Welcome to Physics 264L
Professor Henry Greenside
Monday 2015-08-24
Please mute cellphones, put cellphones and laptops away.
Key Resources
http://www.phy.duke.edu/~hsg/264L/
Has everyone read the 264L syllabus carefully?
Has everyone purchased the 3 required textbooks?
I will hand these out now, fill them out in last minute of class. I will
then post answers to as many questions as I can within a few days.
Note: your name is optional.
Announcements
All future announcements via Piazza.
Please ask questions via Piazza.
First homework assignment due before class on
Monday, August 31, place in 264L homework box.
Assigned Reading: Chapters 1 and 2 of French, 1
and 2 of Taylor, also an assigned video.
Any Questions?
Today’s Goal: Pep talk and Big Picture
What is “Modern Physics”?
Why is it useful?
Why is it exciting?
Is there anything left to do?
What is Modern Physics?
Three major changes starting 1900-1930 in our understanding of
the universe that are deeply incompatible with the “classical”
physics of Newton and Maxwell:
1. Special relativity, how laws of mechanics must change for
objects moving close to the speed of light.
2. Quantum mechanics: wave-particle-spin properties of particles
and groups of particles.
3. General relativity: Einstein’s theory of gravity and cosmology.
These topics and their applications crucial for 21st century physics,
science, and technology.
Discussion Question:
What is “time”?
Clocks
Pre-Modern (Classical) Versus Modern
Understanding of Time
Newton: time is universal absolute, all clocks
measure the same amount of passage of time, flow
of time unchanged by events in the universe.
Einstein: time depends on the observer, and can
pass at different rates for different observers.
(Predicted before confirmation by experiments!)
It IS Possible to Time Travel Into Future
1. Accelerate to high constant speed relative to
Earth, and then return.
2. Enter a stronger gravitational field (say go close,
but not too close, to event horizon of black hole)
That rate of passage of time depends on relative
speed and on strength of gravitational field have
been quantitatively confirmed by many
experiments.
Proof of Massive Galactic Black Hole:
Stars Orbiting Near Center of Milky Way
Global Positioning Systems Takes Into Account
Gravitational Time Shifts
Time Travel into the Past?
Currently unknown whether this is possible, even in
principle. It is a major unsolved scientific question.
What is light?
“Classical” pre-modern late-19th-century answer: light is a superposition of
electromagnetic waves such that the electric field E(t,x,y,z) and magnetic
field B(t,x,y,z) satisfy the four Maxwell equations in vacuum (no charges, no
currents):
Classical View of Light in Trouble
Equililbrium Blackbody Radiation
From Opaque Warm Objects
Cosmological Blackbody Radiation
Deviations of T from mean 2.73 K
Proof that Big Bang occurred, details of dark matter, dark
energy, curvature of space, still being intensely studied,
very “modern”
Modern View Started in 1905:
Light Consists of Non-interacting Point Particles
Biology Knew About Photons
Long Before Einstein
Photon View Requires Randomness
Consider light reflected from water surface
From particle point of view, reflection
coefficient R=.04 must be probability for
photon to reflect rather than refract: where
does this randomness come from? How
does photon “decide” to reflect or refract?
æn -n ö
R=ç 1 2 ÷
è n1 + n2 ø
2
Similar issue for linearly polarized light
striking second linear polarizer, probability
of transmission is cos2(q) which is easy to
understand for electromagnetic wave, but
requires probabilistic understanding for
photons.