp l e , i f ′ t h ed i me n s i onofKwe r edi ffe r e n t( e i t h e rl a r g e rors ma l l e rt h a nt h a tofK)wemi g h twe l lb e u n a b l et or e p r e s e n ts omeoft h ep h y s i c st h a ti n v ol v e dt h emi s s i n gc oor d i n a t eorh a v ea c e r t a i nd e g r e eofa r b i t r a r i n e s sa s s oc i a t e dwi t han e wc oor d i n a t ea d d e don .As e c on d p os s i b l ep r ob l e mi n v ol v e sr e g i on soft h et woc oor d i n a t ef r a me st h a tc a n n otb ema d et o c or r e s p on d– i ft h e r ei sap a t c hoft h eKf r a met h a ts i mp l yd oe sn otma pi n t oa ′ c or r e s p on di n gp a t c hoft h eK f r a mewec a n n ote x p e c tt oc or r e c t l yde s c r i b ea n yp h y s i c s t h a td e p e n d sonc oor d i n a t e si n s i d et h ep a t c hi nt h en e wf r a me . Th e s ea r en o ti r r e l e v a n tma t h e ma t i c a li s s u e st ot h ep h y s i c i s t .Ap e r p e t u a lop e n qu e s t i o ni np h y s i c si swh e t h e rorn ota n yp a r t sofi ti n v ol v ea d d i t i o n a lv a r i a bl e s .Th os e v a r i a b l e smi g h tj u s tb e“ p a r a me t e r s ”t h a tc a nt a k eons omer a n g eofv a l u e s , ort h e ymi g h t b es u p p or t e don l ywi t h i ns p a c e t i mes c a l e st h a ta r et oos ma l lt ob ed i r e c t l yob s e r v e d ( l e a v i n gu st oi n f e rwh a th a p p e n si nt h e s emi c r os c a l e“ p a t c h e s ”f r omob s e r v a t i on sma d eon t h ema c r os c a l e ) , t h e yma yb ema c r os c op i cd oma i n sov e rwh i c hf r a met r a n s f or ma t i on sa r e s i n g u l a r( t h i n k“ b l a c kh ol e s ” )ort h e yma yb ea c t u a le x t r adi me n s i on s–h i d d e nv a r i a b l e s , i f y oul i k e–i nwh i c hi n t e r a c t i on sa n ds t r u c t u r ec a noc c u rt h a ti son l yv i s i b l et ou si nou rf ou r d i me n s i on a ls p a c e t i mei np r o j e c t i o n .Wi t hn oap r i or ir e a s ont oi n c l u d eore x c l u dea n yof t h e s ep os s i b i l i t i e s , t h ewi s es c i e n t i s tmu s tb ep r e p a r e dt ob e l i e v eord i s b e l i e v et h e ma l l a n d t oi n c l u d et h e mi nt h e“ mi x ”ofp os s i b l ee x p l a n a t i on sf orot h e r wi s edi ffic u l tt ou n de r s t a n d p h e n ome n a . Howe v e r , ou rp u r p os e sh e r ea r emor eh u mb l e .Weon l ywa n tt ob ea b l et od e s c r i b e t h er e l a t i v e l ymu n d a n ec oor di n a t et r a n s f or ma t i on st h a tdon oti n v ol v es i n g u l a r i t i e s , u n ma t c h e dp a t c h e s , ora d d i t i on a l ormi s s i n gc oor d i n a t edi me n s i on s .Wewi l l t h e r e f or e r e qu i r et h a tou rc oor d i n a t et r a n s f or ma t i on sb eo n e t oo n e– e a c hp oi n ti nt h e ′ s p a c e t i mef r a meKc or r e s p on d st oon ea n don l yon ep o i n ti nt h es p a c e t i mef r a meK– ′ a n do n t o–n omi s s i n gore x t r ap a t c h e si nt h eK f r a me .Th i ss u ffic e st oma k et h e t r a n s f or ma t i on si n v e r t i b l e .Th e r ewi l lb et wov e r yg e n e r a lc l a s s e soft r a n s f or ma t i on t h a ts a t i s f yt h e s er e qu i r e me n t st oc o n s i de r . I non eoft h e m, t h en e wc oor d i n a t e sc a nb e r e a c h e db yme a n sofap a r a me t r i ct r a n s f o r ma t i onoft h eor i g i n a lon e swh e r et h e p a r a me t e r sc a nb ec on t i n u o u s l yv a r i e df r om as e tof0v a l u e st h a tde s c r i b e“ n o t r a n s f or ma t i on ” . I nt h eot h e r , t h i si sn ott h ec a s e . F ort h emome n t , l e t ’ ss t i c kt ot h ef i r s tk i n d , a n ds t a r tou rdi s c u s s i onb y l ook i n ga tou rf r i e n d st h ec oor d i n a t e st h e ms e l v e s .Byd e f i n i t i on ,t h eu n t r a n s f or me d c oor d i n a t e sofa ni n e r t i a lr e f e r e n c ef r a mea r ec o n t r a v a r i a n tv e c t or s .Wes y mb ol i z e c on t r a v a r i a n tc omp on e n t s( n otj u s t4 v e c t or s–t h i sd i s c u s s i ona p p l i e st ot e n s or qu a n t i t i e sona l lma n i f ol dsont h ep a t c hofc oor d i n a t e st h a ti sl oc a l l yf l a ta r ou n da p oi n t )wi t hs u p e r s c r i p ti n d i c e s : 0 1 2 3 x x, x, x, x. . . ) c o n t r a v a r i a n t=( ( 4 . 1 2 ) wh e r ewea r en otg oi n gt odi s c u s sma n i f ol d s , c u r v e ds p a c e s , t a n g e n torc ot a n g e n tb u n d l e s ( mu c h )a l t h ou g hwewi l ls t i l lu s eaf e woft h e s et e r msi nawa yt h a ti sh op e f u l l yc l e a ri n c on t e x t .Ie n c ou r a g ey o ut oe x p l or et h er e f e r e n c e sa b ov et of i n dd i s c u s s i on st h a te x t e n d i n t ot h e s ea r e a s .Not et h a tI ’ mu s i n gan on b ol dxt os t a n df oraf ou r v e c t or , wh i c hi sp r e t t y a wf u l , b u twh i c hi sa l s ov e r yc ommon . Nowl e tu sd e f i n eama p p i n gb e t we e nap oi n t( e v e n t )xi nt h ef r a meKa n dt h es a me ′ ′ p oi n txde s c r i b e di nt h eK f r a me .xi nKc on s i s t sofas e toff ou rs c a l a rn u mb e r s , i t s f r a mec oor d i n a t e s ,a n dwen e e dt ot r a n s f or mt h e s ef ou rn u mb e r si n t of ou rn e w ′ n u mb e r si nK.F r omt h edi s c u s s i ona b ov e ,wewa n tt h i sma p p i n gt ob eac on t i n u o u s f u n c t i o ni nb ot hdi r e c t i on s . Th a ti s : ′ ′ ′ ′ 0 1 2 0 1 2 0 = x 0( x x, x, x. . . ) x, x, x. . . ) 1 = x 1( x ′ ′ 0 1 2 ( x, x, x. . . ) 2 = x 2 x . . . ( 4 . 1 3 ) ( 4 . 1 4 ) ( 4 . 1 5 ) ( 4 . 1 6 ) a n d 0 00 ′1 ′2 ′ x =x( x, x, x. . . ) ( 4 . 1 7 ) x = x( x, x, x. . . ) ( 4 . 1 8 ) x = x( x, x, x. . . ) . . . ( 4 . 1 9 ) ( 4 . 2 0 ) 1 10 ′1 ′2 ′ 2 20 ′1 ′2 ′ h a v et ob o t he x i s ta n db ewe l lb e h a v e d( c on t i n u ou s l yd i ffe r e n t i a b l ea n ds oon ) .I nt h e mos tg e n e r a lc a s e , t h ec oor d i n a t e sh a v et ob el i n e a r l yi n d e p e n de n ta n ds p a nt h eKor ′ Kf r a me sb u ta r en otn e c e s s a r i l yor t h on or ma l .We ’ l lg oa h e a da n dwor kwi t h or t h on or ma lc oor d i n a t eb a s e s , h owe v e r , wh i c hi sf i n es i n c en on or t h n or ma lb a s e sc a n a l wa y sb eot h og on a l i z e dwi t hGr a mSc h mi d ta n dn or ma l i z e da n y wa y . Gi v e nt h i sf or ma l t r a n s f or ma t i on , wec a nwr i t et h ef ol l owi n gr e l a t i onu s i n gt h e c h a i nr u l ea n dd e f i n i t i onofd e r i v a t i v e : ′ dx0 ′ dx 1 ′ dx2 . ′ 0 ∂x ′ 0 0 ∂x 0 1 ∂x ′ 1 0 ∂x 1 1 ∂x 1 2 ∂x 2 0d 1d 2d x +∂x x + ∂x x = ∂x ′ 1 ∂x 0d x = ∂x ′ 2 ∂x 0 ′ 1d x+ +∂x ′ 2 ∂x ′ 2 2d x ∂x ′ 2 0d x + 1d x + 2 dx = ∂x ∂x ∂x + . . . ( 4 . 2 1 ) + . . . ( 4 . 2 2 ) + . . . ( 4 . 2 3 ) . . wh e r ea g a i n , s u p e r s c r i p t ss t a n df ori n d i c e sa n dn otp owe r si nt h i sc on t e x t . Wec a n wr i t et h i si nat e n s or ma t r i xf or m: ′ 0 ′ ∂x 0 d x 0 ′ ∂x 0 ′ ∂x ′ 1 d x ′ 2 dx . . . = 0 1 ∂x ∂x 2′ ∂x 2′ ∂x 0 1 ∂x 1 ∂x ∂x 1 ∂x 0 1 ∂x 1 ′ ∂x ... dx ... dx . . . d x 2′ ∂x ′ ∂x . . . . 1 ∂x ′ 1 ∂x 1 ∂x . . 0 2 . .. . . 1 Th ede t e r mi n a n toft h ema t r i xa b ov ei sc a l l e dt h eJ a c o b e a noft h et r a n s f or ma t i on a n dmu s tn otb ez e r o( s ot h et r a n s f or ma t i oni si n v e r t i b l e .Th i sma t r i xd e f i n e st h e ′ d i ffe r e n t i a lt r a n s f or ma t i onb e t we e nt h ec oor d i n a t e si nt h eKa n dK f r a me ,g i v e nt h e i n v e r t i b l ema p sd e f i n e da b ov e . Al l f i r s tr a n kt e n s or st h a tt r a n s f or ml i k et h ec oor d i n a t e s , t h a ti st os a ya c c or d i n gt ot h i st r a n s f or ma t i onma t r i xl i n k i n gt h et woc oor d i n a t e s y s t e ms ,a r es a i dt ob ec on t r a v a r i a n tv e c t or swh e r eob v i ou s l yt h ec oor di n a t ev e c t or s t h e ms e l v e sa r ec on t r a v a r i a n tb yt h i sc on s t r u c t i on . Wec a ns i gn i f i c a n t l yc ompr e s st h i se x p r e s s i onu s i n gEi n s t e i n i a ns u mma t i on : ′ i= d x ′ i ∂x j x jd ( 4 . 2 4 ) ∂x i nwh i c hc omp a c tn ot a t i onwec a nwr i t et h ed e f i n i t i onofa na r b i t r a r yc on t r a v a r i a n t v e c t orAa sb e i n gon et h a tt r a n s f or msa c c or d i n gt o: ′ i ∂x Ai= jAj ′ ∂x Th e r e , t h a twa se a s y ! ( 4 . 2 5 ) Ch a p t e r5 Gr o u pTh e o r y On eoft h ef i r s tb i t sof“ ma t h ”y oul e a r n e da sas t u d e n ti sor di n a r ya r i t h me t i c :h owt o a d da n ds u b t r a c tt won u mb e r s , h owt omu l t i p l ya n dd i v i d et won u mb e r s .Al t h ou g hy ou ma yn oth a v er e a l i z e di ta tt h et i me , y ouwe r el e a r n i n gn o ton l yy ou rf i r s ta r i t h me t i c , b u t y ou rf i r s tgr o u pt h e or y !Howe v e r ,g r ou pt h e or yi sal otmor eg e n e r a lt h a n“ j u s t ” a r i t h me t i c . Ag r ou pGi sas e tofe l e me n t st h a ti sc l o s e dwi t hr e s p e c tt oa nop e r a t i onof c omp os i t i o n( t h i n k“ mu l t i p l i c a t i on ” ,a l t h ou g hi tof t e ni s n ’ t ,s op e op l eu s ea mu l t i p l i c a t i on l i k es y mb ol ,◦ ,i n s t e a dof∗wh e nd i s c u s s i n ga b s t r a c tg r ou p s )t h a tt u r n s t woe l e me n t si n t oon e( n otn e c e s s a r i l ydi ffe r e n t )e l e me n t : a◦b=c , wi t ha , b , c∈G ( 5 . 1 ) Th es e tofe l e me n t sh a st oc on t a i non es p e c i a l e l e me n t , t h ei d e n t i t ye l e me n ti , s u c h t h a t : a◦i =a E v e r ye l e me n tmu s th a v eac or r e s p on di n gi n v e r s ee l e me n ti nt h eg r ou p : − 1 a◦a =i , −1 wi t ha , a, i ∈G F i n a l l y , t h ec omp os i t i onr u l eh a st ob ea s s oc i a t i v e : a◦( b◦c )=( a◦b )◦c , wi t ha , b , c∈G Th es i mp l e s t ,a n ds ma l l e s t ,g r ou pc on s i s t sofon l yon ee l e me n t ,t h ei de n t i t y e l e me n t , wh i c hi si t sowni n v e r s e , r e p r e s e n t e db yas i n g l el i n e : i ◦( i ◦i )=( i ◦i )◦i =i ◦i =i wh e r ewes e et h a tt h ei d e n t i t ye l e me n ti sa l wa y si t sowni n v e r s ea n df or msa l l b yi t s e l f as p e c i a lg r ou pc a l l e dt h et r i v i a lg r ou p .Th et r i v i a lg r ou pi sd e n ot e dZ1( ors ome t i me s C1) . 3 3 Youa r ef a mi l i a rwi t han u mb e rofg r ou p sa l r e a d y ,e v e nt h ou g hy ouma yn oth a v e t h ou g h toft h e ma ss u c h .Th es e tofp os i t i v ea n dn e g a t i v ei n t e g e r s ,wi t ht h ea d d i t i o n s y mb olu s e df orc omp os i t i on ,f or msag r ou p ,wi t hz e r ob e i n gt h ei d e n t i t ya n dan e g a t i v e n u mb e rb e i n gt h ei n v e r s eofap os i t i v eon ea n dv i c ev e r s a .Th es e tofi n t e g e r st og e t h e rwi t h mu l t i p l i c a t i onu s e da sac omp os i t i onr u l ei sn o tagr o u p !I ti sa s s oc i a t e , i ti sc l os e d , a n di t h a sa ni d e n t i t y( t h ei n t e g e ron e )b u tt h ei n v e r s eofa l mos ta l le l e me n t si sn oti nt h eg r ou p . Th es e tofa l l r a t i on a ln u mb e r se x c l u d i n gz e r of or msag r ou pwi t hr e s p e c tt omu l t i p l i c a t i on ( wh ymu s twee x c l u d ez e r o? ) .Ma t h e ma t i c i a n sn ot a t i on a l l ywr i t et h i se x c l u s i onwi t ht h e\ s y mb ol ,f ore x a mp l et h eg e n e r a lmu l t i p l i c a t i v eg r ou pov e rt h es e t( f i e l d )ofa l lc omp l e x ∗ n u mb e r sCi sd e n ot e dC =C\ 0 . 5. 0. 1 Subgr ou p s As u b gr o u pi sas u b s e tofe l e me n t si nt h eg r ou pt h a ti si t s e l fag r ou p , f ore x a mp l et h e ∗ s e tofa l l r e a l n u mb e r sl e s sz e r oR =R\ 0i sas u b g r o u p o f C , a n d t h e s e t o f a l l r a t i o n a l ∗ n u mb e r s( l e s sz e r o)i ss i mi l a r l yas u b g r ou pofR.Th ema t h a ma t i c a ln ot a t i onf ora s u b g r ou pi st h es a mea st h a tofas u b s e t : SO( 3 )⊂O( 3 ) or ∗ Z1⊂R Th et r i v i a lg r ou pZ1i sob v i ou s l yas u b g r ou pofa l lg r ou p s .Al s oag r ou pi sa l wa y si t s owns u b g r ou p . As i mp l eg r ou pi son ewi t ho n l yt h e s et wos u b g r ou p s –on ec a n n otf i n da n ys e tofe l e me n t ss ma l l e rt h a nt h ee n t i r eg r ou pe x c e p tt h et r i v i a l g r ou pt h a ti sas u b s e t . 5. 0. 2 Ab e l i a n( Co mmu t a t i v e )Gr o u p s Ag r ou pwi t ht h ec o mmu t a t i v ep r op e r t y : a◦b=b◦a i sc a l l e de i t h e rac ommu t a t i v eg r ou p( wh i c hi sob v i ou s )ora na b e l i a ng r ou p( wh i c h i sn ots oob v i ou s ,b u ty ous h ou l dk n owwh a tt h i swor dme a n s ) .Not ewe l l !Nota l l g r ou p sa r ea b e l i a n !I np a r t i c u l a r ,t h er ot a t i ong r ou pSO( 3 )( d i s c u s s e db e l ow)i s n on a b e l i a n ,b e c a u s et wor ot a t i on st h r ou g haf i n i t ea n g l ea r ou n dt wodi s t i n c ta x e sd o n otp r od u c e st h es a mef i n a lc oor d i n a t ef r a mewh e np e r f or me di ne i t h e ror d e r .Ma n yi f n otmos toft h et r a n s f or ma t i ong r ou p sofp h y s i c sa r en on a b e l i a n ,a n dt h e yp l a ya n e x t r e me l yi mp or t a n tr ol ei nqu a n t u mt h e or y . 5. 0. 3 L i e( Con t i n u ou s )Gr o u p s J u s ta st h e r ei sadi s t i n c t i onb e t we e nt h e( c ou n t a b l e )s e tofi n t e g e r sa n dt h e( u n c ou n t a b l e ) s e tofr e a ln u mb e r s , t h e r ei sad i s t i n c t i onb e t we e nd i s c r e t eg r ou p s( wh e r ea ni d e n t i f i c a t i on c a nb ema d eb e t we e ng r ou pe l e me n t sa n dt h ei n t e g e r s )a n dc on t i n u ou sg r ou p s( wi t ha n ∗ u n c o u n t a b l yi n f i n i t en u mb e rofg r ou pe l e me n t s ) .R i sac on t i n ou sg r ou p , a n di st h eb a s i sof c a l c u l u s ,b e c a u s ei ts u p p or t st h ei de aofd i ffe r e n t i a t i o nu s i n gas u i t a b l el i mi t i n gp r oc e s s s u c ha s l i m x → → d 0 x dx AL i eGr ou pi sac on t i n u ou sg r ou p , wh i c hi sa l s of or ma l l yad i ffe r e n t i a b l ema n i f o l d . Wec ou l de a s i l yg e ts we p tdownt h er a b b i th ol et o“ r e a l ma t h ”a tt h i sp oi n ta n de x p l a i n t h a tadi ffe r e n t i a b l ema n i f ol di sa n ys p a c et h a ti sl oc a l l yi s omor p h i ct oaE u c l i d e a n( f l a t ) 3 s p a c el i k eR ( ar e a ls p a c ei nt h r e eor t h og on a ld i me n s i on s )wh e r e i nd i ffe r e n t i a t i oni s we l l d e f i n e d .Th i sme a n st h a taL i eg r ou pi sg e n e r a t e db yc omp os i n gal a r g en u mb e rof l o c a l“ i n f i n i t e s i ma lt r a n s f or ma t i on s ”i n t oaf i n i t et r a n s f or ma t i on .Con t i n u ou s c oor d i n a t et r a n s f or ma t i on si np h y s i c sof t e nf or mL i eg r ou p s , i np a r t i c u l a rt h es e tofa l l c on t i n ou sr ot a t i on sofac oor d i n a t ef r a me , SO( 3 ) . Al loft h i ss e c t i on s of a r ,i nf a c t ,l e a ds t ot h i s on ec on c l u s i on .Coor d i n a t e t r a n s f or ma t i on sofi n t e r e s tt ou si np h y s i c si ng e n e r a l ,a n de l e c t r ody n a mi c si np a r t i c u l a r , a l mos ta l wa y se n du pb e i n gL i eg r ou p s( wi t ha na s s oc i a t e dL i ea l ge b r af ort h ea b s t r a c t g r ou pop e r a t i on s )g e n e r a t e df r om i n f i n i t e s i ma ll oc a lt r a n s f or ma t i on s .Th ec on t i n ou s g r ou p sa r eof t e ne x t e n d e db ya( u s u a l l ys ma l l / f i n i t e )s e tofd i s c r e t et r a n s f or ma t i on s , s u c h a si n v e r s i on . L e t ’ sd i s c u s st h i sf u r t h e r . 5. 1 Co or d i n a t eTr a n s f o r ma t i o nGr o u p s Coor d i n a t et r a n s f or ma t i on si np h y s i c sf or mag r ou p , ormor ep r op e r l y ,c a nb es p l i tu p i n t os e v e r a ln a me dg r ou p sa n ds u b g r ou p s .I ti sb e y on dt h es c op eoft h i ss h or tr e v i e w t oi n t r od u c ey out oa l l ofs u b t l e t i e sa n dj oy sofg r ou pt h e or yi np h y s i c s( on ec ou l dwr i t e awh ol eb ookont h i sa l on e–ort woort h r e eb ook s ! )s owewi l l j u s tmov ea sd i r e c t l ya s p os s i b l et ot woort h r e ee x a mp l e st h a ts h ou l da l r e a d yb es o me wh a tf a mi l i a rt ot h e r e a d e r . L e tu sd e f i n et h ep os i t i onv e c t or( i na n yc oor d i n a t ef r a meorc oor d i n a t es y s t e m, b u t 3 f orn owwewi l lt h i n kon l yofR,r e a lEu c l i de a ns p a c ei nt h r e ed i me n s i on s )t ob e d e n ot e db y r( d r e s s e dwi t hi n d i c e sorp r i me sa sn e e db e ) .F ore x a mp l e ,i nCa r t e s i a n c oor d i n a t e s : r =x x ˆ+y y ˆ+z z ˆ Th edi s p l a c e me n tv e c t or–ora n yg e n e r a l d i ffe r e n c eofp os i t i onv e c t or s –i sa ne n or mou s l yu s e f u lob j e c ti np h y s i c si ng e n e r a la n de l e c t r od y n a mi c si n p a r t i c u l a r .Wewi l lu s eas p e c i a ln ot a t i onf ori tt h a ts i mp l i f i e sc e r t a i nf or mu l a st h a t oc c u rqu i t eof t e ni nE l e c t r od y n a mi c s( f ol l owi n gGr i ffit h s ) : ′ ′ = rr =( x ′ x) x ˆ+( y − − ′ y) y ˆ+( z − z) z ˆ − I ti sa l s oe s s e n t i a l f ort h ed e f i n i t i onofdi ffe r e n t i a t i on , ma n i f ol ds , t h ec on s t r u c t i onof c a l c u l u sa n dt h ec a l c u l u s b a s e de n t i t i e sofp h y s i c ss u c ha sv e l oc i t y v ora c c e l e r a t i on a, b u tf ort h emome n twewi l l n otwor r ya b ou ta n yoft h i s . Not eWe l l !Av e c t ori sde f i n e dt ob ead i me n s i on e dob j e c tt h a tt r a n s f or msl i k ea d i s p l a c e me n tv e c t orwh e nt h eu n d e r l y i n gc oor d i n a t ef r a mei st r a n s f or me d !Mor eont h i s l a t e r , b u tf i r s t , l e t ’ sl ooka ts omes p e c i f i cL i eg r ou p s . 5. 1. 1 Th eTr a n s l a t i onGr o u p Th et r a n s l a t i ong r ou pi st h es e tofa l lt r a n s f or ma t i on st h a tmov eord i s p l a c et h eor g i n ′ ofac oor di n a t ef r a meSt oan e wl oc a t i on , f or mi n gan e wc oor di n a t ef r a mS.Th i sc a n b ev i s u a l i z e dwi t ht h ef ol l owi n gg r a p h : S’ S R’ R D f r omwh i c hwes e et h a ti fwed i s p l a c eSb yt h ea r b i t r a r yv e c t ord , t h e n : ′ r= r −d [ As i d e :Th i sc a nb ewr i t t e na sama t r i xt of or m ac on t i n ou sg r ou pu s i n gma t r i x mu l t i p i c a t i ona st h eg r ou pc omp os i t i on , b u tdoi n gs oi st r i c k y( i tr e qu i r e se x t e n di n gt h e d i me n s i onof rb yon e )a n dwewi l ll e a v ei ti nt h i se a s i l yu n d e r s t oodf or m,wh e r ei ti s h op e f u l l yob v i ou st h a tt h es e tofa l l s u c ht r a n s f or ma t i on s( i n d e e d, v e c t ora d d i t i oni t s e l f ) f or mag r ou p . ] On ec a ne a s i l yp r ov et h a tt h et r a n s f or ma t i on soft h i sg r ou pl e a v edi s p l a c e me n t v e c t or s u n c h a n g e d .Ne wt on i a nme c h a n i c sa r ei n v a r i a n tu n d e rt h ea c t i onoft h i s g r ou pp r ov i d e dt h a tdi se i t h e rac on s t a n toral i n e a rf u n c t i onoft i me( i n e r t i a lf r a me t r a n s f or ma t i on s )b e c a u s ei nt h i sc a s et h eg r ou pl e a v e sa c c e l e r a t i onu n c h a n g e d. 5. 1. 2 Th eRo t a t i onGr o u p Th er ot a t i ong r ou pi st h es e tofa l l r ot a t i on sofac oor d i n a t ef r a me . On ec a nwr i t ea ′ r e a l i z a t i onoft h i sg r ou pa sas e tof3 dma t r i c e st h a tma p rt or: ′ x ′ y Rxx = Ryx Rxy Ryy Rxz Ryz x y ′ z Rzx Rzy Rzz z Th i si st e d i ou st owr i t eou t !Wewi l lc omp r e s st h i sn ot a t i on a l l yt ot h i se x p r e s s i on f ore a c h( t h ei t h )oft h ev e c t orc oor di n a t e s : 3 ′ r i= Ri jr j j = 1 I nma n yp h y s i c sb ook s–e s p e c i a l l ya th i g h e rl e v e l s–i ti sp oi n t l e s st oe v e nwr i t e t h es u mma t i ons i g n ;p e op l eu s et h eE i n s t e i ns u mma t i onc on v e n t i ont h a tr e p e a t e d i n d i c e si na ne x p r e s s i ona r et ob es u mme d: ′ r i=R i jr j ( t h r e ee qu a t i on s , n ot ewe l l , on ee a c hf ori =1 , 2 , 3 ) . On ec a ne a s i l yp r o v et h a tt h et r a n s f or ma t i on soft h i sg r ou pl e a v et h el e n gt h s ( ma g n i t u de s )b u tn ott h edi r e c t i on sofp os i t i onv e c t or s r ||u n c h a n g e d.I n d e e d , t h e“ c or r e c t ” wa yt od e r i v ear e p r e s e n t a t i onf ort h er ot a t i onma t r i xRi wh i c h j( ⇔ wewi l l a l s owr i t eR)i st of i n dt h es e tofa l l i n f i n i t e s i ma l t r a n s f or ma t i on st h a tl e a v et h e l e n g t hofav e c t oru n c h a n g e d–t h e i rc omp os i t i onf or mst h eL i e( c on t i n u ou s )r ot a t i on g r ou p , SO( 3 ) . 5. 1. 3 Th eI n v e r s i o nGr o u p Th ei n v e r s i ong r ou pc on s i s t sofon l yt woop e r a t i on s :t h ei de n t i t ya n dt h ei n v e r s i on . I n v e r s i oni st h ema t r i xop e r a t i on( i n3s p a t i a l d i me n s i on s ) : ′ x ′ y ′ z − x =− y = − z ′ − 1 0 0 0 − 10 x y 0 z 0− 1 ⇔ wh i c hwemi g h ta l s owr i t ea s r =− r =−Ir.Th ec omb i n a t i onof SO( 3 )a n dt h ei n v e r s i ons y mme t r yf or msO( 3 ) ,t h eOr t h o go n a lGr o u pi nTh r e e Di me n s i on s , wh i c hi st h es e tofa l lc oor d i n a t et r a n s f or ma t i on st h a tl e a v et h el e n g t hof av e c t oru n c h a n g e d . Th i si sn owh e r en e a ra l l oft h eg r ou p sofi n t e r e s ta n du s ei np h y s i c s ! I ti s n ’ te v e na l l oft h eL i eg r ou p sofc oor d i n a t et r a n s f or ma t i on sofi n t e r e s ta n du s ei np h y s i c s .Aswe wi l ls e ei ns omede t a i li nl a t e rc h a p t e r s ,t h et h e or yofs p e c i a lr e l a t i v i t yi smos t b e a u t i f u l l yd e f i n e db yl ook i n gf ort h es e toft r a n s f o r ma t i on soff ou rdi me n s i on a l s p a c e t i met h a tl e a v eap a r t i c u l a rde f i n i t i onoft h el e n g t hofaf ou r v e c t ori n v a r i a n t , a l t h ou g ht h a ti sb e y on dt h es c op eoft h i si n t r od u c t i onorr e v i e w.Ra t h e r ,i ti st h e mot i v a t i onf ort h i sr e v i e w–y ouwon ’ tu n d e r s t a n dwh a tI ’ mt a l k i n ga b ou twh e nIg e tt o t h eL or e n t zg r ou pi fy oud on ’ tk n owwh a tag r ou pi s ! Ch a p t e r6 Sc a l a ra n dVe c t o rCa l c u l u s Tos u mma r i z ewh a twe ’ v ec ov e r e ds of a r :Ou rs t u dyofe l e c t r od y n a mi c si sg oi n gt ob e f ou n d e donr e a l a n dc omp l e xn u mb e r st h a tr e p r e s e n tp h y s i c a lqu a n t i t i e swi t hu n i t s , s o wel e a r n e dab i ta b ou tt h e s ek i n d sof( s c a l a r )n u mb e r s .Si n c ei ti sak i n dofama pof wh a th a p p e n si ns p a c ea n dt i me ,wen e e dt ou n de r s t a n dc oor d i n a t e s ,v e c t or si na c oor d i n a t es y s t e m, a n dv a r i ou swa y st omu l t i p l yv e c t or s .Th a tl e du st oc on s i d e rb ot h t e n s orf or msa n dc oor d i n a t et r a n s f or ma t i on ,a sb ot hoft h e s ewi l lp r ov et ob ev e r y u s e f u li fn ote s s e n t i a l .Coor d i n a t et r a n s f or ma t i on s( a tl e a s t )of t e nf or mg r ou p ss owe l e a r n e dwh a tag r ou pwa s( a n dr e a l i z e dt h a twe ’ v eb e e nu s i n ge . g .t h emu l t i p l i c a t i on g r ou pa l l ofou rl i v e swi t h ou tr e a l i z i n gi t . I ts h ou l dc omea sn os u r p r i s et h a tt h er e ma i n i n gc h u n kofma t hwewi l ln e e di s c a l c u l u s .Af t e ra l l ,Ne wt on i n v e n t e dc a l c u l u ss oh ec ou l di n v e n tp h y s i c s ,a n d e l e c t r od y n a mi c si sv e r ymu c hap a r tofp h y s i c s .I ’ mn otg oi n gt oc ov e re v e r ys i n g l e t h i n gy oul e a r n e di nc a l c u l u sc l a s s e si nt h ep a s th e r e( t h ec h a p t e rwou l db ea sl on gor l on g e rt h a nt h ee n t i r eb ooki fId i d )b u tr a t h e rwi l lf oc u sons h owi n gy out h ep a t h b e t we e nt h ep l a i nol dc a l c u l u sy oua l r e a d yk n ow( Ip r of ou n d l yh op e )a n dt h ev e c t or c a l c u l u sy oup r ob a b l yd on ’ tk n owa n y wh e r en e a rwe l le n ou g hu n l e s sy ouh a dar e a l l y e x t r a or d i n a r yc ou r s ei nmu l t i v a r i a t ec a l c u l u sa n dr e me mb e ri ta l l . L e t ’ sb e g i np r e t t yc l os et ot h eb e g i n n i n g ,wi t hor d i n a r yd i ffe r e n t i a t i on .E v e nh e r eou r t r e a t me n twon ’ tqu i t eb eor d i n a r y ,b e c a u s ewewi l ln otb er e v i e wi n gt h i sp u r e l yi nt h e a b s t r a c t .I na l lc a s e s , wh e r eIr e f e rt ov a r i ou s( s c a l a ra n dv e c t ora n dp os s i b l ye v e nt e n s or ) f u n c t i o n s ,y ous h ou l db et h i n k i n goft h os ef u n c t i on sa sn u me r i c a l l yr e p r e s e n t i n gd e f i n i t e p h y s i c a l qu a n t i t i e s , wi t hu n i t s .Th ec a l c u l u swen e e di sn ota b s t r a c t , i ti sd e s c r i p t i v e , a n di t i st h i s( p os s i b l ys u b t l e )d i ffe r e n t i a t i ont h a ts e p a r a t e st h ema t h e ma t i c i a nf r omt h ep h y s i c i s t . Bot hama t h e ma t i c i a na n dap h y s i c i s tma yt a l ka b ou td oi n gt h i n g st oorwi t ha f u n c t i onf, b u tt h ep h y s i c i s ti sa l wa y st h i n k i n ga b ou tf u n c t i on sft h a ta c t u a l l y“ s t a n df or s ome t h i n g ”a n dfwi l lu s u a l l yb er e p l a c e db yt r a d i t i on a ls y mb ol si na p p l i c a t i on .To ma n yma t h e ma t i c i a n s , fi sj u s tf–s omef u n c t i on , a n yf u n c t i on–a n di tma yorma yn ot me a na n y t h i n ga ta l l b e s i d e si t sown 3 9 s h a p eorf or mi fe v e nt h a ti ss p e c i f i e d . 6. 1 Sc a l a rDi ffe r e n t i a t i o n Re c a l lt h ed e f i n i t i onofor d i n a r yd i ffe r e n t i a t i on .I nl i g h toft h et r e a t me n ta b ov e ,wen ow r e c og n i z et h a tt h e“ or di n a r y ”di ffe r e n t i a t i onwel e a r n e di nt h ef i r s ty e a rofc a l c u l u swa s or d i n a r yb e c a u s ei twa ss c a l a rd i ffe r e n t i a t i on–d i ffe r e n t i a t i onoff u n c t i on st h a tr e p r e s e n t s c a l a rqu a n t i t i e s .Gi v e na( c on t i n u ou s ,di ffe r e n t i a b l e–wewi l la s s u met h i su n l e s ss t a t e d ot h e r wi s ef ora l l f u n c t i on sd i s c u s s e d)f u n c t i o n f( t ) : d f =l t+ i m f( t →0 dx t )−f( t ) t Not emye x p l i c i ta n dde l i b e r a t eu s eofta st h ei n d e p e n de n tv a r i a b l eu p onwh i c hf d e p e n d s .Th i si n v i t e su st ot h i n koft h i sa sar a t eo fc h a n gei np h y s i c swh e r efi ss ome p h y s i c a l qu a n t i t ya saf u n c t i onoftt h et i me . F r omt h i son ec a ne a s i l yd e r i v ea l l s or t sofa s s oc i a t e dr u l e s , t h emos ti mp or t a n tof wh i c ha r e : •Th eCh a i nr u l e . Su p p os eweh a v eaf u n c t i onf( x )wh e r ex ( t )i si t s e l faf u n c t i onof t( a n dt h e r ei sn o“ s e p a r a t e ”t i mede p e n d e n c ei nf) . Th e n : d f =d fdx xd t dt d •Th eSu mr u l e . Su p p os eweh a v et wof u n c t i on s , f( t )a n dg ( t ) . Th e n : d ( f+g ) df dg = + dt dt dt •Th ePr od u c tr u l e . Su p p os eweh a v et wof u n c t i on s , f( t )a n dg ( t ) . Th e n : d( d f d g +f d d tfg)=g d t t Wewi l l of t e ne x p r e s st h e s er u l e si nt e r msofd i ffe r e n t i a l s , n otd e r i v a t i v e swi t h r e s p e c tt os p e c i f i cc o or d i n a t e s . F ore x a mp l e : d f d f d t d t d f=d x dx= d ( fg )=gd f+fdg Mos toft h e s es i mp l es c a l a rr u l e sh a v ec ou n t e r p a r t swh e nwec on s i d e rd i ffe r e n t k i n d sofv e c t ord i ffe r e n t i a t i on . 6. 2 Ve c t o rDi ffe r e n t i a t i o n Wh e nwec on s i d e rv e c t orf u n c t i on sofc oor d i n a t e s ,weh a v ead ou b l eh e l p i n gof c omp l e x i t y .F i r s t , t h e r ea r et y p i c a l l ys e v e r a lc oor di n a t e s–( x , y , z , t )f ore x a mp l e–t h a t t h e ms e l v e sma yf or mav e c t or .Se c on d , t h ef u n c t i on( p h y s i c a l qu a n t i t yofi n t e r e s t )ma y b eav e c t or , ore v e nat e n s or .Th i sme a n st h a twec a nt a k eav e c t or l i k ede r i v a t i v eofa s c a l a rf u n c t i onofv e c t orc oor d i n a t e sa n dp r od u c eav e c t or !Al t e r n a t i v e l y ,wec a nt a k e d e r i v a t i v e st h a tb o t ha c tont h eu n d e r l y i n gv e c t orc oor d i n a t e sa n ds e l e c tou ta n d t r a n s f or ms p e c i f i cc omp on e n t soft h ev e c t orqu a n t i t yi t s e l fi ns p e c i f i cwa y s .Aswa s t h ec a s ef ormu l t i p l i c a t i onofs c a l a r sa n dv e c t or s ,wewon ’ th a v ej u s ton ek i n d–we ma ye n du pwi t ht h r e e ,orf ou r , ormor e !I n d e e d , s omeofou rd e r i v a t i v e swi l le c h ot h e mu l t i p l i c a t i onr u l e sa l g e b r a i c a l l ys p e c i f i e da b ov e . 6. 2. 1 Th ePa r t i a l De r i v a t i v e Th ep a r t i a ld e r i v a t i v ei swh a twet y p i c a l l yu s ewh e nweh a v eaf u n c t i onofmu l t i p l e c oo r d i n a t e s .Su p p os eweh a v ef( x , y , z ) , b u twi s ht os e eh owt h i sf u n c t i onv a r i e swh e n wev a r yon l yx , h ol d i n gt h eot h e rv a r i a b l e sc on s t a n t . Th i sd e f i n e st h ep a r t i a l de r i v a t i v e : ∂f =l x+ i m f( t →0 ∂x x , y , z )−f( x , y , z ) x Not et h a tt h i si sj u s tt a k i n gt h eo r d i n a r ys c a l a rd e r i v a t i v e , wh i l et r e a t i n gt h eot h e r v a r i a b l e sa sc on s t a n t s . I n de e d , ou rs c a l a rd e r i v a t i v ea b ov ei sa l s oap a r t i a l d e r i v a t i v ei n t h ec a s ewh e r et h e r ea r en oot h e rv a r i a b l e s ! F or mi n gt h et ot a ldi ffe r e n t i a l ,h o we v e r ,n owr e qu i r e su st oc on s i de rwh a th a p p e n s wh e nwev a r ya l l t h r e ec oor d i n a t e s : d f= ∂f ∂f x+ ∂y d y+ ∂x d ∂f ∂z dz Th e s ea r en otn e c e s s a r i l ys p a t i a l v a r i a t i on s–wec o u l dt h r owt i mei nt h e r ea swe l l , b u tf ort h emome n twewi l lc on s i d e rt i mea ni n de p e n d e n tv a r i a b l et h a twen e e d c on s i d e ron l yv i at h ec h a i nr u l e . Wec a nwr i t et h i sa sad otp r od u c t : ∂f d f= ∂f ∂f ∂x x ˆ+ ∂y y ˆ+ ∂z z ˆ·{ dx x ˆ+dy y ˆ+dz z ˆ } wh i c hwewr i t ea s : d f=( ∇f)·dℓ wh e r eweh a v ei mp l i c i t l yd e f i n e d∇fa n ddℓ . 6. 3 Th eGr a d i e n t Th eg r a d i e n tofaf u n c t i on : ∂f ∇f= ∂f ∂f ˆ+ ∂y y ˆ+ ∂z zˆ ∂x x i sav e c t orwh os ema g n i t u d ei st h ema x i mu ms l op e( r a t eofc h a n g ewi t hr e s p e c tt ot h e u n d e r l y i n gc oor d i n a t e s )oft h ef u n c t i oni na n yd i r e c t i o n , wh i c hp o i n t si nt h ed i r e c t i on i nwh i c ht h ema x i mu ms l o peoc c u r s . Weu s u a l l ye x p r e s s∇a sad i ffe r e n t i a l op e r a t or: ∇= ∂ ∂ ∂ ∂x x ˆ+ ∂y y ˆ+ ∂z zˆ t h a ta c t sona nob j e c tont h er i g h t , a n dwh i c hf ol l owst h eu s u a lp a r e n t h e s e sr u l e s t h a tc a nl i mi tt h es c op eoft h i sr i g h ta c t i on : ( ∇f) g=g ( ∇f)=g ∇f Nowweg e tt ot h ei n t e r e s t i n gs t u ff. 6. 4 Ve c t o rDe r i v a t i v e s Re c a l lt h a tweh a v et h r e er u l e sf orv e c t ormu l t i p l i c a t i on( n oti n c l u d i n gt h eou t e r p r od u c t ) : Ab , A·B, A×B wh e r ebi sas c a l a r , a n dAa n dBa r ev e c t or sa su s u a l .Wee v i d e n t l ymu s th a v et h r e es i mi l a r r u l e sf ort h eg r a d i e n top e r a t ort r e a t e da si fi ti sav e c t or( op e r a t or ) : ∇f , ∇·A, ∇×A wh e r efi samu l t i v a r i a t es c a l a rf u n c t i on ,a n dAi samu l t i v a r i a t ev e c t orf u n c t i on .We c a l lt h e s e ,r e s p e c t i v e l y ,t h eg r a d i e n tofas c a l a rf u n c t i on ,t h ed i v e r g e n c eofav e c t or f u n c t i on , a n dt h ec u r l ofav e c t orf u n c t i on . Th eg r a d i e n ti st h edi r e c t e ds l op eoffa tap oi n t .Th edi v e r g e n c ei same a s u r eof t h ei n / ou t f l owofav e c t orf i e l dAr e l a t i v et oap oi n t .Th ec u r li same a s u r eoft h e r ot a t i onofav e c t orf i e l dAa b ou tap oi n t .Al lt h r e ea r ed e f i n e da t( i nt h en e i g h b or h ood of )ap oi n ti ns p a c eb yme a n soft h el i mi t i n gp r oc e s si n d i c a t e da b ov ea n dp r e s u met h a t t h eob j e c t st h e ya c tona r ewe l l b e h a v e de n ou g ht op e r mi tl i mi t st ob et a k e n . I nCa r t e s i a nc omp on e n t s , t h eg r a d i e n tofav e c t orVi s : ∇· V=∂Vx+∂Vy+∂Vz∂x ∂y ∂ z a n dt h ec u r l i s : ∂Vz ∇× V= ∂Vy ∂Vx ∂Vz ∂Vy ∂Vx ∂y − ∂z x ˆ+ ∂z − ∂x y ˆ+ ∂x − ∂y zˆ Wh a ta r et h ea n a l og u e soft h es c a l a rr u l e swel i s t e da b ov e ?Wen owh a v et h r e e v e r s i on sofe a c hoft h e m.Th ec h a i nr u l ei sf or me db yc omp os i t i onoft h er u l ef ort h e t ot a l d i ffe r e n t i a l wi t hr u l e sf ort h ec omp on e n td i ffe r e n t i a l sa n dwewon ’ th a v emu c hu s e f ori t . Th es u mr u l e , h owe v e r , i si mp or t a n t( a l l t h r e ewa y s )i fob v i ou s . 6. 4. 1 Th eSu mRu l e s Su p p os efa n dga r es c a l a rf u n c t i on sa n dAa n dBa r ev e c t orf u n c t i on s . Th e n : ∇( f+g )=∇f+∇g ∇· ( A+ B) = ∇· A+ ∇· B ∇× ( A+B) = ∇× A+ ∇× B 6. 4. 2 Th ePr o d u c tRu l e s Th ep r odu c tr u l e sa r emu c hmor ed i ffic u l t .Weh a v et wowa y sofma k i n gas c a l a r p r od u c t–fga n dA·B.Wec a nma k et wov e c t orp r od u c t sa swe l l –fAa n dA×B( n ot e t h a twewi l ln otwor r ya b ou tt h e“ p s e u do”c h a r a c t e roft h ec r os sp r od u c tu n l e s si t ma t t e r st ot h ep oi n twea r et r y i n gt oma k e ) .Th e r ea r ea si tt u r n sou ts i xd i ffe r e n t p r od u c tr u l e s ! ∇( fg )=f∇g+g ∇f ∇( A· B) = A× ( ∇× B) + B× ( ∇× A) + ( A· ∇) B+( B· ∇) A Th ef i r s ti sob v i ou sa n ds i mp l e ,t h es e c on di sdi ffic u l tt op r ov eb u ti mp or t a n tt o p r ov ea sweu s et h i si de n t i t yaf a i rb i t . Not ewe l l t h a t : ( A·∇)=Ax ∂ ∂ ∂ ∂x+Ay ∂y+Az ∂z Weh a v et wodi v e r g e n c er u l e s : ∇·( fA)=f( ∇·A)+( A·∇) f ∇· ( A× B) = B· ( ∇× A) − A· ( ∇× B) Th ef i r s ti sa g a i nf a i r l yob v i ou s .Th es e c on don ec a ne a s i l yb ep r ov e nb yd i s t r i b u t i n gt h e d i v e r g e n c ea g a i n s tt h ec r os sp r od u c ta n dl ook i n gf ort e r mst h a ts h a r ea nu n di ffe r e n t i a t e d c omp on e n t ,t h e nc ol l e c t i n gt h os et e r mst of or mt h et woc r os sp r od u c t s .I tc a na l mos tb e i n t e r p r e t e da sa nor d i n a r yp r od u c tr u l ei fy oun ot et h a twh e ny oup u l l ∇“ t h r ou g h ”Ay oua r e e ffe c t i v e l yc h a n g i n gt h eor de roft h ec r os sp r odu c ta n dh e n c en e e dami n u ss i g n .Th e p r od u c th a st ob ea n t i s y mme t r i ci nt h ei n t e r c h a n g eofAa n dB,s ot h e r eh a st ob eas i g n d i ffe r e n c eb e t we e nt h eot h e r wi s es y mme t r i ct e r msf r omd i s t r i b u t i n gt h ede r i v a t i v e s . F i n a l l y , weh a v et woc u r l r u l e s : ∇×( fA)=f( ∇×A)−( A×∇) f ∇× ( A× B) = ( B· ∇) A− ( A· ∇) B+ A( ∇· B) − B( ∇· A) Th ef i r s ti sa g a i nr e me mb e r a b l ea st h eu s u a lp r od u c tr u l eb u twi t hami n u ss i g n wh e nwep u l l At ot h eo t h e rs i d eof∇.Th es e c on don ei sn a s t yt op r ov eb e c a u s et h e r e a r es ov e r yma n yt e r msi nt h ef u l l ye x p a n d e dc u r loft h ec r os s p r od u c tt h a tmu s tb e c ol l e c t e da n dr e a r r a n g e d , b u ti sv e r yu s e f u l .Not et h a ti ne l e c t r od y n a mi c swewi l l of t e n b ema n i p u l a t i n gors ol v i n gv e c t orp a r t i a l d i ffe r e n t i a l e qu a t i on si nc on t e x t swh e r ee . g .∇ ·E=0or∇·E=0 , s os e v e r a l oft h e s et e r msmi g h tb ez e r o. 6. 5 Se c o n dDe r i v a t i v e s Th e r ea r ef i v es e c on dd e r i v a t i v e s .Twoa r ei mp or t a n t , a n dat h i r dc ou l dc on c e i v a b l yb e i mp or t a n tb u twi l l of t e nv a n i s hf ort h es a mer e a s on .Th ef i r s tr u l ed e f i n e sa n dop e r a t or t h a ti sa r g u a b l yt h emos ti mp or t a n ts e c on dd e r i v a t i v ei np h y s i c s : 2 ∇·∇f=∇f 2 Th e∇ op e r a t ori sc a l l e dt h eL a p l a c i a na n di te n or mou s l yi mp or t a n ti nb ot h d e l e c t r od y n a mi c sa n dq u a n t u m me c h a n i c s .I ti st h e3 de qu i v a l e n tofdx22 ,g i v e n e x p l i c i t l yb y : ∇2= ∂ 2∂ 2∂ 2 ++ 2 ∂x 2 ∂ y 2 ∂ z Ne x tweh a v e : ∇×( ∇f)=0 ( n otp r e c i s e l yt r i v i a l t op r ov eb u ti mp or t a n t ) . Al s o: ∇( ∇·A) wh i c hh a sn os i mp l e rf or mb u twh i c hi sof t e nz e r of orA=E , Bi ne l e c t r od y n a mi c s . Ne x t : ∇· ( ∇× A) = 0 ( n otp r e c i s e l yt r i v i a l t op r ov eb u ti mp or t a n t ) . F i n a l l y : 2 ∇× ( ∇× A) = ∇( ∇· A) − ∇A wh i c hi sv e r yi mp or t a n t–ak e ys t e pi nt h ed e r i v a t i onoft h e3 dwa v ee qu a t i onf r om Ma x we l l ’ se qu a t i on si ndi ffe r e n t i a l f or m! 6. 6 Sc a l a rI n t e gr a t i o n I n t e g r a t i oni sb a s e dond i ffe r e n t i a t i on ,b u tr u n st h ep r oc e s sb a c k wa r d s .Th i si st h e b a s i sf ort h ef u n d a me n t a l t h e or e mofc a l c u l u s . 6. 6. 1 Th eF u n d a me n t a l Th e o r e mofCa l c u l u s Re c a l l t h a tt h ef u n d a me n t a l t h e or e mofc a l c u l u sb a s i c a l l yde f i n e st h ei n t e g r a l : b d f b a d f= dxd x=f( b )−f( a ) a d f Top u ti ta n ot h e rwa y , i fF=dx : b a Fd x=f( b )−f( a ) Th i sj u s t i f i e sr e f e r r i n gt oi n t e g r a t i ona s“ a n t i d i ffe r e n t i a t i on ”– d i ffe r e n t i a t i onr u n b a c k wa r ds .I n t e g r a t i onc on s i s t soff i n d i n gaf u n c t i onwh os ed e r i v a t i v ei st h ef u n c t i on b e i n gi n t e g r a t e d . Asb e f or e , wh a twec a nd owi t hs c a l a r s , wec a nd owi t hv e c t or s–wi t hb e l l son , t wo ort h r e edi ffe r e n twa y s . 6. 7 Ve c t o rI n t e gr a t i on Wen e e dt og e n e r a l i z et h es c a l a rt h e or e mt oaf u n d a me n t a lt h e or e mf orv e c t or de r i v a t i v e s .Howe v e r ,wema ye n du ph a v i n gmor et h a non e !Th a ti sb e c a u s ewec a n i n t e g r a t eov e r1 ,2ora l lt h r e ed i me n s i on a ld oma i n sf ors c a l a ra n dv e c t orf u n c t i on s d e f i n e di n3 dEu c l i d e a ns p a c e .He r ei san on e x h a u s t i v el i s tofi mp or t a n ti n t e g r a l t y p e s ( s omeofwh i c hy ouh a v ee n c ou n t e r e di ni n t r od u c t or yp h y s i c sc ou r s e s ) : Al i n ei n t e g r a l a l on gs omes p e c i f i e dc u r v i l i n e a rp a t hora r ou n ds omes p e c i f i e dl oop C: C V·d ℓ or C V·dℓ Yous h ou l dr e c og n i z et h i st y p eofi n t e g r a lf r om wh a ty ouh a v el e a r n e da b ou t p ot e n t i a l orp ot e n t i a l e n e r g yorc e r t a i nf i e l di n t e g r a l si nMa x we l l ’ sE qu a t i on sl e a r n e di n i n t r od u c t or ye l e c t r i c i t ya n dma g n e t i s m. Ne x tweh a v es u r f a c ei n t e g r a l s( oft h ep a r t i c u l a rk i n da s s oc i a t e dwi t ht h ef l u xofa v e c t orf i e l d ) : S V·n ˆ d A= V·a d S or V·a d S f ort woc ommonn ot a t i on s , t h es e c on don ef a v or e db ye . g . Gr i ffit h sa l t h ou g hI p e r s on a l l yp r e f e rt h ef i r s ton ea n di ti smor ec ommoni np h y s i c st e x t b ook s . I n t h ef i r s tc a s e , Si sa nop e ns u r f a c e , wh i c hme a n si ti sa )( p i e c e wi s e )b ou n d e db yac l os e d c u r v eCa n dt h ed i r e c t i onoft h en or ma lt ot h es u r f a c ei sa r b i t r a r y .I nt h es e c on d ,Si sa c l os e ds u r f a c e–as u r f a c et h a ti st op ol og i c a l l ye qu i v a l e n tt os oa pb u b b l e–i nwh i c hc a s e i te n c l os e sav ol u me .F ore x a mp l ei fwel e tSb eas qu a r eont h ex y p l a n e , wemi g h tc h os e t oma k en ˆ dA=a d=z ˆ d x d y ,s oy ouc a ns e et h a ti na l mos ta l lc a s e sy ouwi l lh a v et oa t l e a s tme n t a l l ye x p r e s sn ˆe x p l i c i t l yi nor d e rt oe v a l u a t ea da n y wa y . [ As i d e :Ac l os e dl i n eb ou n dsa nop e ns u r f a c e .Ac l os e ds u r f a c eb ou n d sa nop e n v ol u me .I fy ouwa n tt oma k ey ou rh e a dh u r t( i nc on s t r u c t i v ewa y s–wewi l ln e e dt o t h i n ka b ou tt h i n g sl i k et h i si nr e l a t i v i t yt h e or y )t h i n ka b ou twh a tac l os e dv ol u memi g h t b ou n d. . . ] F i n a l l y , weh a v ei n t e g r a t i onov e rav ol u me : 3 Fd V= Fdr= V V Fdτ V wh e r eVi st h e( op e n )v ol u met h a tmi g h th a v eb e e nb ou n d e db yac l os e dS,a n dI ’ v e i n d i c a t e dt h r e ed i ffe r e n twa y sp e op l ewr i t et h ev ol u mee l e me n t .Gr i ffit h sf a v or se . g .dτ =d xd yd z . On edo e s n ’ th a v et oi n t e g r a t eon l ys c a l a rf u n c t i on s , a n dt h e r ea r eot h e rl i n ea n ds u r f a c e i n t e g r a l son ec a nd e f i n eors e n s i b l ye v a l u a t e . F ore x a mp l ea l l of : Vd ℓ or fd a or C S Fdτ V mi g h tma k es e n s ei ns o mec on t e x t . 6. 8 Th eF u n d a me n t a l Th e o r e m( s )o fVe c t o rCa l c u l u s 6. 8. 1 ASc a l a rF u n c t i ono fVe c t o rCo or d i n a t e s L e t ’ sr e t u r nt oou re x p r e s s i onf orat ot a l di ffe r e n t i a l ofas c a l a rf u n c t i on , g i v e na b ov e : d f=∇f·d ℓ Th e n b a d f= b a ∇f·d ℓ=f( b )−f( b ) i n d e p e n d e n tofp a t h !Th ei n t e g r a ld e p e n d son l yont h ee n dp oi n t sf ora n yt ot a l di ffe r e n t i a l t h a ti si n t e g r a t e d ! He n c ewek n owt h a t : C ∇f·d ℓ=0 Th i ss h ou l ds e e mv e r yf a mi l i a rt oy ou .Su p p os eF=− ∇Uf orawe l l b e h a v e ds c a l a r f u n c t i onU. Th e n : b W( a→ b )= a b F·dℓ=− a ∇U·d ℓ i n d e p e n d e n tofp a t h .I ni n t r od u c t or yme c h a n i c sy oup r ob a b l ywe n tf r omt h ep r op os i t i on t h a tt h ewor ki n t e g r a lwa si n de p e n d e n tofp a t hf orac on s e r v a t i v ef or c et oad e f i n i t i on oft h ep ot e n t i a le n e r g y , b u ta sf a ra sv e c t orc a l c u l u si sc on c e r n e d , t h eot h e rd i r e c t i on i sat r i v i a li d e n t i t y .An yv e c t orf or c et h a tc a nb ewr i t t e na st h e( n e g a t i v e )g r a d i e n tofa s moot h , di ffe r e n t i a b l ep ot e n t i a l e n e r g yf u n c t i oni sac on s e r v a t i v ef or c e ! 6. 8. 2 Th eDi v e r ge n c eTh e or e m Th i si sas e c on d , v e r y , v e r yi mp or t a n ts t a t e me n toft h eF u n d a me n t a l Th e or e m: V/ S ( ∇·V) dτ= V·n ˆ dA S I nt h i se x p r e s s i onV/ Ss h ou l db er e a di ny ou rmi n da s“ ov e rt h eop e nv ol u meV b ou n d e db yt h ec l os e ds u r f a c eS” ,a n dVi sa na r b i t r a r yv e c t orqu a n t i t y ,t y p i c a l l ya v e c t orf i e l dl i k eEorBorav e c t orc u r r e n tde n s i t ys u c ha sJ .Not ewe l lt h a tt h er i g h t h a n ds i dey ous h ou l db er e a d i n ga s“ t h ef l u xoft h ev e c t orf u n c t i onVou tt h r ou g ht h e c l os e ds u r f a c eS” . Yo umi g h ta l s os e et h i swr i t t e na s : ( ∇·V) dτ= V V·n ˆ dA ∂V wh e r e∂Vi sr e a da s“ t h es u r f a c eb ou n d i n gt h ev ol u meV” .Th i si ss l i g h t l ymor e c omp a c tn ot a t i on , b u tas t u d e n tc a ne a s i l yb ec on f u s e db ywh a ta p p e a r st ob eap a r t i a l d i ffe r e n t i a l i nt h es u r f a c el i mi t s . As i mp l ec on s e qu e n c eoft h ed i v e r g e n c et h e or e mi s : ∇fdτ= V/ S fn ˆ d A= S fa d S Pr oof : As s u me A =fc ˆ t h e n ∇·A=( c ˆ·∇) f+f( ∇·c ˆ )=( c ˆ·∇) f s ot h a t V/ S ∇·Ad τ= V/ S ( c ˆ·∇) fd τ= A·n ˆ d A= s c ˆ f·n ˆ d A s Si n c ec ˆi sc on s t a n ta n da r b i t r a r y , wec a nf a c t ori tou tf r omt h ei n t e g r a l : c ˆ· V/ S ∇fd τ=c ˆ· fn ˆ d A s Si n c et h i sh a st ob et r u ef ora n yn on z e r oc ˆ,wec a ne s s e n t i a l l yd i v i deou tt h e c on s t a n ta n dc on c l u det h a t : ∇fdτ= V/ S fn ˆ dA s Q. E . D. Yous h ou l dp r ov eony ou rown( u s i n ge x a c t l yt h es a mes or tofr e a s on i n g ) t h a t : V/ S ∇×Adτ= n ˆ×Ad A s Th e r et h u si son es u c ht h e or e mf or∇( a c t i n gona n ys c a l a rf) , ∇·A( a c t i n gona n y v e c t orf u n c t i onA)or∇×Aa c t i n gona n yv e c t orf u n c t i onA.Wec a nu s ea l loft h e s e f or msi ni n t e g r a t i onb yp a r t s . 6. 8. 3 St o k e s ’ Th e o r e m St ok e s ’ t h e or e m( wh i c hmi g h twe l lb ec a l l e dt h ec u r lt h e or e mi fwewa n t e dt ob emor e c on s i s t e n ti nou rt e r mi n ol og y )i se qu a l l yc r i t i c a l t oou rf u t u r ewor k : S/ C ( ∇×V)·n ˆ dA= C V·d ℓ Ag a i n ,r e a dS/ Ca s“ t h eop e ns u r f a c eSb ou n d e db yt h ec l os e dc u r v eC,a n dn ot e t h a tt h e r ei sa ni mp l i c i td i r e c t i o ni nt h i se q u a t i on .I np a r t i c u l a r , y oumu s tc h oos e( f r om t h et wop os s i b l ec h oi c e s )t h ed i r e c t i onf orn ˆt h a tc or r e s p on d st ot h er i g h t h a n de d d i r e c t i ona r ou n dt h el oopC.I nwor d s , i fy ouc u r l t h ef i n g e r sofy ou rr i g h th a n da r ou n dC i nt h ed i r e c t i oni nwh i c hy ouwi s ht od ot h ei n t e g r a l , y ou rt h u mbs h ou l dp oi n t“ t h r ou g h ” t h el oopCi nt h ed i r e c t i ony oumu s ts e l e c tf ort h en or ma l . Wec a non c ea g a i nd e r i v ea na d di t i on a l f or moft h ec u r l t h e or e m/ St ok e s ’ t h e or e m: S/ C ( n ˆ×∇f)·d A= Not ewe l l t h a tt h en ˆh a sb e e nmov e dt ot h ef r on t ! C fdℓ 6. 9 I n t e gr a t i o nb yPa r t s I n t e g r a t i onb yp a r t si son eoft h emos ti mp or t a n tt op i c si nt h i sc h a p t e r .I n d e e d ,y oumi g h t h a v eb e e nab i tb or e db yt h er e c i t a t i onoft h i n g st h a tp r ob a b l ywe r ec ov e r e di ny ou r mu l t i v a r i a t ec a l c u l u sc l a s s e s .Th i smi g h th a v eb e e na swe l l , b u tc h a n c e sa r ev e r yg oodt h a t y oud i d n ’ tf i n i s hl e a r n i n gh ow t oma k ei twor ki nt h eg e n e r a lc on t e x toft h ev a r i ou s f u n da me n t a l t h e or e msl i s t e da b ov e . 6. 9. 1 Sc a l a rI n t e g r a t i onb yPa r t s Weh a v ea l r e a d ydon ea l mos ta l loft h ewor kh e r e .St a r twi t ht h ep r od u c tr u l ef ort h e d i ffe r e n t i a l : d ( fg )=fd g+gd f I n t e g r a t eb ot hs i d e s . b b a b b d ( fg )=fg = a fdg+ a gd f a a n dr e a r r a n g e : b b fdg=fg b a gd f − a a Th i si son ewa yofwr i t i n gi n t e g r a t i onb yp a r t s , b u twea r e n ’ tu s u a l l yg i v e nb ot h“ d f” a n d / or“ d g ” .Not ewe l l t h a twec a ne x p r e s sd fa n ddgi nt e r msoft h ec h a i nr u l e , t h ou g h , wh i c hi se x a c t l ywh a twewi l lu s u a l l yb ed oi n gt oe x p r e s st h ei n t e g r a lofk n own f u n c t i on sf( x )a n dg ( x ) : b a d g f dxd x=fg b a b − a d f gdxdx I n t e g r a t i onb yp a r t si sa ne n or mou s l yv a l u a b l et ooli ns c a l a r / on edi me n s i on a li n t e g r a l c a l c u l u s . I ti sj u s ta si mp o r t a n ti nmu l t i v a r i a t ei n t e gr a l c a l c u l u s ! 6. 9. 2 Ve c t orI n t e gr a t i o nb yPa r t s Th e r ea r ema n ywa y st oi n t e g r a t eb yp a r t si nv e c t orc a l c u l u s .Soma n yt h a tI c a n ’ ts h ow y oua l l oft h e m.Th e r ea r e , a f t e ra l l , l o t sofwa y st op u tav e c t ord i ffe r e n t i a l f or mi n t oa n e qu a t i on ,a n d( a tl e a s t )t h r e ed i me n s i on a l i t i e sofi n t e g r a ly oumi g h tb et r y i n gt od o!I wi l lt h e r e f or ed e mon s t r a t eh owt ot h i n ka b ou ti n t e g r a t i n gb yp a r t si nv e c t orc a l c u l u s , e x p l oi t i n gt h eg r a di e n tp r odu c tr u l e ,t h ed i v e r g e n c et h e or e m,orSt ok e s ’t h e or e m.I n a l mo s ta l l oft h e s ec a s e s , t h e yr e s u l tf r omi n t e g r a t i n gat ot a l d e r i v a t i v eofs omes or tor a n ot h e rov e rs omep a r t i c u l a rd oma i n( a sy ouc a ns e ef r omt h e i ri n t e r n a ld e r i v a t i on sor p r oof s , b e y on dt h es c op eoft h i sc ou r s e ) . I ti se a s i e s tt ot e a c ht h i sb ye x a mp l e . L e t ’ swr i t eon eofou rp r od u c tr u l e s : ∇·( fA)=f( ∇·A)+( A·∇) f Not et h a tt h el e f th a n ds i d ei sap u r ed i v e r g e n c e .L e t ’ si n t e g r a t ei tov e rav ol u me b ou n d e db yac l os e ds u r f a c e : V/ S ∇·( fA) d τ= f( ∇·A) dτ+ V/ S V/ S ( A·∇) fd τ Nowwewi l la p p l yt h edi v e r g e n c et h e or e m( on eofou r“ f u n d a me n t a lt h e or e ms ” a b ov e )t ot h el e f th a n ds i d eon l y : fA·n ˆ d A= f( ∇·A) d τ+ ( A·∇) fd τ V/ S S V/ S F i n a l l y , l e t ’ sr e a r r a n g e : V/ S ( A·∇) fd τ= fA·n ˆ d A− S V/ S f( ∇·A) d τ 3 I ne l e c t r od y n a mi c s , i ti so f t e nt h ec a s et h eV=R, a l lofr e a ls p a c e , a n de i t h e rfor Av a n i s ha ti n f i n i t y , wh e r ewewou l dg e t : V/ S ( A·∇) fd τ=− V/ S f( ∇·A) d τ or∇·A=0( ad i v e r g e n c e l e s sf i e l d )i nwh i c hc a s e : V/ S ( A·∇) fd τ= fA·n ˆ dA S Bot hoft h e s ee x p r e s s i on sc a nb ea l g e b r a i c a l l yu s e f u l . Th i si sn otb ya n yme a n st h eon l yp os s i b i l i t y .Wec a ndoa l mos te x a c t l yt h es a me t h i n gwi t h∇×( fA)a n dt h ec u r lt h e or e m.Wec a nd oi twi t ht h edi v e r g e n c eofac r os s p r od u c t , ∇·( A×B) .Youc a ns e ewh yt h e r ei sl i t t l ep oi n ti nt e d i ou s l ye n u me r a t i n ge v e r y s i n g l ec a s et h a ton ec a nb u i l df r oma p p l y i n gap r od u c tr u l ef orat ot a ld i ffe r e n t i a lor c on n e c t e dt oon eoft h eot h e rwa y sofb u i l d i n gaf u n d a me n t a l t h e or e m. Th ema i np oi n ti st h i s :I fy oun e e dt oi n t e gr a t ea ne x p r e s s i oni nv e c t orc a l c u l u s c o n t a i n i n gt h e∇o p e r a t or ,t r yt of i n dap r od u c tr u l ec o n n e c t e dt oav e r s i o no ft h e f u n d a me n t a l t h e o r e mt h a tp r o d u c e st h ee x p r e s s i ona son eofi t st wot e r ms . Th e ng ot h r ou g ht h ec o n c e p t u a lp r oc e s sofwr i t i n gou tt h edi ffe r e n t i a lp r odu c t e x p r e s s i on , i n t e g r a t i n gb ot hs i d e s , a p p l y i n ge . g .t h ed i v e r g e n c et h e or e m, t h ec u r lt h e or e m, org e n e r a l i z a t i on sors p e c i a l c a s e soft h e mi n d i c a t e da b ov e : Th e r ea r et womod e r a t e l yi mp or t a n t( a n df a i r l ye a s yt od e r i v e ,a tt h i sp oi n t ) c on s e qu e n c e sofa l loft h ewa y sofmi x i n gt h ef u n d a me n t a lt h e or e msa n dt h ep r od u c t r u l e si n t os t a t e me n t sofi n t e g r a t i onb yp a r t s .On ei st h es l i g h t l yl e s su s e f u lGr e e n ’ s F i r s tI d e n t i t y( ort h e or e m) . Su p p os efa n dga r e , a su s u a l , s c a l a rf u n c t i on s . Th e n : 2 f∇g−∇g·∇f V d τ= f ∇g·n ˆ ) dA ∂V ∂g =n ˆ·∇gi st h er a t eofc h a n g eoft h ef u n c t i ongi nt h ed i r e c t i onoft h e ou t g oi n gn or ma l ( a n dd i t t of ort h es i mi l a re x p r e s s i onf orf ) . Hi n tf orp r oof : Con s i d e ri n t e g r a t i n g∇·f( ∇g ) . On eu s eoft h i si st op r ov et h ev e r yu s e f u l Gr e e n ’ sSe c on dI d e n t i t y( ort h e or e m) : ∂ g ∂ f 2 2 f∇g−g ∇fd τ= f∂n −g∂n dA wh e r e∂n V ∂V Youc a nj u s twr i t et h ef i r s ti d e n t i t yt wi c ewi t hfa n dgr e v e r s e da n ds u b t r a c tt h e m t h e mt og e tt h i sr e s u l t . Att h i sp oi n ti ti si mp or t a n tt oc on n e c tt h i s“ t ooa b s t r a c t ”r e v i e wofr u l e sa n d t h e or e msa n df or mst or e a l p h y s i c s . Ane x a mp l ei si nor d e r . 6. 10 I n t e gr a t i o nByPa r t si nE l e c t r od y n a mi c s Th e r ei son ee s s e n t i a l t h e or e mofv e c t orc a l c u l u st h a ti se s s e n t i a l t ot h ede v e l op me n t ofmu l t i p ol e s–c omp u t i n gt h ed i p ol emome n t .I nh i sb o ok , Cl a s s i c a lEl e c t r ody n a mi c s , J a c k s on( f ore x a mp l e )b l i t h e l yi n t e g r a t e sb yp a r t s( f orac h a r g e / c u r r e n td e n s i t ywi t h c omp a c ts u p p or t )t h u s l y : 3 3 3 J dx=− I R x (∇·J ) dx 3 ( 6 . 1 ) I R Th e n ,u s i n gt h ec on t i n u i t ye qu a t i ona n dt h ef a c tt h a tρa n dJa r ep r e s u me d ∂ρ h a r mon i cwi t ht i med e p e n de n c te x p ( − i ωt ) , wes u b s t i t u t e∇·J=− ∂t=i ωρt oob t a i n : J3 x x 3 3 3 I R I R i ω x=− i ωp dx=− ρ( )d ( 6 . 2 ) wh e r e pi st h ed i p ol emome n toft h ef ou r i e rc omp on e n toft h ec h a r g ed e n s i t y d i s t r i b u t i on . Howe v e r , t h i sl e a v e san a s t yqu e s t i on : J u s th o wd oe st h i si n t e g r a t i onb yp a r t swor k ? Wh e r ed oe st h ef i r s te qu a t i onc omef r om?Af t e ra l l , wec a n ’ tr e l yona l wa y sb e i n ga b l e t ol ooku par e s u l tl i k et h i s , weh a v et ob ea b l et od e r i v ei ta n dh e n c el e a r name t h odwe c a nu s ewh e nweh a v et od ot h es a met h i n gf orad i ffe r e n tf u n c t i on a l f or m. Wemi g h tg u e s st h a td e r i v i n gi twi l lu s et h ed i v e r g e n c et h e or e m( orGr e e n ’ s t h e or e m( s ) ,i fy oul i k e ) ,b u ta n yn a i v ea t t e mp tt oma k ei td os owi l ll e a dt op a i na n d s u ffe r i n g . L e t ’ ss e eh owi tg oe si nt h i sp a r t i c u l a r l yn a s t y( a n dy e tqu i t es i mp l e )c a s e . Re c a l lt h a tt h ei d e ab e h i n di n t e g r a t i onb yp a r t si st of or mt h ed e r i v a t i v eofa p r od u c t , d i s t r i b u t et h ed e r i v a t i v e , i n t e g r a t e , a n dr e a r r a n g e : d ( u v ) =ud v+vdu d ( u v ) = b b a b ud v+ vd u a b a ud v a b b u v ) | a − a vd =( u ( 6 . 3 ) wh e r ei ft h ep r od u c t su ( a ) v ( a )=u ( b ) v ( b )=0( a swi l l of t e nb et h ec a s ewh e na=− ∞, b =∞ a n dua n dvh a v ec omp a c ts u p p or t )t h ep r oc e s s“ t h r owst h ed e r i v a t i v ef r omon e f u n c t i onov e rt ot h eot h e r ” : b a b ud v=− a vd u ( 6 . 4 ) wh i c hi sof t e ne x t r e me l yu s e f u l i ne v a l u a t i n gi n t e g r a l s . Th ee x a c ts a mei d e ah ol d sf orv e c t orc a l c u l u s ,e x c e p tt h a tt h ei d e ai st ou s et h e d i v e r ge n c et h e o r e mt of or m as u r f a c ei n t e gr a li n s t e a dofab ou n d a r yt e r m.Re c a l lt h a t t h e r ea r ema n yf or msoft h ed i v e r g e n c et h e or e m, b u tt h e ya l l ma p ∇t on ˆi nt h ef ol l owi n gi n t e g r a l f or m: 3 2 ∇. . . dx→nˆ. . . dx V ( 6 . 5 ) S/ V ori nwor d s , i fwei n t e g r a t ea n yf or mi n v ol v i n gt h ep u r eg r a d i e n top e r a t ora p p l i e dt oa ( p os s i b l yt e n s or )f u n c t i on a lf or mi n d i c a t e db yt h ee l l i p s i s. . .i nt h i se qu a t i on ,wec a n c on v e r tt h er e s u l ti n t oa ni n t e g r a lov e rt h es u r f a c et h a tb ou n d st h i sv ol u me , wh e r et h e g r a d i e n top e r a t ori sr e p l a c e db ya nou t wa r ddi r e c t e dn or ma lb u to t h e r wi s et h e f u n c t i on a l f or moft h ee x p r e s s i o ni sp r e s e r v e d . Sowh i l et h edi v e r ge n c et h e or e mi s : 3 2 ∇·Adx=nˆ·Adx V ( 6 . 6 ) S/ V t h e r ei sa“ g r a d i e n tt h e or e m” : 3 V ∇fdx= 2 S/ V n ˆ fdx ( 6 . 7 ) a n ds oon . Top r ov eJ a c k s on ’ se x p r e s s i onwemi g h tt h e r e f or et r yt of i n das u i t a b l ep r od u c twh os e di v e r g e n c ec on t a i n sJa son et e r m.Th i si s n ’ tt ooe a s y , h owe v e r .Th ep r ob l e mi sf i n d i n gt h e r i g h tt e n s orf or m. L e tu sl ooka tt h ef ol l owi n gd i v e r g e n c e : ∇·( x J )=∇x·J+x ∇·J ∇·J x+x =J ( 6 . 8 ) Th i sl ook sp r omi s i n g ; i ti st h ex c omp on e n tofar e s u l twemi g h tu s e .Howe v e r , i ft r yt o a p p l yt h i st oama t r i xd y a d i cf or mi nwh a tl oo k sl i k ei tmi g h tb et h er i g h twa y : ∇· x (J )=( ∇x ·) J+x( ∇·J ) J+ x( ∇·J ) =3 ( 6 . 9 ) weg e tt h ewr o n ga n s we r . Toa s s e mb l et h er i gh ta n s we r , weh a v et os u mov e rt h et h r e es e p a r a t es t a t e me n t s : ∇·( x J )x ˆ= J ∇·Jx ˆ x+x +∇·( y J )y ˆ=+J ∇·Jy ˆ y+y +∇·( z J )z ˆ=+J ∇·Jz ˆ z+z ( 6 . 1 0 ) or x ˆ ( x J )=J+x( ∇·J ) i∇· i i ( 6 . 1 1 ) wh i c hi st h es u m oft h r e ed i v e r ge n c e s ,n otad i v e r g e n c ei t s e l f .I fwei n t e g r a t eb ot h s i d e sov e ra l l s p a c eweg e t : ∇ 3 I R x i i x ˆ i 3 ·( x J )d x = i n J 3 I R J 2 ( x i )d x = ˆ i S( ∞) ˆ· x 0 = i i ˆ 3 3 I R x ∇ J 3 3 3 ( · ) dx ( 6 . 1 3 ) J3 x ∇ J 3 3 3 I R I R dx+ ( · ) dx ( 6 . 1 4 ) dx+ I R J 0 = x ∇ J 3 ( · ) dx ( 6 . 1 2 ) 3 J d x+ 3 I R I R x ∇ J 3 R dx+ I ( · ) dx ( 6 . 1 5 ) 3 3 wh e r eweh a v eu s e dt h ef a c tt h a tJ( a n dρ)h a v ec omp a c ts u p p o r ta n da r ez e r o e v e r y wh e r eonas u r f a c ea ti n f i n i t y . Wer e a r r a n g et h i sa n dg e t : J 3 I R 3 dx=− x ∇ J 3 ( · ) d x 3 I R ( 6 . 1 6 ) wh i c hi sj u s twh a twen e e d e dt op r ov et h ec on c l u s i on . Th i si l l u s t r a t e son eoft h emos td i ffic u l te x a mp l e sofu s i n gi n t e g r a t i onb yp a r t si n v e c t orc a l c u l u s .I ng e n e r a l ,s e e kou tat e n s orf or mt h a tc a nb ee x p r e s s e da sap u r e v e c t ord e r i v a t i v ea n dt h a te v a l u a t e st ot wot e r ms , on eofwh i c hi st h et e r my ouwi s ht o i n t e g r a t e( b u tc a n ’ t )a n dt h eot h e rt h et e r my ouwa n tc ou l di n t e g r a t ei fy ouc ou l don l y p r oc e e da sa b ov e .Ap p l yt h eg e n e r a l i z e dd i v e r g e n c et h e or e m,t h r owou tt h eb ou n d a r y t e r m( orn ot–i fon ek e e p si ton ede r i v e se . g .Gr e e n ’ sTh e or e m( s ) ,wh i c ha r en ot h i n g mor et h a ni n t e g r a t i onb yp a r t si nt h i sma n n e r )a n dr e a r r a n g e ,a n dy ou ’ r eofft ot h e r a c e s . Not ewe l l t h a tt h et e n s orf or msma yn o tb et r i v i a l !Some t i me sy oud oh a v et owor k ab i tt of i n dj u s tt h er i g h tc omb i n a t i ont odot h ej ob . Ch a p t e r7 Co o r d i n a t eSy s t e ms Th ef ol l owi n ga r es t r a i g h tu ps u mma r i e sofi mp or t a n tr e l a t i on sf ort h et h r e emos t i mp or t a n tc oor d i n a t es y s t e ms : Ca r t e s i a n , Sp h e r i c a l Pol a r , a n dCy l i n d r i c a l .I d on ’ td e r i v e t h ev a r i ou se x p r e s s i on s , b u ti naf e wc a s e sI i n d i c a t eh owon ec ou l dd os o. Someoft h ef ol l owi n gy ous h ou l dwor kt oj u s t“ l e a r n ” , s oy ouk n owi tf or e v e r . Ot h e r p a r t sy ouc a np r ob a b l yl ooku pwh e ny oun e e dt h e m.I ’ v et r i e dt oc on c e n t r a t eont h e f or me rh e r e ,a n dwi l ll i k e l yp r ov i d eaf or mu l as h e e twi t ht h el a t t e rf oru s eone x a ms . Howe v e r , y ous t i l l h a v et ol e a r nt owo r kwi t hs t u ffofft h ef or mu l as h e e t s , a n dt h a tt a k e s p r a c t i c e . Th ek e yt ou n d e r s t a n d i n g( a n dmos ts i mp l yd e r i v i n g )di ffe r e n t i a lop e r a t or si na l l c oor d i n a t es y s t e msb e s i de sf l a tE u c l i d e a nCa r t e s i a nc oor d i n a t e si st h ed e f i n i t i onof t h ed i r e c t e dl e n g t he l e me n td ℓ .I nt h emos tg e n e r a lt e r ms , f orac oor d i n a t ef r a mewi t h or t h o n or ma l c omp on e n t sofap oi n tP=( u , v , w) , d ℓa tt h ep oi n tPi sg i v e nb y : d ℓ=fd u u ˆ+gd v v ˆ+hd wwˆ wh e r ef, g , a n dha r ef u n c t i on sof( u , v , w)e v a l u a t e da tt h ep oi n tP.Ac oor d i n a t e s y s t e mi sc h a r a c t e r i z e db yt h et h r e ef u n c t i on s . F ore x a mp l e : f=g=h=1 Ca r t e s i a n ( x , y , z ) f=1 , g=r , h=rs i nθ Sphe r i c a l Pol ar ( r , θ, φ) f=h=1 , g=s Cy l i n dr i c a l ( s , φ, z ) I nor d e rt od oag r a di e n top e r a t or , weh a v et ou s et h eg e n e r a l e x p r e s s i onf orat ot a l d e r i v a t i v eofas c a l a rf u n c t i onT: d T=∇T·dℓ=( ∇T) fd u+( ∇T) v+( ∇T) w u vgd whd 1∂ T c . Th eg r a d i e n ti st h e ng i v e ng e n e r a l l yb y : ∂ u,et wh e r e( ∇T) u=f 5 5 ˆ1∂ +wˆ1∂ ∇=u ˆ 1 ∂ +v g∂v f∂u h∂ w wh e r eweh a v ep l a c e dt h eu n i tv e c t or st ot h el e f tt oma k ei tv e r yc l e a rt h a tt h e p a r t i a l sdon ota c tont h e m. Wen e e dt h eDi v e r g e n c eTh e or e mt oh ol df orou rd e f i n i t i onofd i v e r g e n c e . Todot h ed i v e r g e n c eofav e c t orv a l u e df u n c t i onB=Buu ˆ+Bvv ˆ+Bwwˆ e x p r e s s e di nc u r v i l i n e a rc oor d i n a t e s , wet h u sh a v et os t a r twi t hav ol u mee l e me n t f ort h es p a c e : d V=d τ=dℓ ℓ ℓ fg h ) d ud vdw ud vd w=( wher ewehav et ous et hecomponent sofdℓbecaus ei ngener al , t hec ur v i l i near c oor d i n a t e sma ywe l l n oth a v edi me n s i on sofl e n g t h( wh i c hi son eofs e v e r a l t h i n g st h ef u n c t i on sf , g , hd of oru si nd ℓ ) . Th eb ou n d a r yoft h i si n f i n i t e s i ma l v ol u mei sas or tofd e f or me dr e c t a n g u l a rs ol i d . I t sd i r e c t e ds u r f a c e sa r et h i n g s l i k e( i nt h e± ud i r e c t i on ) : nˆdA=ad =− g hd vd wu ˆ or +g hd vdwu ˆ f ort h eb a c korf r on ts u r f a c e sr e s p e c t i v e l y . I fwee x a mi n et h ef l u xt h r ou g h s u c has u r f a c e : ( g h Bu)dvd w B·ad =± wh e r et h ef u n c t i onF=g h Bumu s tb ee v a l u a t e da te i t h e r( u , v , w)( f or( ) ) or( u+d u , v , w)( f or( +) )r e s p e c t i v e l y ! Not ewe l l t h a tt h e s ea r eb ot hou t wa r d d i r e c t e dn or ma l c on t r i b u t i on s !I fwes u b t r a c tt h e s et woi nt h ed i r e c t i onofu a n dt a k et h eu s u a l l i mi t s : dF F( u+d u )−F( u )= du d u ort h et ot a l c on t r i b u t i ont oB·a di nt h i sd i r e c t i oni s : ∂g h Bu ∂u du d vd w= ∂( g h Bu) ∂u d ud vd w= 1 ∂( g h Bu) ( fgh ) ∂u d τ 1 ( fg h ) Su mmi n gov e ra l l s i xf a c e s , weg e tf ort h i si n f i n i t e s i ma l c u r v i l i n e a rv ol u me : ∂( g h Bu) 1 B·a d= ( fg h ) ∂u ∂( fh Bv) + ∂v ∂( fg Bw) + ∂w d τ=∇·Bd τ Wet h u sa r r i v ea tt h eg e n e r a l ( l oc a l )d i ffe r e n t i a l f or mf ort h ed i v e r g e n c eof av e c t orv a l u e df u n c t i on : + + ∇· B= 1 ∂( g hBu) ∂( fhBv) ∂( fgBw) ∂ ( g h Bu) ∂u dτ ( fg h ) ∂u ∂v ∂w Th i si s , i n c i de n t a l l y , ag e n e r a l d e r i v a t i onoft h ed i v e r g e n c et h e or e m, s i n c ei ti se a s y t os h ow t h a ton ec a nb u i l di n t e g r a l sov e rag e n e r a l( n on l oc a l )v ol u mewi t hi t s b ou n d i n gs u r f a c eb ys u mmi n gov e ri n t e r n a l c u r v i l i n e a rc h u n k swi t hc a n c e l l a t i onoff l u x c on t r i b u t i on sa c r os sa l l i n f i n i t e s i ma l s u r f a c e si n s i d et h ev ol u me .F oraf i n i t ev ol u meV, t h e n : ∂V B·a d= ∇·Bd τ V wh e r et h en on c a n c e l l i n gp a r toft h es u r f a c ei n t e g r a li son l yov e rt h ee x t e r n a l s u r f a c e∂Vwi t ha nou t wa r ddi r e c t e dn or ma l . Todot h ec u r l , on eh a st or e p e a tt h i sa r g u me n tf ort h ef or mu l a : B·dℓ e v a l u a t e da r ou n da ni n f i n i t e s i ma l d i r e c t e dr e c t a n g u l a ra r e aa da l on ge a c hof t h e( u , v , w)di r e c t i on s . Th ea l ge b r ai st e di ou s( b u ti sr e v i e we di ns i mi l a rde t a i l i nGr i ffit h ’ s , Ap p e n d i xAi fouwa n tt os e ei t )a n dl e a d st o: 1∂ ( h Bw) ∇× B= gh ∂v ∂( g Bv) − ∂w fBu) 1 ∂( uˆ +fh ∂w − ∂( h Bw) ∂u g Bv) 1 ∂( v ˆ +fg ∂u ∂( fBu) − ∂v B·dℓ=∇×B·a d. wh i c ha ppl i e st oage n e r a l i n f i n i t e s i ma l s u r f a c ea s Asb e f or e , i fwec h opaf i n i t es u r f a c eSb ou n d e db yac l os e dc u r v eCu pi n t o ma n ydi ffe r e n t i a l pi e c e s , a l l oft h ei n t e r n a l l oopc on t r i b u t i on sb e t we e na dj a c e n t i n f i n i t e s i ma l p i e c e sc a n c a l a n don eg e t sSt ok e ’ sTh e or e m: C B·dℓ= ∇×B·a d S/ C Th i sou t l i n e sh owt oe v a l u a t et h eg r a di e n t , d i v e r g e n c ea n dc u r l f ort h et h r e ep r i ma r y c oor d i n a t es y s t e mswewi l l u s ei nt h i sc ou r s e . Be l owwes u mma r i z et h er e s u l t 7. 1 Ca r t e s i a n Re c a l l , Ca r t e s i a nc omp on e n t sh a v eu=x , v=y , w=za n df=g=h=1 . Th e r e f or e : •Ve c t or s : Wes t a r twi t ht h eb a s i cde f i n i t i onofav e c t orf r om t h eor i g i nt oap oi n ti n Ca r t e s i a nc oor d i n a t e s , wh i c ha l s ot e l l su sh owt owr i t ea n yot h e rv e c t orqu a n t i t y e . g . B: r =xx ˆ+yy ˆ+zz ˆ B=Bxx ˆ+Byy ˆ+Bzz ˆ wˆ •Di r e c t e dL e n g t h Th el e n g t he l e me n ti sd e f i n e da c c or d i n gt ot h er u l eg i v e na b ov ea s : d ℓ=dxx ˆ+d yy ˆ+d zz ˆ •Di r e c t e dAr e a d A=da=d xd yord yd zord zd xa n df or e x a mp l e , a d =n ˆdA=z ˆd xd y ( e t c . )BUTt h e s ea s s u mer e c t i l i n e a rs u r f a c e sp a r a l l e l t oon eoft h ep r i n c i p l e p l a n e s . Th e r ea r eot h e ra d’ son ec a nf or m. •Vol u meEl e me n t : d V=d τ=dxdyd z •Gr a d i e n t : ∂f ∇f= •Di v e r g e n c e : ∇· B= ∂f ∂f ˆ+ ∂y y ˆ+ ∂z zˆ ∂x x ∂Bx ∂x + ∂By ∂y ∂Bz + ∂z •Cu r l : ∂Bz ∂By ∂Bx ∂Bz ∂By ∂Bx ˆ+ ∂z − ∂x y ˆ+ ∂x − ∂y zˆ ∂ y − ∂z x ∇× B= •L a p l a c i a n : 2 ∂x ∂2f ∂2f ∂ f 2 ∇2f = + + 2 2 ∂ y ∂ z 7. 2 Sp h e r i c a l Po l a r •Ve c t or s : r =rr ˆ Bu tNo t eWe l l :r ˆi sn owaf u n c t i onof( θ , φ) ! Si mi l a r l y : A=Arr ˆ wi t hr ˆaf u n c t i onoft h ea n g l e s( θ, φ)t h a td e f i n et h edi r e c t i onofA. Sp e c i f i c a l l y : •Un i tv e c t or s( r e l a t i v et oCa r t e s i a nx ˆ , y ˆ , z ˆ : r ˆ=s i nθc osφx ˆ+s i nθs i nφy ˆ ˆ ˆ+cosθsi nφyˆ−si nθzˆ θ =cosθcosφx ˆ ˆ i nφx ˆ+cosφy ˆ φ =−s ˆ Th ef a c tt h a tr ˆ ( θ , φ) , θ( θ, φ) ,φ( φ)c omp l i c a t e sa l loft h es p h e r i c a lc oor d i n a t e v e c t ordi ffe r e n t i a l f or ms , a l t h ou g hwei n d i c a t ea b ov ead i ffe r e n t , mor ed i r e c twa y ofe v a l u a t i n gt h e mt h a na p p l y i n gd e r i v a t i v e st ot h eu n i tv e c t or st h e ms e l v e s b e f or ec omp l e t i n gt h et e n s orc on s t r u c t i onoft h ev e c t ordi ffe r e n t i a l op e r a t or s . •Di r e c tL e n g t h d=d rˆ+r d θˆ+rs i nθdφˆ ℓ r θφ •Di r e c t e dAr e a 2 d A=rs i nθd θd φ 2 d An ˆ=a d =rs i nθdθd φr ˆ=d Ar ˆ An da g a i n ,t h e r ea r ema n yot h e rp os s i b l ea d’ s ,f ore x a mp l e ,f ort h eb ou n di n g s u r f a c ef orh e mi s p h e r i c a lv ol u mewh e r eon ep i e c eofi twou l db eac i r c u l a rs u r f a c e wi t ha nn or ma ll i k e( f ore x a mp l e )z ˆ .Th i si sp r e c i s e l yt h es u r f a c en e e d e df orc e r t a i n p r ob l e msy ouwi l l t a c k l et h i ss e me s t e r . •Vol u meEl e me n t 2 d V=d τ=rs i nθd θd φdr=a d •Gr a d i e n t : 1∂ fˆ ∂ f ·dr r ˆ 1 ∂fˆ ∂r nθ∂φφ r ˆ+r∂θθ +rsi ∇f= •Di v e r g e n c e ;Th ed i v e r g e n c ei sc on s t r u c t e db yt h es a mea r g u me n tt h a tp r ov e st h e d i v e r g e n c et h e or e mi nag e n e r a lc u r v i l i n e a rc oor d i n a t es y s t e m, ora l t e r n a t i v e l yp i c k s u pp i e c e sf r om∇r ˆ , e t c , h e n c ei t sc omp l e x i t y : 2 1∂ rBr ∇· B= 2 r s i nθBθ) 1 ∂( + rsi nθ ∂r 1 ∂( Bφ) +rsi nθ ∂φ ∂θ Not et h a tt h i sf ol l owsf r om: ∂ ( g hBu) 1 ∇· B= ( fg h ) ∂u ∂( fhBv) + wi t hu=r , v=θ , w=φ, a n df c on t r i b u t i onf r omr : 2 ∂v ∂w + =1 , g=r , h=rs i nθ .Ta k et h e 2 1∂ ( rBr ) = 2 2 rsi nθ r ∂r ∂r b e c a u s es i nθd oe sn otd e p e n donr , s i mi l a r l yf ort h eot h e rt wop i e c e s . 1 ∂( rs i nθBr ) ∂( fgBw) •Cu r l Th ec u r l i se v a l u a t e di ne x a c t l yt h es a mewa yf r omt h ee x p r e s s i ona b ov e , b u ti te n d su pb e i n gmu c hmor ec omp l e x: s i nθBφ) 1 ∂( ∇× B= rsi nθ ∂θ ∂Bθ 1∂ Br 1 ∂r Bφ ˆ1 i nθ∂φ − ∂r θ+r − ∂φ r ˆ +r s ∂( r Bθ) ∂r − ∂θ φ •L a p l a c i a nTh eL a p l a c i a nf ol l owsb ya p p l y i n gt h ed i v e r g e n c er u l et ot h eg r a di e n t r u l ea n ds i mp l i f y i n g : 1∂ 2 ∂f 2 2 ∇f= r∂r r ∂r 1 ∂ ∂f 2 i nθ∂θ si +rs nθ ∂θ 7. 3 Cy l i n d r i c a l Cy l i n d r i c a lc oor d i n a t e sa r eof t e ng i v e na sP=( s ,φ,z )s ot h a tφi sa z i mu t h a li nt h es a me s e n s ea ss p h e r i c a l p ol a r , a n ds ot h a tsi sd i ffe r e n t i a t e df r omr .Howe v e r , ma n yot h e rs i mi l a r c on v e n t i on sa r eu s e d.F ore x a mp l e ,P=( r ,θ ,z )orP=( r ,φ,z )orP=( ρ,θ,z )a r en ot u n c ommon .Wewi l l u s e( s , φ, z )i nt h i sr e v i e wt oa v oi da smu c hc on f u s i ona sp os s i b l ewi t h s p h e r i c a l p ol a rc oor d i n a t e s . •Ve c t or s : r =rr ˆ+z z ˆ Bu tNo t eWe l l :r ˆi sn owaf u n c t i onof( θ ) ! Si mi l a r l y : A=Arr ˆ+Azz ˆ wi t hr ˆaf u n c t i onoft h ea n g l eAθ =θt h a td e f i n e st h ed i r e c t i onofr ˆ . Sp e c i f i c a l l y : •Un i tv e c t or s( r e l a t i v et oCa r t e s i a nx ˆ , y ˆ , z ˆ : s ˆ=c osφx ˆ+s i nφy ˆ ˆ =−s i nφˆ+c osφˆ φ x y z ˆ=z ˆ •Di r e c tL e n g t h d ˆ =dsˆ+s dφ +dzˆ ∂Br ˆ ℓ s φz •Di r e c t e dAr e a d A=s d φdz d An ˆ=a d =sdφdzs ˆ=dAs ˆ An da g a i n , t h e r ea r ema n yot h e rp os s i b l ea d’ s , f ore x a mp l e : d An ˆ=a d =sd φd sz ˆ f ora ne n dc a pofac y l i n d r i c a l v ol u me . •Vol u meEl e me n t d V=d τ=sd φdsd z=a d ·dz z ˆ f ort h es e c on doft h e s ea r e ae l e me n t s . •Gr a d i e n t : ∂f 1∂fˆ ∂f ∇f= ∂ss ˆ+s∂φφ +∂zzˆ •Di v e r g e n c e ; Th ed i v e r g e n c ei sc on s t r u c t e db yt h es a mea r g u me n tt h a tp r ov e st h e d i v e r g e n c et h e or e mi nag e n e r a l c u r v i l i n e a rc oor di n a t es y s t e mg i v e na b ov e . 1 s Bs)+1∂( Bφ)+∂Bz ∇· B= ∂( s ∂s s∂φ∂z •Cu r l Th ec u r l i se v a l u a t e di ne x a c t l yt h es a mewa yf r omt h ee x p r e s s i ona b ov e , b u ti te n d su pb e i n gmu c hmor ec omp l e x: 1∂ Bz ∇× B= ∂Bφ ∂Bs ∂Bz ˆ 1 ∂( s Bφ) s∂φ − ∂z s s ˆ+ ∂z − ∂s φ + ∂Bs ∂s − ∂φ zˆ •L a p l a c i a nTh eL a p l a c i a nf ol l owsb ya p p l y i n gt h ed i v e r g e n c er u l et ot h eg r a di e n t r u l ea n ds i mp l i f y i n g : 1∂ 2 ∇f= ∂f s∂ s s∂s 2 1 ∂f 2 +s 2 ∂φ 2 ∂f 2 + ∂z Ch a p t e r8 Th eDi r a cδF u n c t i o n δ( x ) f ( x ) x x Th eDi r a cδ f u n c t i oni su s u a l l yd e f i n e dt ob eac on v e n i e n t( s moot h , i n t e g r a b l e , n a r r ow) d i s t r i b u t i one . g .χ ( x )t h a ti ss y mme t r i ca n dp e a k e di nt h emi d d l ea n dwi t hap a r a me t r i c wi d t hx . Th ed i s t r i b u t i oni sn or ma l i z e d( i nt e r msofi t swi d t h )s ot h a ti t si n t e g r a l i son e : ∞ χ ( x ) d x=1 − ∞ On et h e nt a k e st h el i mi tx→ 0wh i l ec on t i n u i n gt oe n f or c et h en or ma l i z a t i on c on d i t i ont od e f i n et h eδf u n c t i on : δ( x )=l i mχ ( x ) x →0 6 3 Th eδf u n c t i oni t s e l fi st h u sn ots t r i c t l ys p e a k i n ga“ f u n c t i on ” , b u tr a t h e rt h el i mi tof ad i s t i b u t i on .F u r t h e r mor e ,i ti sn e a r l yu s e l e s si na n dofi t s e l f–a sa“ f u n c t i on ” s t a n d i n ga l on ei tc a nb et h ou g h tofa sa ni n f i n i t e l yn a r r ow, i n f i n i t e l yh i g hp e a ka r ou n dx =0wi t hac on s e r v e da r e aofu n i t y .I t ’ sp r i ma r yp u r p os ei np h y s i c si st ob emu l t i p l i e db y a na c t u a l f u n c t i ona n di n t e g r a t e d , wi t ht h e x→ 0l i mi tt a k e na f t e rd oi n gt h ei n t e g r a l .Howe v e r ,t h er e s u l tofa p p l y i n gt h i s p r oc e s si sg e n e r a l , a n du s e f u le n ou g ht ob et r e a t e da sas t a n d a l on ea n dr e u s a b l es e t ofi n t e g r a l d e f i n i t i on sa n dr u l e s . He r ea r ei t sp r i n c i p l ed e f i n i t i on sa n dp r op e r t i e s : •I n t e g r a t i ona g a i n s taf u n c t i on : b f( x ) δ ( x ) d x=f( 0 ) f ora l l a , b>0 −a •Di s p l a c e me n toft h eδf u n c t i on : x +b 0 f( x ) δ ( x−x ) d x=f( x ) 0 0 f ora l l a , b>0 x −a 0 •u s u b s t i t u t i on( u=k x ) : b f( x ) δ ( k x ) d x= −a 1 k b f(u) δ ( u ) d u= 1f( 0 ) k −a f ora l l a , b>0, kc on s t a n t k •I n t e g r a t i onb yp a r t s / de r i v a t i v eofaδf u n c t i on : b dδ( x ) b x=f( x ) δ( x ) | − −a f( x ) dx d −a b df( x ) d f d x δ( x ) d x=− dx ( 0 ) f ora l l a , b>0 −a ( t h eδf u n c t i oni sz e r oe v e r y wh e r eb u ta tx=0s ot h ef i r s tt e r mi ni n t e g r a t i onb y p a r t sv a n i s h e s ) . •Th e3 Dδf u n c t i on : δr ()=δ( x ) δ( y ) δ( z ) s u c ht h a t : f r () δr () d τ=f( 0 ) ρ>0 Bρ( 0 ) Not e :Bρ( 0 )s t a n dsf ort h eop e nb a l lofr a d i u sρi nt h en e i g h b or h oodof r=0. Mor ep r op e r l y , t h er e s u l th ol d sf ora n yi n t e g r a t i onv ol u met h a tc on t a i n sa nop e n b a l l ofa tl e a s ti n f i n i t e s i ma l r a d i u sa r ou n dt h eor i g i n . Th i sr e s u l tc a na l s ob ed i s p l a c e d : f r () δr (− r0) d τ=f r (0) ρ>0 Bρr (0) a sl on ga st h ei n t e g r a t i onv ol u me( n ow)c on t a i n sa nop e nb a l l a r ou n d r0. •Awa r n i n g :Wh e non et r i e st ob u i l daδf u n c t i oni nt woort h r e ed i me n s i on si n c u r v i l i n e a rc oor di n a t e s ,y oun e e dt ot a k ei n t oa c c ou n tt h ea p p r op r i a t et e r msi n t h eJ a c ob i a n( ort h i n ka b ou tt h ec h a i nr u l e , i fy oup r e f e r ) .Th e s ea r ej u s tt h ef , g , hd i s c u s s e de x t e n s i v e l ya b ov e . F ors p h e r i c a l c oor d i n a t e s , t h e n : δr (− r0)= 1 1 1 r−r )· 0 fδ( θ−θ )· hδ( φ−φ0) 0 gδ( 2 1 r − r ) δ( θ − θ ) δ ( φ− φ0) i nθδ( 0 0 =rs Th i ss e l e c t i v e l yc a n c e l st h efg hp r od u c ti nt h ev ol u mee l e me n t : 2 f( r , θ, φ)δr (− r0)rs i nθd rdθd φ=f( r , θ0, φ0) 0 Bρr (0) a se x p e c t e d . Si mi l a r l yi nc y l i n d r i c a l c oor d i n a t e s : δr (− r0)= 1 1 1 r−r )· 0 fδ( φ−φ0)· hδ( z−z )= 0 gδ( 1 ( s − s ) δ( φ− φ0) δ( z − z ) 0 0 sδ Th i se n ds( f ort h emome n t )ou rt e r s es u mma r ya n dd i s c u s s i onoft h ema t hn e e de d f ori n t e r me d i a t ee l e c t r ody n a mi c s . Ch a p t e r9 Ma t hRe f e r e n c e s •www. g r c . n a s a . g ov / WWW/ K1 2 / Nu mb e r s / Ma t h / d oc u me n t s / . . . ...Te n s or sTM2 0 0 2 2 1 1 7 1 6 . p d f .Th i si saNASAwh i t ep a p e rb yJ os e p hC. Kol e c k i ont h eu s eoft e n s or si np h y s i c s( i n c l u d i n ge l e c t r od y n a mi c s )a n di squ i t e l ov e l y .I tp r e s e n t st h emod e r nv i e woft e n s or sa se n t i t i e sl i n k e db ot ht r a d i t i on a l b a s e sa n dma n i f ol d smu c ha sI h op et od oh e r e . •Ma t h e ma t i c a lPh y s i c sb yDon a l dH.Me n z e l ,Dov e rPr e s s ,I SBN0 4 8 6 6 0 0 5 6 4 . Th i sb ookwa swr i t t e ni n1 9 4 7a n dh e n c ep r e s e n t sb ot ht h e“ ol dwa y ”a n dt h e “ n e wwa y ”ofu n d e r s t a n d i n gt e n s or s .I ti sc h e a p( a sa r ea l lDov e rPr e s sb ook s ) a n da c t u a l l yi sar e a l l ye x c e l l e n td e s kr e f e r e n c ef orb ot hu n d e r g r a d u a t ea n d g r a d u a t el e v e l c l a s s i c a l p h y s i c si ng e n e r a l ! Se c t i on2 7i nt h i sb ookc ov e r ss i mp l e c a r t e s i a nt e n s or s , s e c t i on3 1t e n s or sde f i n e di nt e r msoft r a n s f or ma t i on s . •Sc h a u m’ sOu t l i n es e r i e sh a sav ol u meonv e c t or sa n dt e n s or s .Ag a i na ne x c e l l e n t d e s kr e f e r e n c e ,i th a sv e r yc omp l e t es e c t i on sonv e c t orc a l c u l u s( e . g .d i v e r g e n c e t h e or e m,s t ok e st h e or e m) ,mu l t i d i me n s i on a li n t e g r a t i on( i n c l u di n gd e f i n i t i on soft h e J a c ob i a na n dc oor d i n a t et r a n s f or ma t i on sb e t we e nc u r v i l i n e a rs y s t e ms )a n dt e n s or s ( t h eol dwa y ) . •h t t p : / / www. ma t h p a g e s . c om/ r r / s 5 0 2 / 5 0 2 . h t mTh i sp r e s e n t st e n s or si nt e r msof t h ema n i f ol dc oor d i n a t ede s c r i p t i ona n di sa c t u a l l yqu i t el ov e l y .I ti sa l s oj u s ta p a r tofh t t p : / / www. ma t h p a g e s . c om/ , ar a t h e rh u g ec ol l e c t i onofs h or ta r t i c l e son a l ls or t sofr e a l l yc oolp r ob l e mswi t ha b s ol u t e l yn oor g a n i z a t i ona sf a ra sIc a n t e l l . F u nt ol ookov e ra n ds ome t i me sv e r yu s e f u l . •Wi k i p e di a :h t t p : / / www. wi k i p e di a . or g / wi k i / Ma n i f ol dTe n s or st e n dt ob ed e s c r i b e di n t e r msofc oor di n a t e sonama n i f o l d.Ann di me n s i on a lma n i f ol di sb a s i c a l l ya ma t h e ma t i c a ls p a c ewh i c hc a nb ec ov e r e dwi t hl oc a l l yE u c l i d e a n“ p a t c h e s ”of c oor d i n a t e s .Th ep a t c h e smu s tov e r l a ps ot h a ton ec a nmov ea b ou tf r omp a t c ht o p a t c hwi t h ou te v e rl os i n gt h ea b i l i t y 6 7 t od e s c r i b ep os i t i on i nl oc a l“ p a t c hc oor di n a t e s ”t h a ta r e Eu c l i de a n( i n ma t h e ma t e s e , t h i ss or tofn e i g h b or h oodi ss a i dt ob e“ h ome omor p h i ct oa nop e n E u c l i d e a nn b a l l ” ) .Th ema n i f ol dsofi n t e r e s tt ou si nou rd i s c u s s i onoft e n s or s a r ed i ffe r e n t i a b l ema n i f ol ds ,ma n i f o l d sonwh i c hon ec a nd oc a l c u l u s ,a st h e t r a n s f or ma t i on a ld e f i n i t i onoft e n s or sr e qu i r e st h ea b i l i t yt ot a k ed e r i v a t i v e son t h eu n d e r l y i n gma n i f ol d . •Wi k i p e d i a :h t t p : / / www. wi k i p e d i a . or g / wi k i / Te n s or Th i s r e f e r e n c e i s ( f or Wi k i p e d i a )s ome wh a tl a c k i n g . Th eb e t t e rma t e r i a l i sl i n k e dt ot h i sp a g e , s e ee . g . Wi k i p e d i a : h t t p : / / www. wi k i p e d i a . or g / wi k i / Cov a r i a n tv e c t ora n d Wi k i p e d i a : h t t p : / / www. wi k i p e d i a . or g / wi k i / Con t r a v a r i a n tv e c t ora n d mu c hmor e . •h t t p : / / www. mt h . u c t . a c . z a / ome i / g r / c h a p 3 / f r a me 3 . h t mlTh i si sap a r tofa “ c omp l e t eon l i n ec ou r s ei nt e n s or sa n dr e l a t i v i t y ”b yPe t e rDu n s b y .I t ’ sa c t u a l l y p r e t t yg ood , a n di sd e f i n i t e l ymod e r ni ni t sa p p r oa c h . •h t t p : / / g r u s . b e r k e l e y . e d u / ∼j r g / a y 2 0 2 / n ode 1 8 3 . h t ml Th i si sas e c t i onofa non l i n e a s t r op h y s i c st e x tors e tofl e c t u r en ot e s .Th et e n s orr e v i e wi sr a t h e rb r i e fa n d n oth or r i b l yc omp l e t e , b u ti ti sa d e qu a t ea n di si nt h emi dd l eofot h e ru s e f u l s t u ff. An y wa y ,y oug e tt h ei d e a–t h e r ea r ep l e n t i f u lr e s ou r c e si nt h ef or m ofb ook sb ot h p a p e ra n don l i n e , wh i t ep a p e r s , we bp a g e s , a n dwi k i p e d i aa r t i c l e st h a ty ouc a nu s et or e a l l y g e tt owh e r ey ouu n d e r s t a n dt e n s ora l g e b r a ,t e n s orc a l c u l u s( d i ffe r e n t i a lg e ome t r y ) ,a n d g r ou pt h e or y .Asy oud os oy ou ’ l lf i n dt h a tma n yoft h et h i n g sy ou ’ v el e a r n e di n ma t h e ma t i c sa n dp h y s i c sc l a s s e si nt h ep a s tb e c omes i mp l i f i e dn ot a t i on a l l y( e v e na st h e i r c or ec on t e n tofc ou r s ed oe sn otc h a n g e ) . Asf oot n ot e da b ov e ,t h i ss i mp l i f i c a t i onb e c ome se v e ng r e a t e rwh e ns omeoft h e i d e a sa r ef u r t h e re x t e n de di n t oag e n e r a lg e ome t r i cd i v i s i ona l g e b r a ,a n dIs t r on g l y u r g ei n t e r e s t e dr e a d e r st oob t a i na n dp e r u s eL a s e n b y ’ sb ookonGe ome t r i cAl ge b r a . On ed a yIma ya t t e mp tt oa d das e c t i ononi th e r ea swe l la n dt r yt op r op e r l yu n i f yt h e g e ome t r i ca l g e b r a i cc on c e p t se mb e d d e di nt h ep a r t i c u l a rt e n s orf or msofr e l a t i v i s t i c e l e c t r od y n a mi c s . Pa r tI I No n Re l a t i v i s t i c E l e c t r o d y n a mi c s 6 9 Ch a p t e r10 Ma x we l l ’ sE q u a t i o n s 10. 1 Th eMa x we l l Di s p l a c e me n tCu r r e n t Ma x we l l ’ sEqu a t i on s( ME )c on s i s toft woi n h omog e n e ou sp a r t i a ld i ffe r e n t i a le qu a t i on s a n dt woh omog e n e ou sp a r t i a ld i ffe r e n t i a le qu a t i on s .Att h i sp oi n ty ous h ou l db e f a mi l i a ra tl e a s twi t ht h e“ s t a t i c ”v e r s i on soft h e s ee qu a t i on sb yn a mea n df u n c t i on : ∇ ∇ · H × ∇ B · ∂B ∇× E +∂t ′ u s ssL a wf orEl e c t r os t a t i c s D=ρ Ga ′ e r esL a w = JAmp ′ u s ssL a wf orMa g n e t os t a t i c s = 0 Ga ′ a r a d a ysL a w =0 F ( 1 0 . 1 ) ( 1 0 . 2 ) ( 1 0 . 3 ) ( 1 0 . 4 ) i nSI u n i t s , wh e r eD=ǫEa n dH=B/ µ. Th ea s t u t er e a d e rwi l li mme d i a t e l yn ot i c et wot h i n g s .On ei st h a tt h e s ee qu a t i on sa r e n ota l l ,s t r i c t l ys p e a k i n g ,s t a t i c–F a r a d a y ’ sl a wc on t a i n sat i med e r i v a t i v e ,a n dAmp e r e ’ s l a wi n v ol v e smov i n gc h a r g e si nt h ef or mofac u r r e n t .Th es e c on di st h a tt h e ya r ea l mos t s y mme t r i c .Th e r ei sad i v e r g e n c ee qu a t i ona n dac u r le qu a t i onf ore a c hk i n doff i e l d .Th e i n h omog e n ou se qu a t i on s( wh i c ha r ec on n e c t e dt os o u r c e si nt h ef or mofe l e c t r i cc h a r g e ) i n v ol v et h ee l e c t r i cd i s p l a c e me n ta n dma g n e t i cf i e l d,wh e r et h eh omog e n e ou se qu a t i on s s u g g e s tt h a tt h e r ei sn oma g n e t i cc h a r g ea n dc on s e qu e n t l yn os c r e e n i n goft h ema g n e t i ci n d u c t i onore l e c t r i cf i e l dd u et o ma g n e t i cc h a r g e .On ea s y mme t r yi st h e r e f or et h e p r e s e n c e / e x i s t e n c eofe l e c t r i cc h a r g ei nc on t r a s twi t ht h ea b s e n c e / n on e x i s t e n c eof ma g n e t i cc h a r g e . Th eot h e ra s y mme t r yi st h a tF a r a d a y ’ sl a wc on n e c t st h ec u r loft h eEf i e l dt ot h e t i med e r i v a t i v eoft h eBf i e l d , b u ti t sa p p a r e n tp a r t n e r , Amp e r e ’ sL a w, d oe sn o tc on n e c t t h ec u r l ofHt ot h et i med e r i v i a t i v eofv Da son emi g h te x p e c tf r oms y mme t r ya l on e . 7 1 I fon ee x a mi n e sAmp e r e ’ sl a wi ni t si n t e g r a l f or m, h owe v e r : C B·dℓ=µ S/ C J·n ˆd A ( 1 0 . 5 ) on equ i c k l yc on c l u d e st h a tt h ec u r r e n tt h r ou g ht h eop e ns u r f a c eSb ou n d e db yt h ec l os e d c u r v eCi sn oti n v a r i a n ta son ec h oos e sd i ffe r e n ts u r f a c e s .L e tu sa n a l y z et h i sa n dd e d u c e a ni n v a r i a n tf or mf ort h ec u r r e n t( d e n s i t y ) , t wowa y s . J ρ n1 n2 S1 S2 C F i g u r e1 0 . 1 : Cu r r e n tf l owi n gt h r ou g hac l os e dc u r v eCb ou n de db yt wos u r f a c e s , S1a n d S2. Con s i d e rac l os e dc u r v eCt h a tb ou n dst wod i s t i n c top e ns u r f a c e sS1a n dS2t h a t t og e t h e rf or mac l os e ds u r f a c eS=S1+S2.Nowc on s i d e rac u r r e n t( d e n s i t y )“ t h r ou g h ” t h ec u r v eC, mov i n gf r oml e f tt or i g h t .Su p p os et h a ts o meoft h i sc u r r e n ta c c u mu l a t e s i n s i d et h ev ol u meVb ou n d e db yS.Th el a wofc h a r gec o n s e r v a t i o ns t a t e st h a tt h ef l u x oft h ec u r r e n td e n s i t you toft h ec l os e ds u r f a c eSi se qu a lt ot h er a t et h a tt h et ot a l c h a r g ei n s i d ed e c r e a s e s . E x p r e s s e da sa ni n t e g r a l : d J·nˆdA=− S dtV/ S ρd V ( 1 0 . 6 ) Wi t ht h i si nmi n d , e x a mi n et h ef i g u r ea b ov e . I fwer e a r r a n g et h ei n t e g r a l sont h el e f t a n dr i g h ts ot h a tt h en or ma ln ˆ oi n t si nt ot h ev ol u me( s owec a nc omp u t et h e 1p c u r r e n tt h r ou g ht h es u r f a c eS1mov i n gf r oml e f tt or i g h t )wec a ne a s i l ys e et h a tc h a r g e c on s e r v a t i ont e l l su st h a tt h ec u r r e n ti nt h r ou g hS1mi n u st h ec u r r e n tou tt h r ou g hS2 mu s te qu a lt h er a t ea twh i c ht h et ot a lc h a r g ei n s i det h i sv ol u mei n c r e a s e s .I fwe e x p r e s st h i sa si n t e g r a l s : Jn 1d A− S1 · ˆ dQ Jn 2d A= d S2 · ˆ t = d d t V/S ρd V ( 1 0 . 7 ) I nt h i se x p r e s s i ona n df i g u r e ,n ot ewe l lt h a tn ˆ n dn ˆ oi n tt h r ou g ht h el oopi nt h e 1a 2p s a mes e n s e( e . g .l e f tt or i g h t )a n dn ot et h a tt h ev ol u mei n t e g r a li sov e rt h ev ol u meV b ou n de db yt h ec l o s e ds u r f a c ef or me db yS1a n dS2t og e t h e r . Us i n gGa u s s ’ sL a wf ort h ee l e c t r i cf i e l d , wec a ne a s i l yc on n e c tt h i sv ol u mei n t e g r a l oft h ec h a r g et ot h ef l u xoft h ee l e c t r i cf i e l di n t e g r a t e dov e rt h e s et wos u r f a c e swi t h ou t wa r dd i r e c t e dn or ma l s : ρd V = ǫ E·nˆdA V/ S S =− ǫ E·n ˆdA+ǫ E·n ˆd A S1 S2 ( 1 0 . 8 ) Comb i n i n gt h e s et woe x p r e s s i on s , weg e t : Jn 1d A− S1 · ˆ Jn 2d A= S2 · ˆ d d t −ǫ Jn d nˆ A+ǫ 1d S1 E· n ˆ A 2d S2 E· ( 1 0 . 9 ) En 1d A+ dtǫ S1 · ˆ S1 · ˆ 1d A= d ˆ A 2d S2 J· t S2 ǫE·n nˆ2dA+ d d E S1 J+ǫ dt ( 1 0 . 1 0 ) d E ·nˆ1d A= S2 ˆ A 2d t ·n J+ǫ d ( 1 0 . 1 1 ) F r omt h i swes e et h a tt h ef l u xoft h e“ c u r r e n td e n s i t y ”i n s i d et h eb r a c k e t si si n v a r i a n t a swec h oos ed i ffe r e n ts u r f a c e sb ou n d e db yt h ec l os e dc u r v eC. I nt h eor i g i n a lf or mu l a t i onofAmp e r e ’ sL a wwec a nc l e a r l yg e tadi ffe r e n ta n s we r ont h er i g h tf ort h ec u r r e n t“ t h r ou g h ”t h ec l os e dc u r v ed e p e n d i n gonwh i c hs u r f a c ewe c h oos e .Th i si sc l e a r l yi mp os s i b l e .Wet h e r e f or emod i f yAmp e r e ’ sL a wt ou s et h e i n v a r i a n tc u r r e n td e n s i t y : J i n v=J+ǫ d E d t ( 1 0 . 1 2 ) wh e r et h ef l u xoft h es e c on dt e r mi sc a l l e dt h eMa x we l ld i s p l a c e me n tc u r r e n t( MDC) . Amp e r e ’ sL a wb e c ome s : J n n v· ˆ d A C i d ℓ= µS/ CB· = µ d E J+ǫ ·n ˆd A ( 1 0 . 1 3 ) S/ C d t or d D d ℓ= CH· S/ C J+ dt ·n ˆd A ( 1 0 . 1 4 ) i nt e r msoft h ema g n e t i cf i e l dHa n de l e c t r i cd i s p l a c e me n tD.Th eor i g i noft h et e r m “ d i s p l a c e me n tc u r r e n t ”i sob v i ou s l yc l e a ri nt h i sf or mu l a t i on . Us i n gv e c t orc a l c u l u sonou rol df or mofAmp e r e ’ sL a wa l l owsu st oa r r i v ea tt h i s s a mec on c l u s i onmu c hmor es i mp l y . I fwet a k et h ed i v e r g e n c eofAmp e r e ’ sL a wweg e t : ( 1 0 . 1 5 ) ∇· ( ∇× H) = 0 = ∇· J I fwea p p l yt h ed i v e r g e n c et h e or e mt ot h el a wofc h a r g ec on s e r v a t i one x p r e s s e da sa f l u xi n t e g r a l a b ov e , weg e ti t sd i ffe r e n t i a l f or m: ∇· J− ∂ρ ∂t =0 ( 1 0 . 1 6 ) a n dc on c l u d et h a ti ng e n e r a lwec a nn o tc on c l u det h a tt h ed i v e r g e n c eofJv a n i s h e si n ∂ρ g e n e r a la st h i se x p r e s s i onr e qu i r e s ,a st h e r ei sn og u a r a n t e et h a t ∂tv a n i s h e s e v e r y wh e r ei ns p a c e .I ton l yv a n i s h e sf or“ s t e a dys t a t ec u r r e n t s ”onab a c k g r ou n dof u n i f or m c h a r g ed e n s i t y ,j u s t i f y i n g ou rc a l l i n gt h i sf or m ofAmp e r e ’ sl a wa ma g n e t os t a t i cv e r s i on . I fwes u b s t i t u t ei nρ=∇·D( Ga u s s ’ sL a w)f orρ, wec a ns e et h a ti ti st r u et h a t : ∂D ∇· ( ∇× H) = 0 = ∇· J+ ∂t ( 1 0 . 1 7 ) a sa ni de n t i t y . As u ffic i e n t( b u tn otn e c e s s a r y ! )c on d i t i onf ort h i st ob et r u ei s : ∂D ∇× H= J + ∂t ( 1 0 . 1 8 ) or ∇× H− ∂D ∂t =J . ( 1 0 . 1 9 ) Th i se x p r e s s i oni si d e n t i c a l t ot h ema g n e t os t a t i cf or mi nt h ec a s e swh e r eDi sc on s t a n t i nt i meb u tr e s p e c t sc h a r g ec on s e r v a t i onwh e nt h ea s s oc i a t e d( di s p l a c e me n t )f i e l di s c h a n g i n g . Wec a nn owwr i t et h ec omp l e t es e tofMa x we l l ’ se qu a t i on s ,i n c l u d i n gt h eMa x we l l di s p l a c e me n tc u r r e n td i s c ov e r e db yr e qu i r i n gf or ma li n v a r i a n c eoft h ec u r r e n ta n du s i n g c h a r gec o n s e r v a t i ont od e du c ei t sf or m.Ke e pt h el a t t e ri nmi n d ; i ts h ou l dn otb es u r p r i s i n g t ou sl a t e rwh e nt h el a wofc h a r g ec on s e r v a t i onp op so u tofMa x we l l ’ se qu a t i on swh e nwe i n v e s t i g a t et h e i rf or ma l p r op e r t i e swec a ns e et h a twed e l i b e r a t e l ye n c ode di ti n t oAmp e r e ’ s L a wa st h eMDC. An y wa y , h e r et h e ya r e .L e a r nt h e m.Th e yn e e dt ob es e c on dn a t u r ea swewi l ls p e n da c on s i d e r a b l ea mou n toft i meu s i n gt h e mr e p e a t e d l yi nma n y , ma n y c on t e x t sa swei n v e s t i g a t ee l e c t r oma g n e t i cr a di a t i on . ∇· D=ρ ∂D ( GL E ) ( 1 0 . 2 0 ) ∂t =J ( AL ) ( 1 0 . 2 1 ) ∇·B ∂B =0 ( GL M) ( 1 0 . 2 2 ) =0 ( F L ) ( 1 0 . 2 3 ) ∇× H− ∇× E + ∂t ( wh e r eIi n t r od u c ea n dob v i ou sa n dp e r ma n e n ta b b r e v i a t i on sf ore a c he qu a t i onb y n a mea su s e dt h r ou g h ou tt h er e s toft h i st e x t ) . Ar e n ’ tt h e yp r e t t y !Th en omon op ol e sa s y mme t r yi ss t i l lp r e s e n t ,b u twen owh a v et wo s y mme t r i cd y n a mi ce qu a t i on sc ou p l i n gt h ee l e c t r i ca n dma g n e t i cf i e l d sa n da r er e a dyt o s t a r ts t u d y i n ge l e c t r od y n a mi c si n s t e a dofe l e c t r o s t a t i c s . Not ewe l l t h a tt h et woi n h omo ge n e o u se qu a t i on su s et h ei n me di af or msoft h ee l e c t r i c a n dma g n e t i cf i e l d .Th e s ef or msa r ea l r e a dyc oa r s e g r a i na v e r a g e dov e rt h emi c r os c op i c di s t r i b u t i onofp oi n tc h a r g e st h a tma k eu pb u l kma t t e r .I nat r u l ymi c r os c op i cd e s c r i p t i on , wh e r ewec on s i d e ron l yb a r ec h a r g e swa n de r i n ga r ou n di nf r e es p a c e ,wes h ou l du s et h e f r e es p a c ev e r s i on s : 1 ∇× B− µ0ǫ0 ∇× E + ∇·E ∂E = ǫ0ρ ( 1 0 . 2 4 ) ∂t =µ0J ( 1 0 . 2 5 ) ∇·B ∂B =0 ( 1 0 . 2 6 ) ∂t =0 ( 1 0 . 2 7 ) I ti st i met oma k et h e s ee qu a t i on sj u mpt h r ou g hs omeh oop s . 10. 2 Pot e n t i a l s Web e g i nou rd i s c u s s i onofp ot e n t i a l sb yc on s i d e r i n gt h et woh o mo ge n e o u se qu a t i on s . F ore x a mp l e ,i fwewi s ht oa s s oc i a t eap ot e n t i a lwi t hBs u c ht h a tBi st h er e s u l tof d i ffe r e n t i a t i n gt h ep ot e n t i a l , weob s e r v et h a twec a ns a t i s f yGL Mb yc o n s t r u c t i o ni fwe s u p p os eav e c t o rp o t e n t i a l As u c ht h a t : B= ∇× A ( 1 0 . 2 8 ) ∇· B= ∇· ( ∇× A) = 0 ( 1 0 . 2 9 ) I nt h a tc a s e : a sa ni d e n t i t y . Nowc on s i d e rF L . I fwes u b s t i t u t ei nou re x p r e s s i onf orB: ∇ ∂∇× A ∂t ∂A E+ × = 0 ∂t ) = 0 ∇× ( E + ( 1 0 . 3 0 ) Wec a ns e et h a ti fwed e f i n e : ∂A ∇φ E+ ∂t =− t h e n ( 1 0 . 3 1 ) ∂A ∇× ( E + i sa l s oa ni d e n t i t y . Th i sl e a dst o: = − ∇× ∇φ= 0 ∂t) ( 1 0 . 3 2 ) ∂A ∂t ( 1 0 . 3 3 ) E= − ∇φ− Ou rn e x tc h or ei st ot r a n s f or mt h ei n h omoge n e ou sMEsi n t oe qu a t i on sofmot i on f ort h e s ep ot e n t i a l s–mot i onb e c a u s eME s( a n di n d e e dt h ep ot e n t i a l st h e ms e l v e s )a r e n owp ot e n t i a l l yd y n a mi c a le qu a t i on sa n dn otj u s ts t a t i c .Wed ot h i sb ys u b s t i t u t i n gi n t h ee qu a t i onf orEi n t oGL E,a n dt h ee qu a t i onf orBi n t oAL .Wewi l lwor k( f ort h e mome n t )i nf r e es p a c ea n dh e n c ewi l lu s et h ev a c u u mv a l u e sf ort h ep e r mi t t i v i t ya n d p e r me a b i l i t y . Th ef i r s t( GL E )y i e l d s : ∂A ∇· ( − ∇φ− 2 φ+ ∇ ∂t) ∂( ∇·A) ∂t ρ = ǫ0 = ρ ( 1 0 . 3 4 ) ǫ0 − Th es e c on d( AL )i sab i tmor ewor k .Wes t a r tb ywr i t i n gi ti nt e r msofBi n s t e a dofH b ymu l t i p l y i n gou tt h eµ : 0 2 2 c∂ t ∇×B ∇× ( ∇× A) 2 − ∇A+∇( ∇·A) 1∂ A 2 ∇2A+ − 2 2 c∂ t 1∂2A ∇2A+ − ∂E =µ0J+µ0ǫ0 ∂t ∂ ∂A − ∇φ− =µ0J+µ0ǫ0 ∂t( 1 2 ∂t) ∂φ 1∂A = µ0J− ∇ − 2 ∂ 2∂ 2 c tc t 1∂φ J ∇∇ A ∇ = − µ 0 + ( · ) + c 2∂ t 1∂ φ =− µ J+ ∇∇· A+ 0 c2 ∂t ( 1 0 . 3 5 ) 10. 2. 1 Ga u geTr a n s f or ma t i o n s Nowc ome st h et r i c k yp a r t .Th ef ol l owi n gi sv e r yi mp or t a n tt ou n d e r s t a n d , b e c a u s ei ti s ac ommonf e a t u r et on e a r l ya l ld i ffe r e n t i a lf or mu l a t i on sofa n ys or tofp ot e n t i a l b a s e d f i e l dt h e or y , qu a n t u morc l a s s i c a l . Wek n owf r om ou re x t e n s i v es t u d yofe l e me n t a r yp h y s i c st h a tt h e r emu s tb es ome f r e e d omi nt h ec h oi c eofφa n dA.Th ef i e l dsa r ep h y s i c a la n dc a nb e“ d i r e c t l y ”me a s u r e d, wek n owt h a tt h e ya r eu n i qu ea n dc a n n otc h a n g e .Howe v e r , t h e ya r eb ot hd e f i n e di nt e r ms ofd e r i v a t i v e soft h ep ot e n t i a l s ,s ot h e r ei sa ni n f i n i t ef a mi l yofp os s i b l ep ot e n t i a l st h a t wi l la l ll e a dt ot h es a mef i e l d s .Th et r i v i a le x a mp l eoft h i s ,f a mi l i a rf r omk i d di ep h y s i c s ,i s t h a tt h ee l e c t r os t a t i cp ot e n t i a li son l yd e f i n e dwi t ha na r b i t r a r ya d d i t i v ec on s t a n t .No p h y s i c sc a nd e p e n dont h ec h oi c eoft h i sc on s t a n t , b u ts omec h oi c e sma k ep r ob l e msmor e e a s i l ys ol v a b l et h a not h e r s .I fy oul i k e ,e x p e r i me n t a lp h y s i c sd e p e n d sonp ot e n t i a l d i ffe r e n c e s , n ott h ea b s ol u t ema g n i t u d eoft h ep ot e n t i a l . Soi ti sn owi ng r own u pe l e c t r o d y n a mi c s ,b u tweh a v et ol e a r nan e wt e r m.Th i s f r e e d om t oa ddac on s t a n tp ot e n t i a li sc a l l e dga u gef r e e d o ma n dt h ed i ffe r e n t p ot e n t i a l son ec a nob t a i nt h a tl e a dt ot h es a mep h y s i c a lf i e l da r eg e n e r a t e db yme a n s ofaga u get r a n s f o r ma t i on .Ag a u g et r a n s f or ma t i onc a nb eb r oa d l yd e f i n e da sa n y f or ma l ,s y s t e ma t i ct r a n s f or ma t i ono ft h ep ot e n t i a l st h a tl e a v e st h ef i e l d si n v a r i a n t ( a l t h ou g hi nqu a n t u mt h e or yi tc a nb ep e r h a p sab i tmor es u b t l et h a nt h a tb e c a u s eof t h ea dd i t i on a l d e g r e eoff r e e d omr e p r e s e n t e db yt h equ a n t u mp h a s e ) . Aswa sof t e nt h ec a s ei ne l e me n t a r yp h y s i c swe r ewef r e e l ymov e da r ou n dt h eor i g i nofou r c oor d i n a t es y s t e m( ag a u g et r a n s f or ma t i on ,wen ow r e c og n i z e )orde c i d e dt oe v a l u a t eou r p ot e n t i a l ( d i ffe r e n c e s )f r omt h ei n n e rs h e l l ofas p h e r i c a l c a p a c i t or( a n ot h e rc h oi c eofg a u g e )we wi l l c h o os eaga u gei ne l e c t r od y n a mi c st oma k et h es ol u t i ont oap r ob l e ma se a s ya sp os s i b l eor t ob u i l das ol u t i onwi t hs omede s i r e dc h a r a c t e r i s t i c st h a tc a nb ee n f or c e db ya“ g a u g ec on d i t i on ” –ac on s t r a i n tont h ef i n a l p ot e n t i a l sob t a i n e dt h a ton ec a ns h owi swi t h i nt h er a n g eof p os s i b i l i t i e sp e r mi t t e db yg a u g et r a n s f or ma t i on s . Howe v e r , t h e r e ’ sap r i c et op a y .Ga u g ef r e e d omi nn on e l e me n t a r yp h y s i c si sawe eb i t b r o a d e rt h a n“ j u s t ”a d d i n gac on s t a n t ,b e c a u s eg r a d i e n t s ,di v e r g e n c e sa n dc u r l si n mu l t i v a r i a t ec a l c u l u sa r en ots i mp l ed e r i v a t i v e s . Con s i d e rB=∇× A. Bmu s tb eu n i q u e , b u tma n yA’ se x i s tt h a tc or r e s p on d ′ t oa n yg i v e nB.Su p p os eweh a v eon es u c hA.Wec a nob v i ou s l yma k ean e wAt h a th a s t h es a mec u r l b ya d d i n gt h eg r a d i e n tofa n ys c a l a rf u n c t i o nΛ. Th a ti s : ′ Wes e et h a t : B= ∇× A= ∇× ( A+ ∇Λ) = ∇× A ′ A= A+∇Λ ( 1 0 . 3 6 ) ( 1 0 . 3 7 ) i saga u get r a n s f o r ma t i o noft h ev e c t orp ot e n t i a l t h a tl e a v e st h ef i e l di n v a r i a n t . Not et h a ti t p r ob a b l yi s n ’ tt r u et h a tΛc a nb ea n ys c a l a rf u n c t i on–i ft h i swe r eama t hc l a s sI ’ da dd c a v e a t sa b ou ti tb e i n gn on s i n g u l a r , s moot h l yd i ffe r e n t i a b l ea tl e a s ton et i me , a n ds oon . E v e ni fap h y s i c sc l a s sI mi g h ts a yawor d ort woa b ou ti t , s oI j u s tdi d .Th ep oi n tb e i n gt h a tb e f or ey oup r op os eaΛt h a ti s n ’ t , y ou a tl e a s tn e e dt ot h i n ka b ou tt h i ss or toft h i n g .Howe v e r ,g r e a tp h y s i c i s t s( l i k eDi r a c ) h a v es u b t r a c t e dou ti r r e l e v a n ti n f i n i t i e sf r om p ot e n t i a l si nt h ep a s ta n dg ot t e na wa y wi t hi t( h ei n v e n t e d“ ma s sr e n or ma l i z a t i on ”–b a s i c a l l yag a u g et r a n s f or ma t i on–wh e n t r y i n gt od e r i v ear a di a t i onr e a c t i ont h e or y ) ,s od on ’ tb et ooc l os e dmi n d e da b ou tt h i s e i t h e r . I ti sa l s owor t hn ot i n gt h a tt h i son l ys h owst h a tt h i si sap os s i b l eg a u g et r a n s f or ma t i on ofA, n ott h a ti ti ss u ffic i e n t l yg e n e r a lt oe n c omp a s sa l lp os s i b l eg a u g et r a n s f or ma t i on sof A.Th e r ema ywe l lb et e n s ord i ffe r e n t i a lf or msofh i g h e rr a n kt h a tc a n n otb er e d u c e dt o b e i n ga“ g r a d i e n tofas c a l a rf u n c t i on ”t h a ts t i l lp r e s e r v eB.Howe v e r ,wewon ’ th a v et h e a l g e b r a i ct ool st ot h i n ka b ou tt h i sa tl e a s tu n t i lwer e f or mu l a t eME si nr e l a t i v i t yt h e or ya n d l e a r nt h a tEa n dBa r en ot ,i nf a c t ,v e c t o r s !Th e ya r ec omp on e n t sofas e c on dr a n kt e n s or , wh e r eb ot hφa n dAc omb i n et of or maf i r s tr a n kt e n s or( v e c t or )i nf ou rd i me n s i on s . Th i si squ i t es t a r t l i n gf ors t u d e n t st ol e a r n ,a si tme a n st h a tt h e r ea r ema n y qu a n t i t i e st h a tt h e ymi g h th a v et h ou g h ta r ev e c t or st h a ta r en ot , i nf a c t , v e c t or s .An di t ma t t e r s–t h et e n s orc h a r a c t e rofap h y s i c a lqu a n t i t yi sc l os e l yr e l a t e dt ot h ewa yi t t r a n s f or mswh e nwee . g .c h a n g et h eu n de r l y i n gc oor d i n a t es y s t e m.Don ’ twor r ya b ou t t h i squ i t ey e t , b u ti ti ss ome t h i n gf oru st ot h i n kd e e p l ya b ou tl a t e r . Ofc ou r s e , i fwec h a n g eAi na r b i t r a r ywa y s , Ewi l l c h a n g ea swe l l ! Su p p os eweh a v e a nAa n dφt h a tl e a dst os omep a r t i c u l a rEc omb i n a t i on : ∂A ∂t E= − ∇φ− ( 1 0 . 3 8 ) ′ I fwet r a n s f or mAt oAb yme a n sofag a u g et r a n s f or ma t i on( s oBi sp r e s e r v e d ) , we( i n ′ g e n e r a l )wi l l s t i l l g e tadi ffe r e n tE: ′ ∂A ′ E = − ∇φ− ∂t ∂ =− ∇φ− ∂t( A+∇Λ) ∂∇Λ = E− ∂t =E ( 1 0 . 3 9 ) a st h e r ei sn or e a s ont oe x p e c tt h eg a u g et e r mt ov a n i s h .Th i si sb a a a a a d .Wewa n tt o g e tt h es a meE . ′ ′ Toa c c omp l i s ht h i s , a swes h i f tAt oAwemu s ta l s os h i f tφt oφ.I fwes u b s t i t u t e ′ ′ a nu n k n ownφi n t ot h ee x p r e s s i onf orEweg e t : ∂ ′ E ′ E ′ ∇φ− ∂t( =− A+∇Λ) ′ =− ∇φ− ∂A −∇ ∂Λ ( 1 0 . 4 0 ) ∂t ∂t ′ Wes e et h a ti nor d e rt o ma k eE =E( s oi td oe s n ’ tv a r ywi t ht h eg a u g e ′ t r a n s f or ma t i on )weh a v et os u b t r a c tac omp e n s a t i n gp i e c et oφt of or mφ: ′ φ= φ s ot h a t : − ∂Λ ∂t ( 1 0 . 4 1 ) ′ ∂A ∂Λ =− ∇φ− ∂t = − ∇φ+∇ ∂A ∂t− ∂t−∇ ∂t =− ∇φ− ∂t =E ′ ′ E ∂A ∂Λ ( 1 0 . 4 2 ) I ns u mma r y ,wes e et h a taf a i r l yg e n e r a lg a u g et r a n s f or ma t i ont h a tp r e s e r v e sb ot hE a n dBi st h ef ol l owi n gp a i rofs i mu l t a n e o u st r a n s f or ma t i on sofφa n dA.Gi v e na na r b i t r a r y ( b u twe l l b e h a v e d )s c a l a rf u n c t i onΛ: ′ φ ′ A − ∂Λ =φ ( 1 0 . 4 3 ) ∇Λ =A+ ( 1 0 . 4 4 ) ∂t wi l l l e a v et h ed e r i v e df i e l d si n v a r i a n t . Asn ot e da tt h eb e g i n n i n g ,we ’ dl i k et ob ea b l et ou s et h i sg a u g ef r e e d omi nt h e p ot e n t i a l st oc h oos ep ot e n t i a l st h a ta r ee a s yt oe v a l u a t eort h a th a v es omed e s i r e d f or ma lp r op e r t y .Th e r ea r et wo c h oi c e sf org a u g et h a ta r ev e r yc ommon i n e l e c t r od y n a mi c s , a n dy ous h ou l db ef a mi l i a rwi t hb ot hoft h e m. 10. 2. 2 Th eL or e n t zGa u ge Th eL or e n t zg a u g e ,f orav a r i e t yofr e a s on s ,i si nmyop i n i ont h e“ n a t u r a l ”g a u g eof e l e c t r od y n a mi c s .F oron et h i n g ,i ti se l e g a n ti nf ou rd i me n s i on a ls p a c e t i me ,a n dwea r e g r a d u a l l ywor k i n gt owa r d st h ee p i p h a n yt h a twes h ou l dh a v ef or mu l a t e da l lofp h y s i c si n f ou rd i me n s i on a ls p a c e t i mef r omt h eb e g i n n i n g ,e v e ni fwe ’ r ec on s i d e r i n gn on r e l a t i v i s t i c p h e n ome n a .Wor k i n gi ni t , mos tp r ob l e msa r er e l a t i v e l yt r a c t i b l ei fn ota c t u a l l ye a s y .Wewi l l t h e r e f or ec on s i d e ri tf i r s t . Ab ov ewede r i v e df r omME sa n dt h e i rd e f i n i t i on st h et woe qu a t i on sofmot i onf or t h ep ot e n t i a l sφa n dA: 2 ∇ φ+ 2 ∇A+− ∂( ∇·A) ∂t 2 1∂A 2 2 c ∂t ρ = −ǫ0 ( 1 0 . 4 5 ) 1∂ φ µ J+∇ =− 0 ∇· A+ 2 c ∂t ( 1 0 . 4 6 ) I fwec a ng u a r a n t e et h a twec a na l wa y sf i n dag a u g et r a n s f or ma t i onf r om ag i v e n s ol u t i ont ot h e s ee qu a t i on sofmot i on , φ0, A0, an e won es u c ht h a tn e wφ, As u c ht h a tt h e n e won e ss a t i s f yt h ec on s t r a i n t( t h eL or e n t zg a u g ec on d i t i on ) : 1∂ φ ∇· A+ 2∂ c t =0 ( 1 0 . 4 7 ) t h e nt h et woe qu a t i on sofmot i onb ot hb e c a met h ei n h o mo ge n e o u swa v ee q u a t i onf or p ot e n t i a lwa v e st h a tp r op a g a t ea tt h es p e e dofl i g h ti n t oorou toft h ec h a r g e c u r r e n t s ou r c ei n h omog e n e i t i e s .Th i sp r e c i s e l yc or r e s p on d st oou ri n t u i t i onofwh a ts h ou l db e h a p p e n i n g , i se l e g a n t , s y mme t r i c , a n ds oo n .L a t e rwe ’ l l s e eh owb e a u t i f u l l ys y mme t r i c i tr e a l l yi s . Wemu s t , h owe v e r , p r ov et h a ts u c hag a u g ec on d i t i ona c t u a l l ye x i s t s . Wep r op os e : ∂Λ φ =φ0− ∂t ∇Λ A =A0+ ( 1 0 . 4 8 ) ( 1 0 . 4 9 ) a n ds u b s t i t u t ei ti n t ot h ede s i r e dg a u g ec on d i t i on : 1∂ φ0 1∂ φ 2 2 A0+∇Λ+ c ∂ t = ∇· = 0 ∇· A+ or 2 1∂Λ 2 2 2 c ∂ t− c ∂t ( 1 0 . 5 0 ) 1∂φ0 2 2 2 1∂Λ 2 ∂ t= ∇· A0+ ∇Λ− c c2 ∂t =f x (, t ) f ors omec omput a bl ei nhomoge ne ou ss ou r c e v f u n c t i onf x ( , t ) . Thi sequat i oni ss ol v abl ef oranenor mousr angeofpos s i bl ef x ( ( 1 0 . 5 1 ) , t ) s( ba s i c a l l y , a l lwe l l b e h a v e df u n c t i on swi l ll e a dt os ol u t i on s ,wi t hi s s u e sa s s oc i a t e dwi t ht h e i r s u p p or torp os s i b l es i n g u l a r i t i e s )s oi ts e e msa tt h ev e r yl e a s t“ l i k e l y ”t h a ts u c ha g a u g e t r a n s f or ma t i on a l wa y s e x i s t s f or r e a s on a b l e / p h y s i c a lc h a r g e c u r r e n t d i s t r i b u t i on s . I n t e r e s t i n g l y , t h eg a u g ef u n c t i onΛt h a tp e r mi t st h eL or e n t zc on d i t i ont ob es a t i s f i e ds o t h a tφ,As a t i s f ywa v ee qu a t i on si si t s e l ft h es ol u t i ont oawa v ee qu a t i on !I ti sa l s o i n t e r e s t i n gt on ot et h a tt h e r ei sa d d i t i o n a lg a u g ef r e e d om wi t h i nt h eL or e n t zg a u g e .F or e x a mp l e , i fon e ’ sor i g i n a l s ol u t i onφ0, A0i t s e l fs a t i s f i e dt h eL or e n t zg a u g ec on d i t i on , t h e na g a u g et r a n s f or ma t i ont oφ, Awh e r e Λi sa nyf r e es c a l a rwa v e : ∂Λ φ =φ0− ∂t ( 1 0 . 5 2 ) A ∇Λ =A0+ ( 1 0 . 5 3 ) =0 ( 1 0 . 5 4 ) 2 2 1∂Λ 2 2 ∇ Λ− c ∂t c on t i n u e st os a t i s f yt h eL or e n t zg a u g ec on di t i on .Noton l ya r ewen e a r l yg u a r a n t e e dt h a t s ol u t i on st h a ts a t i s f yt h eL or e n t zg a u g ec on d i t i one x i s t , weh a v ed i s c ov e r e da ni n f i n i t yoft h e m, c on n e c t e db yar e s t r i c t e dga u get r a n s f or ma t i on . I nt h eL or e n t zg a u g e ,t h e n ,e v e r y t h i n gi sawa v e .Th es c a l a ra n dv e c t orp ot e n t i a l s ,t h e de r i v e df i e l d s ,a n dt h es c a l a rg a u g ef i e l dsa l ls a t i s f ywa v ee qu a t i on s .Th er e s u l ti s i n d e p e n d e n tofc oor d i n a t e s , f or mu l a t e sb e a u t i f u l l yi ns p e c i a l r e l a t i v i t y , a n de x h i b i t s( a swewi l l s e e )t h ec a u s a l p r op a g a t i onoft h ef i e l d sorp ot e n t i a l s a tt h es p e e dofl i g h t . Th eot h e rg a u g ewemu s tl e a r ni sn ots op r e t t y .I nf a c t ,i ti sr e a l l yp r e t t yu g l y ! Howe v e r ,i ti ss t i l lu s e f u la n ds owemu s tl e a r ni t .Att h ev e r yl e a s t ,i th a saf e w i mp or t a n tt h i n g st ot e a c hu sa swewor kou tt h ef i e l d si nt h eg a u g e . 10. 2. 3 Th eCou l ombo rTr a n s v e r s eGa u ge L e tu sr e t u r nt ot h ee qu a t i on sofmot i o n : 2 ∇ φ+ 2 ∂( ∇·A) ∂t 2 1∂A ρ = −ǫ0 ( 1 0 . 5 5 ) 1∂ φ 2 c2 ∂t µ J+ ∇∇· A+ =− ( 1 0 . 5 6 ) 0 Th e r ei sa n ot h e rwa yt oma k ea tl e a s ton eoft h e s et woe qu a t i on ss i mp l i f y . Wec a nj u s ti n s i s tt h a t : ∇A+− c2 ∂t ∇· A= 0 . ( 1 0 . 5 7 ) I ti s n ’ ts oob v i ou st h a twec a na l wa y sc h oos eag a u g es u c ht h a tt h i si st r u e .Si n c ewe k n owwec a ns t a r twi t ht h eL or e n t zg a u g e , t h ou g h , l e t ’ sl ookf orΛs u c ht h a ti ti s .Th a t i s , s u p p os ewe ’ v ef ou n dφ, As u c ht h a t : 1∂ φ ∇· A+ 2∂ c t Asb e f or e , wep r op os e : =0 ∂ Λ ′ φ = φ− ( 1 0 . 5 9 ) ∂ t s u c ht h a t ′ ( 1 0 . 5 8 ) 2 ( 1 0 . 6 0 ) ( 1 0 . 6 1 ) ∇· A= ∇· A+ ∇Λ= 0 . I fwes u b s t i t u t ei nt h eL or e n t zg a u g ec on d i t i on : ∇· A= − 1∂ φ ( 1 0 . 6 2 ) 2∂ c t weg e t : 2 1∂ φ′ ∇Λ=− ∇·A= =g x (, t ) 2 ( 1 0 . 6 3 ) c∂ t Asb e f or e ,p r ov i d e dt h a tas ol u t i ont ot h ee qu a t i on sofmot i oni nt h eL or e n t zg a u g e e x i s t s ,wec a ni np r i n c i p l es ol v et h i se qu a t i onf oraΛt h a tma k e s∇· A=0t r u e .I ti s t h e r e f or eal e g i t i ma t eg a u g ec on d i t i on . I fweu s et h eCou l ombg a u g ec on d i t i on( wh i c hwea r en owj u s t i f i e di nd oi n g ,a swe k n owt h a tt h er e s u l t i n gp ot e n t i a l swi l ll e a dt ot h es a mep h y s i c a lf i e l d )t h ep ot e n t i a l si nt h e Cou l ombg a u g emu s ts a t i s f yt h ee qu a t i on so fmot i on : ρ 2 ∇φ 2 =−ǫ0 1∂A 2 2 2 ∂ µ J+ t 0 ∇A+− c =− Th ep ot e n t i a l φi st h e r e f or et h ewe l l k n o wns ol u t i o n φx ()= 1 4πǫ0 1 ∂φ c2∇ ∂t ρx (0) 3 dx 0 x |− x0| ( 1 0 . 6 4 ) ( 1 0 . 6 5 ) ( 1 0 . 6 6 ) t h a ty oup r ob a b l yor i g i n a l l ys a wi ne l e me n t a r yi n t r odu c t or yp h y s i c sa n ds ol v e de x t e n s i v e l y l a s ts e me s t e ru s i n gt h eGr e e n ’ sf u n c t i onf ort h ePoi s s one qu a t i on : Gx (x , )=− 0 1 4πx x ( 1 0 . 6 7 ) |− 0| t h a ts ol v e st h e“ p oi n ts ou r c e ”d i ffe r e n t i a l e qu a t i on : 2 ∇Gx (x , )=δx (− x0) 0 ( 1 0 . 6 8 ) I nt h i se qu a t i onon eu s e st h ev a l u eoft h ec h a r g ed e n s i t yona l l s p a c ea saf u n c t i on oft i meu n d e rt h ei n t e g r a l ,a n dt h e na d d sas ou r c et e r mt ot h ec u r r e n td e n s i t yi nt h e i n h omog e n e ou swa v ee qu a t i on sf ort h ev e c t orp ot e n t i a lde r i v e df r omt h a td e n s i t ya s we l l . Th e r ea r es e v e r a l v e r y , v e r yoddt h i n g sa b ou tt h i ss ol u t i on .On ei st h a tt h eCou l omb p ot e n t i a li si n s t a n t a n e o u s–c h a n g e si nt h ec h a r g edi s t r i b u t i oni n s t a n t l ya p p e a ri ni t s e l e c t r i cp ot e n t i a lt h r o u g h ou ta l ls p a c e .Th i sa p p e a r st ov i ol a t ec a u s a l i t y ,a n di s d e f i n i t e l yn otwh a ti sp h y s i c a l l yob s e r v e d . I st h i sap r ob l e m? Th ea n s we ri s , n o.I fon ewor k sv e r yl on ga n dt e d i ou s l y( a sy ouwi l l , f ory ou rh ome wor k ) on ec a ns h owt h a tt h ec u r r e n td e n s i t yc a nb ede c omp os e di n t ot wop i e c e s–al o n gi t u d i n a l ( n on r ot a t i on a l )on ea n dat r a n s v e r s e( r ot a t i on a l )on e : J=J ℓ+J t ( 1 0 . 6 9 ) ∇× J ℓ = 0 ( 1 0 . 7 0 ) ∇·J t = 0 ( 1 0 . 7 1 ) The s et e r msar ede f i ne dby : E v a l u a t i n gt h e s ep i e c e si sf a i r l ys t r a i g h t f or wa r d . St a r twi t h : 2 ∇× ( ∇× J ) = ∇( ∇· J ) − ∇J ( 1 0 . 7 2 ) Th i se qu a t i onob v i ou s l ys p l i t si n t ot h et wop i e c e s–u s i n gt h ec on t i n u i t ye qu a t i ont o e l i mi n a t et h ed i v e r g e n c eofJi nf a v oro fρ , weg e t : 2 ∇J t 2 ∇J ℓ =− ∇× ( ∇× J ) ( 1 0 . 7 3 ) ∂ρ =∇( ∇· J ) = − ∇ ( 1 0 . 7 4 ) ∂t ( wh i c ha r eb ot hPoi s s one qu a t i on s ) . Wi t hab i tofwor k–s omei n t e g r a t i onb yp a r t st omov et h e∇’ sou toft h ei n t e g r a l s wh i c hi mp os e st h ec on s t r a i n tt h a tJa n dρh a v ec omp a c ts u p p or ts oon ec a ni g n or et h e s u r f a c et e r m–t h ede c omp os e dc u r r e n t sa r e : 3 J t =∇× ( ∇× J ℓ =∇ ∂ J dx 0 4 πxx | − ρ ( 1 0 . 7 5 ) ) 0 | 3 dx 0 =ǫ ∇ ∂φ 0 − x0| ∂t4πx| Su b s t i t u t i n ga n dc omp a r i n gwen ot e : 1 ∂ φ ( 1 0 . 7 6 ) ∂t ∇ =µ J 0 ℓ 2 ∂ c t ( 1 0 . 7 7 ) s ot h a tt h i st e r mc a n c e l sa n dt h ee qu a t i onofmot i onf orAb e c ome s : 1∂ A 2 0 t ∇2A− =− µ J ( 1 0 . 7 8 ) 2 2 c∂ t on l y . I nt h eCou l ombg a u g e ,t h e n ,on l yt h et r a n s v e r s ec u r r e n tg i v e sr i s et ot h ev e c t or p ot e n t i a l , wh i c hb e h a v e sl i k eawa v e . He n c et h eot h e rc ommonn a mef ort h eg a u g e , t h e t r a n s v e r s eg a u g e .I ti sa l s os ome t i me sc a l l e dt h e“ r a d i a t i ong a u g e ”a son l yt r a n s v e r s e c u r r e n t sg i v er i s et op u r e l yt r a n s v e r s er a d i a t i onf i e l d sf a rf r omt h es ou r c e s ,wi t ht h e s t a t i cp ot e n t i a l p r e s e n tb u tn otg i v i n gr i s et or a d i a t i on . Gi v e na l l t h eu g l i n e s sa b ov e , wh yu s et h eCou l ombg a u g ea ta l l ?Th e r ea r eac ou p l e ofr e a s on s .F i r s tofa l lt h ea c t u a le qu a t i on sofmot i ont h a tmu s tb es ol v e da r es i mp l e e n ou g hon c eon ed e c omp os e st h ec u r r e n t .Se c on dofa l l , wh e nc omp u t i n gt h ef i e l d si n f r e es p a c ewh e r et h e r ea r en os ou r c e s ,φ=0a n dwec a nf i n db ot hEa n dBf r om A a l on e : ∂A E =− ∂t ( 1 0 . 7 9 ) B = ∇×A ( 1 0 . 8 0 ) Th el a s tod d i t ya b ou tt h i sg a u g ei st h a ti tc a nb es h o wn–i fon ewor k sv e r yh a r d– t h a ti tp r e s e r v e sc a u s a l i t y .Th et r a n s v e r s ec u r r e n ta b ov ei sn o tl o c a l i z e dwi t h i nt h e s u p p o r tofJb u te x t e n d st h r ou g h ou ta l ls p a c ej u s ta si n s t a n t a n e ou s l ya sφd oe s .On e p a r toft h ef i e l de v a l u a t e df r omt h es ol u t i ont ot h ed i ffe r e n t i a le qu a t i on sf orA,t h e n , mu s tc a n c e lt h ei n s t a n t a n e ou sCo u l ombf i e l da n dl e a v eon ewi t hon l yt h eu s u a l p r op a g a t i n ge l e c t oma g n e t i cf i e l d . Th i si sl e f ta sah ome wor kp r ob l e m. 10. 3 Poy n t i n g’ sTh e o r e m, Wo r ka n dE n e r gy Re c a l lf r om e l e me n t a r yp h y s i c st h a tt h er a t ea twh i c hwor ki sd on eona ne l e c t r i c c h a r g eb ya ne l e c t r oma g n e t i cf i e l di s : P=Fv ·=qEv ·=E·v q ( 1 0 . 8 1 ) I fon ef ol l owst h eu s u a lme t h odofc on s t r u c t i n gac u r r e n td e n s i t yma deu pofma n y c h a r g e s , i ti se a s yt os h owt h a tt h i sg e n e r a l i z e st o: d P d V= E · J ( 1 0 . 8 2 ) f ort h er a t ea twh i c ha ne l e c t r i cf i e l dd oe swor konac u r r e n td e n s i t yt h r ou g h ou ta v ol u me .Th ema g n e t i cf i e l d,ofc ou r s e ,do e sn owor kb e c a u s et h ef or c ei tc r e a t e si s a l wa y sp e r p e n d i c u l a rt ovorJ . I fweu s eALt oe l i mi n a t e ∂D ∂t J = ∇× H− ( 1 0 . 8 3 ) a n di n t e g r a t eov e rav o l u meofs p a c et oc omp u t et h er a t et h ee l e c t r oma g n e t i cf i e l di s d oi n gwor kwi t h i nt h a tv ol u me : ∂D 3 P= J·Edx 0= 3 0 ∂t dx E · ( ∇× H) − E · ( 1 0 . 8 4 ) Us i n g : ( 1 0 . 8 5 ) ∇· ( E × H) = H· ( ∇× E ) − E · ( ∇× H) ( wh i c hc a nb ee a s i l ys h ownt ob et r u ea sa ni d e n t i t yb yd i s t r i b u t i n gt h ed e r i v a t i v e s ) a n dt h e nu s eF Lt oe l i mi n a t e∇×E , on eg e t s : ∂D 3 J·Edx ∇·( E× 0=− ∂B 3 0 ∂t +H· ∂t dx H) + E · ( 1 0 . 8 6 ) I ti se a s yt os e et h a t : ∂E · E ∂t =2 E 1 ∂B· B =2 H ǫ µ ∂t ∂D ∂t ( 1 0 . 8 7 ) ∂B · ∂t ( 1 0 . 8 8 ) · f r om wh i c hwes e et h a tt h e s et e r msa r et h et i mede r i v a t i v eoft h ee l e c t r o ma gn e t i c f i e l de n e r g yd e n s i t y : η= 1ǫE · E + B= 1B· E · D+ B· H) 1( ( 1 0 . 8 9 ) 2 2µ 2 Mov i n gt h es i g nt ot h eot h e rs i d eoft h ep owe re qu a t i ona b ov e , weg e t : ∂η JE3 −V · dx 0=V 3 0 ∇· ( E × H) + ∂t dx a st h er a t ea twh i c hp owe rf l owsou toft h ev ol u meV( wh i c hi sa r b i t r a r y ) . E qu a t i n gt h et e r msu n de rt h ei n t e g r a l : ( 1 0 . 9 0 ) ∂η ∇· S= − J· E ∂t+ ( 1 0 . 9 1 ) wh e r ewei n t r od u c et h ePoy n t i n gv e c t o r S= E × H ( 1 0 . 9 2 ) Th i sh a st h ep r e c i s ea p p e a r a n c eofc on s e r v a t i onl a w.I fwea p p l yt h ed i v e r g e n c e t h e or e mt ot h ei n t e g r a l f or mt oc h a n g et h ev ol u mei n t e g r a l oft h ed i v e r g e n c eofSi n t oa s u r f a c ei n t e g r a l ofi t sf l u x : ∂ nˆdA+ σS· ∂ t Ed V=0 σJ· ηd V+ V/ ( 1 0 . 9 3 ) wh e r eσi st h ec l os e ds u r f a c et h a tb ou n d st h ev ol u meV.Ei t h e rt h edi ffe r e n t i a lor i n t e g r a l f or msc on s t i t u t et h ePo y n t i n gTh e o r e m. I nwor d s , t h es u moft h ewor kd on eb ya l lf i e l d sonc h a r g e si nt h ev ol u me , p l u st h e c h a n g e si nt h ef i e l de n e r g ywi t h i nt h ev ol u me ,p l u st h ee n e r g yt h a tf l owsou toft h e v ol u mec a r r i e db yt h ef i e l dmu s tb a l a n c e–t h i si sav e r s i onoft h ewor k e n e r gy t h e o r e m, b u ton ee x p r e s s e di nt e r mso ft h ef i e l d s . V/ σ I nt h i si n t e r p r e t a t i on ,we s e et h a tS mu s tb et h ev e c t ori n t e n s i t yoft h e e l e c t r oma g n e t i cf i e l d–t h ee n e r g yp e ru n i ta r e ap e ru n i tt i me–s i n c et h ef l u xoft h e Poy n t i n gv e c t ort h r ou g ht h es u r f a c ei st h ep owe rp a s s i n gt h r ou g hi t .I t ’ sma g n i t u d ei s t h ei n t e n s i t yp r op e r , b u ti ta l s ot e l l su st h ed i r e c t i onofe n e r g yf l ow. Wi t ht h i ss a i d ,t h e r ei sa tl e a s ton ea s s u mp t i oni nt h ee qu a t i on sa b ov et h a ti sn ot s t r i c t l yj u s t i f i e d ,a swea r ea s s u mi n gt h a tt h eme d i u mi sd i s p e r s i on l e s sa n dh a sn o r e s i s t a n c e .Wedon ota l l owf ore n e r g yt oa p p e a ra sh e a t , i not h e rwor d s , wh i c hs u r e l ywou l d h a p p e ni fwed r i v ec u r r e n t swi t ht h ee l e c t r i cf i e l d .Wea l s ou s e dt h ema c r os c op i cf i e l d e qu a t i on sa n de n e r g yd e n s i t i e s ,wh i c hi n v ol v eac oa r s e g r a i n e da v e r a g e ov e rt h e mi c r os c op i cp a r t i c l e st h a tma t t e ri sa c t u a l l yma deu pof–i ti st h e i rr a n d ommot i ont h a ti s t h emi s s i n gh e a t . I ts e e ms , t h e n , t h a tPoy n t i n g ’ st h e or e mi sl i k e l yt ob ea p p l i c a b l ei nami c r o s c o p i c d e s c r i p t i onofp a r t i c l e smov i n gi nav a c u u m,wh e r et h e i ri n d i v i d u a le n e r g i e sc a nb e t r a c k e da n dt a l l i e d: ∂η ∂t + ∇· S =− J· E 1 S = µ0E×B ( 1 0 . 9 4 ) ( 1 0 . 9 5 ) 2 η = 1ǫ0E 2 2 + 1B 2µ0 ( 1 0 . 9 6 ) b u tn otn e c e s s a r i l ys ou s e f u l i nma c r os c op i cme d i awi t hd y n a mi c a l di s p e r s i ont h a twe d on oty e tu n d e r s t a n d .Th e r ewec a ni d e n t i f yt h eJ·Et e r ma st h er a t ea twh i c ht h e me c h a n i c a l e n e r g yoft h ec h a r g e dp a r t i c l e st h a tma k eu pJc h a n g e sa n dwr i t e : d E d n ˆd A σS· d t = dt( E f i e l d+E me c h a n i c a l )=− ( 1 0 . 9 7 ) ( wh e r en ˆi s , r e c a l l , a nou t wa r dd i r e c t e dn or ma l )s ot h a tt h i ss a y st h a tt h er a t ea twh i c h e n e r g yf l owsi n t ot h ev ol u mec a r r i e db yt h ee l e c t r oma g n e t i cf i e l de qu a l st h er a t ea t wh i c ht h et ot a lme c h a n i c a lp l u sf i e l de n e r g yi nt h ev ol u mei n c r e a s e s .Th i si sa ma r v e l ou sr e s u l t ! Mome n t u mc a ns i mi l a r l yb ec on s i de r e d , a g a i ni nami c r os c op i cd e s c r i p t i on . Th e r ewes t a r twi t hNe wt on ’ ss e c on dl a wa n dt h eL or e n t zf or c el a w: p d d t F=q( E+ v×B)= ( 1 0 . 9 8 ) s u mmi n gwi t hc oa r s eg r a i n i n gi n t oa ni n t e g r a l a su s u a l : P d me c h d t 3 ρE+J×B) dx =V( ( 1 0 . 9 9 ) Asb e f or e , wee l i mi n a t es ou r c e su s i n gt h ei n h omog e n e ou sMEs( t h i st i mes t a r t i n gf r om t h eb e g i n n i n gwi t ht h ev a c u u mf or ms ) : P d ∂E me c h d t ∇·E ) E−ǫ0 =V ǫ0( 1 3 ∂t×B+ µ0( ∇ ×B)×Bdx ( 1 0 . 1 0 0 ) or ∂E 2 ∂t−cB × ( ∇× B). ρE + J× B= ǫ0E ( ∇· E ) +B× ( 1 0 . 1 0 1 ) Ag a i n , wed i s t r i b u t e : ∂ ∂t( E × B) = or ∂E ∂E ∂B ∂t × B+ E × ∂t ∂ ( 1 0 . 1 0 2 ) ∂B B× ∂t=− ∂t( E× B) + E × ∂t ( 1 0 . 1 0 3 ) 2 s u b s t i t u t ei ti na b ov e , a n da d dcB( ∇·B)=0 : ρE+J×B 2 = ǫ0 E ( ∇·E )+cB( ∇·B) ∂ ∂B ∂ t −∂ E×B)+E× t( 2 − cB×( ∇×B). ( 1 0 . 1 0 4 ) F i n a l l y , s u b s t i t u t i n gi nF L : ρE+J×B 2 = ǫ0 E ( ∇·E )+cB( ∇·B) 2 − E×( ∇×E )−cB×( ∇×B) ∂ − ǫ0∂t( E × B) ( 1 0 . 1 0 5 ) Re a s s e mb l i n ga n dr e a r r a n g i n g : P d me c h d t d EB d V =ǫ0 + dtǫ0 V( × ) 2 V E ( ∇ · E ) − E × ( ∇× E ) + 2 cB( ∇·B)−cB×( ∇×B) d V ( 1 0 . 1 0 6 ) Th equ a n t i t yu n d e rt h ei n t e g r a l ont h el e f th a su n i t sofmome n t u md e n s i t y . Wede f i n e : 1 1 g=ǫ0( E×B)=ǫ0µ ( E×H)= c2 S c2( 0 E × H) = ( 1 0 . 1 0 7 ) t ob et h ef i e l d mome n t u m d e n s i t y .Pr ov i n gt h a tt h er i gh th a n ds i d e oft h i s i n t e r p r e t a t i oni sc on s i s t e n twi t ht h i si sa c t u a l l ya ma z i n g l yd i ffic u l t .I ti ss i mp l e rt oj u s t d e f i n et h eMa x we l l St r e s sTe n s or : 2 Tαβ=ǫ0 EαE Bβ− β+cB α 1 2 ( E·E+cB·B) δ 2 α β ( 1 0 . 1 0 8 ) I nt e r msoft h i s , wi t hal i t t l ewor kon ec a ns h owt h a t : P dP ( fiel d+ me c h a n i c a l ) α= ˆ Tαβnβd A dt ( 1 0 . 1 0 9 ) S β Th a ti s , f ore a c hc omp on e n t , t h et i mer a t eofc h a n g eoft h et ot a l mome n t u m( f i e l dp l u s me c h a n i c a l )wi t h i nt h ev ol u mee qu a l st h ef l u xoft h ef i e l dmome n t u mt h r ou g ht h e c l os e ds u r f a c et h a tc on t a i n st h ev ol u me . Iwi s ht h a tIc ou l dd ob e t t e rwi t ht h i s ,b u ta n a l y z i n gt h eMa x we l lSt r e s sTe n s or t e r mwi s et ou n d e r s t a n dh owi ti sr e l a t e dt of i e l dmome n t u mf l owi ss i mp l yd i ffic u l t .I t wi l la c t u a l l yma k e mor es e n s e ,a n db ee a s i e rt od e r i v e ,wh e n we f or mu l a t e e l e c t r od y n a mi c sr e l a t i v i s t i c a l l ys owewi l l wa i tu n t i l t h e nt od i s c u s st h i sf u r t h e r . 10. 4 Ma gn e t i cMon op o l e s L e tu st h i n kf oramome n ta b ou twh a tME smi g h tb ec h a n g e di n t oi fma g n e t i c mon op ol e s we r ed i s c ov e r e d .We wou l dt h e ne x p e c ta l lf ou re qu a t i on st ob e i n h omog e n e ou s : ∇·D ∂D ∇× H− ∂t ∇·H ∂H ∇× D+ ∂t GL E ) =ρe ( ( 1 0 . 1 1 0 ) e =J ( 1 0 . 1 1 1 ) ( AL ) =ρm ( GL M) ( 1 0 . 1 1 2 ) J m =− ( 1 0 . 1 1 3 ) ( F L ) or , i nav a c u u m( wi t hu n i t sofma g n e t i cc h a r g eg i v e na sa mp e r e me t e r s , a sop p os e dt o we b e r s , wh e r e1we b e r=µ mp e r e me t e r ) : 0a 1 ∇·E ∂E = ǫ0ρe ( GL E ) ( 1 0 . 1 1 4 ) ∇× B− ǫ0µ0 ∂t e =µ0J ( AL ) ( 1 0 . 1 1 5 ) =µ0ρm ( GL M) ( 1 0 . 1 1 6 ) ∇× E + ∇·B ∂B ∂t µ J F L ) 0 m ( =− ( 1 0 . 1 1 7 ) ( wh e r ewen ot et h a ti fwedi s c ov e r e da ne l e me n t a r yma g n e t i cmon op ol eofma g n i t u d e gs i mi l a rt ot h ee l e me n t a r ye l e c t r i cmon op ol a rc h a r g eofewewou l da l mos tc e r t a i n l y n e e dt oi n t r od u c ea d d i t i on a lc on s t a n t s–ora r r a n g e me n t soft h ee x i s t i n gon e s–t o e s t a b l i s hi t squ a n t i z e dma g n i t u der e l a t i v et ot h os eofe l e c t r i cc h a r g ei ns u i t a b l eu n i t s a si sd i s c u s s e ds h or t l y ) . Th e r ea r et woob s e r v a t i on swen e e dt oma k e .On ei st h a tn a t u r ec ou l db er i f ewi t h ma g n e t i cmon op ol e sa l r e a d y .I nf a c t , e v e r ys i n g l ec h a r g e dp a r t i c l ec ou l dh a v eami xof b ot he l e c t r i ca n dma g n e t i cc h a r g e .Asl on ga st h er a t i og / ei sac o n s t a n t , wewou l db e u n a b l et ot e l l . Th i sc a nb es h ownb yl ook i n ga tt h ef ol l owi n gd u a l i t yt r a n s f or ma t i o nwh i c h “ r ot a t e s ”t h ema g n e t i cf i e l di n t ot h ee l e c t r i cf i e l da si tr ot a t e st h ema g n e t i cc h a r g ei n t o t h ee l e c t r i cc h a r g e : ′ ′ os ( Θ)+Z0Hs i n ( Θ) E =Ec ( 1 0 . 1 1 8 ) Z0D =Z0Dc os ( Θ)+Bs i n ( Θ) ( 1 0 . 1 1 9 ) Z0H =− Es i n ( Θ)+Z0Hc os ( Θ) ( 1 0 . 1 2 0 ) ′ ′ ′ ′ ′ ′ Z0Ds i n ( Θ)+Bc os ( Θ) B =− wh e r eZ0= µ0 ǫ0 ( 1 0 . 1 2 1 ) i st h ei mp e da n c eoff r e es p a c e( a n dh a su n i t sofoh ms ) , a qu a n t i t yt h a t( a swes h a l l s e e )a p p e a r sf r e qu e n t l ywh e nma n i p u l a t i n gME s . Not et h a twh e nt h ea n g l eΘ=0 , weh a v et h eor di n a r yME swea r eu s e dt o.Howe v e r , a l l ofou rme a s u r e me n t soff or c ewou l dr e ma i nu n a l t e r e di fwer ot a t e db yΘ=π/ 2a n dE ′ =Z0Hi nt h eol ds y s t e m. Howe v e r , i fwep e r f or ms u c har ot a t i on , wemu s ta l s or ot a t et h ec h a r g e d i s t r i b u t i on si ne x a c t l yt h es a mewa y : ′ ′ Z0ρe =Z0ρ os ( Θ)+ρ i n ( Θ) ec ms ′ ′ ρ = Zρ cos( Θ)+ρ s i n ( Θ) − 0e m m ( 1 0 . 1 2 2 ) ( 1 0 . 1 2 3 ) Z0J J os ( Θ)+J i n ( Θ) e =− ec ms ( 1 0 . 1 2 4 ) ′ ′ ′ ′ J Z0J i n ( Θ)+J os ( Θ) m =− es mc ( 1 0 . 1 2 5 ) I ti sl e f ta sa ne x e r c i s et os h owt h a tt h emon op ol a rf or msofME sa r el e f ti n v a r i a n t –t h i n g sc omei nj u s tt h er i g h tc omb i n a t i on sonb ot hs i d e sofa l le qu a t i on st o a c c omp l i s ht h i s .I nan u t s h e l l ,wh a tt h i sme a n si st h a ti ti sme r e l yama t t e rof c on v e n t i ont oc a l la l lt h ec h a r g eofap a r t i c l ee l e c t r i c .Byr ot a t i n gt h r ou g ha na r b i t r a r y a n g l et h e t ai nt h ee qu a t i on sa b o v e ,we c a nr e c ov e ra ne qu i v a l e n tv e r s i on of e l e c t r od y n a mi c swh e r ee l e c t r on sa n dp r ot on sh a v eon l yma g n e t i cc h a r g ea n dt h e e l e c t r i cc h a r g ei sz e r oe v e r y wh e r e ,b u twh e r ea l lf or c e sa n de l e c t r on i cs t r u c t u r e r e ma i n su n c h a n ge da sl on ga sa l l p a r t i c l e sh a v et h es a meg / er a t i o . Wh e nwes e a r c hf orma g n e t i cmon op ol e s ,t h e n ,wea r er e a l l ys e a r c h i n gf orp a r t i c l e s wh e r et h a tr a t i oi sd i ffe r e n tf r omt h ed omi n a n ton e .Wea r el ook i n gf orp a r t i c l e st h a th a v e z e r oe l e c t r i cc h a r g ea n don l yama g n e t i cc h a r g ei nt h ec u r r e n tf r a mer e l a t i v et oΘ=0 . Mon op ol a rp a r t i c l e smi g h tb ee x p e c t e dt ob eab i toddf orav a r i e t yofr e a s on s–ma g n e t i c c h a r g ei sap s e u dos c a l a rqu a n t i t y , od du n de rt i mer e v e r s a l , wh e r ee l e c t r i cc h a r g ei sas c a l a r qu a n t i t y ,e v e nu n d e rt i mer e v e r s a l ,f ore x a mp l e ,f i e l dt h e or i s t swou l dr e a l l yr e a l l yl i k ef or t h e r et ob ea tl e a s to n emon op ol ei nt h eu n i v e r s e .Nob e l h u n g r yg r a d u a t es t u de n t swou l d n ’ t mi n di ft h a tmon op ol ec a mewa n de r i n gt h r ou g ht h e i rmon op ol et r a p , e i t h e r . Howe v e r , s of a r( d e s p i t eaf e wf a l s ep os i t i v er e s u l t st h a th a v ep r ov e ndu b i ou sora t a n yr a t eu n r e p e a t a b l e )t h e r ei sal a c kofa c t u a le x p e r i me n t a le v i de n c ef ormon op ol e s . L e t ’ se x a mi n ej u s tab i tofwh yt h ei d e aofmon op ol e si se x c i t i n gt ot h e or i s t s . 10. 4. 1 Di r a cMo n o p o l e s Con s i d e rae l e c t r i cc h a r g eea tt h eo r i g i na n da nmon o p ol a rc h a r g ega ta na r b i t r a r y p oi n tont h eza x i s .F r omt h eg e n e r a l i z e df or mofME s , wee x p e c tt h ee l e c t r i cf i e l dt ob e g i v e nb yt h ewe l l k n own : er ˆ 2 E= 4πǫ0r ( 1 0 . 1 2 6 ) a ta na r b i t r a r yp oi n ti ns p a c e .Si mi l a r l y , wee x p e c tt h ema g n e t i cf i e l doft h emon op ol a r c h a r g egt ob e : ′ gr ˆ B= ′ 2 4πµ0r ( 1 0 . 1 2 7 ) ′ wh e r e r= z +r. Th emo me n t u md e n s i t yoft h i sp a i roff i e l d si sg i v e na sn ot e da b ov eb y : 1 2 g= c ( E × H) ( 1 0 . 1 2 8 ) a n di fon ed r a wsp i c t u r e sa n du s e son e ’ sr i g h th a n dt od e t e r mi n ed i r e c t i on s , i ti sc l e a r t h a tt h ef i e l dmome n t u mi sd i r e c t e da r ou n dt h ee−ga x i si nt h er i g h th a n d e ds e n s e .I n f a c tt h emome n t u mf ol l owsc i r c u l a rt r a c k sa r ou n dt h i sa x i si ns u c hawa yt h a tt h ef i e l d h a san on z e r os t a t i ca n gu l a rmome n t u m. Th es y s t e mob v i ou s l yh a sz e r ot ot a lmome n t u mf r oms y mme t r y .Th i sme a n son e c a nu s ea n yor i g i nt oc omp u t et h ea n g u l a rmome n t u m.Todos o,wec omp u t et h e a n g u l a rmome n t u md e n s i t ya s : 1 r 2 c ×E × H ( 1 0 . 1 2 9 ) a n di n t e g r a t ei t : L f i e l d 1 r 2 c = = EH ×( × ) d V µ e1 0 n ˆ×( n ˆ×H) dV4 πr µ0e =− 4π 1 ˆ ( n ˆ·H)d V r H−n ( 1 0 . 1 3 0 ) ov e ra l l s p a c e . Us i n gt h ev e c t ori d e n t i t y : f( r ) ∂f r a{−nˆ ( nˆa·) }+nˆ ( nˆa·) ∂r a (·∇) n ˆ f( r )= ( 1 0 . 1 3 1 ) t h i sc a nb et r a n s f or me di n t o: e L f i e l d=− B·∇) n ˆ d V 4π ( ( 1 0 . 1 3 2 ) I n t e g r a t i n gb yp a r t s : L e e ∇ Bn ′ nB n e g ˆ 4 πz ( ·ˆ) ˆ dV− 4π Sˆ 4π ( · ) d A ( 1 0 . 1 3 3 ) Th es u r f a c et e r mv a n i s h e sf r om s y mme t r yb e c a u s enˆi sr a d i a l l ya wa yf r omt h e= or i g i na n da v e r a g e st oz e r oonal a r g es p h e r e .∇·B ob t a i n : g δr (− z)Th u swef i n a l l y f i e l d= L f i e l d= ( 1 0 . 1 3 4 ) Th e r ea r eav a r i e t yofa r g u me n t st h a ton ec a ni n v e n tt h a tl e a dst oa ni mp or t a n t c on c l u s i on .Th ea r g u me n t sdi ffe ri nd e t a i l sa n di ns ma l lwa y squ a n t i t a t i v e l y , a n ds omea r e mor ee l e g a n tt h a nt h i son e . Bu tt h i son ei sa d e qu a t et oma k et h e p oi n t . I fwer e qu i r et h a tt h i sf i e l da n g u l a rmome n t u mb eq u a n t i z e di nu n i t sof : e g z 4 π ˆ=mz ( 1 0 . 1 3 5 ) wec a nc on c l u det h a tt h ep r o du c tofe gmu s tb equ a n t i z e d .Th i si sa ni mp or t a n t c on c l u s i on !I ti son eoft h ef e wa p p r oa c h e si np h y s i c st h a tc a ng i v eu si n s i g h ta st o wh yc h a r g ei squ a n t i z e d . Th i sc on c l u s i onwa sor i g i n a l l ya r r i v e da tb y( wh oe l s e ? )Di r a c .Howe v e r ,Di r a c ’ s a r g u me n twa smor es u b t l e .Hec r e a t e damon op ol ea sade f e c tb yc on s t r u c t i n ga v e c t orp ot e n t i a lt h a tl e dt oamon o p ol a rf i e l de v e r y wh e r ei ns p a c eb u twh i c hwa s s i n gu l a ronas i n g l el i n e .Th emode lf ort h i sv e c t orp ot e n t i a lwa st h a tofa ni n f i n i t e l y l on gs ol e n oi ds t r e t c h i n gi nf r omi n f i n i t ya l on gt h e− za x i s .Th i ss ol e n oi dwa si nf a c ta s t r i n g–t h i swa si nas e n s et h ef i r s tqu a n t u ms t r i n gt h e or y . Th edi ffe r e n t i a l v e c t orp ot e n t i a l ofadi ffe r e n t i a l ma g n e t i cd i p ol emd =g d ℓ i s : µ0 1 ′ s o | −x| d Ax ()=− 4πmd ×∇ x µ0g ( 1 0 . 1 3 6 ) 1 ′ ℓ×∇ x | −x| πL d Ax ()=− 4 ( 1 0 . 1 3 7 ) Th i sc a na c t u a l l yb ee v a l u a t e di nc oor d i n a t e sf ors p e c i f i cl i n e sL , e . g .al i n ef r om− ∞t o t h eor i g i na l on gt h e− za x i s( t op u ta“ mon op ol e ” )a tt h eor i g i n .I fon et a k e st h ec u r lof t h i sv e c t orp ot e n t i a l on ed oe si n de e dg e taf i e l dof : B= µ0r ˆ 2 4πr ( 1 0 . 1 3 8 ) e v e r y wh e r eb u to nt h el i n eL ,wh e r et h ef i e l di ss i n g u l a r .I fwes u b t r a c ta wa yt h i s s i n g u l a r( b u th i g h l yc on f i n e d–t h ef i e l di s“ i n s i d e ”t h es ol e n oi dwh e r ei tc a r r i e sf l u xi n f r om− ∞)wea r el e f twi t ht h et r u ef i e l dofamon op ol ee v e r y wh e r eb u tont h i sl i n e . Di r a ci n s i s t e dt h a ta ne l e c t r onn e a rt h i smon op ol ewou l dh a v et on ot“ s e e ”t h e s i n g u l a rs t r i n g , wh i c hi mp os e dac on d i t i ononi t swa v e f u n c t i on .Th i sc on d i t i on( wh i c h l e a d st ot h es a meg e n e r a lc on c l u s i o na st h emu c hs i mp l e ra r g u me n tg i v e na b ov e )i s b e y on dt h es c op eoft h i sc ou r s e , b u ti ti sa ni n t e r e s t i n gon ea n di smu c hc l os e rt ot h e r e a la r g u me n t su s e db yf i e l dt h e or i s t swi s h i n gt oa c c omp l i s ht h es a met h i n gwi t ha g a u g et r a n s of or ma t i ona n dI e n c ou r a g ey out or e a di ti ne . g . J a c k s onore l s e wh e r e . Ch a p t e r11 Pl a n eWa v e s 11. 1 Th eF r e eSp a c eWa v eE q u a t i o n 11. 1. 1 Ma x we l l ’ sE qu a t i on s E l e c t r od y n a mi c si st h es t u d yoft h ee n t i r ee l e c t r oma g n e t i cf i e l d .Weh a v el e a r n e df ou r di s t i n c tdi ffe r e n t i a l ( ori n t e g r a l )e qu a t i on sf ort h ee l e c t r i ca n dma g n e t i cf i e l ds : Ga u s s ’ s L a wsf orEl e c t r i c i t ya n df orMa g n e t i s m, Amp e r e ’ sL a w( wi t ht h eMa x we l lDi s p l a c e me n t Cu r r e n t )a n dF a r a d a y ’ sL a w. Col l e c t i v e l y , t h e s ea r ek n owna s : Ma x we l l ’ sE q u a t i o n s( ME ) ∇· D=ρ ∂D ( 1 1 . 1 ) ∂t =J ( 1 1 . 2 ) ∇·B ∂B =0 ( 1 1 . 3 ) =0 ( 1 1 . 4 ) ∇× H− ∇× E + ∂t Th e s ee qu a t i on sa r ef or mu l a t e da b ov ei ni nSI u n i t s , wh e r eD=ǫEa n d H =B/ µ.ǫ, r e c a l l , i st h ep e r mi t t i v i t yoft h eme d i u m, wh e r eµi sc a l l e dt h ep e r me a b i l i t yof t h eme d i u m.E i t h e roft h e mc a ni ng e n e r a lv a r ywi t he . g .p os i t i onorwi t hf r e qu e n c y , a l t h ou g hwewi l l i n i t i a l l yc on s i d e rt h e mt ob ec on s t a n t s .I n −12 de e d, wewi l l of t e nwor kwi t ht he mi nav ac uum, whe r eǫ0=8. 854×10 − 7 a n dµ0=4 π×1 0 r e s p e c t f u l l y . N 2 A a r et h ep e r mi t t i v i t ya n dp e r me a b i l i t yoff r e es p a c e , Th e ya r er e l a t e dt ot h e( c on s i d e r a b l ye a s i e rt or e me mb e r )e l e c t r i ca n dma g - 2 C 2 N−m 9 3 n e t i cc on s t a n t sb y : k = k m = e s ot h a t 1 4πǫ0 µ0 4 π 1 =9 9 × 10 2 N−m ( 1 1 . 5 ) 2 C −7 N 2 =1 0 k e ( 1 1 . 6 ) A 8 m 2 sec 3 × 1 0 m= c=√ ǫ0µ0 = k ( 1 1 . 7 ) Byt h i sp oi n t , r e me mb e r i n gt h e s es h ou l db es e c on dn a t u r e , a n dy ous h ou l dr e a l l yb e a b l et of r e e l yg ob a c ka n df or t hb e t we e nt h e s ea n dt h e i ri n t e g r a lf or mu l a t i on ,a n d d e r i v e / j u s t i f yt h eMa x we l lDi s p l a c e me n tc u r r e n ti nt e r msofc h a r g ec on s e r v a t i on ,e t c . Not et h a tt h e r ea r et woi n h omoge n e ou s( s ou r c e c on n e c t e d )e qu a t i on sa n dt wo h omoge n e ou s( s ou r c e f r e e )e qu a t i on s ,a n dt h a ti ti st h ei n h omo ge n e ou sf or mst h a t a r eme d i u md e p e n d e n t .Th i si ss i g n i f i c a n tf orl a t e r ,r e me mb e ri t .Not ea l s ot h a ti f ma g n e t i cmon op ol e swe r ed i s c ov e r e dt omor r ow,wewou l dh a v et oma k ea l lf ou r e qu a t i on si n h omog e n e ou s , a n di n c i d e n t a l l yc omp l e t e l ys y mme t r i c . F ort h emome n t ,l e tu se x p r e s st h ei n h omog e n e ou sMEsi nt e r msoft h ee l e c t r i c f i e l dE=ǫDa n dt h ema g n e t i ci n du c t i onB=H/ µd i r e c t l y : ρ ∇× B− µǫ ∇× E + ∇·E ∂E ∂t =ǫ ( 1 1 . 8 ) =µJ ( 1 1 . 9 ) ∇·B ∂B =0 ( 1 1 . 1 0 ) ∂t =0 ( 1 1 . 1 1 ) 1 I ti sd i ffic u l tt oc on v e yt oy ouh owi mp or t a n tt h e s ef ou re qu a t i on sa r eg oi n gt ob et o u sov e rt h ec ou r s eoft h es e me s t e r .Ov e rt h en e x tf e wmon t h s , t h e n , wewi l l ma k eMa x we l l ’ s E qu a t i on sd a n c e ,wewi l lma k et h e ms i n g ,wewi l l“ mu t i l a t e ”t h e m( t u r nt h e mi n t od i s t i n c t c ou p l e de q u a t i on sf ort r a n s v e r s ea n dl on g i t u d i n a lf i e l dc omp on e n t s ,f ore x a mp l e ) ,wewi l l c ou p l et h e m,wewi l lt r a n s f or mt h e mi n t oama n i f e s t l yc ov a r i a n tf or m,wewi l ls ol v et h e m mi c r os c op i c a l l yf orap o i n t l i k ec h a r g ei ng e n e r a l mot i on . Wewi l l t r yv e r yh a r dt ol e a r nt h e m. F ort h en e x tt woc h a p t e r swewi l l p r i ma r i l yb ei n t e r e s t e di nt h ep r op e r t i e soft h ef i e l di n r e g i on sofs p a c ewi t h ou tc h a r g e( s ou r c e s ) .I n i t i a l l y , we ’ l l f oc u sonav a c u u m, wh e r et h e r ei s n odi s p e r s i ona ta l l ; l a t e rwe ’ l l l ookab i ta tdi e l e c t r i cme di aa n ddi s p e r s i on .I nas ou r c e f r e e r e g i on , ρ=0a n dJ=0a n dweob t a i n : 1 Ori si tf ou r ?Th e s ea r ev e c t orp a r t i a l d i ffe r e n t i a l e qu a t i on s , s oon ec a nb r e a kt h e mu pi n t oe i gh td i s t i n c t e qu a t i on sr e l a t i n gp a r t i c u l a rc omp on e n t s ,a l t h ou g hi ti s n ’ tc l e a rt h a ta l le i g h twi l lb ei n de p e n d e n t . Al t e r n a t i v e l y ,a swewi l ls e el a t e r ,wec a nr e du c et h e mt oj u s tt wot e n s orp a r t i a ld i ffe r e n t i a le qu a t i on si na r e l a t i v i s t i cf o r mu l a t i on ,a n dwi l lb ea b l et os e eh owon emi g h tb ea b l et owr i t et h e ma sas i n gl et e n s or e qu a t i on . Ma x we l l ’ sE q u a t i o n si naSo u r c eF r e eRe gi o no fSp a c e : ∇× E + ∇·E = 0 ( 1 1 . 1 2 ) ∇·B ∂B = 0 ( 1 1 . 1 3 ) ∂t = 0 ( 1 1 . 1 4 ) = 0 ( 1 1 . 1 5 ) ∂E ∇× B− ǫµ ∂t 11. 1. 2 Th eWa v eE q u a t i o n Af t e ral i t t l ewor k( t a k et h ec u r l oft h ec u r l e qu a t i on s , u s i n gt h ei d e n t i t y : 2 ∇×( ∇×a )=∇( ∇·a )−∇a ( 1 1 . 1 6 ) a n du s i n gGa u s s ’ ss ou r c e f r e eL a ws )wec a ne a s i l yf i n dt h a tEa n dBi nf r e es p a c e s a t i s f yt h ewa v ee q u a t i o n : ∇2u− 1∂2u =0 ( 1 1 . 1 7 ) 2 2 v ∂t ( f oru=Eoru=B)wh e r e 1 2 ( 1 1 . 1 8 ) v=√µ ǫ. Th ewa v ee qu a t i ons e p a r a t e sf orh a r mon i cwa v e sa n dwec a na c t u a l l ywr i t et h e f ol l owi n gh omog e n e ou sPDEf orj u s tt h es p a t i a l p a r tofEorB: 2 ω ∇2 2 +v 2 2 2 2 E=∇ +k E=0 ω2 2 ∇ 2 +v B=∇ +k B=0 − i ωt wh e r et h et i med e p e n d e n c ei si mp l i c i t l ye a n dwh e r ev=ω/ k . Th i si sc a l l e dt h eh omoge n e ou sHe l mh o l t ze qu a t i on( HHE )a n dwe ’ l l s p e n dal otoft i me s t u d y i n gi ta n di t si n h omog e n e ou sc ou s i n .Not et h a ti tr e d u c e si nt h ek→ 0l i mi tt ot h e f a mi l i a rh omog e n e ou sL a p l a c ee qu a t i on , wh i c hi sb a s i c a l l ya s p e c i a l c a s eoft h i sPDE. 3 Ob s e r v i n gt h a t: i k n ˆ · x= i k n ˆ · x ∇e i k n ˆ e ( 1 1 . 1 9 ) 2 i ωt I nc a s ey ou ’ v ef or g ot t e n : Tr yas ol u t i ons u c ha su ( x , t )=X( x ) Y( y ) Z( z ) T( t ) , or( wi t hab i tofi n s p i r a t i on )E ( x ) e − i nt h edi ffe r e n t i a le qu a t i on .Di v i d eb yu .Youe n du pwi t hab u n c hoft e r mst h a tc a ne a c hb ei d e n t i f i e da sb e i n g c on s t a n ta st h e yde p e n donx , y , z , ts e p a r a t e l y .F oras u i t a b l ec h oi c eofc on s t a n t son eob t a i n st h ef ol l owi n gPDEf or s p a t i a l p a r tofh a r mon i cwa v e s . 3 Ye s , y ous h ou l dwor kt h i sou tt e r mwi s ei fy ou ’ v en e v e rd on es ob e f or e . Don ’ tj u s tt a k emywor df ora n y t h i n g. wh e r en ˆi sau n i tv e c t or , wec a ne a s i l ys e et h a tt h ewa v ee qu a t i onh a s( a mon gma n y , 3 ma n yot h e r s )as ol u t i ononI Rt h a tl ook sl i k e : i ( k n ˆ · x −ωt ) 0 u ( x , t )=u e ( 1 1 . 2 0 ) wh e r et h ewa v en u mb e rk=k n ˆh a st h ema g n i t u d e ω k= v=√ µǫω ( 1 1 . 2 1 ) a n dp oi n t si nt h edi r e c t i onofp r op a g a t i onoft h i sp l a n ewa v e . 11. 1. 3 Pl a n eWa v e s Pl a n ewa v e sc a np r op a g a t ei na n yd i r e c t i on .An ys u p e r p os i t i onoft h e s ewa v e s , f ora l l p os s i b l eω, k , i sa l s oas ol u t i ont ot h ewa v ee qu a t i on .Howe v e r , r e c a l l t h a tEa n dBa r e n oti n de p e n d e n t , wh i c hr e s t r i c t st h es ol u t i oni ne l e c t r od y n a mi c ss ome wh a t . Tog e taf e e l f ort h ei n t e r de p e n d e n c eofEa n dB, l e t ’ sp i c kk=± k x ˆs ot h a te . g . : E ( x , t ) i ( k x −ωt )+ i ( −k x −ωt ) + −e e E =E ( 1 1 . 2 2 ) B( x , t ) i ( k x −ωt )+ i ( −k x −ωt ) + e B−e =B ( 1 1 . 2 3 ) wh i c ha r ep l a n ewa v e st r a v e l l i n gt ot h er i g h torl e f ta l on gt h ex a x i sf ora n yc omp l e xE , + E B+, B−. I non ed i me n s i on , a tl e a s t , i ft h e r ei sn od i s p e r s i onwec a nc on s t r u c ta −, f ou r i e rs e r i e soft h e s es ol u t i on sf orv a r i ou skt h a tc on v e r g e st oa n ywe l l –b e h a v e d f u n c t i onofas i n g l ev a r i a b l e . [ Not ei np a s s i n gt h a t : u ( x , t )=f( x−v t )+g ( x+v t ) ( 1 1 . 2 4 ) f ora r b i t r a r ys moot hf( z )a n dg ( z )i st h emo s tg e n e r a l s ol u t i onoft h e1 d i me n s i on a l wa v e e qu a t i on . An ywa v e f or mt h a tp r e s e r v e si t ss h a p ea n dt r a v e l sa l on gt h ex a x i sa ts p e e dv i sas ol u t i ont ot h eon ed i me n s i on a l wa v ee qu a t i on( a sc a nb ev e r i f i e ddi r e c t l y , ofc ou r s e ) . Howb or i n g ! Th e s ep a r t i c u l a rh a r mon i cs ol u t i on sh a v et h i sf or m( v e r i f yt h i s ) . ] I ft h e r ei sd i s p e r s i on( wh e r et h ev e l oc i t yoft h ewa v e si saf u n c t i onoft h ef r e qu e n c y ) t h e nt h ef ou r i e rs u p e r p o s i t i oni sn ol on g e rs t a b l ea n dt h el a s te qu a t i onn ol on ge rh o l d s . E a c hf ou r i e rc omp on e n ti ss t i l la ne x p on e n t i a l ,b u ta l lt h ev e l oc i t i e soft h ef ou r i e r c omp on e n t sa r ed i ffe r e n t .Asac on s e qu e n c e ,a n yi n i t i a l l yp r e p a r e dwa v ep a c k e ts p r e a d s ou ta si tp r op a g a t e s .We ’ l l l ooka tt h i ss h or t l y( i nt h eh ome wor k )i ns omed e t a i lt os e eh ow t h i swor k sf orav e r ys i mp l e( g a u s s i a n )wa v ep a c k e tb u tf orn owwe ’ l l mov eon . Not et h a tEa n dBa r ec on n e c t e db yh a v i n gt os a t i s f yMa x we l l ’ se qu a t i on se v e ni f t h ewa v ei st r a v e l l i n gi nj u s ton ed i r e c t i on( s a y , i nt h ed i r e c t i onofau n i tv e c t orn ˆ ) ; we c a n n otc h oos et h ewa v ea mp l i t u d e ss e p a r a t e l y . Su p p os e i ( k n ˆ · x −ωt ) E x ( )= E e , t i ( k n ˆ · x −ωt ) Bx (, t ) = Be wh e r eE ,B,a n dn ˆa r ec on s t a n tv e c t or s( wh i c hma yb ec omp l e x ,a tl e a s tf ort h e mome n t ) . 2 2 Not et h a ta p p l y i n g( ∇ +k)t ot h e s es ol u t i on si nt h eHHEl e a d su st o: 2 ω 2 2 2 knˆ·nˆ=µǫω = v ( 1 1 . 2 5 ) a st h ec on d i t i onf oras ol u t i on .Th e nar e a ln ˆ·n ˆ=1l e a dst ot h ep l a n ewa v es ol u t i on ω i n d i c a t e da b ov e ,wi t hk= v,wh i c hi st h emos tf a mi l i a rf or moft h es ol u t i on( b u tn ot t h eon l yon e ) ! Th i sh a smos t l yb e e n“ ma t h e ma t i c s ” ,f ol l owi n gmor eorl e s sd i r e c t l yf r om t h ewa v e e qu a t i on .Th es a mer e a s on i n gmi g h th a v eb e e na p p l i e dt os ou n dwa v e s ,wa t e rwa v e s , wa v e sonas t r i n g ,or“ wa v e s ”u ( x ,t )ofn ot h i n gi np a r t i c u l a r .Nowl e t ’ su s es omep h y s i c s a n ds e ewh a ti tt e l l su sa b ou tt h ep a r t i c u l a re l e c t r oma g n e t i cwa v e st h a tf ol l ow f r om Ma x we l l ’ se qu a t i on st u r n e di n t ot h ewa v ee qu a t i on .Th e s ewa v e sa l ls a t i s f ye a c hof Ma x we l l ’ se qu a t i on ss e p a r a t e l y . F ore x a mp l e , f r omGa u s s ’ L a wswes e ee . g . t h a t : ∇ · E · Eei(knˆ·x−ωt) ∇·E =0 =0 · x −ωt ) i ( k n ˆ ∇e =0 i k Ene =0 · x −ωt ) i ( knˆ ( 1 1 . 2 6 ) · or( d i v i d i n gou tn on z e r ot e r msa n dt h e nr e p e a t i n gt h er e a s on i n gf orB) : n ˆ·E=0a n dn ˆ·B=0 . ( 1 1 . 2 7 ) Wh i c hb a s i c a l l yme a n sf orar e a lu n i tv e c t o rn ˆt h a tEa n dBa r ep e r p e n d i c u l a rt on ˆ ,t h e d i r e c t i onofp r op a g a t i on !Ap l a n ee l e c t r oma g n e t i cwa v ei st h e r e f or eat r a n s v e r s ewa v e . Th i ss e e msl i k ei ti sa ni mp or t a n tt h i n gt ok n ow, a n di sn ota ta l l ama t h e ma t i c a l c on c l u s i on oft h ewa v ee qu a t i onp e rs e . Re p e a t i n gt h i ss or toft h i n gu s i n gon eoft h et h ec u r le qn s( s a y , F a r a d a y ’ sl a w)on e g e t s : √ B= µǫn ˆ E ( 1 1 . 2 8 ) × √ ( t h ei c a n c e l s , k / ω=1 / v= ǫµ ) .Th i sme a n st h a tEa n dBh a v et h es a mep h a s ei fn ˆi s 4 r e a l I fn ˆi sar e a l u n i tv e c t ori n3 s p a c e , t h e nwec a ni n t r odu c et h r e er e a l , mu t u a l l y or t h og on a l u n i tv e c t or s( ǫˆ, ǫ ˆ, n ˆ )s u c ht h a tǫ ˆ× ǫ ˆ=n ˆa n du s et h e mt oe x pr e s s 1 2 1 2 t h ef i e l ds t r e n g t h s : ǫE0 E1=ǫˆ ˆ 1E 0,B 1=ǫ 2 √µ ( 1 1 . 2 9 ) a n d ′ ′ ǫE0 E2=ǫˆ ǫˆ 2E 0,B 2=− 1 √µ ( 1 1 . 3 0 ) 4 Wh oop s ! Youme a nn ˆd oe s n ’ th a v et ob er e a l ?Se eb e l ow. Not ea l s ot h a twea r ei mp l i c i t l ya s s u mi n gǫa n dµ a r er e a l a swe l l , a n dt h e ydon ’ th a v et ob ee i t h e r ! ′ wh e r eE n dE r ec on s t a n t st h a tma yb ec omp l e x . I ti swor t hn ot i n gt h a t 0a 0a | E | =v | B| ( 1 1 . 3 1 ) h a v et h es a med i me n s i o n sa n dt h a tt h ema g n i t u d eoft h ee l e c t r i cf i e l di sg r e a t e rt h a n t h a toft h ema g n e t i cf i e l dt owh i c hi ti sc ou p l e dv i aMa x we l l ’ sE qu a t i on sb yaf a c t orof t h es p e e dofl i g h ti nt h eme di u m, a st h i swi l l b eu s e dal oti ne l e c t r ody n a mi c s . Weh a v ec a r e f u l l yc h os e nt h ep ol a r i z a t i ond i r e c t i on ss ot h a tt h e( t i me a v e r a g e d ) Poy n t i n gv e c t orf ora n yp a r t i c u l a rc omp on e n tp a i rp oi n t si nt h ed i r e c t i onofp r op a g a t i on , n ˆ : 1 ∗ S = 2 E×H 1 ( 1 1 . 3 2 ) ∗ = 2µ E×B = √ǫµ 2 µ 1 ∗ E×v B ǫ 2 µ |E0|nˆ = 2 Not ewe l l t h ec omb i n a t i on ( 1 1 . 3 3 ) ( 1 1 . 3 4 ) ( 1 1 . 3 5 ) ǫ , a si twi l l oc c u rr a t h e rf r e qu e n t l yi nou ra l g e b r a µ b e l ow, s omu c hs ot h a twewi l l g i v ei tan a meofi t sownl a t e r .Somu c hf ort h e“ s i mp l e ” mon oc h r oma t i cp l a n ewa v ep r op a g a t i n gc oh e r e n t l yi nad i s p e r s i on l e s sme di u m. 5 Now, k i n k ya si tma ys e e m, t h e r ei sn or e a lr e a s ont h a tk=k n ˆc a n n otb ec omp l e x ( wh i l ekr e ma i n sr e a l ! )Asa ne x e r c i s e ,f i g u r eou tt h ec omp l e xv e c t orofy ou rc h oi c e s u c ht h a t n ˆ·n ˆ=1 . ( 1 1 . 3 6 ) Di dy oug e tt h a t ?Wh a t ,y oud i d n ’ ta c t u a l l yt r y ?Se r i ou s l y ,y ou ’ r eg oi n gt oh a v et oa t l e a s tt r yt h el i t t l emi n i e x e r c i s e sIs u g g e s ta l on gt h ewa yt og e tt h emos tou toft h i s b ook . Ofc ou r s e ,Id i dn ’ tr e a l l ye x p e c tf ory out owor ki tou tons u c has p a r s eh i n t ,a n d b e s i d e s , y oug ot t as a v ey ou rs t r e n g t hf ort h er e a l p r ob l e msl a t e rb e c a u s ey ou ’ l l n e e di t t h e n .Sot h i st i me ,I ’ l lwor ki tou tf ory ou .Th eh i n twa s ,p r e t e n dt h a tn ˆi sc omp l e x . Th e ni tc a nb ewr i t t e na s : nˆ=nˆ nˆ R+i I 2 5 He h , h e h . 2 ( 1 1 . 3 7 ) nR −nI=1 ( 1 1 . 3 8 ) nˆ nˆ . R· I=0 ( 1 1 . 3 9 ) So, n ˆ s tb eor t h og on a l t on ˆ n dt h ed i ffe r e n c eoft h e i rs qu a r e smu s tb eon e . F ore x a mp l e : Rmu Ia √ ˆ ( 1 1 . 4 0 ) ˆ ˆ ˆ n j R=2in I=1 wor k s ,a sd oi n f i n i t e l ymor eMor eg e n e r a l l y( r e c a l l i n gt h ep r op e r t i e sofh y b e r b ol i c s f u n c t i on s ) : nˆ=eˆ1coshθ+i eˆ2si nhθ ( 1 1 . 4 1 ) wh e r et h eu n i tv e c t or sa r eor t h og on a l s h ou l dwor kf ora n yθ. Th u st h emos tge n e r a l Es u c ht h a tn·E=0i s E=( i eˆ1si nhθ−eˆ2coshθ) A+eˆ3B ( 1 1 . 4 2 ) wh e r e( s i g h )Aa n dBa r ea g a i n , a r b i t r a r yc omp l e xc on s t a n t s .Not et h a ti fn ˆi sc omp l e x , t h ee x p on e n t i a l p a r toft h ef i e l d sb e c ome s : n ˆx i ( k e n ˆx n ˆx −k ·e i ( k =e ·−ωt ) I . R·− ωt ) ( 1 1 . 4 3 ) Th i si n h o moge n e ou sp l a v ewa v ee x p on e n t i a l l yg r owsord e c a y si ns omedi r e c t i onwh i l e r e ma i n i n ga“ p l a n ewa v e ”i nt h eot h e r( p e r p e n di c u l a r )d i r e c t i on . F or t u n a t e l y ,n a t u r ep r ov i d e su swi t hf e ws ou r c e sa n da s s oc i a t e dme d i at h a tp r od u c e t h i sk i n dofb e h a v i or( i ma g i n a r yn ˆ ?J u s ti ma g i n e ! )i ne l e c t r od y n a mi c s .Sol e t ’ sf or g e ti tf or t h emome n t ,b u tr e me mb e rt h a ti ti st h e r ef orwh e ny our u ni n t oi ti nf i e l dt h e or y ,or ma t h e ma t i c s , orc a t a s t r op h et h e or y . Wet h e r e f or er e t u r nt oamor emu n d a n ea n dn a t u r a ld i s c u s s i onoft h ep os s i b l e p ol a r i z a t i on sofap l a n ewa v ewh e nn ˆi sar e a lu n i tv e c t or ,c on t i n u i n gt h er e a s on i n g a b ov eb e f or eou rl i t t l ei ma g i n a r yi n t e r l u d e . 11. 1. 4 Pol a r i z a t i o nofPl a n eWa v e s We ’ v er e a l l ydon ea l loft h eh a r dwor ka l r e a d yi ns e t t i n gt h i n g su pa b ov e( a n di twa s n ’ tt oo h a r d ) .I n de e d ,t h eE n dE e f i n e daf e we qu a t i on sb a c ka r ej u s tt woi n d e p e n de n t 1a 2d p ol a r i z a t i on sofat r a n s v e r s ep l a n ewa v e .Howe v e r ,wen e e dt oe x p l or et h er e s toft h e p h y s i c s , a n du n d e r s t a n dj u s twh a ti sg oi n goni nt h ewh ol ee l e c t r ody n a mi cf i e l da n dn otj u s t t h ee l e c t r i cf i e l dc omp on e n tofs a me . L e t ’ ss t a r tb ywr i t i n gEi naf a i r l yg e n e r a l wa y : i ( k · x −ωt ) E ˆ E e i=ǫ i i ( 1 1 . 4 4 ) wh e r ey ouwi l ln ot et h a tweh a v ec o n v e r t e dov e rt ot h en ot a t i onk=k n ˆwi t hn ˆr e a l , s i n c et h e r ei sn or e a l r e a s ont ot r e a tn ˆs e p a r a t e l yf orawh i l e .Th e nwec a nt u r n( a swe wi l l ,ov e ra n dov e ri nt h ep a g e sa h e a d )t ot h ee i t h e roft h ec u r lMEst of i n d( u s i n g F a r a d a y ’ sL a wi nt h i sc a s e ) : d i r e c t i on sofp ol a r i z a t i on kE Bi=√µ ǫ ×i p e r p e n di c u l a rt ok . k wi t hE ǫˆ ori =1 , 2s u c ht h a te ˆ ˆ ˆ ˆ=i n de p e n d e n t i=E i if 1×e 2=e 3=n ( 1 1 . 4 5 ) k ort h et wo kf Th e ng e n e r a l l y , i ( k · x −ωt ) t )=( ǫ ˆ E ˆ E2) e 1 1+ǫ 2 Ex( , 1 i ( k · x −ωt ) ǫˆ2E1 −ǫ ˆ E2) e 1 Bx (, t )= v( ( 1 1 . 4 6 ) ( 1 1 . 4 7 ) wh e r eE n dE r e( a su s u a l )c omp l e xa mp l i t u d e ss i n c et h e r ei sn or e a s on( e v e ni n 1a 2a n a t u r e )t oa s s u met h a tt h ef i e l d sp ol a r i z e di nd i ffe r e n td i r e c t i on sh a v et h es a mep h a s e . ( Not et h a tac omp l e xEc or r e s p on d st oas i mp l ep h a s es h i f ti nt h ee x p on e n t i a l ,s e e p r e l i mi n a r ys e c t i ononc omp l e xn u mb e r si ft h i si sn otc l e a r . ) Th ep ol a r i z a t i onoft h ep l a n ewa v ede s c r i b e st h er e l a t i v ed i r e c t i on ,ma gn i t u d e , a n dp h a s eoft h ee l e c t r i cp a r toft h ewa v e . Weh a v es e v e r a l we l l k n ownc a s e s : a )I fE1a n dE a v et h es a mep h a s e( b u ta r b i t r a r i l ydi ffe r e n tma g n i t u d e s )weh a v eL i n e a r 2h Pol a r i z a t i o noft h eEf i e l dwi t ht h ep ol a r i z a t i onv e c t orma k i n ga na n g l eθ= − 1 2 2 t a n( E / E )wi t hǫ1 a n dma g n i t u d eE=E1 +E2.F r e qu e n t l ywewi l lc h oos e 2 1 c oor d i n a t e si nt h i sc a s es ot h a t( s a y )E . 2=0 b )I fE1a n dE a v edi ffe r e n tp h a s e sa n ddi ffe r e n tma g n i t u d e s ,weh a v eE l l i p t i c a l 2h Pol a r i z a t i o n .I ti sf a i r l ye a s yt os h owt h a tt h ee l e c t r i cf i e l ds t r e n g t hi nt h i sc a s e t r a c e sou ta ne l l i p s ei nt h e1 , 2p l a n e . c )As p e c i a lc a s eofe l l i p t i c a lp ol a r i z a t i onr e s u l t swh e nt h ea mp l i t u d e sa r eou tof p h a s eb yπ/ 2a n dt h ema g n i t u de sa r ee qu a l .I nt h i sc a s eweh a v eCi r c u l a r i π/ 2 Pol a r i z a t i on . Si n c ee t h ef or m: =i , i nt h i sc a s eweh a v eawa v eof E0 E=√( ǫˆ±i ǫˆ )=E ǫˆ . 2 0 ± ( 1 1. 48 ) 1 2 wh e r eweh a v ei n t r od u c e dc omp l e xu n i th e l i c i t yv e c t o r ss u c ht h a t : ∗ ǫ ǫǫ ˆ · ˆ ± 3 ˆ ± ∗ ǫ ∗ ǫ ǫ ·ˆ ∓ = 0 ( 1 1 . 4 9 ) = ˆ · ˆ ± 3 = 0 ( 1 1 . 5 0 ) ǫ ǫ ˆ ˆ ± · ± = 1 ( 1 1 . 5 1 ) Aswec a ns e ef r omt h ea b ov e , e l l i p t i c a lp ol a r i z a t i onc a nh a v ep os i t i v eorn e g a t i v e h e l i c i t yde p e n d i n gonwh e t h e rt h ep ol a r i z a t i onv e c t ors wi n g sa r ou n dt h ed i r e c t i onof p r op a g a t i o nc ou n t e r c l oc k wi s eorc l oc k wi s ewh e nl ook i n gi n t ot h eon c omi n gwa v e . An ot h e rc omp l e t e l yg e n e r a lwa yt or e p r e s e n tap ol a r i z e dwa v ei sv i at h eu n i t h e l i c i t yv e c t or s : i ( k · x −ωt ) E ( x , t )=( E+ǫ ˆ ǫˆ )e ++E − − ( 1 1 . 5 2 ) I ti sl e f ta sa ne x e r c i s et op r ov et h i s . Not et h a ta sa l wa y s , E±a r ec o mp l e xa mp l i t u d e s ! 6 I ’ ml e a v i n gSt ok e sp a r a me t e r so u t ,b u ty ous h ou l dr e a da b ou tt h e m ony ou rowni n c a s ey oue v e rn e e dt h e m( ora tl e a s tn e e dt ok n owwh a tt h e ya r e ) .Th e ya r er e l e v a n tt ot h e i s s u eofme a s u r i n gmi x e dp ol a r i z a t i o ns t a t e s ,b u ta r en omor eg e n e r a lad e s c r i p t i onof p ol a r i z a t i oni t s e l ft h a ne i t h e roft h os ea b ov e . 6Wi k i p e d i a : h t t p : / / www. wi k i p e d i a . or g / wi k i / St ok e sPa r a me t e r s . 11. 2 Re f l e c t i o na n dRe f r a c t i o na taPl a n e I n t e r f a c e Su p p os eap l a n ewa v ei si n c i d e n tu p onap l a n es u r f a c et h a ti sa ni n t e r f a c eb e t we e n ′′ t woma t e r i a l s , on ewi t hµ, ǫa n dt h eot h e rwi t hµ, ǫ. z µ, ε x θθ k il k " x θ r µ’ , ε’ k ’ F i g u r e1 1 . 1 :Ge ome t r yf orr e f l e c t i ona n dr e f r a c t i ona tap l a n ei n t e r f a c eb e t we e nt wo ′′ me d i a , on ewi t hp e r mi t t i v i t y / p e r me a b i l i t yµ, ǫ, on ewi t hp e r mi t t i v i t y / p e r me a b i l i t yµ, ǫ. I nor d e rt od e r i v ea na l g e b r a i cr e l a t i on s h i pb e t we e nt h ei n t e n s i t i e soft h ei n c omi n g wa v e , t h er e f l e c t e dwa v e , a n dt h er e f r a c t e dwa v e , wemu s tb e g i nb yde f i n i n gt h ea l g e b r a i c f or m ofe a c hoft h e s ewa v e si nt e r msoft h ewa v en u mb e r s .Th er e f l e c t e dwa v ea n d i n c i de n twa v ed on otl e a v et h ef i r s tme d i u ma n dh e n c e √ ′ ′ √ ′ r e t a i ns p e e dv=1 /µǫ, µ, ǫa n dk=k=ω µ ǫ=ω/ v .Th er e f r a c t e dwa v ec h a n g e st os p e e dv= √′′ ′ ′ √′′ ′ 1 /µǫ, µ, k=ω µǫ=ω/ v. Not et h a tt h ef r e qu e n c yoft h ewa v e si st h es a mei nb ot hme d i aa sak i n e ma t i c c o n s t r a i n t ! Wh yi st h a t ? Th i sy i e l d st h ef ol l o wi n gf or msf ort h ev a r i ou swa v e s : I n c i d e n tWa v e i ( k · x −ωt ) 0 E=E e B = √µǫ k×E k ( 1 1 . 5 3 ) ( 1 1 . 5 4 ) Re f r a c t e dWa v e i ( k· x − ωt ) ′ =E ′ 0 E e′ B′ = ′×E ′ ′′ k ′ µǫ i ( k· x − ωt ) ′ ′ 0 =E e ′′ B′ ′ = √µǫ ( 1 1 . 5 6 ) k Re f l e c t e dWa v e E ′ ′ ( 1 1 . 5 5 ) ′ ′ ′ ′ k×E ( 1 1 . 5 7 ) ( 1 1 . 5 8 ) k Ou rg oa li st oc omp l e t e l yu n d e r s t a n dh owt oc omp u t et h er e f l e c t e da n dr e f r a c t e d wa v ef r omt h ei n c i d e n twa v e .Th i si sd on eb yma t c h i n gt h ewa v ea c r os st h eb ou n da r y i n t e r f a c e .Th e r ea r et woa s p e c t soft h i sma t c h i n g–as t a t i cork i n e ma t i cma t c h i n gof t h e wa v e f or m i t s e l fa n dad y n a mi c ma t c h i n ga s s oc i a t e d wi t ht h e( c h a n g i n g ) p ol a r i z a t i oni nt h eme d i u m.Th e s et wok i n d sofma t c h i n gl e a dt ot wod i s t i n c ta n dwe l l k n ownr e s u l t s . 11. 2. 1 Ki n e ma t i c sa n dSn e l l ’ sL a w Th ep h a s ef a c t or sofa l l t h r e ewa v e smu s tb ee qu a l ont h ea c t u a l b ou n d a r yi t s e l f , h e n c e : ′ ′ ′ ( kx ·) kx ·) kx ·) z = 0=( z = 0=( z = 0 ( 1 1 . 5 9 ) a sak i n e ma t i cc o n s t r a i n tf ort h ewa v et ob ec on s i s t e n t .Th a ti s , t h i sh a sn ot h i n gt od o wi t h“ p h y s i c s ”p e rs e , i ti sj u s tama t h e ma t i c a l r e qu i r e me n tf ort h ewa v ed e s c r i p t i ont o wor k .Con s e qu e n t l yi ti sg e n e r a l l yc ov e r e de v e ni nk i d d y p h y s i c sc l a s s e s ,wh e r eon e c a nde r i v eSn e l l ’ sl a wj u s tf r om p i c t u r e sofi n c i d e n twa v e sa n dt r i a n g l e sa n da k n owl e d g eoft h ewa v e l e n g t hs h i f ta s s oc i a t e dwi t ht h es p e e ds h i f twi t haf i x e d f r e qu e n c ywa v e . Atz=0 , t h et h r e ek ’ smu s tl i ei nap l a n e .Th ea n g l e sofi n c i d e n c eθ , r e f l e c t i onθ , i l a n dr e f r a c t i onθra ddt ot h ea n g l e si nt h edotp r od u c tt oma k eπ/ 2 , s ot h ec os i n ei nt h e d otp r odu c tb e c ome st h es i n eoft h e s ea n g l e sa n dweob t a i n : ′ ks i n ( θ ) i i n ( θ )=ks i n ( θl ) r =ks ns i n ( θ ) i i n ( θr )=ns i n ( θ ) l =ns ′ wh i c hi sb ot hSn e l l ’ sL a wa n dt h eL a wo fRe f l e c t i o n , ob t a i n e di non ef e l l s woop . ( 1 1 . 6 0 ) ′ ′ ′ ′ ′ Not ewe l l t h a tweu s e dk=ω/ v=n ω/ c=ka n dk=ω/ v=nω/ ct op u ti ti nt e r msof ′ ′ t h ei n d e xofr e f r a c t i on , d e f i n e db yv=c / na n dv=c / n.Th e nwec a n c e l ω/ c , u s i n gt h e f a c tt h a tt h ef r e qu e n c yi st h es a mei nb ot hme d i a . Sn e l l ’ sL a wa n dt h eL a wofRe f l e c t i ona r et h u ss e e nt ob ek i n e ma t i cr e l a t i on st h a ta r e t h er e s u l toft h er e qu i r e me n tofp h a s ec on t i n u i t yont h ep l a n ei n t e r f a c e ′ ′ ′ –a“ wa v e f r on t ”oft h ek( ork)wa v emu s tb et h es a mea st h ewa v e f r on toft h ekwa v e . 11. 2. 2 Dy n a mi c sa n dRe f l e c t i o n / Re f r a c t i o n Nowwed ot h ed y n a mi c s ,t h a ti st os a y ,t h er e a lp h y s i c s .Re a lp h y s i c si sa s s oc i a t e dwi t h t h ee q u a t i o n so fmo t i onoft h eE Mf i e l d,t h a ti s ,wi t hMa x we l l ’ se qu a t i on s ,wh i c hi nt u r n b e c omet h ewa v ee qu a t i on ,s od y n a mi c si sa s s oc i a t e dwi t ht h eb ou n da r yv a l u ep r ob l e m s a t i s f i e db yt h e( wa v ee qu a t i on )PDE s . Sowh a ta r et h os eb ou n d a r yc on d i t i on s ?Re c a l lt h a tt h ee l e c t r i cd i s pl a c e me n t p e r p e n d i c u l a rt ot h es u r f a c emu s tb ec on t i n u ou s ,t h a tt h ee l e c t r i cf i e l dp a r a l l e lt ot h e s u r f a c emu s tb ec on t i n u ou s ,t h a tt h ema g n e t i cf i e l dp a r a l l e lt ot h es u r f a c emu s tb e c on t i n u ou sa n dt h e ma g n e t i ci n d u c t i on p e r p e n d i c u l a rt ot h es u r f a c e mu s tb e c on t i n u ou s . Top u ti ta n ot h e r( mor ep h y s i c a l )wa y ,t h ep e r p e n d i c u l a rc omp on e n t soft h ee l e c t r i c f i e l dwi l l b ed i s c on t i n ou sa tt h es u r f a c ed u et ot h es u r f a c ec h a r g el a y e ra s s oc i a t e dwi t ht h e l oc a lp ol a r i z a t i onoft h eme d i u mi nr e s p on s et ot h ewa v e .Th i sp ol a r i z a t i oni sa c t u a l l yn o t i n s t a n t a n e ou s , a n di sab u l kr e s p on s eb u th e r ewewi l la s s u met h a tt h eme di u mc a nr e a c t i n s t a n t l ya st h ewa v ea r r i v e sa n dt h a tt h ewa v e l e n g t hi n c l u d e sma n ya t omss ot h a tt h e r e s p on s ei sac ol l e c t i v eon e .Th e s ea s s u mp t i on sa r ev a l i df ore . g .v i s i b l el i g h ti n c i d e n ton or d i n a r y“ t r a n s p a r e n t ”ma t t e r .Si mi l a r l y ,s u r f a c ec u r r e n tl oop sc a u s ema g n e t i ci n d u c t i on c omp on e n t sp a r a l l e l t ot h es u r f a c et ob ed i s c on t i n u ou s l yc h a n g e d . Al g e b r a i c a l l y , t h i sb e c ome s( f orE ) : ′ ′ ′′ ′ ′ ′ ′ ′ ′ ǫ( E0+E0)·nˆ =ǫE n ˆ 0· ( 1 1 . 6 1 ) ( E ×nˆ =E ˆ 0+E 0) 0×n ( 1 1 . 6 2 ) wh e r et h el a t t e rc r os sp r odu c ti sj u s taf a n c ywa yoff i n d i n gE c o m p o n e n t s . I n m o s t ⊥ c a s e son ewou l d n ’ ta c t u a l l y“ d o”t h i sd e c omp os i t i ona l g e b r a i c a l l y ,on ewou l dj u s t i n s p e c tt h ep r ob l e ma n dwr i t ed ownt h e| |a n d⊥c omp on e n t sdi r e c t l yu s i n gas e n s i b l e c oor di n a t es y s t e m( s u c ha son ewh e r en ˆ=z ˆ ) . Si mi l a r l yf orB: 1 µ ( B0+B0)·n ˆ =B0·n ˆ ′ ′ 1 ′ ′ ( B0+B0)×n ˆ = µB0 ×n ˆ ( 1 1 . 6 3 ) ( 1 1 . 6 4 ) ( wh e r e , r e c a l l , B=( k×E ) / ( v k )e t c . )Ag a i n , o n eu s u a l l ywou l dn otu s et h i sc r os sp r odu c t a l g e b r a i c a l l y , b u twou l ds i mp l yf or mu l a t et h ep r ob l e mi na c on v e n i e n tc oor d i n a t es y s t e ma n dt a k ea d v a n t a g eoft h ef a c tt h a t : E | =√µǫ E 0 =| B | 0| ( 1 1 . 6 5 ) | 0| v Coo r di n a t ec h oi c ea n dBr e ws t e r ’ sL a w Wh a t , t h e n , i sa“ c on v e n i e n tc oor d i n a t es y s t e m” ?On ewh e r en ˆ=z ˆi sp e r p e n d i c u l a rt ot h e 7 s u r f a c ei sg oodf ors t a r t e r s.Th er e ma i n i n gt woc oor d i n a t e sa r es e l e c t e dt od e f i n et h e p l a n eofr e f l e c t i ona n dr e f r a c t i o na n di t sp e r p e n d i c u l a r .Th i si sp a r t i c u l a r l yu s e f u lb e c a u s e ( a swes h a l ls e e )t h er e f l e c t e da n dr e f r a c t e di n t e n s i t i e sd e p e n dont h e i rp ol a r i z a t i o n r e l a t i v et ot h ep l a n eofs c a t t e r i n g . Ag a i n ,t omot i v a t et h i sb e f or eme s s i n gwi t ht h ea l g e b r a ,y ouh op e f u l l ya r ea l lf a mi l i a r wi t ht h er e s u l tt a u g h ta tt h ek i d d y p h y s i c sl e v e lk n owna sBr e ws t e r ’ sL a w.Th ea r g u me n t wor k sl i k et h i s :b e c a u s et h er e f r a c t e dr a yc on s i s t sof( b a s i c a l l y )d i p ol er e r a d i a t i onoft h e i n c i d e n tf i e l da tt h es u r f a c ea n db e c a u s ed i p ol e sd on otr a d i a t ea l on gt h ed i r e c t i onoft h e d i p o l emome n t , t h ep ol a r i z a t i onc omp on e n twi t hEi nt h es c a t t e r i n gp l a n eh a sac omp on e n t i nt h i sd i r e c t i on . Th i sl e a d st ot h ei n s i g h tt h a ta tc e r t a i na n g l e st h er e f r a c t e dr a ywi l lb ec omp l e t e l y p ol a r i z e dp e r p e n d i c u l a rt ot h es c a t t e r i n gp l a n e( Br e ws t e r ’ sL a w) !Ou ra l g e b r an e e dst o h a v et h i sd e c omp os i t i onb u i l ti nf r om t h eb e g i n n i n gorwe ’ l lh a v et owor kv e r yh a r d i n d e e dt oob t a i nt h i sa sar e s u l t ! L e tu st h e r e f or et r e a tr a y sp ol a r i z e di n orp e r p e n d i c u l a rt ot h ep l a n e of i n c i d e n c e / r e f l e c t i on / r e f r a c t i ons e p a r a t e l y . EPe r p e n d i c u l a rt oPl a n eofI n c i d e n c e Th ee l e c t r i cf i e l di nt h i sc a s ei sp e r f or c ep a r a l l e l t ot h es u r f a c ea n dh e n c eE · n ˆ=0a n d | E×n ˆ | =1( f ori n c i d e n t , r e f l e c t e da n dr e f r a c t e dwa v e s ) .On l yt wooft h ef ou re qu a t i on s a b ov ea r et h u su s e f u l .Th eEe qu a t i oni st r i v i a l .Th eBe qu a t i onr e qu i r e su st o de t e r mi n et h ema g n i t u deoft h ec r os sp r od u c tofBofe a c hwa v ewi t hn ˆ .L e t ’ sd oon e c omp on e n ta sa ne x a mp l e . E x a mi n i n gt h et r i a n g l ef or me db e t we e nB0a n dn ˆf ort h ei n c i d e n twa v e s( wh e r eθi i st h ea n g l eofi n c i d e n c e ) , wen ot et h a tB⊥=B0c os ( θ )a n dt h u s : i 1 B0 µ| 1 os ( θi ) ×n ˆ |= µB0c √µǫ ( θi ) = µ E0cos ǫ = os ( θi ) . µ E0c ( 1 1 . 6 6 ) 7 No t eWe l l ! Th en ˆwea r eu s i n gh e r ei sn ott h ed i r e c t i onofk , i ti st h ed i r e c t i onoft h en or ma l t ot h es u r f a c e , t h a t i st os a yz ˆ . z E F i g u r e1 1 . 2 : Pol a r i z a t i onc omp on e n toft h ei n c i d e n t( a n dr e f l e c t e da n dr e f r a c t e d ) wa v e sp e r p e n d i c u l a rt ot h ep l a n eofi n c i d e n c e . n k θ θ i i Bo E i n ) o( F i g u r e1 1 . 3 : Ge ome t r yofB0×n ˆ . Re p e a t i n gt h i sf ort h eot h e rt wowa v e sa n dc ol l e c t i n gt h er e s u l t s , weob t a i n : ′ ′ ′ E 0+E 0 0 =E ′ ( 1 1 . 6 7 ) ǫ ǫ ′ ′ c os ( θi ) 0) E0 −E µ( ′ ′ µE os ( θr ) 0c = ( 1 1 . 6 8 ) Th i si st woe qu a t i on swi t ht wou n k n own s . Sol v i n gi ti sab i tt e d i ou s . Wen e e d: c os ( θ )= r 2 i n( θ ) r 1−s 2 ( 1 1 . 6 9 ) n = = 2 ′ 2 s i n( θi ) 1− n ′ 2 2 2 n −n s i n( θi ) ′ n ( 1 1 . 7 0 ) ( 1 1 . 7 1 ) ′ Th e nwe( s a y )e l i mi n a t eE s i n gt h ef i r s te qu a t i on : 0u ′ ′ ǫ( E µ 0− 0 Col l e c ta l lt h et e r ms : ′ µ i − ′ ′ µ θi) µcos( ′ 2 n ǫ ǫ E 0 − ′ 2 ′ ′ ′ 0 n ′ 2 ′ 2 n ǫ ′ ′ E= E 0 µ 0 ( 1 1 . 7 2 ) n = 2 2 + µcos ( θi) 2 ns i n( θi ) ′ ′ ′ Sol v ef orE0: ′ 0 − n ǫ 2 ns i n( θi ) µ 0 E 2 ′ ′ i n( θi ) +E) n −n s )= ǫ′( E E)c os ( θ ǫ ( 1 1 . 7 3 ) ′ 2 2 2 √n′ −n s i n( θi ) ǫ′ ′ c os ( θ )− i c os ( θ )+ i µ n √n ′ ǫ ′ µ ǫ ′ ′ 2 2 ( 1 1 . 7 4 ) 2 n( θi ) −n si ′ µ n Th i se x p r e s s i onc a nb es i mp l i f i e da f t e rs omet e d i ou sc a n c e l l a t i on si n v ol v i n g n µǫ ′ ′′ n = µǫ a n de i t h e rr e p e a t i n gt h ep r oc e s sorb a c k s u bs t i t u t i n gt oobt a i n: ′ 2 µ ′ ′ E 0 =E 0 nc os ( θi )− µ′ ′ 2 n −n s i n( θi ) ( 1 1 . 7 6 ) µ ′ 2 2 2 n −n s i n( θi ) nc os ( θi )+ µ′ E 0 2 2 nc os ( θi ) 0 =E ( 1 1 . 7 5 ) µ nc os ( θi )+ µ′ ′ 2 ( 1 1 . 7 7 ) 2 2 n −n s i n( θi ) EPa r a l l e l t oPl a n eo fI n c i d e n c e Nowt h ema gn e t i cf i e l di sp a r a l l e lt ot h es u r f a c es oB·n ˆ=0a n d| B×n ˆ |=1 .Th i st i me t h r e ee qu a t i on ss u r v i v e , b u tt h e yc a n n ota l lb ei n de p e n de n ta sweh a v eon l yt wou n k n own s ( g i v e nSn e l l ’ sl a wa b ov ef ort h er e f l e c t e d / r e f r a c t e dwa v e s ) .Wemi g h ta swe l lu s et h e s i mp l e s tp os s i b l ef or ms ,wh i c ha r ec l e a r l yt h eon e swh e r ewe ’ v ea l r e a d ywor k e dou tt h e g e ome t r y , e . g . E ˆ=E0c os ( θi )( a sb e f or ef orB0) . Th et wos i mp l e s ton e sa r ec l e a r l y : 0×n ′ ′ ′ ( E0−E0)c os ( θi ) =E os ( θr ) 0c ǫ( E µ ′ ′ 0 +E) = 0 ′ ǫ E′ 0 µ′ ( 1 1 . 7 8 ) ( 1 1 . 7 9 ) ( f r omt h es e c on dma t c h i n ge qu a t i on sf orb ot hEa n dBa b ov e ) . I ti sl e f ta samod e r a t e l yt e d i ou se x e r c i s et or e p e a tt h er e a s on i n gp r oc e s sf ort h e s et wo ′ ′ ′ e qu a t i on s– e l i mi n a t ee i t h e rE rE n ds ol v e / s i mp l i f yf ort h eot h e r ,r e p e a tor 0o 0 a b a c k s u b s t i t u t et oob t a i nt h eor i g i n a l l ye l i mi n a t e don e( oru s ey ou rown f a v or i t ewa yofa l g e b r a i c a l l ys ol v i n gs i mu l t a n e ou se qu a t i on s )t oobt a i n : ′ 2 n nc os ( θ ) i ′ E 0 =E 0 µ 2 ′′ os ( θ )+n i µn c µ ′ ′ E 0 =E ′ 2 µn ′ c os ( θ )−n i 0µ 2 ′′ µn c os ( θ )+n i ′ 2 2 2 ′ 2 2 2 ′ 2 2 2 n −n s i n( θ ) i n −n s i n( θi ) ( 11. 80) ( 11. 81) n −n s i n( θ ) i Th el a s tr e s u l tt h a ton es h ou l dn ot eb e f or emov i n goni st h ei mp or t a n tc a s eof n or ma li n c i d e n c e( wh e r ec osθ n ds i n ( θi )=0 ) .Now t h e r es h ou l don l yb e i=1a p e r p e n d i c u l a rs ol u t i on s .I n t e r e s t i n g l y ,e i t h e rt h ep a r a l l e lorp e r p e n d i c u l a rs ol u t i on s a b ov es i mp l i f ywi t hob v i ou sc a n c e l l a t i on sa n dt e d i ou se l i mi n a t i on st o: 2 n ′ ′ E0 =E 0 n+n ′ ′ ′ E 0 =E n−n 0 ( 1 1 . 8 2 ) ( 1 1 . 8 3 ) n′+n Not ewe l lt h a tt h er e f l e c t e dwa v ec h a n g e sp h a s e( i sn e g a t i v er e l a t i v et ot h ei n c i d e n t ′ wa v ei nt h ep l a n eofs c a t t e r i n g )i fn>n. Th i sofc ou r s ema k e ss e n s e –t h e r ea r ema n yi n t u i t i v er e a s on st oe x p e c tawa v et oi n v e r ti t sp h a s ewh e nr e f l e c t i n g 8 f r oma“ h e a v i e r ”me di u m. I n t e n s i t y Wi t h ou twa n t i n gt og e ta l lt e d i ou sa b ou ti t ,y ous h ou l db ea b l et oc omp u t et h e t r a n s mi s s i onc oe ffic i e n ta n dr e f l e c t i o nc o e ffic i e n tf ora l loft h e s ewa v e sf r omt h e s e r e s u l t s .Th e s ea r eb a s i c a l l yt h ef r a c t i onoft h ee n e r g y( p e ru n i ta r e ap e ru n i tt i me )i n t h ei n c i d e n twa v et h a ti st r a n s mi t t e dv sb e i n gr e f l e c t e db yt h es u r f a c e . Th i si sas i mp l ei de a , b u ti ti sab i tt r i c k yt oa c t u a l l yc omp u t ef orac ou p l eofr e a s on s . On ei st h a tweon l yc a r ea b ou te n e r g yt h a tma k e si tt h r ou ght h es u r f a c e .Th ed i r e c t e d i n t e n s i t yoft h ewa v e( e n e r g yp e ru n i ta r e ap e ru n i tt i me )i st h ePoy n t i n gv e c t orS.I n e qu a t i on1 1 . 3 5a b ov e ,wef ou n dt h et i me a v e r a g ePoy n t i n gv e c t ori nt e r msoft h eEf i e l d s t r e n g t ha n ddi r e c t i onofp r op a g a t i on : 1 S= 2 k ǫ 2 E | k µ| ( 1 1 . 8 4 ) 8 I n d e e d , y ous h ou l dh a v el e a r n e ds ome t h i n ga b ou tt h i si ne l e me n t a r yp h y s i c ss t u d y i n gt h er e f l e c t i on sof wa v ep u l s e sonas t r i n g , a n da g a i nwh e ns t u d y i n gt h i nf i l mi n t e r f e r e n c e( ap h e n ome n onwh e r ea c c ou n t i n gf or t h i si n v e r s i oni sc r i t i c a lt og e t t i n gt h er i g h ta n s we r s ) .I fy ou ’ v en e v e rs e et h i sa n di td oe s n ’ tma k es e n s et o y oup l e a s ea s kf orh e l p . ( wh e r eweh a v ewr i t t e nt h ed i r e c t i onofp r op a g a t i oni nt e r msof ˆ k a v oi dc on f u s i onwi t ht h en or ma l t ot h es u r f a c en ˆ , wh i c hwer e c a l l i sz ˆ , n otk ) . =k / kt o ˆ Weon l yc a r ea b ou tt h ee n e r g yf l u xt h r o u ght h ep l a n es u r f a c ea n dt h u smu s t f or mS·nˆf ore ac hwav e : 1 ǫ 2 =Sn = 2 µ| E0|c os ( θi) I 0 ′ ′ I =S 0 n ′ ′ I ′ =S′ 0 n ( 1 1 . 8 5 ) ǫ′ E′2cos( θ ) ( 1 1 . 8 6 ) 2 r 2 ǫ 1 = ′ ′ c os( θ ) E | 0 2 µ| i ( 1 1 . 8 7 ) =1 | µ′ 0| ( 1 1 . 8 8 ) 9 Th i si s“ e a s y ”on l yi ft h ewa v e sa r ei n c i d e n t⊥t ot h es u r f a c e , i nwh i c hc a s eon eg e t s : ′ ′ I T= 0 = I 0 = ǫµ E′2 | ( 1 1 . 8 9 ) 0| ′ 2 µ| E0| ′ǫ 4 n n ′ ( 1 1 . 9 0 ) 2 ( n+n ) R = = ′ ′ I ′ ′ E 2 =| 0| 2 I | E0| 0 0 ′ 2 ′ 2 ( 1 1 . 9 1 ) ( n−n ) ( 1 1 . 9 2 ) ( n+n ) Asami n i e x e r c i s e ,v e r i f yt h a tT+R=1( a si tmu s t ) .Se r i ou s l y ,i tt a k e son l yt h r e eor f ou rl i n e s . Po l a r i z a t i onRe v i s i t e d :Th eBr e ws t e rAn gl e Not ewe l l t h ee x p r e s s i onf ort h er e f l e c t e dwa v ea mp l i t u d ef ori n p l a n ep ol a r i z a t i on : ′ ′ 0 µ E= E ′ 2 nc os ( θ )−n i µ′ ′ 2 2 2 ′ 2 2 2 n −n s i n( θi ) 0µ 2 ′′ µn ( 1 1 . 9 3 ) c os ( θ )+n n −n s i n( θi ) i Th i sa mp l i t u dewi l l b ez e r of orc e r t a i na n g l e s , n a me l yt h os es u c ht h a t : µ ′ 2 ′ 2 2 2 nc os ( θi )=nn −n s i n( θi ) ′ µ ( 1 1 . 9 4 ) 9 Ea s ye n ou g ht owr i t edowni nt h eb ooki na ni n t e l l i g i b l ef or m.Ofc ou r s ei ti ss t r a i g h t f or wa r dt oc o mpu t ei twi t h e . g .ac omp u t e rf ora r b i t r a r yi n c i d e n ta n g l e s–t h i si swh yGodi n v e n t e dc o mp u t e r s , b e c a u s eh u ma nb r a i n swe r en ot r e a l l yu pt ot h et a s k . Un l e s s , ofc ou r s e , t h e yb e l on gt oc omp l e t ema s oc h i s t s . 1 0 Squ a r i n gb ot hs i d e sa n dr e s t or i n gt h ec os i n et e r mt oi t sor i g i n a l f or m : µ 2 ′ 2 2 2 2 nc os( θ )=n c os( θ ) i r ′ µ ( 1 1 . 9 5 ) Wet h e r e f or ee x p e c tt h er e f l e c t e dwa v et ov a n i s hwh e n ′ µn ′ cos( θr ) = µn ( 1 1 . 9 6 ) c os ( θi ) ′ F orop t i c a l f r e qu e n c i e sµ≈µ( t os i mp l i f yt h ea l g e b r as ome wh a t )a n dt h i si se qu i v a l e n t t o: ′ nc os ( θi )=nc os ( θ ) r ( 1 1 . 9 7 ) F r omSn e l l ’ sl a wt h i si nt u r ni s : ′ n n n′ t a n ( θ )= nt a n ( θ ) i r Th i st r a n c e n d e n t a l e qu a t i onc a nb es ol v e db yob s e r v a t i onf r omi t ss y mme t r y . I ti st r u ei fa n don l yi f : ( 1 1 . 9 8 ) ′ n t a n ( θ )= n =c ot ( θ ) i r Th ea n g l eofi n c i d e n c e − 1 θ a n b=t ( 1 1 . 9 9 ) ′ n n ( 1 1 . 1 0 0 ) i sc a l l e dBr e ws t e r ’ sa n gl e .Att h i sp a r t i c u l a ra n g l eofi n c i de n c e , t h er e f l e c t e da n dr e f r a c t e d wa v et r a v e la tr i gh ta n gl e swi t hr e s p e c tt oon ea n ot h e ra c c or d i n gt oSn e l l ’ sl a w.Th i s me a n st h a tt h ed i p ol e si nt h es e c on dme d i u mt h a ta r er e s p on s i b l ef ort h er e f l e c t e dwa v e a r ep a r a l l e lt ot h edi r e c t i onofp r op a g a t i ona n d( a swes h a l ls e e )os c i l l a t i n gdi p ol e sd on o t r a d i a t ei nt h ed i r e c t i onoft h e i rd i p ol emome n t !Howe v e r ,t h er e s u l ta b ov ewa sob t a i n e d wi t h ou ta n ya p p e a lt ot h emi c r os c op i cp r op e r t i e soft h ed i e l e c t r i cmome n t st h a ta c t u a l l y c oh e r e n t l ys c a t t e rt h ei n c i de n twa v ea tt h es u r f a c e–i tf ol l owss t r i c t l ya st h er e s u l tof s ol v i n gab ou n d a r yv a l u ep r ob l e mf ore l e c t r oma g n e t i cp l a n ewa v e s . St u d e n t si n t e r e s t e di nop t i c a l f i b e r sa r ee n c ou r a g e dt or e a df u r t h e ri nJ a c k s on , 7 . 4 a n dl e a r nh owt h ec a n c e l l a t i ona n dr e r a d i a t i onoft h ewa v e st op r od u c ear e f l e c t e d wa v ea ta n g l e swh e r et ot a l i n t e r n a lr e f l e c t i onh a p p e n sd oe sn otoc c u ri n s t a n t a n e ou s l y a tt h er e f r a c t i n gs u r f a c eb u ti nf a c ti n v ol v e st h ep e n e t r a t i onoft h es e c on dme di u m s omes ma l ld i s t a n c eb yn on p r op a g a t i n gf i e l d s .Th i si nt u r ni sr e l a t e dt op ol a r i z a t i on , d i s p e r s i on ,a n ds k i nd e p t h ,wh i c hwewi l ln owt r e a ti ns omede t a i l( s k i p p i n gop t i c a l f i b e r sp e rs e ) . 1 0 Re me mb e rt h ea l g e b r awh e r eweg ott h es qu a r er ooti nt h ef i r s tp l a c e ?We l l , doi tb a c k wa r ds . 11. 3 Di s p e r s i o n Upt on ow,weh a v eob t a i n e da l lofou rr e s u l t swi t ht h ea s s u mp t i o nt h a tt h eme di u mwa s f r e ef r o m di s p e r s i on .Th i sj u s tme a n tt h a twea s s u me dt h a tt h ei n d e xofr e f r a c t i onwa s c o n s t a n ta saf u n c t i onoff r e qu e n c y ,s oa l lwa v e l e n g t h swe r es i mi l a r l ya ffe c t e d .Ofc ou r s e n on eofou rr e s u l t ss of a rde p e n d e dp a r t i c u l a rs t r on g l yont h i sr e s u l t , b u ti na n ye v e n ti ti s n otc o r r e c t .Th ep e r mi t t i v i t y( a n dt o al e s s e re x t e n tf ort r a n s p a r e n tma t e r i a l s ,t h e p e r me a b i l i t y )i saf u n c t i onoft h ef r e q u e n c ya n dt h u st h es p e e dofl i g h tc h a n g e sa sa f u n c t i onoff r e qu e n c ya swe l l f orwa v e sp r op a g a t i n gi nad i s p e r s i v eme d i u m. Byt h ewa y ,wh e nIs a yt h a ti t“ i s n ’ tc or r e c t ”I ’ mn ota s s e r t i n ga nop i n i onor ma t h e ma t i c a lc on c l u s i on .Th a t ’ sn oth owp h y s i c swor k s .Ra t h e ri ti sa l wa y su l t i ma t e l y e mp i r i c a l :r a i n b owsa n dp r i s ms( n ott ome n t i onop a qu eob j e c t s )r e mi n du st h a tmos t p h y s i c a l me d i aa r en o tf r e ef r omdi s p e r s i on .Un d e r s t a n d i n ga n dmode l l i n gt h edy n a mi c sof di s p e r s i oni nawa yt h a tc or r e c t l ye x p l a i n st h e s eob s e r v e dp h e n ome n ai sak e ys t e pi nt h e u n d e r s t a n d i n gofmu c hmod e r np h y s i c s , wh i c hi n v ol v e st h eme a s u r e me n ta n dp r e d i c t i onof v a r i ou ss u s c e pt i b i l i t i e s( y e ta n ot h e rwa yofwr i t i n gt h ep e r mi t t i v i t y ,b a s i c a l l y ,a sy ouc a n s e eb e l ow)i nb ot hc l a s s i c a la n dqu a n t u mc i r c u ms t a n c e s .Af u l lu n d e r s t a n d i n goft h e p a r t i c u l a rd i s p e r s i onofap h y s i c a lme di u mi sp os s i b l eon l yi nt h ec on t e x tofqu a n t u m t h e or y , b u tt ou n d e r s t a n dt h ep h e n ome n oni t s e l fwec a nf or t u n a t e l yr e l yonar a t h e rs i mp l e c l a s s i c a l mod e l t h a te x h i b i t sa l l t h ee s s e n t i a l f e a t u r e sob s e r v e di na c t u a l p h y s i c a l me di a . 11. 3. 1 St a t i cCa s e Re c a l l , ( f r oms e c t i on s4 . 5a n d4 . 6i nJ a c k s on )t h a twh e nt h ee l e c t r i cf i e l dp e n e t r a t e sa me d i u mma d eofb ou n dc h a r g e s ,i tp o l a r i z e st h os ec h a r g e s .Th ec h a r g e st h e ms e l v e s t h e np r od u c eaf i e l dt h a top p os e s ,a n dh e n c eb ys u p e r p os i t i onr e d u c e s ,t h ea p p l i e d f i e l d .Th ek e ya s s u mp t i oni nt h e s es e c t i on swa st h a tt h ep ol a r i z a t i onoft h eme di u m wa sal i n e a rf u n c t i o noft h et ot a l f i e l di nt h ev i c i n i t yoft h ea t oms . L i n e a r i t yr e s p on s ewa se a s i l ymo de l l e db ya s s u mi n gah a r mon i c( l i n e a r ) r e s t or i n gf or c e : 2 F=−mωx 0 ( 1 1 . 1 0 1 ) a c t i n gt op u l lac h a r g eei n t oan e wn e u t r a le qu i l i b r i u mi nt h ep r e s e n c eofa ne l e c t r i c f i e l dv Ea c t i n gonap r e s u me dc h a r g ee . Th ef i e l de x e r t saf or c e Fe=e E , s o: 2 eE−mωx0 =0 ( 1 1 . 1 0 2 ) i st h ec on d i t i onf ore q u i l i b r i u m.Th edi p ol emome n toft h i s( p r e s u me d )mol e c u l a r s y s t e mi s 2 pmol=x e= 2 e 1e E= 2 2 ǫ0E=γ ǫ0E mol mω0 ǫ0mω0 wh e r eγ i st h e“ mol e c u l a rp ol a r i z a b i l i t y ”i ns u i t a b l eu n i t s . mo l ( 1 1 . 1 0 3 ) Re a l mol e c u l e s , ofc ou r s e , h a v ema n yb ou n dc h a r g e s , e a c hofwh i c ha te qu i l i b r i u m h a sa na p p r ox i ma t e l yl i n e a rr e s t or i n gf or c ewi t hi t sownn a t u r a lf r e qu e n c y ,s oamor e g e n e r a l mode l ofmol e c u l a rp ol a r i z a b i l i t yi s : γ = mol 2 e 1 i ǫ0 2 mi ωi . i ( 1 1 . 1 0 4 ) Th i si sf oras i n gl emol e c u l e .Ana c t u a lme di u mc on s i s t sofNmol e c u l e sp e ru n i t v ol u me .F r om t h el i n e a ra p p r ox i ma t i ony ouob t a i n e da ne qu a t i onf ort h et ot a l p ol a r i z a t i on( d i p ol emome n tp e ru n i tv ol u me )oft h ema t e r i a l : 1 3P P=Nγ E+ molǫ 0 ( 1 1 . 1 0 5 ) ( e qu a t i on4 . 6 8 )wh e r et h ef a c t orof1 / 3c ome sf r oma v e r a g i n gt h el i n e a rr e s p on s eov e r a“ s p h e r i c a l ”mol e c u l e . Th i sc a nb ep u ti nma n yf or ms .F ore x a mp l e , u s i n gt h ed e f i n i t i onoft h e ( d i me n s i on l e s s )e l e c t r i cs u s c e p t i b i l i t y : P=ǫ0χ E e wef i n dt h a t : χ e= ( 1 1 . 1 0 6 ) Nγ mol Nγ . ( 1 1 . 1 0 7 ) MOL 1− 3 Th es u s c e p t i b i l i t yi son eoft h emos tof t e nme a s u r e dord i s c u s s e dqu a n t i t i e sof p h y s i c a l me d i ai nma n yc on t e x t sofp h y s i c s . Howe v e r ,a swe ’ v ej u s ts e e n ,i nt h ec on t e x tofwa v e swewi l lmos tof t e nh a v e oc c a s i ont ou s ep ol a r i z a b i l i t yi nt e r msoft h ep e r mi t t i v i t yoft h eme d i u m, ǫ. Re c a l l t h a t : D=ǫE=ǫ0E+P=ǫ0( 1+χ ) E ( 1 1 . 1 0 8 ) e F r omt h i swec a ne a s i l yf i n dǫi nt e r mofχ : e ǫ=ǫ0( 1+χe) ( 1 1 . 1 0 9 ) F r om ak n owl e d g eofǫ( i nt h er e g i meofop t i c a lf r e qu e n c i e swh e r eµ≈µ orma n y 0f ma t e r i a l sofi n t e r e s t )wec a ne a s i l yob t a i n , e . g . t h ei n d e xofr e f r a c t i on : c or √µǫ ǫ n= v =√µ0ǫ0 ≈ n= ǫ0 ≈ 1+χ e γ MOL 1+2N3 Nγ MOL 1− ( 1 1 . 1 1 0 ) ( 1 1 . 1 1 1 ) 3 i fNa n dγ r ek n ownora tl e a s ta p p r ox i ma t e l yc omp u t a b l eu s i n gt h e( s u r p r i s i n g l y mo la a c c u r a t e )e x p r e s s i ona b ov e . Somu c hf ors t a t i cp ol a r i z a b i l i t yofi n s u l a t or s–i ti sr e a di l yu n de r s t a n da b l ei n t e r msofr e a lp h y s i c sofp u s h e sa n dp u l l s , a n dt h es e mi qu a n t i t a t i v emod e l son eu s e s t ou n d e r s t a n di twor kqu i t ewe l l .Howe v e r ,r e a lf i e l d sa r e n ’ ts t a t i c ,a n dr e a lma t e r i a l s a r e n ’ ta l l i n s u l a t or s . Soweg ot t a a )Modi f yt h emod e l t oma k ei tdy n a mi c . b )E v a l u a t et h emod e l ( mor eorl e s sa sa b ov e , b u twe ’ l l h a v et owor kh a r d e r ) . c )Un d e r s t a n dwh a t ’ sg oi n gon . L e t ’ sg e ts t a r t e d . 11. 3. 2 Dy n a mi cCa s e Th eob v i ou sg e n e r a l i z a t i onoft h es t a t i cmod e lf ort h ep ol a r i z a t i oni st oa s s u mead a mpe d l i n e a rr e s p on s et oah a r mo n i c( p l a n ewa v e )d r i v i n ge l e c t r i cf i e l d .Th a ti s ,e v e r ymol e c u l e wi l lb ev i e we da sac ol l e c t i onofd a mp e d ,dr i v e n( c h a r g e d )h a r mon i cos c i l l a t or s .Ma g n e t i c a n dn on –l i n e a re ffe c t swi l l b en e g l e c t e d .Th i si sv a l i df orav a r i e t yofma t e r i a l ss u b j e c t e dt o 1 1 “ we a k ”h a r mon i cE Mf i e l d s wh i c hi np r a c t i c e( wi t hop t i c a lf r e qu e n c i e s )me a n sn e a r l y e v e r y t h i n gb u tl a s e rl i g h t . 1 2 Th ee qu a t i onofmot i on f oras i n g l ed a mp e d , d r i v e nh a r mon i c a l l yb ou n dc h a r g e d e l e c t r oni s : 2 ¨ ˙ mx +γx+ωx0 =− e Ex (, t ) ˙ ( 1 1 . 1 1 2 ) wh e r eγi st h eda mp i n gc on s t a n t( s o− mγ xi st h ev e l oc i t yd e p e n d e n td a mp i n gf or c e ) .I f wea s s u met h a tt h ee l e c t r i cf i e l dEa n d xa r eh a r mon i ci nt i mea tf r e qu e n c yω( orf ou r i e r t r a n s f or mt h ee qu a t i ona n df i n di t ss ol u t i onf oras i n g l ef ou r i e rc omp on e n t )a n d n e g l e c tt h et r a n s i e n t sweg e t : e2 1 3 p=− x e= E ω 2 2 m( ω0 −ω −i ωγ ) ( 1 1 . 1 1 3 ) f ore a c he l e c t r on . Ac t u a l l y , weh a v eNmol e c u l e s / u n i tv ol u mee a c hwi t hZe l e c t r on swh e r ef ft h e mh a v e io f r e qu e n c i e sa n dd a mp i n gc on s t a n t sωia n dγ , r e s p e c t i v e l y( wh e w! )t h e n( s i n c ewewi l l s t i c k i i nt h ed e f i n i t i on sPω=ǫ0χ E n dǫ=1+χ ) e ωa e 2 Ne ǫ( ω) = ǫ01 + m f i 2 2 ωi−ω −i ωγ ) i( i ( 1 1 . 1 1 4 ) whe r et heos c i l l at ors t r engt hss at i s f yt hes umr ul e : f . i=Z ( 1 1 . 1 1 5 ) i 1 1 Wh y ?I fy oudon ’ tu n d e r s t a n dt h i s , y oun e e dt og ob a c kt ob a s i c sa n dt h i n ka b ou te x p a n di n gap ot e n t i a l we l li naTa y l ors e r i e sa b ou tap a r t i c l e ’ se qu i l i b r i u mp os i t i on .Th el i n e a rt e r mv a n i s h e sb e c a u s ei ti s 2 e qu i l i b r i u m, s ot h ef i r s ts u r v i v i n gt e r mi sl i k e l yt ob equ a dr a t i c .Wh i c hi st os a y , p r op or t i on a l t ox wh e r exi s t h edi s p l a c e me n tf r ome qu i l i b r i u m, c or r e s p on d i n gt oal i n e a rr e s t or i n gf or c et ol owe s tor d e r . 1 2 Youdor e me mb e rNe wt on ’ sl a w, don ’ ty ou ?Su r eh o p es o. . . 1 3 I c e r t a i n l yh op ey ouc a nde r i v et h i sr e s u l t , a tl e a s ti fy ou rl i f ed e p e n dsoni t . I nqu a l i f i e r s , wh i l et e a c h i n gk i d dyp h y s i c s , wh e n e v e r . Th e s ee qu a t i on s( wi t h i ns u i t a b l ea p p r ox i ma t i on s )a r ev a l i df orqu a n t u mt h e or i e s , a n di n d e e d , s i n c equ a n t u mos c i l l a t or sh a v ec e r t a i nd i s c r e t ef r e qu e n c i e s , t h e ys e e mt o “ n a t u r a l l y ”b equ a n t u mme c h a n i c a l . 11. 3. 3 Th i n gst oNo t e Be f or eweg oon , wes h ou l du n d e r s t a n daf e wt h i n g s : a )ǫi sn owc o mp l e x ! Th ei ma g i n a r yp a r ti se x p l i c i t l yc on n e c t e dt ot h ed a mp i n g c on s t a n t . b )Con s e qu e n t l ywec a nn ows e eh owt h ei n de xofr e f r a c t i on √µǫ c n= v = √µ0ǫ0 , ( 1 1 . 1 1 6 ) c a nb ea l s ob ec o mp l e x .Ac omp l e xi n d e xofr e f r a c t i onde s c r i b e sa bs o r p t i o n( or a mp l i f i c a t i on ! )a n da r i s e sf r om t h eda mp i n gt e r mi nt h ee l e c t r on s ’E OM ( or n on –l i n e a r , n on –e qu i l i b r i u me ffe c t si nl a s e r s , wh i c hwewi l ln otc on s i de rh e r e ) .Th i s ma k e se n e r g yc on s e r v a t i onk i n dofs e n s e .E n e r g ya b s or b e db yt h ee l e c t r on sa n d d i s s i pa t e dv i at h e“ f r i c t i on a l ”da mp i n gf or c ei sr e mov e df r om t h eE Mf i e l da si t p r op a g a t e st h r ou g ht h eme d i u m.Th i s( c omp l e xd i s p e r s i onofi n c i de n twa v e s )i st h e b a s i sf ort h e“ op t i c a l ”d e s c r i p t i onofs c a t t e r i n gwh i c hi su s e f u l t on u c l e a rp h y s i c i s t s . c )Th et e r m 1 2 2 ωi−ω −i ωγ h a saf or mt h a ty ouwi l l s e ea g a i na n da g a i na n da g a i ni ny ou rs t u d i e s .I ts h ou l d b eme d i t a t e du p on ,s t u d i e d ,dr e a me da b ou t ,me n t a l l yma s t i c a t e da n de n f ol d e d i n t oy ou rb e i n g su n t i ly ouu n d e r s t a n di t .I ti sac omp l e xe qu a t i onwi t hp o l e si n t h ei ma g i n a r y / r e a l p l a n e . I td e s c r i b e s( v e r yg e n e r a l l ys p e a k i n g )r e s on a n c e s . I ti su s e f u lt oc o n v e r tt h i si n t oaf or mwh i c hh a sma n i f e s tr e a la n di ma g i n a r y p a r t s , s i n c ewewi l l h a v eoc c a s i ont oc omp u t et h e mi nr e a lp r ob l e mson ed a y .A b i tofa l g e b r ag i v e su s : 1 2 2 ωi−ω −i ωγ 1 9 2 = 2 ( ωi−ω )+i ωγ 2 22 22 ( ωi−ω )+ω γ d )I fNi s“ s ma l l ”( ∼1 0 mol e c u l e s / c cf orag a s )χ ss ma l l ( j u s tl i k ei nt h es t a t i c ei c a s e )a n dt h eme d i u mi sn e a r l yt r a n s p a r e n ta tmos tf r e qu e n c i e s . 2 3 e )i fNi s“ l a r g e ”( ∼1 0 mol e c u l e s / c cf oral i qu i dors ol i d )χ a nb equ i t el a r g ei n ec p r i n c i p l e , a n dn e a rar e s on a n c ec a nb equ i t el a r g ea n dc omp l e x ! Th e s ep oi n t sa n dmor er e qu i r ean e wl a n g u a g ef ort h e i rc on v e n i e n td e s c r i p t i on . Wewi l l n owp a u s eamome n tt od e v e l opon e . 11. 3. 4 An oma l o u sDi s p e r s i on , a n dRe s o n a n tAb s or p t i on F i g u r e1 1 . 4 :Ty p i c a lc u r v e si n d i c a t i n gt h er e a la n di ma g i n a r yp a r t sofǫ/ ǫ0f ora na t om wi t ht h r e ev i s i b l er e s on a n c e s .Not et h er e g i on sofa n o ma l ou s( d e s c e n d i n g )r e a l d i s p e r s i oni nt h ei mme d i a t ev i c i n i t yoft h er e s on a n c e s ,s e p a r a t e db yl a r g er e g i on sof n o r ma l ( a s c e n di n g )d i s p e r s i on . Th eγ r et y p i c a l l ys ma l lc omp a r e dt ot h eos c i l l a t o rf r e qu e n c i e sωi .( J u s tt og i v e ia 9 − 1 1 5 − 1 y oua ni d e a ,γ 0s e c t oωi∼1 0 s e c f orop t i c a lt r a n s i t i on si na t oms ,wi t h i∼1 s i mi l a rp r op or t i on a l i t i e sf ort h eot h e rr e l e v a n tt r a n s i t i o n s . )Th a tme a n st h a ta tmos t f r e qu e n c i e s , ǫ( ω)i sn e a r l yr e a l Su p p os eweon l yh a v eaf e wf r e q u e n c i e s .Be l owt h es ma l l e s tωi , a l l t h e( r e a l )t e r ms i nt h es u ma r ep os i t i v ea n dReǫ( ω)>1 .Aswei n c r e a s eω, on eb yon et h et e r msi nt h e s u mb e c omen e g a t i v e( i nt h e i rr e a lp a r t )u n t i lb e y on dt h eh i g h e s tf r e qu e n c yt h ee n t i r e s u ma n dh e n c eReǫ( ω)<1 . Aswes we e pp a s te a c h“ p ol e ”( wh e r et h er e a lp a r ti nt h ed e n omi n a t orofas i n g l et e r m i sz e r o)t h a tt e r mi n c r e a s e sr a p i d l yi nt h er e a l p a r t , t h e ndi v e st h r ou g hz e r ot ob e c omel a r g e a n dn e g a t i v e ,t h e ni n c r e a s e smon ot on i c a l l yt oz e r o.Me a n wh i l e ,i t s( u s u a l l ys ma l l ) i ma g i n a r yp a r tg r ows ,r e a c h i n gap e a kj u s twh e r et h er e a lp a r ti sz e r o( wh e nǫ( ω)i sp u r e i ma g i n a r y ) . I nt h ev i c i n i t yoft h e p ol e , t h ec on t r i b u t i onoft h i st e r mc a nd omi n a t et h er e s toft h es u m. Wed e f i n e : Nor ma ld i s p e r s i ona ss t r i c t l yi n c r e a s i n gReǫ( ω)wi t hi n c r e a s i n gω.Th i si st h en or ma l s i t u a t i one v e r y wh e r eb u tn e a rap ol e . An o ma l o u sd i s p e r s i ona sd e c r e a s i n gReǫ ( ω)wi t hi n c r e a s i n gω.Th i si st r u eon l yn e a r as u ffic i e n t l ys t r on gp ol e( on et h a td omi n a t e st h es u m) .Att h a tp oi n t ,t h e i ma g i n a r yp a r toft h ei n d e xofr e f r a c t i onb e c ome s( r e l a t i v e l y )a p p r e c i a b l e . Re s on a n tAb s o r p t i o noc c u r si nt h er e g i on swh e r eI mǫi sl a r g e .Wewi l lp a r a me t r i c a l l y d e s c r i b et h i sn e x t . 11. 3. 5 At t e n u a t i onb yac omp l e xǫ Su p p os ewewr i t e( f orag i v e nf r e qu e n c y ) k=β+i Th e n i k x α . 2 i βx− E ()=e =e e ωx ( 1 1 . 1 1 7 ) α −α x 2 x ( 1 1 . 1 1 8 ) a n dt h ei n t e n s i t yoft h e( p l a n e )wa v ef a l l soffl i k ee .αme a s u r e st h eda mp i n goft h e p l a n ewa v ei nt h eme d i u m. L e t ’ st h i n kab i ta b o u tk : k= ω = ω n v c wh e r e : ( 1 1 . 1 1 9 ) √µǫ n=c / v= ( 1 1 . 1 2 0 ) √µ0ǫ0 I nmos t“ t r a n s p a r e n t ”ma t e r i a l s , µ≈µ0 Th u s : a n dt h i ss i mp l i f i e st on= ǫ/ ǫ0. 2 ω ǫ k =2 ( 1 1 . 1 2 1 ) c ǫ0 Nowe v e r ,n owǫh a sr e a la n di ma g i n a r yp a r t s ,s okma ya swe l l !I nf a c t ,u s i n gt h e e x p r e s s i onf orki nt e r msofβa n dαa b ov e , i ti se a s yt os e et h a t : 2 2 ω2 α 2 2 Rek =β − a n d 2 I mk =βα= 2 2 ǫ 2 R 4 =c eǫ0 ω2 2 I m c ǫ . ( 1 1 . 1 2 2 ) ( 1 1 . 1 2 3 ) ǫ0 Asl on ga sβ > >α ( a g a i n , t r u emos toft h et i mei nt r a s p a r e n tma t e r i a l s ) wec a nt h u swr i t e : α≈ I mǫ( ω) ( 1 1 . 1 2 4 ) β Reǫ( ω) a n d ǫ β≈( ω/ c ) Re ǫ0 ( 1 1 . 1 2 5 ) Th i sr a t i oc a nb ei n t e r p r e t e da saq u a n t i t ys i mi l a rt oQ,t h ef r a c t i on a ld e c r e a s ei n i n t e n s i t yp e rwa v e l e n g t ht r a v e l l e dt h r ou g ht h eme d i u m( a sop p os e dt ot h ef r a c t i on a l d e c r e a s ei ni n t e n s i t yp e rp e r i od ) . Tof i n dαi ns omeu s e f u lf or m,weh a v et oe x a mi n et h ede t a i l sofǫ( ω) ,wh i c hwe wi l l p r oc e e dt od on e x t . Wh e nωi si na mon gt h er e s on a n c e s , t h e r ei sl i t t l ewec a nd ob e s i d e swor kou tt h e de t a i l soft h eb e h a v i or , s i n c et h ep r op e r t i e soft h ema t e r i a lc a nb ed omi n a t e ds t r on g l y b yt h el oc a ldy n a mi c sa s s oc i a t e dwi t ht h en e a r e s t ,s t r on g e s tr e s on a n c e .Howe v e r , t h e r ea r et wol i mi t st h a ta r eofp a r t i c u l a ri n t e r e s tt op h y s i c i s t swh e r et h e“ r e s on a n t ” b e h a v i orc a nb ee i t h e re v a l u a t e dorwa s h e da wa y .Th e ya r et h el owf r e qu e n c yb e h a v i or wh i c hd e t e r mi n e st h ec on d u c t i onp r op e r t i e sofama t e r i a lf a ra wa yf r omt h ee l e c t r on r e s on a n c e sp e rs e , a n dt h eh i ghf r e q u e n c yb e h a v i orwh i c hi s“ u n i v e r s a l ” . 11. 3. 6 L o wF r e q u e n c yBe h a v i or Ne a rω=0t h equ a l i t a t i v eb e h a v i ord e p e n d su p onwh e t h e rorn ott h e r ei sa“ r e s on a n c e ” t h e r e .I ft h e r ei s ,t h e nǫ( ω≈0 )c a nb e g i nwi t hac omp l e xc omp on e n tt h a ta t t e n u a t e s t h ep r op a g a t i onofE Me n e r g yi na( n e a r l ys t a t i c )a p p l i e de l e c t r i cf i e l d . Th i s( a swes h a l l s e e )a c c u r a t e l yd e s c r i b e sc on d u c t i o na n dr e s i s t a n c e .I ft h e r ei s n ’ t , t h e nǫi sn e a r l ya l l r e a l a n dt h ema t e r i a l i sad i e l e c t r i ci n s u l a t or . Su p p os et h e r ea r eb ot h“ f r e e ”e l e c t r on s( c ou n t e db yf h a ta r e“ r e s on a n t ”a tz e r o f)t f r e qu e n c y , a n d“ b ou n d ”e l e c t r on s( c ou n t e db yf ) . Th e ni fwes t a r tou twi t h : b 2 Ne ǫ( ω) = ǫ0 1+ m f i 2 2 ωi−ω −i ωγ ) i( i 2 Ne f b 2 2 ωb −ω −i ωγ ) b b ( = ǫ0 1+m 2 + Nme 2 f f − ω −i ωγ f) f( 2 Nef f γ ω) =ǫb+i ǫ0 mω( 0−i ( 1 1 . 1 2 6 ) wh e r eǫbi sn owon l yt h ec on t r i b u t i onf r oma l l t h e“ b ou n d ”di p ol e s . Wec a nu n d e r s t a n dt h i sf r om ∇× H= J + d D d t ( 1 1 . 1 2 7 ) ( Ma x we l l / Amp e r e ’ sL a w) .L e t ’ sf i r s tofa l lt h i n koft h i si nt e r msofap l a i nol ds t a t i c c u r r e n t , s u s t a i n e da c c or d i n gt oOh m’ sL a w: J = σE . ( 1 1 . 1 2 8 ) I fwea s s u meah a r mon i ct i med e p e n d e n c ea n da“ n or ma l ”d i e l e c t r i cc on s t a n tǫb, weg e t : σ−i ωǫb)E ∇× H=( σ i ω ǫb+i ω E =− . ( 1 1 . 1 2 9 ) Ont h eot h e rh a n d ,wec a ni n s t e a ds e tt h es t a t i cc u r r e n tt oz e r oa n dc on s i de ra l l “ c u r r e n t s ”p r e s e n tt ob et h er e s u l toft h ep ol a r i z a t i onr e s p on s eDt ot h ef i e l dE .I nt h i s c a s e : ∇× H=− i ωǫE 2 Ne i ω ǫb+i ǫ0 =− f f γ ω) E 0−i m ( ( 1 1 . 1 3 0 ) E qu a t i n gt h et wol a t t e rt e r msi nt h eb r a c k e t sa n ds i mp l i f y i n g ,weob t a i nt h e f ol l owi n gr e l a t i onf ort h ec on du c t i v i t y : σ= ǫ0 2 nfe 1 m ( γ ω) 0−i . ( 1 1 . 1 3 1 ) Th i si st h eDr u d eMo d e l wi t hn h en u mb e rof“ f r e e ”e l e c t r on sp e ru n i tv ol u me .I t f=f fNt i sp r i ma r i l yu s e f u lf ort h ei n s i g h tt h a ti tg i v e su sc on c e r n i n gt h e“ c on du c t i v i t y ”b e i n g c l os e l yr e l a t e dt ot h ez e r of r e qu e n c yc omp l e xp a r toft h ep e r mi t t i v i t y .Not et h a ta tω= 0i ti sp u r e l yr e a l , a si ts h ou l db e , r e c ov e r i n gt h eu s u a l Oh m’ sL a w. Wec on c l u d et h a tt h ed i s t i n c t i onb e t we e nd i e l e c t r i c sa n dc on d u c t or si sama t t e rof p e r s p e c t i v ea wa yf r omt h ep u r e l ys t a t i cc a s e .Awa yf r omt h es t a t i cc a s e , “ c on d u c t i v i t y ” i ss i mp l yaf e a t u r eofr e s on a n ta mp l i t u d e s . I ti sama t t e roft a s t ewh e t h e rade s c r i p t i on i sb e t t e rma d ei nt e r mso fd i e l e c t r i cc on s t a n t sa n dc on d u c t i v i t yorc omp l e xd i e l e c t r i c . 11. 3. 7Hi ghF r e qu e n c yL i mi t ; Pl a s maF r e qu e n c y Wa ya b o v et h eh i g h e s tr e s on a n tf r e qu e n c yt h ed i e l e c t r i cc on s t a n tt a k e sonas i mp l e f or m( f a c t or i n gou tω> >ωia n dd oi n gt h es u mt ot h el owe s ts u r v i v i n g or de ri nωp/ ω. Asb e f or e , wes t a r tou twi t h : ǫ( ω)= ǫ0 2 Ne f i 2 2 ωi−ω −i ωγ ) i( i 1+ m =ǫ0 ≈ǫ0 2 Ne f i 1− ω2m γ i 2 ωi 2 1+i ω − ω2) i( NZe 2 1− ω m ≈ǫ0 wh e r e 2 ω ( 1 1 . 1 3 2 ) p 2 1− ω 2 2 n e p m ( 1 1 . 1 3 3 ) ω= . Th i si sc a l l e dt h ep l a s maf r e qu e n c y , a n di td e p e n d son l yonn=NZ, t h et ot a l n u mb e rof e l e c t r on sp e ru n i tv ol u me . Th ewa v en u mb e ri nt h i sl i mi ti sg i v e nb y : 2 2 2 c k= ω −ωp ( 1 1 . 1 3 4 ) 2 22 ( orω =ωp + ck) .Th i si sc a l l e dad i s p e r s i o nr e l a t i onω( k ) .Al a r g ep or t i onof c on t e mp or a r ya n df a mou sp h y s i c si n v ol v e sc a l c u l a t i n gd i s p e r s i on r e l a t i on s( or e qu i v a l e n t l ys u s c e p t i b i l i t i e s , r i g h t ? )f r omf i r s tp r i n c i p l e s . I nc e r t a i np h y s i c a l s i t u a t i on s( s u c ha sap l a s maort h ei on os p h e r e )a l l t h ee l e c t r on sa r e e s s e n t i a l l y“ f r e e ”( i nad e g e n e r a t e“ g a s ”s u r r ou n d i n gt h ep os i t i v ec h a r g e s )a n dr e s on a n t d a mp i n gi sn e g l i b l e .I nt h a tc a s et h i sr e l a t i onc a nh ol df orf r e qu e n c i e swe l lb e l owωp( b u t we l la b ov et h es t a t i cl i mi t , s i n c ep l a s ma sa r el owf r e qu e n c y“ c on d u c t or s ” ) .Wa v e si n c i d e n t onap l a s maa r er e f l e c t e da n dt h ef i e l d si n s i d ef a l l offe x p on e n t i a l l ya wa yf r omt h es u r f a c e . Not et h a t α p≈ 2ωp c ( 1 1 . 1 3 5 ) s h owsh owe l e c t r i cf l u xi se x p e l l e db yt h e“ s c r e e n i n g ”e l e c t r on s . Th er e f l e c t i v i t yofme t a l si sc a u s e db ye s s e n t i a l l yt h es a meme c h a n i s m.Ath i g h f r e qu e n c i e s , t h edi e l e c t r i cc on s t a n tofame t a l h a st h ef or m 2 ω p 2 ǫ( ω)≈ǫ0( ω)− ω ( 1 1 . 1 3 6 ) 2 2 ∗ ∗ wh e r eωp =n e/ m i st h e“ p l a s maf r e qu e n c y ”oft h ec on d u c t i one l e c t r on s .m i st h e “ e ffe c t i v e ma s s ”oft h ee l e c t r on s ,i n t r odu c e dt od e s c r i b et h ee ffe c t s ofb i n d i n g p h e n ome n ol og i c a l l y . Me t a l sr e f l e c ta c c or d i n gt ot h i sr u l e( wi t hav e r ys ma l lf i e l dp e n e t r a t i onl e n g t hof “ s k i nde p t h ” )a sl on ga st h ed i e l e c t r i cc on s t a n ti sn e g a t i v e ; i nt h eu l t r a v i ol e ti tb e c ome s p os i t i v ea n dme t a l sc a nb e c omet r a n s p a r e n t .J u s ton eofma n yp r ob l e msi n v ol v e di n ma k i n gh i g hu l t r a v i ol e t , x –r a ya n dg a mmar a yl a s e r s—i ti ss oh a r dt oma k eami r r or ! 10 8 6 4 2 0 0 2 4 6 8 1 0 F i g u r e1 1 . 5 : Th ed i s p e r s i onr e l a t i onf orap l a s ma . F e a t u r e st on ot e : Ga pa tk=0 , a s y mp t ot i c a l l yl i n e a rb e h a v i or . 11. 4 Pe n e t r a t i o no fWa v e sI n t oaCo n d u c t o r–Sk i n De p t h 11. 4. 1 Wa v eAt t e n u a t i o ni nTwoL i mi t s Re c a l l f r oma b ov et h a t : ∇×H=− i ωǫE=− i ω σ ǫb+iω E . ( 1 1 . 1 3 7 ) 1+i σ ( 1 1 . 1 3 8 ) Th e n : 2 2 2 2 k =ω =µǫω =µǫbω Al s ok=β+i α 2 v2 ωǫb s ot h a t 2 α 2 k= 2 β− 4 σ 2 ωǫb +i α β=µ ǫbω 1+i ( 1 1 . 1 3 9 ) Oop s . Tode t e r mi n eαa n dβ, weh a v et ot a k et h es qu a r er ootofac omp l e xn u mb e r . How d oe st h a twor ka g a i n ?Se et h ea p p e n d i xonCo mp l e xNu mb e r s . . . I nma n yc a s e swec a np i c kt h er i g h tb r a n c hb ys e l e c t i n gt h eon ewi t ht h er i g h t ( d e s i r e d )b e h a v i oronp h y s i c a l g r ou n d s . I fwer e s t r i c tou r s e l v e st ot h et wos i mp l ec a s e s wh e r eωi sl a r g eorσi sl a r g e ,i ti st h eon ei nt h ep r i n c i p l eb r a n c h( u p p e rh a l fp l a n e , a b ov eab r a n c hc u ta l on gt h er e a l a x i s .F r omt h el a s te qu a t i ona b ov e , i fweh a v eap oor c on du c t or( ori ft h ef r e qu e n c yi smu c hh i g h e rt h a nt h ep l a s maf r e qu e n c y )a n dα≪ β, t h e n : β ≈ √ µǫbω µ ( 1 1 . 1 4 0 ) ǫbσ ( 1 1 . 1 4 1 ) α ≈ − i βn ˆ · E a n dt h ea t t e n u a t i on( r e c a l lt h a tE=E e α2 e )i si n d e p e n d e n toff r e qu e n c y .Th eot h e r 0 l i mi tt h a ti sr e l a t i v e l ye a s yi sago o dc on du c t or , σ≫ωǫb. I nt h a t c a s et h ei ma gi n a r yt e r md omi n a t e sa n dwes e et h a t α β≈ 2 ( 1 1 . 1 4 2 ) or µσω β ≈ 2 ( 1 1 . 1 4 3 ) α ≈ 2µσω ( 1 1 . 1 4 4 ) Th u s µσω k=( 1+i ) ( 1 1 . 1 4 5 ) 2 i k ( n ˆ · x −i ωt Re c a l l t h a ti fwea p p l yt h e∇op e r a t ort oE e weg e t : s oE n dH0a r en ot 0a i np h a s e( u s i n gt h e ∇·E = 0 i k E n ˆ = 0 0· i π/ 2 f a c tt h a ti =e ) . E nˆ = 0 0· a n d ∂B − ∂t = ∇×E µσω i ωµH0 ( n ˆ×E ) ( 1+i ) 0 =i 1 σω H0 = ω 1 =ω 1 2 n ˆ×E ) √2( 0 µ( 1+i ) σω i π/ 4 n ˆ×E ) e 0 µ( ( 1 1 . 1 4 6 ) ( 1 1 . 1 4 7 ) I nt h ec a s eofs u p e r c on d u c t or s ,σ→ ∞ a n dt h ep h a s ea n g l eb e t we e nt h e mi sπ/ 4 .I n t h i sc a s eH0≫E( s h owt h i s ! )a n dt h ee n e r g yi smos t l yma gn e t i c . α− 1 F i n a l l y , n ot ewe l lt h a tt h equ a n t i t y 2 =δi sa ne x po n e n t i a ld a mp i n gl e n gt ht h a t d e s c r i b e sh owr a p i d l yt h ewa v ea t t e n u a t e sa si tmov e si n t ot h ec on du c t i n gme d i u m.δ i sc a l l e dt h es k i nde p t ha n dwes e et h a t : 2 1 2 = = α β µσω δ= ( 1 1 . 1 4 8 ) Wewi l l e x a mi n et h i squ a n t i t yi ns omed e t a i l i nt h es e c t i on sonwa v e g u i de sa n dop t i c a l c a v i t i e s , wh e r ei tp l a y sa ni mp or t a n tr ol e . 11. 5 Kr a me r s Kr on i gRe l a t i on s Wef i n dKKr e l a t i on sb yp l a y i n gl oop e dg a me swi t hF ou r i e rTr a n s f or ms .Web e g i nwi t h t h er e l a t i onb e t we e nt h ee l e c t r i cf i e l da n dd i s p l a c e me n ta ts omep a r t i c u l a rf r e qu e n c y ω: Dx (, ω)=ǫ( ω) E x (, ω) ( 1 1 . 1 4 9 ) wh e r ewen ot et h et wo( f or wa r da n db a c k wa r d )f ou r i e rt r a n s f or mr e l a t i on s : 1 Dx( , t )= 2 π Dx (, ω)= 1 √ 2π a n dofc ou r s e : E x (, ω)= √ 2 π 1 1 Dx (, t )= √ = √ =ǫ0 −∞ 2π −∞ ( 1 1 . 1 5 0 ) ∞ ′ Dx (, ′ i ωtd ′ t ) e t ( 1 1 . 1 5 1 ) −∞ −i ωt ∞ , ω) e dω E x ( ( 1 1 . 1 5 2 ) −∞ ∞ ′ E x (, ′ i ωtd ′ t ) e t ( 1 1 . 1 5 3 ) −∞ −i ωt ∞ 2π 1 d ω −∞ √ 2π Th e r e f or e : , ω) e Dx ( 1 Ex( , t )= −i ωt ∞ √ ǫ( ω) Ex (, ω) e dω ∞ ∞ −i ωt ǫ( ω) e d ω 1 √ E x (, t )+ 2π ∞ E x ( ′ ′ i ωtd ′ , t ) e t −∞ t−τ) d τ G( τ) Ex (, −∞ wh e r eweh a v ei n t r od u c e dt h es u s c e p t i b i l i t yk e r n e l : ( 1 1 . 1 5 4 ) 1 ∞ ǫ( ω) ǫ0 −1 G( τ )= 2π −∞ 1 ∞ e d ω= 2π − i ωτ χ ( ω) e e −i ωτ −∞ d ω ( 1 1 . 1 5 5 ) ( n ot i n gt h a tǫ( ω)=ǫ0( 1+χ ( ω) ) ) .Th i se qu a t i oni sn on l oc a li nt i meu n l e s sG( τ)i sa e d e l t af u n c t i on , wh i c hi nt u r ni st r u eon l yi ft h ed i s p e r s i oni sc on s t a n t . Tou n d e r s t a n dt h i s ,c on s i d e rt h es u s c e p t i b i l i t yk e r n e lf oras i mp l eon er e s on a n c e mode l ( mor er e s on a n c e sa r ej u s ts u p e r p os i t i on ) .I nt h i sc a s e , r e c a l l t h a t : ǫ χ e= ǫ0 s o 2 G( τ )= ωp 2 ωp 2 2 γ0ω −1=ω0 −ω −i 1 ∞ 2 2 γ ω 0 2π −∞ ω0 −ω −i −i ωτd e ω ( 1 1 . 1 5 6 ) ( 1 1 . 1 5 7 ) Th i si sa ni n t e g r a lwec a ndou s i n gc on t ou ri n t e g r a t i onme t h od s .Weu s et h e qu a d r a t i cf or mu l at of i n dt h er oot soft h ed e n omi n a t or ,t h e nwr i t et h ef a c t or e d de n omi n a t ori nt e r msoft h er oot s : i γ± ω1,2= − 2 2 −γ +4ω0 2 ( 1 1 . 1 5 8 ) or 2 − i γ ν ω 1 γ = ( 1 1 . 1 5 9 ) 2 2 ± 0 − 4ω0 2 ± 0 1, 2 wh e r eν sl on ga sω0≫γ / 2( a si su s u a l l yt h ec a s e , r e me mb e rβa n dα / 2 ) .Not et h a t 0≈ω0a t h e s ep ol e sa r ei nt h el o we rh a l fp l a n e( L HP)b e c a u s eoft h es i g nofγi nt h eor i g i n a l h a r mon i cos c i l l a t or–i twa sd i s s i p a t i v e . Th i si si mp or t a n t . Th e n 2 1 ωp ω i γ =− −i ωτd e ω G( τ)=( 2πi ) ( 1 1 . 1 6 0) ω−ω1) ( ω−ω2) 2π C( I fwec l os et h ec on t ou ri nt h eu p p e rh a l fp l a n e( UHP) , weh a v et or e s t r i c t τ<0( wh y ?b e c a u s eot h e r wi s et h ei n t e g r a n dwi l ln otv a n i s hont h ec on t ou ra ti n f i n i t y wh e r eωh a sap os i t i v ei ma g i n a r yp a r t .Si n c ei te n c l os e sn op ol e s ,G( τ<0 )v a n i s h e s , a n dweg e tn oc on t r i b u t i onf r o mt h ef u t u r ei nt h ei n t e g r a l a b ov ef orE .Th er e s u l ta p p e a r st ob ec a u s a l , b u tr e a l l ywec h e a t e d–t h e“ c a u s a l i t y ”r e s u l t s f r omt h eda mp i n gt e r m, wh i c hr e p r e s e n t se n t r op ya n dy e a h , g i v e st i mea na r r owh e r e .Bu ti t d oe s n ’ tr e a l l yb r e a kt h es y mme t r yoft i mei nt h i sp r ob l e ma n di fou rmode li n v ol v e da d y n a mi c a l l yp u mp e dme d i u ms ot h a tt h ewa v ee x p e r i e n c e dga i nmov i n gt h r ou g hi t( a n i ma g i n a r yt e r mt h a twa s p os i t i v e )wewou l dh a v eh a dp ol e si nt h eUHPa n dou re x p r e s s i onf orEwou l dn otb e “ c a u s a l ” .Re a l l yi ti se qu a l l yc a u s a li nb ot hc a s e s ,b e c a u s et h ef ou r i e rt r a n s f or ms i n v ol v e ds a mp l ea l l t i me sa n y wa y . I fwec l os et h ei n t e g r a n di nt h eL HP, τ>0a n di fwed ot h er e s toft h e( f a i r l y s t r a i g h t f or wa r d )a l g e b r aweg e t : G( τ)=ωe 2−γτ s i n ( ν 0 ) p 2 ν 0 Θ( τ) ( 1 1. 1 61 ) wh e r et h el a t t e ri saHe a v i s i def u n c t i ont oe n f or c et h eτ>0c on s t r a i n t . Ou rl a s tl i t t l ee x e r c i s ei st ou s ec omp l e xv a r i a b l e sa n dCa u c h y ’ st h e or e ma g a i n .We s t a r tb yn ot i n gt h a tDa n dEa n dG( τ)a r ea l lr e a l .Th e nwec a ni n t e g r a t eb yp a r t sa n d f i n dt h i n g sl i k e : ǫ( ω) ′ G( 0 ) G( 0 ) 2 ǫ0 −1=i ω − ω +. . . ( 1 1 . 1 6 2 ) ∗ ∗ f r om wh i c hwec a nc on c l u d et h a tǫ( − ω)=ǫ( ω )a n dt h el i k e .Not et h ee v e n / od d i ma g i n a r y / r e a los c i l l a t i oni nt h es e r i e s .ǫ( ω)i st h e r e f or ea n a l y t i ci nt h eUHPa n dwe c a nwr i t e : ′ ǫ( z ) ǫ( ω) ǫ0 −1 1 1= ǫ0 − 2 πiC ′ ω −z ′ d ω ( 1 1 . 1 6 3 ) Wel e tz=ω+i δwh e r eδ→ 0 ord e f or mt h ei n t e g r a lab i tb e l owt h es i n g u l a r +( p oi n tont h eRe ( ω)a x i s ) . F r omt h ePl e ml j Re l a t i on : 1 1 ′ πδ( ω −ω) ( 1 1 . 1 6 4 ) ω i δ =P ω ω +i ′ ′ − − − ( s e ee . g .Wy l d , Ar f k i n ) .I fwes u b s t i t u t et h i si n t ot h ei n t e g r a la b ov ea l on gt h er e a la x i s on l y ,d ot h ed e l t a f u n c t i onp a r ta n ds u b t r a c ti tou t ,c a n c e laf a c t orof1 / 2t h a tt h u s a p p e a r s , weg e t : ω ′ ǫ( ω) =1+ 1P ω) ∞ ǫ( ǫ0 1 − dω′ ( 1 1 . 1 6 5 ) ′ ω ǫ0 −∞ ω− i π Al t h ou g ht h i sl ook sl i k eas i n g l ei n t e g r a l , b e c a u s eoft h eii nt h ed e n omi n a t ori ti s r e a l l yt wo.Th er e a lp a r toft h ei n t e g r a n db e c ome st h ei ma g i n a r yp a r toft h er e s u l ta n d v i c ev e r s a . Th a ti s : ′ Re ǫ( ω) ǫ0 ǫ( ω) 1 = 1+ P π 1 ǫ( ω) ∞ I m −∞ ∞ Re ǫ0 ′ ′ ω− ω ′ ǫ( ω) ǫ0 ′ d ω ( 1 1 . 1 6 6 ) −1 ′ ω− ω d ω I m ǫ0 =− π P −∞ ( 1 1 . 1 6 7 ) Th e s ea r et h eKr a me r s Kr on i g Re l a t i on s .Th e yt e l lu st h a tt h ed i s p e r s i v ea n d a b s or p t i v ep r op e r t i e soft h eme d i u ma r en oti n d e p e n d e n t .I fwek n owt h ee n t i r ea b s or p t i v e s p e c t r u mwec a nc omp u t et h ed i s p e r s i v es p e c t r u ma n dv i c ev e r s a .Th e r ei son emor ef or m oft h eKKr e l a t i on sg i v e ni nJ a c k s on , de r i v e df r omt h edi s c ov e r ya b ov et h a tt h er e a lp a r tof ǫ( ω)i se v e ni nωwh i l et h ei ma g i n a r yp a r ti sod d.Se ei fy ouc a nde r i v et h i sony ou rownf or t h ef u nofi ta l l . . . 11. 6 Pl a n eWa v e sAs s i gn me n t Tos t a r tofft h es e me s t e rr i g h t , v i s i tt h eWi k i p e d i aa n dMa t h wor l dwe b s i t e sa n dl ooku p a n du n de r s t a n d: a )Se p a r a t i onofv a r i a b l e s b )Sp h e r i c a l Ha r mon i c s c )Be s s e l F u n c t i on s d )Sp h e r i c a l Be s s e l F u n c t i on s e )Gr e e n ’ sF u n c t i on s f )Wa v eEqu a t i on g )Pl a n eWa v e J u s te x p l or et h ek i n dsoft h i n g sy ouc a nf i n dt h e r e–I ’ md i s c ov e r i n gt h a tt h e s ewe b r e f e r e n c e sa r er a p i dl yb e c omi n gTHEu n i v e r s a lf r e et e x t b ook .I ti sa c t u a l l ya ma z i n gt o wa t c hi th a p p e n( a n dp a r t i c i p a t ei ni ta st i mep e r mi t s ) . J a c k s on , p r ob l e ms : 7 . 4 , 7 . 6 , 7 . 1 9 , 7 . 2 1 Al s o,d e r i v eony ou rowna l lt h ep r i n c i p a lr e s u l t sp r e s e n t e di nt h e s eon l i n el e c t u r e n ot e s .I ti se a s yt or e a da n ds e emed oi t .I ti sn ots oe a s yt od oi t ,e v e nf orme . Wor k i n gt h r ou g ht h i s ,p os s i b l ys e v e r a lt i me su n t i ly our e a l l y“ g e ti t ” ,wi l lt r u l yi mp r ov e y ou ru n de r s t a n di n gofh owe v e r y t h i n gwor k s . Ch a p t e r12 Wa v eGu i d e s 12. 1 Bou n d a r yCon d i t i o n sa taCo n d u c t i n g Su r f a c e : Sk i nDe p t h L e tu sc on s i d e rf oramome n twh a tt i mede p e n d e n tE Mf i e l d sl ookl i k ea tt h es u r f a c eof a“ p e r f e c t ”c on d u c t or .Ap e r f e c tc on d u c t orc a nmov ea smu c hc h a r g ei n s t a n t l ya si s r e qu i r e dt oc a n c e la l lf i e l d si n s i d e .Th es k i nd e p t hδ=l i mσ→∞ 2 / µǫbσ=0a sα di v e r g e s–e ffe c t i v e l ya l lf r e qu e n c i e sa r e“ s t a t i c ”t oap e r f e c tc on d u c t or .Th i si sh ow t y p eI s u p e r c on d u c t or se x p e l a l l f i e l df l u x . I fwee x a mi n et h ef i e l d si nt h ev i c i n i t yofab ou n d a r yb e t we e nap e r f e c tc on d u c t or a n dan or ma l d i e l e c t r i c / d i a ma g n e t i cma t e r i a l , weg e t : ( D−Dc)·n ˆ=n ˆ·D=Σ ( 1 2 . 1 ) wh e r eDca n dE n s i d et h ec on d u c t orv a n i s h . Si mi l a r l y , ci n ˆ×( H−Hc)=n ˆ×H=K ( 1 2 . 2 ) ( wh e r ei nt h e s ee x p r e s s i on s , Σi st h es u r f a c ec h a r g ed e n s i t ys owed on ’ tc on f u s ei twi t ht h e c on du c t i v i t yσ, s i g h , a n ds i mi l a r l yKi st h es u r f a c ec u r r e n td e n s i t y ) . I na d d i t i ont ot h e s et woi n h omog e n e ou se qu a t i on st h a tn or ma l a n dp a r a l l e l f i e l dsa tt h e s u r f a c et os ou r c e s , weh a v et h eu s u a l t woh omog e n e ou se qu a t i on s : n ˆ·( B−Bc) = 0 ( 1 2 . 3 ) n ˆ×( E−E ) = 0 c ( 1 2 . 4 ) Not et h a tt h e s ea r ep r e t t ymu c hp r e c i s e l yt h eb ou n d a r yc on d i t i on sf oras t a t i cf i e l da n d s h ou l dc omea sn os u r p r i s e .F orp e r f e c tc on d u c t or s ,wee x p e c tt h ef i e l d si n s i d et o v a n i s h , wh i c hi nt u r ni mp l i e st h a tEo u t s i d emu s tb en or ma lt ot h ec on du c t i n gs u r f a c e a n dBou t s i d emu s tl i eo n l yp a r a l l e l t ot h ec on d u c t i n gs u r f a c e , a su s u a l . Howe v e r , f orma t e r i a l st h a ta r en o tp e r f e c tc on du c t or s , t h ef i e l d sd on ’ tv a n i s hi n s t a n t l y “ a t ”t h ema t h e ma t i c a l s u r f a c e . I n s t e a dt h e yd i eoffe x p on e n t i a l l y 1 2 7 wi t h i naf e wmu l t i p l e so ft h es k i nd e p t hδ .Ons c a l e sl a r gewi t hr e s p e c tt ot h i s , t h e ywi l l “ l ook ”l i k et h es t a t i cf i e l dc on di t i on sa b ov e , b u tofc ou r s ewi t h i nt h i sc u t offt h i n g sa r e v e r yd i ffe r e n t . F oron et h i n g ,Oh m’ sl a wt e l l su st h a twec a n n oth a v ea na c t u a l“ s u r f a c el a y e rof c h a r g e ”b e c a u s ef ora n yf i n i t ec on du c t i v i t y ,t h er e s i s t a n c es c a l e sl i k et h ec r os s s e c t i on a la r e at h r ou g hwh i c hc h a r g ef l ows .Con s e qu e n t l yt h er e a lb ou n d a r yc on d i t i on onHp r e c i s e l ya tt h es u r f a c ei s : n ˆ×( H−Hc) H | | =0 ( 1 2 . 5 ) = ( 1 2 . 6 ) H c , | | wh e r eH| n ˆ×H)×n ˆ .Howe v e r ,t h i sc r e a t e sap r ob l e m!I ft h i sf i e l dv a r i e sr a p i d l yi n |=( s omed i r e c t i on( a n di tdoe s )i twi l lg e n e r a t ea ne l e c t r i cf i e l da c c or d i n gt oF a r a da y ’ sl a w!I f t h ed i r e c t i onofg r e a t e s tv a r i a t i oni s“ i n t ot h ec on d u c t or ”( a st h ef i e l di sb e i n gs c r e e n e db y i n d u c e ds u r f a c ec u r r e n t s )t h e ni twi l lg e n e r a t eas ma l le l e c t r i cf i e l dp a r a l l e lt ot h es u r f a c e , on ewh i c hi sn e g l e c t e d( orr a t h e r , c a n n otoc c u r )i nt h el i mi tt h a tt h ec on d u c t i v i t yi si n f i n i t e . Th i se l e c t r i cf i e l d , i nt u r n , g e n e r a t e sac u r r e n t , wh i c hc a u s e st h eg r a du a l c a n c e l l a t i onofH| | a sl e s sa n dl e s st h et ot a l b u l kc u r r e n ti se n c l o s e db yad e c e n d i n gl oopb ou n d a r y . I ft h ec on d u c t i v i t yi sl a r g eb u tn oti n f i n i t e ,on ewa yt of i g u r eou twh a th a p p e n si st o e mp l oyas e r i e sofs u c c e s s i v ea p p r ox i ma t i on ss t a r t i n gwi t ht h ea s s u mp t i onofp e r f e c t c on du c t i v i t ya n du s i n gi tt og e n e r a t eaf i r s tor d e rc or r e c t i onb a s e dont h ea c t u a l c on d u c t i v i t ya n dwa v e l e n g t h . Th ewa yi twor k si s : a )F i r s t ,wea s s u met h a tou t s i d et h ec on d u c t orweh a v eon l yE n dH| r omt h e ⊥a |f s t a t e me n toft h eb ou n d a r yc on d i t i on sa s s u mi n gt h a tt h ef i e l d sa r ei n s t a n t l y c a n c e l l e da tt h es u r f a c e . −1 b )As s u meδ≪ k a l o n gt h es u r f a c e–t h es k i nd e p t hi smu c hl e s st h a na wa v e l e n g t ha n dt h ef i e l ds( wh a t e v e rt h e yma yb e )v a n i s ha c r os sr ou g h l yt h i s l e n g t hs c a l e ,s o we c a nn e g l e c tv a r i a t i on ( d e r i v a t i v e s )wi t hr e s p e c tt o c oor d i n a t e st h a tl i ea l on gt h es u r f a c ec omp a r e dt ot h ec oor di n a t ep e r p e n di c u l a r t ot h es u r f a c e . c )Us et h i sa p p r ox i ma t i on i n Ma x we l l ’ sEqu a t i on s ,a l on g wi t ht h ea s s u me d b ou n d a r yc on d i t i on sf orap e r f e c tc on du c t or ,t od e r i v er e l a t i on sb e t we e nt h e f i e l dsi nt h et r a n s i t i onl a y e r . d )Th e s er e l a t i on sd e t e r mi n et h es ma l l c or r e c t i on st ot h ep r e s u me db ou n d a r y f i e l d sb ot hj u s tou t s i dea n dj u s ti n s i d et h es u r f a c e . Th ea s s u mp t i onofr a p i dv a r i a t i onon l ya son ed e c e n d si n t ot h ec on d u c t ori sak e ys t e p , a swes h a l l s e e . Th u s( f r om1 ) : n ˆ×( H−Hc)=0 ( 1 2 . 7 ) orH| ( ou t s i d e )=H| ( i n s i d e )=H| , wh e r et h el a t t e ra s s u mp t i oni sb e c a u s et h er e s u l t | | |=0 i sb or i n gi ft h e r ea r en of i e l d s , r i g h t ? Web ot hAmp e r e ’ sl a w( a s s u mi n gn od i s p l a c e me n ti nt h ec on d u c t ort ol e a d i n g or d e r )a n dF a r a da y ’ sl a wt oob t a i nr e l a t i on sf ort h eh a r mon i cf i e l dsi nt e r msofc u r l sof e a c hot h e r : c=J ∇×Hc = σE ∂Bc ωµcHc ∇×E t =i c =− ∂ ( 1 2 . 8 ) ( 1 2 . 9 ) become 1 E c= σ∇×Hc ( 1 2 . 1 0 ) 1 Hc =− i µcω∇×E c ( 1 2 . 1 1 ) Aswemi g h te x p e c t ,h i g hf r e qu e n c i e sc r e a t er e l a t i v e l yl a r g ei n d u c e de l e c t r i cf i e l d sa s t h ema g n e t i cf i e l d sc h a n g e ,b u th i g hc on d u c t i v i t yl i mi t st h es i z eoft h es u p p or t e d e l e c t r i cf i e l df ora n yg i v e nma g n e t i cf i e l ds t r e n g t hi naf r e q u e n c yi n d e p e n d e n twa y . Nowwen e e dt oi mp l e me n ta s s u mp t i on2ont h e∇op e r a t or . I fwep i c kac oor d i n a t e ξt ob ep e r p e n d i c u l a rt ot h es u r f a c ep oi n t i n gi n t ot h ec on d u c t or( i nt h e− n ˆdi r e c t i on ) a n di n s i s tt h a ton l yv a r i a t i on si nt h i sd i r e c t i onwi l lb es i g n i f i c a n ton l yonl e n g t hs c a l e s ofδ: ∂ ∇≈− n ˆ ∂ξ ( 1 2 . 1 2 ) t he nwege t : 1 ∂Hc E ˆ× ∂ξ c ≈ −σ n 1 ∂E c Hc≈ iµcω nˆ× ∂ξ ( 1 2 . 1 3 ) ( Not ewe l lt h ed e l i b e r a t eu s eofa p p r oxt oe mp h a s i z et h a tt h e r ema ywe l lb e c omp on e n t soft h ef i e l d si nt h en or ma ld i r e c t i onorot h e rc ou p l i n g sb e t we e nt h e c omp on e n t si nt h es u r f a c e ,b u tt h os ec omp on e n t sd on otv a r yp a r t i c u l a r l yr a p i d l y a l on gt h es u r f a c ea n ds oa r en otl a r g ec on t r i b u t or st ot h ec u r l . ) Th e s et woe qu a t i on sa r ev e r yi n t e r e s t i n g .Th e ys h owt h a twh i l et h ema gn i t u d eof t h ef i e l d si nt h ev i c i n i t yoft h ec on d u c t i n gs u r f a c ema yb el a r g eors ma l l ( d e p e n d i n gon t h ec h a r g ea n dc u r r e n t sn e a rt h es u r f a c e )t h ec u r l st h e ms e l v e sa r ed omi n a t e db yt h e p a r t i c u l a rc omp on e n t sofE n dHct h a ta r ei nt h ep l a n ep e r p e n d i c u l a rt on ˆ( a n de a c h ca ot h e r )b e c a u s et h ef i e l ds t r e n g t h s( wh a t e v e rt h e ya r e )a r emos tr a p i dl yv a r y i n ga c r os s t h es u r f a c e . Wh a tt h i sp a i rofe qu a t i on su l t i ma t e l yd oe si ss h owt h a ti ft h e r ei sama g n e t i cf i e l dj u s t i n s i d et h ec on du c t orp a r a l l e lt oi t ss u r f a c e( a n dh e n c ep e r p e n d i c u l a rt on ˆ )H| h a tr a p i d l y |t v a r i e sa son ed e s c e n ds , t h e nt h e r emu s tb ea ne l e c t r i c f i e l dE h a ti si t sp a r t n e r . Ou rz e r ot ha p p r o x i ma t i onb ou n da r yc on di t i onon | |t H| b ov es h owst h a ti ti sa c t u a l l yc on t i n u o u sa c r os st h ema t h e ma t i c a ls u r f a c eoft h e |a b ou n d a r ya n dd oe sn oth a v et ob ez e r oe i t h e rj u s tou t s i d eorj u s ti n s i d eofi t .Howe v e r , i na g oodc on d u c t ort h eE i e l di tp r odu c e si ss ma l l . | |f Th i sg i v e su sab i tofa ni n t u i t i v ef ou n d a t i onf ort h ema n i p u l a t i on sofMa x we l l ’ s e qu a t i on sb e l ow.Th e ys h ou l dl e a du st oe x p r e s s i on sf ort h ec ou p l e dEM f i e l d s p a r a l l e l t ot h es u r f a c et h a ts e l f c on s i s t e n t l yr e s u l tf r omt h e s et woe qu a t i on s . Wes t a r tb yde t e r mi n i n gt h ec omp on e n tofHc( t h et ot a l v e c t orma g n e t i cf i e l dj u s t i n s i det h ec on du c t or )i nt h ed i r e c t i onp e r p e n di c u l a rt ot h es u r f a c e : ∂E c i n ˆ·Hc= µcωnˆ·( nˆ× ∂ξ )=0 ( 1 2 . 1 4 ) Th i st e l l su st h a tHc=H| n ˆ×Hc)×n ˆ–t h ema g n e t i cf i e l dc ou p l e db y |=( Ecb yF a r a d a y ’ sl a wl i e si nt h ep l a n eoft h ec on d u c t i n gs u r f a c et ol owe s tor d e r . Ne x twef or mav e c t ort h a tl i e sp e r p e n di c u l a rt ob ot ht h en or ma l a n dt h ema g n e t i c f i e l d . Wee x p e c tE ol i ea l on gt h i sd i r e c t i onon ewa yort h eot h e r . ct ∂E c 1 n ˆ×Hc =n ˆ×i µcω n ˆ× ∂ξ 1∂ ( nˆ·Ec)−Ec =i µcω ∂ξ nˆ 1∂ E c , | | =− iµcω ∂ξ ( wh e r eE ˆ ( n ˆ·E )a n dE )a n df i n dt h a ti td oe s !Th ef a c tt h a tt h e c , ⊥ =n c=E c , ⊥ +E c , | | e l e c t r i cf i e l dv a r i e smos tr a p i d l yi nt h e− n ˆ( + ξ )d i r e c t i onp i c k sou ti t sc omp on e n ti n t h ep l a n ewh a t e v e ri tmi gh tb ea n dr e l a t e si tt ot h ema g n e t i cf i e l dd i r e c t i ona l s oi nt h e p l a n e . Howe v e r , t h i sd oe sn ots h owt h a tt h et woc on d i t i on sc a nl e a dt oas e l f s u s t a i n i n g s ol u t i oni nt h ea b s e n c eofd r i v i n ge x t e r n a lc u r r e n t s( f ore x a mp l e ) .Tos h owt h a twe h a v et os u b s t i t u t eAmp e r e ’ sl a wb a c ki n t ot h i s : 1∂ n ˆ×Hc =− i 2 ∂Hc) 2 ( nˆ× ∂ξ 2 ∂ ∂Hc µcω ∂ξ − σ ( n ˆ×∂ξ ) 1 n ˆ×Hc 1 2 ∂Hc) 2 n ˆ× ∂ξ =i µcωσ ( i µcωσn ˆ×Hc )=− n ˆ×Hc +i µcωσn ˆ×Hc =0 ) 2 ∂ξ or 2 2 i 2 2 ∂ n ˆ×Hc)+ δ ( n ˆ×Hc)=0 ∂ξ ( 2 ( 1 2 . 1 5 ) wh e r eweu s e dt h ef i r s tr e s u l ta n ds u b s t i t u t e dδ =2 / ( µ ωσ) . c Th i si sawe l l k n ownd i ffe r e n t i a l e qu a t i ont h a tc a nb ewr i t t e na n yofs e v e r a l 2 2 i wa y s . L e tκ = . I ti se qu i v a l e n tt oa l l of : 2 δ 2 ∂ 2 2 +κ) ( n ˆ×Hc) = 0 ∂ξ ( 2 ∂ 2 2 +κ) ( n ˆ×Hc)×n ˆ = 0 ∂ξ ( 2 ∂ 2 2 +κ) H| | = 0 ∂ξ ( 2 ∂ ( 2 ∂ξ 2 +κ) Hc = 0 ( 1 2 . 1 6 ) ( 1 2 . 1 7 ) ( 1 2 . 1 8 ) ( 1 2 . 1 9 ) Wh e r e : ( n ˆ×Hc)×n ˆ=H| | a sn ot e da b ov e . Th es ol u t i ont ot h i sf or mi st h e n : ( 1 2 . 2 0 ) √ ±−κ2ξ Hc( ξ )=H0e ( 1 2 . 2 1 ) wh e r eH0i st h ema g n e t i cf i e l dv e c t o ri nt h ep l a n eo ft h ec on du c t ora tt h es u r f a c ea n d wh e r et h i se qu a t i oni n d i c a t e sh owt h i sv a l u ei sa t t e n u a t e da son ede c e n dsi n t ot h e c on du c t or . Asa l wa y s ,weh a v et wol i n e a r l yi n d e p e n d e n ts ol u t i on s .E i t h e roft h e m wi l lwor k , a n d( g i v e nt h ea l r e a d yde t e r mi n e ds i g n / b r a n c ha s s oc i a t e dwi t ht h et i med e p e n d e n c e − i ωt e )wi l l u l t i ma t e l yh a v et h ep h y s i c a li n t e r p r e t a t i onofwa v e smov i n gi nt h ed i r e c t i on of+ξ( − n ˆ )ori nt h ed i r e c t i onof− ξ( n ˆ ) .L e tu sp a u s ef oramome n tt or e f r e s hou r me mor yoft a k i n gt h es qu a r er ootofc omp l e xn u mb e r s( u s et h es u b s e c t i ont h a tt r e a t s t h i si nt h el a s tc h a p t e roft h e s en ot e sorv i s i tWi k i p e d i aoft h e r ei sa n yp r ob l e m u n d e r s t a n d i n g ) . F ort h i sp a r t i c u l a rp r ob l e m, 2 − κ= 2 i 2 1 − δ =±δ( − 1+i ) ( 1 2 . 2 2 ) ( d r a wt h i sou ti np i c t u r e s ) .Wewa n tt h es ol u t i ont h a tp r op a g a t e si n t ot h es u r f a c eoft h e c on du c t or , d e c e n di n gf r omt h edi e l e c t r i cme d i u m, wh i c hi st h ep os i t i v eb r a n c h : √ Hc=H0e 2 −κ ξ =H0e 1 −1 +i ) ξ δ( ξ ξ − = H0e i δ eδ ( 1 2 . 2 3 ) i ξ / δ − ωt ( c on s i d e re ) . Nowwen e e dt of i n da ne x p r e s s i onf orE , wh i c hwedob yb a c k s u b s t i t u t i n gi n t o c Amp e r e ’ sL a w: ∂Hc 1 E c =− σ n ˆ× ∂ξ 1 E c 1 − 1+i )n ˆ×H0e =− δσ( −1 +i ) ξ δ( µcω ξ − 1−i ) ( n ˆ×H0) e 2 σ( = ξ i δ eδ ( 1 2 . 2 4 ) Not ewe l l t h edi r e c t i on ! Ob v i ou s l yn ˆ·E , ( i nt h i sa p p r ox i ma t i on )s o c=0 Ecmu s tl i ei nt h ep l a n eoft h ec on d u c t ors u r f a c e , j u s tl i k eH| ! As | b e f or e( wh e nwed i s c u s s e df i e l d si nag oodc on d u c t or ) : •E , Hcn oti np h a s e , b u tou tofp h a s eb yπ/ 4 . c •Ra p i dd e c a ya swa v ep e n e t r a t e ss u r f a c e . •Hc≫E σ“ l a r g e ” , δ“ s ma l l ” )s oe n e r g yi sp r i ma r i l yma g n e t i c . c( •n ˆ⊥ E ˆ–f i e l d sa r ep r e d omi n a n t l yp a r a l l e lt ot h es u r f a c ea n d c⊥ Hc⊥ n mu t u a l l yt r a n s v e r s e ,t h e yp r op a g a t e“ s t r a i g h ti n t o”s u r f a c e ,a t t e n u a t i n gr a p i d l y a st h e yg o. •Re c a l l : n ˆ×( E−E )=0 c ( 1 2 . 2 5 ) a tt h es u r f a c e . Si n c eE i e sa p p r ox i ma t e l yi nt h es u r f a c e , t h i sy i e l ds cl µcω ξ ξ − E≈E c≈ 1−i ) ( n ˆ×H0) e 2σ ( i eδ δ ( 1 2 . 2 6 ) j u s to u t s i d et h es u r f a c e–t h ef i e l di sa p p r ox i ma t e l yc on t i n u ou s !Att h i sl e v e lof a p p r ox i ma t i on , ∇×E=i ωB, Ei sp a r a l l e lt ot h es u r f a c e , a n dt h e r ei sas ma l lB⊥ t ot h es u r f a c eoft h es a meg e n e r a l or de rofma g n i t u d ea sE . Si n c eb ot hE n dH| tt h es u r f a c e( ξ=0 )t h e r emu s tb eap owe rf l owi n t o | |=0a |=0a t h ec on du c t or ! dPi n 1 ∗ µcωδ E c×Hc)= d A =− 2Ren ·( H0| 4| 2 ( 1 2 . 2 7 ) wh e r eweHOPEt h a ti tt u r n si n t oh e a t . L e t ’ ss e e : µcωσ J = σE = −ξ ( 1 − i ) / δ 1−i ) ( n ˆ×H0) e 2( ( 1 2 . 2 8 ) s ot h a tt h et i mea v e r a g e dp owe rl os si s( f r omOh m’ sL a w) : dP 1d P dV = Adξ P 1 ∗ 1 ∗ = 2J·E = 2σJ·J 1 ∞ ∗ ( 1 2 . 2 9 ) ξ J·J = A 2σ 0 d µcω ∞ H0| 2 = A 2| µcω 0 −2 ξ / δ d ξ e 2 H0| = A 4| ( 1 2 . 3 0 ) wh i c hj u s th a p p e n st oc or r e s p on dt ot h ef l u xoft h ep oi n t i n gv e c t ort h r ou g has u r f a c eA! F i n a l l y , wen e e dt od e f i n et h e“ s u r f a c ec u r r e n t ” : ∞ Keff= 0 J d ξ=( n ˆ×H) ( 1 2 . 3 1 ) wh e r eHi sd e t e r mi n e dj u s tou t s i de ( i n s i d e )oft h es u r f a c eofa“ p e r f e c t ”c on d u c t ori n a ni d e a l i z e dl i mi t–n ot et h a twea r ej u s ta d d i n gu pt h et ot a l c u r r e n ti nt h es u r f a c el a y e r a n dt h a ti ta l l wor k sou t . Hop e f u l l yt h i se x p os i t i oni sc omp l e t ee n ou g h( a n dc or r e c te n ou g h )t h a ta n y b ob b l e sf r om l e c t u r ea r es moot h e dou t .Youc a ns e et h a ta l t h ou g hJ a c k s onb l i t h e l y p op sa l l s or t sofp u n c hl i n e sd owni nt h et e x t , t h ea c t u a l a l g e b r aofg e t t i n gt h e m, wh i l e s t r a i g h t f or wa r d , i sn ott r i v i a l ! 12. 2 Mu t i l a t e dMa x we l l ’ sE q u a t i o n s( MME s ) Wea r en owp r e p a r e dt ol ooka tt h ep r op a g a t i onofwa v e si nv ol u me sofs p a c eb ou n d e di n s omewa yb yc on d u c t i n gs u r f a c e s .We ’ l lg e n e r a l l ya s s u met h a tt h ec on d u c t or si nqu e s t i on a r e“ p e r f e c t ”a sf a ra sb ou n d a r yc on d i t i on sont h ed i me n s i on soft h ev ol u mei nqu e s t i ona r e c on c e r n e d.Th ep l a c ewh e r et h i swi l ll e a dt oe r r ori si nt h eg r a du a la t t e n u a t i onofa p r op a g a t i n gwa v ea si tl os e se n e r g yt ot h eJ ou l eh e a t i n goft h es u r f a c eoft h eb ou n d i n g c on d u c t or , b u tt h i sp r oc e s swi l l b es l owr e l a t i v et oawa v e l e n g t ha n du s i n gt h er e s u l t soft h e p r e v i ou ss e c t i onwec a na d dt h i sa t t e n u a t i oni nb yh a n da f t e r wa r d si fn e c e s s a r y . Si n c ewea r eg oi n gt oh a v et os ol v eb ou n d a r yv a l u ep r ob l e msf ort h ewa v ee qu a t i on sf or t h ec ou p l e df i e l dc omp on e n t s ,we ’ db e t t e rs e l e c tar e l a t i v e l ys i mp l eg e ome t r yorwe ’ l lb e h e r ea l ls e me s t e r .Th et wog e ome t r i e swewi l le x a mi n ea r ec y l i n d r i c a lwa v e g u i de swh e r e p r op a g a t i oni sa l on gt h eza x i soft h ec y l i n d e ra n dr e c t a n gu l a rwa v e g u i d e swh e r et h e p r op a g a t i oni sa l on gt h eza x i sofawa v e g u i d ewi t har e c t a n g u l a rc r os s s e c t i oni nt h ex−y p l a n eofd i me n s i ona×b .Th et r a n s v e r s ec oor d i n a t e sa r et h e r e f or e( ρ,φ)or( x ,y ) , r e s p e c t i v e l y . Asu s u a l ,wewi l ls t a r tb ya s s u mi n gt h a twe ’ r ede a l i n gwi t hah a r mon i cwa v ewi t ht i me −i ωt d e p e n de n c ee ,wr i t ed ownMa x we l l ’ se qu a t i on si nf r e es p a c e( t h ec a v i t yv ol u me ) ,t u r n t h e mi n t owa v ee qu a t i on sf ort h ef i e l ds e p a r a t e l y , n ot et h a t t h ef i e l dsa r ec ou p l e db yMa x we l l ’ se qu a t i on st h e ms e l v e s ,a n di mp os eb ou n d a r y c on d i t i on s .Th eon l yt h i n gt h a ti s“ s p e c i a l ”a b ou tac y l i n d e ri st h ef or moft h eL a p l a c i a n a n dh owwes e p a r a t et h el a p l a c i a nt or e s p e c tt h eb ou n d a r yc on d i t i on s .L e t ’ ss k i p a h e a dt ot h ewa v ee qu a t i ons i n c eb yn owe v e r y b od ys h ou l db ea b l et odot h i si nt h e i r s l e e p : 2 2 ( ∇ +µǫω ) EorB ( 1 2 . 3 2 ) =0 Wel ooka tp r op a g a t i ona l on gz , ma k i n gi t“ p l a n e wa v e l i k e ” : ± i k z −i ωt E x (, t ) = E( ρ, φ) e ( 1 2 . 3 3 ) Bx (, t ) = B( ρ, φ) e ( 1 2 . 3 4 ) ± i k z −i ωt s ot h a tt h ewa v ee qu a t i onb e c ome s : 2 2 2 ∇⊥+( µǫω −k) 2 2 EorB ( 1 2 . 3 5 ) =0 ∂ ( Not et h a t∇⊥=∇ −∂z22) . Re s ol v ef i e l d si n t oc omp on e n t s⊥a n d| | t oz : E =E ˆ+( z ˆ×E )×z ˆ= zz ( 1 2 . 3 6 ) E z+E ⊥ B =Bzz ˆ+( z ˆ×B)×z ˆ= Bz+B⊥ ( 1 2 . 3 7 ) ( 1 2 . 3 8 ) ( d e f i n i n gE n dE t c .i nf a i r l yob v i ou swa y s ) .Now wet r yt owr i t eMa x we l l ’ s za ⊥e e qu a t i on si nt e r msoft h e s ef i e l dc omp on e n t s ,a s s u mi n gt h a tt h eo n l yz d e p e n d e n c e ± i k z p e r mi t t e di se . Th i si s n ’ tt r i v i a l t od o–l e t ’ ss t a r twi t hF a r a da y ’ sl a w, f ore x a mp l e : ∂B ωB ∂ t =i ∇×E=− I fwep r oj e c tou tt h ezc omp on e n tofb ot hs i de sweg e t : z ˆ· ωBz z ˆ·( ∇×E ) =i ∂E ∂E ∂E ∂E z y x z ∂ y − ∂z x ˆ+ ∂z − ∂x y ˆ+ ∂E ∂E y x ∂x − ∂y z ˆ ωBz =i ∂E ∂E y x ∂x − ∂y ωBz =i z ˆ·( ∇⊥×E ⊥) ωBz =i ( 1 2 . 3 9 ) a son l yt h e⊥c omp on e n t soft h ec u r l c on t r i b u t et ot h ezdi r e c t i on . Si mi l a r l y : ∂E z z ˆ× z ˆ×( ∇×E ) ∂E y ∂E x =i ω( z ˆ×B) ∂E z ∂y − ∂z x ˆ+ ∂z − ∂x y ˆ ∂E ∂E y x ∂y ∂ z z ˆ =i ω( z ˆ×B⊥) ∂x − ∂y ∂E z− ∂E y y ˆ− + ∂E ∂E x z ∂z − ∂x x ˆ=i ω( z ˆ×B⊥) ∂E ⊥ ω( z ˆ×B⊥)= ∂z +i ∇ ⊥E z ( 1 2 . 4 0 ) ( wh e r ez ˆ×B=z ˆ×B⊥, ofc ou r s e ) . Ou c h !L ook sl i k ewor k i n gt h r ou g ht h ec u r lt e r mwi s ei sac e r t a i na mou n tofp a i n ! Howe v e r ,n owt h a twe ’ v ed on ei ton c e( a n ds e eh owi tg oe s )Amp e r e ’ sl a ws h ou l db e s t r a i g h t f or wa r d: i ωDz z ˆ·( ∇×H) = − i ωµǫE z ˆ·( ∇⊥×B⊥) = − z a n d z ˆ×( ∇×H)=− i ω( z ˆ×D) ∂B⊥ ∂z −i ωµǫ( z ˆ×E ⊥) = ∇ ⊥B z F i n a l l y , weh a v eGa u s s ’ sL a w( s ) : ∇·E = 0 ∂E z ∇⊥· E ⊥+ ∂ z =0 ∇⊥· E ⊥ ∂E z = − ∂z a n di d e n t i c a l l y , ∂ Bz ∇⊥·B⊥=− ∂z L e t ’ sc ol l e c ta l l oft h e s ei nj u s ton ep l a c en ow: ∇⊥· E ⊥ = −∂Ez ( 1 2 . 4 1 ) ∇⊥· B⊥ = −∂Bz ( 1 2 . 4 2 ) ∂z z ˆ·( ∇⊥×B⊥) z ˆ·( ∇⊥×E ⊥) ∂ B⊥ ωµǫ( z ˆ×E ⊥) ∂ z −i ∂ E ⊥ ∂z = − i ωµǫE z = i ωBz = ∇⊥Bz +i ω( z ˆ×B⊥) ∂z = ∇⊥Ez ( 1 2 . 4 3 ) ( 1 2 . 4 4 ) ( 1 2 . 4 5 ) ( 1 2 . 4 6 ) Ge e , on l yaf e wp a ge sofa l g e b r at oob t a i ni nas h o r t e n e dwa ywh a tJ a c k s onj u s tp u t s downi nt h r e es h or tl i n e s .Hop e f u l l yt h ep oi n ti sc l e a r–t o“ g e t ”al otoft h i sy ouh a v et o s oon e rorl a t e rwor ki ta l l ou t , h owe v e rl on gi tma yt a k ey ou , ory ou ’ l l e n du pme mor i z i n g( or t r y i n gt o)a l l ofJ a c k s on ’ sr e s u l t s .Some t h i n gt h a tmos tn or ma l h u ma n sc ou l dn e v e rdoi na l i f e t i meoft r y i n g . . . Ba c kt owor k , a st h e r ei ss t i l l p l e n t yt odo. 12. 3 TE MWa v e s Nowwec a ns t a r tl ook i n ga twa v e f or msi nv a r i ou sc a v i t i e s .Su p p os ewel e tEz=Bz=0 . Th e nt h ewa v ei nt h ec a v i t yi sap u r et r a n s v e r s ee l e c t r oma gn e t i c( TE M)wa v ej u s tl i k e ap l a n ewa v e ,e x c e p tt h a ti th a st os a t i s f yt h eb ou n d a r yc on d i t i on sofap e r f e c t c on du c t ora tt h ec a v i t yb ou n da r y ! Not ef r omt h ee qu a t i on sa b ov et h a t : ∇⊥· E ⊥ = 0 ∇⊥× E⊥ f r omwh i c hwec a ni mme d i a t e l ys e et h a t : = 0 2 ∇⊥ E 0 ⊥= ( 1 2 . 4 7 ) a n dt h a t E ∇φ ⊥=− ( 1 2 . 4 8 ) 2 f ors omes u i t a b l ep ot e n t i a lt h a ts a t i s f i e s∇⊥φ =0 .Th es ol u t i onl ook sl i k ea p r op a g a t i n ge l e c t r o s t a t i cwa v e . F r omt h ewa v ee qu a t i onwes e et h a t : 2 2 µǫω =k or √ k=± ω µǫ wh i c hi sj u s tl i k eap l a n ewa v e( wh i c hc a np r op a g a t ei ne i t h e rd i r e c t i on , r e c a l l ) . ( 1 2 . 4 9 ) ( 1 2 . 5 0 ) Ag a i nr e f e r r i n gt oou rl i s tofmu t i l a t e dMa x we l l e qu a t i on sa b ov e , wes e et h a t : i k E i ω( z ˆ×B⊥) ⊥ =− ωµǫ D⊥= − z ˆ×H⊥) k( √ D⊥= ± µǫ( z ˆ×H⊥) ( 1 2 . 5 1 ) orwor k i n gt h eot h e rwa y , t h a t : √ B⊥= ± z ˆ×E ⊥) µǫ( ( 1 2 . 5 2 ) s owec a ne a s i l yf i n don ef r omt h eot h e r . TEM wa v e sc a n n otb es u s t a i n e di nac y l i n d e rb e c a u s et h es u r r ou n d i n g( p e r f e c t , r e c a l l )c on d u c t ori se qu i p ot e n t i a l .Th e r e f or eE sz e r oa si sB⊥.Howe v e r , t h e ya r et h e ⊥i d o mi n a n twa ye n e r g yi st r a n s mi t t e dd ownac oa x i a l c a b l e , wh e r eap ot e n t i a l di ffe r e n c e i sma i n t a i n e db e t we e nt h ec e n t r a lc o n du c t ora n dt h ec oa x i a ls h e a t h e .I nt h i sc a s et h e f i e l dsa r ev e r ys i mp l e , a st h eEi sp u r e l yr a d i a l a n dt h eBf i e l dc i r c l e st h ec on d u c t or( s o t h ee n e r g yg oe swh i c hwa y ? )wi t hn ozc omp on e n t s . F i n a l l y , n ot et h a ta l l f r e qu e n c i e sa r ep e r mi t t e df oraTE Mwa v e .I ti sn ot“ qu a n t i z e d” b yt h ea p p e a r a n c eofe i g e n v a l u e sdu et oac on s t r a i n i n gb ou n d a r yv a l u ep r ob l e m. 12. 4 TEa n dTMWa v e s Not ewe l lt h a tweh a v ewr i t t e nt h emu t i l a t e dMa x we l lEqu a t i on ss ot h a tt h ezc omp on e n t s a r ea l lo nt h er i gh th a n ds i d e .I ft h e ya r ek n ownf u n c t i on s , a n di ft h eon l yzde p e n de n c ei s t h ec omp l e xe x p on e n t i a l( s owec a nd oa l lt h ez de r i v a t i v e sa n dj u s tb r i n gd owna± i k )t h e n t h et r a n s v e r s ec omp o n e n t sE n d ⊥a B⊥a r ed e t e r mi n e d! + i k z −i ωt I nf a c t( f orp r op a g a t i oni nt h e+zd i r e c t i on , e ) : i k E ω( z ˆ×B⊥)=∇⊥Ez ⊥+i i k ( z ˆ×E +i ωz ˆ×( z ˆ×B⊥)=z ˆ×∇⊥E ⊥) z i k ( z ˆ×E ωB⊥+z ˆ×∇⊥E ⊥) =i z i ωµǫE z ˆ·( ∇⊥×B⊥) =− z ( 1 2 . 5 3 ) ( 1 2 . 5 4 ) a n d i k B⊥−i ωµ ǫ( z ˆ×E ⊥)=∇⊥B z i k B⊥−∇⊥Bz = i ωµǫ( z ˆ×E ⊥) 2 k k k ( z ˆ×E ⊥) iωµǫB⊥− ωµǫ ∇⊥Bz = i 2 k k ωB⊥+z ˆ×∇⊥E z iωµǫB⊥− ωµǫ ∇⊥Bz = i ( 1 2 . 5 5 ) or i B⊥ 2 2 = µǫω −k i k ∇⊥Bz+µǫω( z ˆ×∇⊥Ez) ( 1 2 . 5 6 ) 2 2 E ∇⊥E z ˆ×∇⊥Bz) ⊥ z−ω( = µǫω −k k ( 1 2 . 5 7 ) ( wh e r ewes t a r t e dwi t ht h es e c on de qu a t i ona n de l i mi n a t e dz ˆ×B⊥ t og e tt h es e c on d e qu a t i onj u s tl i k et h ef i r s t ) . Nowc ome st h er e l a t i v e l yt r i c k yp a r t .Re c a l lt h eb ou n d a r yc on d i t i on sf orap e r f e c t c on d u c t or : n ˆ×( E−E )=n ˆ×E c n ˆ·( B−Bc)=n ˆ·B = 0 = 0 n ˆ×H = K n ˆ·D = Σ Th e yt e l l u sb a s i c a l l yt h a tE( D)i ss t r i c t l yp e r p e n di c u l a rt ot h es u r f a c ea n dt h a tB( H)i s s t r i c t l yp a r a l l e l t ot h es u r f a c eoft h ec on d u c t ora tt h es u r f a c eoft h ec on d u c t or . Th i sme a n st h a ti ti sn o tn e c e s s a r yf orE rBzb o t ht ov a n i s he v e r y wh e r ei n s i d et h e zo d i e l e c t r i c( a l t h ou g hb ot hc a n , ofc ou r s e , a n dr e s u l ti naTEMwa v eorn owa v ea ta l l ) .Al l t h a t i ss t r i c t l yr e qu i r e db yt h eb ou n d a r yc on d i t i on si sf or E z| S=0 ( 1 2 . 5 8 ) ont h ec on du c t i n gs u r f a c eS( i tc a non l yh a v ean or ma lc omp on e n ts ot h ezc omp on e n tmu s tv a n i s h ) .Th ec on d i t i ononBzi se v e nwe a k e r .I tmu s tl i ep a r a l l e lt ot h e s u r f a c ea n db ec on t i n u ou sa c r os st h es u r f a c e( wh e r eHc a nd i s c on t i n u ou s l yc h a n g e b e c a u s eofK) . Th a ti s : ∂Bz 0 ∂n| S= ( 1 2 . 5 9 ) Wet h e r e f or eh a v et wop os s i b i l i t i e sf orn o n z e r oEzorBzt h a tc a na c ta ss ou r c e t e r mi nt h emu t i l a t e dMa x we l l Equ a t i on s . 12. 4. 1 TMWa v e s Bz = 0 ( 1 2 . 6 0 ) Ez| S = 0 ( 1 2 . 6 1 ) Th ema g n e t i cf i e l di ss t r i c t l yt r a n s v e r s e , b u tt h ee l e c t r i cf i e l di nt h ezd i r e c t i onon l yh a s t ov a n i s ha tt h eb ou n d a r y–e l s e wh e r ei tc a nh a v eazc omp on e n t . Th u s : i 2 2 E ǫω −k ⊥ = µ 1 2 2 2 2 k ∇⊥E z ˆ×∇⊥Bz) z−ω( ( µǫω −k) E k ∇⊥E ⊥ = i z µǫω −k) E ⊥ = ∇⊥E z i k ( ( 1 2 . 6 2 ) wh i c hl ook sj u s tp e r f e c tt os u b s t i t u t ei n t o: i 2 2 B⊥ = µǫω −k k ∇⊥Bz+µǫω( z ˆ×∇⊥Ez) 2 2 ( µ ǫω −k) B⊥ = i µ ǫω( z ˆ×∇⊥E z) µǫω 2 2 2 2 ( µǫω −k) B⊥ = k ( µǫω −k) ( z ˆ×E ⊥) g i v i n gu s : ( 1 2 . 6 3 ) µǫω B⊥= ± or( a st h eb ookwou l dh a v ei t ) : z ˆ×E ⊥) k( ( 1 2 . 6 4 ) ǫω z ˆ×E ⊥) H ⊥=± k( ( 1 2 . 6 5 ) ( wh e r ea su s u a l t h et wos i g n si n d i c a t et h edi r e c t i onofwa v ep r op a g a t i on ) . Ofc ou r s e ,wes t i l lh a v et of i n da tl e a s ton eoft h et wof i e l d sf ort h i st odou sa n y g ood. Ordowe ?L ook i n ga b ov ewes e e : 2 2 ( µǫω −k) E ⊥ E k ∇⊥ψ =i ± i k = 2 2 ( µǫω −k) ⊥ ∇ ψ ⊥ ( 1 2 . 6 6 ) i k z Wh e r eψ( x , y ) e =E Th i smu s ts a t i s f yt h et r a n s v e r s ewa v ef u n c t i on : z. 2 2 2 ∇⊥ +( µǫω −k)ψ=0 a n dt h eb ou n d a r yc on d i t i on sf oraTMwa v e : ( 1 2 . 6 7 ) ψ| 0 S= ( 1 2 . 6 8 ) TEWa v e s E z = 0 ( 1 2 . 6 9 ) ∂Bz S = 0 ∂n | ( 1 2 . 7 0 ) Th ee l e c t r i cf i e l di ss t r i c t l yt r a n s v e r s e , b u tt h ema g n e t i cf i e l di nt h ez d i r e c t i onc a nb e n on z e r o.Doi n ge x a c t l yt h es a mea l g e b r aont h es a met woe qu a t i on sa sweu s e di nt h e TMc a s e , weg e ti n s t e a d: k z ˆ×E ⊥) H⊥=± µω ( ( 1 2 . 7 1 ) a l on gwi t h i k z B = ± i k ⊥ ( µǫω −k) 2 2 ∇ ψ ( 1 2 . 7 2 ) ⊥ wh e r eψ( x , y ) e =Bza n d 2 2 2 µǫω −k) ψ=0 ∇⊥ +( ( 1 2 . 7 3 ) a n dt h eb ou n d a r yc on d i t i on sf oraTEwa v e : ∂ψ 0 S= ∂n| ( 1 2 . 7 4 ) 12. 4. 2 Su mma r yofTE / TMwa v e s Th et r a n s v e r s ewa v ee qu a t i ona n db ou n d a r yc on d i t i on( d i r i c h l e torn e u ma n n ) a r ea ne i ge n v a l u ep r o b l e m. Wec a ns e et wot h i n g sr i g h ta wa y . F i r s tofa l l : 2 2 µǫω ≥k ( 1 2 . 7 5 ) orwen ol on g e rh a v eawa v e , weh a v ea ne x p on e n t i a lf u n c t i ont h a tc a n n otb ema d et o s a t i s f yt h eb ou n d a r yc on d i t i on sont h ee n t i r es u r f a c e . Al t e r n a t i v e l y , ω2 1 2 2 2 v p =k ≥ µ ǫ=v ( 1 2 . 7 6 ) wh i c hh a st h el ov e l yp r op e r t y( a sap h a s ev e l oc i t y )ofb e i n gf a s t e rt h a nt h es p e e dof l i g h ti nt h eme di u m! Top r oc e e df u r t h e ri nou ru n d e r s t a n d i n g , wen e e dt ol ooka ta na c t u a l e x a mp l e–we ’ l lf i n dt h a ton l yc e r t a i nk nf orn=1 ,2 ,3 . . . n l lp e r mi tt h e n=k 0 c u t offwi b ou n d a r yc on d i t i on st ob es ol v e d, a n dwe ’ l l l e a r ns omei mp or t a n tt h i n g s a b ou tt h ep r op a g a t i n gs ol u t i on sa tt h es a met i me . 12. 5 Re c t a n gu l a rWa v e gu i d e s Re c t a n g u l a rwa v e g u i de sa r ei mp or t a n tf ort wor e a s on s .F i r s tofa l l ,t h eL a p l a c i a n op e r a t ors e p a r a t e sn i c e l yi nCa r t e s i a nc oor d i n a t e s , s ot h a tt h eb ou n d a r yv a l u ep r ob l e m t h a tmu s tb es ol v e di sb ot hf a mi l i a ra n ds t r a i g h t f or wa r d .Se c on d ,t h e ya r ee x t r e me l y c ommoni na c t u a la p p l i c a t i oni np h y s i c sl a b or a t or i e sf orp i p i n ge . g .mi c r owa v e s a r ou n da se x p e r i me n t a l p r ob e s . I nCa r t e s i a nc oor d i n a t e s , t h ewa v ee qu a t i onb e c ome s : 2 ∂2 ∂ 2 2 2 2 µǫω −k)ψ=0 ∂x +∂y +( ( 1 2 . 7 7 ) Th i swa v ee qu a t i ons e p a r a t e sa n ds ol u t i on sa r ep r od u c t sofs i n ,c osore x p on e n t i a l f u n c t i on si ne a c hv a r i a b l es e p a r a t e l y .Tod e t e r mi n ewh i c hc omb i n a t i ont ou s ei ts u ffic e st o l ooka tt h eBC’ sb e i n gs a t i s f i e d.F orTMwa v e s ,on es ol v e sf orψ=E u b j e c tt oE , zs z| S=0 wh i c hi sa u t oma t i c a l l yt r u ei f : mπx nπy s i n a b E x , y )=ψmn( x , y )=E i n z( 0s ( 1 2 . 7 8 ) wh e r eaa n dba r et h edi me n s i on soft h exa n dys i d e soft h eb ou n da r yr e c t a n g l ea n d wh e r ei np r i n c i p l em, n=0 , 1 , 2 . . . . Howe v e r , t h ewa v e n u mb e rofa n yg i v e nmod e( g i v e nt h ef r e qu e n c y )i sd e t e r mi n e df r om: m2 2 2 2 k =µǫω −π 2 n 2 a 2 +b + ( 1 2 . 7 9 ) 2 wh e r ek >0f ora“ wa v e ”t oe x i s tt op r op a g a t ea ta l l . I fe i t h e ri n de xmorni sz e r o, t h e r e i sn owa v e , s ot h ef i r s tmod et h a tc a np r op a g a t eh a sad i s p e r s i onr e l a t i onof : 1 2 2 2 1 2 s ot h a t : a k ǫω −π ( 11=µ 2 b ) + ( 1 2 . 8 0 ) 1 1 ) ω≥ √µǫ a + b =ωc, TM( ( 1 2 . 8 1 ) π 1 1 2 2 E a c hc omb i n a t i onofp e r mi t t e dma n dni sa s s oc i a t e dwi t hac u t offoft h i ss or t–wa v e s wi t hf r e qu e n c i e sg r e a t e rt h a nore qu a lt ot h ec u t offc a ns u p p or tp r op og a t i oni na l lt h e mode swi t hl owe rc u t offf r e qu e n c i e s . I fwer e p e a tt h ea r g u me n ta b ov ef orTEwa v e s( a si sdon ei nJ a c k s on , wh i c hi swh y I d i dTMh e r es oy ouc ou l ds e et h e mb ot h )y ouwi l l b el e db yn e a r l yi d e n t i c a l a r g u me n t s t ot h ec on c l u s i ont h a tt h el owe s tf r e qu e n c ymod ec u t offoc c u r sf ora>b , m=1a n dn= 0t op r od u c et h eHz( x , y )=ψ( x , y )s ol u t i ont ot h ewa v ee qu a t i ona b ov e .Th ec u t offi n t h i sc a s ei s : π1=ωc, ( 1 0 )<ωc, 1 1 ) TE TM( ( 1 2 . 8 2 ) ω≥√µ ǫa Th e r ee x i s t s , t h e r e f or e , ar a n g eoff r e qu e n c i e si nb e t we e nwh e r eon l yon eTEmodei s s u p p or t e dwi t hd i s p e r s i o n : 2 2 π2 − . 2 2 a =µǫω k =k10 ( 1 2 . 8 3 ) Not ewe l lt h a tt h i smodea n dc u t offc or r e s p on d st oe x a c t l yon e h a l faf r e e s p a c e wa v e l e n g t ha c r os st h el on gd i me n s i onoft h ewa v e g u i de .Th ewa v es ol u t i onf ort h e r i g h t p r op a g a t i n gTEmod ei s : Hz =H0cos πx i k z −i ωt e ( 1 2 . 8 4 ) a ∂Hz i k i k a πx ( 1 2 . 8 5 ) i k z − i ω t H s i n e = µǫω −k ∂x =− π 0 a µ ω i µ ω a π x ( 1 2 . 8 6 ) E H0s i n i k z −i ωt = Hx = y e k π a 2 2 2 2 2 2 Weu s e dγ =µǫω −k =π/ aa n dE⊥ =i k / γ∇⊥ψt og e tt h es e c on doft h e s e , a n d k H⊥=ωµ ( z ˆ×E )t og e tt h el a s ton e . ⊥) Th e r ei sal otmor eon ec a ns t u dyi nJ a c k s ona s s oc i a t e dwi t hwa v e g u i d e s , b u twe mu s tmov eona tt h i st i met oab r i e fl ooka tr e s on a n tc a v i t i e s( a n ot h e ri mp or t a n tt op i c ) a n dmu l t i p ol e s . Hx 2 2 12. 6 Re s o n a n tCa v i t i e s Wewi l lc on s i d e rar e s on a n tc a v i t yt ob eawa v e g u i deofl e n g t hdwi t hc a p sa tb ot h e n d s .Asb e f or e ,wemu s ts a t i s f yTEorTMb ou n d a r yc on d i t i on sont h ec a ps u r f a c e s , e i t h e rDi r i c h l e ti nEzo rNe u ma n ni nBz.I nb e t we e n ,wee x p e c tt of i n dh a r mon i c s t a n di n gwa v e si n s t e a doft r a v e l l i n gwa v e s . E l e me n t a r ya r g u me n t sf orp r e s u me ds t a n d i n gwa v ez d e p e n d e n c eof : As i nk z+Bc osk z ( 1 2 . 8 7 ) s u c ht h a tt h es ol u t i onh a sn ode sora n t i n od e sa tb ot he n d sl e a don et oc on c l u d et h a t on l y : k=p π ( 1 2 . 8 8 ) d f orp=0 , 1 , 2 . . .a r es u p p or t e db yt h ec a v i t y .F orTMmod e sE s tv a n i s hont h ec a p s ⊥mu b e c a u s et h en on z e r oE i e l dmu s tb et h eon l yEf i e l dc omp on e n ts u s t a i n e d , h e n c e : zf pπz E x , y )c os z=ψ( ( 1 2 . 8 9 ) d F orTEmod e sHzmu s tv a n i s ha st h eon l yp e r mi t t e df i e l dc omp on e n ti sa n on z e r oH⊥, h e n c e : Hz=ψ( x , y )s i n pπz d ( 1 2 . 9 0 ) Gi v e nt h e s ef or msa n dt h er e l a t i o n sa l r e a dyd e r i v e df ore . g .ar e c t a n g u l a rc a v i t y ,on e c a ne a s i l yf i n dt h ef or mu l a ef ort h ep e r mi t t e dt r a n s v e r s ef i e l d s , e . g . : pπ p πz 2 2 E ⊥ µǫω −k)s =− d( i n d i ǫω pπz ∇⊥ψ ( 1 2 . 9 1 ) H⊥ =− µǫω −k c os d ( z ˆ×∇⊥ψ) ( 1 2 . 9 2 ) ( z ˆ×∇⊥ψ) d pπz ( 1 2 . 9 3 ) ∇⊥ψ ( 1 2 . 9 4 ) 2 2 f orTMf i e l d sa n d i µω 2 2 2 2 pπz E ⊥ =− µǫω −k s i n pπ H⊥ µǫω −k) cos = d( d f orTEf i e l d s , wi t hψ( x , y )d e t e r mi n e da sb e f or ef orc a v i t i e s . Howe v e r ,n owki sd ou b l yd e t e r mi n e da saf u n c t i onofb ot hpa n dda n da sa f u n c t i onofma n dn .Th eon l yf r e qu e n c i e st h a tl e a dt oa c c e p t a b l es ol u t i on sa r eon e s wh e r et h et woma t c h ,wh e r et h er e s on a n tki nt h ezd i r e c t i onc or r e s p on d st oa p e r mi t t e dk ( ω)a s s oc i a t e dwi t hawa v e g u i d emod e . I l e a v ey out or e a da b ou tt h ed e f i n i t i onofQ: Q= ω0 ω ( 1 2 . 9 5 ) ort h ef r a c t i on a l e n e r g yl os sp e rc y c l eoft h ec a v i t yos c i l l a t ori nt h el i mi twh e r et h i squ a n t i t y i ss ma l lc omp a r e dt ot h et ot a le n e r g y .Not et h a tωi st h ef u l lwi d t ha th a l fma x i mu moft h e p r e s u me dr e s on a n tf or m( b a s i c a l l yt h es a mea swa sp r e s u me di nou rd i s c u s s i on sof d i s p e r s i on , b u tf ore n e r g yi n s t e a doff i e l d ) . I s t r on g l ya d v i s et h a ty oug oov e rt h i sony ou rown–Qd e s c r i b e st h ed a mp i n gofe n e r g y s t or e di nac a v i t ymod ed u et oe . g .t h ef i n i t ec on d u c t i v i t yoft h ewa l l sort h ep a r t i a l t r a n s p a r e n c yoft h ee n dc a p st oe n e r g y( a smi g h te x i s ti nt h ec a s eofal a s e rc a v i t y ) .I fy ou g oi n t ol a s e rp h y s i c s ,y o uwi l lv e r ymu c hn e e dt h i s .I fn o t ,y ou ’ l ln e e dt ou n d e r s t a n dt h e g e n e r a li d e aofQt ot e a c hi n t r od u c t or yp h y s i c sa n de . g .L RCc i r c u i t sord a mp e dd r i v e n h a r mon i cos c i l l a t or s , wh e r ei ta l s ooc c u r sa n ds h ou l dk n owi ta tl e a s tqu a l i t a t i v e l yf ore . g . qu a l i f i e r s . I a d d e da nop t i on a l p r ob l e mf orr e s on a n tc a v i t i e st ot h eh ome wor ka s s i g n me n ti n c a s ey ouwa n t e ds ome t h i n gs p e c i f i ct owor konwh i l es t u d y i n gt h i s . 12. 7 Wa v eGu i d e sAs s i gn me n t J a c k s on8 . 2 , 8 . 4 ( , 8 . 6op t i on a l ) Ch a p t e r13 Ra d i a t i o n We l l ,n owweh a v el e a r n e dal i t t l ea b ou th owt od e s c r i b ewa v e sp r op a g a t i n gt h r ou g h “ f r e e ”s p a c e–d i e l e c t r i cme d i a ,p os s i b l yb ou n de db yac on d u c t i n gs u r f a c e .Bu th ow d i dt h e yg e tt h e r e ?We l l , s i ty ou r s e l v e sd owna n dI ’ l lt e l ly ou .Th e ywe r er a d i a t e dt h e r e b ya c c e l e r a t i n g, t i med e p e n d e n tc h a r ge –c u r r e n tdi s t r i b u t i on s ! An dn owwe ’ l l l e a r nh ow. . . Not ewe l l ! Th i st r e a t me n td i ffe r ss u b s t a n t i a l l yf r omJ a c k s on ’ s , wh i c ha c t u a l l yk i n da s u c k s .Ul t i ma t e l yi twi l lb emu c hs i mp l e rt ou n d e r s t a n da n di sc on s i s t e n t l yde v e l op e d. Howe v e r , i tr e a l l yi st h es a met h i n ga n don eg e t st h es a meg e n e r a l e x p r e s s i on sf ort h e mu l t i p ol ef i e l d sorp ot e n t i a l s . 13. 1 Ma x we l l ’ sE qu a t i on s , Ye tAga i n Su p p os ewea r eg i v e nas y s t e mofc l a s s i c a lc h a r g e st h a tos c i l l a t eh a r mon i c a l l ywi t h t i me .Not et h a t ,a sb e f or e ,t h i sc a nb ev i e we da st h es p e c i a lc a s eoft h eF ou r i e r t r a n s f or ma tap a r t i c u l a rf r e qu e n c yofag e n e r a lt i med e p e n d e n td i s t r i b u t i on ; h owe v e r , t h i si sav e r yi n v ol v e di s s u et h a twewi l l e x a mi n ei nd e t a i l l a t e ri nt h es e me s t e r . Th ef or moft h ec h a r g ed i s t r i b u t i onwewi l l s t u dyf ort h en e x tf e wwe e k si s : ρx(, t ) Jx(, t ) − i ωt () e =ρx − i ωt x () e =J . ( 1 3 . 1 ) ( 1 3 . 2 ) Th es p a t i a ld i s t r i b u t i oni se s s e n t i a l l y“ a r b i t r a r y ” .Ac t u a l l y , wewa n ti tt oh a v ec o mp a c t s u p p o r twh i c hj u s tme a n st h a ti tdoe s n ’ te x t e n dt oi n f i n i t yi na n yd i r e c t i on . L a t e rwewi l l a l s owa n ti tt ob es ma l l wi t hr e s p e c tt oawa v e l e n g t h . 13. 1. 1 Qu i c k i eRe v i e wofCh a p t e r6 Re c a l lt h ef ol l owi n gmor p h sofMa x we l l ’ se qu a t i on s ,t h i st i mewi t ht h es ou r c e sa n d e x p r e s s e di nt e r msofp ot e n t i a l sb yme a n soft h eh omog e n e ou se qu a t i on s . 1 4 5 Ga u s s ’ sL a wf orma g n e t i s mi s : ( 1 3 . 3 ) ∇· B= 0 Th i si sa ni de n t i t yi fwed e f i n eB=∇×A: ∇· ( ∇× A) = 0 ( 1 3 . 4 ) Si mi l a r l y , F a r a d a y ’ sL a wi s ∂B ∇× E + ∇ ∂t = 0 ( 1 3 . 5 ) = 0 ( 1 3 . 6 ) ∂t) = 0 ( 1 3 . 7 ) A E+ ∂∇× × ∂t ∂A ∇× ( E + a n di ss a t i s f i e da sa ni d e n t i t yb yas c a l a rp ot e n t i a l s u c ht h a t : ∂A E+ ∂t = − ∇φ ( 1 3 . 8 ) ∂A E = − ∇φ− ∂t ( 1 3 . 9 ) Nowwel ooka tt h ei n h omog e n e ou se qu a t i on si nt e r msoft h ep ot e n t i a l s . Amp e r e ’ sL a w: ∂E ( 1 3 . 1 0 ) = µ( J+ǫ ∂ t) ∇×B ∂E ∇× ( ∇× A) ( 1 3 . 1 1 ) = µ( J+ǫ ∂ t) 2 ∂E ∂ t ∇( ∇·A)−∇A = µJ+µ ǫ 2 ∂A ∂ φ −µ ǫ = µJ−µǫ∇ ∂ t 2 ∂ t 2 ∇( ∇·A)−∇A ∂ A 2 ∂ φ ∇2A−µ ǫ µ J+∇( ∇·A+µ ǫ ∂ ) t 2 = − ∂t Si mi l a r l yGa u s s ’ sL a wf ort h ee l e c t r i cf i e l db e c ome s : ρ ∇·E ∇·− ∇φ− 2φ+ ∇ =ǫ ∂A ρ ∂t =ǫ − = ρ ǫ ∂∇· A ( 1 3 . 1 2 ) ( 1 3 . 1 3 ) ( 1 3 . 1 4 ) ( 1 3 . 1 5 ) ( 1 3 . 1 6 ) ( 1 3 . 1 7 ) I nt h et h eL or e n t zg a u g e , ∂Φ ∂t =0 ∇· A+ µǫ ( 1 3 . 1 8 ) t h ep ot e n t i a l ss a t i s f yt h ef ol l owi n gi n h omoge n e ou swa v ee qu a t i on s : 2 ∂Φ 2 ∇Φ−µ ǫ ∂t 2 ∂A ρ =−ǫ ( 1 3 . 1 9 ) ∇A−µǫ ∂t =− µ J ( 1 3 . 2 0 ) 2 2 2 wh e r eρa n dJa r et h ec h a r g ed e n s i t ya n dc u r r e n td e n s i t ydi s t r i b u t i on s ,r e s p e c t i v e l y . F ort h et i meb e i n gwewi l l s t i c kwi t ht h eL or e n t zg a u g e , a l t h ou g ht h eCou l ombg a u g e : ( 1 3 . 2 1 ) ∇· A= 0 i smor ec on v e n i e n tf orc e r t a i np r ob l e ms .I ti sp r ob a b l ywor t hr e mi n d i n gy ’ a l lt h a tt h e L or e n t zg a u g ec on d i t i oni t s e l fi sr e a l l yj u s ton eou tofawh ol ef a mi l yofc h oi c e s . Re c a l l t h a t( ormor ep r op e r l y , ob s e r v et h a ti ni t sr ol ei nt h e s ewa v ee qu a t i on s ) 1 2 µǫ= v ( 1 3 . 2 2 ) wh e r evi st h es p e e dofl i g h ti nt h eme d i u m.F ort h et i meb e i n g , l e t ’ sj u s ts i mp l i f yl i f ea b i ta n da g r e et owor ki nav a c u u m: 1 µ0ǫ0= c 2 s ot h a t : 2 1∂Φ ρ ∂t = −ǫ0 ∇2Φ− c2 2 ( 1 3 . 2 3 ) 2 2 ( 1 3 . 2 4 ) 1∂A 2 2 µ J 0 ∇A− c ∂t = − ( 1 3 . 2 5 ) I f / wh e nwel ooka twa v es ou r c e se mb e d d e di nad i e l e c t r i cme d i u m, wec a na l wa y s c h a n g eb a c ka st h eg e n e r a l f or ma l i s mwi l l n otb ea n yd i ffe r e n t . 13. 2 Gr e e n ’ sF u n c t i o n sf o rt h eWa v eE qu a t i on Asb yn owy ous h ou l df u l l yu n d e r s t a n df r omwor k i n gwi t ht h ePoi s s one qu a t i on ,on e v e r yg e n e r a lwa yt os ol v ei n h omog e n e ou sp a r t i a ldi ffe r e n t i a le qu a t i on s( PDE s )i st o 1 b u i l daGr e e n ’ sf u n c t i on a n dwr i t et h es ol u t i ona sa ni n t e g r a l e qu a t i on . 1 Not et h a tt h i se x p r e s s i ons t a n dsf or :“ Th eg e n e r a l i z e dp oi n ts ou r c ep ot e n t i a l / f i e l dde v e l op e db yGr e e n . ”A n u mb e rofp e op l ec r i t i c i z et h ev a r i ou swa y sofr e f e r r i n gt oi t–Gr e e nf u n c t i on( wh a tc ol orwa st h a ta g a i n ?wh a t s h a deofGr e e n ? ) ,Gr e e n sf u n c t i on( af u n c t i onma deofl e t t u c ea n ds p i n a c ha n dk a l e ? ) ,“ a ”Gr e e n ’ sf u n c t i on( a s i n g u l a rr e p r e s e n t a t i v eofap l u r a lc l a s sr e f e r e n c e da sas i n g u l a rob j e c t ) .Al lh a v ep r ob l e ms .It e n dt og owi t ht h e l a t t e roft h e s ea si ts e e msl e a s tod dt ome . L e t ’ sv e r yqu i c k l yr e v i e wt h eg e n e r a lc o n c e p t( f oraf u r t h e rdi s c u s s i ondon ’ tf or g e t WI YF, MWI YF ) . Su p p os eDi sag e n e r a l ( s e c on dor d e r )l i n e a rp a r t i a l d i ffe r e n t i a l op e r a t or 3 one . g . I Ra n don ewi s h e st os ol v et h ei n h omog e n e ou se qu a t i on : Df x ()=ρx () ( 1 3 . 2 6 ) f orf. I fon ec a nf i n das ol u t i onGx (− x0)t ot h ea s s oc i a t e dd i ffe r e n t i a le qu a t i onf ora 2 po i n ts ou r c ef u n c t i on : DGx (x , )=δx (− x0) 0 ( 1 3 . 2 7 ) t h e n( s u b j e c tt ov a r i ou sc on d i t i on s ,s u c ha st h ea b i l i t yt oi n t e r c h a n g et h edi f f e r e n t i a l op e r a t ora n dt h ei n t e g r a t i on )t os ol u t i ont ot h i sp r ob l e mi saF r e d h ol mI n t e gr a lE qu a t i on( a c on v ol u t i onoft h eGr e e n ’ sf u n c t i onwi t ht h es ou r c et e r ms ) : 3 f x ()=χ x ()+ 3 Gx (x , ) ρ x (0) dx 0 0 ( 1 3 . 2 8 ) I R wh e r eχ x ()i sa na r b i t r a r ys ol u t i ont ot h ea s s oc i a t e dh omog e n e ou sPDE : D[ χ x () ] =0 Th i ss ol u t i onc a ne a s i l yb ev e r i f i e d : f x ()=χx ()+ ( 1 3 . 2 9 ) 3 3 Gx (x , ρx ( 0) dx 0) 0 ( 1 3 . 3 0 ) I R χ x () ] +DI R3 Df x ()=D[ 3 ρx(0) dx 0 3 ρx (0) dx 0 Gx(x ,0) ( 1 3 . 3 1 ) ( 1 3 . 3 2 ) 3 3 DGx (x , ρx ( 0) dx 0) 0 3 δx (−x0) ρx ( 0) dx 0 Df x ()=0+ I R Df x ()=0+ I R 3 ( 1 3 . 3 3 ) ( 1 3 . 3 4 ) Df x ()=ρx () ( 1 3 . 3 5 ) I ts e e ms ,t h e r e f or e ,t h a twes h ou l dt h or ou gh l yu n d e r s t a n dt h ewa y sofb u i l d i n g Gr e e n ’ sf u n c t i on si ng e n e r a l f orv a r i ou si mp or t a n tPDEs .I ’ mu n c e r t a i nofh owmu c hof t h i st od owi t h i nt h e s en ot e s ,h owe v e r .Th i si s n ’ tr e a l l y“ E l e c t r ody n a mi c s ” ,i ti s ma t h e ma t i c a lp h y s i c s ,on e of t h ef u n d a me n t a lt ool s e t sy ou n e e dt od o El e c t r od y n a mi c s ,qu a n t u m me c h a n i c s ,c l a s s i c a lme c h a n i c s ,a n dmor e .Soc h e c kou t Ar f k e n , Wy l d , WI YF, MWI YF a n dwe ’ l lc on t e n tou r s e l v e swi t hav e r yqu i c kr e v i e woft h e p r i n c i p l eon e swen e e d : 2 Not ewe l l t h a tb ot ht h eGr e e n ’ s“ f u n c t i on ”a n dt h ea s s oc i a t e dDi r a cde l t a“ f u n c t i on ”a r en otf u n c t i o n s– t h e ya r ed e f i n e di nt e r msofl i mi t so fadi s t r i b u t i o ni ns u c hawa yt h a tt h ei n t e r c h a n g eofl i mi t sa n dv a l u e sof t h ei n t e g r a l sa b ov ema k es e n s e .Th i si sn e c e s s a r ya sb ot hoft h eob j e c t sa r es i n gu l a ri nt h el i mi ta n dh e n c e a r eme a n i n g l e s swi t h ou tt h el i mi t i n gp r oc e s s .Howe v e r ,we ’ l lg e ti n t or e a lt r ou b l ei fweh a v et owr i t e“ Th e l i mi toft h edi s t r i b u t i onde f i n e db yGr e e nt h a ti st h es ol u t i onofa ni n h omog e n e ou sPDEwi t has ou r c e d i s t r i b u t i ont h a ti nt h es a mel i mi ta p p r oa c h e sau n i ts ou r c es u p p or t e da tas i n g l ep oi n t ”i n s t e a dofj u s t “ Gr e e n ’ sf u n c t i on ” . Sowewon ’ t . 13. 2. 1 Poi s s o nE qu a t i o n Th eGr e e n ’ sf u n c t i onf ort h ePoi s s on( i n h omog e n e ou sL a p l a c e )e qu a t i on : ρ 2 ∇ φ=− ǫ0 ( 1 3 . 3 6 ) (− x0) ∇Gx (x , 0)=δx ( 1 3 . 3 7 ) i st h es ol u t i ont o: 2 Th u sGx (x ,0)s a t i s f i e st h eh omog e n e ou sL a p l a c ePDEe v e r y wh e r eb u ta tt h es i n g l e p oi n t x0.Th es ol u t i ont ot h eL a p l a c ee qu a t i ont h a th a st h er i g h tde g r e eofs i n g u l a r i t yi s t h e“ p ot e n t i a l ofau n i tp oi n tc h a r g e ” : − 1 , )= 0 Gx(x ( 1 3 . 3 8 ) x0| 4πx| − l oc a t e da t x0. He n c e : φx ()=χ0x ( ρx (0) 3 1 dx 0 )+ x0| 4πǫ0 Vx| − ( 1 3 . 3 9 ) wh i c hi sj u s te x a c t l yc or r e c t . Not ewe l lt h a tt h ei n h omo ge n e o u st e r mχ x ()s ol v e st h eh omoge n e ou sL a p l a c e 0 e qu a t i ona n dh a sv a r i ou si n t e r p r e t a t i on s .I tc a nb ev i e we da sa“ b ou n da r yt e r m”( s u r f a c e i n t e g r a lonS=∂V, t h es u r f a c eSb ou n di n gt h ev ol u meV( Gr e e n ’ sTh e or e m)or , a swes h a l l s e e ,a st h ep ot e n t i a lofa l lt h ec h a r g e si nt h ev ol u mee x t e r i ort oV,ora sag a u g e t r a n s f or ma t i onoft h ep ot e n t i a l .Al la r et r u e , b u tt h e“ b e s t ”wa yt ov i e wi ti sa st h ep ot e n t i a l ofe x t e r i orc h a r g e sa st h a ti swh a ti ti si nn a t u r ee v e nwh e ni ti se x p r e s s e d, v i ai n t e g r a t i on b yp a r t s ,a sas u r f a c ei n t e g r a l ,f orav e r ys e n s i b l ec h oi c eofa s y mp t ot i cb e h a v i oroft h e p ot e n t i a l . Not ee q u a l l ywe l lt h a tt h eGr e e n ’ sf u n c t i oni t s e l fh a sp r e c i s e l yt h es a meg a u g e f r e e d om, a n dc a nb ewr i t t e ni ni t smos tg e n e r a l f or ma s : Gx (x , )=F x (x , )+ 0 0 − 1 ( 1 3 . 4 0 ) x0| 4πx| − 2 wh e r e∇F x (x , )=∇0F x (x , )=0i sa n yb i l i n e a r( s y mme t r i ci nb ot hc oor d i n a t e s )s ol u t i on 0 0 2 t ot h eL a p l a c ee qu a t i on !Howe v e r , wewi l l n o tp r oc e e dt h i swa yi nt h i sp a r toft h ec ou r s ea s i ti si nas e n s eu n p h y s i c a lt oe x p r e s st h ePDE st h i swa ye v e nt h ou g hi tdoe su p onoc c a s i on f a c i l i t a t et h es ol u t i ona l g e b r a i c a l l y . 13. 2. 2 Gr e e n ’ sF u n c t i onf ort h eHe l mh ol t zE qu a t i on I fwef ou r i e rt r a n s f or mt h ewa v ee qu a t i on ,ora l t e r n a t i v e l ya t t e mp tt of i n ds ol u t i on s − i ωt wi t has p e c i f i e dh a r mon i cb e h a v i ori nt i mee ,wec on v e r ti ti n t ot h ef ol l owi n g s p a t i a l f or m: 2 2 ()=− ρω ( 1 3 . 4 1 ) ∇ +k φx ǫ0 − i ωt ( f ore x a mp l e , f r omt h ewa v ee qu a t i ona b ov e , wh e r eρx (, t )=ρωx () e − i ωt 22 2 , φx (, t )= φωx () e , a n dkc =ω b ya s s u mp t i on ) . Th i si sc a l l e dt h ei n h o mo ge n e o u sHe l mh ol t z e qu a t i on( I HE ) . Th eGr e e n ’ sf u n c t i ont h e r e f or eh a st os ol v et h ePDE : 2 2 ∇ +k Gx (x , )=δx (− x0) 0 ( 1 3 . 4 2 ) On c ea g a i n ,t h eGr e e n ’ sf u n c t i ons a t i s f i e st h eh o mo ge n e o u sHe l mh ol t ze qu a t i on ( HHE ) .F u r t h e r mor e ,c l e a r l yt h ePoi s s one qu a t i oni st h ek→ 0l i mi toft h eHe l mh ol t z e qu a t i on .I ti ss t r a i g h t f or wa r dt os h owt h a tt h e r ea r es e v e r a lf u n c t i on st h a ta r eg ood c a n d i d a t e sf orG. Th e ya r e : os ( k x |− x0| ) G0x(x, 0) = −c 4πx |−x0| G+x(x, 0) = Gx (x , ) = − 0 ( 1 3 . 4 3 ) +i k x |−x| − e 0 ( 1 3 . 4 4 ) 4πx |−x0| −i k x |−x| − e ( 1 3 . 4 5 ) 0 4πx |−x0| 2 2 Asb e f or e , on ec a na d da r b i t r a r yb i l i n e a rs ol u t i on st ot h eHHE , ( ∇ +k) F x (x , )= 0 2 2 ( ∇0+k) F x (x , )=0t oa n yoft h e s ea n dt h er e s u l ti ss t i l laGr e e n ’ sf u n c t i on .I nf a c t , 0 t h e s ef or msa r er e l a t e db yt h i ss or toft r a n s f or ma t i ona n ds u p e r p os i t i on : Gx(x, )= 1 ( Gx (x , )+Gx ( x , ) ) 2 − 0 0 + 0 0 ( 1 3 . 4 6 ) or G+x( x , ) = Fx (x , ) 0 0 (x , 0)+G0x ( 1 3 . 4 7 ) x0| ) +G0x ( x , ) 0 = − i s i n ( k x |− e t c . I nt e r msofa n yoft h e s e : x0| 4πx| − 1 3 (0) Gx (x , 0) dx 0 φx () = χ0x()− ǫ0 V ρx 1 i k x | − x | 0 ρx ( ) e 0 = χ0x()+ 2 2 ( 1 3 . 4 8 ) 4πǫ0 V x |− x 0| ( 1 3 . 4 9 ) 3 dx 0 ( 1 3 . 5 0 ) wh e r e( ∇ +k) χ x ( )=0a su s u a l . 0 Wen a met h e s et h r e eb a s i cGr e e n ’ sf u n c t i on sa c c or d i n gt ot h e i ra s y mp t ot i ct i me d e p e n d e n c ef a ra wa yf r o mt h ev ol u meV. I nt h i sr e g i onwee x p e c tt os e ea t i mede p e n d e n c ee me r g ef r omt h ei n t e g r a l ofe . g . i k r −i ωt φx (, t )∼e ( 1 3 . 5 1 ) wh e r er= x || .Th i si sa no u t goi n gs p h e r i c a lwa v e .Con s e qu e n t l yt h eGr e e n ’ sf u n c t i on s a b ov ea r eu s u a l l yc a l l e dt h es t a t i o n a r ywa v e ,o u t go i n gwa v ea n di n c o mi n gwa v e Gr e e n ’ sf u n c t i on s . I ti se s s e n t i a lt on ot e ,h owe v e r ,t h a ta n ys ol u t i ont ot h eI HEc a nb ec on s t r u c t e d f r oma n yoft h e s eGr e e n ’ sf u n c t i on s !Th i si sb e c a u s et h ef or moft h es ol u t i on sa l wa y s di ffe rb yah omog e n e ou ss ol u t i on( a sd ot h eGr e e n ’ sf u n c t i on s )t h e ms e l v e s .Th ema i n r e a s ont ou s eon eort h eot h e ri st ok e e pt h ef or moft h es ol u t i ons i mp l ea n di n t u i t i v e ! F ore x a mp l e , i fwea r el ook i n gf oraφx (, t )t h a ti ss u p p os e dt od e s c r i b et h er a d i a t i on ofa ne l e c t r oma g n e t i cf i e l df r om as ou r c e ,wea r el i k e l yt ou s ea nou t g oi n gwa v e Gr e e n ’ sf u n c t i on wh e r ei fwe a r et r y i n gt o de s c r i b et h ea b s o r p t i on ofa n e l e c t r oma g n e t i cf i e l db yas ou r c e ,wea r el i k e l yt ou s et h ei n c omi n gwa v eGr e e n ’ s f u n c t i on ,wh i l ei fwea r el ook i n gf ors t a t i on a r y( s t a n di n g )wa v e si ns omes or tofl a r g e s p h e r i c a lc a v i t yc ou p l e dt oas ou r c en e a rt h emi dd l et h e n( y oug u e s s e di t )t h e s t a t i on a r ywa v eGr e e n ’ sf u n c t i oni sj u s tp e r f e c t . [ Asap a r e n t h e t i c a l a s i d e , y ouwi l l of t e ns e ep e op l eg e tc a r r i e da wa yi nt h el i t e r a t u r ea n d c on n e c tt h eou t g oi n gwa v eGr e e n ’ sf u n c t i onf ort h eI HEt ot h er e t a r d e dGr e e n ’ sf u n c t i onf or t h eWa v eE qu a t i on( f a i r l yd on e–t h e ya r er e l a t e db yac on t ou ri n t e g r a la swes h a l ls e e mome n t a r i l y )a n da r g u ef orac a u s a li n t e r p r e t a t i onoft h er e l a t e di n t e g r a le qu a t i on s ol u t i on s .Howe v e r ,a sy ouc a nc l e a r l ys e ea b ov e ,n oton l yi st h e r en ob r e a k i n goft i me s y mme t r y , t h er e s u l t i n gd e s c r i p t i on sa r ea l l j u s td i ffe r e n twa y sofv i e wi n gt h es a mes ol u t i on ! Th i si s n ’ tc omp l e t e l yas u r p r i s e–t h ep r oc e s soft a k i n gt h eF ou r i e rt r a n s f or ms y mme t r i c a l l y s a mp l e sa l l oft h ep a s ta n da l l oft h ef u t u r ewh e nd oi n gt h et i mei n t e g r a l . Aswewi l ls e ewh e nd i s c u s s i n gr a d i a t i onr e a c t i ona n dc a u s a l i t ya tt h ev e r ye n dof t h es e me s t e r ,i fa n y t h i n gon eg e t si n t ot r ou b l ewh e non ea s s u me st h a ti ti sa l wa y s c or r e c tt ou s ea nou t g oi n gwa v eorr e t a r d e dGr e e n ’ sf u n c t i on , a st h ea c t u a l f i e l da ta n y p oi n ti ns p a c ea ta n yp oi n ti nt i me i st i me r e v e r s a li n v a r i a n ti nc l a s s i c a l e l e c t r od y n a mi c s– a b s or p t i ona n de mi s s i ona r emi r r orp r oc e s s e sa n db ot ha r e s i mu l t a n e ou s l yoc c u r r i n g wh e nac h a r g e dp a r t i c l ei sb e i n ga c c e l e r a t e db ya n e l e c t r oma g n e t i cf i e l d . ] 13. 2. 3 Gr e e n ’ sF u n c t i onf ort h eWa v eE q u a t i on Th i st i mewea r ei n t e r e s t e di ns ol v i n gt h ei n h omo ge n e ou swa v ee q u a t i o n( I WE ) 2 1∂ 2 ρx (, t ) 2 2∂ ∇ −c t φx (, t )=− ǫ0 ( 1 3 . 5 2 ) ( f ore x a mp l e )di r e c t l y ,wi t h ou td oi n gt h eF ou r i e rt r a n s f or m( s )wed i dt oc on v e r ti ti n t o a nI HE. Pr oc e e d i n ga sb e f or e , wes e e kaGr e e n ’ sf u n c t i ont h a ts a t i s f i e s : 2 2 1∂ − 2∂ 2 c t Gx (, t , x ′ δ( t ,t)=δx ( x) ′ t ) . ( 1 3 . 5 3 ) − ∇ − 0 0 Th ep r i ma r yd i ffe r e n c e sb e t we e nt h i sa n dt h ep r e v i ou sc a s e sa r ea )t h ePDEi s h y p e r b ol i c , n ote l l i p t i c a l , i fy ouh a v ea n yc l u ea st owh a tt h a tme a n s ; b )i ti s n owf ou rd i me n s i on a l–t h e“ p oi n ts ou r c e ”i son et h a te x i s t son l ya tas i n g l ep oi n ti n s p a c ef oras i n g l ei n s t a n ti nt i me . Ofc ou r s et h i sma t h e ma t i c a ld e s c r i p t i onl e a v e su swi t hab i tofa ne x i s t e n t i a l d i l e mma ,a sp h y s i c i s t s .Weg e n e r a l l yh a v el i t t l et r ou b l ewi t ht h ei d e aofg r a d u a l l y r e s t r i c t i n gt h es u p p or to fad i s t r i b u t i ont oas i n g l ep oi n ti ns p a c eb yal i mi t i n gp r oc e s s . Wej u s ts qu e e z ei td own , me n t a l l y .Howe v e r , i nas u p p os e d l yc on s e r v a t i v eUn i v e r s e , i t i sh a r df oru st oi ma g i n eon eoft h os es qu e e z e dd ownd i s t r i b u t i on sofc h a r g ej u s t “ p op p i n gi n t oe x i s t e n c e ”a n dt h e np op p i n gr i g h tou t .Wec a n ’ te v e nd oi tv i aal i mi t i n g p r oc e s s ,a si ti sab i tb ot h e r s omet oc r e a t e / de s t r oyc h a r g eou tofn ot h i n g n e s se v e n g r a d u a l l y !Wea r el e f twi t ht h eu n c omf or t a b l ef e e l i n gt h a tt h i sp a r t i c u l a rd e f i n i t i oni s n on p h y s i c a li nt h a ti tc a nd e s c r i b en oa c t u a lp h y s i c a ls ou r c e s–i ti sb yf a rt h emos t “ ma t h e ma t i c a l ”or“ f or ma l ”oft h ec on s t r u c t swemu s tu s e .I ta l s ol e a v e su swi t h s ome t h i n gt ou n d e r s t a n d . On ewa ywec a np r oc e e di st ov i e wt h eGr e e n ’ sf u n c t i on sf ort h eI HEa sb e i n gt h e F o u r i e rt r a n s f or mo ft h ed e s i r e dGr e e n ’ sf u n c t i onh e r e !Th a ti s , wec a ne x p l oi tt h ef a c t t h a t : 1 ∞ δ( t−t )= 2π 0 ( 1 3 . 5 4 ) −i ω( t −t) e d ω 0 −∞ t oc r e a t eaF ou r i e rt r a n s f or moft h ePDEf ort h eGr e e n ’ sf u n c t i on : 2 2 ∇ +k Gx (x , , ω)=δ x ( 0 i ωt − x0) e 0 ( 1 3 . 5 5 ) ( wh e r eI ’ mi n d i c a t i n gt h ee x p l i c i tωd e p e n d e n c ef ort h emome n t ) . F r omt h ep r e v i ou ss e c t i onwea l r e a d yk n owt h e s es ol u t i on s : i ωt 0 G0x(x, 0, ω) = −c os ( k x |− x0| )e 4πx |−x0| G+x(x, 0, ω) = +i k x |−x| e i ωt − e 0 − 3 0 −i k x |−x| e i ωt − e 0 0 4πx |−x0| Att h i sp oi n ti nt i me t h eon l yt h i n gl e f tt od oi st oF ou r i e rt r a n s f or mb a c k– 3 He h , h e h , h e h . . . : ) ( 1 3 . 5 7 ) 0 4πx |−x0| Gx (x , ,ω) = ( 1 3 . 5 6 ) ( 1 3 . 5 8 ) t ot h i sp oi n ti nt i me : 1 G+x (, t , x0, t )= 0 −i ω( t −t) +i k x |−x| − e e dω 0 ∞ ( 1 3 . 5 9 ) 0 2 π−∞ 4 πx |− x0| = 1 ∞ − 1 2π4πx |− x0| = − 1 4πx x | 1 − 0 | −∞ ω +i cx | − e −x0| −i ω( t −t 0) e d ω × ∞ |− x0| i ω( t t) x − −0− c x p 2π −∞ −e x |−x0 | t−t )− c 0 − δ ( = ( 1 3 . 6 0 ) d ω( 1 3 . 6 1 ) ( 1 3 . 6 2 ) 4πx|−x 0| s ot ha t : G±x (, t , x0, t ) 0 Gx (, t , x 0 0 = t−t )∓ − δ ( 0 x |−x0 | c 4πx|−x 0| , t) = 1 ( Gx (, t , x ,t )+Gx (, t , x − 2 + 0 0 0 ( 1 3 . 6 3 ) ,t ) )( 1 3 . 6 4 ) 0 0 Not et h a twh e nwes e tk=ω/ c ,web a s i c a l l ya s s e r t e dt h a tt h es ol u t i oni sb e i n gd e f i n e d wi t h ou tdi s p e r s i on !I ft h e r ei sd i s p e r s i on , t h eF ou r i e rt r a n s f or mwi l ln ol on g e rn e a t l yl i n eu p a n dy i e l dad e l t af u n c t i on ,b e c a u s et h edi ffe r e n tF ou r i e rc omp on e n t swi l ln ott r a v e la tt h e s a mes p e e d .I nt h a tc a s eon emi g h ts t i l le x p e c tap e a k e dd i s t r i b u t i on , b u tn ota ni n f i n i t e l y s h a r pp e a k e dd i s t r i b u t i on . Th ef i r s tp a i ra r eg e n e r a l l yr e a r r a n g e d( u s i n gt h es y mme t r yoft h ed e l t af u n c t i on ) a n dp r e s e n t e da s : ( G ) ± ′ ′′ x(, t x;, t )= δ( t−t∓ ′ x |−x | c ( 1 3 . 6 5 ) x |− x ′| a n da r ec a l l e dt h er e t a r d e d( + )a n da d v a n c e d( )Gr e e n ’ sf u n c t i on sf ort h ewa v e e qu a t i on . Th es e c on df or mi sav e r yi n t e r e s t i n gb e a s t .I ti sob v i ou s l yaGr e e n ’ sf u n c t i onb y c on s t r u c t i on , b u ti ti sas y mme t r i cc omb i n a t i onofa d v a n c e da n dr e t a r d e d .I t su s e“ me a n s ” t h a taf i e l da ta n yg i v e np oi n ti ns p a c e t i mex (, t )c on s i s t soft wop i e c e s–on eh a l fofi ti s d u et oa l lt h es ou r c e si ns p a c ei nt h ep a s ts u c ht h a tt h ef i e l d st h e ye mi ta r ec on t r a c t i n g p r e c i s e l yt ot h ep oi n t xa tt h ei n s t a n tta n dt h eot h e rh a l fi sd u et oa l l oft h os es a mes ou r c e s i ns p a c ei nt h ef u t u r es u c ht h a tt h ef i e l d sc u r r e n t l ye me r g i n gf r omt h ep oi n txa ttp r e c i s e l y a r r i v ea tt h e m.Ac c or di n gt ot h i sv i e w, t h ef i e l da ta l l p oi n t si ns p a c e t i mei sa smu c hdu et o t h ec h a r g e si nt h ef u t u r ea si ti st h os es a mec h a r g e si nt h ep a s t . Ag a i ni ti swor t h wh i l et on ot et h a ta n ya c t u a lf i e l dc o n f i gu r a t i on( s ol u t i ont ot h ewa v e e qu a t i on )c a nb ec on s t r u c t e df r oma n yo ft h e s eGr e e n ’ sf u n c t i o n s a u g me n t e db yt h ea d di t i onofa na r b i t r a r yb i l i n e a rs o l u t i o nt ot h eh omog e n e ou swa v e e qu a t i on( HWE)i np r i me da n du n p r i me dc oor di n a t e s .Weu s u a l l ys e l e c tt h er e t a r de d Gr e e n ’ sf u n c t i ona st h e“ c a u s a l ”on et os i mp l i f yt h ewa ywet h i n kofa ne v a l u a t es ol u t i on s a s“ i n i t i a lv a l u ep r ob l e ms ” ,n otb e c a u s et h e ya r ea n ymor eorl e s sc a u s a lt h a nt h eot h e r s . Ca u s ema yp r e c e d ee ffe c ti nh u ma np e r c e p t i on ,b u ta sf a ra st h ee qu a t i on sofc l a s s i c a l e l e c t r ody n a mi c sa r ec on c e r n e dt h ec on c e p tof“ c a u s e ”i sb e t t e re x p r e s s e da son eof i n t e r a c t i onv i aas u i t a b l ep r op a g a t or( Gr e e n ’ sf u n c t i on )t h a tma ywe l lb et i me s y mme t r i cor a dv a n c e d . Af i n a ln ot eb e f or emov i n goni st h a tt h e r ea r es i mp l yl ov e l yp a p e r s( t h a tweh op e t oh a v et i met os t u d y )b yDi r a ca n db yWh e e l e ra n dF e y n ma nt h a te x a mi n er a di a t i on r e a c t i ona n dt h er a d i a t i onf i e l da sc on s t r u c t e db ya d v a n c e da n dr e t a r d e dGr e e n ’ s f u n c t i on si nc on s i d e r a b l ed e t a i l .Di r a cs h owe dt h a tt h ed i ffe r e n c eb e t we e nt h e a d v a n c e da n dr e t a r de dGr e e n ’ sf u n c t i on sa tt h ep os i t i onofac h a r g ewa sa ni mp or t a n t qu a n t i t y , r e l a t e dt ot h ec h a n gei tma dei nt h ef i e l dp r e s u ma b l yc r e a t e db ya l lt h eot h e r c h a r g e si nt h eUn i v e r s ea tt h a tp oi n ti ns p a c ea n dt i me .Weh a v eal ott os t u d yh e r e , i n ot h e rwor ds . Us i n g( s a y )t h eu s u a l r e t a r d e dGr e e n ’ sf u n c t i on , wec ou l da su s u a l wr i t ea ni n t e g r a l e qu a t i onf ort h es ol u t i o nt ot h eg e n e r a l I WEa b ov ef ore . g . Ax (, t ) : ′ Ax (, t )=χ (, t )−µ0 Ax ′′ 3′ ′ G+x (, t x ;, t ) J x (, t ) dxdt ( 1 3 . 6 6 ) V wh e r eχ ol v e st h eHWE .Th i s( wi t hχ )i se s s e n t i a l l ye qu a t i on( 9 . 2 ) , wh i c hi swh y As A=0 I h a v er e v i e we dt h i s . Ob v i ou s l ywea l s oh a v e 1 φx (, t )=χ (, t )− φx ′ ′′ 3′ ′ (, t x ; , t ) ρx (, t ) dxdt ǫ0 VG+x ( 1 3 . 6 7 ) f orφx (,t )( t h emi n u ss i g n sa r ei nt h ed i ffe r e n t i a le qu a t i on swi t ht h es ou r c e s ,n ot e ) . Yous h ou l df or ma l l yv e r i f yt h a tt h e s es ol u t i on s“ wor k ”g i v e nt h ed e f i n i t i onoft h e Gr e e n ’ sf u n c t i ona b ov ea n dt h ea b i l i t yt or e v e r s et h eor d e rofd i ffe r e n t i a t i ona n d i n t e g r a t i on( b r i n g i n gt h ed i ffe r e n t i a lop e r a t or s , a p p l i e df r omt h el e f t , i nu n d e r n e a t ht h e i n t e g r a l s i g n ) . J a c k s onp r oc e e dsf r om t h e s ee qu a t i on sb yf ou r i e rt r a n s f or mi n gb a c ki n t oak r e p r e s e n t a t i on( e l i mi n a t i n gt i me )a n de x p a n d i n gt h er e s u l tt og e tt omu l t i p ol a r r a d i a t i ona ta n yg i v e nf r e qu e n c y .Howe v e r ,b e c a u s eoft h ewa ywep r oc e e d e da b ov e , wed on ’ th a v et od ot h i s .Wec ou l dj u s ta se a s i l ys t a r tb ywor k i n gwi t ht h eI HEi n s t e a d oft h eI WEa n du s eou rHEGr e e n ’ sf u n c t i on s . I n d e e d , t h a t ’ st h ep l a n , St a n . . . 13. 3 Si mp l eRa d i a t i n gSy s t e ms L e tu ss t a r tb ywr i t i n gt h ei n t e g r a l e qu a t i onf ort h ev e c t orp ot e n t i a l Ax ()wh e r ewep r e s u me t h a twe ’ v ea l r e a dyt r a n s f or me dt h eI WEi n t ot h eI HE.Wewi l lc h oos et ou s et h eou t g oi n g wa v eGr e e n ’ sf u n c t i ont oma k ei tc l e a rt h a tt h e f i e l dwea r el ook i n gf ori st h eon et h a tt h es ou r c ei se mi t t i n g ,n oton et h a ti ti s a b s or b i n g . ′ i k x |−x| e Ax ()=+µ0 ′ 3′ J x () dx. ( 1 3 . 6 8 ) 4πx |−x ′| Th e r ei sn oi n h omog e n e ou st e r mi ft h e r ea r en ob ou n d a r i e swi t hap r i or ik n own b ou n da r yc on d i t i on s . Not et h a tamor eg e n e r a ls ol u t i onwou l db eon et h a ta l l owe df ora b s or p t i onof i n c omi n gwa v e sa swe l l a st h ee mi s s i onofou t g oi n gwa v e s , b u tt h a tt h i swou l dr e qu i r e k n owi n gs ome t h i n ga b ou tt h es ou r c e sou t s i d et h ed oma i nc on s i d e r e dt ob ei n f i n i t e . We wi l l t a l ka b ou tt h i sl a t e r( s c a t t e r i n gt h e or ya n dt h eop t i c a l t h e or e m) . F r omAx ()wec a ne a s i l yf i n dBo rH: B= µ B= ∇× A 0 ( 1 3 . 6 9 ) ( b yd e f i n i t i on ) . Ou t s i d eoft h es ou r c e , t h ou g h( wh e r et h ec u r r e n t sa r ea l l z e r o) Amp e r e ’ sl a wt e l l su st h a t : ( 1 3 . 7 0 ) ∇×H=− i ωD or or ∇×B =− i ωµ ǫ0E 0 ∇×B =− i c E=i cE c ω 2 k ( 1 3 . 7 1 ) ( 1 3 . 7 2 ) E=i k∇×B ( 1 3 . 7 3 ) Doi n gt h ei n t e g r a la b ov ec a nb eq u i t ed i ffic u l ti nt h eg e n e r a lc a s e .Howe v e r ,we ’ l lf i n d t h a tf ormos tr e a s on a b l e ,p h y s i c a ls i t u a t i on s we wi l lb ea b l et oe mp l oyc e r t a i n a p p r ox i ma t i on st h a twi l le n a b l eu st oob t a i nas y s t e ma t i ch i e r a r c h yofd e s c r i p t i on st h a t c on v e r g et ot h ec or r e c ta n s we ra sa c c u r a t e l ya sy oul i k e , a tt h es a met i met h e yi n c r e a s eou r p h y s i c a l i n s i g h ti n t ot h er a di a t i v ep r oc e s s e s . 13. 3. 1 Th eZo n e s Su p p os et h es ou r c el i v e si n s i d ear e g i onofma x i mu ms i z ed≪ λwh e r eλ=2 πc / ω.By t h a tIme a nt h a tas p h e r eofr a d i u sd( a b ou tt h eor i g i n )c omp l e t e l yc on t a i n sa l l c h a r g e –c u r r e n td i s t r i b u t i on s . Th e nwec a nd e f i n et h r e ez on e sofa p p r ox i ma t i on : a )Th en e a r( s t a t i c )z on e d< <r< <λ b )Th ei n t e r me d i a t e( i n d u c t i on )z on e c )Th ef a r( r a d i a t i on )z on e d< <r∼λ d< <λ< <r Th ef i e l dh a sv e r yd i ffe r e n tp r op e r t i e si nt h e s ez on e s . Wewi l l b r i e f l ydi s c u s se a c hof t h e m. •( At omi ca n dMol e c u l a r )s ou r c e sa l ll i v ei n s i d et h e i rownn e a rz on ea top t i c a l f r e qu e n c i e s .I ft h ea t omsa r ei nal i qu i dors ol i d,t h e r ei san e a rf i e l di n t e r a c t i on ( i mp l i c i t l ya l l u d e dt oi nc h a p t e r4a n d7 )t h a tma yb ei mp or t a n ti nd e t e r mi n i n g op t i c a l d i s p e r s i ona n dot h e rob s e r v a b l ep h e n ome n a .On l yf ormi c r owa v ef r e qu e n c i e s a n dl e s sd oe st h en e a rz on eb e c omer e l e v a n tonama c r os c op i cs c a l e .F orr fwa v e s i tb e c ome se x t r e me l yr e l e v a n t , a si tma ye x t e n dah u n d r e dme t e r sormor e . •Th ei n d u c t i onz on ei sa na n n oy i n gr e g i onwh e r emos toft h es i mp l ea p p r ox i ma t i on sf a i l .I ti sd i s t i n g u i s h e db yn otb e i n ge i t h e roft h eot h e rt woz on e s . Th ewa v ec h a n g e sc h a r a c t e rc omp l e t e l yi n s i d et h i sz on e .Be c a u s ec on d e n s e d ma t t e rt h e or yi n v a r i a b l yh a sob j e c t si n t e r a c t i n gwi t h i nt h i sz on ei ti si mp or t a n t t h e r e ,a l t h ou g hi tc a non l yb ec r u d e l yt r e a t e d.Wi t h ou td oi n ga nob n ox i ou s a mou n tofwor k , t h a ti s . •Th ef a rz on ei swh e r ewea l ll i v e ,mos toft h et i me ,wi t hr e s p e c tt ot h ema j or s ou r c e sofE Mr a d i a t i on .Ou rd e t e c t or sa r ema n ywa v e l e n g t h sa wa yf r om t h e a t oms .Ou rr a d i osa r ema n ywa v e l e n g t h sa wa yf r om t h et r a n s mi t t e r s .An d , 4 g e n e r a l l y ,t h es ou r c e sa r es ma l l e r ,i fn otmu c hs ma l l e r ,t h a nawa v e l e n g t h.I n t h ef a rz on e , t h ee mi t t e dEMf i e l dsa r ec h a r a c t e r i s t i c a l l yt r a n s v e r s ea n df a l loff i na mp l i t u dea s1 / rorf a s t e r ,a n dof t e nf a re n o u gha wa yt h e yl ookl oc a l l yl i k e p l a n ewa v e s !Th i si st y p i c a lofr a di a t i onf i e l d sf r omc omp a c ts ou r c e s .Wewi l l s p e n dmos tofou rt i mec on s i de r i n gs ol u t i on si nt h ef a rz on e . 13. 3. 2 Th eNe a rZon e Su p p os et h a twea r ei nt h en e a rz o n e . Th e nb yde f i n i t i on ′ k x |− x| < <1 a n d i k x |− x′|≈1 e Th i sma k e st h ei n t e g r a le qu a t i oni n t ot h e“ s t a t i c ”f or ma l r e a d yc on s i d e r e di n ′ c h a p t e r5( c f .e qu a t i on( 5 . 3 2 ) ) . Wes e et h a t− 1 / 4 πx | − x| i sj u s tt h eGr e e n ’ sf u n c t i onf or t h eg oodol dPoi s s one q u a t i oni nt h i sa p p r ox i ma t i ona n dc a nb ee x p a n de di nh a r mon i c f u n c t i on sj u s tl i k ei nt h eg oodol dd a y s : − 1 ′ G0x (x ,)= ′ ℓ r ℓ + 1 L 2 ℓ+1r ˆ ′∗ YL( ˆ r ) YL( r). ( 1 3 . 7 4 ) Not eWe l l :I wi l l u s eL≡( ℓ , m)f r e e l ya n dwi t h ou twa r n i n gi nt h i sc ou r s e . Th es u mi s ov e ra l l ℓ , m. Hop e f u l l y , b yn owy ouk n owwh a tt h e yr u nov e r . I f 4 Wewi l l l e a r nt ot r e a tc e r t a i ne x c e p t i on s , b e l i e v eme . n ot , r e a dt h ec h a p t e ri nWy l dons p h e r i c a l h a r mon i c sa n dr e v i e wJ a c k s ona swe l l .Th i s i si mp o r t a n t ! Th i sme a n st h a t( i fy oul i k e ) 1 l i mAx ()= ′ ℓ ) r Y Y ( ˆ r ) Jx (′ ℓ + 1 2 ℓ+1 ) r L( k r →0 L ∗ 3′ ( r̂)dr . L ( 1 3 . 7 5 ) ′ Wewi l l u s ee x p r e s s i on sl i k et h i s( de r i v e df r omt h emu l t i p ol a re x p a n s i onoft h eGr e e n ’ s f u n c t i on )f r e qu e n t l yi nwh a tf ol l ows .F ort h a tr e a s onIs u g g e s tt h a ty ous t u dyi t c a r e f u l l ya n db es u r ey ouu n d e r s t a n di t . Si n c e( f orf i x e drou t s i d et h es ou r c e ) l i m→ l i m k →0 k r →0 wes e et h a tt h i sl i mi ti sr e a c h e d( a mon got h e rt i me s )wh e n k→ 0 ( r e l a t i v et ot h es i z eoft h es ou r c ea n dp oi n tofme a s u r e me n t ) !Bu tt h e nt h eI HEt u r n s b a c ki n t ot h ePoi s s one qu a t i on( ori n h omog e n e ou sL a p l a c ee qu a t i on , I L E )a si ts h ou l d , c omet ot h i n ka b ou ti t .Th en e a rf i e l d sos c i l l a t eh a r mon i c a l l yi nt i me , b u ta r es p a t i a l l y i d e n t i c a lt ot h ef i e l d sp r od u c e db ya“ s t a t i c ”c u r r e n twi t ht h eg i v e ns p a t i a ld i s t r i b u t i on . Th a t ’ swh ywea l s oc a l l t h en e a rz on et h e“ s t a t i cz on e ” . 13. 3. 3 Th eF a rZo n e E x a c t l yt h eop p os i t ei st r u ei nt h ef a rz on e .He r ek r> >1a n dt h ee x p on e n t i a l os c i l l a t e s r a p i d l y . Wec a na p p r ox i ma t et h ea r g u me n toft h ee x p on e n t i a l a sf ol l ows : xx |− ′ 2 ′ 2 r +r = | 2r nx′ 2 − · ′ 2 r 2 rnx·′+ r 1 ′ ·+O = r−nx r =r1− 1 / 2 ( 1 3 . 7 6 ) ′ wh e r eweh a v ea s s u me dt h a tr <ra n du s e dab i n omi a le x p a n s i onoft h er oot ma x<d< s u m.Wen e g l e c th i g h e ror d e rt e r ms .Not et h a tt h i sa p p r ox i ma t i oni sg oodi n d e p e n d e n t ofka n dma yb eg oode v e ni nt h en e a rz on e . Th e n i kr ′ ′ −i k n x ˆ ·d 3 ′ l i m v Ax ()= µ0e J x ( ) e x . ( 1 3 . 7 7 ) 4 πr I nt h ef a rz on e ,t h es ol u t i onb e h a v e sl i k ea nou t g oi n gs p h e r i c a lwa v et i me sa n a mp l i t u d et h a td e p e n d soni n t e g r a lov e rt h es ou r c et h a td e p e n d sona n g l e si na n i n t r i c a t ef a s h i on . ( k ) r →∞ Att h i sp oi n tI c ou l dc on t i n u ea n de x t r a c t i kr l i m v Ax ()= µ0e ( k ) r→∞ 4 πr n ( − i k ) Jx ( n ! n ′ ′n 3′ ) ( n ˆx)dx · ( 1 3 . 7 8 ) ( i ft h es ou r c ei sa c t u a l l ys ma l le n ou g ht oa l l owe x p a n s i onoft h ee x p on e n t i a li na 5 s e r i e s) . Th i swou l dg i v eu sac h e a pi n t r od u c t i oni n t omu l t i p ol e s . Bu ti ti ss os l op p y ! I n s t e a dwea r eg oi n gt od oi tr i g h t .Wewi l lb e g i nb yr e v i e wi n gt h es ol u t i on st ot h e h omo ge n e ou sHe l mh ol t ze qu a t i on( wh i c hs h ou l dr e a l l yb edi s c u s s e db e f or ewes we a t s ol v i n gt h ei n h o moge n e o u se qu a t i on ,d on ’ ty out h i n k ? )a n dwi l lc on s t r u c tt h emu l t i p ol a r e x p a n s i o nf ort h eou t g oi n ga n di n c omi n g( a n ds t a t i on a r y )wa v eGr e e n ’ sf u n c t i on .Us i n g t h i s , i twi l lb eat r i v i a l ma t t e rt owr i t ed ownaf or ma l l ye x a c ta n dc on v e r g e n ts ol u t i ont ot h e i n t e g r a l e qu a t i ono na l ls p a c et h a twec a nc h opu pa n da p p r ox i ma t ea swep l e a s e .Th i swi l l p r ov i d eamu c hmor en a t u r a l ( a n da c c u r a t e )p a t ht omu l t i p ol a rr a d i a t i on . Sol e t ’ ss t a r t . 13. 4 Th eHo mo ge n e o u sHe l mh o l t zE q u a t i o n Re c a l la sy our e a dt h i st h a tWI YFa n dMWI YF i na d di t i ont ot h et r e a t me n toft h i s a v a i l a b l ei nJ a c k s on , c h a p t e r s2 , 3 , 6 , a n d8ofWy l d , a n ddou b t l e s sAr f k i n , Mor s ea n d F e s h b a c k , a n dp r ob a b l ys i xot h e rs ou r c e si fy oul ook .Ve r yi mp or t a n ts t u ff, c a n ’ tk n ow i tt oowe l l . Re c a l l f r oma b ov et h eHomoge n e o u sHe l mh ol t zE qu a t i on( HHE ) : 2 6 2 ( ∇ +k) χ x ()=0 . ( 1 3 . 7 9 ) χ x ()= f ( r ) YL( θ , φ) . ℓ ( 1 3 . 8 0 ) Wea s s u met h a t: L Wer e d u c et h eHHEwi t ht h i sa s s u mp t i ont ot h er a di a l d i ffe r e n t i a l e q u a t i o n 2 d 2d ℓ ( ℓ+1 ) 2 2 2 d r +rd r+k − I fwes u b s t i t u t e r f ( r )=0 . ℓ ( 1 3 . 8 1 ) 1 f ( r )= r ℓ 1 / 2u ( r ) ℓ ( 1 3 . 8 2 ) wet r a n s f or mt h i si n t oa ne qu a t i onf oru ( r ) , ℓ 2 d 2 5 1d 2 d r + rd r+k − ( ℓ+ 1) 2 2 2 r u ( r )=0 . ℓ ( 1 3 . 8 3 ) Ta y l or ?Powe r ?L a u r e n t ?Wh oc a nr e me mb e r . . . 6 2 Th i sr e a l l yi s n ’ ta na s s u mp t i on .Wec ou l de qu a l l ywe l l wr i t e∇ i ns p h e r i c a l p ol a rc oor d i n a t e s , s e p a r a t e v a r i a b l e s ,n ot et h a tt h ea n g u l a rODE sh a v es p h e r i c a lh a r mon i c sa se i g e n s t a t e s( “ qu a n t i z e d”b yt h e r e qu i r e me n tofs i n g l e v a l u e d n e s sone . g .r ot a t i on sof2 πi nφ)a n dr e c on s t r u c tt h es e p a r a t e ds ol u t i on .Bu t t h a t ’ st oomu c hwor ka n dwea l r e a d yd i di ta tl e a s ton c ei nou rl i v e s , r i g h t ?Sowe ’ l l “ a s s u me ” . Th ei sBe s s e l ’ sdi ffe r e n t i a le qu a t i on .Se eWy l d , ( 2 6 )orJ a c k s oni nv a r i ou sp l a c e s( s e e k e yonb a c ki n s i dec ov e r )f ormor ede t a i l . Ory ou rownf a v or i t eMa t hPh y s i c sb ook . 3 Twol i n e a r l yi n d e p e n d e n ts ol u t i on sonI R mi n u st h eor i g i nt ot h i sr a d i a l DEa r e : f ( r ) ℓ ( k r )a n d ℓ =j ( 1 3 . 8 4 ) f ( r ) ℓ ( k r ) , =n ℓ ( 1 3 . 8 5 ) t h es p h e r i c a lb e s s e lf u n c t i ona n ds p h e r i c a ln e u ma n nf u n c t i on sr e s p e c t i v e l y .Th e ya r e b ot hr e a l , a n dh e n c ea r es t a t i on a r yi nt i me( wh y ? ) .Th ej ( k r )a r er e gu l a r( f i n i t e )a tt h e ℓ or i g i nwh i l et h en ( k r )a r ei r r e gu l a r( i n f i n i t e )a tt h eor i g i n .Th i si si ne x a c ta n a l og ywi t h ℓ t h es i t u a t i onf ort h eh omog e n e ou sL a p l a c ee qu a t i on( wh i c hi sas p e c i a lc a s eoft h i s s ol u t i on ) . Th ef ol l owi n gi saMI NI MALt a b l eoft h e i ri mp or t a n tp r op e r t i e s .Ab e t t e rt a b l ec a n b ef ou n di nWy l db e t we e nc h p s .6a n d7a n di nMor s ea n dF e s h b a c h( Ic a n ’ tr e me mb e r wh i c hv ol u me ) . 13. 4. 1 Pr o p e r t i e so fSp h e r i c a l Be s s e l F u n c t i on s Re c u r s i o nRe l a t i o n L e tz ( x )b ee i t h e rs ol u t i onoral i n e a rc omb i n a t i onoft h et wo.xi sac omp l e xs c a l a r ℓ i n d e p e n de n tv a r i a b l e( i np r a c t i c e , x=k r ) . Th e n 2 ℓ +1 ( x )−z ( x ) . x z ℓ ℓ −1 z ( x )= ℓ + 1 ( 1 3 . 8 6 ) Th i sr e l a t i oni ss t a b l ef ori n c r e a s i n gℓf orz . I ti ss t a b l ef ord e c r e a s i n g ℓ=n ℓ ± ℓf orz .F ort h a tr e a s oni ti su n s t a b l ei nb ot hd i r e c t i on sf orhℓ( d e f i n e db e l ow) . ℓ=j ℓ How wou l dy ouma k ei t ?Se eAb r a mowi t za n dSt e g u n ,Ha n d b oo ko fMa t h ma t i c a l F u n c t i o n sf ordi s c u s s i onofr e c u r s i v ea l g or i t h ma n dd e f i n i t i onofp owe rs e r i e s e x p a n s i on s . . Th eL o we s tF e wF u n c t i on s j 0 ( x ) = j 1 ( x ) = . s i n ( x ) x s i n ( x ) c os ( x ) x x 2 − . . n 0 ( x ) c os ( x ) =− x c os ( x ) s i n ( x ) 2 n ( x )=− x 1 . . − x ( 1 3 . 8 7 ) ( 1 3 . 8 8 ) ( 1 3 . 8 9 ) ( 1 3 . 9 0 ) As y mp t ot i cF or ms Sma l l x : ℓ 2ℓ ! = l i mj ( x ) ℓ ( 2 ℓ+1 ) ! ( 2 ℓ ) !1 x →0 l i mn( x ) x 0ℓ → ( 1 3 . 9 1 ) ℓ x = − . ℓ ℓ +1 2 ℓ ! x ( 1 3 . 9 2 ) ℓ Not et h a tf ors ma l lx( r< <k )j ( k r )i sp r op or t i on a lt ora n dn ( k r )i sp r op or t i on a lt o ℓ ℓ ℓ + 1 1 / r , wh i c ha r et h er e g u l a ra n di r r e g u l a rs ol u t i on st ot h es e p a r a t e dL a p l a c ee qu a t i on . Th i si st h ec or r e c twa yt oob t a i nt h es t a t i cl i mi t . L a r gex : 1 π i m→∞ j ℓ( x) = xc xl os ( x−( ℓ+1 ) 2) 1 π l i mn ℓ ( x ) = si x n ( x−( ℓ+1 ) ) . x 2 ( 1 3 . 9 3 ) ( 1 3 . 9 4 ) →∞ Not et h a tb ot hs ol u t i on sa r er e g u l a r( g ot oz e r os moot h l y )a ti n f i n i t ya n da r et h es a me ( t r i g )f u n c t i ons h i f t e db yπ/ 2ov e rxt h e r e .Not et h a tt h e ya r en o ts qu a r ei n t e g r a b l eon 3 I R( f ory ou rqu a n t u mc ou r s e )b u ta r es t i l lb e t t e rt h a np l a n ewa v e si nt h a tr e g a r d . Some t h i n gt ot h i n ka b o u t. . . Ha n k e l F u n c t i on s E x a mi n i n gt h ea s y mp t ot i cf or ms ,we s e et h a tt wo p a r t i c u l a rc omp l e xl i n e a r c omb i n a t i on soft h es t a t i on a r ys ol u t i onh a v et h eb e h a v i or , a ti n f i n i t y , ofa nou t g oi n gor i n c omi n gs p h e r i c a l wa v ewh e nt h et i mede p e n d e n c ei sr e s t or e d : + h x ) ℓ( − h x ) ℓ( 1 ( x )+i n ( x ) ℓ ℓ =j ( =h x ) ) ℓ( ( 1 3 . 9 5 ) ( x )−i n ( x ) ℓ ℓ =j 2 ( =h x ) ) ℓ( ( 1 3 . 9 6 ) t h es p h e r i c a lh a n k e lf u n c t i o n soft h ef i r s t( + )( ou t g oi n g )a n ds e c o n d( − )( i n c omi n g ) ℓ + 1 k i n ds .Bo t hoft h e s es ol u t i on sa r es i n g u l a ra tt h eor i g i nl i k e1 / x ( wh y ? )a n db e h a v e l i k e + l i mh( x ) x →∞ ℓ − i ) =( − l i mh( x ) x →∞ ℓ + 1 ℓ eix x −i x ℓ +1e =( i ) x ( 1 3 . 9 7 ) ( 1 3 . 9 8 ) a ti n f i n i t y . Twop a r t i c u l a r l yu s e f u l s p h e r i c a l h a n k e l f u n c t i on st ok n owa r et h ez e r ot h or d e ron e s : + h x ) 0( − h x ) 0( = i x e i x −i x e ( 1 3 . 9 9 ) = ( 1 3 . 1 0 0 ) − i x Pl a n eWa v eE x p a n s i on Pl a n ewa v e sa n df r e es p h e r i c a lwa v e sb ot hf or ma n( on –s h e l l )c omp l e t eor t h n or ma l 3 s e to nI R( wi t horwi t h o u tt h eor i g i n ) .Th a tme a n st h a ton emu s tb ea b l et oe x p a n don e i nt e r msoft h eot h e r . Pl a n ewa v e sc a nb ee x p a n d e di nt e r msoff r e es p h e r i c a l wa v e sb y : i k · r=e i k rc os ( Θ) e = ℓ ˆ ∗ 4 πiYL( k) j ( k r ) YL( ˆ r ). ℓ L ( 1 3 . 1 0 1 ) Th i si sd u et oL or dRa y l e i g ha n di ss ome t i me sc a l l e dt h eRa y l e i g he x p a n s i on . Re c a l l t h a tΘi st h ea n g l eb e t wi x tt h e r a n dt h eka n dt h a tc os ( Θ)=c os ( − Θ) . Th e r ei ss i mi l a r l ya n( i n t e g r a l )e x p r e s s i onf orj ( k r )i nt e r msofa ni n t e g r a lov e rt h e ℓ i k · r e b u twewi l ln otu s ei th e r e .I tf ol l owsf r omt h ec omp l e t e n e s sr e l a t i ononp a g e2 1 4 i nWy l d , t h eRa y l e i g he x p a n s i on ,a n dt h ec omp l e t e n e s sr e l a t i ononp a g e2 1 2 .De r i v ei t f orh ome wor k( orf i n di ts ome wh e r ea n dc op yi t , b u ty ous h ou l d n ’ th a v et o) .Ch e c ky ou r r e s u l tb yf i n d i n gi ts ome wh e r e .It h i n ki tmi g h tb es ome wh e r ei nWy l d , b u tIk n owi ti s e l s e wh e r e . Th i swi l l b eh a n d e di n . ± 13. 4. 2 J ( r ) , NL( r ) , a n dHL( r ) L F orc on v e n i e n c e , wed e f i n et h ef ol l owi n g : J ( r )=j ( k r ) YL( ˆ r ) L ℓ ( 1 3 . 1 0 2 ) NL( r ) =n ( k r ) YL( ˆ r ) ℓ ( 1 3 . 1 0 3 ) ± ± HL( r ) =h k r ) YL( ˆ r ) ℓ( ( 1 3 . 1 0 4 ) 2 Th e s ea r et h eb a s i cs ol u t i on st ot h eHHEt h a ta r ea l s oe i g e n f u n c t i on sofL a n dL z. 2 Cl e a r l yt h e r ei sa ni mp l i c i tl a b e lofk( ork)f ort h e s es ol u t i on s .Ag e n e r a ls ol u t i on( on as u i t a b l ed oma i n )c a nb ec on s t r u c t e dou tofal i n e a rc omb i n a t i onofa n yt wooft h e m. 13. 4. 3 Ge n e r a l Sol u t i o n st ot h eHHE On“ s p h e r i c a l ”doma i n s( t h ei n t e r i ora n de x t e r i orofas p h e r e , ori nas p h e r i c a l s h e l l )t h e c omp l e t e l yg e n e r a l s ol u t i ont ot h eHHEc a nt h e r e f or eb ewr i t t e ni ns t a t i o n a r yf or ma s : ALJ ( r )+BLNL( r ) L ( 1 3 . 1 0 5 ) L or( f ors c a t t e r i n gt h e or y , mos t l y )i nt h eo u t go i n gwa v ef or m + CLJ ( r )+SLHL( r ) . L L ( 1 3 . 1 0 6 ) I n s i d eas p h e r e , BLa n dSLmu s tb ez e r o.Ou t s i d eas p h e r e , ori nas p h e r i c a l a n n u l u s , a l l t h ec oe ffic i e n t sc a nb en on –z e r ou n l i k et h es i t u a t i onf ort h eL a p l a c ee qu a t i on( wh y ? ) . [ Th i ss h ou l dp r ov ok ed e e pt h ou g h t sa b ou tt h ef u n da me n t a ls i g n i f i c a n c eoft h e L a p l a c ee qu a t i on .Ar et h e r ea n y“ r e a l l y ”s t a t i on a r ys ou r c e si nt h ed y n a mi c a l , c ov a r i a n t , u n i v e r s e ?Dowee x p e c tt oh a v eac on t r i b u t i ont ot h ez e r of r e qu e n c yc h a r g e / c u r r e n t d e n s i t yd i s t r i b u t i oni na n yr e g i onofs p a c e ?Wh a twou l dt h i sc or r e s p on dt o? ] 13. 4. 4 Gr e e n ’ sF u n c t i on sa n dF r e eSp h e r i c a l Wa v e s 7 Wee x p e c t ,f orp h y s i c a lr e a s on st h a tt h ewa v ee mi t t e db yat i med e p e n d e n ts ou r c e s h ou l db e h a v el i k ea nou t g oi n gwa v ef a rf r om t h es ou r c e .Not et h a ti n s i d et h e b ou n d i n gs p h e r eoft h es ou r c et h a tn e e dn otb et r u e .E a r l i e ri nt h i sc h a p t e r , weu s e da n “ ou t g oi n gwa v eGr e e n ’ sf u n c t i on ”t oc on s t r u c tt h es ol u t i ont ot h eI HEwi t ht h i s a s y mp t ot i cb e h a v i or . We l l , l oa n db e h ol d: ′ xx i k ± x x′ h π0 ( k |− )=∓ 4 G±(, F ors t a t i on a r ywa v e s( u s e f u l i nqu a n t u mt h e or y ) | ) ( 1 3 . 1 0 7 ) k ′ ,)= G0x( x 4πn0 ( k x |− x ′ | ) . ( 1 3 . 1 0 8 ) Th i se x t r e me l yi mp or t a n tr e l a t i onf or mst h ec on n e c t i onb e t we e nf r e es p h e r i c a l wa v e s( r e v i e we da b ov e )a n dt h ei n t e g r a le qu a t i ons ol u t i on swea r ei n t e r e s t e di n c on s t r u c t i n g . Th i sc on n e c t i onf ol l owsf r omt h ea d d i t i ont h e or e msormu l t i p ol a re x p a n s i on sof t h ef r e es p h e r i c a l wa v e sde f i n e da b ov e . F ort h es p e c i a l c a s eofL=( 0 , 0 )t h e s ea r e : 1 N0( r−r )=n ( k | r−r ) √4π 0 ′ ′ √ =4 π a n d ± H( r 0 ′ − ± ′ r )=h( kr r ) 1 0 √ |− 4π ∗ L √ NL( r ) J ( r ) > L < ± = 4π ( 1 3 . 1 0 9 ) ∗ H( r ) J ( r ). L L > L < ( 1 3 . 1 1 0 ) F r om t h i sa n dt h ea b ov e ,t h ee x p a n s i onoft h eGr e e n ’ sf u n c t i on si nf r e es p h e r i c a l mu l t i p ol a rwa v e si mme d i a t e l yf ol l ows : ′ ∗ G0( r−r )=k NL( r ) J ( r ) > L < L 7 ( 1 3 . 1 1 1 ) Ac op –ou tp h r a s ei ft h e r ee v e rwa son e .I tt r a n s l a t e sa s : b e c a u s et h a t ’ st h ewa yi tt u r n sou ta tt h ee n d. a n d ′ ± G ( r r)= i k ∓ ± − H( r L L ∗ ) J( r) . > L < ( 1 3 . 1 1 2 ) Not eWe l l :Th ec omp l e xc on j u g a t i onop e r a t i onu n d e rt h es u mi sa p p l i e dt ot h e s p h e r i c a l h a r mo n i c( on l y ) , n o t t h e H a n k e l f u n c t i o n ( s ) . T h i s i s b e c a u s e t h e o n l y f u n c t i o n ′∗ oft h ep r od u c tYL( r ˆ ) YL( r ˆ) i st or e c on s t r u c tt h ePℓ( Θ)v i at h ea d d i t i ont h e or e mf or s p h e r i c a l h a r mon i c s . St u d yt h i sp oi n ti nWy l dc a r e f u l l yony ou rown . Th e s er e l a t i on swi l l a l l owu st oe x p a n dt h eHe l mh ol t zGr e e n ’ sf u n c t i on se x a c t l yl i k ewe e x p a n d e dt h eGr e e n ’ sf u n c t i onf ort h eL a p l a c e / Poi s s one qu a t i on .Th i s , i nt u r n , wi l l a l l owu s t op r e c i s e l ya n db e a u t i f u l l yr e c on s t r u c tt h emu l t i p ol a re x p a n s i onoft h ev e c t orp ot e n t i a l , a n d h e n c et h eEMf i e l d si nt h ev a r i ou sz on e s 8 e x a c t l y. Th i se n dsou rb r i e fma t h e ma t i c a lr e v i e woff r e es p h e r i c a lwa v e sa n dwer e t u r nt o t h ed e s c r i p t i onofRa d i a t i on . 13. 5 E l e c t r i cDi p o l eRa d i a t i on Nowt h a tweh a v et h a tu n d e rou rb e l t swec a na d d r e s st h emu l t i p ol a re x p a n s i onoft h e v e c t orp ot e n t i a li n t e l l i g e n t l y .Tob e g i nwi t h ,wewi l lwr i t et h ege n e r a ls ol u t i o nf ort h e v e c t orp ot e n t i a li nt e r msoft h emu l t i p ol a re x p a n s i onf ort h eou t g oi n gwa v eGr e e n ’ s f u n c t i onde f i n e da b ov e : ∞ J r () L Ar ()=i k L r r + ′ ′( ∗ )3′ µ0J r () HLr () dr ′ ′( ∗ )3′ r () J r () dr +HLr () µ0J L 0 ( 1 3 . 1 1 3 ) wh e r e ,b yc on v e n t i on ,( ∗ )me a n st h a tt h eYL( ˆ r )i sc on j u g a t e db u tt h eb e s s e l / n e u ma n n / h a n k e lf u n c t i oni sn o t .Th i si sb e c a u s et h eon l yp oi n toft h ec on j u g a t i oni st o c on s t r u c tPℓ( Θ)f r om t h em–s u mf ore a c hℓv i at h ea d di t i ont h e or e mf ors p h e r i c a l + − h a r mon i c s .Wec e r t a i n l yd on ’ twa n tt oc h a n g eh i n t oh,wh i c hc h a n g e st h et i me 9 d e p e n de n tb e h a v i oroft h es ol u t i on.Not et h a tt h ei n t e g r a lov e ra l ls p a c ei sb r ok e nu p i ns u c hawa yt h a tt h eGr e e n ’ sf u n c t i one x p a n s i on sa b ov ea l wa y sc on v e r g e .Th i s s ol u t i oni se x a c te v e r y wh e r ei ns p a c ei n c l u d i n gi n s i d et h es ou r c ei t s e l f! Wec a nt h e r e f or es i mp l i f you rn ot a t i onb yd e f i n i n gc e r t a i nf u n c t i on soft h er a d i a l v a r i a b l e : Ar ()= i kC L + L r ) Hr (). ( r ) J r)+S ( ( L L ( 1 3 . 1 1 4 ) 8 We l l , i nau n i f or ml yc on v e r g e n te x p a n s i on , wh i c hi sk i n dofe x a c t , i nt h el i mi tofa ni n f i n i t es u m. I nt h eme a n t i me , i ti sad a mng ooda p p r ox i ma t i on . Us u a l l y . 9 Th i ss u g g e s t st h a tt h e r ea r es omei n t e r e s t i n gc on n e c t i on sb e t we e nt h ec on j u g a t i ons y mme t r ya n dt i me r e v e r s a l s y mme t r y . Toob a dwewon ’ th a v et i met oe x p l or et h e m. Youma yo ny ou rown , t h ou g h . I nt h i se qu a t i on , ∞ CL( r ) = SL( r ) = ′ r r 0 ′( ∗ )3′ µ0J r () HLr () dr ′ ′( ∗ )3′ µ0J r () J r () dr . L ( 1 3 . 1 1 5 ) ( 1 3 . 1 1 6 ) Cl e a r l ySL( 0 )=0a n df orr>d ,CL( r )=0 .Att h eor i g i nt h es ol u t i oni sc omp l e t e l y r e g u l a ra n ds t a t i o n a r y .Ou t s i det h eb ou n di n gs p h e r eoft h es ou r c edi s t r i b u t i ont h e s ol u t i onb e h a v e sl i k eal i n e a rc omb i n a t i onofou t g oi n gs p h e r i c a lmu l t i p ol a rwa v e s . F r omn owonwewi l lc on c e n t r a t eont h el a t t e rc a s e , s i n c ei ti st h eon er e l e v a n tt ot h e z on e s . 13. 5. 1 Ra d i a t i o no u t s i d et h es o u r c e Ou t s i d et h eb ou n d i n gs p h e r eoft h es ou r c e , + Ar ()=i k HLr () L ∞ 0 ′ ′( ∗ )3′ µ0J r () J r () dr . L ( 1 3 . 1 1 7 ) Atl a s tweh a v ema d ei tt oJ a c k s on ’ se qu a t i on9 . 1 1 , b u tl o okh owe l e ga n to u ra p pr o a c h wa s .I n s t e a dofaf or mt h a ti son l yv a l i di nt h ef a rz on e ,wec a nn ows e et h a tt h i si sa l i mi t i n gf or mofac on v e r g e n ts ol u t i ont h a twor k si na l lz on e s ,i n c l u d i n gi n s i det h es ou r c e i t s e l f ! Th ei n t e g r a l st h a tg oi n t ot h eCL( r )a n dSL( r )ma ywe l l b ed a u n t i n gt oap e r s ona r me d ′ wi t hp e na n dp a p e r( d e p e n d i n gonh ow n a s t yJ x ()i s )b u tt h e ya r ev e r yd e f i n i t e l y c omp u t a b l ewi t hac omp u t e r ! Now,wemu s tu s es e v e r a li n t e r e s t i n gob s e r v a t i on s .F i r s tofa l l ,J r ()g e t ss ma l l L r a p i d l yi n s i deda sℓi n c r e a s e s( b e y on dk d ) .Th i si st h ea n gu l a rmome n t u mc u t –o ffi n d i s g u i s ea n dy ous h ou l dr e me mb e ri t .Th i sme a n st h a ti fJ ( r )i ss e n s i b l yb ou n d e d , t h e ′ i n t e g r a l ont h er i g h t( wh i c hi sc u toffa tr=d )wi l l g e ts ma l l f or“ l a r g e ”ℓ .I nmos tc a s e s ofp h y s i c a l i n t e r e s t , k d< <1b yh y p ot h e s i sa n dwen e e don l yk e e pt h ef i r s tf e wt e r ms( ! ) . I np r a c t i c a l l ya l loft h e s ec a s e s ,t h el owe s tor d e rt e r m( ℓ=0 )wi l ly i e l da ne x c e l l e n t a p p r ox i ma t i on . Th i st e r mp r od u c e st h ee l e c t r i cd i p o l er a d i a t i onf i e l d. 13. 5. 2 Di p o l eRa d i a t i o n L e tu se v a l u a t et h i st e r m. I ti s( c . f . J 9 . 1 3 ) : i kr Ar ()= µ0e 4 πr r ′ 3 ′ J r ( ) d r 0 ( 1 3 . 1 1 8 ) √ ∗ ( n ot e :Y00( ˆ r )=Y00( ˆ r )=1 /4 π) .I fwei n t e g r a t et h i st e r mb yp a r t s( as u r p r i s i n g l yd i ffic u l t c h or et h a twi l l b ea ne x e r c i s e )a n du s et h ec on t i n u i t ye qu a t i on a n dt h ef a c tt h a tt h es ou r c ei sh a r mon i cweg e t : i k r Ar ()=− i µ0ω e ( 1 3 . 1 1 9 ) p 4 π r wh e r e p=r′ ρr (′ ) d 3r ′( 1 3 . 1 2 0 )i st h ee l e c t r i cd i p o l emo me n t ( s e eJ 4 . 8 ) . Not et h a ti fwed e f i n eρr ()t ob ea “ p r ob a b i l i t yd e n s i t y ”f ort h ee l e c t r on sd u r i n gat r a n s i t i ont h i se x p r e s s i oni ss t i l l v a l i d. Th i si swon d e r f u l l ys i mp l e .I fon l ywec ou l dqu i twi t ht h ev e c t orp ot e n t i a l .Al a s , n o. Wemu s tr e c on s t r u c tt h ee l e c t r oma gn e t i cf i e l db e i n gr a d i a t e da wa yf r omt h es ou r c e f r omt h ee x p r e s s i on sp r e v i ou s l yg i v e n a n d Af t e rat r e me n dou sa mou n tofs t r a i g h t f or wa r db u tn on e t h e l e s sd i ffic u l ta l g e b r at h a t y ouwi l l doa n dh a n di nn e x twe e k( s e ep r ob l e ms )y ouwi l l ob t a i n : 2 i k r e 1 n× p) r H= 4π( k r 1− i c k ( 1 3 . 1 2 1 ) a n d 1 2 i k r e 1 3 i k 2 i kr n× p)×n r +[ e( 13. 122) E= 4πǫ0 k( 3 n( np· )− p] r − r Th ema g n e t i cf i e l di sa l wa y st r a n s v e r s et ot h er a d i a l v e c t or .E l e c t r i cd i p o l er a d i a t i oni s t h e r e f or ea l s oc a l l e dt r a n s v e r s ema gn e t i cr a di a t i on .Th ee l e c t r i cf i e l di st r a n s v e r s ei n t h ef a rz on e , b u ti nt h en e a rz on ei twi l l h a v eac omp on e n t( i nt h e pd i r e c t i on )t h a ti sn ot g e n e r a l l yp e r p e n d i c u l a rt on . As y mp t ot i cp r op e r t i e si nt h eZo n e s I nt h en e a rz o n eweg e t : i ωµ0 B =µ0H= 1 2 4 π( nˆ× p)r 1 1 ( 1 3 . 1 2 3 ) 3 3nˆ( nˆp·)− p]r ( 1 3 . 1 2 4 ) E = 4πǫ0 [ a n dc a nu s u a l l yn e g l e c tt h ema g n e t i cf i e l dr e l a t i v et ot h ee l e c t r i cf i e l d( i ti ss ma l l e rb ya f a c t orofk r< <1 ) .Th ee l e c t r i cf i e l di st h a tofa“ s t a t i c ”d i p ol e( J 4 . 1 3 )os c i l l a t i n g h a r mon i c a l l y . I nt h ef a rz on eweg e t : 2 c kµ0 i kr e B =µ0H= 4π ( n ˆ× p) r i c ( 1 3 . 1 2 5 ) E = k∇×B=c ( 1 3 . 1 2 6 ) B×n ˆ. Th i si st r a n s v e r s eEM r a di a t i on .Ex p a n d e da b ou ta n yp oi n t ,i tl ook sj u s tl i k eap l a n e wa v e( wh i c hi sh ow“ p l a n ewa v e s ”a r eb or n ! ) .Wea r emos ti n t e r e s t e d , a sy ouk n ow, i n t h er a d i a t i onz on ea n ds owewi l l f oc u soni tf oramome n t . E n e r gyr a d i a t e db yt h ed i p o l e Re c a l l ou rol db u d d yt h ec o mp l e xPoy n t i n gv e c t o rf orh a r mon i cf i e l d s( J 6 . 1 3 2 ) : 1 ∗ S= 2ReE×H . ( 1 3 . 1 2 7 ) Th ef a c t orof1 / 2c ome sf r omt i mea v e r a g i n gt h ef i e l d s .Th i si st h ee n e r g yp e ru n i ta r e a p e ru n i tt i met h a tp a s s e sap oi n ti ns p a c e .Tof i n dt h et i mea v e r a g ep owe rp e rs ol i d a n g l e ,wemu s tr e l a t et h en or ma la r e at h r ou g hwh i c ht h ee n e r g yf l u xp a s s e st ot h e s ol i da n g l e : 2 d An=rd Ω ( 1 3 . 1 2 8 ) a n dp r oj e c tou tt h ea p p r op r i a t ep i e c eofS, i . e . —n·S. Weg e t( wi t hµ=1 ) dP 1 2 ∗ [ rn·( E×H) ] . dΩ =2Re ( 1 3 . 1 2 9 ) wh e r ewemu s tp l u gi nEa n dHf r omt h ee x p r e s s i on sa b ov ef ort h ef a rf i e l d . Af t e rab u n c hofa l g e b r at h a tI ’ ms u r ey ouwi l l e n j oyd oi n g , y ouwi l l ob t a i n : 2 c dP dΩ µ0 2 =32π 4 2 ǫ0k | ( n× p)×n| . ( 1 3 . 1 3 0 ) Th ep ol a r i z a t i onoft h er a d i a t i oni sde t e r mi n e db yt h ev e c t ori n s i d et h ea b s ol u t ev a l u es i g n s . Byt h i son eme a n st h a ton ec a np r oj e c tou te a c hc omp on e n tof p ( a n dh e n c et h er a d i a t i on )b e f or ee v a l u a t i n gt h es qu a r ei n d e p e n d e n t l y , i fs o d e s i r e d .Not et h a tt h ed i ffe r e n tc omp on e n t sof pn e e dn oth a v et h es a mep h a s e ( e l l i p t i c a l p ol a r i z a t i on , e t c . ) . I fa l l t h ec omp on e n t sof p( i ns omec oor d i n a t es y s t e m)h a v et h es a mep h a s e , t h e n p n e c e s s a r i l yl i e sa l on gal i n ea n dt h et y p i c a la n g u l a rd i s t r i b u t i oni st h a tof( l i n e a r l y p ol a r i z e d)di p ol er a di a t i o n : dP dΩ 2 c 2 =32π µ0 4 2 2 ǫ0 k p || si nθ ( 1 3 . 1 3 1 ) wh e r eθi sme a s u r e db e t we e npa n dn .Wh e ny oui n t e g r a t eov e rt h ee n t i r es ol i da n g l e ( a sp a r tofy ou ra s s i g n me n t )y ouob t a i nt h et ot a l p owe rr a d i a t e d : 24 ck P= 12π 4 µ0 2 || ǫ0 p ( 1 3 . 1 3 2 ) Th emos ti mp or t a n tf e a t u r eoft h i si st h ek de p e n d e n c ewh i c hi s , a f t e ra l l , wh yt h es k y i sb l u e( a swes h a l l s e e , n e v e rf e a r ) . E x a mp l e :Ac e n t e r f e d , l i n e a ra n t e n n a I nt h i sa n t e n n a , d< <λa n d I ( z , t )=I −i ωt e . 1 2|z| − d 0 ( 1 3 . 1 3 3 ) F r omt h ec on t i n u i t ye qu a t i on( a n dal i t t l es u b t l eg e ome t r y ) , ′ −i ωt ∇· J= ∂ρ( z ) e d I dz =− ∂t ′ =i ωρ( z ) ( 1 3 . 1 3 4 ) a n dwef i n dt h a tt h el i n e a rc h a r g ede n s i t y( p a r t i c i p a t i n gi nt h eos c i l l a t i on ,wi t ha p r e s u me dn e u t r a l b a c k g r ou n d )i si n d e p e n d e n tofz : 2 i I 0 ′ ρ( z )= ±ωd ( 1 3 . 1 3 5 ) wh e r et h e+ / −s i g ni n di c a t e st h eu p p e r / l owe rb r a n c hoft h ea n t e n n aa n dt h e ′ me a n st h a twea r er e a l l yt r e a t i n gρ/ ( d x d y )( wh i c hc a n c e l st h er e l a t e dt e r msi nt h e v ol u mei n t e g r a l b e l ow) .Wec a nt h e ne v a l u a t et h ed i p ol emome n toft h ee n t i r ea n t e n n a f ort h i sf r e qu e n c y : d / 2 p z= i I d. 0 2ω ′ z ρ( z ) dz= −d / 2 ( 1 3 . 1 3 6 ) Th ee l e c t r i ca n dma g n e t i cf i e l dsf orr>di nt h ee l e c t r i cd i p ol ea p p r ox i ma t i ona r e n owg i v e nb yt h ep r e v i ou s l yd e r i v e de x p r e s s i on s .Th ea n g u l a rd i s t r i b u t i onofr a d i a t e d p owe ri s 2 2 2 µ0 I d P 0 = d Ω a n dt h et ot a l r a d i a t e dp owe ri s 2 128π 2 P= ( k d ) s i n θ ( 13 . 1 37 ) ǫ0 2 I k d) 0( 4 8 π µ0 ǫ0 . ( 1 3 . 1 3 8 ) Re ma r k s .F orf i x e dc u r r e n tt h ep owe rr a d i a t e di n c r e a s e sa st h es qu a r eoft h ef r e qu e n c y ( a tl e a s twh e nk d< <1 ,i .e .–l on gwa v e l e n g t h sr e l a t i v et ot h es i z eoft h ea n t e n n a ) .Th e t ot a lp owe rr a d i a t e db yt h ea n t e n n aa p p e a r sa sa“ l os s ”i n“ Oh m’ sL a w”f ort h ea n t e n n a . 2 F a c t or i n gou tI 2 ,t h er e ma i n d e rmu s th a v et h eu n i t sofr e s i s t a n c ea n di sc a l l e dt h e 0/ r a d i a t i onr e s i s t a n c eoft h ea n t e n n a : Rrad= 2 ( k d) = 24π 2P 2 I 0 µ0 ǫ0 2 ≈5 ( k d ) oh ms ) ( 1 3 . 1 3 9 ) wh e r ewed ot h el a t t e rmu l t i p l i c a t i ont oc on v e r tt h er e s u l t i n gu n i t st ooh ms .Not et h a t t h i sr e s i s t a n c ei st h e r ef orh a r mon i cc u r r e n t se v e ni ft h ec on du c t i v i t yoft h eme t a li s p e r f e c t .Not ef u r t h e rt h a tb yh y p ot h e s i st h i se x p r e s s i onwi l lon l yb ev a l i df ors ma l l v a l u e sofRr . a d Goodg ol l y ,t h i si swon d e r f u l .Weh op e f u l l yr e a l l yu n d e r s t a n de l e c t r i cdi p ol e r a d i a t i ona tt h i sp oi n t .I twou l db et r u l ys u b l i mei fa l lr a d i a t or swe r ed i p ol er a d i a t or s . Ph y s i c swou l db es oe a s y .Bu t( a l a s )s ome t i me st h ec u r r e n td i s t r i b u t i onh a sn oℓ=0 mo me n ta n dt h e r ei st h e r e f or en od i p ol et e r m!I nt h a tc a s ewemu s tl ooka tt h en e x t t e r mors oi nt h emu l t i p ol a re x p a n s i on s L e s ty out h i n kt h a tt h i si sawh ol l yu n l i k e l yoc c u r r a n c e , p l e a s en ot et h a tah u mb l e l oopc a r r y i n gac u r r e n tt h a tv a r i e sh a r mon i c a l l yi son es u c hs y s t e m.Sol e tu sp r oc e e d t o: 13. 6 Ma gn e t i cDi p o l ea n dE l e c t r i cQu a d r u p o l eRa d i a t i o n F i e l d s Th en e x tt e r mi nt h emu l t i p ol a re x p a n s i oni st h eℓ=1t e r m: Ax ()=i k µ 0 1 + h( k r ) 1 Y 1 , m ( ˆ r ) m= −1 ∞ ′ ′ J ( x) j ( k r ) Y 1 0 ∗ 3′ 1 , m ( r̂)dx ′ ( 1 3 . 1 4 0 ) Wh e ny ou( f orh ome wor k , ofc ou r s e ) a )m–s u mt h ep r od u c toft h eYℓ, s m’ ′ b )u s et h es ma l l k re x p a n s i onf orj ( k r )i nt h ei n t e g r a l a n dc omb i n ei twi t ht h e 1 e x p l i c i tf or mf ort h er e s u l t i n gP1( θ )t of or madotp r od u c t c )c a n c e l t h e2 ℓ+1 ’ s d )e x p l i c i t l ywr i t eou tt h eh a n k e l f u n c t i oni ne x p on e n t i a l f or m y ouwi l l g e te qu a t i on( J 9 . 3 0 , f or–r e c a l l –d i s t r i b u t i on swi t hc omp a c ts u p p or t ) : i kr µ0e πr Ax ()= 4 1 k r−i ∞ 0 ′ ′ 3′ J ( x) ( n·x) dx. ( 1 3 . 1 4 1 ) Ofc ou r s e , y ouc a ng e ti td i r e c t l yf r omJ 9 . 9( t oal owe ra p p r ox i ma t i on )a swe l l , b u t t h a td oe sn o ts h owy ouwh a tt odoi ft h es ma l lk ra p p r ox i ma t i oni sn otv a l i d( i ns t e p2 a b ov e )a n di tn e g l e c t sp a r toft h eou t g oi n gwa v e ! Th e r ea r et woi mp or t a n ta n di n de p e n d e n tp i e c e si nt h i se x p r e s s i on .On eoft h et wo ′ p i e c e si ss y mme t r i ci nJa n d xa n dt h eot h e ri sa n t i s y mme t r i c( g e tami n u ss i g nwh e n t h ec oor d i n a t es y s t e mi si n v e r t e d ) .An yv e c t orqu a n t i t yc a nb ed e c omp os e di nt h i s ma n n e rs ot h i si sav e r yg e n e r a l s t e p : ′ J ( n·x)= 1 ′ ′ ( n·x) J+( n·J ) x] + 2[ 1 ′ x×J )×n . 2( ( 1 3 . 1 4 2 ) 13. 6. 1 Ma gn e t i cDi p o l eRa d i a t i on L e t ’ sl ooka tt h ea n t i s y mme t r i cb i tf i r s t , a si ti ss ome wh a ts i mp l e ra n dwec a nl e v e r a g e ou re x i s t i n gr e s u l t s .Th es e c on dt e r mi st h ema g n e t i z a t i on( d e n s i t y )d u et ot h ec u r r e n t J : 1 M= 2x (×J ) ( 1 3 . 1 4 3 ) ( s e eJ 5 . 5 3 , 5 . 5 4 )s ot h a t ′ 3′ M( x) dx m= ( 1 3 . 1 4 4 ) wh e r e mi st h ema g n e t i cd i p ol emome n toft h e( f ou r i e rc omp on e n tof )t h ec u r r e n t . Con s i d e r i n gon l yt h i sa n t i s y mme t r i ct e r m, wes e et h a t : i k µ0 AM1x ()= i k r e 4π ( n×m) r 1 1− i k r . ( 1 3 . 1 4 5 ) HMMMMMMM,( y ouh a db e t t e rs a y ) !Th i sl ook s“ j u s tl i k e ”t h ee x p r e s s i onf ort h e ma g n e t i cf i e l dt h a tr e s u l t e df r om t h ee l e c t r i cd i p ol ev e c t orp ot e n t i a l .Su r ee n ou g h , wh e ny ou( f orh ome wor k )c r a n kou tt h ea l g e b r a , y ouwi l l s h ow t h a t i k r e µ0 B= 4π 2 k( n×m)×n 1 i k r +[ 3 n ( n·m)−m] a n d 1 µ0 E=− 4π 2 i k r e ǫ0 k( n×m) r 3 2 r − r i k r e ( 1 3 . 1 4 6 ) 1 1− i k r. ( 1 3 . 1 4 7 ) Cl e a r l y , wed on ’ tn e e dt od i s c u s st h eb e h a v i oroft h ef i e l d si nt h ez on e ss i n c et h e y a r ec omp l e t e l ya n a l og ou s .Th ee l e c t r i cf i e l di sa l wa y st r a n s v e r s e ,a n dt h et ot a lf i e l d a r i s e sf r omah a r mon i cma g n e t i cd i p ol e .F ort h i sr e a s on , t h i sk i n dofr a di a t i oni sc a l l e d e i t h e rma gn e t i cd i p o l e( M1 )r a d i a t i onort r a n s v e r s ee l e c t r i cr a d i a t i on .F orwh a ti t ’ s wor t h , e l e c t r i cd i p ol er a d i a t i oni sa l s oc a l l e d( E 1 )r a d i a t i on . Howe v e r ,t h i si son l yONEp a r toft h ec on t r i b u t i onf r omℓ=1t e r msi nt h eGr e e n ’ s f u n c t i one x p a n s i on . Wh a ta b ou tt h eot h e r( s y mme t r i c )p i e c e ?Oooo, ou c h . 13. 6. 2 E l e c t r i cQu a d r u p ol eRa d i a t i on Nowl e t ’ st ou n t a n g l et h ef i r s t( s y mme t r i c )p i e c e .Th i swi l lt u r nou tt ob ear e ma r k a b l y u n p l e a s a n tj ob .I nf a c ti ti smyn e f a r i ou sa n ds a d i s t i cp l a nt h a ti tb es ou n p l e a s a n tt h a t i tp r op e r l ymot i v a t e sac h a n g ei na p p r oa c ht oon et h a th a n d l e st h i sn a s t yt e n s ors t u ff “ n a t u r a l l y ” . Weh a v et oe v a l u a t et h ei n t e g r a l oft h es y mme t r i cp i e c e . Weg e t : ′ 1 ′3′ i ω x [ ( n ˆx) J+( n ˆJ x )] dx= · 2 Th es t e p si n v ol v e da r e : · ′ ′ ′ 3′ ( n ˆx) ρx () dx ( 1 3 . 1 4 8 ) · − 2 a )i n t e g r a t eb yp a r t s( wor k i n gt oob t a i nd i v e r g e n c e sofJ ) . b )c h a n g i n g∇·Ji n t oaρt i me swh a t e v e rf r omt h ec on t i n u i t ye qu a t i on( f ora h a r mon i cs ou r c e ) . c )r e a r r a n g i n ga n dr e c omb i n i n g . Don ’ tf or g e tt h eb ou n d a r yc on d i t i ona ti n f i n i t y( Ja n dρh a v ec omp a c ts u p p or t ) !You ’ l l l ov ed oi n gt h i son e . . . Th ev e c t orp ot e n t i a l i st h u s : Ax ()= E2 ′ 2 µ0ck − 8π i kr e r 1 1 − x i k r ′ ′ ′ 3′ ( n ˆx) ρx () dx. ( 1 3 . 1 4 9 ) · Not et h a t xa p p e a r st wi c eu n d e rt h ei n t e g r a l ,a n dt h a ti t sv e c t orc h a r a c t e rs i mi l a r l y ′ a p p e a r st wi c e :on c ei n xi t s e l fa n don c ei ni t sp r oj e c t i ononn ˆ .Th ei n t e g r a li st h e e l e c t r i cq u a d r u p o l emome n toft h eos c i l l a t i n gc h a r g ed e n s i t ydi s t r i b u t i ona n dt h e r e s u l t i n gr a d i a t i onf i e l di sc a l l e da ne l e c t r i cq u a d r u p o l e( r a d i a t i o n )f i e l dora nE 2 r a d i a t i o nf i e l d( f ors h or t ) . Tog e tt h ef i e l dsf r omt h i se x p r e s s i onb yt a k i n gi t sc u r l , a n dt h e nt h ec u r l ofi t sc u r l , i s–a h e m–mos tu n p l e a s a n t .J a c k s o nwi mp sou t ! Ac t u a l l y , t a k i n gt h ec u r l si sn omor e di ffic u l tt h a ni twa sf ort h ema g n e t i ct e r m, b u tu n t a n g l i n gt h ei n t e g r a l swi t ht h er e s u l ti s , b e c a u s eoft h et e n s orf or mst h a ta p p e a r .Con s e qu e n t l ywet oowi l lwi mpou t( i nt h e c omf or t i n gk n owl e d g et h a twewi l ls h or t l yd ot h i sr i gh ta n dn o twi mpou tt oa r b i t r a r y or d e ri nap r e c i s ed e c o mp os i t i on )a n dwi l l r e s t r i c tou ra t t e n t i ont ot h ef a rz on e . Th e r ewen e e don l yc on s i d e rt h el owe s tor d e rs u r v i v i n gt e r m, wh i c ha l wa y sc ome s f r omt h ec u r l oft h ee x p on e n t i a l t i me st h er e s t : B =i k ( n ˆ×A) ( 1 3 . 1 5 0 ) µ0 E =i k ǫ0( n ˆ×A)×n ˆ . ( 1 3 . 1 5 1 ) I fwek e e pon l yt h el owe s tor d e rt e r msoft h i sweg e t B= 2 i kr −i c kµ0e 8 πr ( n ˆx ′ ′ ′ 3′ ) ( n ˆx) ρx () dx. ( 1 3 . 1 5 2 ) × · I fwer e c a l l ( f r omt h eb e g i n n i n gofCh a p t e r4 )t h ed i s c u s s i ona n dd e f i n i t i onofmu l t i p ol e mome n t s , i np a r t i c u l a rt h equ a d r u p ol emome n tt e n s or ′′ ′ 2 ′ 3′ Qαβ= ( 3 x ρx () dx αx β−rδ α β) ( 1 3 . 1 5 3 ) wh os ev a r i ou sc omp on e n t sc a nb er e l a t e dt ot h ef i v es p h e r i c a l h a r mon i c swi t h ℓ=2( ! )wec a ns i mp l i f yma t t e r s . Wec a nwr i t et h eon eme s s yi n t e g r a l i nt e r msof a n ot h e r : ′ ′ ′ 3′ n ˆx) ρ x ( ) dx= 1nˆ Q( nˆ x ( n ˆ ) ( 1 3 . 1 5 4 ) · × wh e r e × 3 Qαβnβxˆβ. Q( n ˆ )= ( 1 3 . 1 5 5 ) β Not et h a tt h e“ v e c t or ”Q( n ˆ )( a n dh e n c et h ef i e l d s )d e p e n d si nb ot ht h ema g n i t u de a n dd i r e c t i onont h ed i r e c t i ont ot h ep oi n tofob s e r v a t i onna swe l l a st h ep r op e r t i e sof t h es ou r c e . Wi t ht h e s ed e f i n i t i on s , 3 i k r i c kµ0e n ˆ ) 2 4 π rnˆ×Q( B=− ( 1 3 . 1 5 6 ) wh i c hl ook s( e x c e p tf ort h ep e c u l i a rf or m ofQ)mu c hl i k et h eE1ma g n e t i cf i e l d.I ti s t r a n s v e r s e .Th ee l e c t r i cf i e l di sob t a i n e db ya p p e n d i n g× na n di sa l s ot r a n s v e r s e .F ol l owi n g e x a c t l yt h es a mea l g e b r a i cp r oc e d u r ea sb e f or e , wef i n d f r om 1 ∗ S= 2ReE×H ( 1 3 . 1 5 7 ) a n dc omp u t i n gt h ef l u xoft h ePoy n t i n gv e c t ort h r ou g has p h e r eofr a d i u sra sa f u n c t i onofa n g l et h a tt h ea n g u l a rp owe rd i s t r i b u t i oni s : dP 2 c 2 dΩ =1152π µ0 6 2 ǫ0k |( nˆ×Q( nˆ ) )×nˆ| ( 1 3 . 1 5 8 ) Th ea n g u l a rd i s t r i b u t i oni st ooc omp l i c a t e dt op l a ywi t hf u r t h e ru n l e s sy oun e e dt o c a l c u l a t ei t ,i nwh i c hc a s ey ouwi l lh a v et owor ki tou t .Th et ot a lp owe rc a nb e c a l c u l a t e di na“ s t r a i g h t f or wa r d ”wa y( t oqu ot eJ a c k s on ) .F i r s ton ec h a n g e st h ec r os s p r od u c tt od otp r odu c t su s i n gt h es e c on dr e l a t i onont h ef r on tc ov e ra n ds qu a r e si t . On et h e nwr i t e sou tt h er e s u l ti nt e n s orc omp on e n t s .On ec a nt h e np e r f or mt h ea n g u l a r i n t e g r a l soft h ep r od u c t soft h ec omp on e n t soft h en( wh i c hi ss t r a i g h t f or wa r d ) .F i n a l l y on et e r mi nt h er e s u l t i n ge x p r e s s i ong oe sa wa yb e c a u s eQαβi st r a c e l e s s . Th er e s u l ti s 26 ck P= 1440π 6 µ0 ǫ0 2 α , β | Qαβ| ( 1 3 . 1 5 9 ) ( n ot ek f r e qu e n c yd e p e n d e n c e ) .F ort h en u me r ol og i s t sa mon gy ou ,n ot et h a tt h e r ei s a l mos tc e r t a i n l ys omes or tofc os mi cs i g n i f i c a n c ei nt h e1 4 4 0i nt h ed e n omi n a t ora s t h i si st h en u mb e rofs e c on d si nad a y . J u s tk i d d i n g . F orc e r t a i ns y mme t r i cd i s t r i b u t i on sofc h a r g et h eg e n e r a lqu a d r u p ol emome n t t e n s ors i mp l i f i e ss t i l l f u r t h e r .At y p i c a l c a s eoft h i soc c u r swh e nt h e r ei sa na d di t i on a l , e .g .a z i mu t h a ls y mme t r ys u c ha sa nos c i l l a t i n gs p h e r oi d a ld i s t r i b u t i onofc h a r g e .I n t h i sc a s e ,t h eoff–d i a g on a lc omp on e n t sofQαβv a n i s ha n don l yt wooft h er e ma i n i n g t h r e ea r ei n d e p e n d e n t . Wec a nwr i t e 1 Q33=Q0, Q11= Q22= − 2 Q0 ( 1 3 . 1 6 0 ) a n dt h ea n g u l a rd i s t r i b u t i onofr a di a t e dp owe ri s 2k 6 dp =c dΩ 2 µ0 512π 2 2 2 Q0 si n θcos θ ( 1 3 . 1 6 1 ) ǫ0 wh i c hi saf ou r –l ob e dr a d i a t i onp a t t e r nc h a r a c t e r i s t i cofa z i mu t h a l l ys y mme t r i c s ou r c e s .I nt h i sc a s ei tr e a l l yi ss t r a i g h t f or wa r dt oi n t e g r a t eov e rt h ee n t i r es ol i da n g l e ( ordot h es u mi nt h ee x p r e s s i ona b ov e )a n ds h owt h a t : 26 P= ck µ0 2 Q0. ( 1 3 . 1 6 2 ) ǫ0 960π Att h i sp oi n ti ts h ou l db ec l e a rt h a twea r eoffont h ewr on gt r a c k . Toqu ot e J a c k s on : Th el a b ori n v ol v e di n ma n i p u l a t i n gh i g h e rt e r msi n( t h emu l t i p ol a r e x p a n s i onofAx () )b e c ome si n c r e a s i n g l yp r oh i b i t i v ea st h ee x p a n s i oni s e x t e n de db e y on dt h ee l e c t r i cqu a d r u p ol et e r ms . Somewou l ds a yt h a twes h ou l dh a v equ i ta f t e rt h ee l e c t r i cd i p ol eorma g n e t i cd i p ol e . Th ep r ob l e mh a ss e v e r a l r oot s .F i r s t , i nt h es e c on da n da l l s u c c e e d i n gt e r msi nt h e e x p a n s i ona swr i t t e n , t h ema g n e t i ca n de l e c t r i ct e r msa r ea l l mi x e du pa n dofd i ffe r e n t t e n s or i a lc h a r a c t e r .Th i sme a n st h a tweh a v et op r oj e c tou tt h ep a r t i c u l a rp a r t swe wa n t ,wh i c hi sn ota l lt h a te a s ye v e ni nt h es i mp l e s tc a s e s .Se c on d ,t h i sa p p r oa c hi s u s e f u lon l ywh e nt h ewa v e l e n g t hi sl on gr e l a t i v et ot h es ou r c e( k d< <1 )wh i c hi sn ot ( a l wa y s )p h y s i c a lf orr a d i oa n t e n n a e .Th i r d,wh a tweh a v ed on ei sa l g e b r a i c a l l y i n e ffic i e n t ;wek e e ph a v i n gt od ot h es a mea l g e b r aov e ra n dov e ra g a i na n di tg e t sn o e a s i e r . Un d e r s t a n d i n gt h ep r ob l e mp oi n t sou tt h ewa yt os ol v ei t .Wemu s ts t a r ta g a i na t t h el e v e loft h eGr e e n ’ sf u n c t i one x p a n s i on ,b u tt h i st i mewemu s tc on s t r u c ta g e n e r a l i z e dt e n s o r i a lmu l t i p ol a re x p a n s i ont ou s ei nt h ei n t e g r a le qu a t i on .Af t e rt h a t , wemu s td o“ on c ea n df ora l l ”t h en e c e s s a r yc u r la n dd i v e r g e n c ea l g e b r a , a n dc l a s s i f y t h er e s u l t i n gp a r t sa c c o r d i n gt ot h e i rf or ma lt r a n s f or ma t i onp r op e r t i e s .F i n a l l y , wewi l l r e a s s e mb l et h es ol u t i o ni nt h en e wv e c t ormu l t i p ol e sa n dg l or yi ni t sf or ma l s i mp l i c i t y . Ofc ou r s e , t h ec a t c hi st h a ti ti sal otofwor ka tf i r s t .Th ep a y offi st h a ti ti sge n e r a l a n d s y s t e ma t i c a l l ye x t e n d a b l et oa l l or de r s . Aswed ot h i s , I ’ ml e a v i n gy out owor kou tt h ev a r i ou se x a mp l ep r ob l e msi nJ a c k s on ( e . g .s e c t i onJ 9 . 4 ,9 . 5 )ony ou rown .We ’ v ea l r e a d yc ov e r e dmos tofJ 9 . 6b u tweh a v e t od oab i tmor er e v i e woft h ea n g u l a rp a r toft h eL a p l a c eop e r a t or ,wh i c hwel a r g e l y s k i p p e db e f or e .Th i swi l lt u r nou tt ob ek e ya swed e v e l opMu l t i p ol a rRa d i a t i onF i e l d s p r o p e r l y . 13. 7 Ra d i a t i o nAs s i gn me n t a )De r i v et h ei n t e g r a l e x p r e s s i onf ors p h e r i c a l b e s s e l f u n c t i on si nt e r msofp l a n e wa v e sa tt h es a mewa v e n u mb e r . b )Th ea d di t i ont h e or e ms : 1 √4π =4 π r )=h( kr r ) 1 =√4π ′ ′ N0( r−r )=n ( k| r−r ) 0 √ ∗ L NL( r ) J ( r ) > L < ( 1 3 . 1 6 3 ) and ± H( r 0 − ′ ± 0 ′ |− √ 4π ± ∗ H( r ) J ( r ). L L > L < ( 1 3 . 1 6 4 ) a r ede r i v e ds ome p l a c e , f orb ot ht h i ss p e c i a lc a s ea n df ort h eg e n e r a lc a s e .F i n d a tl e a s t on es u c hp l a c e( f or L = 0 ,0 ) ,c op yt h ed e r i v a t i on ( wi t h a c k n owl e d g e me n t ) , a n dh a n di ti n .I fy ouwor ki nag r ou p , s e eh owma n yp l a c e s y ouc a nf i n di ta n dc omp a r e .L EARNwh a ty ouc a nf r omt h ep r oc e s s , t h a ti s , r e a d t h et e x ta c c omp a n y i n gt h ed e r i v a t i on ( s )y ouf i n da n dt r yt ou n de r s t a n di t .Wor ki t ou t .F ore x t r ac r e di t ,f i n di nt h el i t e r a t u r et h eor i g i n a lp a p e rt h a td e r i v e st h e g e n e r a l a d di t i ont h e or e m. Hi n t s : J MP, Da n osa n dMa x i mon . St u d yi t . c )De r i v et h eGr e e n ’ sf u n c t i onf ort h eHe l mh ol t ze qu a t i oni nf r e es p a c e( z e r ob ou n d a r y c on d i t i on sa ti n f i n i t y ) . Don otu s et h ea d d i t i ont h e or e m, s i n c ey oudon ot( i np r i n c i p l e ) k n owi t sf or my e ta n ds od on otk n owt h a ti ti saNe u ma n norHa n k e lf u n c t i on . Na t u r a l l y ,y ouc a nf ol l owWy l dorJ a c k s onorAr f k e n ,b u ta c k n owl e d g ey ou rs ou r c e a n ds h owt h ee n t i r ede r i v a t i on . d )Ma k ean e a ts h e e twi t hEv e r y t h i n gYouNe v e rWa n t e dToKn owAb ou tSp h e r i c a l Be s s e l / Ne u ma n n / Ha n k e l F u n c t i on sb u twe r eAf r a i dNotToAs koni t .Don ’ th a n d i ti n , t h i swi l l b ey ou rg u i d et h r ou g hl i f e( f ora tl e a s taf e wwe e k s ) .DoNOTs i mp l y p h ot oc op ymyn ot e s . Doi tb yh a n d. Pool y ou rs h e e t swi t ht h os eofy ou rf r i e n d s— p u tt og e t h e re v e r y t h i n gt oma k ea“ b e s t ”s h e e ta n dt h e np h ot oc op yi t .Iu s et h e t e r m“ s h e e t ”l oos e l y . I e x p e c ti twi l l f i l l s e v e r a l ( i td i di nmyn ot e s ) . e )Us i n gt h ea d d i t i ont h e or e mde r i v e da b ov e( i nt h ef or moft h eGr e e n ’ sf u n c t i on )a n d t h ea s y mp t ot i cr e l a t i on sony ou rwor k s h e e t ,d e r i v et h es t a t i cr e s u l tf ort h ev e c t or p ot e n t i a l Awep r e v i ou s l yob t a i n e df ort h en e a rf i e l d z on e( mye qu a t i o n6 6 ) .F i n dt h el owe s tor d e rc or r e c t i ont ot h i se x p r e s s i on .Th i s wi l l , ofc ou r s e , i n v ol v ef i n d i n gmor eou ta b ou ts p h e r i c a l wa v e st h a nIh a v es of a r t ol dy ou !i t e m Us i n gt h es a mea d d i t i ont h e or e ma n dt h eot h e ra s y mp t ot i c r e l a t i on s ,d e r i v ea ne x p r e s s i onf ort h ev . p .A i nt h ef a rz on e .I st h e r ea c or r e s p on da n c eofs omes or twi t hou rp r e v i ou sr e s u l t( J a c k s on9 . 9 ) ? f )Sh owt h a t 1 + Y ( ˆ r ) Ax ()=i k h( k r ) 1 , m 1 ′ ′ J ( x) j ( k r ) Y 1 ∗ 3′ 1 , m ( r̂)dx ′ m= −1 i se qu i v a l e n tt o i k r1 e πrr −i Ax ()= 4 k ′ ′ 3′ J ( x) ( n·x) dx f ork d< <1 . g )An yv e c t orqu a n t i t yc a nb ed e c omp os e di nas y mme t r i ca n da na n t i s y mme t r i c p i e c e .Pr ov et h a t ,i nt h ec a s eoft h eℓ=1t e r md e r i v e da b ov e ,t h ec u r r e n tt e r m c a nb ed e c omp o s e di n t o ′ 1 ′ ′ 1 ′ ( n·x) J+( n·J ) x] + 2( x×J )×n 2[ J ( n·x)= h )E v a l u a t et h ea n t i s y mme t r i cp i e c e . Sh ow( f r omt h ema g n e t i cd i p ol ev e c t or p ot e n t i a l )t h a t i k r e µ0 B= 4π a n d 2 k( n×m)×n r +[ 3 n ( n·m)−m] 1 µ0 E=− 4π 1 2 i k i kr 3 −r 2 e r 1 i k r e ǫ0 k( n×m) r 1− i k r . Re ma r ku p ont h es i mi l a r i t i e sa n dd i ffe r e n c e sb e t we e nt h i sr e s u l ta n dt h ee l e c t r i c d i p ol er e s u l t . i )Ne x ts t a r tt oe v a l u a t et h ei n t e g r a l oft h es y mme t r i cp i e c e . Sh owt h a ty oug e t : 1 ′ ′3′ ( n·x) J+( n·J ) x] dx=− 2 [ i ω 2 ′ ′ ′ 3′ x( n·x) ρ( x) dx Th es t e p si n v ol v e da r e : A) i n t e g r a t eb yp a r t s( wor k i n gt oob t a i nd i v e r g e n c e sofJ ) . B)c h a n g i n g∇·Ji n t oaρt i me swh a t e v e rf r omt h ec on t i n u i t ye qu a t i on( f ora h a r mon i cs ou r c e ) . C)r e a r r a n g i n ga n dr e c omb i n i n g . Don ’ tf or g e tt h eb ou n d a r yc on d i t i ona ti n f i n i t y ! j )Home ma d et a b l e s ,p a r tI I .Wh a ty oudi df ors p h e r i c a lb e s s e lf u n c t i on s ,d of or s p h e r i c a lh a r mon i c s .I np a r t i c u l a r ,d e r i v et h ec ommu t a t i onr u l e sf ort h er a i s i n g a n dl owe r i n gop e r a t or sf r omt h ec a r t e s i a nc ommu t a t i onr e l a t i on sf orL .F r omt h e c ommu t a t i onr u l e sa n dL e r i v et h e( n or ma l i z e d )a c t i onofL n zY ℓ m =mY ℓ md ±o Yℓ, m. k )J a c k s on , p r ob l e ms9 . 2 , 9 . 3 , 9 . 4 Ch a p t e r14 Ve c t o rMu l t i p o l e s AsIn ot e dj u s ta b ov e ,we ’ r ea l r e a d yh a l fwa yt h r ou g hJ 9 . 6 ,wh i c hi smos t l yt h er e v i e wof s p h e r i c a lb e s s e l , n e u ma n n , a n dh a n k e lf u n c t i on st h a tweh a v ej u s th a d .Th er e ma i n d e ri sa l i g h t n i n gr e v i e wofs c a l a rs p h e r i c a l h a r mon i c s . Si n c ewe ’ r ea b ou tt oge n e r a l i z et h a tc on c e p t , we ’ l l qu i c k l yg oov e rt h eh i g hp a r t s . 14. 1 An gu l a rmome n t u ma n ds p h e r i c a l h a r mo n i c s 2 Th ea n g u l a rp a r toft h eL a p l a c eop e r a t or∇ c a nb ewr i t t e n : 1 2 r 1∂ 2 1∂ ∂ si nθ∂θ si nθ∂θ 2 2 2 n θ∂φ +si 2 L 2 =− r ( 1 4 . 1 ) 2 E l i mi n a t i n g− r( t os ol v ef ort h eL d i ffe r e n t i a le qu a t i on )on en e e d st os ol v ea n e i g e n v a l u ep r ob l e m: 2 Lψ=e ψ ( 1 4 . 2 ) wh e r eea r et h ee i g e n v a l u e s , s u b j e c tt ot h ec on di t i ont h a tt h es ol u t i onb es i n g l ev a l u e d onφ∈[ 0 , 2 π)a n dθ∈[ 0 , π] . Th i se qu a t i one a s i l ys e p a r a t e si nθ , φ.Th eφe qu a t i oni st r i v i a l –s ol u t i on sp e r i od i ci nφ a r ei n d e x e dwi t hi n t e g e rm.Th eθe qu a t i onon eh a st owor ka tab i t–t h e r ea r ec on s t r a i n t s ont h es ol u t i on st h a tc a nb eob t a i n e df o ra n yg i v e nm–b u tt h e r ea r ema n ywa y st os ol v ei t a n da tt h i sp oi n ty ous h ou l dk n owt h a ti t ss ol u t i on sa r ea s s o c i a t e dL e ge n d r ep o l y n o mi a l s Pℓ, x )wh e r ex=c osθ. Th u s m( t h ee i g e n s ol u t i onb e c ome s : 2 LYℓm=ℓ ( ℓ+1 ) Yℓm ( 1 4 . 3 ) wh e r eℓ=0 , 1 , 2 . . .a n dm=− ℓ , − ℓ+1 , . . . , ℓ−1 , ℓa n di st y p i c a l l yor t h on or ma l ( i z e d)on t h es ol i da n g l e4 π. 1 7 7 Th ea n g u l a rp a r toft h eL a p l a c i a ni sr e l a t e dt ot h ea n gu l a rmo me n t u mofawa v ei n qu a n t u mt h e or y . I nu n i t swh e r e=1 , t h ea n g u l a rmome n t u mop e r a t ori s : 1 x (×∇) L= i ( 1 4 . 4 ) a n d 2 2 2 2 L =L x +L y +L z ( 1 4 . 5 ) Not et h a ti na l loft h e s ee x p r e s s i on sL ,L,L e t c .a r ea l lo p e r a t or s .Th i sme a n st h a t z, t h e ya r ea p p l i e dt ot h ef u n c t i on sont h e i rr i g h t( b yc on v e n t i on ) .Wh e ny ous e et h e m a p p e a r i n gb yt h e ms e l v e s , r e me mb e rt h a tt h e yon l yme a ns ome t h i n gwh e nt h e ya r ea p p l i e d , s o∇’ sou tb yt h e ms e l v e sont h er i g h ta r eok . Th ezc omp on e n tofLi s : ∂ L i ∂φ z=− ( 1 4 . 6 ) 2 a n dwes e et h a ti nf a c tYl ms a t i s f i e st h et woe i g e n v a l u ee qu a t i on s : 2 LYℓm=ℓ ( ℓ+1 ) Yℓm ( 1 4 . 7 ) L zY ℓ m =mY ℓ m ( 1 4 . 8 ) a n d Th eYl m’ sc a n n otb ee i g e n s ol u t i on sofmor et h a non eoft h ec omp on e n t sofLa t on c e .Howe v e r , wec a nwr i t et h ec a r t e s i a nc omp on e n t sofLs ot h a tt h e yf or ma nf i r s t r a n kt e n s ora l ge b r aofop e r a t or st h a tt r a n s f or mt h eYℓm,f orag i v e nℓ ,a mon g t h e ms e l v e s( t h e yc a n n o tc h a n g eℓ ,on l ymi xm) .Th i si st h eh op e f u l l yf a mi l i a rs e tof e qu a t i on s : L L + =L x+i y ( 1 4 . 9 ) L L − =L x−i y ( 1 4 . 1 0 ) L 0 =L z ( 1 4 . 1 1 ) Th e Ca r t e s i a nc omp on e n t s ofL d on otc ommu t e .I nf a c t ,t h e yf or m an i c e a n t i s y mme t r i cs e t : [ L , L =i ǫi L ( 1 4 . 1 2 ) i j] j k k wh i c hc a nb ewr i t t e ni nt h es h or t h a n dn ot a t i on L×L=i L . ( 1 4 . 1 3 ) Con s e qu e n t l y ,t h ec omp on e n t se x p r e s s e da saf i r s tr a n kt e n s ora l s odon ot c ommu t ea mon gt h e ms e l v e s : [ L , L ] =2 L + − z ( 1 4 . 1 4 ) [ L , L =∓L ± z] ± ( 1 4 . 1 5 ) a n d 2 b u ta l l t h e s ewa y sofa r r a n g i n gt h ec omp on e n t sofLc ommu t ewi t hL: 2 [ L , L] =0 i ( 1 4 . 1 6 ) a n dt h e r e f or ewi t ht h eL a p l a c i a ni t s e l f : 2 2 [ ∇, L ] =0 i wh i c hc a nb ewr i t t e ni nt e r msofL a s : 2 1∂ 2 2 ∇ ( 1 4 . 1 7 ) 2 L 2 r∂r ( r)− r = ( 1 4 . 1 8 ) Ason ec a ne a s i l ys h ow e i t h e rb yc on s i de r i n gt h ee x p l i c ta c t i onoft h ea c t u a l d i ffe r e n t i a lf or msont h ea c t u a le i g e n s ol u t i on sYℓm ormor es u b t l yb yc on s i d e r i n gt h e a c t i onofL nL Yℓℓ( a n ds h owi n gt h a tt h e yb e h a v el i k er a i s i n ga n dl owe rop e r a t or s zo ± f orma n dp r e s e r v i n gn or ma l i z a t i on )o n eob t a i n s : L Yℓm = ( ℓ−m) ( ℓ+m+1 )Yℓ, + m+ 1 ( 1 4 . 1 9 ) ℓ+m) ( ℓ−m+1 )Yℓ, m− 1 = ( =mYℓm ( 1 4 . 2 0 ) ( 1 4 . 2 1 ) L −Y ℓ m L zY ℓ m F i n a l l y ,n ot et h a tLi sa l wa y sor t h og on a lt orwh e r eb ot ha r ec on s i d e r e da s o p e r a t or sa n dra c t sf r omt h el e f t : r·L=0 . ( 1 4 . 2 2 ) Youwi l ls e ema n yc a s e swh e r ei d e n t i t i e ss u c ha st h i sh a v et ob ewr i t t e ndowni na p a r t i c u l a ror d e r . Be f or eweg oont od oamor el e i s u r e l yt ou rofv e c t ors p h e r i c a lh a r mon i c s ,we p a u s et omot i v a t et h ec on s t r u c t i on . 14. 2 Ma gn e t i ca n dE l e c t r i cMu l t i p o l e sRe v i s i t e d Asweh a v en ows e e nr e p e a t e d l yf r omCh a p t e rJ 6on , i nas ou r c ef r e er e g i onofs p a c e , h a r mon i ce l e c t r oma g n e t i cf i e l d sa r ed i v e r g e n c e l e s sa n dh a v ec u r l sg i v e nb y : ∇×E =i ωB=i k c B k ∇×B =− icE . ( 1 4 . 2 3 ) ( 1 4 . 2 4 ) 2 Byma s s a g i n gt h e s eal i t t l eb i t( r e c a l l ∇×( ∇×X)=∇( ∇·X)−∇Xa n d ∇·X=0f orX=E , B)wec a ne a s i l ys h owt h a tb ot hEa n dBmu s tb ed i v e r g e n c e l e s s s ol u t i on st ot h eHHE: 2 2 ( ∇ +k) X=0 ( 1 4 . 2 5 ) I fwek n owas ol u t i ont ot h i se qu a t i onf orX=Ewec a nob t a i nBf r omi t sc u r l f r omt h e e qu a t i ona b ov e : i B=− ω ∇×E ( 1 4 . 2 6 ) a n dv i c ev e r s a .Howe v e r , t h i si sa n n oy i n gt ot r e a td i r e c t l y , b e c a u s eoft h ev e c t orc h a r a c t or ofEa n dBwh i c hc omp l i c a t et h ed e s c r i p t i on( a sweh a v es e e n –t r a n s v e r s ee l e c t r i cf i e l d sa r er e l a t e dt oma g n e t i cmu l t i p ol e sa n dv i c ev e r s a ) . L e t ’ s e l i mi n a t ei t . Byc on s i d e r i n gt h ea c t i onoft h eL a p l a c i a nont h es c a l a rp r od u c tof r wi t h awe l l –b e h a v e dv e c t orf i e l dX, 2 2 ∇r (·X)= r·( ∇X)+2 ∇·X ( 1 4 . 2 7 ) a n du s i n gt h ed i v e r g e n c e l e s sofEa n dB, wes e et h a tt h es c a l a r sr ( ·E )a n d r (·B)a l s os a t i s f yt h eHHE : 2 2 2 2 ( ∇ +k) ( r ( ∇ +k) ( r · E ) = 0 ( 1 4 . 2 8 ) · B) = 0 ( 1 4 . 2 9 ) Wea l r e a dyk n owh owt owr i t eag e n e r a ls ol u t i ont oe i t h e roft h e s ee qu a t i on si nt e r ms oft h es p h e r i c a l b e s s e l , n e u ma n n , a n dh a n k e l f u n c t i on st i me ss p h e r i c a l h a r mon i c s . Re c a l l , t h a twh e nwep l a y e da r ou n dwi t hmu l t i p ol ef i e l d s ,Ik e p te mp h a s i z i n gt h a t e l e c t r i cn p ol ef i e l dswe r et r a n s v e r s ema g n e t i ca n dv i c ev e r s a ?We l l ,t r a n s v e r s e e l e c t r i cf i e l d sh a v er (·E )=0b yd e f i n i t i on ,r i g h t ?Son owwed e f i n eama gn e t i c mu l t i p o l ef i e l dofor d e rLb y ℓ ( ℓ+1 ) ( M) r·BL = ( k r ) YL( ˆ r ) ℓ k g ( M) r·E L ( 1 4 . 3 0 ) =0 . ( 1 4 . 3 1 ) Si mi l a r l y , ae l e c t r i cmu l t i p o l ef i e l do for d e rL( wh i c hmu s tb et r a n s v e r s ema g n e t i c )i s a n ys ol u t i ons u c ht h a t ℓ ( ℓ+1 ) ( E ) r·E L =− k f ( k r ) YL( ˆ r ) ℓ ( E ) r·BL ( 1 4 . 3 2 ) =0 . ( 1 4 . 3 3 ) I nt h e s et wod e f i n i t i on s , g ( k r )a n df ( k r )a r ea r b i t r a r yl i n e a rc omb i n a t i on sofs p h e r i c a l ℓ ℓ 1 b e s s e lf u n c t i on s, t woa tat i me .J a c k s onu s e st h et woh a n k e lf u n c t i on si n( J 9 . 1 1 3 ) k , b u tt h i si sn otn e c e s s a r y . Now, al i t t l et r i c k e r y . Us i n gt h ec u r l e qu a t i onf orBweg e t : k r (·BL 1 ( M) )=1 ·( i r ∇ ×E L ( M) )=1 ) · EL r (× i ∇ F r omn owon , t h i st e r mi sg e n e r i cu n l e s sc l e a r l yot h e r wi s ei nc on t e x t . ( M) = L · E L ( M) ( 1 4 . 3 4 ) ( s ot h a tL·ELM)i sas c a l a rs ol u t i o nt ot h eHHEf orma g n e t i cmu l t i p ol a rf i e l d s .Di t t o (E ) f orL·BL i nt h ec a s eofe l e c t r i cmu l t i p ol a rf i e l d s . Th u s , ( M) L·E ( ℓ+1 ) g ( k r ) YL( ˆ r ) L =ℓ ℓ ( E ) e t c . f orL·BL . No wweg e tr e a l l yc l e v e r . Re me mb e rt h a t r h a v ea r r a n g e dt h i n g sj u s ts ot h a ti fwewr i t e : ·L=0 . Al s o, L ( 1 4 . 3 5 ) 2 =L·L . We ( M) =g L E ( k r ) L YL( ˆ r ) ℓ i ( M) BL ( 1 4 . 3 6 ) ( M) E =− ω ∇× L ( 1 4 . 3 7 ) wee x a c t l yr e c on s t r u c tt h es ol u t i on sa b ov e .Ne a t o!Th i sg i v e su sac omp l e t e l yg e n e r a l TE, MME MF .ATM, EME MFf ol l owss i mi l a r l ywi t hg→ fa n dE↔ B( a n dami n u ss i g n i nt h es e c on de qu a t i on ) . Th i si sg oodn e wsa n db a dn e ws .Th eg oodn e wsi st h a tt h i si sah e l lofal ot s i mp l e rt h a n s c r e wi n g a r ou n d wi t h s y mme t r i c a n d a n t i s y mme t r i c v e c t or d e c omp os i t i on sa n di n t e g r a t i on sb yp a r t sa dn a u s e a m.Th er a d i a l p a r toft h es ol u t i on s i ss t r a i g h t f or wa r d , a n dt h ea n g u l a rp a r ti swr i t t e ni nac on c i s en ot a t i on .Th eb a dn e ws i sweh a v en e v e rs e e nt h a tn ot a t i on ,g oodorb a d ,e v e rb e f or e .Weh a v et woc h oi c e s . E i t h e rwec a nl a b or i ou s l yc r a n kou tt h eop e r a t orp r odu c t sa n dc u r l sf ore a c hp r ob l e m a swen e e dt o( wh i c hi sr e a l l yj u s ta sb a da swh a tweh a v eb e e nd oi n g )orweh a v et o wor kou tt h ea l g e b r aoft h e s en e wob j e c t son c ea n df ora l l s owec a np l u ga n dc h u gou t t h emos td i ffic u l tofa n s we r swi t hc omp a r a t i v ee a s e . Gu e s swh i c hon ewe ’ r ea b ou tt od o. 14. 3 Ve c t orSp h e r i c a l Ha r mon i c sa n dMu l t i p ol e s Re c a l l t h a t L=− r i×∇. ( 1 4 . 3 8 ) Th i si sa n“ or b i t a l ”r ot a t i onop e r a t or .I ns y s t e mswi t hs p i ni ti smor ec on v e n i e n ti n ma n yc a s e st od e f i n ea“ t ot a l ”r ot a t i onop e r a t ort h a ta dd st h eor b i t a lr ot a t i onop e r a t or t oa“ s p i n ”r ot a t i onop e r a t or( d e f i n e db e l ow) .Si n c et ot a la n g u l a rmome n t u m( a s op p os e dt oor b i t a la n g u l a rmome n t u m)i sar e l a t i v i s t i c a l l yi n v a r i a n tqu a n t i t yt h a t a p p e a r s“ n a t u r a l l y ”i nc ov a r i a n tk i n e ma t i c s ,wea r ei n s p i r e dt of i n dar e p r e s e n t a t i on t h a ti s a )Av e c t orf u n c t i onofi t sc oor d i n a t e s . 2 2 b )Si mu l t a n e ou se i g e n f u n c t i on sofJ, L, a n dJ z. c )Pos s e s s e dofc e r t a i nde s i r a b l ep r op e r t i e swewi l l d e r i v e . Ac t u a l l y , f i g u r i n gou ts ome t h i n gl i k et h i st h ef i r s tt i mei sn otqu i t es oe a s y ; i ti sf u l l off a l s es t a r t sa n de x p l or i n ga l t e r n a t i v e s .Af t e rt h ef a c t , h owe v e r , i ti sc l e a rt h a tt h i si s t h ec or r e c tc h oi c e . I ti sa l s oe x t r e me l yu s e f u l i nqu a n t u mt h e or y . Th et ot a l r ot a t i onop e r a t ori s J = L + S ( 1 4 . 3 9 ) S=i I × ( 1 4 . 4 0 ) wh e r e i st h e“ s p i n ”op e r a t or . As i d e :Th eSp i nOp e r a t or Si nt h i se x p r e s s i oni sat e n s orop e r a t o r .I t( l i k ea l lop e r a t or s )h a sn ome a n i n gb y i t s e l f .I ti s , h owe v e r , qu i t ed i ffe r e n tf r omt h es c a l a rop e r a t or sy oua r eu s e dt o.Amon g ot h e rt h i n g s , wh e nSop e r a t e sonav e c t orA, i tg e n e r a t e san e wv e c t ort h a tp oi n t si na d i ffe r e n td i r e c t i on . L e tu ss e et h i s . I nt h ede f i n i t i onofS, Ii st h ei de n t i t yt e n s or( u n i tdi a g on a l ma t r i x )a n di ti sc r os s e d i n t owh a t e v e rs i t soni t sr i g h t .Tou n d e r s t a n di t sa c t i on ,l e tu se v a l u a t ei t sc a r t e s i a n c omp on e n t sa c t i n gons omev e c t orA: SxA =i I x ˆ×A x×A=i ( 1 4 . 4 1 ) SyA = i y ˆ×A ( 1 4 . 4 2 ) SxA i z ˆ×A ( 1 4 . 4 3 ) = or( e . g . ) SzA=i ( Axy ˆ−Ayx ˆ ) . ( 1 4 . 4 4 ) Not et h a tt h ea c t i onofac omp on e n tofSonav e c t orAs h i f t st h ed i r e c t i onofAt oa d i r e c t i onp e r p e n d i c u l a rt ob ot hSa n dt h ec omp on e n t .On l yb yc on s i de r i n gt h ea c t i onof a l lt h ec omp on e n t sc a nt h et ot a lv e c t ora c t i onofSonAi nag i v e ndi r e c t i onb e e v a l u a t e d . Th e r ea r es e v e r a l i mp or t a n tp r op e r t i e sofS.Th ef i r s ti st on ot et h a ti th a st h ef or m 2 ofa na n g u l a rmome n t u mop e r a t orwi t has p e c i a la c t i ononv e c t or s .I fwef or mS a n d e v a l u a t ei t sa c t i ononA: 2 SA=−x ˆ×( ˆ x×A)+y ˆ×( ˆ y×A)+z ˆ×( ˆ z×A) { A−3 A} =− =2 A=s ( s+1 ) A 2 ( 1 4 . 4 5 ) f ors=1 .S a c t i n gona n yv e c t orp r od u c e s2t i me st h es a mev e c t or , t e l l i n gu st h a ta v e c t orh a s“ s p i na n g u l a rmome n t u m”of1 .Not et h a tt h i sc on n e c t i oni su n i v e r s a l .I n f i e l dt h e or ya“ v e c t orb os on ”h a ss p i n1 .I ne l e c t r od y n a mi c s( qu a n t u morc l a s s i c a l )t h e “ v e c t orr a di a t i onf i e l d”h a ss p i non e . Th es p i nop e r a t ort h u sf or me di smor eg e n e r a l , b e c a u s ei t sa c t i onc a nb ee x t e n d e dt o h i g h e rr a n kt e n s or s .( 2 n dr a n kt e n s or )g r a v i t a t i on a lf i e l dsh a v es p i n2 .Sc a l a r( 0 t hr a n k t e n s or )f i e l d sh a v es p i n0 . Tot r e a tmor eg e n e r a l c a s e s , h owe v e r , weh a v et owor kwi t ht e n s ori n di c e se x p l i c i t l ya n dy ou ’ l l s e ee n ou g hoft h a ti n t h es e c t i ononr e l a t i v i t y .F e e lf r e et os t u d yt h i sma t t e rf u r t h e r .L ou c ka n dBi e de n h a r n ’ s b ook( E n c y c l .ofMa t hPh y s . ,s e emef orr e f . )c on t a i n samu c hd e e p e rd i s c u s s i onof t h i se n t i r es u b j e c t . I tma ys e e mt h a twi t hs u c hap e c u l i a rs t r u c t u r e , Szc a nh a v en oe i g e n v e c t or s .Th i s i sn ott h ec a s e . Yous h o u l dv e r i f yt h a t 1 1 χ 1 = − √2 x ˆ+i y ˆ 0 χ 1 = z ˆ 1 −1 χ1 ˆ−i y ˆ = √2 x a r ee i ge n v e c t or ss u c ht h a t m ( 1 4 . 4 6 ) ( 1 4 . 4 7 ) ( 1 4 . 4 8 ) m Szχ1 s =msχ1 s ( 1 4 . 4 9 ) m m =s S2χ ( s+1 ) χ f orms=− 1 , 0 , 1a n d s ( 1 4 . 5 0 ) s 1 1 f ors=1 .Yous h ou l da l s ov e r i f yt h ec ommu t a t i onr e l a t i on sf ort h ec omp on e n t sofS, t h a ti s , s h owt h a t S×S=i S ma k i n gi ta“ t r u e ”r ot a t i on / a n g u l a rmome n t u mop e r a t or . I na d d i t i on , wewi l l n e e dt ou s et h eop e r a t or s ( 1 4 . 5 1 ) 2 ( e t c . )a n d J =J J J x x+J y y+J zJ z, ( 1 4 . 5 2 ) J z=L z+S z ( 1 4 . 5 3 ) L =L L x x+L yL y+L zL z ( 1 4 . 5 4 ) 2 s ot h a t 2 2 J =L +2+2 i L × ( 1 4 . 5 5 ) wh i c hc a nb ep r ov e na sf ol l ows . Con s i d e ri t sa c t i ononA( a su s u a l ) : 2 JA = 2 2 L+ S+2 L · SA 2 = L +2+2 i L ( ˆ x× x = L +S +2 i ( L× 2 2 )+L ˆ y× y( )+L ˆ z× z( )A ( 1 4 . 5 6 ) wh e r et h eme a n i n goft h el a t t e re x p r e s s i oni sh op e f u l l yn owc l e a r . m Th e nwed e f i n et h ev e c t o rs ph e r i c a l h a r mo n i c sY b y : j , ℓ 2 m 2 m )A m JYj , ℓ ( j +1) Yj , ℓ =j LYj , ℓ ( ℓ+1) Yj , ℓ =ℓ m m JzYj,ℓ ( 1 4 . 5 7 ) ( 1 4 . 5 8 ) ( 1 4 . 5 9 ) Not et h a ti nor d e rf ort h el a t t e re x p r e s s i ont ob et r u e , wemi g h tr e a s on a b l ye x p e c tt h e v e c t ors p h e r i c a lh a r mon i c st ob ec on s t r u c t e dou tofs u msofp r od u c t sofs p h e r i c a l h a r mon i c sa n dt h ee i g e n v e c t or soft h eop e r a t orSzd e f i n e da b ov e .Th i si st h ev e c t or a n a l og u eofc on s t r u c t i n gas p i n orwa v e f u n c t i oni nqu a n t u mt h e or y . I na d d i t i on , wen or ma l i z et h e s eor t h og o n a l f u n c t i on ss ot h a tt h e ya r eor t h o n o r ma l a sad otp r o d u c t . Th i swi l l a l l owu st ou s et h e mt oc on s t r u c tp r oj e c t i on s . ′ m ′ m∗ θ , φ)·Yj′, ′( θ , φ) d Ω=δj ′δ ′δ mm ℓ j ℓ ℓ Yj,ℓ ( ( 1 4 . 6 0 ) 2 Wen own e e dt od e r i v et h ep r op e r t i e soft h e s ef u n c t i on s .Web e g i nb ya p p l y i n gJ m t oY j , ℓ 2 m 2 m JYj i L ×Yj , ℓ =L +2+2 , ℓ ( 1 4 . 6 1 ) s ot h a tweg e t m m 2i L×Yj j ( j +1)−ℓ ( ℓ+1)−2 }Yj , ℓ ={ , ℓ. ( 1 4 . 6 2 ) Mos toft h el a t e rr e s u l t swi l lb eb a s e dont h i son e , s ou n d e r s t a n di tc omp l e t e l y .I fwet a k e L ·ofb ot hs i d e sof( 1 4 . 6 2 ) , u s eav e c t ori d e n t i t ya n dr e c a l l t h a t L×L=i Lweg e t : m [ j ( j +1 )−ℓ ( ℓ+1 ) ] L·Yj . , ℓ =0 ( 1 4 . 6 3 ) Si mi l a r l y , wef or mt h ev e c t orp r od u c tofLwi t hb ot hs i d e sof( 1 4 . 6 2 ) : m m { j ( j +1 )−ℓ ( ℓ+1 )−2 }L×Yj i L×( L×Yj . , ℓ =2 , ℓ) ( 1 4 . 6 4 ) Tor e d u c et h i sf u r t h e r ,wemu s tu s et h eop e r a t orv e c t ori de n t i t y( wh i c hy ous h ou l d p r ov e ) ( 1 4 . 6 5 ) L×( L×V)=L ( L·V)+i L×V−L 2 VL×Yu s i n g a n de l i mi n a t et h e ( 1 4 . 6 2 ) . On eg e t s : [ j ( j +1 )−ℓ ( ℓ+1 ) ] [ j ( j +1 )−ℓ ( ℓ+1 )−2 ] Ymj,ℓ = 4 ℓ ( ℓ+1 ) Y m j , ℓ −4 L ( L ( 1 4 · Y. 6 6 ) . m j , ℓ I fwee l i mi n a t et h eL·Y( u s i n gt h er e s u l ta b ov e )weg e tt h ec h a r a c t e r i s t i ce qu a t i on t h a ti sac on s t r a i n tont h ep os s i b l ev a l u e sofj a n dℓ : 3 2 x −2 x −4 ℓ ( ℓ+1 ) x=0 ( 1 4 . 6 7 ) wh e r e x=j ( j +1 )−ℓ ( ℓ+1 ) b yd e f i n i t i on . Th es ol u t i on st ot h i sf a c t or i z a b l ec u b i ca r e : j =ℓ , ℓ +1 , ℓ−1 , − ℓ−1 , − ℓ−2 , − ℓ . ( 1 4 . 6 8 ) Weon l yn e e dt oc on s i de rt h es ol u t i on swi t hp os i t i v ej i nt h i sp r ob l e ma st h eot h e r sa r e n oti n de p e n d e n ti nt h i sc a s e .Si n c eℓ≥0weon l yn e e dc on s i de rt h ef i r s tt h r e e p os s i b i l i t i e s . Sol u t i on swi t hj =ℓ Th e nx=0a n d m m j ( j +1 ) Yj ( L·Yj j =L j) ( 1 4 . 6 9 ) f r omt h et h i r de qu a t i ona b ov e . I fwet a k et h edotp r od u c tofLwi t ht h i sr e l a t i on , weg e t 2 m m L( L·Yj =j ( j +1 ) ( L·Yj j) j) ( 1 4 . 7 0 ) m a n dwet h u ss e et h a tL·Yj n ds o: j ∝Y j , ma 1 m Yj j= j ( j +1 ) L Yj,m ( 1 4 . 7 1 ) ( wh e r eweh a v en or ma l i z e dt h er e s u l t . Weh a v ea tl a s tf ou n ds ome t h i n gr e c og n i z a b l e .Th i si sp r e c i s e l yt h ec omb i n a t i onofs p h e r i c a lh a r mon i c sa n dLwef ou n di nou rb r i e fe x c u r s i oni n t omu l t i p ol e s ! Wes e et h a twec ou l dh a v ewr i t t e nt h e( e . g . )ma g n e t i cs ol u t i ona s m EL(M) ( k r ) ℓ ℓ =g ( ℓ+1 )Yℓℓ ( 1 4 . 7 2 ) BL E L. =−ω∇ × ( 1 4 . 7 3 ) ( M) i ( M) Wi t hj u s tal i t t l emor ewor k( l a t e r )wewi l lb ea b l et oob t a i nt h ec u r lp a r ta sa g e n e r a lr e s u l t ,wh i c hwi l lr e a l l ys i mp l i f yl i f ef oru s .I ti sat r i v i a le x e r c i s e( l e f tf ort h e r e a d e r )t ov e r i f yt h a t m m J zYj j =mYj j. ( 1 4 . 7 4 ) On es i mp l yp l u g si nt h ee x p l i c i tf or mofJ n dc ommu t e st h er e s u l t a n tL t hLt o za zwi c a n c e l t h e“ s p i n ”p a r t . Sol u t i on swi t hj =ℓ I fj =ℓ , wes e ef r omt h ee qu a t i ona f t e r( 1 4 . 6 2 )t h a tL·Y=0 .Tog of u r t h e rweh a v e t og ob a c kt o( 1 4 . 6 2 )a n df ol l owad i ffe r e n tl i n e . I fwemu l t i p l yb ot hs i de sb yr ˆ ·a n dr ˆ × , m m [ j ( j +1)−ℓ ( ℓ+1)−2]r ˆ·Yj i r ˆ·L×Yj ℓ =2 ℓ ( 1 4 . 7 5 ) a n d m m [ j ( j +1)−ℓ ( ℓ+1)−2] r ˆ×Yj i r ˆ×( L×Yj ℓ =2 ℓ) Wec a nr e d u c et h e s ewi t ht h ev e c t ori d e n t i t i e s ( 1 4 . 7 6 ) r ˆ·( L×A)=2 i r ˆ·A−L·( ˆ r×A) ( 1 4 . 7 7 ) r ˆ×( L×A)=L ( ˆ r·A)+i r ˆ×A. ( 1 4 . 7 8 ) a n d Yous h ou l dg e t m m [ j ( j +1)−ℓ ( ℓ+1)+2] r ˆ·Yj 2i L·( ˆ r×Yj ℓ =− ℓ) ( 1 4 . 7 9 ) a n d m m [ j ( j +1)−ℓ ( ℓ+1) ]r ˆ×Yj i L ( ˆ r·Yj . ℓ =2 ℓ) ( 1 4 . 8 0 ) F i n a l l y , i fwep l u gt h es e c on doft h e s ei n t ot h ef i r s ta n de l i mi n a t et h ec r os sp r od u c t , we g e tt h es c a l a re qu a t i on : 1 m 2 m j ( j +1 )−ℓ ( ℓ+1 ) ] [ j ( j +1 )−ℓ ( ℓ+1 )+2 ] ( ˆ r·Y j )=L( ˆ r·Y j ) . ( 1 4 . 8 1 ) ℓ ℓ 4[ m Th i si mp l i e st h a t( ˆ r ·Yj )i sas p h e r i c a l h a r mon i c : t h a ti sac on s t a n t× Yk, ℓ m. j ( j +1 )−ℓ ( ℓ+1 ) 2 j ( j +1 )−ℓ ( ℓ+1 ) +1=k ( k+1 ) 2 ( 1 4 . 8 2 ) Th i sh a st h es ol u t i on s a )k= j ( j + 1 ) −ℓ ( ℓ + 1 ) 2 b )k= j ( j + 1 ) −ℓ ( ℓ + 1 ) 2 −1 . Si n c ewea l r e a d yk n owt h a tj =ℓ±1 , wec a ni n v e s t i g a t et h e s et woc a s e se x p l i c i t l y . Th ep os i t i v es ol u t i on s( i nb ot hc a s e s )a r ee a s i l ys e e nt ob ek=j . Wec a nt h e nc on s t r u c t t h ec omp l e t es ol u t i on s , s i n c e m m m Yj ˆ ( ˆ r·Yj −r ˆ×( ˆ r×Yj , ℓ =r , ℓ) , ℓ) ( 1 4 . 8 3 ) i sa ni d e n t i t y( r e l a t e dt ot h es y mme t r i c / a n t i s y mme t r i cd e c omp os i t i ona n dh e n c ewor t h p r ov i n g )a n ds i n c eweh a v ea l r e a d ys h ownt h a t m −1 m r ˆ×Yj i [ j ( j +1 )−ℓ ( ℓ+1 ) ]L ( ˆ r·Yj , ℓ =2 , ℓ) ( 1 4 . 8 4 ) m wi t h( ˆ r·Yj ac on s t a n tt i me sYℓ, Weg e t : , ℓ) m. m − 1 Yj c on s t a n t ) r ˆ−2 i [ j ( j +1 )−ℓ ( ℓ+1 ) ]( ˆ r×L )Yℓ, , ℓ =( m. ( 1 4 . 8 5 ) Ane x e r c i s ewi l l b et ov e r i f yt h en or ma l i z a t i onoft h ef i n a l s ol u t i on s : m Y 1 m − j r ˆ+i r ˆ×L ] Yℓ, m =− j ( 2 j +1 )[ 1 ( 1 4 . 8 6 ) j , j + 1 ( j +1 ) ˆ r+i r ˆ×L ] Yℓ, m. =− ( j +1 ) ( 2 j +1 ) [ ( 1 4 . 8 7 ) j , j −1 Y Youmu s ta l s ov e r i f yt h a tt h e ys a t i s f yt h ee qu a t i onf orJ z. F i n a l l y , y oua r ep r ob a b l ywon d e r i n gwh yweh a v eb ot h e r e dt ol e a r na l l oft h i sa b ou t t h ej =ℓc a s e si nt h ef i r s tp l a c e . I ti sb e c a u s e m i ∇×( Yj ( r ) )= jf j +1 f df 2 j +1 ( r j +1 ) r+ d j + m Y j , j −1 f d f 2 j +1 −j r +d r m Y j , j + 1 .( 1 4 . 8 8 ) Th ea c t i onoft h ec u r l mi x e st h ev e c t ors p h e r i c a l h a r mon i c s .I nf a c t , i ta c t st os h i f tj b y on ei na n yp e r mi t t e dd i r e c t i on( s e eh a n dou ts h e e t ) .Th e r e f or e , i nor d e rt oe v a l u a t et h e e n t i r eE Mf i e l da n de x p r e s si tc omp a c t l y ,on emu s tu s et h en ot a t i onoft h ev e c t or s p h e r i c a lh a r mon i c s .Yous h ou l dp r ov et h i s ,a n da tl e a ton eoft h ed i v e r g e n c e e qu a t i on sf orh ome wor k .Youwi l ln e e dt og e tt h ec omp on e n t soft h ev . s . h .a l on ga n d t r a n s v e r s et or ˆi nor de rt od ot h ev e c t ora l g e b r a . Th i si sn ott oob a d , b u t( a swes h a l ls e e )i ti sn ott h eb e s twec a nd o.Byc a r e f u l l y d e f i n i n gap a r t i c u l a rs e tofmu l t i p ol a rs ol u t i on s ,wec a nma k eou rn ot a t i oni t s e l fd o a l mos ta l lt h ewor kofd oi n gt h ec u r l s ,e t c .s ot h a ta l lweh a v et od oa te i t h e re n di s t r a n s l a t eap a t i c u l a rp r ob l e mi n t oa n dou toft h en ot a t i onwi t ht h ef or ma ls ol u t i oni n h a n d . Ne x tt i mewewi l l d oj u s tt h a ta swed e v e l opt h eHa n s e nMu l t i p o l a rSol u t i o n s . Ch a p t e r15 Th eHa n s e nMu l t i p o l e s Weh a v ea l r e a d ys e e nh o wi fwel e tEorBb eg i v e nb y 1 EorB= f ( k r ) L YL( ˆ r ) ℓ ( ℓ+1 ) ℓ ( 1 5 . 1 ) t h e n a )Bot ht h ef i e l d sg i v e na b ov ea n dt h e i rp a r t n e rf i e l d s( g i v e nb yt h ec u r l )h a v ez e r o d i v e r g e n c e . b )Th ef i e l d sg i v e na b ov ea r ec omp l e t e l yt r a n s v e r s e , s i n c er ˆ · L=0( op e r a t or ) . c )Th ep a r t n e rf i e l dsg i v e nb yt h ec u r l a r en otp u r e l yt r a n s v e r s e . d )I nor d e rt ob ec on s i s t e n t , t h ef i e l d sa b ov ea r ea l s ot h ec u r l soft h ep a r t n e rf i e l d s .I n f a c t , t h i sf ol l owsf r omv e c t ori de n t i t i e sf ordi v e r g e n c e l e s sf i e l ds . I ti st h e r e f or es e n s i b l et od e f i n e , on c ea n df ora l l , as e tofmu l t i p ol e st h a te mb ody t h e s ep r op e r t i e s .I na d di t i on , a n t i c i p a t i n gan e e dt ot r e a tl on g i t u d i n a lf i e l d sa swe l la s t r a n s v e r s ef i e l d s ,wewi l ld e f i n eat h i r dk i n dofmu l t i p ol e swi t hz e r oc u r lb u tn on –z e r o d i v e r g e n c e .Th e s ewi l l n e c e s s a r i l yb e“ c on n e c t e d ”t os ou r c e s( wh y ? ) .Wewi l l c a l l t h e s e “ p r e c omp u t e d ”c omb i n a t i on sofb e s s e lf u n c t i on s ,v e c t ors p h e r i c a lh a r mon i c s ,a n d t h e i rc u r l st h eHa n s e nMu l t i p ol e s( f ol l owi n gu n p u b l i s h e dn ot e sf r omL .C.Bi e d e n h a r n a sI h a v eb e e nu n a b l et od e t e r mi n eh i sor i g i n a l r e f e r e n c e ) : 15. 1 Th eHa n s e nMu l t i p o l e s 15. 1. 1 Th eBa s i cSo l u t i on s Th eHa n s e ns ol u t i on st ot h ev e c t orHHE( t h a tc a ne x p a n dt h ef r e es p a c es ol u t i on sf ort h e v e c t orp ot e n t i a lorv e c t orf i e l d s )a r ea sf ol l ows .M Li st h e( n or ma l i z e d )e l e me n t a r y m s ol u t i onc on s i s t i n gofab e s s e l f u n c t i ont i me sL YL=Y l . I ti s l 1 8 9 ( b yc on s t r u c t i on )p u r e l yt r a n s v e r s e :r ˆ · M L=0 .NLi st h es ol u t i onc on s t r u c t e db yt h e t a k i n gt h ec u r lofM L.L st h e“ l on g i t u d i n a l ”s ol u t i onc on s t r u c t e db yt a k i n gt h e Li g r a d i e n toft h es c a l a rs ol u t i on–i ti sl e f ta sa ne x e r c i s et os h owt h a tt h i ss t i l l s a t i s f i e s t h eHHE .Th et h r e eoft h e s ep i e c e ss p a nt h er a n g eofp os s i b l es ol u t i on sa n d r e c on s t r u c ta ni d e n t i t yt e n s ort h a tc a nb eu s e dt oc on s t r u c tav e c t orh a r mon i cGr e e n ’ s f u n c t i one x p a n s i on . Th i si ss u mma r i z e d , wi t hc or r e c t i onf orf a c t or sofki n t r odu c e db yt h ed e r i v a t i v e s , h e r e : ML = 1 L( f ( k r ) YL( r ˆ ) )= ℓ ℓ ( ℓ+1 ) i NL = k∇× ML i L = − L ( k r ) YL( ˆ r ) k∇f ℓ 1 m f ( k r ) Yl ℓ l ( r ˆ )( 1 5 . 2 ) ℓ ( ℓ+1 ) ( 1 5 . 3 ) ( 1 5 . 4 ) 15. 1. 2 Th e i rSi gn i f i c a n tPr o p e r t i e s Th ev i r t u eoft h eHa n s e ns ol u t i on si st h a tt h e y“ a u t oma t i c a l l y ”wor kt od e c omp os e f i e l dc omp on e n t si n t op a r t st h a ta r emu t u a lc u r l s( a sr e qu i r e db yF a r a da y / Amp e r e ’ s l a wsf ort h ef i e l d s )ord i v e r g e n c e s( a sr e qu i r e db yGa u s s ’ sl a wsf ort h ef i e l d s ) : ∇· ML =0 ( 1 5 . 5 ) ∇· NL =0 ( 1 5 . 6 ) ∇· L k f ( k r ) YL( r ˆ ) L =i ℓ ( 1 5 . 7 ) He n c eM La n dNLa r ed i v e r g e n c e l e s s , wh i l et h ed i v e r g e n c eofL sas c a l a rs ol u t i on Li t ot h eHHE ! L sr e l a t e dt ot h es c a l a rf i e l da n dt h eg a u g ei n v a r i a n c eoft h et h e or yi na n Li i n t e r e s t i n gwa ywewi l l d e v e l op . Al s o: ∇× ML =− i k NL ( 1 5 . 8 ) ∇× NL k ML = i = 0 ( 1 5 . 9 ) ( 1 5 . 1 0 ) ∇× L L wh i c hs h owsh owM La n dN La r en owi d e a l l ys u i t e dt of or mt h ec omp on e n t sof e l e c t r i ca n dma g n e t i cmu l t i p ol ef i e l d smu t u a l l yl i n k e db yAmp e r e ’ sa n dF a r a da y ’ sl a w. 15. 1. 3 E x p l i c i tF o r ms Th eb e a u t yoft h ed e f i n i t i on sa b ov ei st h a tt h e yp e r mi tu st odoa l g e b r at h a ti n i t i a l l ys k i p s t h ef ol l owi n gf u l l ye x p a n d e df or msi nt e r msoft h ev e c t ors p h e r i c a lh a r mon i c s .Howe v e r u l t i ma t e l yon eh a st odoc omp u t a t i on s , ofc ou r s e–t h e r e a r en of r e el u n c h e s . Th ef ol l owi n gr e s u l t sc omef r oma c t u a l l ywor k i n gou tt h eg r a di e n t s , d i v e r g e n c e s , a n dc u r l si nt h ede f i n i t i on s : m ML =f ( k r ) Yℓℓ ℓ ( 1 5 . 1 1 ) ℓ+1 NL = L L = ℓ m 2 ℓ +1f ℓ −1 ℓ ( k r ) Y ℓ , ℓ −1 m − k r ) Yℓ, ℓ − 1 + 2 ℓ +1f ℓ −1 ( 2 ℓ +1 f ℓ + 1 m ( k r ) Y ℓ+1 ℓ , ℓ + 1 m k r ) Yℓ, 2 ℓ +1 f ℓ + 1 ℓ + 1( ( 1 5 . 1 2 ) ( 1 5 . 1 3 ) or( i nd i ffe r e n t i a l f or m) m ML = f ( k r ) Yℓℓ ℓ 1 d ( 1 5 . 1 4 ) m ( k r f )i r ˆ×Y ℓℓ −r ˆℓ ( ℓ+1 ) f YL ℓ ℓ NL = k r d ( k r ) 1 d m L L = i r ˆ×f Yℓℓ)−r ˆ d ℓ k r( ( k r )f ℓ ( ℓ+1 ) ℓY L ( 1 5 . 1 5 ) ( 1 5 . 1 6 ) Aswewi l ls e e ,t h e s er e l a t i on sa l l ow u st oc on s t r u c tt h ec o mp l e t e l yge n e r a l s ol u t i ont ot h eE Mf i e l de qu a t i on si nawa yt h a ti si n t u i t i v e ,r e a s on a b l e ,a n d ma t h e ma t i c a l l ya n dn u me r i c a l l yt r a c t i b l e .I not h e rwor d s , we ’ r e( mos t l y )d on ewi t ht h e g r u n twor ka n dc a nb e g i nt or e a pt h er e wa r ds . Wh a tg r u n twor kr e ma i n s ,y oumi g h ta s k ?We l l ,t h e r ea r eas l e wofi d e n t i t i e sa n d e v a l u a t i on sa n dr e l a t i on sd e v e l op e df r om t h ed e f i n i t i on soft h es p h e r i c a lh a r mon i c s t h e ms e l v e s ,t h es p h e r i c a lb e s s e l / n e u ma n n / h a n k e lf u n c t i on st h e ms e l v e s ,a n dt h e v e c t ors p h e r i c a lh a r mon i c sa n dHa n s e ns ol u t i on st h a tc a nb ewor k e dou ta n d a s s e mb l e di nat a b l eofs or t st os i mp l i f yt h ea c t u a lp r oc e s sofdoi n ga l g e b r aor c omp u t a t i on su s i n gt h e m. Su c hat a b l ei sp r e s e n t e da tt h ee n doft h i sc h a p t e r ,a n dp r ov i n gr e l a t i on sont h a t t a b l ec on s t i t u t emos toft h eh ome wo r kr e l a t e dt ot h ec h a p t e r ,s i n c eon c et h i swor ki s d on ed oi n ga c t u a lc omp u t a t i on sf ors p e c i f i cc h a r g e / c u r r e n td e n s i t i e si sr e d u c e dt o qu a d r a t u r e s( a n ot h e rwa yofs a y i n g“ e x p r e s s i b l ea sab u n c hofde f i n i t ei n t e g r a l s ”t h a t c a ne i t h e rb ed on ea n a l y t i c a l l yi ft h e ya r er e l a t i v e l ys i mp l eorn u me r i c a l l yi fn ot ) . Th os er e wa r dsa r emos tr e a d i l ya p p a r e n twh e nwec on s t r u c tt h ev e c t orGr e e n ’ s f u n c t i onf ort h ev e c t orI HE . 15. 2 Gr e e n ’ sF u n c t i o n sf o rt h eVe c t o rHe l mh o l t z E q u a t i on Th ec or r e c tf or mf ort h eGr e e n ’ sf u n c t i onf ort h ev e c t orHe l mh ol t ze qu a t i oni s ⇔ ⇔ ′ ′ G±( r , r )=IG±( r , r ) ( 1 5 . 1 7 ) ′ ( wh e r eG±( r , r )i saGr e e n ’ sf u n c t i onf ort h es c a l a rI HE , t h a ti s : ± i k R e ′ G±( r , r )=− ( 1 5 . 1 8 ) 4πR ′ f orR= | r−r| . Th ei d e n t i t yt e n s ort r a n s f or msav e c t oront h er i g h ti n t ot h es a mev e c t or , s ot h i ss e e msl i k eat r i v i a l d e f i n i t i on .Howe v e r , t h ep oi n ti st h a twec a nn owe x p a n dt h e i d e n t i t yt e n s ort i me st h es c a l a rGr e e n ’ sf u n c t i oni nv e c t ors p h e r i c a lh a r mo n i c sor Ha n s e nf u n c t i on sd i r e c t l y ! Weg e t : ⇔ G± ′ ( r , r )= ± m m∗ ′ h( k r ) Y ( ˆ r ) Y ( ˆ r ) j( k r) i k ∓ > ℓ ℓ j , ℓ , m + 0∗ < + j ℓ j ℓ 0∗ = ∓i k M L( r ) ML( r )+N L( r ) NL ( r )+ > < > < L + 0∗ L r ) L r ) L( > L ( < ( 1 5 . 1 9 ) m I na l lc a s e st h e“ * ” sa r et ob ec on s i d e r e ds l i d i n g , a b l et oa p p l yt ot h eY j ( r ˆ )on l yof l e i t h e rt e r mu n de ra ni n t e g r a l . Id on oti n t e n dt op r o v eak e ye l e me n toft h i sa s s e r t i on( t h a tt h ep r od u c t soft h eY m ( r ˆ )i n v ol v e dr e d u c et oL e g e n d r ep ol y n omi a l si nt h ea n g l eb e t we e nt h ea r g u me n t s j l t i me st h ei d e n t i t yt e n s or )i nc l a s s .I n s t e a d, I l e a v ei ta sa ne x e r c i s e .Tog e ty ous t a r t e d , c on s i de rh ows i mi l a rc omp l e t e n e s s / a d di t i ont h e or e msa r ep r ov e nf ort h es p h e r i c a l h a r mon i c st h e ms e l v e sf r omt h eg i v e nor t h on or ma l i t yr e l a t i on . Wi t ht h e s er e l a t i on si nh a n d,wee n dou rma t h e ma t i c a ld i g r e s s i oni n t ov e c t or s p h e r i c a lh a r mon i c sa n dt h eHa n s e ns ol u t i on sa n dr e t u r nt ot h el a n dofmu l t i p ol a r r a d i a t i on . 15. 3 Mu l t i p o l a rRa d i a t i o n , r e v i s i t e d Wewi l ln ow,a tl on gl a s t ,s t u d yt h ec o mp l e t er a d i a t i onf i e l di n c l u d i n gt h es c a l a r , l on g i t u d i n a l , a n dt r a n s v e r s ep a r t s .Re c a l lt h a twewi s ht os ol v et h et woe qu a t i on s( i n t h eL or e n t zg a u g e ) : 2 2 2 2 ∇ +k ∇ +k ρ Φ( x)= −ǫ0( r ) ( 1 5 . 2 0 ) µ J x () 0 Ax ()= − ( 1 5 . 2 1 ) wi t ht h eL or e n t zc on d i t i on : 1∂Φ ∇· A+ 2 c ∂t =0 ( 1 5 . 2 2 ) wh i c hi sc on n e c t e d( a swes h a l l s e e )t ot h ec on t i n u i t ye qu a t i onf orc h a r g ea n dc u r r e n t . Ea n dBa r en ow ( a su s u a l )d e t e r mi n e df r om t h ev e c t orp ot e n t i a lb yt h ef u l l r e l a t i on s , i . e . –wema k en oa s s u mp t i ont h a twea r eou t s i d et h er e g i onofs ou r c e s : ∂A ∂t ∇Φ− =− E ( 1 5 . 2 3 ) B =∇× ( 1 5 . 2 4 ) A, Us i n gt h eme t h od sdi s c u s s e db e f o r e( wr i t i n gt h es ol u t i ona sa ni n t e g r a le qu a t i on , b r e a k i n gt h ei n t e g r a lu pi n t ot h ei n t e r i ora n de x t e r i oroft h es p h e r eofr a d i u sr ,a n d u s i n gt h ec or r e c tor d e roft h emu l t i p ol a re x p a n s i onoft h eGr e e n ’ sf u n c t i oni nt h e i n t e r i ora n de x t e r i orr e g i on s )wec a ne a s i l ys h owt h a tt h eg e n e r a ls ol u t i ont ot h eI HE ’ s a b ov ei s : Φ( r)=i k e x t i n t + p r ) J r ()+p r ) HLr () L ( L L ( ( 1 5 . 2 5 ) L wher e ∞ e x t p r )= L ( r r i n t p r )= L ( 0 + ′ ∗ ′ ′ 3′ h k r ) YL( r ˆ) ρ r () dr ℓ( ′ ∗ ′ ′ 3′ j ( k r ) YL( r ˆ) ρr () dr ℓ ( 1 5 . 2 6 ) ( 1 5 . 2 7 ) Ou t s i d et h e( b ou n d i n gs p h e r eoft h e )s ou r c e , t h ee x t e r i orc oe ffic i e n ti sz e r oa n dt h e i n t i n t e r i orc oe ffic i e n ti st h es c a l a rmu l t i p ol emome n tp ( ∞)oft h ec h a r g es ou r c e L=p L d i s t r i b u t i on , s ot h a t : Φ( r)= i k ǫ0 + p HLr () L ( 1 5 . 2 8 ) L Th i si sa ni mp or t a n tr e l a t i ona n dwi l lp l a ya ns i g n i f i c a n tr ol ei nt h ei mp l e me n t a t i onof t h eg a u g ec on d i t i onb e l ow. Si mi l a r l ywec a nwr i t et h ei n t e r i ora n de x t e r i ormu l t i p ol a rmome n t soft h ec u r r e n ti n t e r msofi n t e g r a l sov e rt h ev a r i ou sHa n s e nf u n c t i on st oob t a i nac omp l e t e l yg e n e r a l e x p r e s s i onf ort h ev e c t orp ot e n t i a lAr () .Tos i mp l i f yma t t e r s , Ia mg oi n gt oon l ywr i t e downt h es ol u t i onob t a i n e dou t s i d et h ec u r r e n tde n s i t yd i s t r i b u t i on ,a l t h ou g ht h e i n t e g r a t i onv ol u mec a ne a s i l yb es p l i ti n t or n dr i e c e sa sa b ov ea n da ne x a c t <a >p s ol u t i onob t a i n e dona l l s p a c ei n c l u di n gi n s i det h ec h a r gedi s t r i b u t i o n . I ti s : + Ar ()=i k µ0 + mLMLr ()+n NLr ()+l L r () L L L ( 1 5 . 2 9 ) L wher e ′ 0 ′∗3′ ′ 0 ′∗3′ mL = J r ()·MLr ()dr n L = J r ()·NLr ()dr l L = J r ()·L ()dr Lr ′ 0 ′∗3′ ( 1 5 . 3 0 ) ( 1 5 . 3 1 ) ( 1 5 . 3 2 ) Not ewe l lt h a tt h ea c t i onoft h ed otp r odu c twi t h i nt h ed y a d i cf or mf ort h eGr e e n ’ s f u n c t i on( e x p a n d e di nHa n s e ns ol u t i on s )r e d u c e st h edy a d i ct e n s ort oav e c t ora g a i n . I tt u r n sou tt h a tt h e s ef ou rs e t sofn u mb e r s :p ,mL,n ,l r en oti n d e p e n de n t .Th e y L L La a r er e l a t e db yt h er e qu i r e me n tt h a tt h es ol u t i on ss a t i s f yt h eL o r e n t zga u gec o n d i t i on , wh i c h i sac o n s t r a i n tont h ea d mi s s i b l es ol u t i on s .I fwes u b s t i t u t et h e s ef or msi n t ot h eg a u g e c on d i t i oni t s e l fa n du s et h ed i ffe r e n t i a lr e l a t i on sg i v e na b ov ef ort h eHa n s e nf u n c t i on st o s i mp l i f yt h er e s u l t s , weob t a i n : 1∂ Φ + i k L i k ∇· A+ i ω + 2 µ0l ∇·L L L HL − cǫ0p L + L 2 c ∂t + l ∇·L k c p HL L L −i L 2 { l p }HL L−c L L + =0 = 0 + − k = 0 = 0 + ( 1 5 . 3 3 ) ∗ wh e r eweu s e d∇·LL=i k HL i nt h el a s ts t e p . I fwemu l t i p l yf r omt h el e f tb yYℓ′ n d , m′a u s et h ef a c tt h a tt h eYLf or mac omp l e t eor t h on or ma l s e t , wef i n dt h er e l a t i on : l p L−c L=0 ( 1 5 . 3 4 ) l p L=c L ( 1 5 . 3 5 ) or Th i st e l l su st h a tt h ee ffe c toft h es c a l a rmome n t sa n dt h el on g i t u di n a lmome n t sa r e c on n e c t e db yt h eg a u g ec on di t i on . I n s t e a doff ou rr e l e v a n tmome n t sweh a v ea tmos tt h r e e . I nf a c t , a swewi l l s e eb e l ow, weh a v eon l yt wo! Re c a l l t h a tt h ep ot e n t i a l sa r en otu n i qu e–t h e yc a na n dd ov a r ya c c or d i n gt ot h eg a u g e c h os e n .Th ef i e l d s ,h owe v e r ,mu s tb eu n i qu eorwe ’ dg e tdi ffe r e n te x p e r i me n t a lr e s u l t si n d i ffe r e n tg a u g e s . Th i swou l dob v i ou s l yb eap r ob l e m! L e tu st h e r e f or ec a l c u l a t et h ef i e l d s .Th e r ea r et wowa y st op r oc e e d .Wec a n c omp u t eBd i r e c t l yf r omv A: B =∇× A k µ0 =i k µ0 =i + L + + + ml ( − i k NL)+n ( i k ML) l L 2 0 =kµ + ml ( ∇×ML)+n ( ∇×NL)+l ( ∇×L l l L) + + mLNL −n ML L L ( 1 5 . 3 6 ) E ∂ a n du s eAmp e r e ’ sL a w, ∇×B=µ0ǫ0 ∂t =− i ωµ ǫ0Et of i n dE : 0 2 i c ∇×B k c E = + k c µ0 = i + L mL( ∇×NL)−n ( ∇×ML) L 1 + µ0ǫ0 µ 0 =i k mL( i k ML)−n ( − i k NL) L L µ0 2 =− k + + M ǫ0 L 2 =− kZ0 + N mL L+n L + + mLML +nLNL L µ0 wh e r eZ0= L . ( 1 5 . 3 7 ) i st heus ua l i mpe da nc eoff r e es pa c e , a r oun d377ohms . ǫ0 Wow!Re c a l lt h a tt h eMwa v e sa r et r a n s v e r s e ,s ot h emLa n dn r et h ema g n e t i c La ( t r a n s v e r s ee l e c t r i c )a n de l e c t r i c( t r a n s v e r s e ma g n e t i c ) mu l t i p ol e mome n t s r e s p e c t i v e l y .Th ef i e l dou t s i d eoft h es ou r c ei sap u r ee x p a n s i oni ne l e me n t a r y t r a n s v e r s emu l t i p ol e s .( L a t e rwewi l ls h owt h a tt h e( a p p r ox i ma t e )d e f i n i t i on sweh a v e u s e dt od a t ea s” mu l t i p ol e s ”a r et h el i mi t i n gf or msoft h e s ee x a c tde f i n i t i on s . ) Not ewe l l t h a tt h ea c t u a l f i e l d sr e qu i r eon l yt wooft h eb a s i ch a n s e ns ol u t i on s –t h et wot h a ta r emu t u a l l yt r a n s v e r s e .Some t h i n gh a p p e n e dt ot h el on g i t u d i n a lp a r t a n dt h ed e p e n d e n c eoft h ef i e l dont h es c a l a rp ot e n t i a l .Tos e ej u s twh a t ,l e tu sr e e v a l u a t et h ee l e c t r i cf i e l df r om: ∂A ∂ t E =− ∇Φ− i k ∇ ǫ0 = − i k =− ǫ0 2 k = ǫ0 L L + pLHL + +i ωi k µ 0 1 L + p L−cl L−kZ 0 L L + ( Not et h a tweu s e dω=k ca n d∇H ga u gec on di t i on : + −kµ0c M ml L + =i k L. ) L L l p L=c L M 2 l LL 2 L mLMLr ()+n NLr ()+l L r () L L L L L + p ( ∇HL)−i k c µ0ǫ0 L + + N L+n L L ml L + + N +n L L + ( 1 5 . 3 8 ) L F r omt h i swes e et h a ti ft h e ( 1 5 . 3 9 ) i ss a t i s f i e d, t h es c a l a ra n dl o n gi t u d i n a l v e c t o rp a r t soft h ee l e c t r i cf i e l dc a n c e l e x a c t l y ! Al l t h a ts u r v i v e sa r et h et r a n s v e r s ep a r t s : 2 + + E=−kZ0mLML +nLNL ( 1 5 . 4 0 ) L a sb e f or e .Th eL or e n t zg a u g ec on d i t i oni st h u si n t i ma t e l yc on n e c t e dt ot h ev a n i s h i n g ofas c a l a rorl on g i t u di n a l c on t r i b u t i ont ot h eEf i e l d! Al s on ot et h a tt h ema g n i t u d eofE i sg r e a t e rt h a nt h a tofBb yc , t h ev e l oc i t yofl i g h t . Now,wea r ei n t e r e s t e d( a su s u a l )mos t l yi nob t a i n i n gt h ef i e l dsi nt h ef a rz on e , wh e r et h i sa l r e a dys i mp l ee x p r e s s i ona t t a i n sac l e a na s y mp t ot i cf or m.Us i n gt h ek r→ ∞f or moft h eh a n k e l f u n c t i on , i k r −( ℓ +1 ) e + l i m h( k r )= k r →∞ ℓ i π ( 1 5 . 4 1 ) 2 k r weob t a i nt h el i mi t i n gf or ms( f ork r→ ∞) : i k r −( ℓ +1 ) e + ML ∼ i k r − ℓ e NL∼ k r Y − i ) Yℓℓ=( ℓ+1 2 m 2 k r i π + i kr ℓ+1e m i π k r ℓℓ ℓ m Y 2 ℓ +1 ℓ , ℓ −1 ( 1 5 . 4 2 ) m 2 ℓ +1 Yℓ, ℓ +1 + ( 1 5 . 4 3 ) Th eb r a c k e ti nt h es e c on de qu a t i onc a nb es i mp l i f i e d , u s i n gt h er e s u l t soft h et a b l e I h a n de do u tp r e v i ou s l y . Not et h a t ℓ+1 2 ℓ +1 ℓ m Y ℓ , ℓ −1 ℓ +1 + 2 m m Y ℓ , ℓ +1 m π −i =i ( r ˆ×Yℓℓ)=− e ( r ˆ×Yℓℓ)( 15. 44) 2 s ot h a t( s t i l l i nt h ef a rz on el i mi t ) i k r −( ℓ +1 ) e i kr ℓ+1e i π + NL∼− m 2 ( r ˆ×Yℓℓ)=− ( − i ) k r m r ˆ×Yℓℓ) . k r( ( 1 5 . 4 5 ) L e tu sp a u s et oa dmi r et h i sr e s u l tb e f or emos e y i n gon . Th i si sj u s t 2 i k r e B=− kµ0k r i k r 2 ( − i ) L e k µ0 r =− ℓ + 1 ( − i ) L i k r e E=− kZ0k r i k r ℓ + 1 L e k Z0 r =− m r Y ℓ + 1 ( − i ) ℓ + 1 L ( − i ) mL ˆ× ℓ ℓ m mLr ˆ×Yℓℓ Y mL m +n Y L m +nLYℓℓ m ℓ ℓ ℓ ℓ ( 1 5 . 4 6 ) m −n ˆ×Yℓℓ L r m m mLYℓℓ −nLrˆ×Yℓℓ . ( 1 5 . 4 7 ) I fIh a v ema deas ma l le r r ora tt h i sp oi n t , f or g i v eme .Cor r e c tme , t oo.Th i si sap u r e l y t r a n s v e r s eou t g oi n gs p h e r i c a l wa v ewh os ev e c t orc h a r a c t e ri sf i n a l l yt r a n s l u c e n t , i fn ot t r a n s p a r e n t . Th ep owe rf l u xi nt h eou t g oi n gwa v ei ss t i l l n ott ooe a s yt oe x p r e s s , b u ti ti sad a mn s i g h te a s i e rt h a ni twa sb e f or e .Atl e a s tweh a v et h es a t i s f a c t i onofk n owi n gt h a twe c a ne x p r e s si ta sag e n e r a l r e s u l t . Re c a l l i n g( a su s u a l ) 1 ∗ S= 2Re ( E×H) ( 1 5 . 4 8 ) a n dt h a tt h ep owe rd i s t r i b u t i oni sr e l a t e dt ot h ef l u xoft h ePoy n t i n gv e c t ort h r ou g ha s u r f a c ea tdi s t a n c eri nad i ffe r e n t i a l s ol i da n g l edΩ: dP 1 2 ∗ dΩ =2 Re ˆ·( E×H) ] [ rn ( 1 5 . 4 9 ) weg e t S= 2 k ′ 2 ℓ 2 rZ0Re ∗ × i−ℓ mL ′ L L m ′ L Y m ℓ ℓ m −n ˆ×Yℓℓ Lr ∗ ′ ′ r ˆ Ym∗ +n ′Y m∗ × ℓℓ L ℓℓ ′ ′ ( 1 5 . 5 0 ) ′ ′ ( Not e :Un i t sh e r en e e dt ob er e c h e c k e d,b u tt h e ya p p e a rt ob ec on s i s t e n ta tf i r s t g l a n c e ) . Th i si sa ne x t r e me l yc omp l i c a t e dr e s u l t ,b u ti th a st ob e ,s i n c ei te x p r e s s e st h e mos tge n e r a lp o s s i b l ea n gu l a rd i s t r i b u t i o no fr a di a t i on( i nt h ef a rz on e ) .Th ep owe r d i s t r i b u t i onf ol l owst r i v i a l l y .Wec a n , h owe v e r , e v a l u a t et h et ot a lp owe rr a di a t e d , wh i c h i sav e r yu s e f u l n u mb e r . Th i swi l l b ea ne x e r c i s e . Youwi l l n e e dt h er e s u l t s 2 ′ m∗ m ˆ×Yℓ′ℓ′ dΩˆr·Yℓℓ ×r = ′ m m∗ dΩYℓℓ ·Yℓ′ℓ′ 2 = δℓℓ′δmm′ and 2 ( 1 5 . 5 1 ) ′ m∗ m Y Ωˆ r· ℓℓ ×Yℓ′ℓ′ =0 ( 1 5 . 5 2 ) d t oe v a l u a t et y p i c a l t e r ms . Us i n gt h e s er e l a t i on s , i ti sn ott ood i ffic u l tt os h owt h a t 2 k P= 2Z0 2 L 2 | mL|+| n L| ( 1 5 . 5 3 ) wh i c hi st h es u moft h ep owe re mi t t e df r oma l lt h ei n d i v i d u a lmu l t i p ol e s( t h e r ei sn o i n t e r f e r e n c eb e t we e nmu l t i p ol e s ! ) . L e tu se x a mi n ee . g .t h ee l e c t r i cmu l t i p ol a rmome n tn os e eh owi tc omp a r e st o Lt t h eu s u a l s t a t i cr e s u l t s . St a t i cr e s u l t sa r eob t a i n e di nt h ek→ 0( l on gwa v e l e n g t h )l i mi t . ℓℓ I nt h i sl i mi te . g . j ( k r ) ∼k r a n d : ℓ ℓ+1 n c L≈i ℓ k ℓ 3 () rYℓ, r ˆ ) dr ℓ ( 2 ℓ+1 ) ! ρr m( ( 1 5 . 5 4 ) Th edi p ol et e r mc ome sf r omℓ=1 . F oras i mp l ed i p ol e : n 1 , m √2 ≈i c 3k 3 ρr Y1, mdr √ k c2 ≈i 3 3 4π e<r> √ ≈ k c 6 i 3 6 π e<r> i e ≈−√ 6πω <r ¨> ( 1 5 . 5 5 ) 2 wh e r eweu s e<r ¨> =− ω <r> . I nt e r msoft h i st h ea v e r a g ep owe rr a d i a t e db yas i n g l ee l e c t r ondi p ol ei s : P= 1 2 e πǫ0c3 2 6 wh i c hc omp a r e swe l l wi t ht h eL a r morF or mu l a : 2 | r ¨| 2 ( 1 5 . 5 6 ) 2 e 3 P= 3 4πǫ0c 2 | r ¨ | ( 1 5 . 5 7 ) Th el a t t e ri st h ef or mu l af ort h ei n s t a n t a n e ou sp owe rr a di a t e df r omap oi n tc h a r g ea si ti s a c c e l e r a t e d . E i t h e rf l a v ori st h ed e a t hk n e l l ofc l a s s i c a l me c h a n i c s –i ti sv e r yd i ffic u l tt ob u i l damod e lf oras t a b l ea t omb a s e donc l a s s i c a lt r a j e c t or i e s ofa ne l e c t r ona r ou n dan u c l e u st h a tdoe sn oti n v ol v ea c c e l e r a t i onoft h ee l e c t r oni n qu e s t i on . Wh i l ei ti sn ote a s yt os e e ,t h er e s u l t sa b ov ea r ee s s e n t i a l l yt h os eob t a i n e di n J a c k s on( J 9 . 1 5 5 )e x c e p tt h a t( c omp a r i n ge . g .J 9 . 1 1 9 ,J 9 . 1 2 2 ,a n dJ 9 1 . 1 6 5t or e l a t e d r e s u l t sa b ov e )J a c k s o n ’ sa ℓ ,m)mome n t sdi ffe rf r om t h eHa n s e nmu l t i p ol a r E , M( mome n t sb yf a c t or sofs e v e r a l p owe r sofk .I fon ewor k sh a r de n ou g h , t h ou g h , on ec a n s h owt h a tt h er e s u l t sa r ei de n t i c a l , a n de v e nt h ou g hJ a c k s on ’ sa l g e b r ai smor et h a na b i tEv i li ti swor t h wh i l et od ot h i si fon l yt ov a l i d a t et h er e s u l t sa b ov e( wh e r er e c a l l t h e r eh a sb e e nau n i tc on v e r s i ona n dh e n c et h e ydon e e dv a l i d a t i on ) . An ot h e ru s e f u l e x e r c i s ei st or e c ov e rou rol df r i e n d s , t h ed i p ol ea n dqu a d r u p ol er a d i a t i on t e r msofJ 9f r omt h ee x a c td e f i n i t i onoft h e i rr e s p e c t i v emome n t s .On emu s tma k et h el on g wa v e l e n g t ha p p r ox i ma t i onu n d e rt h ei n t e g r a li nt h ed e f i n i t i onoft h emu l t i p ol emome n t s , i n t e g r a t eb yp a r t sl i b e r a l l y , a n du s et h ec on t i n u i t ye qu a t i on .Th i si squ i t ed i ffic u l t , a si tt u r n s ou t , u n l e s sy ouh a v es e e ni tb e f or e , s ol e tu sl ooka ta ne x a mp l e .L e tu sa p p l yt h eme t h od s weh a v ed e v e l op e da b ov et oob t a i nt h er a d i a t i onp a t t e r nofad i p ol ea n t e n n a ,t h i st i me wi t h o u ta s s u mi n gt h a ti t ’ sl e n g t hi ss ma l lw. r . t .awa v e l e n g t h .J a c k s ons ol v e smor eorl e s s t h es a mep r ob l e mi nh i ss e c t i on9 . 1 2 ,s ot h i swi l lp e r mi tt h ed i r e c tc omp a r i s onoft h e c oe ffic i e n t sa n dc on s t a n t si nt h ef i n a le x p r e s s i on sf ort ot a lr a d i a t e dp owe rort h ea n g u l a r d i s t r i b u t i onofp owe r . 15. 4 AL i n e a rCe n t e r F e dHa l f Wa v eAn t e n n a Su p p os ewea r eg i v e nac e n t e r f e ddi p ol ea n t e n n awi t hl e n g t hλ / 2( h a l f wa v ea n t e n n a ) . Wewi l la s s u mef u r t h e rt h a tt h ea n t e n n ai sa l i g n e dwi t ht h eza x i sa n dc e n t e r e dont h e or i g i n , wi t hac u r r e n tg i v e nb y : 2πz λ I =I o s ( ωt )c os 0c ( 1 5 . 5 8 ) Not et h a ti n“ r e a ll i f e ”i ti sn ote a s yt oa r r a n g ef orag i v e nc u r r e n tb e c a u s et h ec u r r e n t i n s t a n t a n e ou s l yde p e n d sont h e“ r e s i s t a n c e ”wh i c hi saf u n c t i onoft h er a d i a t i onf i e l d i t s e l f .Th ec u r r e n ti t s e l ft h u sc ome sou toft h es ol u t i onofa ne x t r e me l yc omp l i c a t e d b ou n da r yv a l u ep r o b l e m.F ora t omi corn u c l e a rr a d i a t i on ,h owe v e r ,t h e“ c u r r e n t s ”a r e g e n e r a l l yma t r i xe l e me n t sa s s oc i a t e dwi t ht r a n s i t i on sa n dh e n c ea r ek n own . I na n ye v e n t , t h ec u r r e n td e n s i t yc or r e s p on d i n gt ot h i sc u r r e n ti s J=z ˆ I os 0c f orr≤λ / 4a n d 2πr δ( 1−| c osθ| ) λ 2πrsi nθ ( 1 5 . 5 9 ) 2 J=0 ( 1 5 . 6 0 ) f orr>λ / 4 . Wh e nweu s et h eHa n s e nmu l t i p ol e s ,t h e r ei sl i t t l ei n c e n t i v et oc on v e r tt h i si n t oa f or m wh e r ewei n t e g r a t ea g a i n s tt h ec h a r g ede n s i t yi nt h ea n t e n n a .I n s t e a dwec a n e a s i l ya n ddi r e c t l yc a l c u l a t et h emu l t i p ol emome n t s . Th ema g n e t i cmome n ti s 0∗ 3 mL = J·M Ldr 2 π λ / 4 m∗ I 0 π0 =2 0 m∗ 0, φ)+zˆ·Yℓℓ ( c os ( k r ) j ( k r ) zˆ·Yℓℓ ( ℓ π, φ) dφ(15dr.61) ( wh e r eweh a v ed on et h ei n t e g r a l ov e rθ) . Now, 1 m zˆ·Yℓℓ = ℓ ( ℓ+1 )mYL ( Wh y ?Con s i d e r( z ˆ·L ) YL) . . . )a n dy e t ( 1 5 . 6 2 ) 1 / 2 YL( 0 , φ) = δm0 2 ℓ +1 4π ( 1 5 . 6 3 ) 2 ℓ +1 ℓ − 1 )δm0 YL( π, φ) = ( 4π 1 / 2 . ( 1 5 . 6 4 ) Cons e que nt l y , wec a nc onc l ude( mδm0=0)t ha t mL=0 . ( 1 5 . 6 5 ) Al lma g n e t i cmu l t i p ol emome n t soft h i sl i n e a rdi p ol ev a n i s h .Si n c et h ema g n e t i c mu l t i p ol e ss h ou l db ec on n e c t e dt ot h er ot a t i on a lp a r toft h ec u r r e n tde n s i t y( wh i c hi s z e r of orl i n e a rf l ow)t h i ss h ou l dn ots u r p r i s ey ou . Th ee l e c t r i cmome n t sa r e 0∗ 3 n L = J·N Ldr I 0 = 2π − 2 π λ / 4 0 ℓ 0 ℓ+1 c os ( k r ) m∗ 2 ℓ +1 j ℓ −1 ( k r ) m∗ zY j ℓ +1 ( k r ) ˆ· 2 ℓ +1 m∗ zˆ· Y ℓ, ℓ−1 m∗ ( 0, φ)+zˆ·Yℓ, ℓ+1 ( π, φ) d φd r z Y ℓ , ℓ +1 ( 0 , φ)+ ˆ· ℓ , ℓ −1 ( π, φ). ( 15. 66) I fwel ooku pt h ed e f i n i t i onoft h ev . s . h . ’ sont h eh a n d ou tt a b l e , t h ezc omp on e n t sa r e g i v e nb y : ℓ m∗ zˆ· Y ℓ, ℓ−1 m∗ zˆ· Y ℓ, ℓ−1 m∗ zˆ· Y ℓ, ℓ+1 4 π ( 0, φ) =δm0 ℓ − 1 ) ( π, φ) =( m0 −δ ℓ+1 ( 0, φ) =−δm0 m∗ ( − 1 ) − δm0 ( π, φ) =− s ot h ee l e c t r i cmu l t i p ol emome n t sv a n i s hf orm=0 , a n d ( 1 5 . 6 9 ) 4π ( 1 5 . 7 0 ) λ / 4 ℓ ( ℓ+1 ) 4 π( 2 ℓ+1 ) ( 1 5 . 6 8 ) ℓ+1 ℓ, ℓ+1 n ℓ , 0=I 0 δm0 4π 4 π ℓ 1 zˆ· Y ( 1 5 . 6 7 ) ℓ 1 ℓ + 1 1+( − 1 ) c os ( k r )j ( k r )+j ( k r )d r . ℓ − 1 ℓ + 1 0 ( 1 5 . 7 1 ) E x a mi n i n gt h i se qu a t i on , wea l ls e et h a ta l l t h ee v e nℓt e r msv a n i s h ! Howe v e r , t h e od dℓ ,m = 0 t e r ms d on otv a n i s h , s owec a n ’ tqu i ty e t .Weu s et h e f ol l owi n gr e l a t i on s : 2 ℓ +1 j ℓ −1+j ℓ +1= ℓ k rj ( 1 5 . 7 2 ) ( t h ef u n d a me n t a l r e c u r s i onr e l a t i on ) , c os ( k r ) n ( k r )=− 0 ( t r u ef a c t )a n d z )= d zf( z ) g′( 2 z k r ( 1 5 . 7 3 ) ′ fg −f g′ ( 1 5 . 7 4 ) ℓ ℓ [ ℓ ( ℓ ′ ′ +1 ) − ℓ ( ℓ+1 ) ] ℓℓ ′ ℓ ℓ ′ f ora n yt wos p h e r i c a lb e s s e lt y p ef u n c t i on s( av a l u a b l et h i n gt ok n owt h a tf ol l owsf r om i n t e g r a t i onb yp a r t sa n dt h er e c u r s i onr e l a t i on ) . F r omt h e s eweg e t πI 0 kδm0 n ℓ , 0= 2 2 ℓ +1 ℓ + 1 − 1 ) j ( π/ 2 ) . ℓ 4 πℓ ( ℓ+1 ) 1+( ( 1 5 . 7 5 ) Na t u r a l l y ,t h e r ei sawe et a dofa l g e b r ai n v ol v e dh e r et h a tIh a v es k i p p e d .You s h ou l dn ’ t .Now, l e t ’ sf i g u r eou tt h ep owe rr a di a t e df r omt h i ss ou r c e .Re c a l l f r oma b ov e t h a t : 2 k µ0 P= 2 ǫ0 2 k µ0 2 2 | mL|+| n L| L 2 nℓ, 0| = 2 ǫ0 ℓodd | 2 πI 2 ℓ +1 µ0 0 = ǫ0 8 ℓ ( ℓ+1 ) ℓodd 1 2 [ j ( π/ 2) ] ℓ ( 15 . 7 6) 2 Nowt h i sa l s oe qu a l s( r e c a l l ) 2I ,f r om wh i c hwec a nf i n dt h er a d i a t i on 0R r a d r e s i s t a n c eoft h eh a l fwa v ea n t e n n a : Rrad= π µ0 4 ǫ0 2 ℓ +1 ℓodd 2 [ j ( π/ 2) ] ℓ ℓ ( ℓ+1 ) . ( 1 5 . 7 7 ) Wea r eb l e s s e db yt h i sh a v i n gma n i f e s tu n i t sofr e s i s t a n c e , a swer e c og n i z eou r µ0 ol df r i e n dZ0= ≈3 7 7 Ω( t h ei mp e d a n c eoff r e es p a c e )a n dab u n c hof ǫ0 d i me n s i on l e s sn u mb e r s ! I nt e r msoft h i s : Rrad=Z0 π 2 ℓ +1 4 ℓ ( ℓ+1 ) ℓodd 2 [ j ( π/ 2 ) ]. ℓ ( 1 5 . 7 8 ) Wec a nob t a i nag oo de s t i ma t eo ft h ema g n i t u d eb ye v a l u a t i n gt h ef i r s tf e wt e r ms . Not i n gt h a t 2 2 ( 1 5 . 7 9 ) j 1 ( π/ 2 )=π 2 j 3 ( π/ 2 )=π 2 6 0 π2 −6 ( 1 5 . 8 0 ) a n dd oi n gs omea r i t h me t i c , y ous h ou l db ea b l et os h owt h a tRr 3 . 1 Ω. Not et h a t a d=7 t h er a t i ooft h ef i r s t( d i p ol e )t e r mt ot h et h i r d( oc t u p ol e )t e r mi s n3 n1 2 72 6 0 2 2 23 π −6 =1 2 7 60 =18 2 π −6 ≈0. 00244 Th a tme a n st h a tt h i si sl i k e l yt ob eago oda p p r ox i ma t i on( t h ea n s we ri sv e r yn e a r l y u n c h a n g e db yt h ei n c l u s i onoft h ee x t r at e r m) .E v e ni ft h el e n g t hoft h ea n t e n n ai son t h eor d e rofλ , t h emu l t i p ol ee x p a n s i oni sa ne x t r e me l ya c c u r a t ea n dr a p i d l yc on v e r g i n g a p p r ox i ma t i on .Th a ti s , a f t e ra l l , wh yweu s ei ts omu c hi na l l k i n d sofl oc a l i z e ds ou r c e wa v et h e or y . Howe v e r , i fwep l u gi nt h e“ l on gwa v e l e n g t h ”a p p r ox i ma t i onwep r e v i ou s l yob t a i n e d f oras h or tdi p ol ea n t e n n a( wi t hd=λ / 2 )weg e t : 2 ( k d) Rrad= µ0 ǫ0 ≈4 8 Ω 24π ( 1 5 . 8 1 ) wh i c hi soffb yc l os et oaf a c t orof5 0 %.Th i si sn ots u c hag oodr e s u l t .Us i n gt h i sf or mu l a wi t hal on gwa v e l e n g t ha p p r ox i ma t i onf ort h ed i p ol emome n t( on l y )of I 0 2 n 1 , 0≈ k 3π ( 1 5 . 8 2 ) y i e l d sRr 0 Ω, s t i l l offb y1 1 %. a d≈8 15. 5 Co n n e c t i o n t o Ol d ( Ap p r o x i ma t e ) Mu l t i p ol e Mo me n t s Toc on c l u d eou rd i s c u s s i onofmu l t i p ol ef i e l ds ,l e tu sr e l a t et h emu l t i p ol emome n t s d e f i n e da n du s e da b ov e( wh i c ha r ee x a c t )t ot h e“ u s u a l ”s t a t i c ,l on gwa v e l e n g t h mome n t swed e d u c e di nou re a r l i e rs t u d i e s . We l l , 3 N0∗d r n =J · L L a n d 1 NL = 1 r (×∇) ( f ( k r ) YL( r ˆ ) ) ℓ ℓ ( ℓ+1 ) k∇× 1 1 ∂ 2 ℓ ( ℓ+1 )kr ∇ −∇r = f ( k r ) YL( r ˆ ) ) ℓ ∂r+1 ( ( u s i n gt h ev e c t ori d e n t i t y 2 ∂ ∇×L=i r∇ − ∇r∂r+1 t os i mp l i f y ) . Th e n n = L 2 ∗ 3 k r ( J k r ) Y( r ˆ ) dr+ − 1 ) j( · ℓ kℓ ( ℓ+1 ) L 3 ∗ · k r ) )dr r ˆ )∂( ( J ∇)Y ( r j( ( 1 5 . 8 3 ) ( 1 5 . 8 4 ) ( 1 5 . 8 5 ) ( 1 5 . 8 6 ) Now, ( f r omt h ec on t i n u i t ye qu a t i on ) ∇·J=i ωρ ( 1 5 . 8 7 ) s owh e nwe( s i g h )i n t e g r a t et h es e c on dt e r mb yp a r t s , ( b yu s i n g ∇·( a B)=B·∇a+a ∇·B ( 1 5 . 8 8 ) s ot h a t ∗ r ˆ ) ∂ ( ( J ∇)Y ( r j ( k r ) )=∇ · ∂r ℓ L ∗ ∗ J Y( r ˆ )∂ ( r ˆ ) ∂( r j( k r ) ) Y( r j( J k r ) )∇ − L ∂r ℓ · ∂r ℓ L ( 1 5 . 8 9 ) a n dt h edi v e r g e n c et h e or e mont h ef i r s tt e r m, ∇ V ∗ r ˆ )∂( J Y ( r j ∂r ℓ · L · ( k r ) )d V= ∗ r ˆ )∂( J Y( r j( k r ) )dA · L ∂r ℓ nˆ = 0 ∂V→∞ ( 1 5 . 9 0 ) f ors o u r c e swi t hc omp a c ts u p p or tt odot h ei n t e g r a t i on )weg e t n = L 2 ∗ 3 k r ( J k r ) Y( r ˆ ) dr ) j( · ℓ − L − 1 kℓ ( ℓ+1 ) 3 ∗ r ˆ )∂ ( ( i ωρr ( ) )Y ( r j( k r ) ) dr L ∂r ℓ i c = ℓ ( ℓ+1 ) 3 ∗ k r ) )dr ρr ()Y ( r ˆ )∂( r j( L ∂r ℓ ∗ k − 3 k r ) Y( r ˆ ) dr r (J ) j ( · ℓ ℓ ( ℓ+1 ) L ( 1 5 . 9 1 ) Th ee l e c t r i cmu l t i p ol emome n tt h u sc on s i s t soft wot e r ms .Th ef i r s tt e r ma p p e a r s t oa r i s ef r om os c i l l a t i o n soft h ec h a r g ede n s i t yi t s e l f ,a n dmi g h tb ee x p e c t e dt o c or r e s p on dt oou ru s u a l d e f i n i t i on .Th es e c on dt e r mi st h ec on t r i b u t i ont ot h er a d i a t i on f r omt h er a d i a los c i l l a t i onoft h ec u r r e n td e n s i t y .( Not et h a ti ti st h ea x i a lort r a n s v e r s e c u r r e n tde n s i t yos c i l l a t i on st h a tg i v er i s et ot h ema g n e t i cmu l t i p ol e s . ) On l yi ft h ewa v e l e n g t hi smu c hl a r g e rt h a nt h es ou r c ei st h es e c on dt e r mofl e s s e r i k or d e r( b yaf a c t orof c) . I nt h a tc a s ewec a nwr i t e i c n L≈ ℓ ( ℓ+1 ) 3 ∗ k r ) ) dr . ρY ∂( r j( L ∂ r ( 1 5 . 9 2 ) ℓ F i n a l l y , u s i n gt h el on gwa v e l e n g t ha p p r ox i ma t i onont h eb e s s e l f u n c t i on s , n L ≈ ℓ ℓ ∗3 i c ℓ+1k ρrY dr ( 2 ℓ+1 ) ! ! ℓ L i c ≈ ℓ+1 ℓ m ( 2 ℓ+1 ) ! ! ℓ kqℓ, ( 1 5 . 9 3 ) ( 1 5 . 9 4 ) a n dt h ec on n e c t i onwi t ht h es t a t i ce l e c t r i cmu l t i p ol emome n t sqℓ, sc omp l e t e .I na mi s i mi l a rma n n e ron ec a ne s t a b l i s ht h el on gwa v e l e n g t hc on n e c t i onb e t we e nt h emLa n d t h ema g n e t i cmome n t sofe a r l i e rc h a p t e r s .Al s on ot ewe l lt h a tt h er e l a t i on s h i pi sn o t e q u a l i t y .Th e“ a p p r ox i ma t e ”mu l t i p ol e sn e e dt ob er e n or ma l i z e di nor d e rt of i tt og e t h e r p r op e r l ywi t ht h eHa n s e nf u n c t i on st or e c o n s t r u c tt h eE Mf i e l d . 15. 6 An gu l a rMo me n t u mF l u x L e tu sc on s i d e rt h ea n g u l a rmome n t u mr a d i a t e da wa ywi t ht h ee l e c t r oma g n e t i cf i e l d. Th ea n g u l a rmome n t u mf l u xd e n s i t yi sb a s i c a l l yv rc r os s e di n t ot h emome n t u m d e n s i t yS/ cor : ∗ × ( E × H) 1 L= Re r 2 c ( 1 5 . 9 5 ) I n t ot h i se x p r e s s i onwemu s ts u b s t i t u t eou re x p r e s s i on sf orEa n dH: 2 + kZ0 E= − ( 1 5 . 9 6 ) L 2 + H=k + mLML +n NL L + mLNL −n ML . L L ( 1 5 . 9 7 ) I fwet r yt ou s et h ea s y mp t ot i cf a rf i e l dr e s u l t s : i k r E e k Z0 r =− ℓ + 1 L m ( − i ) m mLYℓℓ −nLr ˆ×Yℓℓ ( 1 5 . 9 8 ) i k r e H =− k r ℓ + 1 m ( − i ) L m mLr ˆ×Yℓℓ +nLYℓℓ ( 1 5 . 9 9 ) weget : ∗ E H × = 2 kZ0 r ∗ m 2 L r ˆ ′ L ∗ m × r ˆ )+n ′Y Ym′′′∗( ′ ℓ −ℓ i L ℓℓ L L ∗ m r ˆ ) +m n ′Y ( ℓ ℓ × L ( r ˆ ) ℓℓ m r ˆ × ℓ ℓ m ∗ Y ′′′ ( r ˆ ) × ℓℓ r ˆ ) Ym′′′∗ ( ℓℓ ∗ m −n m L′ r ˆ×Y ℓℓ( r ˆ ) L ∗ ∗ ∗ m ′ × ′′ ′ r ˆ) m m ′Y ( L LL m r ˆ ) n r r ˆ ) m Y( ˆ Y( − L × ℓℓ L ℓ ℓ ′ L = kZ0 2 r m ′ ℓ −ℓ i 2 −n nL′ r ˆ×Y ℓℓ( r ˆ ) L ′ m∗ ×r ˆ×Yℓ ′ℓ′ ( r ˆ ) ′ m∗ ×Yℓ ′ℓ′ ( r ˆ ). ( 1 5 . 1 0 0 ) Wi t hs omee ffor tt h i sc a nb es h ownt ob ear a d i a lr e s u l t–t h ePoy n t i n gv e c t orp oi n t s d i r e c t l ya wa yf r om t h es ou r c ei nt h ef a rf i e l dt ol e a di n gor de r .Con s e qu e n t l y ,t h i s l e a d i n gor d e rb e h a v i orc on t r i b u t e sn o t h i n gt ot h ea n g u l a rmome n t u mf l u x .Wemu s t k e e pa tl e a s tt h el e a d i n gc or r e c t i ont e r mt ot h ea s y mp t ot i cr e s u l t . I ti sc on v e n i e n tt ou s ear a d i a l / t a n g e n t i a lde c omp os i t i onoft h eHa n s e ns ol u t i on s . Th eMLa r ec omp l e t e l yt a n g e n t i a l ( r e c a l l r·ML=0 ) . F ort h eNLweh a v e : 1 d ℓ ( ℓ+1 ) m r f ( k r ) )( i r ˆ×Y ℓℓ( r ˆ) )−r ˆ ℓ NLr ()= k rd r( f l l ( k r ) YL( r ˆ ) e k r ( 15. 101) ∗ Us i n gou rf u l l e x p r e s s i on sf orEa n dH: 2 E= − kZ0 H= k + L 2 + mLML +n NL L + ( 15. 102) + mLNL −n ML L ( 15. 103) L wi t ht h i sf or ms u b s t i t u t e df orNLa n dt h eu s u a l f or mf orMLweg e t : L= ∗ ( E × H) 1Re r × 2 c 4 kZ0 =−2c Re + L ′ L m ( k r ) Yℓℓ ( r ˆ ) r ×mLhℓ + + 1 d( r h( k r ) ) h( k r ) m ℓ ( r ˆ ) )−r ˆ ℓ( ℓ ℓ ℓ+1 ) ℓ + n L ∗ m × k r d r ( i r ˆ×Y ′ − m∗ 1 d ( r h( k r ) ) ′ (i r ˆ Yℓ ′ ′ ( r ˆ ) ) −r ˆ ℓ − × k r d r − L m ∗ ′ ′ Y ′ ′ ′ h( k r ) ℓ ′ ℓ ( ℓ+1 ) ℓ ′ YL( r ˆ ) k r k r r ∗ Y( r ˆ ) L ′ ( 1 5 . 1 0 4 ) ∗ − k r ) ℓℓ ( ℓ( +nL h ˆ ) Al l t h ep u r e l yr a d i a l t e r msi nt h eou t e r mos t unde rt hes u mdonotc ont r i but e t ot h ea n g u l a rmome n t u mf l u xd e n s i t y . Th es u r v i v i n gt e r msa r e : L= − k4Z0 Re r − 2 c ′ L L ∗ +n m L n − n − 1 + m m∗ ′h( k r ) × L L ℓ + h ℓ ( k r ) k r + h k r ) ℓ( ( k r ) ) d r ℓ ( ℓ+1 ) L ′ L L × k r ( r ˆ ) r ˆ − k r ′ ℓ ℓ( ℓ +1 ) × ℓℓ ′ ′ ′ i r ˆ 1d( r hℓ( k r ) ) − ∗ ′ ( i r ˆ Y ℓ ( ℓ+1) h( k r ) ( ˆ n ′ m + ℓ r Lk ′ m∗ L d( r h ′ h( k r ) m ′ ℓ( ℓ +1 )ℓ ( Y ( r ˆ ) r ˆ) Y∗(rˆ) × L ′ ′ k r ℓℓ d r rY × ′ m∗ ′′ ( ˆ ) ℓℓ r ( r ˆ − h( k r ) ℓ k r ′ × × Y m ′ ∗ ℓℓ ( r ˆ ) ) ′ ∗ YL′( r ˆ ) ( 1 5 . 1 0 5 ) Th el owe s tor d e rt e r mi nt h ea s y mp t ot i cf or mf ort h es p h e r i c a lb e s s e lf u n c t i on s ma k e sac on t r i b u t i oni nt h ea b ov ee x p r e s s i on s .Af t e ru n t a n g l i n gt h ec r os sp r odu c t s a n ds u b s t i t u t i n gt h ea s y mp t ot i cf or ms , weg e t : k µ0Re L= mm 2 2r ∗ L ′ L ′ ′ ′ L L m ℓ−ℓ i Y∗′( r ˆ ) Y ℓ( ℓ +1 ) ′ L Y ( r ˆ ) × ℓℓ ′ ℓ−ℓ ℓ ( ℓ+1 ) i Y + n m ℓ−ℓ i Y ( ℓ ( ℓ+1 ) r ˆ )r ˆ Ym′ ′∗ ( r ˆ ) × ℓℓ L L L L ( r ˆ )r ˆ ′ ′ ′ ∗ ′ m′∗ ℓ−ℓ r ˆ ) Y Y( ℓ ( ℓ+1 )i ′ + n n′ L L ( r ˆ) m ∗ nm ′ −L L ∗ ℓ ℓ L ( r ˆ ) ( 1 5 . 1 0 6 ) ′′ ℓℓ Th ea n g u l a rmome n t u ma b ou tag i v e na x i se mi t t e dp e ru n i tt i mei sob t a i n e db y s e l e c t i n gap a r t i c u l a rc omp on e n toft h i sa n di n t e g r a t i n gi t sf l u xt h r ou g had i s t a n t 2 s p h e r i c a l s u r f a c e .F ore x a mp l e , f ort h ez c omp on e n twef i n d( n ot i n gt h a trc a n c e l sa s i ts h ou l d) : dLz k µ0 d t = 2Re ′ z ˆ·. . .s i n ( θ) dθ d φ ( 1 5 . 1 0 7 ) L L wh e r et h eb r a c k e t si n d i c a t et h ee x p r e s s i ona b ov e .Wel ooku pt h ec omp on e n t soft h e v e c t orh a r mon i c st ol e tu sd ot h ed otp r od u c ta n df i n d : m m z ˆ·Yℓℓ = ℓ ( ℓ+1 )Yℓ,m ( 15. 108) ℓ+1 m z ˆ·( r ˆ×Yℓℓ) ℓ m i =− ˆ·Yℓ, ℓ −1+ 2 ℓ +1z = i − ( ℓ+1 ) ( ℓ −m ) Y 2 ℓ ( 2 ℓ ˆ·Yℓℓ+1 2 ℓ +1z 2 1 ) ( 2 ℓ+1 ) − m 2 2 [ ( ℓ+1 )−m ] ℓ ℓ −1 , m− ( 2 ℓ+1 ) ( 2 ℓ+3 ) ( ℓ+1 ) k µ0 d t =2 Comp a r et h i st o: 2 L 2 k P= 2Z0 L =k µ0m 2 m| mL|+| n | L 2 t e r mb yt e r m. F ore x a mp l e : d L mL) z( ℓ + 1 , m ( 15. 109) Doi n gt h ei n t e g r a l i sn ows i mp l e , u s i n gt h eor t h on or ma l i t yoft h es p h e r i c a l h a r mon i c s . On eob t a i n s( a f t e rs t i l l mor ewor k , ofc ou r s e ) : d L z Y 2 | mL|+| n L| 2 mL)=| mL| 2 P( ( 1 5 . 1 1 0 ) ( 1 5 . 1 1 1 ) ( 1 5 . 1 1 2 ) ( wh e r emi nt h ef r a c t i oni st h es p h e r i c a lh a r mon i cm,n ott h emu l t i p ol emL) .I not h e r wor d s ,f orap u r emu l t i p ol et h er a t eofa n g u l a rmome n t u ma b ou ta n yg i v e na x i s t r a n s f e r r e di sm/ ωt i me st h er a t eofe n e r g yt r a n s f e r r e d ,wh e r em i st h ea n g u l a r mome n t u ma l i g n e dwi t ht h a ta x i s .( Not et h a ti fwec h os es omeot h e ra x i swec ou l d, wi t he n ou g hwor k , f i n da na n s we r , b u tt h ea l ge br ai son l ys i mp l ea l on gt h ez a x i sa st h e mu l t i p ol e swe r eor i g i n a l l yde f i n e dwi t ht h e i rmi n de xr e f e r r e dt ot h i sa x i s .Al t e r n a t i v e l y wec ou l dr ot a t ef r a me st oa l i g nwi t ht h en e wdi r e c t i ona n dd ot h ee n t i r ec omp u t a t i on ov e r . ) Th i si squ i t ep r of ou n d .I fwei n s i s t , f ore x a mp l e , t h a te n e r g yb et r a n s f e r r e di nu n i t s ofω, t h e na n g u l a rmome n t u mi sa l s ot r a n s f e r r e di nu n i t sofm! 15. 7 Co n c l u d i n gRe ma r k sAb o u tMu l t i p ol e s Th e r ea r es t i l lma n y , ma n yt h i n g swec ou l ds t u d yc on c e r n i n gmu l t i p ol e sa n dr a d i a t i on . F ore x a mp l e , weh a v en oty e tdon eama g n e t i cl oopa n t e n n a , b u td oi n gon es h ou l dn ow b es t r a i g h t f or wa r d( t oob t a i nama g n e t i cdi p ol er a d i a t i onf i e l dt ol e a di n gor de r ) . Hmmm, s ou n d sl i k eah ome wor kore x a mp r ob l e mt ome . . . St i l l , I h op et h a tt h i sh a sl e f ty ouwi t he n ou g hf u n da me n t a l st h a ty ou : a )Un d e r s t a n db e s s e l f u n c t i on s ; b )Un d e r s t a n ds p h e r i c a l h a r mon i c s ; c )Un d e r s t a n da tl e a s ts ome t h i n ga b ou tv e c t ors p h e r i c a l h a r mon i c s ; d )Kn owwh a ta“ mu l t i p ol a re x p a n s i on ”i s ; e )Kn owh owt oe x p a n dav a r i e t yofi mp or t a n tGr e e n ’ sf u n c t i on sf orv e c t ora n d s c a l a rHe l mh ol t ze qu a t i on s( i n c l u d i n gt h ePoi s s one qu a t i on ) . f )Kn ow h ow t of or mu l a t ea ni n t e g r a le qu a t i ons ol u t i ont ot h e s ed i ffe r e n t i a l e qu a t i on sb a s e dont h eGr e e n ’ sf u n c t i on ,a n da tl e a s tf or ma l l ys ol v ei tb y p a r t i t i on i n gt h ei n t e g r a l i n t odoma i n sofc on v e r g e n c e . g )Kn owh owt od e s c r i b et h ee l e c t r oma g n e t i cf i e l da tav a r i e t yofl e v e l s .Th e s el e v e l s h a db e t t e ri n c l u d et h ee l e me n t a r yde s c r i p t i onoft h eE 1 , E 2 , a n dM1“ s t a t i c ”l e v e l sa s we l la se n ou g hk n owl e d g et ob ea b l et odoi tc or r e c t l yf ore x t e n d e ds ou r c e sor s ou r c e swh e r eh i g h e ror de rmome n t sa r ei mp or t a n t , a tl e a s ti fy ou rl i f eorj oborn e x t p a p e rd e p e n doni t . h )Ca np a s sp r e l i ms . I fy ouf e e lde f i c i e n ti na n yoft h e s ea r e a s ,Ir e c omme n dt h a ty out a k et h et i met o r e v i e wa n dl e a r nt h ema t e r i a l a g a i n , c a r e f u l l y .Th i sh a sb e e nt h emos ti mp or t a n tp a r tof t h ec ou r s ea n di st h eon et h i n gy ous h ou l dn otf a i l t ot a k eou tofh e r ewi t hy ou . I h op ey ouh a v ee n j oy e di t . 15. 8 Ta b l eofPr o p e r t i e so fVe c t orHa r mo n i c s a )Ba s i cDe f i n i t i on s Y m ℓ ℓ Yℓ,m ℓ ( ℓ+1 ) 1 m Y L 1 = m ℓ r ˆ+i r ˆ×LYℓ, m =− ℓ ( 2 ℓ+1 ) − 1 ℓ ℓ +1 ℓ+1 ) r ˆ+i r ˆ×LYℓ, m =− ( ℓ+1 ) ( 2 ℓ+1 ) ( ℓ ℓ −1 Y b )Ei g e n v a l u e s( j , ℓ , ma r ei n t e g r a l ) : 2 m 2 m m JYj ( j +1 ) Yj ℓ =j ℓ m LYj ( ℓ+1 ) Yj ℓ =ℓ ℓ m m ℓ = mYj JzYjℓ c )Pr oj e c t i v eOr t h on or ma l i t y : ′ m∗ m Yjℓ·Yj′ℓ′ d Ω=δjj′δ ℓ ℓ ′δ mm′ d )Comp l e xCon j u g a t i on : m ∗ ℓ + 1 −j Y j − 1 ) ℓ =( m −m ( − 1 )Y j ℓ e )Add i t i onTh e or e m( L CBn ot e sc or r u p t–t h i sn e e d st ob ec h e c k e d ) : m Y j ℓ ∗ ′ m′ ·Yj ′ℓ ′ = m+ 1 ( − 1 ) 4 π( 2 n+1 ) n ′′ j ℓ j ℓ; n ) Y ′W( Cℓℓ′nCjj′n 0 0 0 ′ ( 2 ℓ+1 ) ( 2 ℓ+1 ) ( 2 j+1 ) ( 2 j +1 ) 0 , −m, m ′ n , ( m −m) f )F orFa n yf u n c t i onofron l y : m ∇·( Yℓℓ F) = m 0 ℓ F d F ∇·( Yℓℓ−1F) = 2 ℓ +1 ( r Yℓ,m ℓ − 1 ) r− d ℓ+1 F d F ∇·( Yℓℓ+1F) = 2 ℓ +1 ( r Yℓ,m ℓ +2 ) r− d m × g )Di t t o: ℓ+1 m i ∇×( Y F )= ℓ ℓ F d F 2 ℓ + 1 ( r ℓ +1 ) r +d ℓ+1 F d F m ℓ m Y ℓ ℓ −1 m + F d F 2 ℓ +1 − r ℓr +d i ∇×( Yℓℓ−1F)=− 2 ℓ +1 ( ℓ − 1 ) r− d r Yℓℓ m ℓ F d F m i ∇×( Yℓℓ+1F)= 2 ℓ + 1 ( r ℓ +2 ) r −d Y ℓ ℓ h )Th i sp u t st h eVSHsi n t ov e c t orf or m: 2ℓ ( ℓ + 1) − Yℓ, m−1 ( ℓ + m) ( ℓ −m+1 ) Y ℓ ℓ m √ = ℓ , m ℓ ( ℓ + 1 )Y m ( ℓm) ( ℓ +m+1 ) − 2 ℓ ( ℓ +1 ) Yℓ, m+1 2ℓ ( 2ℓ−1) Yℓ−1, m−1 ( ℓ + m−1 ) ( ℓ + m) = ℓ ℓ1 m − ℓ ( 2 ℓ − 1) Yℓ 1, m − ( ℓ −m) ( ℓ + m) Y ( ℓm 1 ) ( ℓ − − − 2 ℓ ( 2 ℓ −1 ) m) Yℓ 1 , m+1 − ( ℓ −m+ 1 ) ( ℓ −m+ 2 ) 2 ( ℓ + 1) ( 2 ℓ +3 ) m Y = ℓ ℓ +1 m+1 ) ( ℓ + m+1 ) ( ℓ − ( ℓ +1 ) ( 2 ℓ +3 ) ( ℓ +m+2 ) ( ℓ + m+ 1) Yℓ+1,m−1 Yℓ+1,m Yℓ+1, m+1 2 ( ℓ + 1) ( 2 ℓ +3 ) i )Ha n s e nMu l t i p ol ePr op e r t i e s ∇· ML=0∇· NL= 0 ∇·L L ∇× ML ∇× NL = i k f ( k r ) YL( r ˆ ) ℓ ∇× L L m Y ℓ ℓ +1 = − i k NL = i k ML = 0 j )Ha n s e nMu l t i p ol eE x p l i c i tF or ms m ML =f ( k r ) Yℓℓ ℓ ℓ+1 NL = L L = 2 ℓ +1f ℓ −1 ℓ ℓ −1 2 ℓ +1f ℓ m ( k r ) Y ℓ , ℓ −1 m ( k r ) Y ℓ , ℓ −1 − + m k r ) Yℓ, 2 ℓ +1f ℓ +1 ℓ + 1 ( ℓ+1 2 ℓ +1f ℓ + 1 m ML = f ( k r ) Y ℓℓ ℓ NL L L m ( k r ) Y 1 d m ( k r f ) i r ˆ × Y ˆℓ ( ℓ+1 ) f YL ℓ ℓ k r d ( k r ) = ℓ ℓ −r 1 d m i r ˆ×f Yℓℓ)−r ˆ d ℓ r( ( k r )f ℓY L = ℓ ( ℓ+1 )k ℓ , ℓ +1 Ch a p t e r16 Op t i c a l Sc a t t e r i n g 16. 1 Ra d i a t i o nRe a c t i onofaPol a r i z a b l eMe d i u m Us u a l l y , wh e nwec on s i d e rop t i c a l s c a t t e r i n g , wei ma g i n et h a tweh a v eamo n o c h r oma t i c p l a n ewa v ei n c i d e n tu p onap o l a r i z a b l eme d i u me mb e d d e di n( f ort h es a k eofa r g u me n t ) f r e es p a c e . Th et a r ge twei ma g i n ei sa“ p a r t i c l e ”ofs omes h a p ea n dh e n c ei s ma t h e ma t i c a l l ya( s i mp l y )c on n e c t e dd oma i nwi t hc omp a c ts u p p or t . Th ep i c t u r ewemu s t d e s c r i b ei st h u s Th ei n c i de n twa v e( i nt h ea b s e n c eoft h et a r g e t )i st h u sap u r ep l a n ewa v e : E i n c H ˆ i kn =eˆ0E0e 0 · r i n c ˆ / Z0. 0×E i n c =n Th ei n c i de n twa v ei n d u c e sat i mede p e n d e n t p ol a r i z a t i on ( 1 6 . 1 ) ( 1 6 . 2 ) d e n s i t yi n t ot h e 2 1 1 me di u m.I fwei ma g i n e( n otu n r e a s on a b l y )t h a tt h et a r g e ti sap a r t i c l eora t ommu c h s ma l l e rt h a nawa v e l e n g t h ,t h e nwec a nd e s c r i b et h ef i e l dr a d i a t e df r om i t si n d u c e d d i p ol emome n ti nt h ef a rz on ea n dd i p o l ea p p r ox i ma t i on( s e ee . g . 4 . 1 2 2 ) : E s c H i kr e 1 2 ( n ˆ× p)×n ˆ−n ˆ× m/ c } = 4πǫ0 k r { ( 1 6 . 3 ) ˆ×E / Z0. s c = n ( 1 6 . 4 ) k0 k , whi l ee ˆ e ˆa r et hepol a r i z a t i onof 0, n dn ˆ= k I nt h e s ee x p r e s s i on s , n ˆ 0=k0 a t h ei n c i d e n ta n ds c a t t e r e dwa v e s , r e s p e c t i v e l y . Wea r ei n t e r e s t e di nt h er e l a t i v ep owe rd i s t r i b u t i oni nt h es c a t t e r e df i e l d( wh i c h s h ou l db ep r op or t i on a lt ot h ei n c i d e n tf i e l di nawa yt h a tc a nb ema d ei n d e p e n d e n tof i t sma g n i t u d ei nal i n e a rr e s p on s e / s u s c e p t i b i l i t ya p p r ox i ma t i on ) .Th ep owe rr a d i a t e d i nd i r e c t i onn ˆwi t hp ol a r i z a t i one ˆi sn e e de dp e ru n i ti n t e n s i t yi nt h ei n c i d e n twa v e s c wi t hn ˆ , e ˆ . Th i squ a n t i t yi se x p r e s s e da s 0 0 2 dσ( nˆ,eˆ,nˆ0,eˆ0)=r d Ω ∗ 1 e 2 Z0 ˆ· 1 2 Z0 ∗ e E 2 s c 2 E ˆ 0· ( 1 6 . 5 ) i n c [ On eg e t st h i sb yc o n s i d e r i n gt h ep owe rd i s t r i b u t i on : dP 1 dΩ ∗ 2 nˆ· = 2 Rer 1 E×H = 2Z0 E×( n ˆ×E ) 1 2 = 2Z0 E ( 1 6 . 6 ) a su s u a l , wh e r et h el a t t e rr e l a t i ons t e p sh ol df ort r a n s v e r s eE Mf i e l d s7 . 1a n d7 . 2on l y a n dwh e r ewe ’ v ep r oj e c t e dou tas i n g l ep ol a r i z a t i onf r om t h ei n c i d e n ta n ds c a t t e r e d wa v e ss owec a nd i s c u s sp ol a r i z a t i onl a t e r . ] 2 Th i squ a n t i t yh a st h eu n i t sofa r e a( r)a n di sc a l l e dt h ed i ffe r e n t i a l c r os s –s e c t i o n : d σ dΩ d P/ d Ω d A dP0/ dA ∝ dΩ ∼A. = ( 1 6 . 7 ) I nqu a n t u mt h e or yas c a t t e r i n gc r os s –s e c t i onon ewou l ds u b s t i t u t e“ i n t e n s i t y ” ( n u mb e rofp a r t i c l e s / s e c on d )f or“ p owe r ”i nt h i sde f i n i t i onb u ti ts t i l lh ol d s .Si n c et h e u n i t sofa n g l e s , s ol i dorn ot , a r ed i me n s i on l e s s ,ac r os s –s e c t i ona l wa y sh a st h eu n i t s ofa r e a .I fon ei n t e g r a t e st h ec r os s –s e c t i ona r ou n dt h e4 πs ol i da n g l e ,t h er e s u l t i n g a r e ai st h e“ e ffe c t i v e ”c r os s –s e c t i on a la r e aoft h es c a t t e r e r , t h a ti s , t h ei n t e g r a t e da r e ofi t se ffe c t i v e“ s h a d ow” .Th i si st h eb a s i soft h eop t i c a lt h e or e m, wh i c hIwi l lme n t i on b u twewi l l n ots t u d y( d e r i v e )f orl a c koft i me . Th ep oi n ti nd e f i n i n gi ti st h a ti ti sg e n e r a l l yap r op e r t yoft h es c a t t e r i n gt a r g e tt h a t l i n e a r l yd e t e r mi n e st h es c a t t e r e dp owe r : dP dσ 0 dΩ = dΩ ×I ( 1 6 . 8 ) wh e r et h el a s tqu a n t i t yi st h ei n t e n s i t yoft h ei n c i d e n tp l a n ewa v eb e a m.Th ec r os s s e c t i oni si n d e p e n d e n t( wi t h i nr e a s on )oft h ei n c i de n ti n t e n s i t ya n dc a nb ec a l c u l a t e d orme a s u r e d“ on c ea n df ora l l ”a n dt h e nu s e dt op r e d i c tt h ep owe rd i s t r i b u t i onf ora g i v e nb e a mi n t e n s i t y . Wen e e dt ou s et h ea p p a r a t u sofc h a p t e r7t oh a n d l et h ev e c t orp ol a r i z a t i on c or r e c t l y .Th a ti s ,t e c h n i c a l l ywen e e dt ou s et h eSt o k e sp a r a me t e r sors ome t h i n g s i mi l a rt oh e l pu sp r oj e c tou tofEap a r t i c u l a rp ol a r i z a t i onc omp on e n t .Th e n( a sc a n e a s i l yb es h ownb yme d i t a t i n gon : ∗ ˆ· e 1 e E sc ∗ kr 2 i e np nnm ˆ· { ( ˆ × =4πǫ0k r { ) × ˆ − ˆ × / c } } ( 1 6 . 9 ) f orat r a n s v e r s ef i e l d) : 1 ∗ e 2 E dσ 2 2Z0 ˆ· s c dΩ =r 1 e∗ E 2 Z0 4 k ∗ ∗ ep = 2 4 πǫ0E0) 2 ( ne m | ˆ· + ( ˆ × ˆ ) × 2 1 6 . 1 0 ) / c |.( ˆ 0· i n c Tog e tt h i sr e s u l t , weh a dt oe v a l u a t e( u s i n gv e c t ori de n t i t i e s ) ∗ e a n d ∗ e np ∗ ep n ˆ· ( ˆ × ) × ˆ = ˆ· nm m ˆ· ( ˆ × ( 1 6 . 1 1 ) ∗ ne / c )=− · ( ˆ × ˆ ) . ( 1 6 . 1 2 ) F r omt h i swei mme d i a t e l ys e eon ei mp or t a n tr e s u l t : dσ 4 1 4 dΩ ∝k ∝ λ . ( 1 6 . 1 3 ) Th i si sc a l l e dRa y l e i gh ’ sL a w;t h es c a t t e r i n gc r os s s e c t i on( a n dh e n c ep r op or t i onof t h ep owe rs c a t t e r e df r om ag i v e ni n c i d e n tb e a m)b yap ol a r i z a b l eme d i u mi s p r op or t i on a lt ot h ei n v e r s ef ou r t hp owe ro ft h ewa v e l e n gt h .Or ,i fy oup r e f e r ,s h or t wa v e l e n g t h s( s t i l ll on gwi t hr e s p e c tt ot h es i z eoft h es c a t t e r e ra n don l yi ft h edi p ol e t e r mi nt h es c a t t e r i n gd omi n a t e s )a r es c a t t e r e dmor es t r on g l yt h a nl on gwa v e l e n g t h s . Th i si st h eor i g i n a l “ b l u es k y ”t h e or ya n dp r ob a b l yt h eor i g i noft h ep h r a s e ! Tog of u r t h e ri nou ru n d e r s t a n d i n g ,a n dt og a i ns omeu s e f u lp r a c t i c ea g a i n s tt h e d a yy ouh a v et ou s et h i st h e or yort e a c hi tt os ome on ewh omi g h tu s ei t ,wemu s t c on s i de rs omes p e c i f i cc a s e s . 16. 2 Sc a t t e r i n gf r o maSma l l Di e l e c t r i cSp h e r e Th i si sar e l a t i v e l ys i mp l e , a n dh e n c ev e r ys t a n d a r dp r ob l e m. 1 Now,weh a v en od e s i r et o“ r e i n v e n tt h es p h e r e ”b u ti ti si mp or t a n tt h a ty ou u n de r s t a n dwh e r eou rr e s u l t sc omef r om.F i r s tofa l l ,l e tu si n t r od u c ed i me n s i on l e s s , s c a l e dv e r s i on soft h er e l a t i v ep e r me a b i l i t ya n dp e r mi t t i v i t y( as t e pt h a tJ a c k s ona p p a r e n t l y p e r f or msi nJ 1 0b u td oe sn otd oc u me n tore x p l a i n ) : ǫr =ǫ( ω) / ǫ0 ( 1 6 . 1 4 ) ω) / µ 0≈1 µr =µ( ( 1 6 . 1 5 ) wh e r ewea s s u met h a twea r en ota tar e s on a n c es ot h a tt h es p h e r e sh a v en or ma l d i s p e r s i on a n dt h a tt h e s en u mb e r sa r eb a s i c a l l yr e a l .Th el a t t e ri sag ood a p p r ox i ma t i onf orn on ma g n e t i c ,n on c on d u c t i n gs c a t t e r e r se . g .ox y g e norn i t r og e n mol e c u l e s . I fy our e f e rb a c kt oJ 4 . 4 , e qu a t i onJ 4 . 5 6a n dt h es u r r ou n di n gt e x t , y ouwi l l s e et h a t t h ei n d u c e ddi p ol emome n ti nad i e l e c t r i cs p h e r ei nt e r msoft h er e l a t i v ep e r mi t t i v i t yi s : p=4 πǫ0 ǫr−1 ǫr+2 3 aE i n c ( 1 6 . 1 6 ) Tor e c a p i t u l a t et h ed e r i v a t i on( u s e f u l s i n c et h i si sac ommo nqu e s t i ononqu a l i f i e r sa n d t h el i k e )wen ot et h a tt h es p h e r eh a sa z i mu t h a l s y mme t r ya r ou n d 1 Hy u k , h y u k , h y u k . . . t h ed i r e c t i onofE , s owec a ne x p r e s st h es c a l a rp ot e n t i a l i n s i d ea n dou t s i d et h es p h e r e a s ℓ AℓrPℓ( c osθ) φi n = ( 1 6 . 1 7 ) ℓ ℓ Bℓr+Cℓ φout = ℓ 1 Pℓ( c osθ ) . ℓ +1 r ( 1 6 . 1 8 ) Wen e e dt oe v a l u a t et h i s .Ati n f i n i t ywek n owt h a tt h ef i e l ds h ou l db e( t ol owe s t or d e r )u n d i s t u r b e d , s ot h ep ot e n t i a l mu s ta s y mp t ot i c a l l yg oov e rt o l i mφout=− E z=− E rc osθ=− E r P1( c osθ) r 0 0 0 ( 1 6 . 1 9 ) →∞ s owec on c l u d et h a tB1=− E n da l lot h e rBℓ>1=0 .Top r oc e e df u r t h e r , wemu s tu s e 0a t h ema t c h i n gc on d i t i o n soft h et a n g e n t i a la n dn or ma lf i e l d sa tt h es u r f a c eoft h e s p h e r e : n 1 ∂φi −a ∂θ ( t a n g e n t i a l c omp on e n t )a n d =− r = a 1 ∂φout a ∂φi n ∂ θ ( 1 6 . 2 0 ) r = a ∂φout 0 = a=−ǫ − ǫ ∂r r ∂r ( 1 6 . 2 1 ) r = a ( n or ma l Don t oE ) . Si n c et h i si st h es u r f a c eofas p h e r e( ! )wec a np r oj e c tou te a c hs p h e r i c a l c omp on e n ti fwewi s ha n dc a u s et h e s ee qu a t i on st ob es a t i s f i e dt e r mb yt e r m.F r om t h ef i r s t( t a n g e n t i a l )e qu a t i onwej u s tma t c hφi t s e l f : ℓ ℓ 1( Aℓa)= 1 Bℓa +Cℓ a a 1 ( 1 6 . 2 2 ) ℓ +1 a or( u s i n gou rk n owl e dg eofBℓ) A1 Aℓ C1 3 − E + 0 a = Cℓ = 2 ℓ +1 a ℓ=1 ( 1 6 . 2 3 ) e l s e ( 1 6 . 2 4 ) ℓ=1 ( 1 6 . 2 5 ) e l s e . ( 1 6 . 2 6 ) F r omt h es e c on d( n or ma l )e qu a t i onwege t C1 ǫrA1 E − 2 a 0 3 = − ( ℓ+1 ) Cℓ ǫr Aℓ = − 2ℓ +1 a Th es e c on de qu a t i onofe a c hp a i ra r ei n c omp a t i b l ea n dh a v eon l yt h et r i v i a l Aℓ=Cℓ=0 ℓ=1 . ( 1 6 . 2 7 ) On l yt h eℓ=1t e r ms u r v i v e s . Wi t hal i t t l ewor kon ec a ns h owt h a t 3E0 ( 1 6 . 2 8 ) A1 =− 2+ǫr C1 = ǫr−1 aE0 3 ( 1 6 . 2 9 ) ǫr+2 s ot h a t 3 φi n φ ( 1 6 . 3 0 ) =− ǫr+2 E0rcosθ 3 = Erc osθ+ ǫr−1 E a cosθ. ( 1 6 . 3 1 ) 0 2 r ǫr+2 − 0 Wh e nwei d e n t i f yt h es e c on dt e r moft h ee x t e r n a lf i e l dwi t ht h ed i p ol ep ot e n t i a l a n dc omp a r ewi t ht h ee x p a n s i onoft h ed i p ol ep ot e n t i a l ou t φr ()= 1p r· ( 1 6 . 3 2 ) 3 4πǫ0 r wec on c l u det h a tt h ei n du c e dd i p ol emome n ti s : p=4 πǫ0 ǫr−1 ǫr+2 3 aE0z ˆ . ( 1 6 . 3 3 ) a sg i v e na b ov e . Th e r ei sn oma g n e t i cd i p ol emome n t ,b e c a u s eµr=1a n dt h e r e f or et h es p h e r e b e h a v e sl i k ea“ d i p ol ea n t e n n a ” .Th u s m =0a n dt h e r ei sn oma g n e t i cs c a t t e r i n gof r a di a t i onf r om t h i ss y s t e m.Th i son ee qu a t i on ,t h e r e f or e ,( t og e t h e rwi t hou ror i g i n a l d e f i n i t i on soft h ef i e l d s )i ss u ffic i e n tt od e t e r mi n et h edi ffe r e n t i a l c r os s –s e c t i on : 4 6 ǫ−1 dσ =k r a dΩ ǫr+2 ∗ 2e ˆ eˆ 2 ( 1 6 . 3 4 ) | · 0| wh e r er e me mb e rt h a tǫr ( ω)( f ordi s p e r s i on )a n dh op e f u l l ye v e r y b od yn ot e st h e d i ffe r e n c eb e t we e nd i e l e c t r i cǫa n dp ol a r i z a t i one ˆ( s i g h–wen e e dmor es y mb ol s ) . Th i se qu a t i onc a nb eu s e dt of i n dt h ee x p l i c i tdi ffe r e n t i a l c r os s –s e c t i on sg i v e n( n ˆ , n ˆ , 0 e ˆ , e ˆ ) , a sde s i r e d . 0 Howe v e r ,t h el i g h ti n c i de n tont h es p h e r ewi l lg e n e r a l l yb eu n p ol a r i z e d .Th e nt h e qu e s t i onn a t u r a l l ya r i s e sofwh e t h e rt h ev a r i ou si n d e p e n d e n tp ol a r i z a t i on soft h e i n c i d e n tl i g h tb e a mwi l lb es c a t t e r e di de n t i c a l l y .Or ,t op u ti ta n ot h e rwa y ,wh a ti st h e a n g u l a rd i s t r i b u t i onf u n c t i onofr a d i a t i onwi t hade f i n i t ep ol a r i z a t i on ?Toa n s we rt h i s , wen e e dt oc on s i d e ras u i t a b l ed e c omp os i t i onoft h ep os s i b l ep ol a r i z a t i ond i r e c t i on s . Th i sd e c omp os i t i oni sa p p a r e n tf r om c on s i d e r i n gt h ef ol l owi n gp i c t u r eoft h e g e n e r a l g e ome t r y : ( 1 ) ( 2 ) L e tn ˆ , n ˆ e f i n et h ep l a n eofs c a t t e r i n g .Weh a v et of i xe ˆ a n de ˆ r e l a t i v et ot h i s 0d ( 1 ) ( 2 ) s c a t t e r i n gp l a n ea n da v e r a g eov e rt h ep ol a r i z a t i on si nt h ei n c i d e n tl i g h t ,e ˆ 0a n de ˆ 0 ( a l s of i x e dr e l a t i v et ot h i sp l a n e ) . Wec a na l wa y sc h oos et h e ( 2 ) ( 2 ) d i r e c t i on sofp ol a r i z a t i ons u c ht h a te ˆ =e ˆ 0i sp e r p e n d i c u l a rt ot h es c a t t e r i n g ( 1 ) ( 1 ) p l a n ea n de ˆ =e ˆ 0a r ei ni t ,a n dp e r p e n d i c u l a rt ot h ed i r e c t i on sn ˆa n dn ˆ 0 r e s p e c t i v e l y . Th ed otp r odu c t sa r et h u s ( 1 ) ( 1 ) ∗ e e ˆ ·ˆ 0 ( 2) ∗ e ( 2) e ˆ ·ˆ 0 nn · ˆ =ˆ osθ 0 =c ( 1 6 . 3 5 ) =1 . ( 1 6 . 3 6 ) Wen e e dt h ea v e r a g eo ft h es qu a r e soft h e s equ a n t i t i e s .Th i si se s s e n t i a l l y 2 2 a v e r a g i n gs i n φa n dc os φov e rφ∈( 0 ,2 π) .1 Al t e r n a t i v e l y ,wec a nme d i t a t eu p on s y mme t r ya n dc on c l u d et h a tt h ea v e r a g ei sj u s t 2.Th u s( f ort h ep ol a r i z a t i oni nt h e p l a n e()a n dp e r p e n d i c u l a rt ot h ep l a n e( ⊥)ofs c a t t e r i n g , r e s p e c t i v e l y )weh a v e : dσ dΩ d σ⊥ dΩ 46 =ka 2 ǫr−1 2 c osθ ǫr+2 2 46 =ka ǫr−1 21 ǫr+2 2 ( 1 6 . 3 7 ) ( 1 6 . 3 8 ) Wes e et h a tl i g h tp ol a r i z e dp e r p e n d i c u l a rt ot h ep l a n eofs c a t t e r i n gh a sn oθ de p e n d e n c e , wh i l el i g h tp ol a r i z e di nt h a tp l a n ei sn ots c a t t e r e dp a r a l l e l t ot h ed i r e c t i on ofp r op a g a t i ona ta l l( a l on gθ=0orπ) .Wewi l li n v e r tt h i ss t a t e me n ti namome n ts o t h a ti tma k e smor es e n s e . Se et h ed i a g r a mb e l ow. Un f or t u n a t e l y ,e v e r y t h i n gt h u sf a ri se x p r e s s e dwi t hr e s p e c tt ot h ep l a n eof s c a t t e r i n g ,wh i c hv a r i e swi t ht h ed i r e c t i onoft h es c a t t e r e dl i g h t .I fwede f i n et h e p o l a r i z a t i o nΠ( θ)oft h es c a t t e r e dr a d i a t i ont ob e Π( θ )= d σ⊥ d Ω − d σ d Ω σ d Ω + dΩ dσ ⊥ 2 i n θ =s 2 1+cos θ ( 1 6 . 3 9 ) t h e nweob t a i naqu a n t i t yt h a ti si na c c or dwi t hou ri n t u i t i on .Π( θ )i sma x i mu ma tθ= π/ 2 .Th er a d i a t i ons c a t t e r e dt h r ou g ha na n g l eof9 0de g r e e si sc omp l e t e l yp ol a r i z e di n ap l a n ep e r p e n d i c u l a rt ot h ep l a n eofs c a t t e r i n g . F i n a l l y , wec a na d dt h et wop i e c e soft h ed i ffe r e n t i a l c r os s –s e c t i ont og e t h e r : dσ 4 6 =ka dΩ ǫ−1 ǫ+2 1 2 2 ( 1+c os θ ) 2 ( 1 6 . 4 0 ) wh i c hi ss t r on g l ya n ds y mme t r i c a l l yp e a k e df or wa r da n db a c k wa r d .F i n a l l y , t h i si se a s y t oi n t e g r a t et oob t a i nt h et ot a l c r os s –s e c t i on : σ= 8π 4 6 ǫr−1 ka ǫr+2 3 2 . ( 1 6 . 4 1 ) Atl a s t ,wec a np u ti ta l lt og e t h e r .Mol e c u l e si nt h ea t mos p h e r eb e h a v e ,f a rf r om r e s on a n c e , l i k ei t t y –b i t t yd i e l e c t r i cs p h e r e st oar e ma r k a b l ea p p r ox i ma t i on .Si n c eb l u e l i g h ti ss c a t t e r e dmor es t r on g l yt h a nr e d ,l i g h ts e e na wa yf r om i t sd i r e c t i onof i n c i d e n c e( t h es k ya n dn ott h es u n )i ss h i f t e di nc ol orf r omwh i t et ob l u e .Wh e nMr .Su n i se x a mi n e dd i r e c t l yt h r ou g hat h i c kl a y e rofa t mos p h e r e( a ts u n s e t )t h eb l u ei sa l l s c a t t e r e dou ta n dt h er e ma i n i n gl i g h tl ook sr e d .F i n a l l y , l i g h tf r omd i r e c t l yov e r h e a da t s u n u pors u n downi sp ol a r i z e di nan or t h –s ou t hdi r e c t i on ;a tn oont h el i g h tf r omt h e h or i z on i sp ol a r i z e dp a r a l l e lt ot h eh or i z on ( a n dh e n c ei sf i l t e r e db yv e r t i c a l t r a n s mi s s i ona x i sp ol a r i z e ds u n g l a s s e s .Yous h ou l dv e r i f yt h i sa ty ou rn e x top p or t u n i t y ou t d oor swi t hap a i ro fp ol a r i z e ds u n g l a s s e s ,a st h i swh ol ed i s c u s s i oni st a u g h ti n e l e me n t a r yt e r msi ns e c on ds e me s t e ri n t r od u c t or yp h y s i c sc ou r s e s . 2 Don ’ ts a yI n e v e rt a u g h ty oua n y t h i n g. Th el a s tr e ma r k sIwou l dma k ec on c e r nt h et ot a lc r os s –s e c t i on .Not et h a ti fwe 2 f a c t orou ta4 πa weg e tt h e“ a r e a ”oft h es p h e r et i me sap u r e( di me n s i on l e s s )n u mb e r 4 ( k a )a s s o c i a t e dwi t ht h er e l a t i v es i z eoft h es p h e r er a d i u sa n dt h ewa v e l e n g t ha n da s e c on dp u r en u mb e ri n v ol v i n gon l yt h edi e l e c t r i cp r op e r t i e soft h eme d i u m: 2 4 σ=( 4 πa) ( k a ) 2 ǫr−1 2 . ( 1 6 . 4 2 ) 3 ǫr+2 Th i se x p r e s s i oni s n ’ ta n ymor eu s e f u lt h a nt h eon ea b ov e , b u ti td oe sma k et h er ol eof t h ed i ffe r e n tt e r mst h a tc on t r i b u t et ot h et ot a l s c a t t e r i n gc r os s s e c t i onmor ec l e a r . 2 E v e ni fi t ’ st r u e. . . 16. 3 Sc a t t e r i n gf r o maSma l l Co n d u c t i n gSp h e r e Pe r f e c tc on d u c t or sa r en otj u s td i e l e c t r i c swh e r et h ee l e c t r i cf i e l di sc omp l e t e l yz e r o i n s i de .Th ee l e c t r i cf i e l di se x a c t l yc a n c e l l e dont h ei n t e r i orb yt h ei n d u c e ds u r f a c e c h a r g e .Asweh a v es e e n ,t h i sc a n c e l l a t i onoc c u r sc l os et ot h es u r f a c e( wi t h i naf e w t i me st h es k i nd e p t h ) .Howe v e r ,t h ei n d u c e dc u r r e n t sa l s ot e n dt oe x p e lt h et i me d e p e n d e n tma g n e t i cf i e l d .Wet h e r e f or eh a v et womod i f i c a t i onofou rr e s u l t sf r omt h e p r e v i ou ss e c t i on .Th ee l e c t r i cp ol a r i z a t i onwi l l h a v eadi ffe r e n tf or m, a n dt h e r ewi l l b ea c on t r i b u t i onf r omt h ei n d u c e dma gn e t i cmome n toft h es p h e r ea swe l l . Re c a l l ( f r omJ 2 . 5 )t h a tt h ei n d u c e dd i p ol emome n tonac on d u c t i n gs p h e r e i s 3 p=4 πǫ0aE . i n c ( 1 6 . 4 3 ) Th i si si n d e e dt h eg e n e r a l i z a t i onoft h er e s u l tf orpl a s tt i me , a sy ous h ou l db ea b l et o de r i v ei naf e wmi n u t e sofwor k .E i t h e rr e v i e wt h a ts e c t i onors ol v et h eb ou n da r yv a l u e p r ob l e mwh e r eE sd i s c on t i n u ou sa tt h es u r f a c ea n dE nt h es u r f a c et oob t a i n : ⊥i | |=0o 3 a φ=− E0 r−r 2c osθ ( 1 6 . 4 4 ) f r omwh i c hwec a ne a s i l ye x t r a c tt h i s p. Bu t , t h ema g n e t i cf i e l di sa l s ov a r y i n g , a n di ti n d u c e sa nEMFt h a tr u n si nl oop sa r ou n d t h ema g n e t i cf i e l dl i n e sa n dop p os e st h ec h a n g ei nma g n e t i cf l u x .As s u mi n gt h a tn of i e l d l i n e swe r et r a p p e di nt h es p h e r ei n i t i a l l y , t h ei n du c e d c u r r e n t sa c tt oc a n c e lc omp on e n toft h ema g n e t i cf i e l dn or ma lt ot h es u r f a c e .Th e s p h e r et h u sb e h a v e sl i k eama g n e t i c a l l yp e r me a b l es p h e r e( s e ee . g .s e c t i onJ 5 . 1 0a n d J 5 . 1 1 , e qu a t i on sJ 5 . 1 0 6 , J 5 . 1 0 7 , J 5 . 1 1 5 ) : M= m 3 4πa/ 3 =3 µ− µ0 H ( 1 6 . 4 5 ) i n c µ+ 2 µ0 wi t hµ / µ0=0s ot h a t : r=µ 3 m =− 2 πaHi . n c ( 1 6 . 4 6 ) Th ede r i v a t i oni sa g a i nv e r ys i mi l a rt ot h ed e r i v a t i onwep e r f or me dl a s tt i me ,wi t h s u i t a b l yc h os e nb ou n da r yc on di t i on sonBa n dH. I fwet h e nr e p e a tt h er e a s on i n ga n da l g e b r af ort h i sc a s eoft h ec on d u c t i n gs p h e r e ( s u b s t i t u t i n gt h i s pa n dm i n t ot h ee x p r e s s i on we d e r i v e df ort h e di ffe r e n t i a l c r os s –s e c t i on ) , weg e t : dσ dΩ 1 ne∗ 4 6e ∗e ne ˆ ˆ × ˆ) · ( ˆ 0 − 2( 0 =ka ˆ· 2 × ˆ ). 0 ( 1 6 . 4 7 ) Af t e rmu c ht e d i ou sb u ts t r a i g h t f or wa r dwor k , wec a ns h ow( orr a t h e ry ouc a ns h ow f orh ome wor k )t h a t : 46 d σ dΩ ka 1 = 2 c osθ− 2 d σ⊥ d Ω ka 1 = 2 1− 2 cosθ 46 2 2 ( 1 6 . 4 8 ) ( 1 6 . 4 9 ) s ot h a tt h et ot a l di ffe r e n t i a l c r os ss e c t i oni s : dσ 46 dΩ =ka a n dt h ep ol a r i z a t i oni s : 5 2 1+cos θ)−cosθ) 8( Π( θ )= 2 3s i nθ ( 1 6 . 5 0 ) ( 1 6 . 5 1 ) 2 5 ( 1+c os θ )−8c osθ F i n a l l y , i n t e g r a t i n gt h edi ffe r e n t i a l c r os ss e c t i ony i e l d st h et ot a l c r os s s e c t i on : 46 10πka 2. 5 2 4 = ( 4 π a ) ( k a ) e l e c t r i c σ= 3 3∼σdi ( 1 6 . 5 2 ) f orǫr> >1c u r i ou s l ye n o u g h . Wh a td ot h e s ee qu a t i on st e l lu s ?Th ec r os s –s e c t i oni ss t r on g l yp e a k e db a c k wa r d s . Wa v e sa r er e f l e c t e db a c k wa r d smor et h a nf or wa r d s( t h es p h e r ea c t u a l l yc a s t sa“ s h a dow” . Th es c a t t e r e dr a di a t i oni sp ol a r i z e dqu a l i t a t i v e l ya l i k et h er a d i a t i ons c a t t e r e df r om t h e d i e l e c t r i cs p h e r e , b u twi t has ome wh a td i ffe r e n t F i g u r e1 6 . 1 : Di ffe r e n t i a l c r os s –s e c t i ona n dp ol a r i z a t i onofas ma l l c on du c t i n gs p h e r e . a n g u l a rd i s t r i b u t i on .I ti sc omp l e t e l yp ol a r i z e dp e r p e n di c u l a rt ot h es c a t t e r i n gp l a n e ◦ ◦ wh e ns c a t t e r e dt h r ou g ha na n g l eof6 0, n ot9 0. 4 Wes e et h a td i p ol es c a t t e r i n gwi l la l wa y sh a v eac h a r a c t e r i s t i ck d e p e n de n c e .By k n owy ous h ou l dr e a d i l yu n d e r s t a n dmewh e nIs a yt h a tt h i si st h er e s u l tofp e r f or mi n g amu l t i p ol a re x p a n s i onoft h er e a c t i onf i e l d( e s s e n t i a l l ya ne x p a n s i oni np owe r sofk d wh e r edi st h ec h a r a c t e r i s t i cma x i mu me x t e n toft h es y s t e m)a n dk e e p i n gt h ef i r s t ( d i p ol e )t e r m. I fon ewi s h e st oc on s i d e rs c a t t e r i n gf r om ob j e c t swh e r ek d∼1org r e a t e r ,on e s i mp l yh a st oc on s i d e rh i g h e ror d e rmu l t i p ol e s( a n don emu s tc on s i d e rt h ep r op e r mu l t i p ol e si n s t e a dofs i mp l ee x p a n s i on si np owe r sofk d ) .I fk d> >1( wh i c hi st h ec a s e f orl i g h ts c a t t e r i n gf r om ma c r os c op i cob j e c t s ,r a d a rs c a t t e r i n gf r om a i r p l a n e sa n d i n c omi n gn u c l e a rmi s s i l e s ,e t c )t h e nawh ol ed i ffe r e n ta p p a r a t u smu s tb eb r ou g h tt o b e a r .Ic ou l ds p e n das e me s t e r( oral e a s tac ou p l eofwe e k s )j u s tl e c t u r i n gont h e s c a t t e r i n gofe l e c t r oma g n e t i cwa v e sf r oms p h e r e s , l e ta l on eot h e rs h a p e s . Howe v e r ,n ou s e f u lp u r p os ewou l db es os e r v e d ,s oIwon ’ t .I fy oue v e rn e e dt o f i g u r ei tou t , y ouh a v et h et ool sa n dc a nf i n da n du n de r s t a n dt h en e c e s s a r yr e f e r e n c e s . 16. 4 Ma n ySc a t t e r e r s I ti s ,h owe v e r ,wor t h wh i l et os p e n damome n tc on s i de r i n gac ol l e c t i on sofi d e n t i c a l s c a t t e r e r sa tf i x e ds p a t i a lp os i t i on s .E a c hs c a t t e r e rt h e na c t si d e n t i c a l l y ,b u ti s s c a t t e r i n ga ne l e c t r oma g n e t i cf i e l dwi t hi t sown( s p a t i a l l yd e p e n d e n t )p h a s ea tag i v e n mome n toft i me .Th es c a t t e r e df i e l d st h e np r op a g a t ef r e e l y ,r e c omb i n e ,a n df or ma t ot a lE Mf i e l dt h a ti sme a s u r e db yt h ed e t e c t or .I nor de rt oe v a l u a t et h et ot a l d i ffe r e n t i a lc r os s –s e c t i o nwemu s ts u mt h ef i e l da mp l i t u d e st i me st h ea p p r op r i a t e p h a s e s , p r oj e c tou tt h ed e s i r e dp ol a r i z a t i onmome n t s , a n dt h e ns qu a r e . 3 Amome n tofqu i e tr e f l e c t i on wi l l c on v i n c ey out h a ti ng e n e r a l : d σ d Ω 2 4 = ∗ k e ˆ p +( nˆ 2 ( 4 πǫ0E ) 0 j ·j ∗ qx eˆ)m / i·j ce ·j × ( 1 6 . 5 3 ) wher e q=k . 0−k ( 1 6 . 5 4 ) a c c omoda t e st h er e l a t i v ep h a s ed i ffe r e n c eb e t we e nt h ef i e l de mi t t e db yt h es c a t t e r e r sa t di ffe r e n tl o c a t i on s . Th eg e ome t r yoft h i ss i t u a t i oni sp i c t u r e db e l ow. I na l ldi r e c t i on sb u tt h ef or wa r dd i r e c t i on ,t h i sd e p e n d sont h ed i s t r i b u t i onof s c a t t e r e r sa n dt h en a t u r eofe a c hs c a t t e r e r .I fwei ma g i n ea l l t h es c a t t e r e r st ob ea l i k e ( a n da s s u met h a twea r ef a rf r omt h ec ol l e c t i on )t h e nt h i se x p r e s s i ons i mp l i f i e s : dσ dσ0 ( q ) ( 1 6 . 5 5 ) dΩ =dΩF 3 Sor r y . . . F i g u r e1 6 . 2 :Ge ome t r yofmu l t i p l es c a t t e r e r s .Th er e l a t i v ep h a s eoft wos ou r c e s d e p e n d sont h ep r oj e c t i onoft h ed i ffe r e n c ei nwa v ev e c t or son t ot h ev e c t orc on n e c t i n g t h es c a t t e r e r s . d σ wh e r e dΩ0i st h es c a t t e r i n gc r os s s e c t i onofas i n g l es c a t t e r e ra n dt h eF ( q)i sc a l l e da “ s t r u c t u r ef a c t or ” : F qx 2 i· e ( q )= ( 1 6 . 5 6 ) j j qx x i· (−) e . = j i ( 1 6 . 5 7 ) i , j Th i sl a s te x p r e s s i oni s1ont h ed i a g on a li=j .I ft h e( e . g . )a t omsa r eu n i f or ml yb u t r a n d oml yd i s t r i b u t e d , t h es u moft h eoffd i a g on a lt e r msa v e r a g e st oz e r oa n dt h et ot a l s u mg oe st oN( t h en u mb e rofa t oms ) .Th i si sa ni n c oh e r e n ts u p e r p os i t i ona n dt h e s c a t t e r e di n t e n s i t i t i e sa d dwi t hn e g l i g i b l ei n t e r f e r e n c e . I ft h ea t omsa r ei n s t e a donar e g u l a rl a t t i c e , t h e n“ Br a g g ”s c a t t e r i n gr e s u l t s .Th e r ewi l l e x i s tc e r t a i nv a l u e sofqt h a tma t c ht h es p a c i n gb e t we e np l a n e si ns u c hawa yt h a twh ol e r owsoft h ema t r i xa r e1 .I nt h os ed i r e c t i on / wa v e l e n g t hc omb i n a t i on s ,t h es c a t t e r e d 2 i n t e n s i t yi sofor de rN a n dh e n c ei smu c hb r i g h t e r .Th es c a t t e r e dp owe rd i s t r i b u t i ont h u s h a sb r i g h ts p ot si ni sc or r e s p on d i n gt ot h e s ed i r e c t i on s , wh e r ec on s t r u c t i v ei n t e r f e r e n c ei n t h es c a t t e r e dwa v e soc c u r s . St r u c t u r ef a c t ors u msoc c u ri nma n yb r a n c h e sofp h y s i c s . I fy out h i n ka b ou ti tf ora mome n t ,y ouc a ne a s i l ys e et h a ti ti sp os s i b l et od oas t r u c t u r ef a c t ors u mu s i n gt h e Gr e e n ’ sf u n c t i one x p a n s i on sy ouh a v es t u d i e d.I ne l e c t r od y n a mi c sa n dqu a n t u m mu l t i p l es c a t t e r i n gt h e or yt h e s es u msa p p e a rf r e qu e n t l yi na s s oc i a t i onwi t hs p a t i a l l y f i x e ds t r u c t u r e s( l i k ec r y s t a ll a t t i c e sormol e c u l e s ) .I nf i e l dt h e or y ,l a t t i c es u msa r e s ome t i me su s e da sad i s c r e t i z e da p p r ox i ma t i onf ort h ec on t i n u u m, a n d“ l a t t i c eg a u g e ” t y p ef i e l dt h e or i e sr e s u l t .I nt h e s et h e or i e s , ou ra b i l i t yt od ot h es t r u c t u r ef a c t ors u ms i su s e dt oc on s t r u c tt h eGr e e n ’ sf u n c t i on sr a t h e rt h a nt h eot h e rwa ya r ou n d. Ei t h e rwa y , y ous h ou l db ef a mi l i a rwi t ht h et e r ma n ds h ou l dt h i n ka b ou tt h ewa y sy oumi g h t a p p r oa c he v a l u a t i n gs u c has u m. Wea r en owd on ewi t hou rd i s c u s s i onofs c a t t e r i n gf r omob j e c t sp e rs e .I ti swe l l wor t h y ou rwh i l et or e a dJ 10. 2o ny ou rown .I h a v eg i v e ny out h es e mi –qu a n t i t a t i v ea r g u me n tf or t h eb l u es k y ;t h i ss e c t i onp u t sou rs i mp l et r e a t me n tonf i r me rg r ou n d .I ta l s ode r i v e st h e p e r t u r b a t i ont h e or yofs c a t t e r i n g( u s i n gt h eBor na p p r ox i ma t i on ) , a n dd i s c u s s e san u mb e r ofi n t e r e s t i n gc u r r e n tr e s e a r c ht op i c s( s u c ha sc r i t i c a lop a l e s c e n c e ) .Iwi l lp r ob a b l ya s s i g n on ep r ob l e mou toft h i ss e c t i ont oh e l py ouou t .Howe v e r , p e r t u r b a t i v es c a t t e r i n gi se a s i e r t ou n d e r s t a n d , a n dmor eu s e f u l , i nt h ec on t e x tof( s c a l a r )qu a n t u mt h e or ya n ds oIwi l l s k i p t h i ss e c t i on , e x p e c t i n gt h a ty ouwi l l s e ee n ou g hofi tt h e r e . Yous h ou l da l s or e a dJ 1 0 . 3 .Th i sp r e s e n t so n ewa yt od e r i v et h eRa y l e i g he x p a n s i onf or a( s c a l a r )p l a n ewa v ei nt e r msoff r e es p h e r i c a l wa v e s( t h e r ea r es e v e r a l ) .Howe v e r , i tg oe s f u r t h e ra n da dd r e s s e se x p a n s i on sofe . g .c i r c u l a r l yp ol a r i z e dp l a n ewa v e si nt e r msof v e c t ors p h e r i c a l h a r mon i c s !L or dk n owswh yt h i si ss t u c koffi nt h i son es e c t i ona l l b yi t s e l f –In e e dt op u tt h ee qu i v a l e n tr e s u l tf ore x p a n s i oni nt e r msofHa n s e ns ol u t i on s( wh i c hof c ou r s ewi l lb emu c hmo r en a t u r a la n dwi l lp r e c omp u t emos toft h ea n n oy i n gp a r t soft h e a l g e b r af or u s )i nt h es e c t i on sont h eHa n s e nf u n c t i on sa n dVSHswh e r ei tb e l on g s ,a si twi l l a c t u a l l yb emu c hs i mp l e rt ou n d e r s t a n dt h e r e . J 1 0 . 4r e doe ss c a t t e r i n gf r omas p h e r e“ r i g h t ”i nt e r msofVSHs ,a n da g a i n ,i fwe wi s h e dt op u r s u et h i swewou l dn e e dt or e dot h i si nt e r msofHa n s e nf u n c t i on st ok e e p i ts i mp l e .Th ep r i ma r ya dv a n t a g eofr e a di n gt h i sc h a p t e ri st h a ti td e f i n e st h epa r t i a l wa v ep h a s es h i f t sofs c a t t e r i n gf r omas p h e r e ,qu a n t i t i e st h a ta r ei nu s ei np r e c i s e l y t h es a mec o n t e x ti nqu a n t u ms c a t t e r i n gt h e or yi ne . g .n u c l e a rp h y s i c s .SO, i fy oup l a n t og oi n t on u c l e a rp h y s i c sy oua r ewe l la d v i s e dt or e a dt h i sc h a p t e ra swe l la n dwor k t h r ou g hi t . Howe v e r ,wec a n n otd ot h i sa tt h i st i meb e c a u s eweh a dt og ob a c ka n dr e doJ 7 a n dJ 8 .Be s i de s ,we ’ r edou b t l e s sab i tb or e dwi t hmu l t i p ol e sa n dwa n tt ob e c ome e x c i t e da g a i n .Wewi l lt h e r e f or en owmov eont oon eofmyf a v or i t et op i c s ,r e l a t i v i t y t h e o r y . Pa r tI I I Re l a t i v i s t i c E l e c t r o d y n a mi c s 2 2 7 Ch a p t e r17 Sp e c i a l Re l a t i v i t y 17. 1 E i n s t e i n ’ sPos t u l a t e s Byt h i st i meI c e r t a i n l yh op et h a ty oua r ef a mi l i a rwi t ht h et wop os t u l a t e s , d u et o E i n s t e i n , t h a tl e a dt ot h et h e or yofs p e c i a l r e l a t i v i t y . Th e ya r e : a )Th el a wsofn a t u r ea r ei n v a r i a n twi t hr e s p e c tt ot h eu n i f or mt r a n s l a t i onoft h e c oor d i n a t es y s t e mi nwh i c ht h e ya r eme a s u r e d . b )Th es p e e dofl i g h ti si n d e p e n d e n toft h emot i onoft h es ou r c e . Pr op e r l ys p e a k i n g , t h es e c on dp os t u l a t ei sac on s e qu e n c eoft h ef i r s t , s i n c ei ft h es p e e d ofl i g h td e p e n d e dont h emot i onofi t ss ou r c et h el a wsofe l e c t r od y n a mi c s( wh i c h d e t e r mi n et h es p e e doff r e e l yp r op a g a t i n ge l e c t r oma g n e t i cwa v e s )wou l dd e p e n dont h e i n e r t i a lf r a meoft h es ou r c e ,wh i c hc on t r a d i c t st h ef i r s tp os t u l a t e .F orwh a ti ti swor t h ,t h e f i r s ti sn ota sob v i ou s l yac on s e qu e n c eoft h es e c on d :i ts e e mse n t i r e l yp os s i b l ef ors ome l a wst od e p e n dont h ev e l oc i t yoft h es ou r c ea n dn otc on t r a d i c tt h es e c on dp os t u l a t e ,a s l on ga st h e ya r en ote l e c t r od y n a mi c a li nn a t u r e .Th i sh a sb e e nt h es u b j e c tofc on s i d e r a b l e d i s c u s s i on , a n dI h e s i t a t et os t a t ear e l i g i ou sv i e wu p oni t . Iwi l l ,h owe v e r ,p oi n tou tt h a ti nt h eop i n i onofDi r a c ,a tl e a s t—t h ed i s c ov e r yoft h e ◦ u n i f or m 3Kb l a c k b od yb a c k g r ou n de x p l i c i t l yc on t r a d i c t e dt h ef i r s tp os t u l a t eb u tn ott h e s e c on d.Youmi g h ta mu s ey ou r s e l f ,s omequ i e te v e n i n g ,b yc on s i d e r i n ge x p e r i me n t st h a t wou l dme a s u r ey ou ra b s ol u t ev e l oc i t yr e l a t i v et ot h e“ r e s t ”f r a meoft h i sr a d i a t i on .Th e s e c on dp os t u l a t e( wh i c hi sa l lwen e e d )t h u ss e e mst ob et h es a f e roft h et wou p onwh i c h t ob a s eou rr e a s on i n g . Is t r on g l yr e c omme n dt h a ty our e a dJ 1 1 . 1—J 1 1 . 2ony ou rown .Th e ya r e“ t r u e f a c t s ”t h a twi l lc omei nh a n d ys omed a y ,a n ds h ou l da s t ou n da n da ma z ey ou .Ye s , Vi r g i n i a , s p e c i a l r e l a t i v i t yr e a l l yr e a l l ywor k s . F orou rp u r p os e s ,wewi l lb e g i n wi t ha b r i e fr e v i e w oft h eb a s i cL or e n t z t r a n s f or ma t i ona n da ni n t r odu c t i ont of ou rv e c t or s .Be c a u s ewewi l ld oi ta g a i n ( c or r e c t l y )i nawe e kors owewon ’ tt a k el on gn ow.Wewi l la l s os t u d yf ou r –v e l oc i t y a n df ou r –mome n t u m. Th i swi l l s u ffic et og i v eu st h e“ f l a v or ”oft h e 2 2 9 t h e or ya n de s t a b l i s ht h eg e ome t r i c a l yg r ou n d sf ort h ema t r i xt h e or ywewi l l t h e nde r i v e . Asa na p p l i c a t i onoft h i s ,wewi l ls t u d yTh oma sp r e c e s s i onb r i e f l ya n dt h e ng oon t op e r f or made t a i l e da p p l i c a t i onoft h et h e or yt ot h ed y n a mi c sofi n t e r a c t i n gc h a r g e d p a r t i c l e sa n df i e l d s .Wewi l l s p e n dt h er e s toft h es e me s t e ront h i ss u b j e c t , i non ef or m ora n ot h e r . 17. 2 Th eE l e me n t a r yL o r e n t zTr a n s f or ma t i o n Tomot i v a t et h eL or e n t zt r a n s f or ma t i on ,r e c a l lt h eGa l i l e a nt r a n s f or ma t i o nb e t we e n mov i n gc oor d i n a t es y s t e ms : ′ x1 t 1−v =x ′ x2 =x 2 ′ x3 =x 3 ′ t ( 1 7 . 1 ) ( 1 7 . 2 ) ( 1 7 . 3 ) =t ( 1 7 . 4 ) ′ ( wh e r eKi sf i x e da n dKi smov i n gi nt h e1 –d i r e c t i ona ts p e e dv ) . Th e n ′ ′ F ¨ ¨ j=mx j=mx j=F j ( 1 7 . 5 ) orNe wt o n ’ sL a wsa r ec ov a r i a n twi t hr e s p e c tt ot h eGa l l i l e a nt r a n s f or ma t i o n . Bu t ∂ ∂ ∂x =∂x′ 1 1 1∂ + v∂t ′ ( 1 7 . 6 ) a n ds o 2 2 ∂x ∂x 2 2 ∂x 2 2 2 ∂x 3 2 ∂ + v2 ∂t2 + v∂x′ ∂t 1 ′ ′ 1 ∂2 = = ( 1 7 . 8 ) ′ 2 ( 1 7 . 9 ) ∂x 3 2 ( 1 7 . 7 ) ′ 2 ∂x 2 2 ∂ 2 ∂ ∂ 2 ∂t Th u si f 2 1∂ ′ 2 = 1 ∂ ∂ 2 2 ∂ ∂ ′ 2 = ∂t . ( 1 7 . 1 0 ) 2 1∂ 2 2 2∂ t ∇ −c t h e n ∇ ′ 2 2 1∂2 − 2∂ c t ψ=0 2 1∂ψ ψ= − 2 v ∂t ′ ( 1 7 . 1 1 ) 2 2 ∂ψ =0 ∂t − 2 v∂x′ ′ 1 ( 1 7 . 1 2 ) ′ ( s ot h a tt h ewa v ee q u a t i on ,a n dh e n c eMa x we l l ’ se qu a t i on swh i c hl e a dd i r e c t l yt o t h ewa v ee qu a t i oni nf r e es p a c e ,a r en otc ov a r i a n twi t hr e s p e c tt ot h eGa l l i l e a n t r a n s f or ma t i on !Th e ya l r e a d yd e t e r mi n et h ep e r mi t t e dv e l oc i t yofal i g h twa v e ,a n dd o n ota l l owt h a tv e l oc i t yt ode p e n dona n y t h i n gb u tt h ep r op e r t i e soft h eme d i u mt h r ou g h wh i c ht h ewa v ei st r a n s mi t t e d . Th es i mp l e s tl i n e a rt r a n s f or ma t i onofc oor d i n a t e si st h a tp r e s e r v e st h ef or m oft h e wa v ee qu a t i oni se a s yt od e t e r mi n e .I ti son et h a tk e e p st h es p e e doft h e( l i g h t )wa v ee qu a l ′ i nb ot ht h eKa n dt h eK f r a me s .Ge ome t r i c a l l y ,i faf l a s hofl i g h ti se mi t t e df r om t h e ′ ( c oi n c i d e n t )or i g i n sa tt i met=t=0 ,i twi l la p p e a rt oe x p a n dl i k eas p h e r eou tf r omb o t h c oor di n a t eor i g i n s , e a c hi ni t sownf r a me : 2 2 2 2 ′2 ′ 2 ′ 2 ′ 2 ( c t )−( x+y +z)=0 ( 1 7 . 1 3 ) a n d ( c t )−( x +y +z)=0 ( 1 7 . 1 4 ) a r es i mu l t a n e o u sc on s t r a i n t sont h ee qu a t i on s . Mos tg e n e r a l l y , 2 2 2 2 2 ( c t )−( x +y +z)=λ ′2 ′ 2 ′ 2 ′ 2 ( c t )−( x +y +z) ( 1 7 . 1 5 ) wh e r e , λ ( v )d e s c r i b e sap os s i b l ec h a n g eofs c a l eb e t we e nt h ef r a me s .I fwei n s i s tt h a t 1 t h ec oor d i n a t et r a n s f or ma t i onb eh omog e n e ou sa n ds y mme t r i cb e t we e nt h ef r a me s, t h e n λ = 1 . ( 1 7 . 1 6 ) 1 I fwer e l a xt h i sr e qu i r e me n ta n da l l owf oru n i f or me x p a n s i on sa n d / orc on t r a c t i on soft h ec oor di n a t es y s t e m, a mor eg e n e r a l g r ou ps t r u c t u r e , t h ec on f or ma l g r ou p , r e s u l t s L e tu sd e f i n e x 0 =c t ( 1 7 . 1 7 ) x 1 =x ( 1 7 . 1 8 ) x 2 =y ( 1 7 . 1 9 ) x 3 =z ( 1 7 . 2 0 ) ( x4 =i c tMi n k ows k i me t r i c ) ( 1 7 . 2 1 ) Th e nwen e e dal i n e a rt r a n s f or ma t i onoft h ec oor d i n a t e st h a tmi x e sxa n d( c t )i n t h edi r e c t i onofvi ns u c hawa yt h a tt h el e n g t h 2 2 2 2 2 s =( x )−( x 0 1 +x 2 +x 3) ( 1 7 . 2 2 ) i sc on s e r v e da n dt h a tg oe si n t ot h eGa l l i l e a nt r a n s f or ma t i ona sv→ 0 .I fwec on t i n u e t oa s s u met h a tvi si nt h e1d i r e c t i on , t h i sl e a dst ot h eL o r e n t zt r a n s f or ma t i o n : ′ x 0 ′ x 1 ′ x 2 ′ x 3 ( x x ) 0−β 1 =γ ( 1 7 . 2 3 ) ( x x ) =γ 1−β 0 ( 1 7 . 2 4 ) =x 2 ( 1 7 . 2 5 ) =x 3 ( 1 7 . 2 6 ) ′ wh e r ea tx , 1=0 v x t→ β= 1=v c. ( 1 7 . 2 7 ) Th e n 2 l e a dst o ′ 2 s s= 2 2 2 2 2 22 2 ( 1 7 . 2 8 ) 2 x x +γβ( x 0 −x 1 =γ( 0 −x 1) 1 −x 0) or 2 2 ( 1 7 . 2 9 ) γ( 1−β)=1 ( 1 7 . 3 0 ) ± 1 ( 1 7 . 3 1 ) s o γ= 2 1 − β wh e r ewec h oos et h e+s i g nb yc on v e n t i on . Th i sma k e sγ ( 0 )=+1 . F i n a l l y , 1 γ ( v )= ( 1 7 . 3 2 ) 2 v 1− c2 a swea l l k n owa n dl ov e . Now, l e tmer e mi n dy out h a twh e nv< <c , 2 1v 2 γ ( v )=1+ 2 c + . . . ( 1 7 . 3 3 ) v t ol owe s ts u r v i v i n gor d e ri n c.Aswes h a l ls e e ,t h i si swh y“ k i n e t i ce n e r g y ”i n n on –r e l a t i v i s t i cs y s t e ms( b e i n gd e f i n e da st h et ot a le n e r g ymi n u st h ep ot e n t i a le n e r g y 1 2 a n dt h er e s tma s se n e r g y )i st h eu s u a l2mv. ′ Th ei n v e r s et r a n s f or ma t i on( f r omKt oK)i sa l s oofs omei n t e r e s t . ′ x 0 ′ ( x x 0+β 1) =γ ( 1 7 . 3 4 ) ′ ′ ( x x =γ 1+β 0) ′ =x 2 ′ 3 =x x 1 x 2 x 3 ( 1 7 . 3 5 ) ( 1 7 . 3 6 ) ( 1 7 . 3 7 ) wh i c hi sp e r f e c t l ys y mme t r i c , wi t hv→ − v .I ta p p e a r st h a twh i c hf r a mei sa t“ r e s t ”a n d wh i c hi smov i n gi sma t h e ma t i c a l l y , a tl e a s t , ama t t e rofp e r s p e c t i v e . F i n a l l y , i fwel e t β= v ( 1 7 . 3 8 ) c ( i na na r b i t r a r yd i r e c t i o n )t h e nweh a v eb u tt ou s edotp r od u c t st oa l i g nt h ev e c t or t r a n s f or ma t i one qu a t i on swi t ht h i sd i r e c t i on : ′ x βx ) · x =x+ γ−1( βx ) β 0 =γ ( x ( 1 7 . 3 9 ) 0− ′ 2 β · γ βx − ( 1 7 . 4 0 ) 0 It h i n kt h a ty ous h ou l dp r ov et h a tt h i si sc or r e c ta sa ne x e r c i s e .Si n c et h ed i r e c t i onofβ i sa r b i t r a r y ,i ts h ou l ds u ffic et os h o wt h a tt h i sr e du c e st h et h ef or ma b ov ef ort h a t d i r e c t i ona n da na r b i t r a r yt r a n s v e r s ed i r e c t i on . Sol u t i on :Not et h a t β·x ) β x= ( ( 1 7 . 4 1 ) β2 L or e n t zt r a n s f or mi ta c c or d i n gt ot h i sr u l ea n don eg e t s( b yi n s p e c t i on ) ′ x =γ ( x−βx ) 0 ( 1 7 . 4 2 ) ′ a son es h ou l d .Th ex0t r a n s f or mi sob v i ou s .F i n a l l y , t h eot h e rt wo( ⊥)c omp on e n t sdo n otg e tac on t r i b u t i onf r omγ . Th a ti s , ′ x=( x +γ x−x−γ βx ⊥+x) 0 ( 1 7 . 4 3 ) ( r e c on s t r u c t i n gt h er e s u l ta b ov edi r e c t l y )QE D. Th i si sn ott h emos tg e n e r a lorc on v e n i e n twa yt owr i t et h ef i n a lt r a n s f or m.Th i si s b e c a u s eγa n dβa r eb ot hr e l a t e df u n c t i on s ;i ts h ou l dn otb en e c e s s a r yt ou s et wo p a r a me t e r st h a ta r en oti n d e p e n d e n t .Al s o, t h el i mi t i n gs t r u c t u r eoft h et r a n s f or ma t i on i sn ota ta l l a p p a r e n twi t h ou tc on s i d e r i n gt h ef u n c t i on a l f or msi nde t a i l . I ti se a s yt os e ef r omt h ede f i n i t i ona b ov et h a t 2 22 ( γ −γβ)=1 . ( 1 7 . 4 4 ) Th er a n g eofβ i sd e t e r mi n e db yt h er e qu i r e me n tt h a tt h et r a n s f or ma t i on b e n on –s i n g u l a ra n di t ss y mme t r y : 0≤β<1 s ot h a t1≤γ<∞. ( 1 7 . 4 5 ) I fwet h i n ka b ou tf u n c t i o n st h a t“ n a t u r a l l y ”p a r a me t e r i z et h e s er a n g e s , t h e ya r e : 2 2 c os h ξ−s i nh ξ=1 ( 1 7 . 4 6 ) wher e − ξ β =t a n hξ γ =c os hξ γ β =s i n hξ ξ e −e = −ξ ξ [ 0, 1) ∈ e +e 1 − ξ ξ e +e) ∈ [ = 2( 1 , ∞) 1 − ξ ξ e −e) = 2( ∈ [ 0 , ∞) . ( 1 7 . 4 7 ) ( 1 7 . 4 8 ) ( 1 7 . 4 9 ) Th ep a r a me t e rξi sc a l l e dt h eb o os tp a r a me t e rorr a p i d i t y .Youwi l ls e et h i su s e d f r e qu e n t l yi nt h ed e s c r i p t i onofr e l a t i v i s t i cp r ob l e ms . Youwi l l a l s oh e a ra b ou t“ b oos t i n g ” b e t we e nf r a me s ,wh i c he s s e n t i a l l yme a n sp e r f or mi n gaL or e n t zt r a n s f or ma t i on( a “ b oos t ” )t ot h en e wf r a me .Th i swi l lb e c o mec l e a r e rl a t e rwh e nweg e n e r a l i z et h e s e r e s u l t sf u r t h e r .Tog i v ey ous ome t h i n gt ome di t a t eu p on ,c on s i d e ri ny ou rmi n dst h e f or ma ls i mi l a r i t yb e t we e na“ b oos t ”a n da“ r ot a t i on ”b e t we e nt h exa n dx oor di n a t e s 0c wh e r et h er ot a t i oni st h r ou g ha ni ma g i n a r ya n g l ei ξ . Hmmmm. Toe l u c i d a t et h i sr e ma r kawe et a dmor e , n ot et h a ti nt h i sp a r a me t e r i z a t i on , ′ x 0 ′ x ′ x ⊥ =x coshξ xs i n hξ − 0 ( 1 7 . 5 0 ) x i n hξ+xc os hξ 0s =− ( 1 7 . 5 1 ) ⊥. =x ( 1 7 . 5 2 ) Wh a ti st h e4×4t r a n s f o r ma t i onma t r i x( i nf ou rd i me n s i on s )f ort h i sr e s u l t ? 2 Doe si tl ookl i k ea“ h y p e r b ol i cr ot a t i on ”orwh a t ? Weh a v ej u s td e t e r mi n e dt h a tt h e( v e c t or )c oor d i n a t es y s t e mt r a n s f or msac e r t a i n wa y .Wh a t , t h e n , ofv e c t orf i e l ds , ora n yot h e rv e c t orqu a n t i t y ?Howdog e n e r a lv e c t or s t r a n s f or mu n d e rab oo s t ?Th i sd e p e n d sont h en a t u r eoft h ev e c t or s .Ma n yv e c t or s , i f n otmos t ,t r a n s f or ml i k et h eu n d e r l y i n gc oor di n a t ed e s c r i p t i onoft h ev e c t or s .Th i s i n c l u d e st h eon e sofg r e a t e s ti n t e r e s ti np h y s i c s .Toma k et h i sob v i ou s , wewi l l h a v et o g e n e r a l i z eav e c t orqu a n t i t yt of ou rd i me n s i on s . 17. 3 4Ve c t o r s Not ewe l lt h a twea r en otYETi n t r od u c i n gp r op e rn ot a t i onf orc o-a n dc on t r a v a r i a n t t e n s or sa swed on ’ tk n o wwh a tt h a tme a n s . Ac t u a l l yt h en ot a t i on 2 2 2 “ Hy p e r b ol i c ”b e c a u s eoft h er e l a t i v emi n u ss i g nb e t we e nx a n dc t. Mor eont h i sl a t e r . 0 1 2 3 f oror d i n a r yc oor d i n a t e ss h ou l db ex, x, x, xa n dwewi l l h a v et ode t e r mi n ewh e t h e ra n y g i v e n4 v e c t orqu a n t i t yt h a ti saf u n c t i onoft h ec oor d i n a t e st r a n s f or msl i k et h ec oor d i n a t e orl i k ead i ffe r e n t i a loft h ec oor di n a t ei nor de rt od e t e r mi n ei fi ti sc o-orc on t r a v a r i a n t . Si mi l a r l y ,weh a v en oty e td i s c u s s e dh owt of or mt h ev a r i ou sdy a d i cp r odu c t sofc o-a n d c on t r a v a r i a n tv e c t or s–s omewi l l f or ms c a l a r s , s omev e c t or s , s omes e c on dr a n kt e n s or s . I n ot h e rwor ds , t h er e s u l t sb e l owa r ea l l c or r e c t , b u tt h en ot a t i ons u c k sa n dt h i ss u c k i n e s swi l l ma k ec e r t a i np a r t sofd oi n gt h ea l g e b r amor ed i ffic u l tt h a ni tn e e d st ob e . Ima yr e wr i t et h i swh ol ec omb i n e dmu l t i c h a p t e rs t r e t c h ,a sI ’ mn otc e r t a i noft h e p e g a g og i c a lv a l u eofp r e s e n t i n gt h i n g si n c or r e c t l yori na ne l e me n t a r yf or ma n dt h e n c or r e c t l yi na ne l e g a n tf or ma sJ a c k s ond oe s .I nt h eme a n t i me ,p l e a s eb e a rwi t ht h e n ot a t i onb e l owa l l owi n gf ort h ef a c tt h a tmu c hofi ti sj u s twr on g . Coo r di n a t e4 –v e c t or sa r e( x , x , x , x ) . 0 1 2 3 Ar b i t r a r y4 –v e c t or sa r e( A0, A1, A2, A3) . I ft h e“ a r b i t r a r y ”v e c t ort r a n s f or msl i k et h ec oor d i n a t e s , t h e n A′ =γ ( A 0 β A) − · ( 1 7 . 5 3 ) 0 ′ A ′ A⊥ ( A−βA0) =γ ( 1 7 . 5 4 ) =A⊥ ( 1 7 . 5 5 ) a n d 2 2 2 2 2 A =A0 −( A1 +A2 +A3) = 2 A0− A· A ( 1 7 . 5 6 ) i sa ni n v a r i a n toft h et r a n s f or ma t i on .Not e :wh e n e v e rIb ol d f a c eav e c t orqu a n t i t y ,I me a nt h e3 De u c l i d e a n( c a r t e s i a n )v e c t ori nor d i n a r ys p a c e .I nt h a tc a s eIwi l l wr i t et h e t i me( 0 )c omp on e n te x p l i c i t l y .Wh e nIwa n tt or e f e rt oa4 –v e c t org e n e r i c a l l y , Iwi l ln ot b ol df a c ei t( e . g . —Av sA) . Ki d s ! Ama z ey ou rf r i e n d s ! As t ou n dy ou rn e i g h b or s ! Sh owt h a t A′ B′ A′ 0 0− ′ · B= AB A B · 0 0− ( 1 7 . 5 7 ) i sa ni n v a r i a n toft h eL or e n t zt r a n s f or ma t i onf ora r b i t r a r y4 –v e c t or sA, B.Th i si s( orwi l l b e )h ome wor k . Now,weh a v eaf e wd e f i n i t i on sof“ n e wwor d s ”t ol e a r n .Mos tofy oup r ob a b l y a l r e a d yk n owt h e mf r omon ec on t e x tora n ot h e r , b u twea l ln e e dt oa g r e ea tt h i sp oi n t t ot a l kac ommonl a n g u a g e ,s owewi l lr e v i e wt h ed e f i n i t i on sc a r e f u l l ya n da v oi d c on f u s i on . E l e c t r oma g n e t i cs i g n a l s( a n da n y t h i n ge l s et r a v e l l i n ga ts p e e dc )t r a v e l ont h el i gh t c o n e .Ane v e n ti sac oor d i n a t ex=( x ,x ) .Wea r eu s u a l l yi n t e r e s t e di nc a u s a l l y 0 c o n n e c t e de v e n t sonawor l dl i n e .Th i smi g h tb e ,f ore x a mp l e ,t h et r a j e c t or yofa ma s s i v ep a r t i c l e( l i k eon eont h et i pofy ou rn os e )wi t hv<c .Ca u s a l l yc on n e c t e dwor l d l i n et r a j e c t or i e smu s tl i v ei n s i d et h el i g h tc on eofe a c he v e n tt h a tl i e su p ont h e m. F i g u r e1 7 . 1 : Th eL i g h tCo n e : Pa s t , n ow, f u t u r e , a n de l s e wh e r e . E v e n t s . Th ewor l dl i n e . Con s i d e rt woe v e n t s . I fwed e f i n et h ei n v a r i a n ti n t e r v a l 2 2 2 2 S12 =c( t )−| x | 1−t 2 1−x 2 ( 1 7 . 5 8 ) t h e nweh a v ea 2 t i me l i k es e p a r a t i o nS12 >0 2 2 2 ⇒ c( t )>| x |. 1−t 2 1−x 2 Bot he v e n t sa r ei n s i d ee a c hot h e r ’ sl i g h tc on e .Th e s ee v e n t sc a nb e“ c a u s a l l y c on n e c t e d ” , b e c a u s eal i g h ts i g n a l g i v e noffb yon ec a nr e a c ht h eot h e rf r omt h e “ i n s i d e ” . I nt h i sc a s e , as u i t a b l eL or e n t zt r a n s f or ma - ′ ′ ′ ′ t i onc a nma k ex1=x2, b u tt l wa y s . 1=t 2a 2 s p a c e l i k es e p a r a t i o nS12 <0 2 2 2 ⇒ c( t )<| x |. 1−t 2 1−x 2 Bot he v e n t sa r eou t s i d ee a c hot h e r ’ sl i g h tc on e .Th e s ee v e n t sa r e“ c a u s a l l y d i s c on n e c t e d” , b e c a u s eal i g h ts i g n a l g i v e noffb yon ec a nn otr e a c ht h eot h e r .I f n ot h i n gg oe sf a s t e rt h a nl i g h t , t h e nt h os ep a r t i c u l a re v e n t sdi dn ots p e a kt oon e a n ot h e r .Not et h a tt h i sdoe sn otme a nt h a te a r l i e r( a n dl a t e r )e v e n t sone a c h wor l dl i n et on otc on n e c t .Th ee v e n t sa r ed i s c on n e c t e d ,n ott h ewor l dl i n e s t h e ms e l v e s . ′ ′ ′ ′ I nt h i sc a s e ,as u i t a b l eL or e n t zt r a n s f or ma t i onc a nma k et ,b u tx1=x2 1=t 2 a l wa y s . 2 l i gh t l i k es e p a r a t i onS12 =0 2 2 2 ⇒ c( t )=| x |. 1−t 2 1−x 2 Bot he v e n t sa r eone a c hot h e r ’ sl i g h tc on e . Th e s ee v e n t sa r e“ c a u s a l l yc on n e c t e d” b ye l e c t r oma g n e t i cr a d i a t i on .Th ef i e l dp r od u c e db yc h a r g e sa ton ee v e n ta r e d i r e c t l yi n t e r a c t i n gwi t hc h a r g e sa tt h eot h e re v e n t , a n dv i c ev e r s a . Not ewe l lt h a tt h ee v e n tp a i r sc o n s i de r e da b ov ec a nb ema d es p a t i a l l yc oi n c i d e n t , t e mp or a l l yc oi n c i d e n t ,orb ot h ,b ys u i t a b l yb oos t i n gt h ef r a me .E v e n t swi t hat i me l i k e s e p a r a t i onc a nb ema d es p a t i a l l yc oi n c i d e n t .Ev e n t swi t has p a c e l i k es e p a r a t i onc a nb e ma det ooc c u ra tt h es a met i me , ori ne i t h e ror d e r . E v e n t swi t hal i g h t l i k es e p a r a t i onwi l l a l wa y sh a v eal i g h t l i k es e p a r a t i oni na l l f r a me s . Wea r ea b ou tt or u ni n t oap r of ou n dp h i l os op h i c a ld i ffic u l t y .Ph y s i c si sd e d i c a t e d t ody n a mi c s– t y p i c a l l ys ol v i n gi n i t i a lv a l u ep r ob l e msa n dh e n c ep r e d i c t i n gt h e d y n a mi c a le v ol u t i onofs y s t e msi nt i me .Un f or t u n a t e l y ,wej u s te l i mi n a t e dt i mea sa n i n de p e n d e n tv a r i a bl e .Byma k i n gi tap a r tofou rg e ome t r y , i ti sn ol on g e ra v a i l a b l ea s a ni n d e p e n d e n tp a r a me t e rt h a twec a nu s et owr i t et r a d i t i on a l e qu a t i on sofmot i on . Th e r ea r el i k e l yt oot h e rs i g n i f i c a n tc on s e qu e n c e soft h i sd e c i s i on , a sma n yoft h e qu a n t i t i e ss t u d i e di np h y s i c sa r et e n s o rf or msde f i n e dwi t hr e s p e c tt os p a t i a l ge ome t r y .Th a ti s ,wh e nIc omp u t e“ c h a r g e ”or“ mome n t u m”or“ e l e c t r i cf i e l d ”ora “ r ot a t i onma t r i x ” ,I ’ mc omp u t i n g0 t h ,1 s tor2 n dr a n kt e n s or st h a ti n h e r i tt h e i r d i r e c t i on a l c h a r a c t e r( orl a c kofi t )f r omt h eu n d e r l y i n gs p a t i a l c oor d i n a t es y s t e m.We l l , we ’ v ej u s tma d et h a tu n de r l y i n gc oor d i n a t es y s t e mf o u rd i me n s i on a l a n ds oqu a n t i t i e s l i k e“ mome n t u m”a n d“ e l e c t r i cf i e l d ”wi l l h a v et ob er e c on s i d e r e d .Wema yn e e dt of i n d n e w“ t i me l i k e ”c oor d i n a t e st oa s s oc i a t ewi t hs omeoft h e s e ,a n dp e r h a p sr e c l a s s i f y ot h e r sa sdi ffe r e n ts or t soft e n s or s . F i n a l l y ,wen e e dt or e c ov e ra“ t i me ”t h a tc a nb eu s e dt owr i t ed owns o mes or tof e qu a t i on sofmot i onorwec a n ’ tma k ea“ p h y s i c s ” .Th i swi l lp r ov et ob ev e r yd i ffic u l t . F oron et h i n g ,wec a nn ol on g e re x p e c tt ob ea b l et os ol v ei n i t i a lv a l u ep r ob l e ms ,a s t i mei sn owas y mme t r i cc oo r d i n a t e .Th et r a j e c t or i e sofp a r t i c l e sa r ed e t e r mi n e db y t h e i rr e l a t i v i s t i ci n t e r a c t i onc on n e c t i on sa n ddi ffe r e n t i a l“ e qu a t i on sofmot i on ”wi t h b ou n da r yc on di t i on sonac l os e df ou rd i me n s i on a lh y p e r s u r f a c ea tf ou r –i n f i n i t y !Th a t me a n st h a ti ti si mp os s i b l ei np r i n c i p l et op r e d i c tf u t u r et r a j e c t or i e sf r om on l ya k n owl e d g eoft h os et r a j e c t or i e si nt h ep a s t .I ti sa ma z i n gh owf e wp e op l ei np h y s i c s a r ewi l l i n gt oi n t e r n a l l ya c k n owl e d g et h a tf a c t .Ac c e p ti t .I ti st r u e .Youwi l lb eh a p p i e r f ori t . An y wa y , t h e r ea r ea tl e a s tt wowa y sa r ou n dt h i s( ma t h e ma t i c a l )d i ffic u l t y .On ei st o i n t r od u c ea“ h y p e r t i me ”–y e ta n ot h e rd i me n s i onc on t a i n i n gap a r a me t e rt h a tc a n 3 s e r v eu sa st i meh a ss e r v e di nt h ep a s t.Th i s ,h owe v e r ,i n t r od u c e saf i f t hd i me n s i on wh i c hwen e e d( c u r r e n t l y )l i k eaf i f t hwh e e l .Ma y b eGodl i v e si nh y p e r t i me ,b u tt h e r e a r ei n f i n i t edi ffic u l t i e sa s s oc i a t e dwi t hou rt r y i n gt oi mp l e me n ti ti nt h ec omp l e t e a b s e n c eo fp h y s i c a lp r ob e s .Sa yh e l l ot oPl a n eJ oef r om F l a t l a n d .L e a v ei tt o ma s oc h i s t i ct h e or i s t st op l a yg a me swi t h1 0 , 2 6 , ore v e n4 0 9 6di me n s i on a lp r oj e c t i v e ma n i f ol d sa tl e a s tu n t i l y oua r er e a d yt ob e c omeon eoft h e m. Th es e c on dwa yi st oi n t r odu c et h ep r op e rt i me .Th i si st h et i meme a s u r e di nt h e “ r e s tf r a me ”o fap a r t i c l ea si tmov e sa l o n gi t swo r l dl i n e .Ass u c h ,i ti ss t i l ln ota n “ a b s ol u t e ”t i mel i k ewea r eu s e dt ob u ti ti st h ec l os e s tt h a twec a nc omet oi t . Not ewe l lt h a tp r op e rt i med oe sn otr e a l l ys ol v eou rp h i l o s o p h i c a lp r ob l e ms ,b e c a u s e on emu s ts t i l l a s kh owt h e“ p a r t i c l e ”me a s u r e st i me . I fi tc a r r i e swi t hi t 3 Don ’ tt h i n kt ooh a r da b ou tt h i ss e n t e n c eory ou ’ l l s t a r tt og os l i g h t l yn u t sb e c a u s ei ti ss e l f r e f e r e n t i a l a n dh e n c eG¨ od e l i a n . al i t t l e“ c l oc k ” , t h a tc l oc kmu s th a v emov i n gp a r t sa n ds omes or tofa s s oc i a t e dp e r i od, a n d t h os ep a r t sh a v ei nt u r nt h e i rownp r op e rt i me .I fi ti sap oi n tp a r t i c l e , i t sc l oc kmu s te i t h e r b ei ni n t e r n a lde g r e e soff r e e d om–y oub e g i nt os e ewh yt h os et h e or i s t sme n t i on e da b ov e wor kt h e i rwa yu pt oh i g h e rd i me n s i on a ls p a c e s–ore l s et h ep a r t i c l ei n f e r st h ep a s s a g eof t i mef r om wh a ti t“ s e e s ”oft h er e s toft h eUn i v e r s ev i ai t si n t e r a c t i onc on n e c t i on sa n d doe s n ’ tr e a l l yh a v eap r op e rt i mea ta l l b e c a u s ei tc a n n oth a v ei t sownp r op e rc l oc k . I td oe s ,h owe v e r ,s ol v eou ri mme d i a t ema t h e ma t i c a lp r ob l e m( t h a toff i n di n ga s u i t a b l ep a r a me t e ri nt e r msofwh i c ht ode s c r i b et h ee v o l u t i onofas y s t e m)s owe ’ l l g o wi t hi ta n y wa y . 17. 4 Pr o p e rTi mea n dTi meDi l a t i on Su p p os eweh a v eap a r t i c l emov i n gwi t hav e l oc i t yvi nag i v e nc oor d i n a t es y s t e mK.I n at i med t( i nt h a ts y s t e m)i tmov e sd x=v d t . Th e ni t si n v a r i a n ti n f i n i t e s i ma l i n t e r v a l i s 2 2 2 22 2 ( d s )=( c dt )−| dx |=cd t( 1−β) . ( 1 7 . 5 9 ) ′ I nt h ep a r t i c u l a rf r a mewh e r et h ep a r t i c l ei sa tr e s t( d x=0 )wed e f i n et h ep r op e rt i me t ob e ′ d τ=d t s ot h a t 2 2 ( 1 7 . 6 0 ) 2 ( ds )=c( dτ). ( 1 7 . 6 1 ) Th u st h ep r op e rt i mei sj u s tt h et i mee x p e r i e n c e db yt h ep a r t i c l ei ni t sownr e s tf r a me . F r omt h er e l a t i on sa b ov e , i ti se a s yt os e et h a t d t γ ( t ) 2 d τ=d t1−β( t )= ( 1 7 . 6 2 ) a n dt of i n dt h ei n t e r v a lb e t we e nt woe v e n t sons omewor l dl i n ei ti sn e c e s s a r yt o i n t e g r a t e : τ 2 t 2−t 1= d τ 2 1−β( τ ) τ 1 τ 2 = γ ( τ) d τ . ( 1 7 . 6 3 ) τ 1 I fβi sc on s t a n t( s ot h ef r a me sa r ei n e r t i a l )t h e nweg e tt h eu s u a l t i med i l a t i on t=γ τ or ( 1 7 . 6 4 ) t τ= γ ( 1 7 . 6 5 ) ′ ′′ ′ ′ ′ F i g u r e1 7 . 2 :Ph a su=( u, θ, φ )i nKf r a me .Ki smov i n gi nt h e1d i r e c t i ona tv=c β. γ ( v )c h a n g e sf r a me s . Wewa n tu ( u , θ , φ) . b u tt h i si sn ott r u ei ft h ep a r t i c l ei sa c c e l e r a t i n g .Ap p l y i n gi twi t h ou tt h ou g h tl e a dst ot h e “ t wi np a r a dox ” .Howe v e r , t h ef u l l i n t e g r a l r e l a t i on swi l l b ev a l i de v e ni ft h et wop a r t i c l e sa r e a c c e l e r a t i n g( s ot h a tβ( τ) ) .Youwi l ln e e dt oe v a l u a t et h e s er e l a t i on st os ol v et h et wi n p a r a d oxf oron eofy ou rh ome wor kp r ob l e ms . F i n a l l y ,Iwa n tt on ot e( wi t h ou td i s c u s s i n gi tf u r t h e ra tt h i st i me )t h a tp r op e rt i me d i l a t i onl e a d st oar e l a t i v i s t i cc or r e c t i ont ot h eu s u a l d op p l e rs h i f t .Ors h ou l dIs a yt h a t t h en on –r e l a t i v i s t i cdop p l e rs h i f ti sj u s tal owv e l oc i t yl i mi toft h ec or r e c t , t i med i l a t e d r e s u l t . Nowt h a tweh a v es omen ot i onofwh a ta ni n f i n i t e s i ma lt i mei n t e r v a li s ,wec ou l dg o a h e a da n dt r yt ode f i n c e4 –d i me n s i on a lg e n e r a l i z a t i on sofmome n t u ma n de n e r g y .F i r s t , h owe v e r , wewi l l l e a r nh owv e l o c i t i e sL or e n t zt r a n s f or m. 17. 5 Ad d i t i o no fVe l o c i t i e s I fwef or mt h ei n f i n i t e s i ma l v e r s i onoft h eL or e n t zt r a n s f or ma t i onofc oor d i n a t e s : ′ ′ ′ ′ d x 0 ( d x dx 0+β 1) =γ ( 1 7 . 6 6 ) d x 1 ( d x dx =γ 1+β 0) ( 1 7 . 6 7 ) d x 2 d x 3 ′ ′ x =d 2 ′ x 3 =d ′ Poi n tPi smov i n ga tv e l oc i t yui nf r a meK, wh i c hi si nt u r nmov i n ga t v e l oc i t yv=v 1wi t hr e s p e c tt ot h e“ r e s t ”f r a meK. Wen e e dt od e t e r mi n eu ˆ ( 1 7 . 6 8 ) ( 1 7 . 6 9 ) ( t h ev e l oc i t yofPi nK) . Wewi l l e x p r e s st h ep r ob l e m, a su s u a l , i nc oor d i n a t e sa n d⊥t o t h ed i r e c t i onofmot i on , e x p l oi t i n gt h eob v i ou sa z i mu t h a l s y mme t r y oft h et r a n s f or ma t i ona b ou tt h e1di r e c t i on . Not et h a t ˆ d x i u i=c d x 0 ( 1 7 . 7 0 ) f ori =0...3.Th e n ′ ′ 0 1 γ ( d x dx 1+β 0) ′ ′ u = cγ( d x +βdx) ′ dx 1 =c +β ′ dx 0 1+β dx ′ u 1+ ( 1 7 . 7 1 ) v. 2 · c Si mi l a r l y , u e . g . —u )i sg i v e nb y ⊥( 2 u 2 1 ′ d x 0 u +v = ′ ′ cdx2 = ′ ′ γ ( dx dx 0+β 1) ′ u2 = ′ γ ( 1+ u · v ( 1 7 . 7 2 ) c 2 or u ⊥= u ⊥ γ1+ ′ u c v · . 2 Wes e e , t h e n , t h a tt h ev e l oc i t yc h a n g e si nb o t ht h eNot e ( 1 7 . 7 3 ) a n dt h e⊥di r e c t i on s . ′ a l s ot h a ti f| u| a n d| v | < <c , t h e n ′ u·v < <1 c2 a n d s ot h a twer e c ov e rt h eGa l l i l e a nr e s u l t , ( 1 7 . 7 4 ) ( 1 7 . 7 5 ) γ≈1 ( 1 7 . 7 6 ) ′ u = u+v u ⊥ ′ ⊥. = u ( 1 7 . 7 7 ) ′ Wh a ta b ou tt h eot h e rl i mi t ?I f| u| =c , t h e n | u | =c ( 1 7 . 7 8 ) a sy ous h ou l dv e r i f yony ou rown .Th i si sE i n s t e i n ’ ss e c o n dp o s t u l a t e !Weh a v et h u s p r ov e ne x p l i c i t l yt h a tt h es p e e dofl i g h t( a n dt h es p e e dofa n y t h i n ge l s et r a v e l l i n ga tt h e s p e e dofl i g h t )i si n v a r i a n tu n d e rL o r e n t zc oor d i n a t et r a n s f or ma t i on s .Th i si st h e i r e n t i r emot i v a t i on . F i g u r e1 7 . 3 :Not et h a tγ ot h a te a c hc omp on e n toft h e4 –v e l oc i t yi sa l wa y s u≥1s “ l a r g e r ”t h a na s s oc i a t e dCa r t e s i a nc omp on e n t s ,e v e nt h ou g h( a su s u a l )t h el e n g t hof t h ef ou rv e l oc i t yi si n v a r i a n t . Wh a ti si t si n v a r i a n tl e n g t h ? Weob s e r v et h a tt h et h r e es p a t i a lc omp on e n t sof“ v e l oc i t y ”d on o ts e e mt o t r a n s f or ml i k eaf ou rv e c t or .Bot ht h ea n dt h e⊥c omp on e n t sa r emi x e db yab oos t .We c a n , h owe v e r , ma k et h ev e l oc i t yi n t oaf ou rv e c t ort h a td oe s . Wed e f i n e U0 U = dx0 d τ =c γ ( u ) dx 0d t = d tdτ dx dx d t = d τ d tdτ =u γ ( u ) ( 1 7 . 7 9 ) = ( 1 7 . 8 0 ) wh e r eγ ( u )i se v a l u a t e du s i n gt h ema g n i t u deofu .I ti sa ne x e r c i s et os h owt h a tt h i s t r a n s f or msl i k et h ec oor d i n a t e4 –v e c t orx . Nowwec a n“ g u e s s ”t h a tt h e4 –mome n t u mofap a r t i c l ewi l lb e∼mU.Top r e p a r e u sf ort h i s , ob s e r v et h a t U=( U0, U)=( γ c , γ u ) u u ( 1 7 . 8 1 ) a r ej u s tt h eγ –s c a l e d“ v e l oc i t i e s ”oft h ep a r t i c l e : u 17. 6 Re l a t i v i s t i cE n e r gya n dMome n t u m Wes e e kar e l a t i v i s t i cg e n e r a l i z a t i onofmo me n t u m( av e c t orqu a n t i t y )a n de n e r g y .We k n owt h a ti nt h el ows p e e dl i mi t , v< <c , p=mu ( 1 7 . 8 2 ) 1 2 mu 2 E= E( 0 ) + ( 1 7 . 8 3 ) wh e r eE ( 0 )i sac on s t a n ta l l owe db yNe wt on ’ sl a ws( s i n c ef or c e sde p e n don l yone n e r g y d i ffe r e n c e s ) . Th eon l yp os s i b l ef or mf ort h i sg e n e r a l i z a t i onoft h e s ee qu a t i on sc on s i s t e n twi t h ou rr e qu i r e me n tt h a tt h el a wsofn a t u r er e ma i ni n v a r i a n ta r e : p=M( u ) u ( 1 7 . 8 4 ) E=E ( u ) , ( 1 7 . 8 5 ) t h a ti s ,t h ema s sa n dt h ee n e r g ymu s tb e c omef u n c t i on soft h es p e e don l y ,a n dl e a v e t h ev e c t orc h a r a c t e roft h ev e l oc i t ya l on e .Ab oos tc a n n otc h a n g et h ed i r e c t i onoft h e mome n t u mofap a r t i c l e ,a n da n y( s c a l a r )f u n c t i on a lv a r i a t i oni ni t sma g n i t u dec a nb e t h r owni n t ot h e“ ma s s ”t e r m. Th i si mme d i a t e l yy i e l d st h el i mi t i n gf or ms : M( 0 )=m ∂E ( 1 7 . 8 6 ) m 2 ∂u ( 0 )=2 ( 1 7 . 8 7 ) wh e r eweh a v ea s s u me dt h a tt h e r ei sn op a t h ol og yi nt h ef u n c t i on sa tt h eor i g i n . Th e r ea r es e v e r a lp os s i b l ewa y st oe v a l u a t et h ef u l lf or msoft h e s ef u n c t i on s . J a c k s on ’ s( b a s e dons c a t t e r i n gt h e or y )i st e d i ou sa n dc on c e a l st h es t r u c t u r eoft h e r e s u l t .F u r t h e r mor e , a f t e rt e l l i n gu st h a ts e l e c t i n gc l e v e ri n i t i a l d i r e c t i on swi t ha ne y et o s i mp l i f y i n gt h ea l g e b r a“ l a c k smot i v a t i on ”h ede r i v e sar e s u l tb ys e l e c t i n gp a r t i c u l a r i n i t i a l di r e c t i on s . Th eg u yl ov e sa l g e b r a , wh a tc a nI s a y . F e e l f r e et os t u d yh i sa p p r oa c h . I twor k s . I , ont h eot h e rh a n d , a mt ool a z yt os p e n dmos tofap e r i odd e r i v i n gar e s u l tt h a ti s “ ob v i ou s ”i nt h ec or r e c tn ot a t i on .Ia mt h e r e f or eg oi n gt o“ g i v e ”y out h er e s u l ta n d mot i v a t ei t ,a n dt h e nv e r i f yi tt r i v i a l l yb ee x p r e s s i n gi ta saf ou r –v e c t or .Th i swor k s n e a r l ya swe l l a n di sn ota n y wh e r en e a ra sp a i n f u l . Web e g i nb yc on s i d e r i n ge l a s t i cs c a t t e r i n gt h e or y .Ane l a s t i cc ol l i s i onoft wo i d e n t i c a lp a r t i c l e smu s tc on s e r v emome n t u ma n de n e r g yi na l li n e r t i a lf r a me s .I nt h e ′ c e n t e rofma s sf r a me( wh i c hwewi l l c on s i de rt ob eK) ′ ′ ′ ′ ′ ′ ′ ′ p i a+p i b=p fa+p fb ( 1 7 . 8 8 ) E i a+E i b=E fa+E fb ( 1 7 . 8 9 ) r e l a t et h ei n t i a l a n df i n a l mome n t aa n de n e r g yoft h et woi d e n t i c a l p a r t i c l e s . Now, ′ ′ u u i a=v=− i b ( 1 7 . 9 0 ) ′ ◦ 2 1 F i g u r e1 7 . 4 :θ =3 0a n dβ = 3.Th eda s h e dl i n e sa r et h er e s u l t sofaGa l l i l e a n ′ t r a n s f or ma t i onf r omK t oK.Not et h a tt h es c a t t e r i n gi smor ef or wa r dt h a ne x p e c t e d b e c a u s eo ft h eL or e n t zc on t r a c t i onoft h el on g i t u d i n a l d i s t a n c e ss e e nb yt h ep a r t i c l e s . a n d ′ ′ u fa=v=u fb ( 1 7 . 9 1 ) b yd e f i n i t i oni nt h ec e n t e rofma s ss y s t e m. Amome n t squ i e tr e f l e c t i on( e g a d , a n ot h e rp u n ! )s h ou l dc on v i n c ey out h a ti nt e r ms oft h eg e n e r a l t r a n s f or ma t i on : ′′ ′′ M( v ) v−M( v ) v=M( v) v−M( v) v ′ ( 1 7 . 9 2 ) ′ E ( v )+E ( v )=E ( v)+E ( v) . ( 1 7 . 9 3 ) F orwh a ti ti swor t h , i ft h ec ol l i s i oni se l a s t i ca n dt h ep a r t i c l e sa r ei d e n t i c a lb e f or ea n d ′ a f t e rt h ec ol l i s i on ,v=va n da l lt h ema s st e r msa r et h es a me .Wewi l ld e n ot et h e ′ ′ s c a t t e r i n ga n g l ei nKa sθ. Wet h u sb e g i nwi t h M( v ) v−M( v ) v=M( v ) v−M( v ) v ( 1 7 . 9 4 ) E ( v )+E ( v )=E ( v )+E ( v ) ( 1 7 . 9 5 ) wh e r evi st h es p e e do ft h ei n c omi n ga n dou t g oi n gp a r t i c l e s .Now,M( v )mu s tb ea s c a l a rf u n c t i onofv , a n di nt h el i mi tv→ 0mu s tt u r ni n t o l i mM( v )=m. v →0 ( 1 7 . 9 6 ) Th eon l ys c a l a rf u n c t i onofvweh a v ee n c ou n t e r e ds of a rwi t ht h i sb e h a v i ori s γ ( v ) , s owes h ou l ds e n s i b l yg u e s s M( v )=γ ( v ) m ( 1 7 . 9 7 ) wh i c hh a st h ee x a c t l yc or r e c tl i mi t i n gb e h a v i or . Th u s p=γ mu ( 1 7 . 9 8 ) i sar e a s on a b l eg u e s st ob et h eg e n e r a l i z a t i onofmome n t u m wes e e k .I ti se a s yt o v e r i f yt h a tt h i si sac on s i s t e n tc h oi c e ,a n dt h a ti ti n de e dr e s u l t si nc on s e r v a t i onof mome n t u mi na l l i n e r t i a l f r a me s . Tog e tt h ee n e r g ye qu a t i on ,weu s et h es a mea p p r oa c h .Re c a l lt h a tab i n omi a l e x p a n s i onofγi sg i v e nb y l i mγ ( v )= v → 1− 2 c 0 2 v −1 / 2 2 1v ( 1 7 . 9 9 ) =1+2c .. 2+. Wen e e dt ok e e pt h ef i r s tn on –c on s t a n tt e r mb e c a u s ewer e c a l lt h a tp h y s i c si sa l wa y s “ i n d e p e n d e n t ”ofa b s ol u t ee n e r g ys c a l e . Th e ni ts h ou l db ec l e a rt h a t l i m v 0E → ( v )=γ ( v ) E 1 ( 0 ) ≈E ( 0 )+ 2E 2 v ( 0 )2 c ≈E 1 2 mv ( 0 )+ ( 1 7 . 1 0 0 ) 2 a si tmu s ti nor d e rt oy i e l dt h el owv e l oc i t yl i mi tofk i n e t i ce n e r g yi fa n don l yi f 2 E ( 0 )=mc. ( 1 7 . 1 0 1 ) Th e r ea r es e v e r a lqu e s t i on st ob ea n s we r e da tt h i sp oi n t ,s omee x p e r i me n t a l l ya n d s omet h e or e t i c a l l y .Wen e e dt ome a s u r et h er e s tma s s e sa n dt h e or e t i c a l l yv e r i f yt h a ton l y t h i st r a n s f or ma t i onc or r e c t l yp r e s e r v e st h ee n e r g ymome n t u mc on s e r v a t i onl a wsi ne l a s t i c c ol l i s i on sa sr e qu i r e d.Be y on dt h a t ,t h e r ea r es t i l ls omeu n c e r t a i n t i e s .F ore x a mp l e ,t h e r e c ou l di np r i n c i p a lb ea na d d i t i on a lc on s t a n te n e r g ya d d e dt ot h ee n e r g yt e r mt h a twa sn ot s c a l e db yγa n dt h el a wsofp h y s i c swou l ds t i l lb ee x p r e s s i b l e , s i n c et h e ya r en ots e n s i t i v e t oa b s ol u t ee n e r g ys c a l e .Wewi l lt a k ea d v a n t a g eoft h a tf r e e domi ns e v e r a li n s t a n c e st o a d dors u b t r a c ta ni n f i n i t et h e or e t i c a lc on s t a n ti nor d e rt oma k et h er e s tma s sc omeou tt o t h eob s e r v e de x p e r i me n t a l ma s sm. Th i si sc a l l e dr e n or ma l i z a t i on . Toob t a i nt h es a mer e s u l tad i ffe r e n twa y , wet u r nt ot h en ot a t i onof4 –v e c t or s .We ob s e r v et h a tt h ec ommonf a c t orofγa b ov ei nb ot hEa n dpa l s ooc c u r swh e non e ma k e sv e l oc i t yi n t oaf ou rv e c t or .Th i ss u g g e s t st h a te n e r g ya n dmome n t u mc a n s i mi l a r l yb ema d ei n t of ou rv e c t or st h a tt r a n s f or ml i k et h ec oor di n a t e su n d e rab oos t .I f wet r yt h ec omb i n a t i on p0 E U0= c =mc ( 1 7 . 1 0 2 ) p =mU ( 1 7 . 1 0 3 ) wes e et h a ti twor k se x a c t l y . I tr e s u l t si na ni n v a r i a n t 2 2 p p=( mc). 0 −p· ( 1 7 . 1 0 4 ) I ti se a s yt os e et h ev a l u eoft h ei n v a r i a n twh e nv=0 ;y ous h ou l dv e r i f ye x p l i c i t l yt h a ti t h ol d swh e nv=0a swe l l .Pr a c t i c a l l ys p e a k i n g ,i ts u ffic e st os h owt h a tt h i sl e n g t hi s i n v a r i a n twh e non ewi s h e st os h owt h a ti t sc omp on e n t st r a n s f or ml i k et h ec oor d i n a t e s u n d e rt h ea c t i onofab oos t( wh yi st h a t ? ) . Th et ot a l e n e r g yc a nt h u sb ee x p r e s s e di nt e r msoft h et h r e emome n t u ma s 22 24 E= cp +m c. ( 1 7 . 1 0 5 ) F i n a l l y , i ti ss ome t i me sc on v e n i e n tt ob ea b l et og e tt h ev e l oc i t yoft h ep a r t i c l ei nt e r ms ofi t se n e r g ya n dmome n t u m 2 u= cp E ( 1 7 . 1 0 6 ) wh i c hf ol l owsdi r e c t l yf r omt h ed e f i n i t i on s . Th i sc omp l e t e sou rr e v i e wof“ e l e me n t a r yr e l a t i v i t yt h e or y ” .Wes h a l ln owp r oc e e d t od e v e l opt h et h e or yi nan e w, ge o me t r i cl a n g u a g ewh i c hi ss u i t a b l et oou rmu c hmor e s op h i s t i c a t e dn e e ds .Tod ot h i s , wewi l ln e e dt ob e g i nb yg e n e r a l i z i n gt h en ot i onofa f ou rdi me n s i on a l v e c t ors p a c ewi t has e toft r a n s f or ma t i on st h a tl e a v ea na p p r op r i a t e l y d e f i n e d“ l e n g t h ”i n v a r i a n t . Ch a p t e r18 Th eL o r e n t zGr o u p 18. 1 Th eGe ome t r yofSp a c e –Ti me Re c a l l t h a tag r e a td e a l ofs i mp l i f i c a t i onoft h ek i n e ma t i c sofc l a s s i c a l n on –r e l a t i v i s t i c me c h a n i c soc c u r swh e non ec on s i d e r st h egr ou ps t r u c t u r eoft r a n s f or ma t i on swi t h r e s p e c tt ot h eu n d e r l y i n gc oor d i n a t e s .Sp e c i f i c a l l y ,t h eg r ou p ofi n v e r s i on s , t r a n s l a t i o n sa n dr o t a t i o n sofag i v e nc oor d i n a t es y s t e ml e a v et h en o r m( l e n g t h )ofa g i v e nv e c t ori n v a r i a n t .Th e s et r a n s f or ma t i on sf or mt h eE u c l i d e a ng r ou pi nt h r e e d i me n s i on s , E3. F ort h os eofy ouwh ol e dde p r i v e dc h i l d h ood s , agr o u pGi sas e tofma t h e ma t i c a l ob j e c t s( a , b , c. . . )wi t har u l eofc omp os i t i on , org r ou pp r od u c t , ( a◦b )s u c ht h a t : a )E v e r yp r od u c tofap a i rofe l e me n t si nt h eg r ou pi sa l s oi nt h eg r ou p . Th a ti s , i fa , b∈Gt h e nc=a◦b∈G) . Th i sp r op e r t yi sc a l l e dc l o s u r e . b )Th eg r ou pmu s tc on t a i nas p e c i a l e l e me n tc a l l e dt h ei d e n t i t yI ∈Gs u c ht h a ta◦I =af ora l l a∈G. −1 c )E v e r ye l e me n toft h eg r ou pGmu s th a v ea ni n v e r s e , a l s oi nG. I fa∈Gt h e n∃a −1 ∈Gs u c ht h a ta◦a =I . d )Th eg r ou pp r od u c tmu s tb ea s s oc i a t i v e . Th a ti s , a◦( b◦c )=( a◦b )◦c , ∀a , b , c∈ G. 1 I ft h eg r ou pp r od u c tc o mmu t e s( a◦b=b◦a )t h eg r ou pi ss a i dt ob eAb e l i a n ot h e r wi s e t h eg r ou pi ss a i dt ob en on –Ab e l i a n , wh i c hi ss e n s i b l ee n ou g h .AL i eg r ou pi sac o n t i n u o u s 2 g r ou p s u c ha st h eg r ou p ofi n f i n i t e s i ma lt r a n s f or ma t i on s .I tn e c e s s a r i l yh a sa n u n c o u n t a b l ei n f i n i t yofe l e me n t s .Th e r ea r ea l s od i s c r e t e( b u tc ou n t a b l yi n f i n i t e )g r ou p s , f i n i t eg r ou p s ,a n de v e r y t h i n gi nb e t we e n .Th e r ea r ea l s o“ s e mi –g r ou p s ”( wh i c hd on ot ,f or e x a mp l e , c on t a i n 1Wi k i p e d i a : h t t p : / / www. wi k i p e d i a . or g / wi k i / Ab e l i a ng r ou p . ; 2Wi k i p e di a : h t t p : / / www. wi k i pe di a . or g / wi k i / L i eg r ou p . , 2 4 7 a ni n v e r s e ) .F i n a l l y ,on ec a nc on s t r u c t“ n on –a s s oc i a t i v e ”s t r u c t u r e sl i k eg r ou p sf r om n on –a s s oc i a t i v ea l g e b r a sl i k et h eoc t on i on s .Mu l t i p l i c a t i onov e rt h er e a l sf or msa c on t i n u ou sAb e l i a ng r ou p .Rot a t i on sf or man on –Ab e l i a nL i eg r ou p .Mu l t i p l i c a t i onov e r r a t i on a ln u mb e r sf or msac ou n t a b l yi n f i n i t eg r ou p .Th es e tofr ot a t i on sa n di n v e r s i on s t h a tl e a v eas qu a r ei n v a r i a n tf or maf i n i t e( p oi n t )g r ou p .Th e“ r e n or ma l i z a t i ong r ou p ” y ouwi l l h e a rmu c ha b ou tov e rt h ey e a r si sn otag r ou pb u tas e mi –g r ou p—i tl a c k sa n i n v e r s e . Howe v e r , ou rp u r p o s eh e r ei sn ot , h owe v e r , t os t u d yg r ou pt h e or yp e rs e .On ec ou l d s t u d yg r ou pt h e or yf orf ou ry e a r ss t r a i g h ta n ds t i l lon l ys c r a t c ht h es u r f a c e .I ti s s ome wh a ts u r p r i s i n gt h a t ,g i v e nt h ei mp or t a n c eofg r ou pt h e or yi np h y s i c s ,wed on ’ t offe ras i n g l ec ou r s ei ni t , b u tt h e na g a i n , i t ’ sn ott h a ts u r p r i s i n g . . . Wi t ht h a ti nmi n d , wec a nd e c i dewh a twea r el ook i n gf or .Wes e e ki n i t i a l l yt h es e t oft r a n s f or ma t i on si nf ou rdi me n s i on st h a twi l l l e a v e 2 2 s =x x·x ) 0 −( ( 1 8 . 1 ) i n v a r i a n tf oras i n g l ee v e n txwi t hr e s p e c tt oap a r t i c u l a rc oor di n a t eor i g i n .Th e s e t r a n s f or ma t i on sf or mag r ou pc a l l e dt h eh o mo ge n e o u sL or e n t zgr ou p .I tc on s i s t sof or d i n a r yr ot a t i on si nt h es p a t i a lp a r t , t h eL or e n t zt r a n s f or ma t i on sweh a v ej u s tl e a r n e d t h a tmi xs p a c ea n dt i me ,a n ds e v e r a ld i s c r e t et r a n s f or ma t i on ss u c ha ss p a c e i n v e r s i on ( s )a n dt i mei n v e r s i on . Th es e toft r a n s f or ma t i on st h a tl e a v et h equ a n t i t y 2 2 2 2 2 s( x , y )=( x )−( x )+( x )+( x )) 0−y 0 1−y 1 2−y 2 3−y 3 ( 1 8 . 2 ) 3 i n v a r i a n tf or mt h ei n h omo ge n e ou sL o r e n t z orPoi n c a r ´ egr o u p .I tc on s i s t soft h e h omog e n e ou sg r ou p( i n c l u d i n gt h e“ i mp r op e r ”t r a n s f or ma t i on st h a ti n c l u de s p a t i a l r e f l e c t i ona n dt i mer e v e r s a l )a n du n i f or mt r a n s l a t i on soft h eor i gi n .I fa n y on ec a r e s ,t h e L or e n t zg r ou pi st h eg e n e r a l i z e dor t h og on a lg r ou pO( 1 , 3 ) .Th ep r op e rs u b g r ou poft h e L or e n t zg r ou p( t h eon et h a ti ss i mp l yc on n e c t e ds p a t i a l l y( n ooddi n v e r s i on s )a n dc on t a i n s t h ei de n t i t y )i sSO( 1 , 3 )t h es p e c i a lor t h og on a lg r ou p .I ft i me ’ sdi r e c t i oni sa l s op r e s e r v e d + wea dda+ , SO ( 1 , 3 ) .Th i sn ome n c l a t u r ei sde f i n e dh e r ef ory ou rc on v e n i e n c eb u tofc ou r s e t h ewi k i n ot er e f e r e n c ec on t a i n sa c t i v el i n k st oal otoft h i si nde t a i l . Wewi l ld e f i n es ( x , y )t ob et h en or mofr e l a t i v i s t i cs p a c e –t i me .Th i squ a n t i t yma y b ec on s i d e r e dt ob et h ei n v a r i a n t“ d i s t a n c e ”( s qu a r e d )b e t we e nt woe v e n t s ,xa n dy , a n dofc ou r s ei son eoft h ef u n da me n t a lob j e c t sa s s oc i a t e dwi t ht h ec on s t r u c t i onof d i ffe r e n t i a l s .Si n c equ a n t i t i e st h a ta r eu n c h a n g e db yag e ome t r i ct r a n s f or ma t i ona r e c a l l e ds c a l a r si ti se v i d e n tt h a ts ( x ,y )i sa4 –s c a l a r .Si n c et h ef i r s tp os t u l a t es t a t e s t h a tt h el a wsofp h y s i c smu s tb ei n v a r i a n tu n d e rh omog e n e ou s( a tl e a s t )L or e n t z t r a n s f or ma t i on s ,t h e ymu s tu l t i ma t e l yb eb a s e donL or e n t zs c a l a r s .I n d e e d ,t h e L a g r a n g i a nd e n s i t i e su p onwh i c hf i e l dt h e or i e sa r eb a s e da r eg e n e r a l l yc on s t r u c t e dt o b eL or e n t zs c a l a r s . Th i si sas t r on gc on s t r a i n tona l l owe dt h e or i e s . 3Wi k i p e d i a : h t t p : / / www. wi k i p e di a . or g / wi k i / L or e n t zg r ou p . , Th e s es c a l a r sa r e ,h owe v e r ,f or me dou tof4 –v e c t or s( a swes e ea b ov e )or ,mor e g e n e r a l l y ,t h ec on t r a c t i onof4 –t e n s or s .Wemu s t ,t h e r e f or e ,d e t e r mi n et h eg e n e r a l t r a n s f or ma t i onp r op e r t i e sofat e n s orofa r b i t r a r yr a n kt oc omp l e t e l yd e t e r mi n eat h e or y . I nt h ep a r toft h i sb ookd e v ot e dt oma t h e ma t i c a lp h y s i c si sa ne n t i r ec h a p t e rt h a t d i s c u s s e st e n s or s ,i np a r t i c u l a rt h ed e f i n i t i on sofc ov a r i a n ta n dc on t r a v a r i a n tt e n s or s , h owt oc on t r a c t( E i n s t e i ns u m)p a i r soft e n s or st of or mt e n s or sofl owe rr a n k , a n dt h e r ol eoft h eme t r i ct e n s ori nde f i n i n gt e n s orc oor di n a t ef r a me sa n dt r a n s f or ma t i on s t h e r e u p on .Wewi l ln o tr e p e a tt h i sr e v i e wori n t r od u c t i on( d e p e n di n gont h es t u d e n t ) a n du r g es t u d e n t st oa tt h i st i mes p e n da nh ou rors owor k i n gt h r ou g ht h i sc h a p t e r b e f or ec on t i n u i n g( e v e ni fy ou ’ v es e e ni tb e f or e ) . 18. 2 Te n s or si n4Di me n s i o n s L e tu sn owc on s i d e rt h es p e c i f i cn a t u r eoft e n s or sonf ou r d i me n s i on a ls p a c e t i me . 4 Te n s or sofr a n kk a r ec a t e g or i z e d( f o re a c hc oor d i n a t ei n de x )b yt h e i rt r a n s f or ma t i on ′ p r op e r t i e sr e l a t i v et oat r a n s f or ma t i onoft h eu n d e r l y i n gc oor d i n a t es y s t e mx→ xa s de f i n e da b ov e . Th i st r a n s f or ma t i oni si mp l i c i ti na l l t h ed i s c u s s i onb e l ow. As c a l a r( t e n s orofr a n kz e r o)i su n c h a n g e db ys u c hat r a n s f or ma t i on .Th i si sn ota t r i v i a ls t a t e me n t !I ti st r i v i a lf ors c a l a rn u mb e r sl i k eπ,n od ou b t ,b u ti np h y s i c st h e i n t e r e s t i n gp a r toft h i sr e qu i r e me n toc c u r swh e nd i s c u s s i n gt h es c a l a r st h a tr e s u l t a l ge b r a i c a l l yf r omf u l l yc on t r a c t i n gp r od u c t soft e n s or sov e ra l loft h e i ri n di c e su s i n gt h e me t r i ct e n s or . Th i swi l l b ema d equ i t ec l e a rb e l ow. F orav e c t or( t e n s orofr a n kon e )weh a v et wop os s i b i l i t i e s . E i t h e ri tt r a n s f or msl i k e t h ec oor d i n a t ei t s e l fa n dweh a v ea 0 1 2 3 c on t r a v a r i a n tv e c t o r( A, A, A, A)s u c ht h a t α A= α ∂x¯ Aβ β ∂x ( 1 8 . 3 ) ( n ot i n gt h a ta l lt h ei n d i c e sa r eont o p ,a l on gwi t ht h en e wp r i me dc oor di n a t e ) .Th i s ma k e st h e di ffe r e n t i a lt r a n s f or ma t i on r e l a t i on s h i pt ot h eu n d e r l y i n g or d i n a r y ( c on t r a v a r i a n t )c oor d i n a t e se x p l i c i ta n di sob v i ou s l ya ni d e n t i t yf ort h os ec oor d i n a t e s . Al t e r n a t i v e l y , weh a v ea c ov a r i a n tv e c t o r( B0, B1, B2, B3)s u c ht h a t β ∂x Bα= Bβ α ∂ x ¯ ( 1 8 . 4 ) 4 Th er a n kofat e n s ori sd e t e r mi n e db yt h en u mb e rofi n di c e si th a s .Sc a l a r sa r e0 t hr a n k , v e c t or sa r e1 s tr a n k , 2 Dma t r i c e sa r e2 n dr a n k , a n dou rol df r i e n dǫi sat h i r dr a n kf u l l ya n t i s y mme t r i ct e n s or . j ki ( wi t ht h ec oor d i n a t ei n d i c e sont opa n dt h en e wp r i me dc oor d i n a t eont h eb ot t om) .Ag a i n , n ot et h a tt h i si sp r e c i s e l ywh a twee x p e c t–t h et r a n s f or ma t i oni si nt h eo p p os i t es e n s eof t h a toft h eu n d e r l y i n gc oor d i n a t e s .Wen e e di nb ot hc a s e s ,ofc ou r s e ,t of i g u r eou tt h e ∂ xβ ma t r i xofe . g . ∂x α e x p l i c i t l y . I namome n twewi l l s e ee x p l i c i t l ywh a te x a c t l yt h ed i ffe r e n c ei sb e t we e nt h e s et wo t y p e soff i r s tr a n kt e n s or s . F i r s t , h owe v e r , wes h ou l dn ot et h a t c on t r a v a r i a n tt e n s or sofr a n k2t r a n s f or ml i k e α β F = α β ∂x¯ ∂x¯ γ δ γ δF . ∂x ∂ x ( 1 8 . 5 ) Si mi l a r l y , weh a v e c ov a r i a n tt e n s o r so fr a n k2 γ δ ∂x ∂ x α β Gαβ= ∂x¯ ∂x¯ Gγδ ( 1 8 . 6 ) a n d mi x e dt e n s o r sofr a n k2 α δ α γ Hβ = ∂x¯ ∂x H . γ β ∂x ∂x ¯ ( 1 8 . 7 ) δ I ti sc l e a r l yat r i v i a le x e r c i s et od e t e r mi n et h ec o/ c on t r av a r i a n tt r a n s f or ma t i on p r op e r t i e sofh i g h e rr a n kt e n s or s .Wec a nf or mh i g h e rr a n kt e n s or sb yme a n sofa n ou t e r( d y a d i c )p r od u c t ,wh e r ewes i mp l yt a k et wot e n s or sofs omer a n ka n dmu l t i p l y t h e mou tc omp on e n t wi s e , p r e s e r v i n gp r od u c t sofa n yu n de r l y i n gb a s i sv e c t or sa st h e y oc c u r . F ore x a mp l ewec a nc on s t r u c tas e c on dr a n kt e n s orb y : α β αβ F =AB ( 1 8 . 8 ) wh e r eαa n dβr u nov e rt h ef u l lr a n g eofi n d e xv a l u e s .Not ewe l lt h a tt h i sd e f i n e sa s q u a r ema t r i xi nt h i sc a s eofb a s i sv e c t ordy a d sa sob j e c t ss u c ha sx ˆ x ˆ ,x ˆ y ˆ ,. . . oc c u r . On ei mp or t a n tqu e s t i oni swh e t h e ra l le . g .s e c on dr a n kt e n s or sc a nb ewr i t t e na s p r od u c t soff i r s tr a n kt e n s or s .I ti sn ott h eg e n e r a l c a s et h a tt h i si sp os s i b l e , b u ti nma n yof ou ru s e soft h e s ei d e a si np h y s i c si twi l lb e .I nt h i sc a s et h eg e n e r a l i z e dp r od u c tf or msa d i v i s i o na l ge b r awh e r ewec a nf a c t o re . g .s e c on dr a n kt e n s or si n t of i r s tr a n kt e n s or si n v a r i ou swa y s .Di v i s i ona l g e b r a sa r ed i s c u s s e di nt h eMa t h e ma t i c a l Ph y s i c ss e c t i ona swe l l , a n di n t e r e s t e ds t u de n t ss h ou l dr e t u r nt h e r et or e a da b ou tge o me t r i ca l ge b r a s , t h er e s u l tof f u l l yg e n e r a l i z i n gt h en ot i onofc omp l e xn u mb e r st oc omp l e xs p a c e sofa r b i t r a r yd i me n s i on wh i l ep r e s e r v i n gt h ef a c t or i z a b i l i t yoft h ea l g e b r a i cob j e c t s . I na d d i t i ont oe x t e n di n gt h er a n koft e n s orob j e c t sb yf or mi n gd y a di c , t r i a d i c , orn a d i c p r odu c t soft e n s or s , wec a nr e d u c et h er a n koft e n s or sb yme a n s ofap r oc e s sc a l l e dc o n t r a c t i o n .Ac on t r a c t i onoft wot e n s or si st h er e s u l tofs e t t i n g t wooft h ei n d i c e s( t y p i c a l l yt h e ymu s tb eac ov a r i a n t / c on t r a v a r i a n tp a i r )t ob ee qu a l a n dp e r f or mi n gt h eE i n s t e i ns u mma t i onov e rt h es h a r e dr a n g e . Th i sr e d u c e st h er a n kof t h ee x p r e s s i onb yon er e l a t i v et ot h a tofi t sc on s t i t u e n t s , h e n c et h et e r m“ c on t r a c t i on ” . Ane x p r e s s i onc a nb ec on t r a c t e dov e rs e v e r a lc omp on e n t sa tat i mewh e ndoi n g a l g e b r as os e c on dr a n kt e n s or sc a nb ec on t r a c t e dt of or ma4 s c a l a r , f ore x a mp l e , or t h i r dr a n kt e n s or sc a nb ec on t r a c t e dt of i r s t . Ou rf a mi l i a rn ot i onofmu l t i p l y i n gav e c t orb yama t r i xt op r od u c eav e c t ori np r op e r t e n s orl a n g u a g ei st of or mt h eou t e rp r od u c toft h ema t r i x( s e c on dr a n kt e n s or )a n dt h e v e c t or( f i r s tr a n kt e n s or ) , s e tt h er i g h t mos ti n di c e st ob ee qu a l a n ds u mov e rt h a ti n d e x t op r od u c et h er e s u l t i n gf i r s tr a n kt e n s or . He n c ewed e f i n eou rs c a l a rp r o d u c tt ob et h ec o n t r a c t i onofac ov a r i a n ta n d c on t r a v a r i a n tv e c t or . α B· A= BαA ( 1 8 . 9 ) Not et h a tI ’ v ei n t r od u c e das or tof“ s l o p p y ”c on v e n t i ont h a tas i n g l equ a n t i t yl i k eBorA c a nb eaf ou r v e c t ori nc on t e x t .Cl e a r l yt h ee x p r e s s i onont h er i g h ts i d ei sl e s s a mb i g u ou s ! Then: γ ′′ α ∂x ∂ x ¯ δ δ B· A = ∂x¯α Bγ ∂x A γ = ∂x δ A γ δB ∂x δ =δγδBγA δ B· A =BδA = ( 1 8 . 1 0 ) a n dt h ede s i r e di n v a r i a n c ep r op e r t yi sp r ov e d. Hmmm, t h a twa sp r e t t ye a s y ! Ma y b et h e r ei ss ome t h i n gt ot h i sn ot a t i ont h i n ga f t e ra l l ! 18. 3 Th eMe t r i cTe n s o r Th es e c t i ona b ov ei ss t i l lv e r yg e n e r i ca n dl i t t l eofi td e p e n dsonwh e t h e rt h et e n s or sa r e t h r e eorf ou rort e nd i me n s i on a l . Wen o wn e e dt oma k et h e mwor kf ort h es p e c i f i cg e ome t r y wea r ei n t e r e s t e di n , wh i c hi son ewh e r ewewi l lu l t i ma t e l yb es e e k i n gt r a n s f or ma t i on st h a t p r e s e r v et h ei n v a r i a n ti n t e r v a l : 2 02 12 22 32 ( d s )=( d x)−( d x)−( d x)−( dx) ( 1 8 . 1 1 ) a st h i si st h eon et h a td i r e c t l ye n c od e sa ni n v a r i a n ts p e e dofl i g h t . 2 2 F r om t h i sp oi n ton ,wemu s tb ec a r e f u ln ott oc on f u s ex·x=x a n dx =y ,e t c . Con t r a v a r i a n ti n d i c e ss h ou l db ec l e a rf r omc on t e x t ,a ss h o u l db ep owe r s .Tos i mp l i f yl i f e , a l g e b r a i c a l l yi n di c e sa r ea l wa y sg r e e k( 4 –v e c t or )orr oma ni t a l i c( 3 –v e c t or )wh i l ep owe r s a r es t i l l p owe r sa n dh e n c ea r eg e n e r a l l yi n t e g e r s . µ L e tu swr i t et h i si nt e r msofon l yc on t r a v a r i a n tp i e c e sd x.Th i sr e qu i r e st h a twe i n t r od u c ear e l a t i v emi n u ss i gnwh e nc on t r a c t i n gou tt h ec o mp on e n t soft h e s p a t i a lp a r to ft h ed i ffe r e n t i a lo n l y .Wec a nmos te a s i l ye n c od et h i sr e qu i r e me n ti n t oa s p e c i a l ma t r i x( t e n s or )c a l l e dt h eme t r i ct e n s ora s : 2 α β ( d s )=g xd x α βd ( 1 8 . 1 2 ) Th et e n s orgob v i ou s l ys a t i s f i e st h ef ol l owi n gp r op e r t y : g α β=g βα ( 1 8 . 1 3 ) ( t h a ti s ,i ti ss y mme t r i c )b e c a u s et h emu l t i p l i c a t i oni nt h eE i n s t e i ns u mma t i oni s or d i n a r ymu l t i p l i c a t i ona n dh e n c ec ommu t a t i v e .I ti sc a l l e dt h eme t r i ct e n s o rb e c a u s e i td e f i n e st h ewa yl e n gt hi sme a s u r e d . Att h i sp oi n ti fwewe r eg oi n gt od i s c u s sg e n e r a l r e l a t i v i t ywewou l dh a v et ol e a r nwh a t 5 ama n i f ol ds .Te c h n i c a l l y , ama n i f ol di sac oor d i n a t es y s t e mt h a tma yb ec u r v e db u twh i c h i sl oc a l l yf l a t .Byl oc a l l yf l a tIme a nv e r ys p e c i f i c a l l yt h a ton ec a nc ov e rt h ee n t i r es p a c e wi t h“ p a t c h e s ”i nt h en e i g h b or h oodofp o i n t swh e r et h ec oor di n a t es y s t e mi sl oc a l l y E u c l i d e a n( e . g .Ca r t e s i a n ) .Ane x a mp l eofac u r v e ds p a c ema n i f ol di st h es u r f a c eofa s p h e r e( t h i n kt h es u r f a c eoft h ee a r t h ) .Wh e nwel ookd owna tt h eg r ou n db e n e a t hou rf e e t , i tl ook squ i t ef l a ta n dwec a nd r a wt r i a n g l e soni tt h a ta p p e a rt oh a v ei n t e r i ora n g l e st h a t s u mt oπa n dwec a ndr a wama pof( s a y )ou rc ou n t yt h a tmor eorl e s sa c c u r a t e l ye n c ode s d i s t a n c e sont h eg r ou n di nt e r msofd i s t a n c e sme a s u r e dont h ema p .Howe v e r ,i fwet a k e t oob i gap a t c ha l l oft h i sb r e a k sd own .Th ea n g l e si nat r i a n g l es u mt os t r i c t l ymo r et h a nπ r a d i a n s .Ma p sh a v et ob ed i s t or t e da n dc h op p e di n t op i e c e st oc or r e c t l yr e p r e s e n t d i s t a n c e sont h eg r ou n da sd i s t a n c e sont h ef l a t2 d i me n s i on a lma p .Th i si sh ow a ma n i f ol dwor k s–wec a nwor kwi t hi ti nt h el o c a ln e i gh b o r h o odofa n yp oi n ta si fi ti sf l a t , b u ti fweg ot oof a rweh a v et owor kh a r d e ra n dc or r e c tf ori t sc u r v a t u r e , wh e r e“ t oof a r ”i s ob v i ou s l yd e f i n e di nt e r msoft h es c a l eofi t sc u r v a t u r ea n ds omec ommons e n s e . Ge n e r a lr e l a t i v i t yi n t r od u c e st h eh y p ot h e s i st h a tg r a v i t a t i on a lf i e l d sb e n ds p a c e t i me .Howe v e r ,t h i sb e n d i n gi sv e r y ,v e r ys l i g h tu n l e s son ei si nav e r ys t r on g g r a v i t a t i on a lf i e l d,a n dt h i sb e n d i n gp r e s e r v e sal oc a ls moot h n e s sofs p a c e t i mes o t h a ts p a c e t i me ,a l t h ou g hi ti sn ol on g e rs t r i c t l yEu c l i d e a n ,i ss t i l lama n i f ol da n dwe c a ndoa l ls or t soft r a n s f or ma t i on si nav e r yg e n e r a lwa ya sl on ga swer e s t r i c tt h e r e s u l t st oal oc a l l yf l a tp a t c h . I nou rd i s c u s s i onofs p e c i a lr e l a t i v i t ywewi l la s s u mef r omt h eb e g i n n i n gt h a tou r s p a c e –t i mei sf l a ta n dn otb e n tb ys t r on gg r a v i t a t i on a lf i e l d s .I nt h i sc a s et h eme t r i c t e n s orc a nb ee x p r e s s e di nav e r ys i mp l ef or m.Wewi l lu s et h eL or e n t zme t r i c( a s 4 0 op p os e dt ot h eMi n k ows k i me t r i ct h a tu s e sx =i c ti n s t e a dofx) . Us i n gou rde f i n i t i on s oft h eµ=0 , 1 , 2 , 3c oor d i n a t e s , gi nt h ed i ffe r e n t i a l sa b ov ei sj u s t : g , g 1 0 0=1 1 1=g 2 2=g 3 3=− ( 1 8 . 1 4 ) a n dwes e et h a ti ti sn otj u s ts y mme t r i c , i ti sd i a g on a l . Th ec on t r a v a r i a n ta n dmi x e dme t r i ct e n s or sf orf l a ts p a c e –t i mea r et h es a me( t h i s ∂x f ol l owsb yc on s i d e r i n gt h e ∂xαβc oor d i n a t et r a n s f or ma t i onma t r i c e st h a t 5Wi k i p e d i a : h t t p : / / www. wi k i p e di a . or g / wi k i / Ma n i f ol d. i d e f i n ec oa n dc on t r a v a r i a n c e ) : β α β g α β=g α =g . ( 1 8 . 1 5 ) F i n a l l y , t h ec on t r a c t i onofa n yt wome t r i ct e n s or si st h e“ i d e n t i t y ”t e n s or , γ β β α β g α γg =δ α =δ α β=δ . ( 1 8 . 1 6 ) 2 Si n c ewewa n t( d s )t ob e( t oc on t r a c tt o)as c a l a r , i ti sc l e a rt h a t : β x α α βx =g ( 1 8 . 1 7 ) x =g x β ( 1 8 . 1 8 ) α α β ort h eme t r i ct e n s orc a nb eu s e dt or a i s eorl owe ra r b i t r a r yi n d i c e s ,c on v e r t i n g c ov a r i a n ti n d i c e st oc on t r a v a r i a n ta n dv i c e –v e r s a : α βF µν Fµαν=g ( 1 8 . 1 9 ) β Th i si sa ni mp or t a n tt r i c k !Not ewe l l t h a ti nor de rt op e r f or mac on t r a c t i ont h a tr e d u c e s t h er a n koft h ee x p r e s s i onb yon e ,t h ei n d i c e sb e i n gs u mme dmu s toc c u ra sa c o/ c on t r ap a i r( i ne i t h e ror de r ) .I fb ot ha r ec ov a r i a n t , orb ot ha r ec on t r a v a r i a n t , on eor t h eot h e rmu s tb er a i s e dorl owe r e db yc on t r a c t i n gi twi t ht h eme t r i ct e n s orb e f or e c on t r a c t i n gt h eov e r a l l p a i r ! Weu s et h i sr e p e a t e d l yi nt h ea l g e b r ai ns e c t i on sb e l ow. F i n a l l ywea r ei nap os i t i ont os e eh owc ov a r i a n ta n dc on t r a v a r i a n tv e c t or sd i ffe r ( i nt h i sme t r i c ) .Weh a v ea l r e a d ys e e nt h a t“ or di n a r y ”v e c t or smu s tl i n e a r l yt r a n s f or m 0 1 2 3 l i k ec on t r a v a r i a n tv e c t or s .Gi v e nac on t r a v a r i a n tv e c t or( A,A,A,A)wet h u ss e e t h a t 0 1 2 3 A0=A, A1=− A, A2=− A, A3=− A ( 1 8 . 2 0 ) or α 0 0 A =( A, A) , Aα=( A, − A) . ( 1 8 . 2 1 ) Cov a r i a n tv e c t or sa r ej u s ts p a t i a l l yi n v e r t e dc on t r a v a r i a n tv e c t or s .Not et h a tt h i s d e f i n i t i on ,t og e t h e rwi t hou rd e f i n i t i onoft h eg e n e r a ls c a l a rp r od u c t ,r e c on s t r u c t st h e d e s i r e di n v a r i a n t : α 00 B· A= BαA = ( BA− B· A) ( 1 8 . 2 2 ) Th i st e l l su sh owor d i n a r yqu a n t i t i e st r a n s f or m.Howe v e r , wea r ea l s oi n t e r e s t e di n h owt e n s ord i ffe r e n t i a l st r a n s f or m,s i n c et h e s ea r ei n v ol v e di nt h ec on s t r u c t i onofa d y n a mi c a l s y s t e m. Byc on s i d e r i n gt h ec h a i nr u l ewes e et h a t β ∂ ∂x ∂ ∂x¯ =∂x¯ ∂x α α β ( 1 8 . 2 3 ) or ,d i ffe r e n t i a t i onb yac on t r a v a r i a n tc oor d i n a t et r a n s f or msl i k eac ov a r i a n tv e c t o r op e r a t o r .Th i si s mor e orl e s st h ed e f i n i t i on ofc ov a r i a n t ,i nf a c t .Si mi l a r l y , d i ffe r e n t i a t i onwi t hr e s p e c tt oac o v a r i a n tv e c t orc oor d i n a t et r a n s f or ms l i k eac on t r a v a r i a n tv e c t orop e r a t o r .Th i sa l s of ol l owsf r om t h ea b ov eb yu s i n gt h e me t r i ct e n s or , ∂ ∂ β x . ∂x α =g α β ∂ ( 1 8 . 2 4 ) I ti st e d i ou st owr i t eou ta l loft h ep i e c e sofp a r t i a lde r i v a t i v e sw. r . t .v a r i ou s c omp on e n t s , s owe( a su s u a l , b e i n gt h el a z ys or t st h a twea r e )i n t r odu c ea “ s i mp l i f y i n g ”n ot a t i on . I td oe s , t oo, a f t e ry oug e tu s e dt oi t . α ∂ ∂ ∂ 0 ∂x , α =( − ∇) = ∂x ∂ ∂ α ( 1 8 . 2 5 ) 0 ∂α = ∂x ∂x , +∇) . =( ( 1 8 . 2 6 ) Not et h a tweh a v ec l e v e r l yi n d i c a t e dt h ec o/ c on t r an a t u r eoft h ev e c t orop e r a t or sb y t h ep l a c e me n toft h ei n d e xont h eb a r ep a r t i a l . Wec a n n otr e s i s twr i t i n gdownt h e4–di v e r ge n c eofa4–v e c t or : 0 0 ∂A α α ∂Aα=∂ A= α 1 ∂A 0 ∂x + c ∂ ∇· A= t+ ∇· A ( 1 8 . 2 7 ) wh i c hl ook sal o tl i k eac on t i n u i t ye qu a t i onorac e r t a i nwe l l –k n owng a u g ec on d i t i on . µ ( Me d i da t eonj u s twh a tA wou l dn e e dt ob ef ore i t h e roft h e s ee qu a t i on st ob er e a l i z e d a saf ou r s c a l a r ) . Hmmmmmm, I s a y . E v e nmor ee n t e r t a i n i n gi st h e4 –L a p l a c i a n , c a l l e dt h eD’ L a mb e r t i a nop e r a t or : ∂2 α 02 2 ∂ = ∂x −∇ α ✷ =∂ ( 1 8 . 2 8 ) 2 1∂ 2 2 2 ( 1 8 . 2 9 ) = c ∂t −∇ wh i c hj u s th a p p e n st ob et h e( n e g a t i v eoft h e )wa v eo p e r a t or !Hmmmmmmmm!By s t r a n g ec oi n c i d e n c e ,c e r t a i nob j e c t sofg r e a ti mp or t a n c ei ne l e c t r od y n a mi c s“ j u s t h a p p e n ”t ob eL or e n t zs c a l a r s !Re me mb e rt h a tId i ds a ya b ov et h a tp a r toft h ep oi n tof i n t r odu c i n gt h i sl ov e l yt e n s orn ot a t i onwa st oma k et h ev a r i ou st r a n s f or ma t i on a l s y mme t r i e sofp h y s i c a lqu a n t i t i e sma n i f e s t ,a n dt h i sa p p e a r st ob et r u ewi t ha v e n g e a n c e ! Th a twa st h e“ e a s y ”p a r t .I twa sa l lg e ome t r y .Nowweh a v et od ot h eme s s yp a r t a n dd e r i v et h ei n f i n i t e s i ma l t r a n s f or ma t i on st h a tl e a v es c a l a r si nt h i sme t r i ci n v a r i a n t . 18. 4 Ge n e r a t o r so ft h eL o r e n t zGr o u p L e t 0 x x= 1 x 3 x 2 x ( 1 8 . 3 0 ) b eac ol u mnv e c t or .No t et h a twen ol on g e ri n d i c a t eav e c t orb yu s i n gav e c t ora r r ow a n d/ orb ol d f a c e–t h os ea r er e s e r v e df ort h es p a t i a l p a r toft h ef ou r v e c t oron l y .Th e na “ ma t r i x ”s c a l a rp r od u c ti sf or me di nt h eu s u a l wa yb y ( a , b )=a b ˜ ( 1 8 . 3 1 ) wh e r ea ˜i st h e( r owv e c t or )t r a n s p os eofa . Th eme t r i xt e n s ori sj u s tama t r i x : g=0 − 1 0 0 10 0 0 00 0 1 ( 1 8 . 3 2 ) 00 − 1 0 − andg 2 ⇔ =I. F i n a l l y , 0 g x= x − x1 x −x 2 =x0 . ( 1 8 . 3 3 ) x 1 2 x 3 x 3 − I nt h i sc omp a c tn ot a t i onwede f i n et h es c a l a rp r od u c ti nt h i sme t r i ct ob e α β α a·b=( a , g b )=( g a , b )=a g b ˜=ag . α βb =ab α ( 1 8 . 3 4 ) Wes e e kt h es e t( g r o u p , weh op e )ofl i n e a rt r a n s f or ma t i on st h a tl e a v e s( x , g x )=x·x i n v a r i a n t .Si n c et h i si st h e“ n or m”( s qu a r e d )ofaf ou rv e c t or ,t h e s ea r e“ l e n g t h p r e s e r v i n g ”t r a n s f or ma t i on si nt h i sf ou rd i me n s i on a lme t r i c .Th a ti s ,wewa n ta l l ma t r i c e sAs u c ht h a t ′ ( 1 8 . 3 5 ) ′′ ( 1 8 . 3 6 ) x=Ax l e a v e st h en or mofxi n v a r i a n t , ′ ′ x·x=x ˜g x=x g x ˜=x·x or ˜ x ˜ Ag Ax=x g x ˜ or ˜ Ag A=g . Cl e a r l yt h i sl a s tc on d i t i o ni ss u ffic i e n tt oe n s u r et h i sp r op e r t yi nA. Now, ˜ 2 t| g| ( de t| A| ) =de t| g| d e tAg A =de ( 1 8 . 3 7 ) ( 1 8 . 3 8 ) ( 1 8 . 3 9 ) wh e r et h el a s te qu a l i t yi sr e qu i r e d. Bu tde t| g | =− 1=0 , s o d e t| A| =± 1 ( 1 8 . 4 0 ) i sac o n s t r a i n tont h ea l l owe dma t r i c e s( t r a n s f or ma t i on s )A. Th e r ea r et h u st wo c l a s s e soft r a n s f or ma t i on swec a nc on s i d e r . Th e p r op e rL o r e n t zt r a n s f o r ma t i o n swi t hd e t| A| =+ 1 ; a n d i mp r op e rL o r e n t zt r a n s f or ma t i o n swi t hd e t| A| =± 1 . Pr op e rL .T. ’ sc on t a i nt h ei de n t i t y( a n dt h u sc a nf or m ag r ou pb yt h e ms e l v e s ) ,b u t i mp r o p e rL .T. ’ sc a nh a v ee i t h e rs i g noft h ed e t e r mi n a n t .Th i si sas i g n a lt h a tt h e me t r i cwea r eu s i n gi s“ i n d e f i n i t e ” .Twoe x a mp l e sofi mp r op e rt r a n s f or ma t i on st h a t i l l u s t r a t et h i sp oi n ta r es p a t i a l i n v e r s i on s( wi t hde t| A|=− 1 )a n dA=− I( s p a c ea n dt i me i n v e r s i on , wi t hd e t| A| =+ 1 ) . I nv e r yg e n e r a l t e r ms , t h ep r op e rt r a n s f or ma t i on sa r et h ec on t i n u ou s l yc on n e c t e don e s t h a tf or maL i eg r ou p , t h ei mp r op e ron e si n c l u deon eormor ei n v e r s i on sa n da r en ote qu a l t ot h ep r odu c tofa n yt wop r op e rt r a n s f or ma t i on s .Th ep r op e rt r a n s f or ma t i on sa r ea s u b g r ou poft h ef u l lg r ou p—t h i si sn ott r u eoft h ei mp r op e ron e s ,wh i c h ,a mon got h e r t h i n g s , l a c kt h ei d e n t i t y .Wi t ht h i si nmi n d, l e tu sr e v i e wt h ep r op e r t i e sofi n f i n i t e s i ma l l i n e a r t r a n s f or ma t i on s ,p r e p a r a t or yt od e d u c i n gt h ep a r t i c u l a ron e st h a tf or mt h eh omog e n e ou s L or e n t zg r ou p . 18. 4. 1 I n f i n i t e s i ma l Tr a n s f o r ma t i on s Wes e e k( L i e )g r ou p sofc on t i n ou sl i n e a rt r a n s f or ma t i on s , ′ x=Tax ( 1 8 . 4 1 ) or ′ µ µ x =f ( x ; a ) ( 1 8 . 4 2 ) f orµ=1 , 2 , ...n .Wer e q u i r et h a tt h ea=a , ..., a r err e a ln u mb e r s( p a r a me t e r s )t h a t 1 ra c h a r a c t e r i z et h et r a n s f or ma t i on . rmu s tb emi n i ma l ( “ e s s e n t i a l ” ) . E x a mp l e soft r a n s f o r ma t i on sofi mp or t a n c ei np h y s i c s( t h a ty ous h ou l da l r e a d yb e f a mi l i a rwi t h )i n c l u d e ′ x =Tdx =x+d ( 1 8 . 4 3 ) wh e r ed=( d ,...,dn) .Th i si st h e( np a r a me t e r )t r a n s l a t i ong r ou pi nnd i me n s i on s . 1 Al s o, ′ i j x =Ri jx wh e r e ( 1 8 . 4 4 ) ˜ RR=I , de t| R|>0, i =1, 2, 3 i st h e( t h r e ep a r a me t e r )r ot a t i ongr ou p . Ani n f i n i t e s i ma l t r a n s f o r ma t i o ni non eoft h ep a r a me t e r si sd e f i n e db y 2 Ta( ǫ) . 0 ) + ǫ=I ǫ+O( ( 1 8 . 4 5 ) ( 1 8 . 4 6 ) I nt h i sd e f i n i t i on ,a ( 0 )a r et h e( r –p a r a me t e r )v a l u e sa s s oc i a t e d wi t ht h ei d e n t i t y t r a n s f or ma t i onI . Th e s ec a nb ec h os e nt ob ez e r ob ys u i t a b l yc h oos i n gt h e 2 p a r a me t e rc oor di n a t e s . Th ei n f i n i t e s i ma l p a r a me t e r sǫua r et a k e nt oz e r o, s ot h a tǫ = ǫuǫu( s u mme d)i sn e g l i b l e . Th u s I +ǫuQu ǫ=I wh e r e ( 1 8 . 4 7 ) ∂ µ ( 1 8 . 4 8 ) Qu=fu( x )∂ xµ a n d µ ∂f( x , a ) µ f u( x )= . ∂a u ( 1 8 . 4 9 ) a = a ( 0 ) Pu t t i n gt h i sa l l t og e t h e r , ′ x = ( Ta( x=( I +ǫuQu)x 0 ) + ǫ) = I x+ǫuQux µ ∂f( x , a ) =x+ǫu ∂x ( 1 8 . 5 0 ) ∂ x µ a = a ( 0 ) ∂a u ( s u mme dov e rµ=0 ,...,3i nf ou rdi me n s i on a ls p a c e –t i mea n du=0 ,...,r ) .Th u s ( u n s u r p r i s i n g l y ) ′ µ νµ x =x δ +ǫ ν ν µ ∂f u g ∂aua=a(0) ( 1 8 . 5 1 ) ν wh i c hh a st h ef or moft h ef i r s tt wot e r msofaTa y l ors e r i e s .Th i si sc h a r a c t e r i s t i cof i n f i n i t e s i ma l l i n e a rt r a n s f or ma t i on s . On ec a ne a s i l yv e r i f yt h a t I ǫI ǫ′ =I ǫ′I ǫ ( 1 8 . 5 2 ) ( i n f i n i t e s i ma l t r a n s f or ma t i on sc ommu t e )a n dt h a t −1 I ǫ =I −ǫ 2 ( 1 8 . 5 3 ) ( t oor d e rǫ) .Th e yt h u sh a v ea ni d e n t i t y ,a ni n v e r s e ,a n dc a nb es h ownt ob e a s s oc i a t i v e . Th ec on t i n u ou st r a n s f or ma t i ong r ou p( me n t i on e da b ov e )f ol l owsi mme d i a t e l yf r om ma k i n g du ( t h ed i s p l a c e me n tofc oor d i n a t e s )i n f i n i t e s i ma la n df i n d i n gf i n i t e di s p l a c e me n t sb yi n t e g r a t i on .Th er ot a t i ong r ou p( ma t r i c e s )a r eal i t t l et r i c k i e r .Th e y a r e I +g S ǫ=I wh e r e ( 1 8 . 5 4 ) ˜ ∂Ri j S=− S, Si j=ǫ k ∂a ka ( 0 . ( 1 8 . 5 5 ) Th ei n f i n i t e s i ma lSa r ea n t i s y mme t r i ca n dt r a c e l e s s( i n3 D) ,s ot h e yh a v eon l yt h r e e i n de p e n d e n tp a r a me t e r s( t h a ta r et h u s“ e s s e n t i a l ” ) . Wec a nwr i t et h e mg e n e r a l l ya s Sij=ǫijkd ωk ( 1 8 . 5 6 ) wh e r et h ed ωki st h ei n f i n i t e s i ma lp a r a me t e ra n dwh e r eǫi st h ea n t i s y mme t r i cu n i t j ki t e n s or . Th u s , i f ′ d x i=x i−x i=S i jx j=ǫ i j k x ωk jd ( 1 8 . 5 7 ) wes e et h a t x d= x×ωd ( 1 8 . 5 8 ) Amome ntoft houghts houl dc onv i nc ey out ha tωd i st h ei n f i n i t e s i ma l ( v e c t or ) r ot a t i ona n g l e , wi t hd i r e c t i ont h a tp oi n t sa l on gt h ea x i sofr ot a t i on . Toob t a i nt h er ot a t i ongr ou pwemu s ts h owt h a te v e r yr ot a t i onc a nb eob t a i n e db y i n t e g r a t i n gI Th i sf ol l owsb ywr i t i n ga na r b i t r a r yr ot a t i onorp r od u c tofr ot a t i on sa sa d ω. s i n g l er ot a t i ona b ou taf i x e da x i s .F orωdp a r a l l e lt ot h i sa x i sΩ, t h i si sob v i ou s l yt r u e , a sIs h own e x t .Si n c ea n yr ot a t i onc a nb ewr i t t e nt h i swa y , t h er ot a t i on si n d e e df or ma g r ou p . Th ei n t e g r a t i onp r oc e e d sl i k e : Ω/ω RΩ=l i m( Rω) ( 1 8 . 5 9 ) ω→0 wh e r e ω=| ω| a n dΩ=Ω. Wec a np a r a me t e r i z et h i sa s 1 m ΩS RΩ=l i m( I + ΩS0) =e m→∞ 0 ( 1 8 . 6 0 ) m wh e r e ( S0) i j=ǫ i j k ωk . ( 1 8 . 6 1 ) ω Be l i e v ei torn ot ,t h i swa son eoft h ep r i ma r yt h i n g swewa n t e dt os h owi nt h i sa s i de . Wh a ti ts h owsi st h a tr ot a t i on sa b ou ta na r b i t r a r ya x i sc a nb ewr i t t e na sa ne x p o n e n t i a l t h a tc a nb et h ou g h tofa st h ei n f i n i t ep r o d u c tofa s e r i e s ofi n f i n i t e s i ma l t r a n s f o r ma t i o n swh e r ee a c ht r a n s f or ma t i onh a sv a r i ou sn i c ep r op e r t i e s . Wi t ht h e s ek n ownr e s u l t sf r om s i mp l e rd a y sr e c a l l e dt omi n d ,wer e t u r nt ot h e h omog e n e ou s ,p r op e rL or e n t zg r ou p .He r e we s e e kt h ei n f i n i t e s i ma ll i n e a r t r a n s f or ma t i on s , e t c .i nf o u rd i me n s i on s .Al g e b r a i c a l l yon ep r oc e e d sa l mos ti d e n t i c a l l y t ot h ec a s eofr ot a t i on ,b u tn owi nf ou rd i me n s i on sa n dwi t ht h eg oa lofp r e s e r v i n g l e n g t hi nad i ffe r e n tme t r i c .Ag e n e r a li n f i n i t e s i ma lt r a n s f or ma t i onc a nb ewr i t t e n c omp a c t l ya s : ˜ I +g L ǫ=I ( 1 8 . 6 2 ) wh e r e( a sb e f or e )g L=− g L( a n dh e n c eg Li st r a c e l e s s ) , Li si n f i n i t e s i ma l , a n dwh e r eg i st h eu s u a lme t r i ct e n s or( t h a tf ol l owsf r oma l lt h ea n n oy i n gd e r i v a t i v e swi t hr e s p e c t t ot h ep a r a me t e r sa n dc oor d i n a t e s ) . Th u s m L A=l i m I + 1L =e ( 1 8 . 6 3 ) m→∞ m d e f i n e st h ef or mofag e n e r a l t r a n s f or ma t i o nma t r i xa s s oc i a t e dwi t hag i v e n“ d i r e c t i on ” i nt h ep a r a me t e rs p a c ec on s t r u c t e df r om a ni n f i n i t ep r odu c tofi n f i n i t e s i ma l t r a n s f or ma t i on s , e a c hofwh i c hi sb a s i c a l l yt h el e a di n gt e r mofa Ta y l ors e r i e soft h eu n d e r l y i n gc oor d i n a t ef u n c t i ont r a n s f or ma t i oni nt e r msoft h e p a r a me t e r s .Th i sj u s t i f i e st h e“ a n s a t z ”ma d eb yJ a c k s on .Th ema t r i c e sLa r ec a l l e dt h e ge n e r a t o r soft h el i n e a rt r a n s f or ma t i on . Th u s , wh e n e v e rwewr i t e L A=e ( 1 8 . 6 4 ) wh e r et h eL ’ sa r e( t ob e )t h eg e n e r a t or soft h eL or e n t zg r ou pt r a n s f or ma t i on swe s h ou l dr e me mb e rwh a ti ts t a n d sf or .L e t ’ sf i n dt h ed i s t i n c tL .E a c hon ei sa4×4r e a l , t r a c e l e s sma t r i xt h a ti s( a swes h a l ls e e )a n t i s y mme t r i ci nt h es p a t i a lp a r t( s i n c eg Li s a n t i s y mme t r i cf r omt h ea b ov e ) . Toc on s t r u c tA( a n df i n dt h ed i s t i n c tc omp on e n t sofL )wema k eu s eofi t s p r op e r t i e s . I t sd e t e r mi n a n ti s Tr L L d e t| A| =d e t( e)=e =± 1 ( 1 8 . 6 5 ) ( Th i sf ol l owsf r om d oi n gas i mi l a r i t yt r a n s f or ma t i ont op u tAi nd i a g on a lf or m.Li s n e c e s s a r i l yt h e nd i a g on a l .Si mi l a r i t yt r a n s f or ma t i on sd on ota l t e rt h ed e t e r mi n a n t , b e c a u s e −1 −1 d e tS MS=d e tS d e tM d e t S =d e tM . ( 1 8 . 6 6 ) || || || I fLi sd i a g on a l ,t h e nt h el a s te qu a t i onf ol l owsf r om t h eu s u a lp r op e r t i e soft h e e x p on e n t i a l a n dt h ed e f i n i t i onoft h ee x p on e n t i a l ofama t r i x . ) I fLi sr e a lt h e nde t| A|=− 1i se x c l u d e db yt h i sr e s u l t .I fLi st r a c e l e s s( a n don l yi f , g i v e nt h a ti ti sr e a l ) , t h e n d e t| A| =+ 1 ( 1 8 . 6 7 ) wh i c hi sr e q u i r e dt ob et r u ef orp r op e rL or e n t zt r a n s f or ma t i on s( r e c a l lf r oml a s tt i me ) . Ma k i n gLat r a c e l e s s4 x 4ma t r i xt h e r e f or es u ffic e st oe n s u r et h a twewi l lf i n don l y p r op e rL or e n t zt r a n s f or ma t i on s . Th i n kb a c kt ot h er e qu i r e me n tt h a t : ˜ Ag A=g i nor d e rt op r e s e r v et h ei n v a r i a n ti n t e r v a l wh e r e g= 0 − 1 0 1 0 0 0 0 − 1 ( 1 8 . 6 8 ) 0 0 0 ( 1 8 . 6 9 ) 0 0 0 − 1 a n dLi sar e a l , t r a c e l e s s , 4×4ma t r i x . −1 I fwemu l t i p l yf r omt h er i g h tb yA a n dt h el e f tb yg , t h i se qu a t i oni s e qu i v a l e n ta l s ot o ˜ − 1 g Ag =A . ˜ ( 1 8 . 7 0 ) ˜ L − 1 − L 2 Si n c eA =e, A =e , a n dI =g: ˜ 2˜ gL gAg =e ˜ gL g= −L =e e ( 1 8 . 7 1 ) or ˜ g L g=− L . ( 1 8 . 7 2 ) ( Th i sc a na l s oe a s i l yb ep r ov e nb yc on s i d e r i n gt h e“ p owe rs e r i e s ”orp r od u c t e x p a n s i on soft h ee x p on e n t i a l soft h ea s s oc i a t e dma t r i c e sa b ov e ,c h a n g i n gt h e s i g n / d i r e c t i onoft h ei n f i n i t e s i ma l s e r i e s . ) F i n a l l y , i fwemu l t i p l yb ot hs i d e sf r omt h el e f tb yga n de x p r e s st h el e f th a n ds i d e a sat r a n s p os e , weg e t ˜ g L=− g L . ( 1 8 . 7 3 ) F r omt h i swes e et h a tt h ema t r i xg Li st r a c e l e s sa n da n t i s y mme t r i ca sn ot e d / e x p e c t e df r o m a b ov e . I fweme n t a l l yf a c t orou tt h eg , wec a nwi t h ou tl os sofg e n e r a l i t ywr i t eLa s : L 0 1 L 0 2 L 0 3 0 L= L 0 1 L 0 2 L 0 4 0 − L L 12 L 1 2 L 1 3 0 L 23 L 0 − − 13 . ( 1 8 . 7 4 ) 23 Th i sma t r i xf or ms a t i s f i e sa l lt h ec on s t r a i n t swede du c e da b ov ef ort h eg e n e r a t or s . An yLoft h i sf or mwi l l ma k ea nAt h a tp r e s e r v e st h ei n v a r i a n ti n t e r v a l ( l e n g t h )ofaf ou r v e c t or .Th e r ea r ee x a c t l ys i xe s s e n t i a lp a r a me t e r sa se x p e c t e d .F i n a l l y ,i fweu s eou r i n t u i t i on , wewou l de x p e c tt h a tt h eL ori , j=1 ,2 , 3f or mt h er o t a t i o ns u b gr o u pa n d i jf d e s c r i b ep h y s i c a l r ot a t i on s . Sot h i si sj u s tg r e a t .L e tu sn ow s e p a r a t eou tt h ei n di v i d u a lc ou p l i n g sf orou r a p p r e c i a t i ona n de a s yma n i p u l a t i on .Tod ot h a twed e f i n es i xf u n da me n t a l ma t r i c e s( c a l l e d t h ege n e r a t o r soft h eg r ou pf r omwh i c hwec a nc on s t r u c ta na r b i t r a r yLa n dh e n c eA.Th e y a r eb a s i c a l l yt h ei n d i v i du a l ma t r i c e swi t hu n i torz e r oc omp on e n t st h a tc a nb es c a l e db yt h e s i xp a r a me t e r sL h ep a r t i c u l a rc h oi c e sf ort h es i g n sma k ec e r t a i nr e l a t i on swor kou t µν.T n i c e l y : S1 = 0 0 0 0 0 0 0 0 S2 = 0 0 0 0 0 0 0 1 0 0 ( 1 8 . 7 5 ) 0 0 − 1 0 0 0 1 1 0 0 0 0 ( 1 8 . 7 6 ) 0 0 − 0 0 0 S3 = 0 0 0 0 0 0 − 1 1 0 0 0 0 0 ( 1 8 . 7 7 ) 0 1 0 0 1 0 0 0 K1 = 0 0 0 0 0 0 0 0 ( 1 8 . 7 8 ) 0 0 1 0 0 0 0 0 K2 = 0 0 1 0 0 0 ( 1 8 . 7 9 ) 0 0 0 0 0 1 0 0 0 0 K3 = 1 0 0 0 0 0 . 0 0 ( 1 8 . 8 0 ) Th ema t r i c e sSig e n e r a t er ot a t i on si nt h es p a t i a lp a r ta n dt h ema t r i c e sKig e n e r a t e b oo s t s .Not et h a tt h es qu a r e soft h e s ema t r i c e sa r ed i a g on a l a n de i t h e r+1or− 1i nt h e s u b ma t r i xi n v ol v e d : ( 1 8 . 8 1 ) S12= 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0− 0 − a n d K2 1 1 0 0 0 0 1 0 0 = 0 0 0 0 0 0 0 0 , ( 1 8 . 8 2 ) e t c . F r omt h i swec a nd e du c et h a t 3 Si 3 Ki Si =− ( 1 8 . 8 3 ) . =Ki ( 1 8 . 8 4 ) Not et h a tt h e s er e l a t i on sa r ev e r ys i mi l a rt ot h emu l t i p l i c a t i onr u l e sf oru n i tp u r e c omp l e xorp u r er e a l n u mb e r s . Th er e a s ont h i si si mp or t a n ti st h a ti fwef or mt h edotp r od u c tofav e c t oroft h e s e g e n e r a t or swi t has p a t i a lv e c t or( e ffe c t i v e l yde c omp os i n gav e c t orp a r a me t e ri nt e r ms oft h e s ema t r i c e s )i nt h ee x p on e n t i a l e x p a n s i on , t h ef ol l owi n gr e l a t i on sc a nb eu s e dt o r e du c ep owe r soft h eg e n e r a t or s . 3 ( ǫ ˆ ·S)=− ǫ ˆ ·S ( 1 8 . 8 5 ) a n d 3 ( ǫ ˆ ·K)=ǫ ˆ ·K ( 1 8 . 8 6 ) I nt h e s ee x p r e s s i on s ,ǫˆa na r b i t r a r yu n i tv e c t or ,a n dt h e s ee x p r e s s i on se ffe c t i v e l y ma t c hu pt h eg e n e r a t ora x e s( wh i c hwe r ea r b i t r a r y )wi t ht h ed i r e c t i onoft h ep a r a me t e r v e c t orf orr ot a t i onorb oos tr e s p e c t i v e l y .Af t e rt h er e d u c t i on( a swes h a l ls e eb e l ow) t h ee x p on e n t i a l i s , i nf a c t , awe l l b e h a v e da n de a s i l yu n d e r s t oodma t r i x ! I ti se a s y( a n di mp or t a n t ! )t od e t e r mi n et h ec ommu t a t i onr e l a t i on soft h e s e g e n e r a t or s . Th e ya r e : [ Si , Sj]=ǫijkSk ( 1 8 . 8 7 ) [ Si , Kj]= ǫijkKk ( 1 8 . 8 8 ) [ Ki , Kj]= − ǫijkSk. ( 1 8 . 8 9 ) Th ef i r s ts e ta r ei mme di a t e l yr e c og n i z a b l e .Th e yt e l l su st h a t“ t wor ot a t i on sp e r f or me d i nb ot hor d e r sd i ffe rb yar ot a t i on ” .Th es e c on da n dt h i r ds h owt h a t“ ab oos ta n da r ot a t i ondi ffe rb yab oos t ”a n d“ t wob oos t sd i ffe rb yar ot a t i on ” , r e s p e c t i v e l y .I nqu ot e s b e c a u s et h a ti ss ome wh a tov e r s i mp l i f i e d , b u ti tg e t ss omeoft h ei d e aa c r os s . Th e s ea r et h eg e n e r a t or sf ort h eg r ou p sSL ( 2 , C)orO( 1 , 3 ) .Th el a t t e ri st h eg r ou p ofr e l a t i v i t ya swea r ec u r r e n t l ys t u dy i n gi t . Aqu e s t i ont h a th a sb e e nb r ou g h tu pi nc l a s si s“ wh e r ei st h ef a c t orii nt h eg e n e r a t or s ofr ot a t i on ”s ot h a tS×S=i Sa swemi g h te x p e c tf r om c on s i d e r i n gs p i na n da n g u l a r 2 mome n t u mi not h e rc on t e x t s .I ti st h e r e ,b u ts u b t l yh i d d e n ,i nt h ef a c tt h a tSi =− I nt h e ii p r o j e c t i v eb l oc koft h er ot a t i onma t r i c e son l y .Ma t r i c e sa p p e a rt ob eawa yt or e p r e s e n t g e ome t r i ca l g e b r a s ,a smos tr e a d e r soft h i st e x ts h ou l da l r e a d yk n owf r omt h e i rs t u dyof t h e( qu a t e r n i on i c )Pa u l is p i nma t r i c e s .Wewon ’ td we l lont h i sh e r e ,b u tn ot ewe l lt h a tt h e Pa u l i ma t r i c e sI , σ1, σ2, σ r ei s omor p h i ct ot h eu n i tqu a t e r n i on s1 , i , j , kv i at h ema p p i n g 3a 6 I→ 1 , σ1σ2→ i , σ3σ1→ j , σ σ3→ ka st h er e a de rc a ne a s i l yv e r i f y Not ewe l l 2 t h a t : σ3σ1 = 0 1 ( 1 8 . 9 0 ) − 1 0 i sb ot hr e a la n d ,n ota ta l lc oi n c i de n t a l l y ,t h es t r u c t u r eofa nSs u b b l oc k .Wi t ht h e s e d e f i n i t i on si nh a n d, wec a ne a s i l yd e c omp os eLi nt e r msoft h e Sa n dt h eKma t r i c e s . Weg e t : L = − ω· S− ξ · K ( 1 8 . 9 1 ) wh e r e ω i sa( f i n i t e )r ot a t i ona r ou n da na x i si nd i r e c t i onωˆa n dwh e r eξi sa ˆ ( f i n i t e )b oos ti nd i r e c t i onξ . Th u st h ec o mp l e t e l yge n e r a l f or mofAi s −ω· S−ξ · K. A=e ( 1 8 . 9 2 ) Th e( c a r t e s i a n )c omp on e n t sof ωa n dξa r en ow t h es i xf r e ep a r a me t e r soft h e t r a n s f or ma t i on . L e tu ss e et h a tt h e s ea r ei n d e e dt h ef a mi l i a rb oos t sa n dr ot a t i on swea r eu s e dt o. Af t e ra l l , t h i se x p on e n t i a l n ot a t i oni sn ott r a n s p a r e n t . Su p p os et h a t 6 An ds h o u l d!Th a t ’ sr i g h t , y ous t u de n t s , y ouk n owwh oI ’ mt a l k i n gt o.Soh e r e ’ saqu e s t i onf ory ou :Ar eI , σ3σ1ar e a li s omor p h i s mt oc omp l e xn u mb e r s ?Wh a twou l dt h ev a r i ou sr e s u l t soft h ei n t r od u c t i ont o c omp l e xn u mb e r sl ookl i k ee x p r e s s e di nt e r msoft h e s et woma t r i c e s ?Wh a ti npa r t i c u l a rd oe smu l t i p l y i n gb y au n i mod u l a r“ c omp l e xn u mb e r ”s u c ha sc os ( θ) I +s i n ( θ ) σ3σ ookl i k e ?Hmmm. . . v e e e e r yi n t e r e s t i n g . 1l ω =0a n dξ=ξ x ˆ . Th e nL=− ξ K1a n d 1 L 1 2 3 A =e =I −ξ K1+2 ξ K1)−3 ξ K1)+. . . !( !( 13 2 12 2 = ( I − K1) − K1( ξ+3 . . ) + K1( I +2 . . ) !ξ +. !ξ +. 2 2 =( I −K1)−K1s i n h ( ξ )+K1 c os h ( ξ ) ( 1 8 . 9 3 ) or( i nma t r i xf or m) A= −s i n h ( ξ ) c os h ( ξ ) 0 0. i n h ( ξ ) c os h ( ξ ) −s 0 00 0 0 ( 1 8 . 9 4 ) 0 0 0 0 0 wh i c h( oh my g os h ! )i sou rol df r i e n dt h eL or e n t zt r a n s f or ma t i on , j u s tl i k ewed e r i v e di ta l ak i dd y –p h y s i c s –wi s e .Asa ne x e r c i s e , s h owt h a tt h e ω=ωx ,̂ξ=0r e s u l ti sar ot a t i on 2 a r ou n dt h exa x i s .Not et h a tt h es t e pof“ a dd i n ga n ds u b t r a c t i n g ”S1 i se s s e n t i a lt o r e c o n s t r u c t i n gt h es e r i e soft h es i n ea n dc os i n e ,j u s tl i k et h eK1wa sa b ov ef orc os h a n ds i n h . Now, ab oos ti na na r b i t r a r ydi r e c t i oni sj u s t ξ K − · A=e ( 1 8 . 9 5 ) . Wec a nc e r t a i n l yp a r a me t e r i z ei tb y ( 1 8 . 9 6 ) ξ=βˆt a n h −1β ˆ ( s i n c ewek n owt h a tβ=ξt a n hξ , i n v e r t i n gou rf or me rr e a s on i n gf or Th e n ˆ −1 β I c a nd on ob e t t e rt h a nqu ot eJ a c k s onont h er e ma i n d e r : “ I ti sl e f ta sa ne x e r c i s et ov e r i f yt h a t. . . ” − 1+ β2 · β1 A( β)= −γ β2 · − γ β1 2 ( γ −1) β1 ( γ − β2 − − γ β2 · · · · ( e t c . )wh i c hi sj u s tt h ee x p l i c i tf u l l ma t r i xf or mof 0 βx) 0 ′ =γ ( x x x =x+ ( 1 8 . 9 8 ) − γ β3 ( γ1) β1β2 1 ) β1β2 ′ ( 1 8 . 9 7 ) . −β· Kt a n h A( β)=e γ β ∈[ 0 ,1 ] . · · − · 0 βx ( γ − 1 ) ( βx) β−γ ( 1 8 . 9 9 ) ( 1 8 . 1 0 0 ) β2 f r omb e f or e . · Now,weh a v ee n ou g hi n f or ma t i ont oc on s t r u c tt h ee x a c tf or mofas i mu l t a n e ou s b oos ta n dr ot a t i on ,b u tt h i sp r e s e n t sad u a lp r ob l e m.Wh e nweg ot of a c t or i z et h e r e s u l t s( l i k eb e f or e )t h ec omp on e n t sofi n d e p e n de n tb oos t sa n dr ot a t i on sd on ot c ommu t e ! I fy oul i k e , A( β, 0 ) A( 0 ω, )=A( 0 ω, ) A( β, 0 ) ( 1 8 . 1 0 1 ) a n dwec a n n ots a ya n y t h i n gt r i v i a l l i k e A( βω, )=A( β, 0 ) A( 0 ω, ) ( 1 8 . 1 0 2 ) s i n c ei td e p e n dsont h eor d e rt h e ywe r ep e r f or me di n !E v e nwor s e , t h ep r od u c toft wo b oos t si se qu a lt oas i n g l eb oos ta n dar ot a t i on( i ft h eb oos t sa r en oti nt h es a me d i r e c t i on ) ! Th ewor s tp a r t ,o fc ou r s e ,i st h ea l g e b r ai t s e l f .A u s e f u le x e r c i s ef ort h e a l g e b r a i c a l l yi n c l i n e dmi g h tb ef ors ome on et oc on s t r u c tt h eg e n e r a l s ol u t i onu s i n g , e . g . –ma t h e ma t i c a . Th i ss u g g e s t st h a tf orr o t a t i n gr e l a t i v i s t i cs y s t e ms( s u c ha sa t omsoror b i t sa r ou n d n e u t r ons t a r s )wema yn e e dak i n e ma t i cc or r e c t i ont oa c c ou n tf ort h es u c c e s s i v e f r a mec h a n g e sa st h es y s t e mr ot a t e s . Th ea t om p e r c e i v e si t s e l fa sb e i n g“ e l l i p t i c a l l yd e f or me d” .Th ec on s e qu e n c e sof t h i sa r eob s e r v a b l e . Th i si sk n owna s“ Th oma sp r e c e s s i on ” . 18. 5 Thoma sPr e c e s s i o n Wemu s tb e g i nou rd i s c u s s i onb yn ot i n gt h a tt h ema g n e t i cmome n tofa ne l e c t r oni s ( a c c or di n gt ot h e“ Uh l e n b e c k Gou d s mi th y p ot h e s i s ” ) ge µ= s 2mc ( 1 8 . 1 0 3 ) wh e r e si st h e( h a l fi n t e g e r )s p i noft h ee l e c t r oni nu n i t sof a n dwh e r egi s t h e“ g –f a c t or ”i n t r od u c e dt oa c c omod a t et wodi s t i n c tr e s u l t s . Th es p l i t t i n gof t heobs e r v e ds pe c t r ai na na ppl i e dma gne t i cf i e l dBv i at hea noma l ousZe e ma n i n t e r a c t i on : ge UAZ= − 2mcs·B ( 1 8 . 1 0 4 ) wa sc or r e c t l yp r e di c t e don l yi fg=2 .Ont h eot h e rh a n d( a swes h a l ls e e ) ,t h es i mp l e c l a s s i c a l a r g u me n tt h a tl e dt ot h i sr e s u l ta l s ol e dt oas p i nor b i ti n t e r a c t i on g 22 1d V USO= 2m c ( s·L )r d r ( 1 8 . 1 0 5 ) ( wh e r eL=m( r×v )i st h eor b i t a l a n g u l a rmome n t u moft h ee l e c t r on )t h a twa saf a c t or of ,c u r i ou s l ye n ou g h ,2t ool a r g e .Th a ti s ,t h ef i n e –s t r u c t u r ei n t e r v a l sob s e r v e di n n a t u r ewe r eon l yh a l ft h et h e or e t i c a l l yp r e di c t e dv a l u e s .I fg=1wa sc h os e ni n s t e a d , t h es p l i t t i n g swe r ec or r e c tb u tt h eZe e ma ne ffe c twa st h e nn or ma l( i n s t e a dof a n oma l ou s , a sob s e r v e d ) . I d on ’ th a v et h et i met og oi n t omo r ed e t a i l on , f ore x a mp l e , wh a tt h eZe e ma ne ffe c t ( s p l i t t i n gofe n e r g yl e v e l si na na p p l i e dma g n e t i cf i e l d )i s .I na n ye v e n t ,i ti ss t r i c t l ya qu a n t u me ffe c t ,a n dy ous h ou l ds t u d yi ts ooni ne l e me n t a r yqu a n t u mt h e or y ,i fy ou h a v e n ’ ta l r e a d y . Th oma s( wh ot a u g h tf ory e a r sov e ra tNC St a t e )s h owe di n1 9 2 7t h a tt h e di s c r e p a n c yi sd u et oar e l a t i v i s t i ck i n e ma t i cc or r e c t i onl i k et h a twep r e v i ou s l y c on s i de r e d .I nan u t s h e l l ,t h er e s tf r a meoft h ee l e c t r onr ot a t e sa swe l la st r a n s l a t e s ( b oo s t s )a n dwemu s tt h e r e f or et a k ei n t oa c c ou n tb ot hk i n e ma t i c a l e ffe c t s .Th i sr e s u l t s i na na d d i t i on a l( Th oma s )“ p r e c e s s i on ”oft h ef r a me s .Wh e nTh oma sp r e c e s s i oni s t a k e ni n t oa c c ou n t , n oton l ya r eb ot ht h ef i n es t r u c t u r ea n da n oma l ou sZe e ma ne ffe c t i na t omsa c c omoda t e d,b u tad e e p e ru n d e r s t a n d i n goft h es p i n –or b i ti n t e r a c t i oni n n u c l e a rp h y s i c s( a n dr ot a t i n gf r a me si ng e n e r a l )a l s or e s u l t s . L e tu sb e g i nb y( n a ´ ı v e l y )d e r i v i n gt h es p i n –i n t e r a c t i one n e r g y .Su p p os et h e e l e c t r onmov e swi t hv e l oc i t yvi ne x t e r n a lf i e l d sEa n dB.Th e nt h et or qu eont h e e l e c t r oni ni t sr e s tf r a mei sj u s t ds d t ′ r e s tf r a me ′ = µ ×B ( 1 8 . 1 0 6 ) wh e r eBi st h ema g n e t i cf i e l di nt h a tf r a me . Aswewi l l s h owv e r ys oon , t h ema g n e t i cf i e l dt r a n s f or msl i k e ′ B= 2 2 B v −c× E ( 1 8 . 1 0 7 ) t oor d e rv/ c. Th e n d s v d t r e s tf r a me = µ ×B− c×E. As s oc i a t e dwi t ht h i st or qu et h e r ei sa ni n t e r a c t i one n e r g y ′ U=µ B v − E. ( 1 8 . 1 0 8 ) ( 1 8 . 1 0 9 ) c× − · Th ee l e c t r i cf or c ee Ei sv e r yn e a r l yt h en e g a t i v eg r a d i e n tofas p h e r i c a l l ya v e r a g e d p ot e n t i a le n e r g yV( r ) .F oron ee l e c t r ona t omst h i si se x a c t ; i ti sag ooda p p r ox i ma t i on f ora l l t h eot h e r s . Th u swewi l l t r yu s i n g rdV r e E=− rd ( 1 8 . 1 1 0 ) i nt h ee qu a t i onf ort h es p i ni n t e r a c t i o ne n e r g y : ge ′ U= − g 22 1dV 2mcs·B+ 2m c ( r s·L)rd ( 1 8 . 1 1 1 ) ( wh e r eL=m( r×v )f ort h eor b i t i n ge l e c t r on ) .Th i sg i v e st h ea n oma l ou sZe e ma ne ffe c t c or r e c t l y( f r omt h ef i r s tt e r m)b u tt h es p i nor b i t( f i n es t r u c t u r e )s p l i t t i n gi saf a c t orof t wot ool a r g e . Toob a d ! Th ee r r ori s , i nan u t s h e l l , t h a tweh a v ea s s u me dt h ee l e c t r ont ob ei na“ r e s t ”f r a me ( t h a ti s , af r a met r a v e l l i n gi nas t r a i g h tl i n e )wh e nt h a tf r a mei s , i nf a c t , r ot a t i n g .Th e r e i sa na d d i t i on a lc or r e c t i ont ov e c t orqu a n t i t i e st h a ta r i s e sf r om t h er ot a t i onoft h e f r a me . Th i sc or r e c t i on , i nma c r os c op i cs y s t e ms , g i v e sr i s et ot h i n g sl i k ec or i ol i sf or c e . L e tu sr e c a l l( f r om c l a s s i c a lme c h a n i c s )t h a ti fac oor d i n a t es y s t e mr ot a t e sa t s omea n g u l a rv e l oc i t y ω, t h et ot a l r a t eofc h a n g eofa n yv e c t orqu a n t i t yi sg i v e nb y d G d G = dt + ω× G. ( 1 8 . 1 1 2 ) d t n on −r ot r e s tf r a me Th i si sag e ome t r i cr e l a t i ont h a ts a y st h a tav e c t ori nan on –r ot a t i n gf r a mei sr e l a t e dt o t h es a mev e c t ore x p r e s s e di na( r ot a t i n g )“ r e s t ”f r a meb ya dd i n gi t st i mer a t eof c h a n g ei nd i r e c t i o nr e s u l t i n gf r om t h er ot a t i onoft h ef r a me .Amome n tofqu i e t r e f l e c t i ons h ou l dc on v i n c ey out h a tt h i ss h ou l dh a v et h ema g n i t u d e d θ d t G a n ds h ou l db ep e r p e n d i c u l a rt oωa n dG.Th i sj u s ta d dst h er ot a t i ono ft h ef r a met ot h e v e c t ori nt h ef r a met og e tt h ev e c t ori nan on –r ot a t e df r a me . We l l ,a sIn ot e da b ov e ,t h ee x p r e s s i onweh a v eg i v e na b ov ef ort h et i mer a t eof c h a n g eoft h es p i nwa sc or r e c tf ort h ef i e l da n dmome n te x p r e s s e di nt h er e s tf r a me o ft h ee l e c t r on .I nt h el a b( n on –r ot a t i n g )f r a me , wh i c hi swh e r eweme a s u r ei t se n e r g y , wet h e r e f or es h ou l dh a v e : ′ g e B d s d t =s× n on −r ot ωT 2mc − ( 1 8 . 1 1 3 ) wh e r eωTi st h ea n g u l a rv e l oc i t yoft h ep r e c e s s i onoft h ef r a me s .Th i sa dd sa( sω·T) c or r e c t i ont ot h ei n t e r a c t i one n e r g y : ge g 1dV 22 sω·T) . U=− 2mcs·B+ 2m c ( s·L )rd r+( ( 1 8 . 1 1 4 ) Ui st h u st h el a b or a t o r yp ot e n t i a l e n e r g yofi n t e r a c t i on .Wh a t , t h e n , i st h ec or r e c tv a l u e of ωT? Toa n s we rt h a twemu s tc on s i de rc a r e f u l l ywh a td e f i n e st h e“ r e s t ”f r a meoft h e a c c e l e r a t i n ge l e c t r on .Wewi l ldos ob yc h op p i n gt h emot i onoft h ee l e c t r oni n t o i n f i n i t e s i ma l s e g me n t s . I ft h ee l e c t r oni smov i n ga tv e l oc i t yv ( t )=c βa ta n yi n s t a n toft i met , t h e na tt+δ tt h ee l e c t r oni smov i n ga tv ( t )=c ( β+δβ) .Tog e tf r omt h el a bf r a me( x )t ot h e i n s t a n t a n e ou sr e s tf r a meoft h ee l e c t r on ′ ( x)wemu s tt h e r e f or eb oos t : ′ x=A( β) x ( 1 8 . 1 1 5 ) ( a tt )or ′ ′ x=A( β+δ β) x ( 1 8 . 1 1 6 ) ( a tt+δt ) .Not et h a tf ore a c hoft h e s et r a n s f or ma t i on s ,t h e r ei sn or ot a t i on ,j u s tt h e b oos t . Th ec oor d i n a t ef r a mep r e c e s s i oni sg oi n gt ob ed e t e r mi n e db yt h eL or e n t z t r a n s f or ma t i onb e t we e nt h e s et wo( i n f i n i t e s i ma l l ys e p a r a t e d )r e s u l t s : ′ ′ ′ x=ATx ( 1 8 . 1 1 7 ) wh e r e( a sI h op ei sob v i ou s ) −1 AT=A( β+δβ) A ( β)=A( β+δβ) A( − β) . ( 1 8 . 1 1 8 ) Toe v a l u a t et h i s( i nt h el i mi tofv a n i s h i n gδt )wewi l lp i c ka ni n i t i a lβa l on gt h e1 d i r e c t i ona n da d dt oi tδ βi nt h e1 –2p l a n e .Cl e a r l yt h i si sg e n e r a l , f oras u i t a b l ei n i t i a l or i e n t a t i onoft h ec oor di n a t es y s t e m. Th e n γ γ β 0 0 − 0 0 1 0 A(β) = γ β γ 0 0 0 ( 18. 119) 0 01 a n d( k e e p i n gon l yf i r s tor d e rt e r msi nδ β) 3 3 γ+γβδβ1 3 A( β+δβ)= ( γ β+γδ β1) − − γ δ β2 − ( γ β+γδ β1) − γ δ β2 0 γ + γβδβ1 ( γ−β 1) δβ2 0 1 0 3 ( γ−β 0 1) δβ2 0 0 .( 1 8 . 1 2 0 ) 1 Wemu l t i p l yt h e s ema t r i c e st og e t h e rt oob t a i n : 2 − γδβ1 1 2 − γδβ1 A( β+δβ)= − γ δ β2 0 1 ( γ −β 1 δβ2 ) 0 − γ δ β2 0 ( γ−β 1) δβ2 0 0 1 0 . ( 1 8 . 1 2 1 ) 1 ( Not et h a tt h ea c t i onofA( − β)i son l yi nt h eu p p e rl e f tc or n e r ) .F i n a l l y , i fwed e c omp os e t h i si nt e r msoft h eSa n dKma t r i c e s , weg e t : 2 γ−1 ( γδβ+γ δβ β δβ) S ( A=I T − β2 × ·− )K ⊥ ( 1 8 . 1 2 2 ) · wh e r eδβ a n dδβ⊥a r et h ec omp on e n t sofδβp a r a l l e l t oa n dp e r p e n d i c u l a rt o β, r e s p e c t i v e l y . Tof i r s tor de ri nδβ, wes e et h a tt h et ot a l t r a n s f or ma t i onATi se qu i v a l e n tt oab oos t a n dar ot a t i on : AT=A( Δβ) R( ΔΩ) ( 1 8 . 1 2 3 ) wh i c hc a nb ep e r f or me di ne i t h e ror d e r( b e c a u s et h e ya r e“ i n f i n i t e s i ma l ”a n dh e n c e c ommu t et of i r s tor de r . I nt h i se x p r e s s i on , A( Δβ)=I − β·K ( 1 8 . 1 2 4 ) R( ΔΩ)=I − Ω· S. ( 1 8 . 1 2 5 ) a n d Ob v i ou s l y , 2 β=γδβ+γ δ β⊥ a n d ( 1 8 . 1 2 6 ) 2 γ Ω= γ−1 ( β δβ)= β δβ. ( 1 8 . 1 2 7 ) 2 β × γ+1 × F i n a l l ywes e ee x p l i c i t l yt h a ta tl e a s tf ori n f i n i t e s i ma lt r a n s f or ma t i on s ,ap u r eL or e n t z b oos tA( β+δβ)i se qu i v a l e n tt oab oos tt oa ni n f i n i t e s i ma l l ydi ffe r i n gf r a meA( β)f ol l owe d b yas i mu l t a n e ou si n f i n i t e s i ma l b oos ta n dr ot a t i on . Nowc ome st h et r i c k yp a r t .Th ee qu a t i onofmot i onf ort h es p i nt h a tweb e g a nwi t h ( i nt h e“ r e s tf r a me ” )c a nb ee x p e c t e dt oh ol dp r ov i de dt h a tt h ee v ol u t i onoft h er e s t f r a mei sd e s c r i b e db yas e r i e sofi n f i n i t e s i ma lb oos t sa l on e( wi t h o u tr ot a t i on s ) .I n ot h e rwor d s ,weh a v et oa d dt h er e l a t i v i s t i ce qu i v a l e n tofc ou n t e r r ot a t i n gt h ef r a me s ( l i k e wed i da b ov e wi t ht h e ω T ×G t e r m) .Th e s e“ r e l a t i v i s t i c a l l yn on r ot a t i n g c oor d i n a t e s ”a r er e l a t e dt ot h ei n s t a n t a n e ou sr e s tf r a mec oor d i n a t e soft h ee l e c t r onb y t h ei n f i n i t e s i ma l b oos t ′ ′ ′ ′ x =A( Δβ)A( β) x=x ( 1 8 . 1 2 8 ) a l on e . I nt e r msoft h el a bc oor d i n a t e s , ′ ′ ′ x =R( − Ω) A( β+δβ) x . ( 1 8 . 1 2 9 ) ′ ′ ′ Th u st h e“ r e s t ”s y s t e mofc oor di n a t e soft h ee l e c t r ona r ed e f i n e db yx.Th e ya r e ′ ′ r ot a t e db y−Ωr e l a t i v et ot h eb oos t e dl a b or a t or ya x e sx.I fap h y s i c a lv e c t orGh a sa ( p r op e r )t i mer a t eofc h a n g eofdG/ d τi nt h er e s tf r a me ,t h ep r e c e s s i onoft h er e s t f r a mea x e swi t hr e s p e c tt ot h el a b or a t or yma k e st h et ot a l t i mer a t eofc h a n g e d G d G d t = dt n on −r ot a sb e f or ewi t h + ω× G ( 1 8 . 1 3 0 ) r e s tf r a me 2 γ a×v ωT=l i m Ω = 2 δt →0 δ t γ+1 c − 1 ( 1 8 . 1 3 1 ) ( Re c a l lt h a tt h ec on n e c t i ont ol a b or a t or yt i mei sdG/ d t=γ d G/ dτi nt h er e s tf r a me i t s e l f ) . Th ea c c e l e r a t i onp e r p e n d i c u l a rt ot h ei n s t a n t a n e ou sv e l oc i t ya p p e a r si nt h i se x p r e s s i on b e c a u s ei ti st h i squ a n t i t yt h a tp r od u c e st h e“ r ot a t i on ”i nt h ei n f i n i t e s i ma lt r a n s f or ma t i on b e t we e nf r a me st h a toc c u r e di nt h ei n f i n i t e s i ma l t i me i n t e r v a l .Not et h a tt h i si sap u r e l yk i n e ma t i c a le ffe c t ,a n dh a sn ot h i n gt od owi t ht h e l a wsofn a t u r e ,j u s tl i k et h en on r e l a t i v i s t i c“ c or i ol i sf or c e ”a n d“ c e n t r i f u g a lf or c e ” .I f on ewi s h e st or e l a t et h el a wsofn a t u r ea sme a s u r e di ns omea c c e l e r a t i n gf r a met o t h os eme a s u r e di nan on a c c e l e r a t i n gf r a me ,t h e ni ti sn e c e s s a r yt oi n s e r taf i c t i t i ou s “ f or c e ”( ormor ep r op e r l yi n t e r a c t i on“ e n e r g y ” )t h a ti sk i n e ma t i ci nor i g i n . I nt h i sc a s e ,c u r i ou s l ye n ou g h ,t h el a wsofn a t u r ea r ek n owni nt h ea c c e l e r a t i n g f r a me ,a n dt h ef i c t i t i ou sf or c ea p p e a r si nt h el a bf r a me ,wh e r ei ti sn otp r op e r l y s p e a k i n gf i c t i t i ou s .Howe v e r ,i ti ss t i l lk i n e ma t i c .Th a ti s ,t h e r ei sn oa c t u a le n e r g y a s s oc i a t e d wi t ht h ef i c t i t i ou si n t e r a c t i on ( wh a t e v e rt h a tme a n s ) ;h owe v e r ,t h i s i n t e r a c t i oni sn e c e s s a r yn on e t h e l e s si fwewi s ht oob t a i nt h ee qu a t i onofmot i onf r om t h ee n e r g ye qu a t i ona l on ewi t h ou te x p l i c i tc on s i d e r a t i onoft h et r a n s f or ma t i on sof f r a me s . Toc on c l u d e ,f ore l e c t r on st h ea c c e l e r a t i oni sc a u s e db yt h e( s c r e e n e d )Cou l omb f or c eont h ee l e c t r ont h a tk e e p si tb ou n d . Th u s 1 r×v 1dV = − 1 L1dV . ωT= − 3 22 2 c m rd r 2m c rd r ( 1 8 . 1 3 2 ) Th i sh a se x a c t l yt h es a mef or ma st h e“ r e s tf r a me ”s p i nor b i ti n t e r a c t i onwi t hh a l ft h e ma g n i t u d ea n dt h eop p o s i t es i g n .I tb e a u t i f u l l yc a n c e l st h ee x t r af a c t orof2 .Th ef i n a l r e s u l ti s : ′ U= ge s B+ ( g−1 )( s L )1dV . −2mc · 22 2m c ( 1 8 . 1 3 3 ) · rd r Wi t hg=2 ,b o t ht h es p i n –or b i ti n t e r a c t i ona n dt h ea n oma l ou sZe e ma ne ffe c ta r e c or r e c t l yp r e di c t e di na c c or dwi t hwh a ti se x p e r i me n t a l l yob s e r v e d .Re l a t i v i s t i ce ffe c t s , wh i c ha r eg e n e r a l l yt h ou g h tofa sb e i n g“ s u b t l e ” , a r en ots u b t l ea ta l l wh e ni tc ome st o k i n e ma t i c s .Th er e l a t i v i s t i ck i n e ma t i cc or r e c t i oni sa sl a r g ea st h eot h e rqu a n t i t i e s n a t u r a l l yp r e s e n ti n de p e n d e n toft h ep a r t i c u l a ror b i tors p e e doft h ee l e c t r on . Th i se ffe c ti se v e nmor ep r on ou n c e di na t omi cn u c l e i .Th e r et h ee l e c t r oma g n e t i c f or c e sa r emu c hwe a k e rt h a nt h eb i n di n gn u c l e a rf or c e s ,a n dc a nb en e g l e c t e dt o l owe s tor d e r . Howe v e r , e v e nu n c h a r g e dn e u t r on se x p e r i e n c eas p i n –or b i ti n t e r a c t i on 1 1d VN 22 Ur M c s·Lrd mN=− 2 r ( 1 8 . 1 3 4 ) t h a ti sn ow p u r e l yk i n e ma t i ca n dh a sn ot h i n gwh a t s oe v e rt od owi t ht h ee l e c t r oma g n e t i cf or c e !Th e r ewi l l b eas ma l l e l e c t r oma g n e t i cc or r e c t i ont ot h i sf orp r ot on s . Th i ss i mp l ep r e d i c t i oni si nr e a s on a b l ea g r e e me n twi t hwh a ti sob s e r v e di nma n y n u c l e i if ors i mp l emode l sf orVN.Un f or t u n a t e l y , t h es y s t e mi sa c t u a l l ys oc omp l i c a t e d t h a tt h i ss i mp l emi n d e d , s i n g l ep a r t i c l ed e s c r i p t i oni t s e l fi sn otr e a l l yv a l i d . Th i si sj u s tadr opi nt h ep r ov e r b i a l b u c k e tofa c c e l e r a t e ds y s t e ms .Cl e a r l y , a c c e l e r a t e d, r e l a t i v i s t i cs y s t e msh a v eamu c hmor ei n v ol v e ds t r u c t u r et h a nt h a td e s c r i b e db yt h eL or e n t z t r a n s f or ma t i on sa l on e .Th i sb e c ome se v e nmor es owh e nEi n s t e i n ’ sr e v e r e de qu i v a l e n c e p r i n c i p a l i si n v ok e d , s ot h a tg r a v i t a t i on a l f or c ea n d“ r e a l ”a c c e l e r a t i ona r en ot( l oc a l l y )di s t i n g u i s h a b l e .Bu tt h a ti sg e n e r a l r e l a t i v i t ya n df a rb e y on dt h es c op eoft h i sc ou r s e . 18. 6 Co v a r i a n tF o r mu l a t i onofE l e c t r o d y n a mi c s Wea r en owr e a d yt og e ts e r i ou sa b ou te l e c t r od y n a mi c s .Weh a v ede v e l op e dab e a u t i f u l , g e ome t r i cs y s t e mf ord e s c r i b i n gt h ec o or d i n a t e si nt e r msofwh i c he l e c t r od y n a mi c smu s t b ef or mu l a t e df ort h es p e e dofl i g h tt ob ea ni n v a r i a n t .Weh a v ed e v e l op e dag r ou pof c oor d i n a t et r a n s f or ma t i on st h a tp r e s e r v e st h a ti n v a r i a n c e .Nowwemu s tf a c eu pt ot h ef a c t t h a tou ror i g i n a le qu a t i on sofe l e c t r ody n a mi c sa r en oti na“ c ov a r i a n t ”f or mu l a t i ont h a t ma k e st h e s ec on s t r a i n t sa n dt r a n s f or ma t i onp r op e r t i e sma n i f e s t .F ore x a mp l e ,wed on ot y e tk n owh owt h ee l e c t r i ca n dma g n e t i cf i e l d st h e ms e l v e st r a n s f or mu n de raL T! L e tu st h e nr e f or mu l a t eou rb a s i ce qu a t i on si n4 –t e n s orf or m.Wewi l lma k et h e e qu a t i on st h e ms e l v e s4 –s c a l a r s ,4 –v e c t or s ,or4 –t e n s or sofh i g h e rr a n ks ot h a twe c a ns i mp l yl ooka tt h e ma n dd e du c et h e i rt r a n s f or ma t i onp r op e r t i e s .I na d d i t i on ,we wi l l s i mp l i f yt h en ot a t i onwh e np os s i b l e . Web e g i na tt h eb e g i n n i n g .Al lwer e a l l yk n owa b ou te l e c t r oma g n e t i cf i e l d si st h e i r ( d e f i n e d)a c t i ononac h a r g e dp a r t i c l e : dp v d t=q E+ c ×B ( 1 8 . 1 3 5 ) ( i n3 –v e c t orn ot a t i on ) .We l l ,wek n owt h a tt h e3 –v e c t ormome n t u mi sj u s tp a r tofa 4 –v e c t ormome n t u m: α 0 0 p =( p, p )=m( U, U) ( 1 8 . 1 3 6 ) 0 ( wh e r ep =E / c ) .Al s o, wed on ’ tk n owwh a t“ t ”i s( s i n c et h a td e p e n d sont h ec h oi c eof f r a me )s owen e e dt ou s e“ τ”i n s t e a di nou rd e f i n i t i on . Th u swec a nwr i t e dp q 0 + U× B. d τ =c UE ( 1 8 . 1 3 7 ) Th el e f th a n ds i d et e l l su st h er a t eofc h a n g eoft h e( s p a t i a l )mome n t u m, wh i c hi sj u s t p a r tofaf ou rv e c t or .Th et i mec omp on e n ts h ou l dt e l lu sh owt h ee n e r g yc h a n g e swi t h p r op e rt i me : 0 d p q d τ = cU· E . ( 1 8 . 1 3 8 ) Now, i ft h i se n e r g y –f or c e4 –v e c t ore qu a t i oni st ob ec ov a r i a n t( s oi t st r a n s f or me d f or mi ss t i l la4 –v e c t or )t h e nt h er i g h th a n ds i d e smu s tf or ma4 –v e c t ort oo.Th u swe mu s tb ea b l et oe x p r e s si t( a sac on t r a c t i onofc oa n dc on t r av a r i a n tt e n s or s )s ot h a t t h i sp r op e r t yi s“ ma n i f e s t ” .Wek n ow( e x p e r i me n t a l l y )t h a tc h a r g ei saL or e n t zs c a l a r ; 0 t h a ti s , c h a r g ei si n v a r i a n tu n d e rL T’ s . ( U, U)f or msac on t r a v a r i a n t4 –v e c t or . F r omt h i swec a nd e d u c et h e4 –t e n s orf or mf ort h ee l e c t r oma g n e t i cf i e l d! Si n c et h e s p a c ep a r t sU·Ef or mt h et i mec omp on e n tofaf ou rv e c t or , Emu s t b et h et i me –s p a c ep a r tofat e n s orofr a n kt wo. Th a ti s , 0 β E·U=F Uβ. ( 1 8 . 1 3 9 ) Wec ou l de a s i l yf i n dB i nas i mi l a rf a s h i ona n dc ou l de v e n t u a l l ywor kou tt h e e l e c t r oma g n e t i cf i e l ds t r e n g t ht e n s or .Howe v e r ,i ti smor ec on s t r u c t i v et ok e e pon ma k i n gf ou rv e c t or s , e t c . ou toft h er e s toft h er e l a t i on sa th a n d . F ore x a mp l e ,wea l r e a d yh a v eob s e r v e dt h a tt h ec on t i n u i t ye qu a t i oni sac ov a r i a n t 4 –s c a l a r : ∂ρ ( 1 8 . 1 4 0 ) ∇· J = 0 . ∂t+ Toma k ei t ’ sc ov a r i a n c ema n i f e s t , wed e f i n ea4 –c u r r e n t α J =( c ρ, J ) s ot h a t ( 1 8 . 1 4 1 ) α ∂αJ =0 ( 1 8 . 1 4 2 ) i st h ec on t i n u i t ye qu a t i on .Not et h a t( a sJ a c k s onr e ma r k s )t h i son l ywor k sb e c a u s e e l e c t r i cc h a r g ei saL or e n t zi n v a r i a n ta n ds oi saf o u r –d i me n s i on a lv ol u mee l e me n t ( s i n c ed e tA=+ 1 ) . Ne x t ,c on s i d e rt h ewa v ee qu a t i on sf ort h ep ot e n t i a l si nt h eL or e n t zg a u g e( n ot ewe l l t h a tJ a c k s onf orn oob v i ou sr e a s onIc a ns e es t i l lu s e sGa u s s i a nu n i t si nt h i sp a r tof c h a p t e r1 1 , wh i c hi sg oi i n gt oma k et h i sap a i nt oc on v e r tb e l ow –b e a rwi t hme ) : 2 1∂φ 2 2 c∂ t 0 J ρ 2 − ∇φ= ǫ0 =ǫ0c 0 = µ0 J 0 =µ0( c J) µ0ǫ0c s ot h a t : 1∂2( φ/ c ) 2 0c −∇2( φ/ c )=µ0J 2 ∂t 1∂ A 2 −∇2A 2 2 0 J = µ c∂ t Th e r e f or e , i fwef or mt h e4 –v e c t orp ot e n t i a l α φ c,A) A =( t h e nwec a nwr i t et h ev a r i ou s4 –r e l a t i on s : ( 1 8 . 1 4 3 ) ( 1 8 . 1 4 4 ) α ∂ A = 1 ∂ α A0 + ∇· A c∂t ( 1 8 . 1 4 5 ) ( 1 8 . 1 4 6 ) ( 1 8 . 1 4 7 ) ( 1 8 . 1 4 8 ) ( wh i c hi st h e4 –s c a l a rL or e n t zg a u g ec on d i t i on ) α βα α ✷A =∂ J β∂A =µ 0 ( 1 8 . 1 4 9 ) ( t h e4 –v e c t ori n h omog e n e ou se l e c t r oma g n e t i cwa v ee qu a t i onc on s t r u c t e df r om t h e 4 –s c a l a rD’ L a mb e r t i a nwa v eop e r a t or–t h es e toff ou rwa v ee qu a t i on sf orφa n dt h e c omp on e n t sofAa b ov e ) . Nowwec a nc on s t r u c tt h ec omp on e n t sofEa n dBf r om t h ec ov a r i a n t4 –v e c t or p ot e n t i a l . F ore x a mp l e , wek n owt h a t : ∂A E= − ∇φ− ( 1 8 . 1 5 0 ) ∂t 0 wh e r eφ=c A, s o 0 ∂Ax ∂c A 01 10 E c ∂( c ( ∂A −∂A) x=− c t )− ∂x =− ( 1 8 . 1 5 1 ) a n ds i mi l a r l y , s i n c eB=∇×A: ∂Az ∂Ay 23 32 ( ∂A −∂A) Bx= ∂y − ∂z =− ( 1 8 . 1 5 2 ) e t c . Th ec omp on e n t soft h ee l e c t r i ca n dma g n e t i cf i e l d s( a l l s i xoft h e m)t h u st r a n s f or m l i k et h ec omp on e n t sofas e c o n dr a n k , a n t i s y mme t r i c , t r a c e l e s s 7 f i e l ds t r e n gt ht e n s or: α β αβ βα F =∂A −∂A. ( 1 8 . 1 5 3 ) I ne x p l i c i tc omp on e n tf or m, E / c− E c− E c x y/ z/ 0 F =Ex/c α β E/ c − 0 − Bz By Bx Bz y E/ c z By 0 ( 1 8 . 1 5 4 ) . Bx − 0 − Th et e n s orwi t ht woc ov a r i a n ti n d i c e s( f or me db yt woc on t r a c t i on swi t hg )i sob t a i n e d b yr e p l a c i n gEwi t h− E . E / cEy/ cEz/ c x 0 F α β= − E / c x − Bz 0 − y / c B E z E/ c − Bz − y B x 0 By − 0 Bx . ( 1 8 . 1 5 5 ) 7Wi k i p e di a :h t t p : / / www. wi k i p e d i a . or g / wi k i / El e c t r oma g n e t i ct e n s or .Not et h a tI ’ m n otc omp l e t e l y c omf or t a b l ewi t ht h es i g n sf ort h ec ov a r i a n tf or moft h ep ot e n t i a li nt h eWi k i p e di aa r t i c l e , a l t h ou g hi t sma i n c on c l u s i on sa r es ou n de n ou g h . α β An ot h e ri mp or t a n tv e r s i onoft h i st e n s ori st h ed u a lf i e l ds t r e n gt ht e n s orF .I n t e r msoft h et ot a l l ya n t i s y mme t r i ct e n s oroft h ef ou r t hr a n ka n dt h en or ma lf i e l d s t r e n g t ht e n s ori ti sg i v e nb y : 1 B α β= ǫ α βγ δF 0 E c − Ey/ c. ( 1 8 . 1 5 6 ) γ δ= x z/ 0 F B 2 z − Bx − By E/ c − Bz E/ c y x 0 Ex/ c − Th i si sob t a i n e df r omt h eb a s i cc on t r a v a r i a n tf i e l ds t r e n g t ht e n s orb yt h es u b s t i t u t i on s E→ B, B→ − E .Con s i d e r a t i onoft h es e c t i ononma g n e t i cmon op ol e ss h owst h a tt h i s i si n d e e dap a r t i c u l a rd u a l i t yt r a n s f or ma t i onob t a i n e di nf r e es p a c ewi t ht h e“ r ot a t i on ” p a r a me t e re qu a l t oπ/ 2( i nJ 6 . 1 5 1 ) . F i n a l l y , wemu s twr i t eMa x we l l ’ se qu a t i on si nc ov a r i a n tf or m.Th ei n h omog e n e ou s e qu a t i on sa r e( r e c a l l ) ρ ∇·E 1∂ E 2 c ∂ t ∇× B− = ǫ0 ( 1 8 . 1 5 7 ) =µ0J ( 1 8 . 1 5 8 ) Th equ a n t i t yont h er i g h ti sp r op or t i on a lt ot h ef ou rc u r r e n t .Th equ a n t i t yont h el e f t mu s tt h e r e f or ec on t r a c ta4 –d e r i v a t i v ewi t ht h ef i e l ds t r e n g t ht e n s or .Yous h ou l dv e r i f y t h a t α β β ∂αF =µ J 0 β ( 1 8 . 1 5 9 ) e x a c t l yr e c on s t r u c t st h ei n h omog e n e ou se qu a t i onf ore a c hc omp on e n tofJ . Th eh omog e n e ou se qu a t i on s ∇× E + ∇·B ∂B = 0 ( 1 8 . 1 6 0 ) ∂t = 0 ( 1 8 . 1 6 1 ) a l s of or m af ou rv e c t or( ofz e r o’ s )a n dmu s th e n c eb et h ec on t r a c t i onofaf i e l d s t r e n g t ht e n s or .Bu twh i c hon e ?We l l , t h es e c on dh omog e n e ou se qu a t i onr e qu i r e st h a t B→ − Ea n db ot hr e qu i r et h a tE→ B, s oi tmu s tb et h ed u a l : α β ∂αF =0 . ( 1 8 . 1 6 2 ) I fwef e e lc omp e l l e dt owr i t ee v e r y t h i n gi nt e r msoft h ef i e l ds t r e n g t ht e n s ori t s e l f , t h i s c a nb ed on e . Th er e s u l ti st h ef ou re qu a t i on s ∂αFβγ+∂βFγα+∂ γ Fαβ =0 ( 1 8 . 1 6 3 ) wh e r eα , β, γa r ea n yt h r e eoft h ef ou ri n d i c e s0 , 1 , 2 , 3 . Howe v e r , t h i se qu a t i on i sat h i r dr a n kt e n s oront h el e f t , a n di t sr e d u c t i onb ys y mme t r yt oat e n s oroff i r s tr a n k i sn otma n i f e s t . I ti su g l y , i fy ouwi l l . Nowt h a tweh a v ewr i t t e nMa x we l l ’ se qu a t i on s( a n dt h ec on s e qu e n c e sofME )i n f ou rd i me n s i on a l f or m( r e ma r k i n ga l l t h ewh i l et h a tt h e ya r eu n u s u a l l y b e a u t i f u la n dc on c i s ei nt h i sn ot a t i on )wea r ed on e .Be f or eweg oont od e d u c e( f r om t h e s er e s u l t s )h owe l e c t r i ca n dma g n e t i cf i e l d sL T,h owe v e r ,wes h ou l dc omp l e t et h e qu e s t i onwi t hwh i c hweb e g a nt h ed i s c u s s i on , n a me l y , h owd oe sNe wt on ’ sl a wb e c ome c ov a r i a n t ?Th ea n s we ri s( n owt h a twek n owwh a tt h ef i e l ds t r e n g t ht e n s ori s ) α dp dτ α dU =m q = d τ α β F Uβ. c ( 1 8 . 1 6 4 ) Th et i me –c omp on e n te qu a t i oni sj u s tt h ewor k –e n e r g yt h e or e m,a n dt h es p a c e e qu a t i on sa r eNe wt on ’ sl a w. Asap os t s c r i p tt oou rd i s c u s s i on( r e c a l l i n gt h a ts ome t i me st h ef i e l d sp r op a g a t ei n s omeme d i u ma n dn o tf r e es p a c e )wen ot et h a ti nt h i sc a s et h eh omog e n e ou s e qu a t i on ( s )r e ma i nu n c h a n g e d ,b u tt h ei n h omg e n e ou se qu a t i on sa r emod i f i e d( b y u s i n gHa n dDi n s t e a dofBa n dE ) . Th ei n h omog e n e ou se qu a t i oni st h e n α β β ∂αG =µ J ( 1 8 . 1 6 5 ) α β wh e r eh op e f u l l yt h ed e f i n i t i onofG i sob v i ou s( t h a ti s ,s u b s t i t u t ev=1 / ǫµf orc t h r ou g h ou ti na p p r op r i a t ep l a c e s ,ori fy oup r e f e rr e c a p i t u l a t et h ee n t i r ed e r i v a t i on u s i n gHa n dDf r omt h eb e g i n n i n g ) . L e tu sp a u s ef oramome n tofr e l i g i ou ss i l e n c ea n dc on t e mp l a t eag r e a twon d e rof n a t u r e . Th i si st h es c i e n t i s t ’ sv e r s i onof“ p r a y e ri ns c h ool ” . 18. 7 Th eTr a n s f o r ma t i o no fE l e c t r o ma gn e t i cF i e l d s Nowt h a tweh a v et h i si nh a n d,wec a ne a s i l ys e eh owt ot r a n s f or mt h ee l e c t r i ca n d ma g n e t i cf i e l d swh e nweb oos taf r a me .Ofc ou r s e ,t h a td oe sn otg u a r a n t e et h a tt h e r e s u l twi l l b es i mp l e . α β ′ Toc on v e r tF f r o mKt oK, wemu s tc on t r a c ti t si n d i c e swi t ht h et r a n s f or ma t i on t e n s or s , ′ α ′ β ∂x ∂x γ δ ′ α β F= γ δF . ( 1 8 . 1 6 6 ) ∂x ∂ x Not et h a ts i n c eAi sal i n e a rt r a n s f or ma t i on : α Aγ= ′ α ∂ x ( 1 8 . 1 6 7 ) γ ∂x ( wh e r eIh a v ede l i b e r a t e l yi n s e r t e das p a c et odi ffe r e n t i a t et h ef i r s ti n d e xf r om t h e s e c on d )wec a nwr i t et h i si nt e r msoft h ec omp on e n t sofAa s : α ′ α β =A F γδA β F γ α δ γ δ˜ β =AγF A δ ( 1 8 . 1 6 8 ) or( i nac ompr e s s e dn ot a t i on ) : ′ ˜ F=AF A ( 1 8 . 1 6 9 ) Th i si sj u s tas p e c i f i cc a s eoft h eg e n e r a lr u l et h a tAc a nb eu s e di ng e n e r a lt o t r a n s f or ma n yn t hr a n kt e n s orb yc on t r a c t i n gi ta p p r op r i a t e l ywi t he a c hi n d e x . Aswes a wi nou rd i s c u s s i onofTh oma sp r e c e s s i on ,wewi l lh a v eoc c a s i ont ou s e t h i sr e s u l tf ort h ep a r t i c u l a rc a s eofap u r eb oos ti na na r b i t r a r ydi r e c t i ont h a twec a n wi t h ou tl os sofg e n e r a l i t yp i c kt ob et h e1d i r e c t i on . L e t ’ ss e eh owt h i sg oe s . Re c a l l t h a t Af orap u r eb oos ti nt h eon edi r e c t i oni st h ema t r i xf or me dwi t hal owe rr i g h tqu a d r a n t i d e n t i t ya n da nu p p e rl e f tqu a d r a n t2×2wi t hγont h ed i a g on a l a n d− γ βont h ec or n e r s ) . Th u s : s o: ′ 0 1 F ′ E 0 0 1 1 1 2E 1 2 2E γ c = − −γβ c 1 −c 0 1 0 1 = A0F A1 +A1F A1 ′ 2 22 E1 =( γ +γβ) E1 ′ E1 = E 1 ( 1 8 . 1 7 0 ) Not et h a tweh a v ee x t r a c t e dt h eor d i n a r yc a r t e s i a nc omp on e n t sofEa n dBf r omFa f t e r t r a n s f or mi n gi t .Il e a v et h er e s toft h e mt owor kou ty ou r s e l f .Yous h ou l db ea b l et o s h owt h a t : ′ E 1 ′ E 2 ′ E 3 ′ B1 ′ B2 ′ B3 1 =E ( 1 8 . 1 7 1 ) ( E B3) =γ 2−β ( 1 8 . 1 7 2 ) ( E B2) =γ 3+β ( 1 8 . 1 7 3 ) =B1 ( 1 8 . 1 7 4 ) ( B2+βE ) =γ 3 ( 1 8 . 1 7 5 ) ( B3−βE2) =γ ( 1 8 . 1 7 6 ) Th ec omp on e n toft h ef i e l d si nt h ed i r e c t i onoft h eb oos ti su n c h a n g e d,t h e p e r p e n d i c u l a rc omp on e n t soft h ef i e l da r emi x e d( a l mos ta si ft h e ywe r es p a c e –t i me p i e c e s )b yt h eb oos t .I fy ouu s ei n s t e a dt h eg e n e r a lf or mofAf orab oos ta n de x p r e s s t h ec omp on e n t si nt e r msofdotp r od u c t s ,y ous h ou l da l s os h ow t h a tt h eg e n e r a l t r a n s f or ma t i oni sg i v e nb y : ′ E ′ B 2 γ =γ ( E + β× B) − γ+1β( β·E ) γ ( 1 8 . 1 7 7 ) 2 =γ ( B− β× E ) − γ+1β( β·B) . ( 1 8 . 1 7 8 ) Ap u r e l ye l e c t r i corma g n e t i cf i e l di non ef r a mewi l lt h u sb eami x t u r eofe l e c t r i ca n d ma g n e t i cf i e l d si na n ot h e r .Wes e et h a tt r u l y , t h e r ei sl i t t l er e a s ont od i s t i n g u i s ht h e m.We h a v et ob eal i t t l ec a r e f u l ,ofc ou r s e .I ft h e r ei samon op ol a r( s t a t i c )e l e c t r i cf i e l di na n y f r a me , wec a n n ott r a n s f or mi tc omp l e t e l yi n t oama g n e t os t a t i cf i e l di na n ot h e r , f ore x a mp l e . Wh y ?Be c a u s et h ee qu a t i on sa b ov ewi l ll e a dt os omemi x t u r ef ora l lβ<1 ,a n dβ<1i n n a t u r ea sac on s t r a i n t . I e n c ou r a g ey out or e v i e wt h ee x a mp l eg i v e ni nJ a c k s ona n dme d i t a t eu p ont h er e ma r k s t h e r e i n . Wewi l l n ots p e n dv a l u a b l ec l a s st i meont h i s , h owe v e r . I n s t e a dwewi l le n dt h i s ,a f t e ra l l ,p u r e l yma t h e ma t i c a l / g e ome t r i c a lk i n e ma t i c a l i n t e r l u de( n oHa mi l t on i a n sorL a g r a n g i a n s=n op h y s i c s )a n dd os omep h y s i c s .L e tu s d e d u c et h ec ov a r i a n td y n a mi c s of r e l a t i v i s t i cp a r t i c l e si n( a s s u me df i x e d ) e l e c t r oma g n e t i cf i e l d s . Ch a p t e r19 Re l a t i v i s t i cDy n a mi c s 19. 1 Co v a r i a n tF i e l dTh e or y Wea r ei n t e r e s t e di nd e d u c i n gt h ed y n a mi c sofp oi n tc h a r g e dp a r t i c l e si n“ g i v e n ”( i . e . — f i x e d )e l e c t r oma g n e t i cf i e l ds . Wea l r e a d y“ k n ow”t h ea n s we r , i ti sg i v e nb yt h ec ov a r i a n t f or mofNe wt on ’ sl a w, t h a ti s : α dp dτ α =m dU q = FαβUβ. dτ c ( 1 9 . 1 ) F r omt h i swec a nf i n dt h e4 –a c c e l e r a t i on , α dU dτ = q α β F Uβ mc ( 1 9 . 2 ) wh i c hwec a ni n t e g r a t e( i np r i n c i p l e )t of i n dt h e4 –t r a j e c t or yoft h ep a r t i c l ei nqu e s t i on . Howe v e r , t h i si sn otu s e f u lt ou s .Re a lp h y s i c i s t sd o n ’ tu s eNe wt on ’ sl a wa n y mor e . Th i si sn ot h i n ga g a i n s tNe wt on ,i ti sj u s tt h a twen e e dHa mi l t on ’ sorL a g r a n g e ’ s f or mu l a t i onofd y n a mi c si nor de rt oc on s t r u c taqu a n t u mt h e or y( ore v e na ne l e g a n t c l a s s i c a lt h e or y ) .Ou rf i r s tc h or e ,t h e r e f or e ,wi l lb et og e n e r a l i z et h ea r g u me n t st h a t l e a dt ot h eE u l e r –L a g r a n g eorHa mi l t one qu a t i on sofmo t i ont of ou rd i me n s i on s . 19. 1. 1 Th eBr u t eF o r c eWa y Re c a l lt h a tt h eL a g r a n g i a np a t ht ot h ed y n a mi c sofap a r t i c l e( wh i c hi smos te a s i l yma de c ov a r i a n t , s i n c ei tu s e s( q( t ) , q˙ ( t ) , t )a si t sv a r i a b l e s )i sb a s e dont h e Ac t i on t 1 A= L ( q( t ) , q˙ ( t ) , t ) d t . t 0 ( 1 9 . 3 ) 2 7 7 Byr e qu i r i n gt h a tAb ea ne x t r e mu ma saf u n c t i o n a l oft h es y s t e mt r a j e c t or y , weob t a i n t h eE u l e r –L a gr a n gee q u a t i o n s d ∂L ∂L dt ∂qi̇ − ∂qi =0 . ( 1 9 . 4 ) Th e s ea r ee qu i v a l e n tt oNe wt on ’ sl a wf ors u i t a b l ed e f i n i t i on sofLa n dt h ef or c e .Th e s i mp l e s twa yt oma k et h i sr e l a t i v i s t i ci st oe x p r e s si ti nt e r msoft h ep r op e rt i mea n dt h e n r e qu i r et h a tt h ea c t i onAb ee x t r e ma l i na l l f r a me s . Th e n , τ 1 A= γ L d τ ( 1 9 . 5 ) τ 0 i st h ea c t i on , s i n c ed t=γ d τ. Wen owmu s tr e ma r k a )I ft h ee x t r e ma l c o n d i t i oni st ob ei n v a r i a n twi t hr e s p e c tt oL T’ s , t h e nAmu s tb e i n v a r i a n t( a n dh e n c ea4 –s c a l a r ) ; b )Th e r e f or e( s i n c edτi si n v a r i a n t )γ Lmu s tb ei n v a r i a n t ; c )F i n a l l y , s i n c eγi sj u s tan u mb e r , Lmu s tb ea4 –s c a l a r . Th i sf i n a l c on c l u s i ong r e a t l yc on s t r a i n st h ep os s i b l ef or msofL . Not ewe l l t h a ti ti sn otc l e a rt h a tt h i sa r g u me n t( f r omJ a c k s on )i sv a l i d. γ , wh i l ean u mb e r , i sn oti n v a r i a n tu n d e rb oos t s–i n d e e d , i ti sp a r a me t r i c a l l yr e l a t e dt o t h eb oos tp a r a me t e r !I ti sa l s op e r f e c t l yc l e a rt h a tt h ef i r s ts t a t e me n ti sf a l s e–wh i l ei t i st r u et h a ti fAi saf ou r s c a l a rwi t hr e s p e c tt ob oos t st h a ti t se x t r e mu msmu s tb e p r e s e r v e d ,i ti se qu a l l yt r u et h a tt h i sc on d i t i oni sn otn e c e s s a r i l yu n i qu e–a l lt h a ti s n e c e s s a r yi st h a tab oos tmon ot on i c a l l ys c a l et h ea c t i oni ns u c hawa yt h a tt h e e x t r e ma l p r op e r t yi sp r e s e r v e d ! Awe a k e r( b u ts t i l l s u ffic i e n t )a r g u me n tmi g h tt h e nb e : I fLi sa4 s c a l a r ,a n dγi samon ot on i cf u n c t i oni n de p e n d e n toft h e4 c oor di n a t e s ,4 v e l oc i t i e s ,a n dτ,t h e nt h ep r op e r t yofag i v e nt r a j e c t or y r e s u l t i n gi na ne x t r e mu moft h ea c t i oni sp r e s e r v e d. I nmyop i n i ont h i si sc l e a r e ra n ds t i l la d e qu a t ef orou rp u r p os e s .Lb e i n ga4 s c a l a r( 0 t h r a n kt e n s ori n4 s p a c ew. r . t .t h eL or e n t zt r a n s f or ma t i on )i ss u ffic i e n tt op r od u c ea n i n v a r i a n te x t r e mu moft h ea c t i onA, e v e ni ft h en u me r i c a l v a l u e sofAv a r yu n de rab oos t . To p r ov et h a ti ti sa l s on e c e s s a r yv e r yl i k e l yi n v ol v e sa ne x e r c i s ei nt h ec a l c u l u sofv a r i a t i on s t h a td i s t r i b u t e st h ed e r i v a t i v e sov e rγ L–s i mi l a re x e r c i s e sa r ea l r e a d yi nt h eh ome wor kf or t h i sc h a p t e ri not h e rc on t e x t s . E i t h e rwa y ,wewi l ln owa s s e r tt h a tt h eL a g r a n g i a nofaf r e ep a r t i c l emu s tb ea4 s c a l a r( a n dh e n c emu s tb ef or me dou toft h ef u l l c on t r a c t i onoft e n s or sofh i g h e rr a n k ) , a n dwi l lr e ma i na l e r ti nt h ewor kb e l owf ora n ys or tofi n c on s i s t e n c yt h a tmi g h tb e r e l a t e dt oγ . Ob v i ou s l y ,wewa n ti tt or e p r od u c et h ec l a s s i c a ln on –r e l a t i v i s t i ct h e or yi nt h e a p p r op r i a t el i mi t , t h a ti s , af r e ep a r t i c l es h ou l dh a v ec on s t a n te n e r g ya n d mome n t u m, or , e qu i v a l e n t l y , 4 –v e l oc i t y . Th es i mp l e s t( n ot“ on l y ”a sJ a c k s ons t a t e s ) L or e n t zi n v a r i a n tf u n c t i onoft h e4 –v e l oc i t yi si t ’ squ a dr a t i cf or m: α 2 U Uα=c ( 1 9 . 6 ) Ofc ou r s e ,a n yp ol y n omi a lf u n c t i on a lofb u tt h i squ a d r a t i ci sa l s oap os s i b l es c a l a r , Th u s t h e ya r en ott h et h i n gt ot r yf i r s t . g r a n g i a ni s ar e a s on a b l eg u e s sf ort h eL a 2 u 2−1 2 2 L=( c on s t a n t ) cγ =− mc 1− c . ( 1 9 . 7 ) I fwen owc r u n c ht h r ou g ht h eE u l e r –L a g r a n g ee qu a t i onwef i n dt h a tt h i sc h oi c ef or t h ec on s t a n tl e a dst o d ( γ mu )=0 dt ( 1 9 . 8 ) wh i c hi si n de e dNe wt on ’ sl a wf oraf r e ep a r t i c l e ,b u twi t ht h er e l a t i v i s t i cf or moft h e t h r e e –mome n t u m. I fon ec h oos e saf r a mewh e r et h ep a r t i c l ei si n i t i a l l ya tr e s t ,at r a j e c t or ywh e r ei t −1 r e ma i n sa tr e s twi l ly i e l dt h el e a s ta c t i on( y ous h ou l dc h e c kt h i s ) .Th i si sb e c a u s eγ i s ma x i ma l wh e nβ=0( a n dt h eL a g r a n g i a nh a sami n u ss i g n ) . Now,s u p p os et h a tt h ec h a r g e dp a r t i c l ei si nae l e c t r oma g n e tp ot e n t i a l .I fi twe r e mov i n gs l owl yi nas c a l a rp ot e n t i a lΦon l y ,i t sp ot e n t i a le n e r g ywou l db eV=qΦ.Th e n on –r e l a t i v i s t i cL a g r a n g i a ni nt h i sc a s es h ou l db ej u s tT−V( wh e r eTi st h ef r e e p a r t i c l eL a g r a n g i a n ) .Th ei n t e r a c t i onp a r toft h er e l a t i v i s t i cL a g r a n g i a nmu s tt h e r e f or e r e d u c et o− qΦi nt h i sn on –r e l a t i v i s t i cl i mi t . Wemu s tf i n daL or e n t zi n v a r i a n t( s c a l a r )f or mf orγ L h a tr e d u c e st o− qΦ f or i n tt n on –r e l a t i v i s t i cv e l oc i t i e s .Si n c eΦi st h et i mec omp on e n tofaf ou rv e c t orp ot e n t i a l , wec a n α g u e s st h a tt h ec or r e c tg e n e r a l i z a t i onmu s ti n v ol v et h ef ou rv e c t orp ot e n t i a lA.Si n c ei t α mu s tb eas c a l a r , i tmu s ti n v ol v et h es c a l a rp r od u c tofA wi t hs omef ou rv e c t or ( s ) . Th eon l y α on e sa v a l i a b l ea r ex a n d α U. α Th ec or r e c tγ L e p e n d son l yont h eU .I fi td e p e n d e dont h ec oor di n a t e sa swe l l , i n td t h e nt h ep h y s i c swou l dn otb et r a n s l a t i on a l l yi n v a r i a n ta n dt h er e s u l t sofou rc a l c u l a t i on mi g h twe l ld e p e n donwh e r ewec h os et h eor i g i n .Th i sd oe sn ots e e mr e a s on a b l e .On c e a g a i n ,t h i sd oe sn otu n i qu e l yd e t e r mi n ei t ,i ton l yd e t e r mi n e st h es i mp l e s t( l i n e a r )f or mt o wi t h i nas i g na n dac on s t a n t : q α γ L i n t=− cU A α or q u A. ( 1 9 . 9 ) ( 1 9 . 1 0 ) L i n t=− qΦ+c · Th e r ec ou l db ea d d i t i on a lt e r msi n v ol v i n gp ol y n omi a l soft h i squ a n t i t y ,t h ep r od u c t α AAα( wh i c hi si n d e e dp r e s e n ti ns omet h e or i e s )a n dot h e rs c a l a rr e d u c t i on soff i e l d a n dc h a r g e / f i e l dt e n s o rqu a n t i t i e s .L i n e a r i t y ,i ne i t h e rt h ev e c t orp ot e n t i a lort h e v e l oc i t y , i sa na x i oma n dn otl og i c a l l yn e c e s s a r y . Th ec omp l e t er e l a t i v i s t i cL a g r a n g i a nf orac h a r g e dp a r t i c l ei st h u s 2 u 2 q 1− +u A qΦ. 2 c· − c L=− mc ( 1 9 . 1 1 ) I ts h ou l dt a k ey oua b ou ton eh ou rt os h owt h a tt h i sy i e l d st h ec or r e c tr e l a t i v i s t i c L or e n t zf or c el a w. Th ef r e ep a r t i c l ep a r ti sob v i ou s , t h ee l e c t r i cf i e l di sob v i ou s . Youwi l l h a v et owor kab i t , u s i n g d ∂ d t = ∂t+ u·∇ ( 1 9 . 1 2 ) t os qu e e z e− u×( ∇×A)ou toft h er e ma i n d e r .Is u g g e s tt h a ty ous i mp l ywor kou tt h e t e r msb ye x p a n d i n gt h e ma sf a ra st h e yg oa n dr e a s s e mb l i n gt h ep i e c e s , b u ts omeof y ouma yk n owe n ou g hv e c t ora l g e b r at od oi tb e t t e rwa y s .Th i swi l lb eont h en e x t a s s i g n me n t , s of e e l f r e et os t a r t . Th ec a n on i c a lmome n t u mPc on j u g a t et ot h ep os i t i onc oor di n a t e sxi sob t a i n e d ( a su s u a l )f r om ∂L q Pi= ∂u mu . i =γ i+ cAi ( 1 9 . 1 3 ) Th i sr e s u l t , P= p q +A c ( 1 9 . 1 4 ) ( wh e r e pi st h er e l a t i v i s t i ck i n e t i cmome n t u moft h ep a r t i c l e )i se x t r e me l yi mp or t a n tt o r e me mb e r ,a si ti san e c e s s a r yi n g r e d i e n ti nt h ec on s t r u c t i onofe i t h e raqu a n t u m t h e or yora ne l e g a n tc l a s s i c a lt h e or y .Pl a c i n gt h ep a r t i c l ei naf i e l da l t e r si t sc a n on i c a l “ mome n t u m” . Wema k et h eHa mi l t on i a nou toft h eL a g r a n g i a na n dt h ec a n on i c a l mome n t u mv i a H=P· u−L ( 1 9 . 1 5 ) Th eb a s i cr e s u l th e r eh a st ooma n yv a r i a b l e s .Wemu s te l i mi n a t e ui nf a v orofAa n dP. Not et h a t c P−qA u= q P− cA 2 ( 1 9 . 1 6 ) 22 +m c ( s ome t h i n gt h a ti sawe ec h or et op r ov e ,ofc ou r s e ,b u ti ti ss t r a i g h t f or wa r da l g e b r a ) . Wi t he v e nmor et e d i ou sa l g e b r a , y ouc a ns h owt h a tt h eHa mi l t on i a ni s : 2 24 H= ( c P−qA)+m c +qΦ=W. ( 1 9 . 1 7 ) F r omt h i sr e s u l t , Ha mi l t on ’ se qu a t i on sofmot i ons h ou l dr e p r odu c et h eL or e n t zf or c el a w. Se et h a ti td oe s( a l t h ou g ht h er e l a t i on s h i pb e t we e nt h eELe qu a t i on sa n dHa mi l t on ’ s e qu a t i on sma k e st h er e s u l tob v i ou s ) .Not et h a ti fwei n t e r p r e tt h eHa mi l t on i a n( a su s u a l )a s t h et ot a l e n e r g yWoft h ep a r t i c l e , t h i sr e s u l ti sr e l a t e dt ot h ef r e ep a r t i c l ee n e r g yb y p→ ( P− q a n dt h ea d di t i onoft h es c a l a r cA) p ot e n t i a l e n e r g yqΦ. Th i si sa c t u a l l yj u s tas i n g l ec h a n g ei nt h ef ou r –v e c t ormome n t u m: 2 2 24 α ( W−qΦ)−( c P−qA)=m c =pp α ( wh i c hh a st h eu s u a l f or mi f E α p= cp , ( 1 9 . 1 8 ) 1 q W−e Φ) , P− = c( c A ( 1 9 . 1 9 ) ) . Th i sa l s oma k e st h ei n v a r i a n c ep r op e r t i e soft h eHa mi l t on i a nma n i f e s t . I ti sr e a l l ya n n oy i n gt oob t a i nt h ei n v a r i a n c ep r op e r t i e soft h i n g sa f t e rt h ef a c t .I ti s a l s oa n n oy i n g( a l t h ou g hp e r h a p su s e f u l )t oh a v et h et h r e ev e c t orc oor d i n a t e s x , u h a n g i n ga r ou n da tt h i sp oi n t .Sol e tu sr e d e r i v et h e s er e s u l t su s i n gon l yf ou r –v e c t or s a n ds u i t a b l es c a l a rr e d u c t i on s . 19. 1. 2 Th eE l e ga n tWa y Wec anwr i t et hef r e epa r t i c l eL agr angi anus i ngonl ys c al a rr e duc t i onsofs ui t a bl e 4 –v e c t o r s : mc α γ UαU L f r e e=− 2− 1 ( 1 9 . 2 0 ) . Th ea c t i o ni st h u s ( wh i c hi ss t i l l mc γ ) − τ 1 A=− mc τ 0 α UαU dτ . ( 1 9 . 2 1 ) Th ev a r i a t i on sont h i sa c t i onmu s tb ec a r r i e dou ts u b j e c tt ot h ec on s t r a i n t α 2 ( 1 9 . 2 2 ) UαU =c wh i c hs e v e r e l yl i mi t st h ea l l owe ds ol u t i on s . Wewr i t et h i sa s α d( UαU ) d τ α dUα Uα +UαdU dτ d τ α dUα αβ dU α g U + U α g βα dτ dτ β dU Uβ dτ Now, α UαU d τ= α dU +Uαdτ α dU 2Uαdτ α dU Uαdτ d τ = 0 = 0 = 0 = 0 = 0 α β g d x d x α β α d x x αd = 0 d τ= d τ ( 1 9 . 2 3 ) ( 1 9 . 2 4 ) wh i c hi sa ni n f i n i t e s i ma l l e n g t hi nf ou r –s p a c e .Th el a t t e re x p r e s s i ond oe sn ote x p l i c i t l y c on t a i nd τ.Wec a nt h u sp a r a me t e r i z et h ea c t i oni nt e r msofap a t h –p a r a me t e rst h a t i n c r e a s e smon ot on i c a l l ywi t hτb u ti sot h e r wi s ea r b i t r a r y . Th e n s 1 A=− mc g dx x αd β αβ s 0 d sds d s . ( 1 9 . 2 5 ) Wea r ec l e a r l yma k i n gp r og r e s s .Weh a v et a k e nap e r f e c t l yg oode x p r e s s i ona n dma d e i nu n r e c og n i z a b l e . Toma k ey oual i t t l eh a p p i e r , n ot et h a tt h i sh a sn owg ott h ef or mof ˜ ( 1 9 . 2 6 ) A= L d s ˜ wh e r eLi sas c a l a r“ L a g r a n g i a n ”wr i t t e ni nt e r msofa ni n d e p e n d e n tf r e ep a r a me t e r .Th i s mi g h tb ep r og r e s sa f t e ra l l , s i n c eweh a v equ a s h e dt h ea n n oy i n g −1 γ . I fwen owd ot h ec a l c u l u sofv a r i a t i on st h i n ga n dg e tt h eE u l e r L a g r a n g ee qu a t i on s i nf o u rd i me n s i on s : d d s ˜ ∂ d L dx α ds α −∂ ˜ L=0 ( 1 9 . 27 ) ( f orα=0 , 4 ) . Ap p l y i n gt h e mt ot h eL a n g r a n g i a ni nt h i sa c t i on , t h e yt u r nou tt ob e : 1 2 δβdx βd x δ d∂ g mc ds ∂ d s d s dx α d s =0 ( 1 9 . 2 8 ) α α dx mcd ds ds 2d s + dx ds ds β dx βd x =0 ( 1 9 . 2 9 ) =0 . ( 1 9 . 3 0 ) α d x d mcds dsds ds β dx βd x Th i ss t i l ld oe sn oth a v et h ec on s t r a i n ta b ov ei mp os e doni t .Wei mp os et h e c on s t r a i n tb yi de n t i f y i n gd swi t hdτi ns u c hawa yt h a tt h ec on s t r a i n ti ss i mu l t a n e ou s l y s a t i s f i e d : α dx x αd d sds d d τ α d s= c dτ = 2 c d α d x xd s αd d s ds ( 1 9 . 3 1 ) ( wh i c hr e qu i r e sb ot hd s=dτa n dUαU =c) .I fy oul i k e , t h i sc on s t r a i n tp i c k sou tofa l l p os s i b l ep a t hp a r a me t e r i z a t i on st h eon et h a tf ol l owst h ep r op e rt i me wh i l ek e e p i n gt h ef ou rv e c t orv e l oc i t ys c a l a rp r od u c tL or e n t zi n v a r i a n t .F orf r e ep a r t i c l e s t h i si sal otofwor k , b u ti ti sp a i db a c kwh e nwei n c l u d ea ni n t e r a c t i on . I fwemu l t i p l yt h eE u l e r L a g r a n g ee qu a t i on( i nt e r msofs )f r omt h el e f tb y : c α d x x αd d s ds a n du s et h ec on s t r a i n tt oc on v e r tt oτ, t h er e s u l t( f ort h ee qu a t i onofmot i on ) i s : ( 1 9 . 3 2 ) c m d c dx βd s βd x d sd s or d α x =0 dx βd s βd x d s ( 1 9 . 3 3 ) ds 2 α d x md τ2=0 ( 1 9 . 3 4 ) wh i c hc e r t a i n l yl ook si th a st h er i g h tf or m. Wec a ni n c l u d ea ni n t e r a c t i on .J u s ta sb e f or e , γ L s tb eaL or e n t zs c a l a r .Wh e n i n tmu wema k eap a r a me t e r i z e dv e r s i ono ft h eL a g r a n g i a n ,t h ep a r tu n d e rt h ei n t e g r a lmu s t b ea4 –s c a l a r . Th ec ov a r i a n tf or moft h er e s u l ti s( h op e f u l l yob v i ou s l y ) s 1 dδdx β d sds δ β mcg A=−s0 qdx β β s . c d + sA d ( 1 9 . 3 5 ) qdx β ( 1 9 . 3 6 ) Th e“ f ou rL a g r a n g i a n ”i nt h i se qu a t i oni s δd x β δβd ˜ d sds L=−mcg β sA +c d . Asb e f or ewec on s t r u c tt h eE u l e r L a g r a n g ee qu a t i on . β mcd s dxβ dx +cA −cd s∂A =0 α d x d ds ds d s q qx β αβ α ( 1 9 . 3 7 ) ( 1 9 . 3 8 ) Ag a i nwemu l t i p l yt h r ou g hf r omt h el e f tb y c ( 1 9 . 3 9 ) α d x x αd d sd s a n dc on v e r tt oτt og e t : α d2x 2 m dτ α qdA qdx β αβ cd τ −c d τ∂A =0 + ( 1 9 . 4 0 ) α Th ede r i v a t i v ed A/ d τi sab i tj a r r i n g .Howe v e r , i fwee x p r e s st h i st ot a l d e r i v a t i v ei n t e r msofp a r t i a l sweob s e r v et h a t : α d A dτ dx dx β ∂ α β βα A= ∂A = β dτ ∂x d τ ( 1 9 . 4 1 ) Su b s t i t u t i n g , t h ee qu a t i onofmot i onb e c ome s : α α d2x d( mU ) 2 =m dτ d τ q α β dx β β α q α β Uβ. c τ =F =c( ∂ A −∂ A )d ( 1 9 . 4 2 ) wh i c hi s , a se x p e c t e d , t h eL or e n t zf or c el a wi nc ov a r i a n tf or m! Howl ov e l y ! Toma k eaHa mi l t on i a ni nt h i sn ot a t i on ,wemu s tf i r s tma k et h ec a n on i c a l mome n t u m: ˜ q α α x ( 1 9 . 4 3 ) α P= − ∂ ds =mU + cA α ∂L wh i c hi sac ov a r i a n tv e r s i onoft h ec o mp l e t es e tofi n t e r a c t i one qu a t i on sf r om t h e p r e v i ou ss e c t i on( i td oe sb ot he n e r g ya n d3 –mome n t u m) . Th e r ea r es e v e r a lwa y st oma k eaHa mi l t on i a n( r e c a l lt h a ti ng e n e r a lt h e r ei swh a t a mou n t st og a u g ef r e e d om,mi n i ma l l yt h ea b i l i t yt oa dda na r b i t r a r yc on s t a n twh i c h 1 n a t u r a l l yd oe sn ota ffe c tt h er e s u l t i n gd i ffe r e n t i a l e qu a t i on s ) . On ei s: ˜ H= UαP Aga i n , wemu s te l i mi n a t e : ˜ α ( 1 9 . 4 4 ) +L 1 q α Uα = m α P − cA ( 1 9 . 4 5 ) α α i nf a v orofP , A. Th u s : ˜ 1 L=− mc 2 m q α Pα− cAα q α P − cA q q α − mc Pα − cAα A ( 1 9 . 4 6 ) a n d ˜ q α 1 H=m Pα− cAα P −mc q q 1 2 m q Pα− cAα q α α P − cA α 1 −mc Pα− cAα A q qα α = m Pα− cAα P − cA q −c Pα− cAα α P q ( 1 9 . 4 7 ) α −c A( 19. 48) Th i sHa mi l t on i a ni nf ou rd i me n s i on si sn ol on g e ra ne n e r g ys i n c ei ti sob v i ou s l ya 4 –s c a l a ra n de n e r g yt r a n s f or msl i k et h et i me –c omp on e n tofaf ou rv e c t or . 1 Not et h a tI ’ v er e a r r a n g e dt h i ss l i g h t l yt oa v oi dh a v i n gt odol ot sofs t u ffwi t hgs a n d wi c h e sb e l ow. Howe v e r , i twor k s . Ha mi l t on ’ se qu a t i on s( i nf ou rd i me n s i on s )l e a da g a i nd i r e c t l yt ot h e r e l a t i v i s t i cL or e n t zf or c el a w: ˜ α dx α dP 1 ∂H =∂Pα d τ ˜ ∂H q α q α˜ q ∂ H= α =− d τ =− ∂x mc Pα− cAα Th e r ei sab i tofa l g e b r ai n v ol v e di nd e r i v i n gt h i sr e s u l t . h a st or e c og n i z et h a t : α α α q α cA p =mU =P − α 22 ( 1 9 . 4 9 ) α = m P − cA αβ ∂A ( 1 9 . 5 0 ) F ore x a mp l e , on e ( 1 9 . 5 1 ) a n dp p =m c a n da p p l yt h i st oe l i mi n a t eu n wa n t e dt e r msj u d i c i ou s l y ,t h a ti sa f t e r α d i ffe r e n t i a t i on .I fy oua p p l yi tt ooe a r l y( f ore x a mp l ea tt h eb e g i n n i n g )y ouob s e r v et h e p u z z l i n gr e s u l tt h a t : ˜ H= 1 α √ α m pαp −c pαp 1 22 √ 2 = m m c−cm2c 2 2 = mc −mc = 0 ( 1 9 . 5 2 ) wh i c hl e a d son et ot h ev e r yZe nc on c l u s i ont h a tt h ec a u s eofa l lt h i n g si sNot h i n g( i n f ou rdi me n s i on s , y e t ) ! Wea r el e f twi t har a t h e rmy s t i f i e df e e l i n gt h a tt h ea l g e b r a i ch a n di squ i c k e rt h a n t h ee y e .Some h owa ne qu a t i onwh os ef o u r s c a l a rv a l u ei sz e r oh a saf u n c t i on a lf or m, as t r u c t u r e ,t h a tl e a d st on on –z e r o,c ov a r i a n te qu a t i on sofmot i on .Al s o( a sa l r e a d y r e ma r k e d)t h i sHa mi l t on i a ni sn otu n i qu e .Ob v i ou s l yon ec a na dda na r b i t r a r yf ou r s c a l a rc on s t a n tt ot h ee qu a t i ona n dg e tn oc on t r i b u t i onf r omt h ed e r i v a t i v e s( j u s ta s on ec a ni nn on r e l a t i v i s t i cc l a s s i c a lp h y s i c s ) .Th e r ea r eot h e rg a u g ef r e e d oms– u l t i ma t e l yt h e r es e v e r a lot h e rwa y sofwr i t i n gt h eHa mi l t on i a na s s oc i a t e dwi t ht h e g i v e nL a g r a n g i a n ;a l loft h e my i e l dac on s t a n tv a l u et h a ti sn ott h ee n e r g ywh e n e v a l u a t e da n dy i e l dt h ec or r e c te qu a t i on sofmot i onwh e np r oc e s s e d . F i n a l l yt h e r ee x i s twh a ta r ec a l l e ds i n gu l a rL a gr a n gi a n s–L a g r a n g i a n sf orwh i c h t h eg e n e r a l i z e dc oor d i n a t e sd on ota l wa y sma pi n t og e n e r a l i z e dc on j u g a t ev a r i a b l e s ! Di r a cwa s( u n s u r p r i s i n g l y )t h ef i r s tt of or ma l l yi d e n t i f yt h i si nt h ec on t e x tof c on s t r a i n e ds y s t e ms( s y s t e msd e s c r i b e db yaL a g r a n g i a na n dc on s t r a i n t swi t h L a g r a n g emu l t i p l i e r sf orwh i c ht h eHe s s ed e t e r mi n a n tv a n i s h e s ) ;Be r g ma n n( a t Sy r a c u s e )a l s oma d ema j orc on t r i b u t i on st ot h ef or ma ld e v e l op me n toft h ec on c e p t . Howe v e rt h er oot soft h ep r ob l e mda t emu c hf u r t h e rb a c kt oe . g .Noe t h e r ’ st h e or e m.I h a v eac ou p l eofp a p e r sont h i st h a tI ’ v ec ol l e c t e df r omt h ewe b ,a l t h ou g ht h ei de ai s a l s odi s c u s s e di nv a r i ou smon og r a p h sa n dt e x t b ook sonme c h a n i c s . I ti swor t hp oi n t i n gou tt h a tt h e r ewa sa ton ep oi n tc on s i d e r a b l ewor kb e i n gd on e h e r ea tDu k eont h ei d e a–N. Mu k u n d a , Ma xL oh e , ( b ot hf r i e n dsof mi n e )wor k e dont h ei d e awi t hL a r r yBi e de n h a r n( mya d v i s or ) ;Bi e d e n h a r na l s o p u b l i s h e dwor kwi t hL ou c kont h es u b j e c t ,a n dofc ou r s eMu k u n d aa n dSu d a r s h a n ’ s b ookonc l a s s i c a l me c h a n i c sr e ma i n sa“ c l a s s i c ” .Si n c eDi r a c ’ st i met h en ot i ont h a tt h e “ r i g h t ”u n i f i e df i e l dt h e or ywi l lh a v ec e r t a i ni n t e r e s t i n gp r op e r t i e sr e l a t e dt ot h i sh a s b e e nb a t t e da r ou n d . Th i sp oi n t sou ta non g oi n gp r ob l e mi nr e l a t i v i s t i cqu a n t u mt h e or i e s .Th e s et h e or i e s a r eg e n e r a l l yb a s e donaHa mi l t on i a n ,b u tma n i f e s t l yc ov a r i a n tHa mi l t on i a n sf ora g i v e ns y s t e mc a n n oti ng e n e r a lb eu n i qu e l yd e r i v e df r om f i r s tp r i n c i p l e sa st h e ma p p i n gb e t we e nv e l oc i t i e sa n dmome n t ai sn ota l wa y son e t oon e .Th u se v e nwh e na c ov a r i a n tL a g r a n g i a nd e n s i t yc a nb ec on s t r u c t e d ,t h ea s s oc i a t e dHa mi l t on i a ni sn ot ob v i ou sorn e c e s s a r i l yu n i qu e .Th i si sj u s ton e( a l t h ou g hi ti son eoft h emos t f u n da me n t a l )ob s t a c l e st oov e r c omewh e nde v e l op i n gar e l a t i v i s t i cqu a n t u mf i e l d t h e or y . 19. 2 Mot i o no faPo i n tCh a r gei naSt a t i cMa gn e t i c F i e l d Nowt h a tweh a v eob t a i n e dt h ev a r i ou sc ov a r i a n tf or msoft h eL or e n t zf or c el a w,we c a ne a s i l yd e t e r mi n et h et r a j e c t or i e sofc h a r g e dp a r t i c l e si nv a r i ou sf i x e df i e l ds . I nf a c t , wec ou l dh a v edon et h i swe e k sa g o( i fn oty e a r s )e v e nwi t h ou tk n owi n gt h ec ov a r i a n t f or ms . I nas t a t i cma g n e t i cf i e l d , t h ee qu a t i on sofmot i ona r e : dE d t d p = =0 q v ( 1 9 . 5 3 ) B ( 1 9 . 5 4 ) d t c × f ort h ee n e r g ya n dmo me n t u m,r e s p e c t i v e l y( a r r a n g e dl i k ep i e c e sofaf ou rv e c t orf or c l a r i t y ) .Cl e a r l yt h es p e e doft h ep a r t i c l ei sc on s t a n ts i n c et h ef or c ei sp e r p e n d i c u l a rt o t h emot i ona n dd oe sn owor k . γi st h e r e f or ea l s oc on s t a n t . Th u s d v v× ωB d t= ( 1 9 . 5 5 ) wh e r e qB qc B ωB= γ mc = E ( 1 9 . 5 6 ) i st h eg y r a t i onorp r e c e s s i on( c y c l ot r on )f r e qu e n c y .Th emot i ond e s c r i b e db yt h i s e qu a t i oni sac i r c u l a rmo t i onp e r p e n d i c u l a rt oBc ou p l e dt oau n i f or mmot i onp a r a l l e l t o B. Th i si st ood r ol lf orwor d s( a n di nf a c ty ouh a v ep r ob a b l ya l r e a d yt a u g h ti tt oy ou r k i d si nk i d d yp h y s i c s )b u ti td oe sy i e l do n ei mp or t a n tr e s u l t .Th ema g n i t u d eoft h e mome n t u mp e r p e n d i c u l a rt oBi s c p Ba ⊥=q ( 1 9 . 5 7 ) wh e r eai st h er a di u soft h ec i r c u l a rh e l i x .F r om t h i s( i n ,f ore x a mp l e ,ab u b b l ec h a mb e r , wh e r et h et r a c kc a nb ep h ot og r a p h e d )a n dak n owl e d g e( org u e s s )a st h et h ec h a r g e ,t h e t r a n s v e r s emome n t u mc a nb eme a s u r e d .Me a s u r i n got h e rt h i n g s( l i k et h er a t eofc h a n g eof t h ec u r v a t u r eoft h et r a c k )c a ny i e l dt h ema s soft h ep a r t i c l ef r om ak n owl e d g eofi t s mome n t u m.F r om t h e s eh u mb l et r a c e st h ee n t i r ep i c t u r ewec u r r e n t l yh a v eoft h e s u b –a t omi cz ooh a sb e e nb u i l tu p . Se c t i on s1 2 . 2 1 2 . 4a r et oos i mp l et owa s t et i meon .1 2 . 5 1 2 . 6a r ei n t e r e s t i n gb u t i mp or t a n ton l yt op l a s map e op l e .1 2 . 7i sr e d u n d a n toft h i n g swewi l l d oc or r e c t l yl a t e r . Th u swes k i pt o1 2 . 8 , l e a v i n gy out or e a da n yora l l oft h ei n t e r me di a t ema t e r i a l ony ou r own . Wewi l l s k i p1 2 . 9 . F i n a l l y , wewi l l do1 2 . 1 0 –1 2 . 1 1t oc omp l e t ec h a p t e r1 2 . 19. 3 Bu i l d i n gaRe l a t i v i s t i cF i e l dTh e o r y Weh a v en otqu i t ef i n i s h e dt h ej obofb u i l d i n gap r op e rr e l a t i v i s t i cf i e l dt h e or yof e l e c t r oma g n e t i s m.Th a ti sb e c a u s ewewou l dl i k et ob ea b l et oob t a i na l loft h e e qu a t i on sofmot i on( t h a ti s ,p h y s i c s )d e s c r i b i n gt h es y s t e mf r omac ov a r i a n ta c t i on p r i n c i p l e .Weh a v ed on et h a tf ort h ep a r t i c l e si nt h ef i e l d s ,b u twh a ta b ou tt h ef i e l d s t h e ms e l v e s ?I nf a c t ,s i n c et h ep a r t i c l e sp r odu c e( a n dh e n c emodi f y )t h ef i e l d s ,wed o n ote v e nh a v et h ec or r e c ts ol u t i on sf ort h ep a r t i c l e sa l on e ,y e t .L e tu ss e ei fwec a n d e v e l opas u i t a b l eL a g r a n g i a nf ort h ef i e l dst h a tl e a d s , i d e a l l y , t oMa x we l l ’ se qu a t i on s . Th eRu l e sf orb u i l di n gaf i e l dt h e or yL a g r a n g i a na r eofi n t e r e s ti na n doft h e ms e l v e s , s i n c et h e ya r equ i t eg e n e r a l . Th er u l e sa r e : a )Ta k et h ep os i t i ona n dv e l oc i t yc oor d i n a t e sf orc on t i n u ou ss p a c et i mea n d r e p l a c et h e mwi t hf i e l dv a r i a b l e s . b )L a b e l t h ef i e l dv a r i a b l e swi t hd i s c r e t e( c oor d i n a t ed i r e c t i on )l a b e l sa n dwi t h c on t i n u ou s( p os i t i on )v a r i a b l e s . c )Re p l a c et h e“ v e l oc i t y ”wi t ht h e4 –g r a d i e n t . d )Re qu i r et h ea c t i ont ob es t a t i on a r yw. r . t . v a r i a t i on si nt h ef i e l dv a r i a b l e s t h e ms e l v e sa n dt h e i rg r a d i e n t s . Th a ti s , α k i → x, ( 1 9 . 5 8 ) qi → φk( x ) ( 1 9 . 5 9 ) α x ) qi̇ → ∂φk( L= d i ∂L d t ∂qi̇ Li ( qi , qi̇ )→ =∂L ∂qi → ( 1 9 . 6 0 ) α 3 L ( φk, ∂φk) dx ∂β ∂L β ∂∂ φk = ∂L . ∂φk ( 1 9 . 6 1 ) ( 1 9 . 6 2 ) Wh e nwema k ea na c t i oni n t e g r a l ,wei n t e g r a t eLov e rt i me ,ma k i n gt h et ot a l i n t e g r a lf ou rd i me n s i o n a l .Wet h e r e f or ec a l lL t h eL a gr a n gi a nd e n s i t yi nf ou r d i me n s i on s .Not et h a tt h ea c t i onwi l l b ec ov a r i a n tp r ov i d e dt h eL a g r a n g i a nd e n s i t yi sa 4 –s c a l a r .Th i si swh a tIh a v eme a n twh e n e v e rIh a v ei n a d v e r t a n t l yc a l l e dt h e “ L a g r a n g i a n ”as c a l a r .Good ,c l e a n ,r e l a t i v i s t i ct h e or i e swi t horwi t h ou tp a r t i c l e sa r e ma d eou tofs c a l a rL a g r a n g i a nd e n s i t i e s , n otL a g r a n g i a n sp e rs e : A= 3 4 L dx d t= L dx . ( 1 9 . 6 3 ) Wen owd ot h eu s u a lda n c e .Wek n owt h a tLf ort h ef i e l d smu s tb eas c a l a r .Wea l s o k n owt h a tMa x we l l ’ se qu a t i on sr e l a t et h ef i e l d st ot h ec u r r e n t st h a tp r od u c et h e m, a n da l s o α β l i n kt h ee l e c t r i ca n dma g n e t i cf i e l ds .Wet h u sn e e dt ob u i l dat h e or you tofF Va r i ou swa y swec a ndot h i si n c l u d e α α ,A,J. α β F α βF α J A α α β F α βF a n ds t i l l me s s i e rp i e c e sl i k e αβ F α βJA . Th ef i r s tt wot e r msa r ei n v a r i a n tu n d e rt h et r a n s f or ma t i on soft h ef u l lL or e n t zg r ou p .Th e t h i r di sn otas c a l a ru n d e ri n v e r s i on , b u tap s e u d os c a l a r( od du n d e ri n v e r s i on ) .Wer e j e c ti t . Th el a s ti same s s .Wer e j e c ti t .Wewa n tat e r mqu a d r a t i ci nt h e4 g r a d i e n t soft h ef i e l ds . Th i si st h ef i r s tt e r m.Wewa n tas ou r c et e r mt oc ou p l et h ef i e l d sa n dt h ep a r t i c l e s .Th e s e c on dt e r md oe st h a t . So, wet r yaL a g r a n g i a nd e n s i t ywi t hj u s tt h e s et wot e r ms , wi t hu n k n ownc on s t a n t s Qa n dRt h a th a v et ob ed e t e r mi n e ds ot h a tt h e yc or r e c t l yr e c on s t r u c tMa x we l l ’ s e qu a t i on si nwh a t e v e rs y s t e mofu n i t swel i k e : α β α L=− QF J A. α βF −R α ( 1 9 . 6 4 ) β α Wen e e dt ot a k ed e r i v a t i v e sofLwi t hr e s p e c tt o∂ A, s oi ti su s e f u lt owr i t et h i s L a g r a n g i a ni nt h ef or m: µσ σµ λν νλ α L=− Qg ∂A −∂A ) ( ∂A −∂ A)−RJ A. λ µg ν σ( α ( 1 9 . 6 5 ) β α Wh e nwef or m∂L / ∂( ∂ A)weg e td e l t af u n c t i on swh e n e v e rαa n dβa r ee qu a l t oap a i r oft h ei n di c e sa b ov e . Wet h e r e f or eg e tf ou rt e r ms : ∂L = Qg ∂( ∂A) − βα g λ µ ν σ δµδσFλν βα ν δσδµFλν+δλδνFµσ − βα βα λ Fµσ δδ − ( 1 9 . 6 6 ) βα wh e r et h ef i r s tt wot e r msc omef r omd e l t af u n c t i on sf or me df r omt h ef i r s tt e r ma n dt h e s e c on dt wot e r msc omef r omd e l t af u n c t i on sf or me df r omt h es e c on dt e r m. α β g ss y mme t r i c( i nf a c t ,d i a g on a l ) .Th eF i sa n t i s y mme t r i c .Wh e nwed ot h es u ms α βi a g a i n s tt h eδ–f u n c t i on s , t h ef ou rt e r msma k ei de n t i c a l c on t r i b u t i on s : ∂L βA α ∂ ( ∂ )=−4QFβα=4QFαβ. ( 1 9 . 6 7 ) Th eot h e rp a r toft h eE –Le qu a t i on( wh i c hc or r e s p on dsi np os i t i ons p a c et ot h e “ p ot e n t i a l ” , or“ f or c e ”t e r m)i s ∂L = RJ . α ( 1 9 . 6 8 ) ∂A − α Th e r e f or et h ee qu a t i on sofmot i onf ort h ee l e c t r oma g n e t i cf i e l dc a nb ewr i t t e n β 4 Q∂F J . βα=R α ( 1 9 . 6 9 ) I fon ec h e c k sb a c ki non e ’ sn ot e s , on es e e st h a tt h i si si n d e e dt h ec ov a r i a n tf or moft h e i n h omog e n e ou sMa x we l l ’ se qu a t i on si fQ=1 / 4a n dR=µ : 0 β ∂F J βα=µ 0 α ( 1 9 . 7 0 ) f ol l owsf r omt h eL a g r a n g i a nd e n s i t y : 1 α β α J A. α βF −µ 0 α L=− 4F ( 1 9 . 7 1 ) Th e r e f or et h eL a g r a n g i a nweh a v ec on s t r u c t e dy i e l d st h ei n h omog e n e ou sMa x we l l e qu a t i on s ,b u tn ott h eh omog e n e ou son e s .Th a ti sok a y ,t h ou g h ,b e c a u s eweh a v e α β α c on s t r u c t e dt h eF i nt e r msoft h eA i ns u c hawa yt h a tt h eh omog e n e ou son e sa r e s a t i s f i e da u t oma t i c a l l y !Toob s e r v et h a tt h i smi r a c l ei st r u e ,wer e c a l lt h ec ov a r i a n t f or moft h eh omog e n e ou se qu a t i on s : α β ∂ F =0 . α Al s o, 1 F= αβ Th u s α β ∂αF ( 1 9 . 7 2 ) α βγ δ 2ǫ 1 F . γ δ α βγ δ = 2∂αǫ α βγ δ =∂ ǫ α F γ δ ∂γAδ α βγ δ =ǫ ( 1 9 . 7 3 ) ∂α∂γAδ ( 1 9 . 7 4 ) α βγ δ i st h ef i r s tt e r m.Bu t∂α∂γi ss y mme t r i c ,wh i l eǫ i sa n t i s y mme t r i ci nt h es a met wo i n d i c e s , s ot h ec on t r a c t i onont h et woi n d i c e sv a n i s h e s( wor ki tou tt e r mb yt e r mi fy ou dou b ti t ) . α β Th u st h eh omog e n e ou se qu a t i on sa r es a t i s f i e db you rde f i n i t i o n ofF qu i t e i n d e p e n d e n tofa n yd y n a mi c s .I nf ou rd i me n s i on s ,a l loft h ei n h omog e n e ou ss ou r c et e r ms mu s ta p p e a ri ne qu a t i on swi t ht h ef or moft h ei n h omog e n e o u s e qu a t i ona b ov e ,a n don l yon eoft h e s ee qu a t i on sc a nr e s u l tf r omt h ea c t i onp r i n c i p l e . Th es i mi l a r i t yt r a n s f or ma t i ont ot h ef i e l dsweob s e r v ei st h u st h e“ n a t u r a l ”f or moft h e ME ’ s ,a n di nf ou rd i me n s i on st h eh omog e n e ou se qu a t i on sa r er e a l l yn oti n d e p e n d e n t a sf a ra st h ea c t i onp r i n c i p l ei sc on c e r n e d.Not et h a tt h i si sf u n d a me n t a l l yb e c a u s eou r f i e l ds t r e n g t ht e n s orde r i v e sf r omt h ede f i n i t i onoft h ema g n e t i cf i e l da st h ec u r l oft h e v e c t orf i e l dA( wh i c hi sd i v e r g e n c e l e s s )wh i c hi sb u i l ti n t ot h ed e f i n i t i on . Asaf i n a lqu i x ot i cn ot e , ob s e r v et h a ti fwet a k et h e4 –di v e r g e n c eofb ot hs i d e sof t h ei n h omog e n e ou sMa x we l l e qu a t i on s : αβ α ∂∂F ∂J βα=µ 0 α ( 1 9 . 7 5 ) t h el e f th a n ds i d ev a n i s h e sb e c a u s ea g a i n ,as y mme t r i cd i ffe r e n t i a lop e r a t ori s c on t r a c t e dwi t hac omp l e t e l ya n t i s y mme t r i cf i e l ds t r e n g t ht e n s or . Th u s α ∂J , α=0 ( 1 9 . 7 6 ) wh i c h ,b ys omes t r a n g ec oi n c i de n c e ,i st h ec h a r g e –c u r r e n tc on s e r v a t i one qu a t i oni nf ou r di me n s i on s .Doy oug e tt h ef e e l i n gt h a ts ome t h i n gv e r yd e e pi sg oi n gon ?Th i si swh a tI l ov ea b ou tp h y s i c s . Be a u t i f u l t h i n g sa r er e a l l yb e a u t i f u l ! Wewi l ln owb l owo fft h e“ p r oc a ”L a g r a n g i a n ,wh i c hwou l db ea p p r op r i a t ei ft h e p h ot onh a dama s s . I td oe s n ’ t , b u ti fi tdi dy ouwou l dn e e dt or e a dt h i sc h a p t e r . I tmi g h t , ofc ou r s e ,s oy ous h ou l dp r ob a b l yr e a dt h ec h a p t e ra n y wa y ,b u ti tc u r r e n t l y( b a dp u n ) d oe s n ’ ts oI ’ mg oi n gt oma k el i g h tofi t( wo r s ep u n )a n dc on t i n u e . I fweh a don emor emon t h , wewou l dn ows t u dyt h ec ov a r i a n tf or msoft h es t r e s s t e n s or .I ti si mp or t a n t ,b u ti ti sa l s oqu i t ed i ffic u l t ,a n dn e c e s s i t a t e sab r oa de r di s c u s s i ont h a nwec a nn owa ffor d .Tot r e a tt h es u b j e c tp r op e r l y ,wewou l dn e e dt o t r e a tp a r t sofc h a p t e r1 7s i mu l t a n e ou s l y ,a n dwewou l dn e e dt odoal otofa l g e b r a . Th i swou l dme a nt h a twewou l dmi s s( i na l lp r ob a b i l i t y )b e i n ga b l et ol e a r nt h e L íe n a r d –Wi e c h a r tp ot e n t i a l ,wh i c hi sf a rmor ei mp or t a n t .Wewi l lt h e r e f or ec on t e n t ou r s e l v e swi t hme r e l yd e f i n i n gt h es t r e s st e n s or ,r e ma r k i n gons omeofi t sp r op e r t i e s wi t h ou tp r oof ,a n dmov i n gon .Youa r er e s p on s i b l ef orwor k i n gy ou rwa yt h r ou g ht h i s c h a p t e r , a c c or d i n gt oy ou rn e e d s , i n c l i n a t i on s , a n da b i l i t i e s , ony ou rown . 19. 4 Th eSy mme t r i cSt r e s sTe n s o r I ma g i n eab i gb l obofj e l l y .I ma g i n ep ok i n gi tonas i d e .Th ewh ol et h i n gwi g g l e sa n d di s t or t s ,a st h ef or c eofy ou rp ok ea c t sont h ee n t i r eb l obofj e l l y .Th ema t h e ma t i c a l me c h a n i s mt h a td e s c r i b e sh owy ou rp ok ei sd i s t r i b u t e di sc a l l et h es t r e s st e n s o roft h e ma t e r i a l . I tt e l l sh owe n e r g ya n dmome n t u ma r ec on n e c t e db yt h eme di u mi t s e l f . Th es a mec on c e p tc a nb eg e n e r a l i z e dt oaf ou rdi me n s i on a lme d i u m, wh e r et h e“ j e l l y ” i ss p a c et i mei t s e l f . L e tu sn ows t u dywh a ta ne l e c t r oma g n e t i cs t r e s s t e n s ori s , a n dh owi tr e l a t e st oe l e c t r o ma g n e t i c“ p ok e s ” . Re c a l l t h a t ∂L p i= ( 1 9 . 7 7 ) ∂qi̇ i st h ec a n on i c a l mome n t u mc or r e s p o n d i n gt ot h ev a r i a b l eqi L a g r a n g i a n . Th eHa mi l t on i a ni sg i v e n , i nt h i sc a s e , b y H=pi qi̇−L i na na r b i t r a r y ( 1 9 . 7 8 ) i a su s u a l .I f∂L / ∂t=0t h e non ec a ns h owt h a t∂H/ ∂ t=0 .F orf ou rd i me n s i on a l f i e l d swe s h ou l dp r ob a b l yh a v eaL a g r a n g i a na n dHa mi l t on i a nd e n s i t ywh os e3 –i n t e g r a la r et h e u s u a lL a g r a n g i a na n dHa mi l t on i a n s .Th eHa mi l t on i a ni st h ee n e r g yofap a r t i c l eor s y s t e m, s oi ts h ou l dt r a n s f or ml i k et h ez e r ot hc omp on e n tofaf ou rv e c t or . Th u s , s i n c e 3 H= Hdx 4 ( 1 9 . 7 9 ) 3 a n ddx=d x dx ,t h e nHmu s tt r a n s f or ml i k et h et i mec omp on e n tofas e c on dr a n k 0 t e n s or .I fwed e f i n et h eHa mi l t on i a nd e n s i t yHi nt e r msoft h eL a g r a n g i a nd e n s i t yLofa f i e l d , t h e n H = ∂ k ∂L ∂φk ∂φk ∂t ∂t −L . ( 1 9 . 8 0 ) We l l , g r e a t ! Th ef i r s tf a c t ori nt h es u mi st h ec on j u g a t emome n t u mb yd e f i n i t i on , a n dt h e s e c on di st h eg e n e r a l i z e d“ v e l oc i t y ” .Si n c eHmu s tt r a n s f o r ml i k et h et i mec omp on e n tofa s e c on dr a n kt e n s or( a n dt h et i med e r i v a t i v ea p p e a r si nt h i se qu a t i on )i ta p p e a r st h a tt h e c ov a r i a n tg e n e r a l i z a t i onoft h eHa mi l t on i a nd e n s i t yi ss ome t h i n gt h a tp u t sac ov a r i a n t de r i v a t i v et h e r e , i n s t e a d . Wet r y β Tαβ= α β . ∂L ∂ φk−g L k ( 1 9 . 8 1 ) ∂( ∂αφk) Th i si sc a l l e dt h ec a n on i c a ls t r e s st e n s o r ,a n di sr e l a t e dt ot h es t r e s st e n s orde f i n e d a n ds t u d i e di nCh a p t e r6 .Th i st e n s o rh a st h ec ov a r i a n tf u n c t i onoft e l l i n gu sh owt h e e n e r g ya n dmome n t u mc a r r i e db yt h ee l e c t r oma g n e t i cf i e l dt r a n s f or m. Wh a ti st h i st e n s or ?I ti s , i nf a c t , h i g h l yn on –t r i v i a l .Th eb e s twec a nd oi sn ot et h a t i fwea s s u met h a ton l yf r e ef i e l d sa r ep r e s e n ta n dt h a tt h ef r e ef i e l d sa r el oc a l i z e di n s omef i n i t er e g i onofs p a c e( n e i t h e ra s s u mp t i oni sp a r t i c u l a r l yp h y s i c a l ) ,t h e nwec a n s h owt h a t 0 03 T dx= 1 2 2 ( ǫ0E + 12 3 B) dx=E f i e l d µ0 ( 1 9 . 8 2 ) a n d 1 3 i E×H) dx=c Pf c ( i i e l d 0 i3 3 T dx=ǫ0c( E×B) dx= i ( 1 9 . 8 3 ) wh i c ha r et h e“ u s u a l ”e x p r e s s i on sf ort h ee n e r g ya n dmome n t u moft h ef r e ef i e l d .At l e a s ti fI g ott h ec h a n g et oSI u n i t sr i g h t . . . Wh a t , y oumi g h ta s k , i st h i sg oodf or ?We l l , a s i d ef r omt h i sc or r e s p on d a n c e( wh i c h i sf u l l ofh ol e s , b yt h ewa y ) , wec a nwr i t et h ee n e r g y –mome n t u mc on s e r v a t i onl a w α β ∂αT =0 . ( 1 9 . 8 4 ) Th i si sp r ov e ni nJ a c k s on , wi t had i s c u s s i onofs omeofi t ss h or t c omi n g s . On eoft h e s ei st h a ti ti sn ots y mme t r i c .Th i sc r e a t e sd i ffic u l t i e swh e nwec on s i d e r t h ea n g u l a rmome n t u mc a r r i e db yt h ef i e l d.Si n c et h ea n g u l a rmome n t u md e n s i t yi s i mp or t a n twh e nweg ot oc r e a t ep h ot on s( wh i c hmu s th a v equ a n t i z e da n g u l a r mome n t a ) , i ti swor t h wh i l et oc on s t r u c tt h es y mme t r i cs t r e s st e n s or 1 αβ α β α µ λ β µλ Θ =g F F + g F F µλ µλ ( 1 9 . 8 5 ) 4 i nt e r msofwh i c hwec a nc or r e c t l yc on s t r u c tac ov a r i a n tg e n e r a l i z a t i onoft h ee n e r g y mome n t u mc on s e r v a t i onl a w α β ∂αΘ =0 ( 1 9 . 8 6 ) a n dt h ea n g u l a rmome n t u mt e n s or γ− β Mαβγ=Θαβx Θαγx ( 1 9 . 8 7 ) wh i c hi st h e r e f or ec on s e r v e d .Th i sf or m oft h es t r e s st e n s orc a na l s ob ed i r e c t l y c ou p l e dt os ou r c et e r ms , r e s u l t i n gi nt h ec ov a r i a n tf or moft h ewor ke n e r g yt h e or e mf or t h ec omb i n e ds y s t e mo fp a r t i c l e sa n df i e l d s . Th i si sa b ou ta l l wewi l l s a ya b ou tt h i sa tt h i st i me .I r e a l i z et h a ti ti su n s a t i s f a c t or y a n da p ol og i z e .I fweh a don emor es e me s t e rt og e t h e r , wec ou l dd oi tp r op e r l y , b u twe d on ’ t . Th e r e f or e , i ti sont o 19. 5 Co v a r i a n tGr e e n ’ sF u n c t i o n s J u s twh e ny out h ou g h ti twa ss a f et og ob a c ki n t ot h ec l a s s r oom,a l on gc ome sJ a ws h i ms e l f . Gr e e n ’ sf u n c t i o n sa r ey ou rf r i e n d s ! Th ei n h omog e n e ou sMa x we l l e qu a t i on sa r en owc omp a c t l ywr i t t e na s α β β ∂αF =µ J. 0 ( 1 9 . 8 8 ) F r omt h ed e f i n i t i onoft h ef i e l ds t r e n g t ht e n s or , t h i si s β β α β ✷A −∂( ∂αA)=µ0J ( 1 9 . 8 9 ) α I ft h ep ot e n t i a l ss a t i s f yt h eL or e n t zc on d i t i on , ∂ A =0a n dt h e r e f or e α β β ✷A =µ0J ( 1 9 . 9 0 ) Doy oug e tt h ef e e l i n gt h a tt h e r ei ss ome t h i n gmy s t i c a la b ou ts p a c e –t i men ot a t i on s ? Doy our e me mb e rwh a tap a i ni nt h eb u t tt h i swa st od e r i v et h eh a r dwa y ? Tos ol v et h i si n h omog e n e ou sd i ffe r e n t i a le qu a t i on , wec on s t r u c ts i mu l t a n e ou s l ya Gr e e n ’ sf u n c t i on ′ ( 4 ) ′ ✷D( x , x)=δ ( x−x) ( 1 9 . 9 1 ) a n dt h ea s s oc i a t e di n t e g r a l e qu a t i onov e rt h es ou r c et e r m: α α 4′ ′α ′ A( x )=AI+µ0 dxD( x−x) J( x) ( 1 9 . 9 2 ) α ( wh e r et h ei n h omog e n e ou st e r m AId e p e n d sont h eGr e e n ’ sf u n c t i ona n di st h e “ b ou n da r y ”t e r mort h ef r e ep ot e n t i a lf r omi n h omog e n e ou ss ou r c e sou t s i d et h er e g i on ofi n t e g r a t i on ) . Ne x twe e kwewi l lc on c e n t r a t eont h ei n t e g r a le qu a t i ons ol u t i on st h e ms e l v e s .Nowl e t u ss e eh owt oc on s t r u c tt h ea p p r op r i a t e( c ov a r i a n t )Gr e e n ’ sf u n c t i on .Asu s u a l , t h ep r i n c i p l e p a r toft h eGr e e n ’ sf u n c t i onc a ni n v ol v eon l yt h ea b s ol u t edi s t a n c eb e t we e nt h ep oi n t s . Th u s α α ′ α i fy =x −x wes e e ks ol u t i on st o ( 4 ) ✷D( y )=δ ( y ) . ( 1 9 . 9 3 ) Th e r ea r es e v e r a lwa y swec ou l dg oa b ou ts ol v i n gt h i se qu a t i on .Th e ya r ea l l e qu i v a l e n ta ts omel e v e l ora n ot h e r .F ore x a mp l e , weh a v ea l r e a d ys ol v e dt h i se qu a t i on f oras i n g l ef ou r i e rc omp on e n ti nCh a p t e r9 .Wec ou l dt r a n s f or mt h i sr e s u l ta n dob t a i n af ou rd i me n s i on a lr e s u l t .Howe v e r ,amor eg e n e r a lp r oc e d u r ei st oc on s t r u c tt h e s ol u t i onf r oms c r a t c h . Th ef ou rd i me n s i on a lf ou r i e rt r a n s f or moft h ede s i r e dGr e e n ’ sf u n c t i oni sd e f i n e d b y 1 4 ( 2π) D( y )= 4 − i k · y dk D˜( k ) e wh e r ek·y=k y ·. Th ef ou rd i me n s i on a l d e l t af u n c t i oni s 0 0−ky 4 δ ( y )= 1 4 4 −i k · y d k d ( 2 π) s o( t a k i n gt h e✷ofD( y )u n d e rt h ei n t e g r a l a n de qu a t i n gf a c t or s ) 1 D˜ ( k )=−k· k. Wet h e r e f or ek n owt h a tt h eGr e e n ’ sf u n c t i onh a st h ef or m D( y )= − 1 4 ( 2π) 4 dk −i k · y e k·k . ( 1 9 . 9 4 ) ( 1 9 . 9 5 ) ( 1 9 . 9 6 ) ( 1 9 . 9 7 ) ( I mz ) −κ −i ε k0 ( Rez ) κ −i ε C F i g u r e1 9 . 1 : Con t ou r sf ore v a l u a t i n gt h eGr e e n ’ sf u n c t i oni n4 –d i me n s i on s . Th ei n t e g r a n di nt h i se x p r e s s i oni ss i n g u l a rwh e nk·k=0 .Re c a l lt h a tt h ep r e s e n c eof s i n g u l a r i t i e sme a n st h a tweh a v et od e c i deh owt ot r e a tt h e mt og e tawe l l d e f i n e dr e s u l t . Th e r ea r es e v e r a lwa y st od ot h i s ,a n de a c hh a sap h y s i c a li n t e r p r e t a t i on .I fwei n t e g r a t e ov e rt h e“ t i me ”c omp on e n tk i r s t , weg e t 0f D( y )=− 1 4 ( 2 π) 3 i k y · d k e −i ky e 00 dk 0 2 k 2 κ 0− ( 1 9 . 9 8 ) wh e r e| k |=κ .Nowt h es i n g u l a r i t i e sl i v ei nas i n g l e1 –Di n t e g r a lt h a twec a ne a s i l y e v a l u a t ev i ac on t ou ri n t e g r a t i ona n dt h eme t h odofr e s i d u e sp r ov i d e dt h a twes e l e c ta s u i t a b l ec on t ou r . L e t ’ sdot h ei n t e g r a lc a r e f u l l y( i nc a s ey ou rc on t ou ri n t e g r a t i oni sb i tr u s t y ) .Not et h a t t h ep ol e soft h i si n t e g r a la r eb ot hr e a l .Th i sme a n st h a tt h ei n t e g r a li sa mb i g u ou s–i tc a n b ea s s i g n e da n yofs e v e r a l p os s i b l ev a l u e sde p e n di n gonh owwec h oos et oe v a l u a t i oni t .I t i sb e y on dt h es c op eoft h e s en ot e st oe v a l u a t et h ec on s e qu e n c e sofma k i n ga n dp h y s i c a l l y i n t e r p r e t i n ge a c hoft h e s ec h oi c e s .I n s t e a dwewi l l c h oos et oi n c l u d eb ot hp ol e sc omp l e t e l y wi t h i nas t a n d a r dc on t ou rc l os e di nt h eu p p e rorl owe rh a l fp l a n er e s p e c t i v e l y , a n dt h e nt a k e l i mi t ss u c ht h a tt h ep ol e sr e t u r nt ot h er e a la x i sa f t e rt h ei n t e g r a lb e c a u s et h i sp a r t i c u l a r c h oi c el e a d su sv e r ys i mp l yt ot h ea dv a n c e da n dr e t a r de df or msoft h eGr e e n ’ sf u n c t i on t h a twea l r e a d yob t a i n e dwh e nd i s c u s s i n gt h ef ou r i e rt r a n s f or moft h ei n c omi n gorou t g oi n g s p h e r i c a l Gr e e n ’ sf u n c t i on sf ort h eHe l mh ol t ze qu a t i on . F i r s tweh a v et ode c i d ewh i c hwa yt oc l os et h ec on t ou r .Ex a mi n i n gt h ei n t e g r a n d , 0 0 ′ 0 wen ot et h a ti fy =x −x >0t h ei n t e g r a n dv a n i s h e sonal owe r h a l fc on t ou rl i k eCi n t h ef i g u r ea b ov e . Wed i s p l a c et h ep ol e sdowns l i g h t l ys o t h a tt h e yl i ei n s i d et h ec on t ou r: ± κ→ ± κ−i ǫ. F i n a l l y , l e tz=k k eac omp l e x 0+i ib v a r i a b l es u c ht h a tt h er e a l a x i si sk . 0 −i yz e 2 −i ky e ∞ 0 2 d z z −κ = −i yz e 00 0 2 2 k 0 −κ 0k − ∞d 2 2 +Cdz z −κ ( 1 9 . 9 9 ) Asn ot e d , t h ei n t e g r a l ov e rCc l e a r l yv a n i s h e sf ory . Th u s : 0>0 ∞ −i k y e −i yz e 2 2 k 0 k −∞ d 0 −κ = d z z −κ 0 0 0 2 = 2 −i z y 0 e l i m( − 2 πi ) Re s ( z−( κ−i ǫ) ) ( z+( κ+i ǫ) ǫ→0 −i κy 0 e =− 2 πi i κy 0 e 2 κ 2 κ + − s i n ( κ y ) 0 κ 2 π =− ( 1 9 . 1 0 0 ) Wec a nt h e nwr i t et h eGr e e n ’ sf u n c t i ona s s i n ( κz 0) y 0 ) d D( z )=θ( 3k eik·z 3 κ ( 2 π) = = = θ ( y 0 ) 3 ( 2 π) θ ( y 0 ) 2 ( 2 π) θ ( y 0 ) 2 2 πR ∞ 2 1 κd κ −1 0 ∞ 2 κ d κ 0 ∞ 0 d ( c os ( θ ) ) i n( κy 0) i κRcos ( θ)s dφe κ 0 i κR −i κR 2 π i n ( κ y ) 0 d ( i κ Rc os ( θ) ) eiκRcos(θ) s i κ R κ d κs i n ( κ R)s i n ( κ y ) 0 ( 1 9 . 1 0 1 ) ′ ′ wh e r eR=| x−x| i st h es p a t i a l s e p a r a t i onoft h ep oi n t sxa n dx. Us i n gat r i gi d e n t i t y( ori fy oup r e f e re x p a n di n gt h es i n ’ si nt e r msofe x p on e n t i a l s a n dmu l t i p l y i n gou t ,t h e nc h a n g i n gv a r i a b l e sa n de x p l oi t i n gt h ef a c tt h a ton l ye v e n t e r mss u r v i v e )t oe x t e n dt h ei n t e g r a l t o− ∞ wec a nwr i t et h i s a s : 0 θ( y) 1 ∞ D( z )= 4πR i ( y−R) κ− i ( y+R) κ. d κe e 2π 0 0 ( 1 9 . 1 0 2 ) −∞ Th e s er e ma i n i n gi n t e g r a l sa r ej u s ton ed i me n s i on a l Di r a cd e l t af u n c t i on s . E v a l u a t i n g , weg e t : ′ 0 ′ 0 0 0 ′ 0 R ′ 0 +R x ( x −x) δ( x x )+δ( x D( x x)= θ ) − − − 4πR r − ( 1 9 . 1 0 3 ) ′ wh e r eweh a v en owl a b e l l e di twi t h“ r ”f or“ r e t a r de d” .Th es ou r c ee v e n txi sa l wa y sa t a ne a r l i e rt i met h a nt h eob s e r v a t i one v e n tx . Th i sme a n st h a tt h e d oma i noft h es u p p or toft h eHe a v i s i d ef u n c t i onj u s th a p p e n st ob ed i s j oi n tf r om t h e s u p p or toft h es e c on dde l t af u n c t i on . Wec a nt h e r e f or es i mp l i f yt h i st o: ′ 0 ′ 0 0 ′ 0 ( x −x)δ( x x D( x x)= θ R) − − 4πR r − ( 1 9 . 1 0 4 ) wh i c hi sj u s twh a tweg otb e f or ef r om F ou r i e rt r a n s f or mi n gt h eou t g oi n gs t a t i on a r y wa v eGr e e n ’ sf u n c t i on , a si ts h ou l db e . I fweh a dc h os e nt h eot h e rc on t ou r ,i d e n t i c a la r g u me n t swou l dh a v el e du st ot h e a d v a n c e dGr e e n ’ sf u n c t i on : ′ D( x x)= ′ θ [ − ( x ]δ( 0−x 0) x ′ +R) −x ( 1 9 . 1 0 5 ) 0 − 4πR 0 Th eot h e rp os s i b l ec on t ou r s( e n c l os i n gon l yon eort h eot h e roft h et wos i n g u l a r i t i e s , u s i n gac on t ou rt h a ta v oi d st h es i n g u l a r i t i e sont h er e a la x i si n s t e a dofdi s p l a c i n gt h e s i n g u l a r i t i e s )wou l dy i e l ds t i l lot h e rp os s i b l eGr e e n ’ sf u n c t i on s .J u s ta sa na r b i t r a r y n or ma l i z e ds u mofou t g o i n ga n di n c omi n gGr e e n ’ sf u n c t i on sr e s u l t e di na na c c e p t a b l e Gr e e n ’ sf u n c t i onb e f or e , a na r b i t r a r ys u mofa d v a n c e da n dr e t a r d e dGr e e n ’ sf u n c t i on s a r ea c c e p t a b l eh e r e .Howe v e r ,t h ei n h omog e n e ou st e r moft h ei n t e g r a le qu a t i oni sa f u n c t i on a l oft h eGr e e n ’ sf u n c t i ons e l e c t e d ! a F orwh a ti ti swor t h , t h eGr e e n ’ sf u n c t i on sc a nb ep u ti nc ov a r i a n tf or m.On ea l mos t n e v e ru s e st h e mi nt h a tf or m,a n di ti s n ’ tp r e t t y , s oIwon ’ tb ot h e rwr i t i n gi td own .We c a nn owe a s i l ywr i t edo wnf or ma l s ol u t i on st ot h ewa v ee qu a t i onf ora r b i t r a r yc u r r e n t s ( n o tj u s th a r mon i con e s ) : α α 4′ ′α ′ A( x )=Ai x )+µ0 dxDr ( x−x) J( x) n( ( 1 9 . 1 0 6 ) a n d α α 4′ ′α ′ A( x )=Aout( x )+µ ( x−x) J( x) . 0 dxDa ( 1 9 . 1 0 7 ) I nt h e s ee qu a t i on s ,t h ei n h omog e n e ou st e r msa r et h er a d i a t i onf i e l di n c i d e n tu p on ( r a d i a t e df r om)t h ef ou r v ol u meofs p a c e t i mec on t a i n i n gt h ef ou r c u r r e n tt h a ta r en ot c on n e c t e dt ot h ef ou r c u r r e n ti nt h a tf ou r v ol u meb yt h er e t a r d e dGr e e n ’ sf u n c t i on . I ti sawor t h wh i l ee x e r c i s et ome d i t a t eu p onwh a tmi g h tb eas u i t a b l ef or mf ort h e i n h omog e n e ou st e r msi fon ec on s i d e r s tt h ei n t e g r a t i onf ou r v ol u met ob ei n f i n i t e( wi t h n oi n h omog e n e ou st e r ma ta l l )a n dt h e ns p l i tt h ei n f i n i t ev ol u meu pi n t ot h ei n t e r i or a n de x t e r i orofaf i n i t ef ou r v ol u me ,a swedi dwi t hi n c omi n ga n dou t g oi n gwa v e s b e f or e , e s p e c i a l l ywh e nt h e r ea r ema n yc h a r g e sa n dt h e ya r ep e r mi t t e dt oi n t e r a c t . Di r a cn ot e dt h a tc h oos i n ga“ r e t a r d e d ”Gr e e n ’ sf u n c t i on , j u s ta sc h oos i n ga n“ ou t g oi n g wa v e ”Gr e e n ’ sf u n c t i onb e f or e ,r e s u l t si nas ome wh a tmi s l e a d i n gp i c t u r eg i v e nt h a tt h e a c t u a lp h y s i c si sc omp l e t e l yt i me r e v e r s a ls y mme t r i c( i n de e d ,i n d e p e n d e n tofu s i n ga mi x e dv e r s i onoft h eGr e e n ’ sf u n c t i on si ne i t h e rc a s e ) .Het h e r e f or ei n t r od u c e dt h e “ r a d i a t i onf i e l d ”a st h ed i ffe r e n c eb e t we e n t h e“ ou t g oi n g ”a n dt h e“ i n c omi n g ”i n h omog e n ou st e r msg i v e nt h ec on t r a i n tt h a tt h e a c t u a l v e c t orp ot e n t i a l i st h es a mer e g a r d l e s soft h ec h oi c eofGr e e n ’ sf u n c t i onu s e d : : A α 4π α α r a di at i on =A out −Ai n= c 4′ ′α ′ dxD( x−x) J( x) ( 1 9 . 1 0 8 ) wh e r e D( z )=Dr ( z )−Da( z ) . ( 1 9 . 1 0 9 ) I ns omef u n d a me n t a ls e n s e ,on l yt h er a d i a t i onf i e l d sa r e“ p h y s i c a l ”–t h e ya r et h e c h a n gei nt h ev e c t orp ot e n t i a la ta ne v e n tp r od u c e ds y mme t r i c a l l yb ya n yg i v e nf ou r c u r r e n td u et oi t sp a s ta n di t sf u t u r emot i on .Th i si sac r i t i c a la s p e c toft h e i n t e r p r e t a t i onofr a di a t i onr e a c t i ona sb e i n gp r od u c e db yt r a n s f e rofmome n t u mb ot ht o ac h a r g e( e v e n t )f r om ot h e rc h a r g e si ni t sp a s ta n df r om ac h a r get ot h os es a me c h a r g e si ni t sf u t u r e . Ch a p t e r20 Ra d i a t i o nf r o mPo i n t Ch a r ge s Tos u mma r i z ef r om t h el a s tc h a p t e r ,t wou s e f u lGr e e n ’ sf u n c t i on sf ort h ei n h omog e n e ou swa v ee qu a t i on : α α ✷A =µ J 0 a r e 0 ′ ′ 0 ( 2 0 . 1 ) 0 ( x −x)δ( x x ′ 0 D( x x)= θ R) − − 4πR r − ( 2 0 . 2 ) ( t h er e t a r d e dGr e e n ’ sf u n c t i on )a n d ′ D ( x x)= − a 0 ′ 0 0 ′ 0 θ [ − ( x −x) ]δ( x x +R) − 4πR ( 2 0 . 3 ) ( t h ea d v a n c e dGr e e n ’ sf u n c t i on ) . Th ei n t e g r a l e qu a t i on sa s s oc i a t e dwi t ht h e s e Gr e e n ’ sf u n c t i on swe r e : α α A( x )=Ai x )+µ n( 0 4′ ′α ′ 4′ ′α ′ dxDr ( x−x) J( x) ( 2 0 . 4 ) a n d α α A( x )=Aout( x )+µ 0 dxDa( x−x) J( x) . ( 2 0 . 5 ) F ort h emome n t ,l e tu si g n or eDi r a c ’ sob s e r v a t i on sa n dt h er a di a t i onf i e l da n df oc u s i n s t e a donon l yt h e“ n or ma l ”c a u s a l l yc on n e c t e dr e t a r de dp ot e n t i a lp r od u c e db yas i n g l e c h a r g e dp a r t i c l ea si tmov e si nt h ea b s e n c eofe x t e r n a l p ot e n t i a l s .Th i sp ot e n t i a l i s“ c a u s a l ” i nt h a tt h ee ffe c t( t h ep ot e n t i a lf i e l d )f ol l owst h ec a u s e( t h emot i onoft h ec h a r g e )i nt i me , wh e r et h ea d v a n c e dp ot e n t i a lh a st h ee ffe c tp r e c e d i n gt h ec a u s e ,s ot os p e a k .L e tme e mp h a s i z et h a tt h i si sn otap a r t i c u l a r l yc on s i s t e n ta s s u mp t i on( a g a i n ,wet h et h e or yi s ma n i f e s t l yt i mes y mme t r i cs o“ p a s t ”a n d“ f u t u r e ”a r ep r e t t ymu c ha r b i t r a r yn a mi n g sof 2 9 9 t woop p os e dd i r e c t i on s ) , b u ti ty i e l d ss omev e r yn i c er e s u l t s , a swe l l a ss omep r ob l e ms . I nt h a tc a s e : α 4′ A( x )=µ0 ′α ′ ( 2 0 . 6 ) dxDr ( x−x) J( x) wh e r et h ef ou r –c u r r e n tofap oi n tc h a r g eei sf ou n df r om ′ ′ J x (, t )= ′ c ρx (, t ) cδx [− r( t ) ] = e ′ v α ( 2 0 . 7 ) r x t ) ] eδ[ −( Jx (, t ) i nt h el a b / r e s tf r a meKor( i nc ov a r i a n tf or m) : α′ ′ ( 4 ) ′ J( x)=e cd τU ( τ) δ ( [ x−r ( τ) ] ) ( 2 0 . 8 ) c v ( 2 0 . 9 ) wh e r e U=γ r =d d τ Not et h a tt h eδf u n c t i o ni nt h e s ee x p r e s s i on ss i mp l yf or c e st h ep a r t i c l et ob ef ou n da t t h ec or r e c tl oc a t i ona te a c h( p r op e r )t i me .Th er ( τ)f u n c t i oni st h et r a j e c t or yoft h e p a r t i c l e .I t sτd e r i v a t i v ei st h ef ou r –v e l oc i t y .Th i sy i e l d s( wh e nt h eγ ’ sh a v ea l lb e e n a c c ou n t e df or )t h er e s tf r a mee x p r e s s i on . Todot h ei n t e g r a l , wen e e dt h e“ ma n i f e s t l yc ov a r i a n t ”f or moft h er e t a r d e d Gr e e n ’ sf u n c t i on . Not et h a t : ′2 0 ′ 02 ′2 δ[ ( x x)]=δ[ ( x x) x v x ] −|− | − − 0 1 0 ′ 0 ′ ′ [ δ( x )+δ( x ) ] 0−x 0−R 0−x 0+R 2R = ′ 0 =δ[ ( x −x −R) ( x −x +R) ] ( 2 0 . 1 0 ) ′ ( wh e r eR= x |− x| ) . I nt e r msoft h i s , Dri sg i v e nb y 1 ′ Dr ( x−x)= ′ ′2 θ ( x δ [ ( x−x)] . 0−x 0) 2π ( 2 0 . 1 1 ) Ag a i n ,t h es e c on dd e l t a –f u n c t i onma k e sn oc on t r i b u t i onb e c a u s eoft h eop p os i n g θ–f u n c t i on . Th u s α µ0c A( x )= 2π 4′ ′ ′ 2 dxθ ( x δ ( [ x−x]) 0−x 0) α ( 4 ) ′ × ed τU ( τ) δ ( [ x−r ( τ) ] ) ( 2 0 . 1 2 ) e µ0c = 2π α 2 d τU ( τ) θ [ x ( τ) ] δ[ x−r ( τ) ]. 0−r x ( 2 0 . 1 3 ) Th ev e c t orp ot e n t i a la tap oi n tg e t sac on t r i b u t i onon l ywh e r e –wh e nt h a tp oi n tl i e sont h e l i g h tc on ei nt h ef u t u r e( p i c k e dou tb yt h eθf u n c t i on )oft h ewor l dl i n eoft h ec h a r g e( p i c k e d ou tb et h eδf u n c t i on ) . Th ec on t r i b u t i oni sp r op or t i on a l t o α e U( τ)a tt h a t( r e t a r de d)t i me . I td i e soffl i k e1 / R, a l t h ou g ht h a ti sob s c u r e db yt h e f or moft h eδf u n c t i on . Toe v a l u a t et h i s( a n ddi s c ov e rt h ee mb e d d e dR) , weu s et h er u l e( f r omwa yb a c ka t t h eb e g i n n i n goft h eb ook , p . 3 0i nJ 1 . 2 ) δ( x−x ) i δ[ f( x ) ] = ( 2 0 . 1 4 ) i df d x x = x i wh e r et h ex=x r et h en on –d e g e n e r a t ez e r osoff( x ) . f( x )i sa s s u me dt ob e“ s moot h ” . ia Th e ni fwel e t 2 f( τ)=[ x−r ( τ) ] ( 2 0 . 1 5 ) ( wh i c hi sz e r owh e nτ=τ nt h ep a s t )t h e n pi d 2 β x−r ( τ) ]=− 2 [ x−r ( τ) ] τ) βU ( dτ[ ( 2 0 . 1 6 ) a n dt h e r e f or e 2 ( τ) ])= δ( [ x r δ( τ−τ ) p δ( τ−τ ) p = β β ( 2 0 . 1 7 ) | − 2[ x−r ( τ) ] τ) |2[ x−r ( τ) ] τ) βU ( βU ( − F r omt h i swes e et h a t α eµ0c α A( x )= 4π U( τ ) U·[ x−r ( τ) ] τ=τp ( 2 0 . 1 8 ) wh e r eτ st h ep r op e rt i mei nt h ep a s tofxwh e nt h el i g h tc on eoft h ec h a r g ec on t a i n s pi t h ee v e n tx . Th i sp ot e n t i a l ( a n di t sot h e rf or msa b ov e )a r ec a l l e dt h eL íe n a r d –Wi e c h e r t p o t e n t i a l s . I nn on –c ov a r i a n tf or m, t h e ya r eob t a i n e df r omt h ei de n t i t y x ( τ ) ] −U·[ x−r ( τ ) ] 0−r 0 p p U·( x−r )=U0[ c R( 1−β·n ) =γ ( 2 0 . 1 9 ) wh e r eni sau n i tv e c t ori nt h edi r e c t i o nofx−r ( τ)a n dwh e r eβ=v ( τ) / ca su s u a l . Re c a l l t h a tA=( φ/ c , A) . Th u s : 0 A( x )= eµ0c γ c ( 2 0 . 2 0 ) c R( 1−β·n ˆ )r e t 4π γ a n d e 0 φx (, t )=c A= 1 1−β·n ˆ )r e t 4πǫ0 R( ( 2 0 . 2 1 ) wh e r ea l l qu a n t i t i e s( e . g . β, R)mu s tb ee v a l u a t e da tt h er e t a r d e dt i mewh e r et h ee v e n tx i sont h el i g h tc on eofap oi n tont h ep a r t i c l et r a j e c t or y . Si mi l a r l y Ax (, t )= = eµ0c γ c β c R( 1−β·n ˆ )r e t 4 π γ µ e β 0 ( 2 0 . 2 2 ) 4 π ǫ0 R( 1−β·n ˆ )r e t wh e r ea g a i nt h i n g smu s tb ee v a l u a t e da tr e t a r d e dt i me sont h ep a r t i c l et r a j e c t or y . Not ewe l lt h a tb ot hoft h e s ema n i f e s t l yh a v et h ec or r e c tn on r e l a t i v i s t i cf or mi nt h e l i mi t| β| < <1 . Wec a ng e tt h ef i e l d sf r omt h e4 –p ot e n t i a l i na n yoft h e s ef or ms .Howe v e r , t h el a s tf e w f or msweh a v ewr i t t e na r ec omp a c t ,b e a u t i f u l ,i n t u i t i v e ,a n dh a v ev i r t u a l l yn oh a n d l e swi t h wh i c ht ot a k ev e c t ord e r i v a t i v e s . I ti ss i mp l e rt or e t u r n α t ot h ei n t e g r a l f or m, wh e r ewec a nl e t∂ a c tont h eδa n dθf u n c t i on s . eµ0c ∂A = 2π αβ β α 2 dτU ( τ) θ[ x τ) ] ∂δ 0−r 0( wh e r e α d α ∂δ[ f] =∂f· [ x−r ( τ) ] d τ α ( 20. 23) d d fδ[ f· d τδ[ f] =∂f· d f] . ( 20. 24) 2 Ag a i n , wel e tf=[ x−r ( τ) ]. Th e n α α ( x−r ) ∂δ[ f] = −U ( x d δ[ f] ( 20. 25) r )d τ · − Th i si si n s e r t e di n t ot h ee x p r e s s i ona b ov ea n di n t e g r a t e db yp a r t s : e µ0c ∂αAβ =− 2 π β dτU ( τ) θ[ x α x−r ) r( τ) ]( 0− 0 U ( x dδ[ f] r )d τ α · − ) d β ( x−r 2 U ( τ ) dτ θ[ x r ( τ) ] δ( [ x r ( τ ( 20 ) ] ) . . 2 6) − − 0 d τ U ( x r ) 0 2 π · − Th e r ei sn oc on t r i b u t i onf r omt h eθf u n c t i onb e c a u s et h ed e r i v a t i v eofat h e t af u n c t i on i sad e l t af u n c t i onwi t ht h es a mea r g u me n t s = eµ0c d ( x ( τ) )=δ[ x ( τ) ] d τθ 0 −r 0 0 −r 0 ( 2 0 . 2 7 ) 2 wh i c hc on s t r a i n st h eot h e rd e l t af u n c t i ont ob eδ( − R) .Th i son l yg e t sac on t r i b u t i ona t R=0( ont h ewor l dl i n eoft h ec h a r g e ) ,b u twea l r e a d yf e e lu n c omf or t a b l ea b ou tt h e f i e l dt h e r e , wh i c hwes u s p e c ti si n f i n i t ea n dme a n i n g l e s s , s owee x c l u d et h i sp oi n tf r om c on s i d e r a t i on . An y wh e r ee l s et h er e s u l ta b ov ei se x a c t . Wec a nn owdot h ei n t e g r a l s( wh i c hh a v et h es a mef or ma st h ep ot e n t i a li n t e g r a l s a b ov e )a n dc on s t r u c tt h ef i e l ds t r e n g t ht e n s or : α β F = eµ0c e d αβ β α ( x−r )U −( x−r )U ( 2 0 . 2 8 ) x−r )dτ 4π U·( U·( x−r ) r e t Th i swh ol ee x p r e s s i onmu s tb ee v a l u a t e da f t e rt h ed i ffe r e n t i a t i ona tt h er e t a r d e dp r op e r t i meτ . p Th i sr e s u l ti sb e a u t i f u l l yc ov a r i a n t ,b u tn otp a r t i c u l a r l yt r a n s p a r e n tf ora l loft h a t . Ye twewi l l n e e dt of i n de x p l i c i ta n du s e f u l f or msf ort h ef i e l dsf orl a t e ru s e , e v e ni ft h e y a r en ota sp r e t t y .J a c k s ong i v e sa“ l i t t l e ”l i s tofi n g r e d i e n t s( J 1 4 . 1 2 )t op l u gi n t ot h i s e x p r e s s i onwh e nt a k i n gt h ede r i v a t i v et og e tt h er e s u l t , wh i c hi sob v i ou s l yqu i t eap i e c e ofa l g e b r a( wh i c hwewi l l s k i p ) : µ 03 ( n ˆ−β) n ˆ×( n ˆ−β)×β +e ˙ e µ0 E x (, t )= r e t 2 4 πc 2 32 4 πc γ( 1−β·nˆ )R 3 r e t ( 1−β·nˆ )R ( 2 0 . 2 9 ) a n d 1 Bx (, t )= c ( n ˆ×E ) ( 2 0 . 3 0 ) “ Ar r r g h ,ma t e y s !Sh i v e rmet i mb e r sa n da v a s t ! ” ,y ouc r you ti nd i s ma y .“ Th i si s e a s i e r ?Non s e n s e ! ”Ac t u a l l y , t h ou g h , wh e ny out h i n ka b ou ti t( s ot h i n ka b ou ti t )t h ef i r s t t e r mi sc l e a r l y( i nt h el owv e l oc i t y , l owa c c e l e r a t i onl i mi t s )t h eu s u a l s t a t i cf i e l d : en ˆ 2 πǫ0R E≈ 4 ( 2 0 . 3 1 ) I n t e r e s t i n g l y , i th a sa“ s h or t ”r a n g ea n di si s ot r op i c . Th es e c on dt e r mi sp r op or t i on a l t ot h ea c c e l e r a t i o noft h ec h a r g e ; b ot hEa n dBa r e − 1 t r a n s v e r s ea n dt h ef i e l dsd r opoffl i k eR a n dh e n c ea r e“ l on gr a n g e ”b u th i g h l y d i r e c t i on a l . I fy oul i k e , t h ef i r s tt e r msa r et h e“ n e a r ”a n d“ i n t e r me d i a t e ”f i e l d sa n dt h es e c on di s t h ec o mp l e t e“ f a r ”f i e l d ;on l yt h ef a rf i e l di sp r od u c e db yt h ea c c e l e r a t i onofac h a r g e . On l yt h i sf i e l dc on t r i b u t e st oan e tr a d i a t i onofe n e r g ya n dmome n t u ma wa yf r omt h e c h a r g e . Wi t ht h a t( wh e w! )b e h i n du s we c a np r oc e e dt od i s c u s ss ome i mp or t a n t e x p r e s s i on s . F i r s tofa l l , wen e e dt oob t a i nt h ep owe rr a di a t e db yamov i n gc h a r g e . 20. 1 L a r mo r ’ sF o r mu l a I fon ei sf a r( e n ou g h )a wa yf r omt h ea na c c e l e r a t i n gc h a r g ei nt h er i g h td i r e c t i on ,t h e f i e l di sg i v e nb yp r i ma r i l yb yt h es e c on d( a c c e l e r a t i o n )t e r m.Th i si st h e“ u s u a l ” t r a n s v e r s eEMf i e l d . I ft h ep a r t i c l ei smov i n gs l owl ywi t hr e s p e c tt oc( s oβ< <1 ) , t h e n ˙ E = ˆ×( nˆ×β) e 1 n 4πǫ0c R ˙ n ˆ × β 1 e B = 2 4πǫ0c R r e t ( 2 0 . 3 2 ) r e t ( 2 0 . 3 3 ) Th ee n e r g yf l u xi sg i v e nb yt h e( i n s t a n t a n e ou s )Poy n t i n gv e c t or : 1 S= µ0( E ×B) 2 e 2 11 2 ˙2 3 | nˆ×( nˆ×β) |nˆ = 16πǫ0R µ0ǫ0c 2˙ 2 2 1 e 2 23 = 16πǫ0R c | nˆ×( nˆ×cβ) | nˆ 2 e 1 ˙2 2 23 nˆ×( nˆ×v) |nˆ = 16πǫ0R c | ( 2 0 . 3 4 ) Asa l wa y s , t h epowe rc r os s s e c t i on( e ne r gype runi ts ol i da ngl e )i s dP dΩ 2 =S·n ˆ R 2 e 1 ˙2 nˆ×( nˆ× v) | = 16πǫ0c | 2 1 ˙ e 2 3 2 2 ||s i n( Θ) = 16πǫ0c v 2 3 ˙ ( 2 0 . 3 5 ) wh e r eΘi st h ea n g l eb e t we e nn ˆa n d v. 2 Ah a !wes a y .Th ec h a r a c t e r i s t i cs i n Θ!Ah aa g a i n !I n s p e c t i n gt h ev e c t orp r odu c t s ,we s e et h a tt h er a d i a t i oni sp ol a r i z e di nt h ep l a n eofn , v,̇ p e r p e n d i c u l a rt on . F i n a l l y , t h ei n t e g r a l ov e ra n g l e sy i e l ds8 π/ 3 , s ot h a t 2 e | v ˙ | 2. 3 πǫ0c P= 6 ( 2 0 . 3 6 ) Th i si st h eL a r morf or mu l af ort h ep owe rr a di a t e df r oman on r e l a t i v i s t i ca c c e l e r a t e d p oi n tc h a r g e . Th i sh a sac ov a r i a n tg e n e r a l i z a t i ont h a ti sv a l i df ora n yv e l oc i t yofc h a r g e . 2 F i r s twef a c t orou ta nm a n dc on v e r tt h i st omome n t u mc oor d i n a t e s .Th e nwer e a l i z e t h a tt h ee n e r g yc a r r i e db yt h i sf i e l d( p e ru n i tt i me )i si n d e e dr e l a t e dt ot h emome n t u m b yaf a c t orof1 / ca n dc on v e r tt h ewh ol et h i n gt o4 –v e c t orf or m.L a s t , wec on v e r tti n t o τ: 2 P= e 1 3 2 6πǫ0c m 2 = e 23 6πǫ0m c 2 d ( mv) dt d( mv) γ d τ 2 e 23 2 pd 2 2 1−β) dτ = 6πǫ0m c ( 2 2 2 e pd 1d E 23 = 6πǫ0m c d τ 2 τ − cd 2 e 23 =−6πǫ0m c α dpαdp d τ d τ ( 2 0 . 3 7 ) 2 Th i sc a nb ewr i t t e non emor ewa y ,( s u b s t i t u t i n gE=γ mc a n dp=γ mva n du s i n g s omev e c t ori d e n t i t i e s )d u et oL íe n a r d : 2 e ˙2 6 3 ˙ 2 ( β× β) P= 6πǫ0c γ [ ( β) − ] ( 2 0 . 3 8 ) Wea r ea l l b e t t e rp e op l ef ork n owi n gt h i s . Wh y , y ouma ya s k , i st h i st or t u r en e c e s s a r y ?Be c a u s equ i t eaf e wofy ouwi l l s p e n d u n r e a s on a b l ea mou n t sofy ou rl i v e sc a l c u l a t i n gt h i n g sl i k er a di a t i v el os s e si n a c c e l e r a t or s .Af t e ra l l , i fwec ou l db u i l dGe Va c c e l e r a t or si nal i t t l eb i t t yt e nf ootr i n gi t wou l db eawh ol el otc h e a p e rt h a n6b i l l i onb u c k s , p l u si n f l a t i on .Un f or t u n a t e l y , n a t u r e s a y st h a ti fy out r yi tt h en a s t yt h i n gwi l l g i v eoffs y n c h r ot r o nr a d i a t i o n !L e tu ss e et h a t 1 t a n s t a a f l. Th er a d i a t e dp owe ri sp r op or t i on a l t ot h ea c c e l e r a t i on .Th ewor ki sp r op or t i on a l t o t h et a n g e n t i a lf or c et i me st h ev e l oc i t y .L i g h tp a r t i c l e sa c c e l e r a t et h emos tf orag i v e n t a n g e n t i a lf or c ea n dh a v et h eh i g h e s tv e l oc i t yf orag i v e ne n e r g y ;r a d i a t i v el os s e sa r e t h u st h emos ti mp or t a n tf ort h os ep a r t i c l e sa ta l le n e r g i e s .Wewi l le v a l u a t et h e r a d i a t i v ep owe rl os sf ora ne l e c t r oni nal i n e a ra c c e l e r a t or . Web e g i nwi t h 2 dp 2 e 23 t P= 6πǫ0m c d ( 2 0 . 3 9 ) wh e r e− ei sn owr e a l l yt h ec h a r g eont h ee l e c t r on .Si n c et h ea c c e l e r a t ori sl i n e a r ,wec a n f i n dt h ef or c ed i r e c t l yf r omt h er a t ea twh i c hwor ki sd on eont h ee l e c t r on( ot h e r wi s ewe wou l dh a v et oi n c l u d et h ef or c eb e n d i n gi ti nac u r v e dp a t h ,wh i c hdoe sn owor k ) .I ti s r e l a t e dt ot h e“ g r a d i e n t ”oft h et ot a l e n e r g y , 2 e dE 23 P= 6πǫ0m c 2 d x . ( 2 0 . 4 0 ) F orl i n e a ra c c e l e r a t i onwedon ’ tc a r ewh a tt h ea c t u a le n e r g yoft h ep a r t i c l ei s ; weon l y c a r eh owt h a te n e r g yc h a n g e swi t hd i s t a n c e . Wewi l l t u r nt h i si n t oar a t ee qu a t i onb yu s i n gt h ec h a i nr u l e : 2 e d Ed Ed t 23 Prad= 6πǫ0m c dxdtdx ( 2 0 . 4 1 ) Th u st h er a t i oofp owe rr a di a t e dt op owe rs u p p l i e db yt h ea c c e l e r a t orPacc=dE / dti s : Prad 2 e 23 1dE 2 2 1 e/ mc dE 2 6πǫ0m c vdx ≈ 6πǫ0 mc dx Pacc = wh e r et h el a t t e rf or mi sv a l i dwh e nt h ee l e c t r oni st r a v e l l i n ga tv≈c . ( 2 0 . 4 2 ) 2 2 Th i squ a n t i t ywi l lb el e s st h a non ewh i l et h eg a i ni ne n e r g yi nad i s t a n c ee/ mc = − 1 3 2 . 8 2×1 0 2 c mi soft h eor d e ro fmc =. 5Me V.Th a twou l dr e qu i r eap ot e n t i a l 1 4 d i ffe r e n c e( orot h e rf or c e )ont h eor de rof1 0 MV/ me t e r . 1 Th e r eAi n ’ tNoSu c hTh i n gAsAF r e eL u n c h . Nok i d di n g . Ma y b ea tt h es u r f a c eofap os i t r on .Comet ot h i n kofi t ,f a l l i n gi n t oap os i t r ont h e r e c ome sap oi n twh e r et h i si st r u ea n da tt h a tp oi n tt h et ot a lma s se n e r g yoft h ep a i ri s r a d i a t e da wa y .Bu tn owh e r ee l s e .Wec a nc omp l e t e l yn e g l e c tr a di a t i v el os s e sf orl i n e a r a c c e l e r a t i ons i mp l yb e c a u s et h ef or c e sr e qu i r e dt op r odu c et h er e qu i s i t ec h a n g e si ne n e r g y wh e nt h ep a r t i c l ei smov i n ga tn e a r l yt h es p e e do fl i gh ta r el u di c r ou s l yl a r g e . F orac h a r g e d p a r t i c l emov i n gi nas t r a i g h tl i n e , r a d i a t i v el os s e sa r emor ei mp or t a n ta tl owv e l oc i t i e s .Th i s i sf or t u n a t e , orr a d i osa n dt h el i k ewi t hl i n e a rdi p ol ea n t e n n a swou l dn otwor k ! Howe v e r ,i ti si n c ov e n i e n tt ob u i l dl i n e a ra c c e l e r a t or s .Th a ti sb e c a u s eal i n e a r a c c e l e r a t orl on ge n ou g ht oa c h i e v er e a s on a b l ee n e r g i e sf ore l e c t r on ss t a r t s( t h e s ed a y s )a t a r ou n d1 0 0 –5 0 0mi l e sl on g .Att h a tp oi n t , i ti ss t i l l n ot“ s t r a i g h t ”b e c a u s et h ee a r t hi s n ’ tf l a t a n dwed on ’ tb ot h e rt u n n e l l i n gou tas e c a n t .Al s o,i ts e e mss e n s i b l et ol e tac h a r g e d p a r t i c l ef a l lma n yt i me st h r ou g ht h e“ s a me ”p ot e n t i a l ,wh i c hi sp os s i b l eon l yi ft h e a c c e l e r a t ori sc i r c u l a r .Un f or t u n a t e l y ,weg e ti n t or e a lt r ou b l ewh e nt h ea c c e l e r a t ori sn ot s t r a i g h t . I nac i r c u l a ra c c e l e r a t or ,t h e r ei san on –z e r of or c ep r op or t i on a lt oi t sv e l oc i t y s qu a r e d , e v e nwh e nl i t t l eorn owo r ki sb e i n gd on et oa c c e l e r a t et h ep a r t i c l e !I nf a c t , t h ec e n t r i p e t a l f or c eont h ep a r t i c l ei s dp d τ =γ ω| p | > > 1dE cdτ ( 2 0 . 4 3 ) a l lofwh i c hi n c r e a s ea st h es p e e doft h ep a r t i c l ei n c r e a s e s .I fwec omp l e t e l yn e g l e c t t h er a d i a t i v el os sd u et ot a n g e n t i a la c c e l e r a t i on( wh i c hi sc omp l e t e l yn e g l i g i b l eon c e r e l a t i v i s t i cv e l oc i t i e sh a v eb e e nr e a c h e d )wes e et h a t 2 e2 ec 23 2 2 2 2 44 p |= 6πǫ0r βγ P= 6πǫ0m c γω | ( 2 0 . 4 4 ) wh e r eweh a v eu s e dω=( c β/ r ) .Th el os sp e rr e v o l u t i oni sob t a i n e db ymu l t i p l y i n gb yT ( t h ep e r i odofar e v ol u t i on ) . Th i sy i e l ds E= 2 2 πr P= e 3 4 ( 2 0 . 4 5 ) β γ 3ǫ0r cβ wh i c hi ss t i l l d e a dl yi fri ss ma l l a n d / orγa n dβa r el a r g e . I fon edoe ss omea r i t h me t i c( s h u d de r ) , on ec a ns e et h a tf orh i g he n e r g y e l e c t r on s( wh e r eβ≈1 ) , t h i si s 4 [ E ( Ge V) ] − 2 E ( Me V)=8 . 8 5×1 0 r ( me t e r s ). ( 2 0 . 4 6 ) Ata r ou n d1Ge V,on en e e d sr ou g h l y1 / ( 1 0 r )oft h a te n e r g yg a i np e rc y c l ei nor de rt ot u r n ( h e h , h e h )an e tp r of i t . Th a ti sn ots ob a d , b u tt h ep owe rof4s a y st h a ta t1 0Ge V, on en e e ds ag a i np e rc y c l eof1 0 0 0 / rGe V( ! )i nor de rt ot u r nap r of i t .Now, i ti st r u et h a tt h eb i g g e rt h e r a d i u st h el on g e rt h ec i r c u mf e r e n c e( l i n e a r l y )a n dt h el on g e rt h ec i r c u mf e r e n c et h emor e wor kon ec a ndowi t hag i v e nf i x e dp ot e n t i a li nac y c l e .Soi nt e r msoff or c et h i sr e l a t i oni s n ota sb a d a si ts e e ms .Bu ti ti sb a de n ou g h ,b e c a u s ey ous t i l lh a v et odot h ewor k ,wh i c hc os t s y out h es a men oma t t e rh owh a r dy ouh a v et op u s ht od oi t .Cl e a r l ye v e na t1 0Ge V, a n or b i to fr a d i u s∼1 0 0me t e r sorb e t t e ri sn e c e s s a r y .I ne l e c t r on –p os i t r ons t or a g er i n g s , wor kmu s tb ed on ea tt h i sg e n e r a l r a t ej u s tt ok e e pt h ep a r t i c l e smov i n g . Th os eofy ouwh on e e dt ok n owc a nr e a ds e c t i on1 4 . 3ony ou rown .Th er e s u l t sa r e s t r a i g h t f or wa r db u ta l g e b r a i c a l l yt e d i ou s ,a n da r eofu s eon l yi fy oup l a nons t u d y i n g a c c e l e r a t orde s i g norn e u t r ons t a r s .Don ’ tg e tmewr on g .Nob e lp r i z e sh a v eb e e nwonf or a c c e l e r a t orde s i g na n dma yb ea g a i n . Gof ori t . Di t t of or1 4 . 4 .Th i si sh i g h l yr e a da b l ea n dc on t a i n sn oa l g e b r a .I nan u t s h e l l , ap a r t i c l e mov i n gi nas y n c h r ot r one mi t si t sr a d i a t i oni ni t si n s t a n t a n e ou sd i r e c t i onofmot i on( wh i c h i si n d e e dp e r p e n d i c u l a rt ot h ea c c e l e r a t i on ) .Si n c ei tmov e si nac i r c l e , as t a t i on a r yob s e r v e r i nt h ep l a n eofmot i ons e e ss h or tb u r s t sofr a di a t i ona tt h ec h a r a c t e r i s t i cf r e qu e n c yc / r .Th e l e n g t h( i nt i me )oft h ep u l s e si sL / ci nt i me ,a n dt h u swi l lc on t a i nf r e qu e n c i e su pt oc / L∼ 3 3 ( c / r ) γi naf ou r i e rd e c omp os i t i onoft h e i r“ wa v ep a c k e t ”wh e r eL≈r / ( 2 γ )i st h el e n g t hof t h ep u l s ei ns p a c e .F orh i g h l yr e l a t i v i s t i cp a r t i c l e smov i n gi nb i gc i r c l e s ,t h ec h a r a c t e r i s t i c f r e qu e n c yc a nb ema n yor de r sofma g n i t u d es ma l l e rt h a nt h eh i g hf r e qu e n c yc u toff,a si n AM r a d i of r e qu e n c i e st oX–r a y sorwo r s e .Sy n c h r ot r onr a d i a t i oni sap ot e n t i a ls ou r c eof h i g hf r e qu e n c ye l e c t r oma g n e t i ce n e r g y . Ofc ou r s e ,i ti s n ’ tt u n a b l eorc oh e r e n t( i nf a c t ,i t sh i g h l yi n c oh e r e n ts i n c et h e s p e c t r u mi ss owi de ! )a n dwe ’ dl ov et ou s et h es a mek i n doft r i c kt oma k ec oh e r e n t , t u n a b l e ,h i g hf r e qu e n c yl i g h t .Someofy oup r ob a b l ywi l lu s et h es a mek i n doft r i c k b e f or ey oul e a v e ,s i n c ef r e ee l e c t r onl a s e r sp r od u c ee n e r g yf r om as i mi l a rp r i n c i p l e ( a l t h ou g hwi t hat ot a l l yd i ffe r e n ts p e c t r u m! ) .Se c t i on1 4 . 6d e a l swi t ht h es p e c t r u m, a n d wewi l lb l owt h a toff, t oo.Su ffic ei tt os a yt h a ti tc a nb ec a l c u l a t e d , a n dy ouc a nl e a r n 3 h ow,i fy oun e e dt o.Your e a l l ys h ou l dr e me mb e rt h a tωc≈ω0γ,a n ds h ou l dt a k ea p e e ka tt h ed i s t r i b u t i onc u r v e s .Th e s ec u r v e sl e ton ed e t e c ts y n c h r ot r onr a d i a t i onf r om c os mol og i c a ls ou r c e s .Th e s es ou r c e sa r eg e n e r a l l yc h a r g e dp a r t i c l e sf a l l i n gi n t oda r k s t a r s ,r a d i a t i onb e l t sa r ou n dp l a n e t s ,s u n s p ot s ,ora n y p l a c ee l s et h a tr e l a t i v i s t i c e l e c t r on sa r es t r on g l ya c c e l e r a t e di nac i r c u l a r , orh e l i c a l , p a t h .F i n a l l y , wewi l ln e g l e c t 1 4 . 5t oo, wh i c ha n a l y z e sr a d i a t i one mi t t e db yp a r t i c l e smov i n gi nwi e r dwa y s .J a c k s on i se n c y c l op a e di a c , b u twen e e d n ’ tb e . Wewi l lc omeb a c ki n t of oc u sa ts e c t i on1 4 . 7 ,Th oms onSc a t t e r i n gofRa d i a t i on . Th i si ss c a t t e r i n gofr a d i a t i onb yc h a r g e dp a r t i c l e sa n di sc l os e l yr e l a t e dt oComp t on s c a t t e r i n g . I ti si mp or t a n t , a si ti sac o mmonp h e n ome n on . 20. 2 Th o ms o nSc a t t e r i n go fRa d i a t i o n Su p p os et h a tap l a n ewa v eofmon oc h r oma t i ce l e c t r oma g n e t i cr a d i a t i oni si n c i d e n ton af r e ep a r t i c l eofc h a r g eea n dma s sm.Th ep a r t i c l ewi l le x p e r i e n c eaf or c ef r omt h i s f i e l d , a n dwi l la c c e l e r a t e .Asi ta c c e l e r a t e s , i twi l le mi tr a di a t i oni ndi ffe r e n tdi r e c t i on s , d i s p e r s i n gt h ei n c i d e n tb e a m. F oran on –r e l a t i v i s t i cp a r t i c l ea c c e l e r a t e db yaf or c ewec a ns e et h a t : 2 ∗ e 1 e ˙2 dP 2 ∗ e ˙2 ˙2 i n ( wh e r e| ˆ ·v | =v| |s 2 3 ˆv · dΩ =16πǫ0c ( 2 0 . 4 7 ) Θf orap a r t i c u l a rp ol a r i z a t i onp e r p e n di c u l a rt ot h e ˙ p l a n eofn ˆa n d v) . Th e( l e a d i n gor d e r )a c c e l e r a t i oni sdu et ot h ep l a n ewa v ee l e c t r i cf i e l dwi t h p ol a r i z a t i one ˆ , wa v ev e c t ork , a n dNe wt on ’ sL a w: 0 0 ˙ e k x i ˆ0e v = mE0e ( 2 0 . 4 8 ) ·−ωt 0 I ft h ec h a r g emov e smu c hl e s st h a non ewa v e l e n g t hd u r i n gac y c l e( t r u ef ora l l b u tt h e l i g h t e s tp a r t i c l e sa n ds t r on g e s tf i e l d s )t h e n v˙ ˙˙ ∗ =1 Rev( v ) || a v 2 Th u st h ea v e r a g ep owe rf l u xd i s t r i b u t i oni s d P · 2 2 ∗ 2 e e 2 c 2 E0 | mc = 32πǫ0 | d Ω av ( 2 0 . 4 9 ) 2 e 2 | ˆ· ˆ 0 | 2 ∗ ǫ0cE0 e e 2 e 2 2 ˆ·ˆ | 0 2| = 4πǫ0mc ( 2 0 . 5 0 ) Th i si sc l e a r l yoft h es a meg e n e r a lf or ma st h es c a t t e r i n ge x p r e s s i on swe 2 d e s c r i b e da n dd e r i v e de a r l i e r .Si n c et h er e s u l tc on t a i n sE0 i tma k e ss e n s et odi v i d e ou tt h ei n c i d e n ti n t e n s i t ya n dt h u sob t a i nad i ffe r e n t i a lc r os ss e c t i ont h a twor k sf ora l l b u tt h es t r on g e s tf i e l d s .Wet h u sd i v i d eou tt h et i me a v e r a g e df l u xoft h ePoy n t i n g v e c t oroft h ei n c i d e n tp l a n ewa v e : 2 I = h e n c e dσ ǫ0c E ( 2 0 . 5 1 ) 0 2 2 e 2 ∗ e 2 e 2 | ˆ· ˆ 0 | = 4πǫ0mc ( 2 0 . 5 2 ) I fwel e tt h ep l a n ewa v eb ei n c i d e n ta l on gt h eza x i s ,l e tn ˆf or ma na n g l eθwi t h t h a ta x i s , a n dp i c kt wop ol a r i z a t i ondi r e c t i o n si na n dp e r p e n di c u l a rt ot h e( n ˆ , z ˆ )p l a n e ( a sb e f or e ) , a n da v e r a g eov e rp ol a r i z a t i on st h e nt h i sd otp r od u c ty i e l d s : 2 d σ e 21 2 ( 1+c os θ) = . ( 2 0 . 5 3 ) dΩ 2 4πǫ0mc d Ω 2 a si tdi db a c ki nou re a r l i e rwor kons c a t t e r i n g , b u tn owf orap oi n tp a r t i c l e . Th i si st h e Th oms onf or mu l af ors c a t t e r i n gofr a d i a t i onb yf r e ec h a r g e . I twor k sf orX–r a y sf or e l e c t r on sorγ –r a y sf orp r ot on s . I td oe sn otwor kwh e nt h e p h ot onmome n t u ma n dt h er e c oi loft h ec h a r g e dp a r t i c l ec a n n otb en e g l e c t e d.Th e i n t e g r a l oft h i s , 2 e 8π σT = 2 3 4πǫ0mc 2 ( 2 0 . 5 4 ) − 2 9 2 i sc a l l e dt h eTh o ms onc r o s s –s e c t i o n . I ti s0 . 6 6 5×1 0 m f ore l e c t r on s .Th equ a n t i t y i np a r e n t h e s e sh a st h eu n i t sofl e n g t h . I ft h et ot a l “ ma s s – e n e r g y ”oft h ee l e c t r onwe r edu et oi t sc h a r g eb e i n gc on c e n t r a t e di nab a l l ,t h e nt h i s wou l db et h ec l os eor d e roft h er a d i u soft h a tb a l l ;i ti sc a l l e dt h ec l a s s i c a le l e c t r o n r a d i u s .Th i sn u mb e rc r op su pqu i t ef r e qu e n t l y , s oy ous h ou l dr e me mb e ri t .Wh a ti tt e l l s u si st h a te v e np o i n tp a r t i c l e sh a v eaf i n i t es c a t t e r i n gc r os s s e c t i ont h a ta p p e a r si nt h i s l i mi tt ob ei n d e p e n d e n toft h ewa v e l e n g t hoft h el i g h ts c a t t e r e d . Howe v e r , t h i si sn otr e a l l yt r u ei fy o ur e c a l lt h ea p p r ox i ma t i on sma d e–t h i se x p r e s s i on wi l lf a i li ft h ewa v e l e n g t hi sont h es a meor d e ra st h ec l a s s i c a lr a d i u s ,wh i c hi sp r e c i s e l y wh e r ep a i rp r odu c t i onb e c ome sas i g n i f i c a n tp r oc e s squ a n t u m me c h a n i c a l l y .I nqu a n t u m 2 me c h a n i c s ,i ft h ee n e r g yoft h ei n c i d e n tp h ot onω ≈mc f ort h ee l e c t r on ,s i g n i f i c a n t mome n t u mi st r a n s f e r r e dt ot h ee l e c t r onb yt h ec ol l i s i ona n dt h ee n e r g yoft h es c a t t e r e d p h ot onc a n n otb ee qu a l t ot h ee n e r g yo ft h ei n c i de n tp h ot on .Wh a t e v e rap h ot oni s. . . Wec a na c t u a l l yf i xt h a twi t h ou tt oo mu c hd i ffic u l t y ,d e r i v i n gt h eComp t on s c a t t e r i n gf or mu l a( wh i c ht a k e sov e rf r omTh oms oni nt h i sl i mi t ) .Th i sf or mu l aa d d sa wa v e l e n g t h / a n g l ed e p e n d e n c et oTh oms on ’ sg e n e r a lr e s u l ta n dy i e l d st h eKl i e n Ni s h i n af or mu l a ,b u tt h i si sb e y on dou rs c op ei nt h i sc ou r s et od e r i v eordi s c u s si n f u r t h e rde t a i l . Wea r ea l mos tf i n i s h e dwi t hou rs t u d yofe l e c t r od y n a mi c s .Ou rf i n a l ob j e c tofs t u d y wi l l b et ot ot r yt oa d d r e s st h ef ol l owi n gob s e r v a t i on : Ac c e l e r a t e dc h a r g e sr a di a t e .Ra d i a t i on a c c e l e r a t e sc h a r g e .E n e r g ymu s tb e c on s e r v e d . Th e s et h r e et h i n g sh a v en otb e e nc on s i s t e n t l yma i n t a i n e di nou rt r e a t me n t s . Wes t u d yon e ,t h e nt h eot h e r ,a n dr e qu i r et h et h i r dt ob et r u ei non l yp a r toft h e d y n a mi c s . Wh a ti smi s s i n gi sr a d i a t i o nr e a c t i o n .Asc h a r g e sa c c e l e r a t e ,t h e yr a d i a t e .Th i s r a d i a t i onc a r r i e se n e r g ya wa yf r omt h es y s t e m.Th i s ,t h e nme a n st h a tac ou n t e r f o r c e mu s tb ee x e r t e dont h ec h a r g e swh e nwet r yt oa c c e l e r a t et h e mt h a tda mp sc h a r g e os c i l l a t i on s . Atl a s tt h ef ol l yofou rwa y si sa p p a r e n t .Ou rb l i n di n s i s t e n c et h a ton l yr e t a r d e df i e l d s a r eme a n i n g f u l ( s ot h a twec a ni ma g i n et h ef i e l dst ob ez e r ou pt os omet i mea n dt h e ns t a r t mov i n gac h a r g e ,wh i c hs u b s e qu e n t l yr a d i a t e s )h a sl e f tu swi t hon l yon ec h a r g et h a tc a n p r od u c et h ef i e l dt h a tp r od u c e st h ef or c et h a td a mp sa p p l i e de x t e r n a lf or c e s—t h ec h a r g e i t s e l ft h a ti sr a d i a t i n g . Noot h e rc h a r g ep r od u c e saf i e l dt h a tc a na c tont h i sc h a r g e“ i nt i me ” . Weh a v ei n v e n t e dt h emos ts u b l i meofv i ol a t i on sofNe wt on ’ sl a ws–a nob j e c tt h a tl i f t ’ s i t s e l fu pb yi t sownb oot s t r a p s ,a nAr i s t ot e l i a nob j e c tt h a tmi g h te v e nb ea b l et oc omet o r e s toni t so wni nt h ea b s e n c eofe x t e r n a l f or c e s . Cl e a r l ywemu s ti n v e s t i g a t er a d i a t i onr e a c t i ona sas e l f –f or c ea c t i n gona ne l e c t r ond u e t oi t sownr a d i a t i onf i e l d , a n ds e ei fi ti sp os s i b l et os a l v a g ea n y t h i n g l i k eaNe wt on i a nd e s c r i p t i onofe v e nc l a s s i c a l d y n a mi c s .Wea l r e a d yk n owt h a tL a r mor r a d i a t i onp l u ss t a b l ea t omss p e l l st r ou b l ef orNe wt on ,b u tNe wt ons t i l lwo r k s c l a s s i c a l l y , d oe s n ’ ti t ? L e t ’ st a k eal ook .Uh –oh , y ous a y .Wa s n ’ tt h e , we l l , wa s n ’ te v e r y t h i n gs i n g u l a rona p oi n tc h a r g e ?Won ’ tweg e ti n f i n i t i e sa te v e r yt u r n ?Howwi l lwer e a l i z ef i n i t er e s u l t s f r omi n f i n i t ef i e l d s , p ot e n t i a l s , s e l f e n e r g i e s , a n ds oon ? Ye s !Ic r ywi t hg l e e .Th a t ’ st h ep r o b l e m.F i n a l l ywewi l l l e a r nh owt ot a k eas i n g u l a r f i e l d ,as i n g u l a rc h a r g e ,a n di n f i n i t ee n e r g y ,a n dma k eap h y s i c a l l yr e a l i z e d( a l mos t ) r a di a t i onr e a c t i onf or c eou tofi t . Ch a p t e r21 Ra d i a t i o nRe a c t i o n 21. 1 Th eDe a t ho fCl a s s i c a l Ph y s i c s Th u sf a rweh a v el e a r n e dh owt os o l v et wok i n d sofp r ob l e ms .E i t h e rt h ef i e l d swe r e a s s u me dt ob eg i v e n , i nwh i c hc a s et h er e l a t i v i s t i cL or e n t zf or c el a wy i e l d e dc ov a r i a n t e qu a t i on sofmot i onf orap oi n tc h a r g e dma s s i v ep a r t i c l ei n t e r a c t i n gwi t ht h e s ef i e l ds o rt h et r a j e c t or yofac h a r g e d,p oi n tp a r t i c l ewa sg i v e na n dt h ef i e l d sr a di a t e db yt h i s p a r t i c l ewe r ed e t e r mi n e d . Th i s ,h owe v e r ,wa sc l e a r l yn ote n ou g h ,ora tl e a s twa sn otc on s i s t e n t .Th a ti s b e c a u s e( a saf e ws i mp l eme n t a lp r o b l e mswi l ls h ow)e a c hoft h e s ep r oc e s s e si son l y h a l fofa ni n t e r a c t i on— ac omp l e t e ,c on s i s t e n tf i e l dt h e or ywou l di n c l u d et h e s e l f –c o n s i s t e n ti n t e r a c t i onofac h a r g e dp a r t i c l ewi t ht h ef i e l di ni t sv i c i n i t y ,orb e t t e r y e t ,t h es e l f c on s i s t e n ti n t e r a c t i onofa l lp a r t i c l e sa n df i e l d s .Wen e e dt ob ea b l et o c a l c u l a t et h et ot a lf i e l d( i n c l u di n gt h er a di a t e df i e l d )a tt h ep os i t i onofa n yg i v e np oi n t c h a r g e .Someoft h a tf i e l di sdu et ot h ec h a r g ei t s e l fa n ds omei sdu et ot h ef i e l d p r od u c e db yt h eot h e rc h a r g e s .Bu twedon otk n owh owt odot h i s , r e a l l y , s i n c et h eon e wi l l a ffe c tt h eot h e r , a n dt h e r ea r ec l e a r l yi n f i n i t i e sp r e s e n t . Th i ss or tofp r ob l e mc a na l s ol e a dt oNe wt on i a np a r a d ox e s ,p a r a d ox e st h a ts ma c kof t h er e s u r r e c t i onofAr i s t ot e l i a nd y n a mi c s .Tos e et h i s , l e tu sa s s u me( n on –p h y s i c a l l y )t h a t weh a v eaUn i v e r s ec on s i s t i n gofas i n g l ep oi n tc h a r g eor b i t i n ga r ou n da nu n c h a r g e d g r a v i t a t i on a lma s s( ors o meot h e rf or c ec e n t e rt h a tc a u s e st h ec h a r g et omov ei nab ou n d or b i t ) .I nt h a tc a s e ,t h ep oi n tc h a r g emu s t( a c c or d i n gt ot h el a wsofe l e c t r od y n a mi c st h a t weh a v et h u sf a rd e du c e d )r a d i a t ee n e r gya n dmo me n t u mi n t ot h ee l e c t r oma gn e t i cf i e l d . Asi ta c c e l e r a t e s , i tmu s tr a d i a t e .Asi tr a d i a t e s , e n e r g ya n dmome n t u mmu s tb ec a r r i e d a wa yf r om t h ep oi n tp a r t i c l et o“ i n f i n i t y ” .Th ep a r t i c l emu s tt h e r e f or ed e c r e a s ei t st ot a l e n e r g y .I ft h ep a r t i c l ei sb ou n di na na t t r a c t i v e ,n e g a t i v ep ot e n t i a lwe l l ,t h eon l ywa yt h a t t ot a le n e r g yc a nb ec on s e r v e di si fi t st ot a le n e r g yde c r e a s e s .Th ep a r t i c l emu s tt h e r e f or e s p i r a li n wa r d st h ec e n t e r ,c on v e r t i n gi t sp ot e n t i a le n e r g yi n t or a d i a t i v ee n e r g yi nt h ef i e l d , u n t i l i tr e a c h e s 3 1 1 t h ep ot e n t i a l mi n i mu ma n dc ome st or e s t . Th e r ei son l yon ed i ffic u l t ywi t ht h i sp i c t u r e .Th e r ei son l yon ec h a r g e dp a r t i c l ei n t h eUn i v e r s e , a n di ti si n t e r a c t i n gwi t hon l yon ea t t r a c t i v ec e n t e r . Wh a ta c t st os l o wt h ep a r t i c l ed own ? Th i si san on –qu e s t i on ,ofc ou r s e–at h ou g h te x p e r i me n tde s i g n e dt oh e l pu s u n d e r s t a n dwh e r eou re qu a t i on sofmot i ona n dc l a s s i c a lp i c t u r ea r ei n c omp l e t eor i n c on s i s t e n t .Th er e a l u n i v e r s eh a sma n yc h a r g e dp a r t i c l e s , a n dt h e ya r ea l l c on s t a n t l y i n t e r a c t i n gwi t ha l lt h eo t h e rc h a r g e dp a r t i c l e st h a tl i ewi t h i nt h e“ e v e n th or i z on ”ofa n e v e n tr e l a t i v et ot h et i meoft h eb i gb a n g , wh i c hi st h es e toft h emos td i s t a n te v e n t si n s p a c e –t i mei nt h ep a s ta n di nt h ef u t u r et h a tc a ni n t e r a c twi t ht h ec u r r e n te v e n tont h e 1 wor l dl i n eofe a c hp a r t i c l e . I ti st h ee dg eoft h e“ b l a c kh ol e ”t h a ts u r r ou n dsu s. Howe v e r ,i nou rs i mp l i e dUn i v e r s et h i squ e s t i oni sv e r yr e a l .Weh a v es y s t e ma t i c a l l yr i dou r s e l v e soft h ef i e l d sofa l lt h eot h e rp a r t i c l e s , s on owwemu s tf i n da f i e l db a s e dont h ep a r t i c l ei t s e l ft h a ty i e l d st h en e c e s s a r y“ r a d i a t i onr e a c t i on ”f or c et o b a l a n c et h ee n e r g y –mome n t u mc on s e r v a t i one qu a t i on s .Th i sa p p r oa c hwi l l h a v ema n y u n s a t i s f a c t or ya s p e c t s , b u ti twor k s . F i r s t , wh e nwi l lr a di a t i onr e a c t i onb e c omei mp or t a n t ?Wh e nt h ee n e r g yr a di a t e db y ap a r t i c l ei sar e a s on a b l ef r a c t i onoft h et ot a lr e l e v a n te n e r g yE ft h ep a r t i c l eu n d e r 0o c on s i d e r a t i on . Th a ti s 22 2eaT 2 3c4πǫ0c E r a d∼ ( 2 1 . 1 ) wh e r eai st h et ot a l( e . g .c e n t r i p e t a l )a c c e l e r a t i ona n dTi st h ep e r i odoft h eor b i t a s s oc i a t e dwi t hE rt h et i meau n i f or ma c c e l e r a t i oni sa p p l i e d .I fEr <E h e nwe 0o a d< 0t c a nn e g l e c tr a d i a t i onr e a c t i on . Asb e f or e , i fap a r t i c l ei su n i f or ml y( l i n e a r l y )a c c e l e r a t e df orat i meτ , t h e nwec a n r n e gl e c tr a d i a t i onr e a c t i o nwh e n 22 2eaτ r 2 E a τ )≫ 0∼m( r 2 3c4πǫ0c ( 2 1 . 2 ) Ra d i a t i onr e a c t i oni st h u son l ys i gn i f i c a n twh e nt h eop p os i t ei st r u e , wh e n : 2 2e 2 c4 πǫ0mc τ r∼ 3 2 2 / c= 3τ e e ∼ 3r ( 2 1 . 3 ) On l yi fτ n dai sl a r g ewi l lr a di a t i onr e a c t i onb ea p p r e c i a b l e .F ore l e c t r on st h i s r∼τ ea − 2 3 t i mei sa r ou n d1 0 s e c on d s .Th i swa st h es i t u a t i onwee x a mi n e db e f or ef orl i n e a r a c c e l e r a t or sa n de l e c t r on –p os i t r ona n i h i l l a t i on .On l yi nt h el a t t e rc a s ei sr a d i a t i on r e a c t i onl i k e l y . 1 I ti si n t e r e s t i n gt ome d i t a t eu p ont h ef a c tt h a ty o u re v e n th or i z ona n dmye v e n th or i z ona r en ot c oi n c i d e n t , wh i c hl e a d si nt u r nt oa ni n t e r e s t i n gp r ob l e mwi t hl og i c a l p os i t i v i s m. Th es e c on dc a s et oc on s i d e ri swh e r et h ea c c e l e r a t i oni sc e n t r i p e t a l .Th e nt h e p ot e n t i a l a n dk i n e t i ce n e r g ya r ec omme n s u r a t ei nma g n i t u d e( v i r i a l t h e or e m)a n d 22 E 0∼mω0d 2 wh e r ea∼ω0da n dτ / ω0. Asb e f or e , wec a nn e g l e c tr a d i a t i onr e a c t i oni f r∼1 2 42 2 22 mω0d ≫ 2 eω d 2 0 2 ( 2 1 . 4 ) 2e 2 3c4πǫ0cω0=ω0d 3c4πǫ0c ( 2 1 . 5 ) Ra d i a t i onr e a c t i oni st h u sa g a i ns i g n i f i c a n tp e rc y c l eon l yi f ω0τ r∼1 ( 2 1 . 6 ) ( i g n or i n gf a c t or sofor d e ron e )wh e r eτ sg i v e na b ov e–a n ot h e rwa yofs a y i n gt h e ri − 1 2 s a met h i n g .ω0 i s( wi t h i ni r r e l e v a n tf a c t orof2 πa n d 3)t h et i mea s s oc i a t e dwi t ht h e mot i on ,s oon l yi ft h i st i me s c a l ec or r e s p on d st oτ l lr a d i a t i onr e a c t i onb e r≈τ ewi s i g n i f i c a n t . Sof a r ,ou rr e s u l t sa r ej u s tar e s t a t e me n toft h os eweob t a i n e ddi s c u s s i n gL a r mor r a d i a t i one x c e p tt h a twea r eg oi n gt ob emor ei n t e r e s t e di ne l e c t r on si na t omi cs c a l e p e r i od i cor b i t sr a t h e rt h a na c c e l e r a t or s .E l e c t r on si na na t omi cor b i twou l db ec on s t a n t l y a c c e l e r a t i n g , s oa n ys ma l ll os sp e rc y c l ei ss u mme dov e rma n yc y c l e s .Ab i tofv e r ys i mp l e or de r of ma g n i t u d ea r i t h me t i cwi l l s h owy out h a tr a di a t i v ep owe rl os sn e e dn otb en e g l i g i b l e −1 −1 5 a sar a t ec omp a r e dt oh u ma nt i me s c a l e swh e nω0 i sv e r ys ma l l( e . g .or de rof1 0 s e c on dsf ore . g .op t i c a lf r e qu e n c yr a d i a t i on ) .Ch a r g e dp a r t i c l e s( e s p e c i a l l ye l e c t r on s )t h a t mov ei nac i r c l ea tah i g he n ou g h( a n g u l a r )s p e e dd oi n d e e dr a d i a t eas i g n i f i c a n tf r a c t i onof t h e i re n e r g yp e rs e c o n dwh e nt h el os si ss u mme dov e rma n yc y c l e s .Th el os sp e rc y c l ema y b es ma l l , b u ti ta d d su pi n e x or a b l y . Howdowee v a l u a t et h i s“ r a d i a t i onr e a c t i onf or c e ”t h a th a sn oob v i ou sp h y s i c a l s ou r c ei nt h ee qu a t i on st h a tr e ma i n ?Th ee a s ywa yi s :t r yt ob a l a n c ee n e r g y( a n d mome n t u me t c )a n da d dar a d i a t i onr e a c t i onf or c et oa c c ou n tf ort h e“ mi s s i n ge n e r g y ” . Th i swa st h ea p p r oa c ht a k e nb yAb r a h a ma n dL or e n t zma n ymoon sa g o. 21. 2 Ra d i a t i o nRe a c t i ona n dE n e r gyCon s e r v a t i o n Wek n owt h a t ˙ Ft ot=mv ( 2 1 . 7 ) i s( n on r e l a t i v i s t i c )Ne wt on ’ s2 n dL a wf orac h a r g e dp a r t i c l eb e i n ga c c e l e r a t e db ya( f or t h emome n t ,n on –e l e c t r oma g n e t i c )g i v e ne x t e r n a lf or c e .Th ewor ke n e r g yt h e or e m d i c t a t e sh owf a s tt h ep a r t i c l ec a ng a i nk i n e t i ce n e r g yi ft h i si st h eon l yf or c ea c t i n g . Howe v e r ,a tt h es a met i mei ti sb e i n ga c t e donb yt h ee x t e r n a lf or c e( a n di s a c c e l e r a t i n g ) , i ti sa l s or a d i a t i n gp o we ra wa ya tt h et ot a l r a t e : 2 2 P( t )= 2 e v ˙ 2 3 c4 πǫ0c = 2mr e v 3 c 2̇ 2̇ v r = mτ ( 2 1 . 8 ) ( t h eL a r mo rf or mu l a ) .Th e s ea r et h et wop i e c e swe ’ v et h u sf a rt r e a t e di n d e p e n d e n t l y , n e g l e c t i n gt h eon et oob t a i nt h eot h e r . Howe v e r , i nor d e rf o rNe wt on ’ sl a wt oc or r e c t l yl e a dt ot h ec on s e r v a t i onofe n e r g y , t h ewor kd on eb yt h ee x t e r n a lf or c emu s te qu a lt h ei n c r e a s ei nk i n e t i ce n e r g yp l u st h e e n e r g yr a d i a t e di n t ot h ef i e l d . E n e r g yc on s e r v a t i onf ort h i ss y s t e ms t a t e st h a t : Wext=E e+E f ( 2 1 . 9 ) ort h et ot a lwor kd on eb yt h ee x t e r n a lf or c emu s te qu a lt h ec h a n g ei nt h et ot a le n e r g y oft h ec h a r g e dp a r t i c l e( e l e c t r on )p l u st h ee n e r g yt h a ta p p e a r si nt h ef i e l d .I fwe r e a r r a n g et h i st o: Wext−E f=E e ( 2 1 . 1 0 ) a n dc on s i d e rt h ee l e c t r o non l y ,wea r ef or c e dt oc on c l u det h a tt h e r emu s tb ea n ot h e r f or c ea c t i n gont h ee l e c t r on , on ewh e r et h et ot a l wor kd on eb yt h ef or c ed e c r e a s e st h e c h a n g ei ne n e r g yoft h ee l e c t r ona n dp l a c e st h ee n e r g yi n t ot h er a d i a t e df i e l d .Wec a l l t h a tf or c eFr , t h er a d i a t i o nr e a c t i o nf o r c e . a d Th u s( r e wr i t i n gNe wt on ’ ss e c on dl a wi nt e r msoft h i sf or c e ) : F F e x t+ r a d F r a d ˙ =mv ˙ =mv −Fext ( 2 1 . 1 1 ) d e f i n e st h er a d i a t i o nr e a c t i onf o r c et h a tmu s ta c tont h ep a r t i c l ei nor de rf ore n e r g y c on s e r v a t i ont oma k es e n s e .Th er e a c t i onf or c eh a san u mb e rofn e c e s s a r yor 2 d e s i r e a b l ep r op e r t i e si nor de rf oru st on otg e ti n t o“ t r ou b l e ”. •Wewou l dl i k ee n e r g yt ob ec on s e r v e d( a si n d i c a t e da b ov e ) ,s ot h a tt h ee n e r g y t h a ta p p e a r si nt h er a d i a t i onf i e l di sb a l a n c e db yt h ewor kd on eb yt h er a d i a t i on r e a c t i onf or c e( r e l a t i v et ot h et ot a lwor kd on eb ya ne x t e r n a lf or c et h a tma k e s t h ec h a r g ea c c e l e r a t e ) . •Wewou l dl i k et h i sf or c et ov a n i s hwh e nt h ee x t e r n a lf or c ev a n i s h e s ,s ot h a t p a r t i c l e sd on ots p on t a n e ou s l ya c c e l e r a t ea wa yt oi n f i n i t ywi t h ou ta ne x t e r n a l a g e n ta c t i n gont h e m. 2 Tr ou b l es u c ha sp a r t i c l e sc a p a b l eofl i f t i n gt h e ms e l v e su pb yt h e i rownme t a p h or i c a l b oot s t r a p s . . . 2 •Wewou l dl i k et h er a d i a t e dp o we rt ob ep r op or t i on a lt oe,s i n c et h ep owe ra n d 2 i t ss p a c ed e r i v a t i v e si sp r op or ot i on a l t oe a n ds i n c et h ef or c ema g n i t u d es h ou l d b ede p e n d e n toft h es i g noft h ec h a r g e . •F i n a l l y ,wewa n tt h ef or c et oi n v ol v et h e“ c h a r a c t e r i s t i ct i me ”τ( wh e r e e v e ri t n e e dsap a r a me t e rwi t ht h ed i me n s i on soft i me )s i n c en oot h e rt i me s c a l e d p a r a me t e r sa r ea v a i l a b l e . L e t ’ ss t a r twi t ht h ef i r s toft h e s e .Wewa n tt h ee n e r g yr a di a t e db ys ome“ b ou n d ” c h a r g e( on eu n d e r g oi n gp e r i od i cmo t i oni ns omeor b i t , s a y )t oe qu a lt h ewor kd on eb y t h er a d i a t i onr e a c t i onf or c ei nt h ep r e v i ou se qu a t i on .L e t ’ ss t a r tb ye x a mi n i n gj u s tt h e r e a c t i onf or c ea n dt h er a d i a t e dp owe r , t h e n , a n ds e tt h et ot a lwor kdon eb yt h eon et o e qu a l t h et ot a l e n e r g yr a d i a t e di nt h eot h e r , ov e ras u i t a b l et i mei n t e r v a l : t 2 t 1 t 2 t 2 Fr ·d adv t=− t 1 Pd t=− t 1 ˙ ˙ mτvr · vdt ( 2 1 . 1 2 ) f ort h er e l a t i onb e t we e nt h er a t e s ,wh e r et h emi n u ss i g ni n d i c a t e st h a tt h ee n e r g yi s r e mov e df r omt h es y s t e m. Wec a ni n t e g r a t et h er i g h th a n ds i d eb yp a r t st oob t a i n t 2 t 2 ¨ ˙ t 2 mτv ·dt−mτr v (v ·)| 1 rv t Fr ·d t= t1 a dv ( 2 1 . 1 3 ) F i n a l l y , t h emot i oni s“ p e r i od i c ”a n dweon l ywa n tt h er e s u l tov e rap e r i od ; we t 1 ˙ c a nt he r e f or epi c kt hee ndpoi nt ss uc ht ha t vv · t 2 t 1 =0 . Th u sweg e t ¨ Fr v · d a d−mτ r v t=0 . ( 2 1 . 1 4 ) On e( s u ffic i e n tb u tn otn e c e s s a r y )wa yt oe n s u r et h a tt h i se qu a t i onb es a t i s f i e di s t ol e t ¨ Fr v a d=mτ r Th i st u r n sNe wt on ’ sl a w( c or r e c t e df orr a d i a t i onr e a c t i on )i n t o F ( 2 1 . 1 5 ) ˙ e x t= mv −Fr a d ˙ ¨ ( −τ v r) =mv ( 2 1 . 1 6 ) Th i si sc a l l e dt h eAb r a h a m–L o r e n t ze qu a t i onofmo t i ona n dt h er a di a t i onr e a c t i on f or c ei sc a l l e dt h eAb r a h a m–L or e n t zf o r c e .I tc a nb ema d er e l a t i v i s t i cb ec on v e r t i n gt o p r op e rt i mea su s u a l . Not et h a tt h i si sn otn e c e s s a r i l yt h eon l ywa yt os a t i s f yt h ei n t e g r a lc on s t r a i n ta b ov e . An ot h e rwa yt os a t i s f yi ti st or e qu i r et h a tt h ed i ffe r e n c eb eor t h og on a l t ov.E v e nt h i si st oo s p e c i f i c , t h ou g h .Th eon l yt h i n gt h a ti sr e q u i r e di st h a tt h et ot a li n t e g r a lb ez e r o, a n ds h or t ofd e c omp os i n gt h ev e l oc i t yt r a j e c t or yi na nor t h og on a ls y s t e ma n dp e r h a p su s i n gt h e c a l c u l u sofv a r i a t i on s ,i ti sn otp os s i b l et oma k ep os i t i v es t a t e me n t sa b ou tt h en e c e s s a r y f or mofFr . a d Th i s“ s u ffic i e n t ”s ol u t i oni sn otwi t h ou tp r ob l e msofi t sown ,p r ob l e mst h a ts e e m u n l i k e l yt og oa wa yi fwec h oos es omeot h e r“ s u ffic i e n t ”c r i t e r i on .Th i si sa p p a r e n t f r omt h eob s e r v a t i ont h a tt h e ya l ll e a dt oa ne qu a t i onofmot i ont h a ti st h i r do r d e ri n t i me . Now, i tma yn ots e e mt oy ou( y e t )t h a tt h a ti sad i s a s t e r , b u ti ti s . Su p p os et h a tt h ee x t e r n a l f or c ei sz e r oa ts omei n s t a n toft i met=0 . Th e n ˙ ¨ v ≈τv or ˙ ( 2 1 . 1 7 ) t / τ v( t )= a0e ( 2 1 . 1 8 ) wh e r e a0i st h ei n s t a n t a n e ou sa c c e l e r a t i onoft h ep a r t i c l ea tt=0 . Re c a l l i n gt h a t v ˙ · v =0a tt n dt , wes e et h a tt h i sc a non l yb et r u ei f 1a 2 a0=0( orwec a nr e l a xt h i sc on d i t i ona n dp i c ku pa na d d i t i on a lb ou n d a r yc on d i t i ona n d wor kmu c hh a r d e rt oa r r i v ea tt h es a mec on c l u s i on ) .Di r a ch a das i mp l yl ov e l yt i me wi t ht h et h i r dor d e re qu a t i on .Be f or ea t t a c k i n gi t , t h ou g h ,l e tu sob t a i nas ol u t i ont h a t d oe s n ’ th a v et h ep r ob l e msa s s oc i a t e dwi t hi ti nad i ffe r e n t( mor eu p f r on t )wa y . L e tu sn ot et h a tt h er a d i a t i onr e a c t i onf or c ei na l mos ta l lc a s e swi l lb ev e r ys ma l l c omp a r e dt ot h ee x t e r n a l f or c e . Th ee x t e r n a l f or c e , i na dd i t i on , wi l l g e n e r a l l yb e“ s l owl y −2 4 v a r y i n g ” , a tl e a s tonat i me s c a l ec omp a r e dt oτ 0 s e c on d s .I fwea s s u met h a tF r≈1 ( t )i ss moo t h( c on t i n u ou s l yd i ffe r e n t i a b l ei nt i me ) , s l owl yv a r y i n g , a n ds ma l le n ou g h e x t t h a tFr a nu s ewh a ta mou n t st op e r t u r b a t i ont h e or yt ode t e r mi n eFr a d≪ Fe x twec a d a n dob t a i nas e c on dor d e re qu a t i onofmot i on . ˙ Un d e rt h e s ec i r c u ms t a n c e s , wec a na s s u met h a tFext≈mv, s ot h a t : ˙ ¨ F e x t ( −τ v r) = mv ˙ F d ext t ≈ mv −τr d or ( 2 1 . 1 9 ) F ˙F mv = ex t F +τ r d e x t dt ∂ ( 2 1 . 2 0 ) = +τ t+v( ·∇)Fext r ∂ Th i sl a t t e re qu a t i onh a sn or u n a wa ys ol u t i on sora c a u s a l b e h a v i ora sl on ga s ex t Fexti sd i ffe r e n t i a b l ei ns p a c ea n dt i me . Wewi l ld e f e rt h ed i s c u s s i onoft h ec ov a r i a n t ,s t r u c t u r ef r e eg e n e r a l i z a t i onoft h e Ab r a h a m–L or e n t zd e r i v a t i onu n t i ll a t e r .Th i si sb e c a u s ei ti n v ol v e st h eu s eoft h ef i e l d s t r e s st e n s or , a sd oe sDi r a c ’ sor i g i n a l p a p e r—wewi l l di s c u s st h e ma tt h es a met i me . Wh a ta r et h e s er u n a wa ys ol u t i on so ft h ef i r s t( Ab r a h a mL or e n t z )e qu a t i onof mot i on ?Cou l dt h e yr e t u r nt op l a g u eu swh e nt h ef or c ei sn ots ma l la n dt u r n son q u i c k l y ?L e t ’ ss e e . . . 21. 3 I n t e gr o d i ffe r e n t i a l E q u a t i o n sofMo t i on Wes e e ks ol u t i on st ot h et h i r dor d e rALe qu a t i onofmot i o nt h a te v ol v ei n t ot h e“ n a t u r a l ” on e swh e nt h ed r i v i n gf or c ei st u r n e doff.I not h e rwor d s ,r a di a t i onr e a c t i onmu s t ,b y h y p ot h e s i s , on l yda mpt h es y s t e ma n dn otd r i v ei t .Cl e a r l ye v e nt h i sr e qu i r e me n tma k e s n os e n s ewh e nt i mer e v e r s a ls y mme t r yi sc on s i d e r e d .On c ewef a l li n t ot h et r a pof c h oos i n gr e t a r d e di n t e r a c t i onon l y ,wea r es u n ka n da n y t h i n gwed ot of i xi twi l lb ea b a n d–a i d . L e tu si n t r od u c ea n“ i n t e g r a t i n gf a c t or ”i n t ot h ee qu a t i on sofmot i on .I fwea s s u me ( qu i t eg e n e r a l l y )t h a t ˙ t / τ r v( t )=e t ) u( ( 2 1 . 2 1 ) wh e r e u( t )i st ob ed e t e r mi n e d , t h e nt h ee qu a t i on sofmot i ons i mp l i f yt o ˙ 1 −t/τ mu=− τ t ) . re F( ( 2 1 . 2 2 ) Wec a nf or ma l l yi n t e g r a t et h i ss e c on de qu a t i on , ob t a i n i n g ˙ mv( t )= C t / τ e r τ r ′ −t/ τ rF ′ ′ e ( t ) d t t ( 2 1 . 2 3 ) Th ec on s t a n tofi n t e g r a t i oni sd e t e r mi n e db you rr e qu i r e me n tt h a tn or u n a wa y s ol u t i on se x i s t ! Not ewe l l t h a ti ti sac on s t r a i n tt h a tl i v e si nt h ef u t u r eoft h ep a r t i c l e . I n or d e rt ou s et h i st of i n d v( t ) , wemu s tk n owt h ef or c eF( t )f ors omet i me( ofor d e rτ )i n r t h ef u t u r e ! Af t e rt h i s , t h ei n t e g r a n di s“ c u toff”b yt h ed e c a y i n ge x p on e n t i a l . Th i ss u g g e s t st h a twec a ne x t e n dt h ei n t e g r a lt oC=∞ wi t h ou td i ffic u l t y .I nt h e l i mi tτ , wer e c ov e rNe wt on ’ sl a w, a swes h ou l d . Tos e et h i s , l e t r→ 0 1 ′ t−t ) r( s=τ s ot h a t ˙ ∞ mv( t )= 0 ( 2 1 . 2 4 ) − s e F( t+τ s ) d s . r ( 2 1 . 2 5 ) Th ef or c ei sa s s u me dt ob es l owl yv a r y i n gwi t hr e s p e c tt oτ( orn on eoft h i sma k e ss e n s e , j u s ta swa st h ec a s ea b ov e )s ot h a taTa y l ors e r i e se x p a n s i onc on v e r g e s : ∞ F( t +τs )= F n 2 ( τ r s )ad ( t ) n n ! n = 0 dt wh i c h , u p ons u b s t i t u t i ona n di n t e g r a t i onov e rs , y i e l d s ˙ mv = ∞ n n τ dF n d t . ( 2 1 . 2 6 ) ( 2 1 . 2 7 ) ˙ F i g u r e2 1 . 1 :F( t ) , v( t )a n dv( t )onat i me s c a l eofτ .Not et h a tt h ep a r t i c l e r “ p r e a c c e l e r a t e s ”b e f or e“ t h ef or c eg e t st h e r e ” , wh a t e v e rt h a tme a n s . I nt h el i mi tτ→ 0o n l yt h el owe s tor de rt e r ms u r v i v e s .Th i si sNe wt on ’ sl a wwi t h ou t r a d i a t i onr e a c t i on .Th eh i g h e ror d e rt e r msa r es u c c e s s i v er a d i a t i v ec or r e c t i on sa n d ma t t e ron l yt ot h ee x t e n tt h a tt h ef o r c ev a r i e si nt i me .Not et h a tt h i sf or c eob e y sa “ L e n z ’ sL a w”s or tofb e h a v i or ; wh e nt h ea p p l i e df or c ei sc h a n g e d( s a y , i n c r e a s e d)t h e r e i sa na d di t i on a l“ f or c e ”i nt h ed i r e c t i o no ft h ec h a n get h a ta c t sont h ep a r t i c l e .A p a r t i c l emov i n gi nac i r c l eh a saf or c et h a tc h a n g e sd i r e c t i o nb u tn otma g n i t u d e .Th i s c h a n g ei s( t h i n ka b ou ti t )t a n g e n tt ot h emot i ona n di nt h eop p os i t ed i r e c t i on .I ta c t st o s l o wt h ec h a r ge dp a r t i c l ed o wn . Hmmmmmm. Th e r ea r et woe x t r e me l ya n n oy i n ga s p e c t st ot h i sot h e r wi s en ob l es ol u t i on .F i r s t , a swe h a v er e p e a t e d l yn ot e d , i tr e qu i r e sak n owl e d g eofF( t )i nt h ef u t u r eoft h ep a r t i c l et oob t a i n i t sa c c e l e r a t i onn o w.Tr u t h f u l l y ,t h i si s n ’ tr e a l l yap r ob l e m –ob v i ou s l yt h i si sa b s ol u t e l y e qu i v a l e n tt os a y i n gt h a tF( t )c a nb ee x p a n d e di naTa y l ors e r i e s( i sa na n a l y t i cf u n c t i on ) . Se c on d,( a n de v e nwor s e )i tr e s p o n d st oaf or c et h a ti sc omp l e t e l yi ni t sf u t u r ewi t ha n a c c e l e r a t i onn ow. I t“ k n o ws ”t h a taf or c ei sg oi n gt oa c toni tb e f o r et h a tf or c eg e t st h e r e . −2 4 Mi n dy ou ,n otl o n gb e f or et h ef or c eg e t st h e r e .Ab ou t1 0 s e c on d sb e f or e( f or r e a s on a b l ef or c e s ) .Cl a s s i c a l l yt h i si sv e r yb a d ,b u tqu a n t u mt h e or yf u z z e sp h y s i c sov e ra mu c hl a r g e rt i mes c a l e .Th i si sv i e we db yma n yp h y s i c i s t sa sa ne x c u s ef orn otwor k i n gou t ac on s i s t e n t l yc a u s a l c l a s s i c a l t h e or y . Youc a nma k e u py ou rownmi n da b ou tt h a t , b u tn o t ewe l lt h a te v e ni ft h ei n t e g r odi ffe r e n t i a le qu a t i on h a di n v ol v e dp a s tv a l u e soft h ef or c ey ous h ou l dh a v eb e e ne q u a l l yb ot h e r e d–e i t h e r on ema k e sNe wt on ’ sl a wn on l oc a l i nt i me ! Not ewe l lt h a twe ’ v ea l r e a d ys e e n( n on l oc a l )i n t e g r od i ffe r e n t i a le qu a t i on si nt i mei na s ome wh a ts i mi l a rc on t e x t !Re me mb e rou rd e r i v a t i onofofd i s p e r s i onr e l a t i on s , i np a r t i c u l a r Kr a me r s Kr on i g ?Weh a dak e r n e lt h e r et h a te ffe c t i v e l ys a mp l e dt i me si nt h ef u t u r eorp a s t ofas y s t e m’ smot i on .Th i swor k e db e c a u s ewec ou l di n t e g r a t eov e rf r e q u e n c i e swi t ha c on s t r a i n tofa n a l y t i c i t y– ou rf i e l d swe r ep r e s u me df ou r i e rde c omp os a b l e .F ou r i e r t r a n s f or msa r e ,ofc ou r s e ,i n f i n i t e l yc on t i n u ou s l ydi ffe r e n t i a b l ea sl on ga swea v oi ds h a r p c h a n g e sl i k e( p u r e )h e a v i s i d ef u n c t i onf or c e sorf i e l dc h a n g e s ,a n dy e s ,t h e ye x p l i c i t y p r ov i d eak n owl e d g eoft h equ a n t i t i e si nt h ef u t u r ea n dp a s toft h e i rc u r r e n tv a l u e s . Ip e r s on a l l yt h i n kt h a tt h i si sy e ta n ot h e ra s p e c toft h emi s t a k ema d eb yr e qu i r i n g t h a tou rd e s c r i p t i onofe l e c t r od y n a mi c sa l wa y sp r oc e e df r omt h ep a s ti n t ot h ef u t u r e wi t har e t a r d e di n t e r a c t i on .Asweh a v es e e n , t h i si ss i l l y–on ec ou l de qu a l l ywe l lu s e on l ya d v a n c e di n t e r a c t i on sorami xo ft h et woa n dt h es ol u t i on sob t a i n e df orag i v e n b ou n d a r yv a l u ep r ob l e mwi l lb ei de n t i c a l ,wh e r et h e“ b ou n d a r y ”i sn owaf ou r v ol u me a n dh e n c er e qu i r e sf u t u r ec on di t i on st ob es p e c i f i e da swe l la st h ep a s tc on di t i on son as p a t i a l t h r e e s u r f a c eb ou n d i n gt h ef ou r v ol u me . 21. 4 Ra d i a t i o nDa mp i n go fa nOs c i l l a t i n gCh a r ge Th emos ti mp or t a n ta p p l i c a t i onoft h eAb r a h a m–L or e n t zf or c el a wi st h er a di a t i on r e a c t i onofb ou n de l e c t r on si na t omsa st h e yr a d i a t e .Th i si st h ep r ob l e m or i g i n a l l y s t u d i e db yL or e n t z , i nt h ec on t e x tofac l a s s i c a los c i l l a t o r , a n dy e s , wea r er e t u r n i n gt o ou rd i s c u s s i onofdi s p e r s i onb u tn owwi t hap h y s i c a lmo d e lf orwh ywee x p e c tt h e r et o b eada mp i n gt e r mi n s t e a dofas t r i c t l yp h e n ome n ol og i c a l on e . Tos i mp l i f yl i f e ,wec on s i d e raL or e n t z“ a t om”t ob ea ne l e c t r ononas p r i n gwi t h 2 c on s t a n tk=mω0;aon e –d i me n s i on a lc l a s s i c a los c i l l a t orwi t har e s on a n tf r e qu e n c y ω0.I ft h eos c i l l a t ori sd i s p l a c e df r om e qu i l i b r i u m,i tr a d i a t e se n e r g ya wa ya n di s s i mu l t a n e ou s l yda mp e d.Th i si sac l a s s i c a la n a l og u eoft h ee mi s s i onofap h ot onb ya qu a n t u ma t om, wh i c hi sa c c omp a n i e db yt h ea t ome n t e r i n gal owe re n e r g yl e v e l . Th ee qu a t i onofmot i onf ort h ee l e c t r oni s( f r omt h eALf or c el a wa b ov e , i n t e g r a t e d a sd e s c r i b e df oroffs e tt i me s ) : ∞ 2−s x ¨ ( t )+ω0e x ( t+τs ) d s=0 0 ( 2 1 . 2 8 ) wh e r eweh a v eu s e dHook e ’ sl a w.I fwet r yt h eu s u a l s on ga n dd a n c e( a s s u met h a tx ( t ) − α t =x e weg e tt h ec h a r a c t e r i s t i ce qu a t i on 0 −α t 2 x e 0 2 α +ω0 ∞ 0 −( 1 +α τ) s e d s=0 . ( 2 1 . 2 9 ) I nor d e rf ort h ei n t e g r a l t oe x i s t , i tmu s td a mpa ti n f i n i t y , s oRe ( 1+α τ)>0 . I nt h a tc a s e , weg e t : ω2 2 α+ ∞ −x ed x = 0 − ( 1+α τ ) 0 2 ω 2 α + 2 = 0 ( 1+α τ ) 2 α( 1+α τ )+ω0 3 0 2 0 =0 2 τ α +α +ω0 =0 3 2 2 ( τ α )+( τ α )+( ω0τ ) = 0 3 2 22 z +z +ω0τ = 0 ( 2 1 . 3 0 ) wh e r ewe ’ v ed e f i n e dz=α τ. Th i si st h es a mec u b i ct h a twou l da r i s ed i r e c t l yf r omt h eor i g i n a lALe qu a t i onof mot i onb u tt h er e s t r i c t i o nont h ei n t e g r a le l i mi n a t e st h e“ r u n a wa y ”s ol u t i on s( α=− ( 1+ 2 2 ω0τ ) / τ)a tt h ee x p e n s eofi n t r od u c i n gp r e a c c e l e r a t e don e s .Th e r ei sn op oi n ti n g i v i n gt h ep h y s i c a lr oot si nc l os e df or mh e r e , b u ty ous h ou l df e e lf r e et oc r a n ku pe . g . Ma t h e ma t i c aa n dt a k eal ook . I fω0τ< <1( wh i c hi st h ep h y s i c a l l yr e l e v a n tr a n g e ) , t h e nt h ef i r s tor d e r r e s u l ti s ( ω0 + α= 2 ±i wh i t h ω) ( 2 1 . 3 1 ) 2 =ω0τ ( 2 1 . 3 2 ) a n d 53 ω 2 ω=− 8 0τ . ( 2 1 . 3 3 ) Th ec on s t a n ti st h ed e c a yc o n s t a n ta n dt h eωi st h el e v e ls h i f t .Not et h a tt h er a d i a t i v e f or c eb ot hi n t r od u c e sda mp i n ga n ds h i f t st h ef r e qu e n c y ,j u s tl i k ei tdoe sf orac l a s s i c a l d a mp e d os c i l l a t or .I fwee v a l u a t et h ee l e c t r i cf i e l dr a d i a t e db ys u c ha nos c i l l a t or ( n e g l e c t i n gt h et r a n s i e n ts i g n a l a tt h eb e g i n n i n g )wef i n d t h a tt h ee n e r g yr a di a t e da saf u n c t i onoff r e qu e n c yi s d I ( ω) ω− ω0− 0 2π( dω =I 1 2 2 ω)+(/ 2 ) ( 2 1 . 3 4 ) wh i c hi st h ec h a r a c t e r i s t i cs p e c t r u mofab r o a d e n e d,s h i f t e dr e s on a n tl i n e .Th i sc on c l u de s ou rdi s c u s s i onoft h ec o n s e qu e n c e sofr a d i a t i onr e a c t i on . You wi l l n ot et h a tt h ed e r i v a t i on sweh a v es e e na r en otp a r t i c u l a r l ys a t i s f y i n gorc on s i s t e n t . Nowwewi l l e x a mi n et h e“ b e s t ”oft h ede r i v a t i on s( Di r a c ’ sa n dWh e e l e ra n dF e y n ma n ’ s ) a n dt r yt oma k es omes e n s eofi ta l l . Th ef ol l owi n gs e c t i o n sa r ea l a ss t i l l i n c omp l e t eb u twi l l b ea d d e ds h or t l y . F i g u r e2 1 . 2 : At y p i c a l b r oa d e n e da n ds h i f t e dr e s on a n tl i n ed u et or a d i a t i onr e a c t i on . 21. 5 Di r a c ’ sDe r i v a t i o no fRa d i a t i o nRe a c t i on 21. 6 Wh e e l e ra n dF e y n ma n ’ sDe r i v a t i ono fRa d i a t i on Re a c t i o n 21. 7 MyOwnF i e l d F r e eDe r i v a t i o no fRa d i a t i o n Re a c t i o n