Luminiferous Ether
Luminiferous Ether
Assumptions



Mass less but rigid
Had no effect on the motion of planets or
other object.
Was everywhere even free space.
Luminiferous Ether
E


ether is pictured as a ghost wind blowing through
the earth
preferred frame (or absolute frame) is the one at rest
with respect to the ether
Luminiferous Ether


Many experiments developed to detect the
ether
The most is the Michelson-Morley experiment
Michelson-Morley
Experiment
Michelson-Morley Experiment(1887)


Michelson developed a device called an inferometer.
Device sensitive enough to detect the ether.
Michelson-Morley Experiment(1887)

Apparatus at rest wrt the ether.
Michelson-Morley Experiment(1887)


Light from a source is split by a half silvered mirror
(M)
The two rays move in mutually perpendicular
directions
Michelson-Morley Experiment(1887)


The rays are reflected by two mirrors (M1 and M2)
back to M where they recombine.
The combined rays are observed at T.
Michelson-Morley Experiment(1887)


The path distance for each ray is the same (l1=l2).
Therefore no interference will be observed
Michelson-Morley Experiment(1887)
ut
u

Apparatus at moving through the ether.
Michelson-Morley Experiment(1887)
ut
u
First consider the time required for the parallel ray

Distance moved during the first part of the path is
ct||  L  ut|| (distance moved by light to meet the mirror)

 t|| 
L
(c  u )
Michelson-Morley Experiment(1887)
ut
u
ct||  L  ut|| (distance moved by light to meet the mirror)
 t|| 
L
(c  u )
Similarly the time for the return trip is
The total time t|| 
L
L

(c  u ) (c  u )
t|| 
L
(c  u )
Michelson-Morley Experiment(1887)
ut
u
The total time t|| 
L
L

(c  u ) (c  u )

2 Lc
(c 2  u 2 )

2L / c
1 u2 c2
Michelson-Morley Experiment(1887)
ut
For the perpendicular ray
u
we can write,
ct2  L2  (ut) 2 (initial leg of the path)
L2  c 2t 2  u 2t 2
 (c 2  u 2 )t 2
L
t 
c2  u2
ct
vt
The return path is the same as the
initial leg therefore the total time is
2L
t 
c2  u2
Michelson-Morley Experiment(1887)
ut
t 
u
 t 
2L
c2  u2
2L / c
1 u2 c2
The time difference between the
two rays is,
2 1
2  2




2L
u
u 
t  t   t||  1  2   1  2  
c  c   c  


After a binomial expansion
1
ct
vt
2L u 2
Lu 2
t 
 2  3
c 2c
c
Michelson-Morley Experiment(1887)


The expected time difference is too small to
be measured directly!
Instead of measuring time, Michelson and
Morley looked for a fringe change.
Michelson-Morley Experiment(1887)



The expected time difference is too small to
be measured directly!
Instead of measuring time, Michelson and
Morley looked for a fringe change.
as the mirror (M) was rotated there should be
a shift in the interference fringes.
Michelson-Morley Experiment(1887)
Results of the Experiment
 A NULL RESULT


No time difference was found!
Hence no shift in the interference patterns
Conclusion from Michelson-Morley
Experiment

the ether didn’t exist.
Attempts to save the ether




Three of theories developed to explain the
Michelson-Morley experiment:
ether drag
Lorentz contraction
Emission theory
Lorentz Contraction(1890)


we very briefly look at Lorentz contraction
because of its significance to the
development of relativity.
His hypothesis assumed that that all objects
moving in the ether contracted by a factor
1  u 2 c 2  in the direction of the motion.
1
2
Consequences of Lorentz Contraction


Maxwell’s equations are invariant.
The laws of mechanics must be changed so
that they are invariant.

Solution: Relativistic mass
Problem with Lorentz’s Hypothesis

The Michelson-Morley experiment still gives a
null effect even when the Lorentz contraction
is included.