faster than light?Einstein, light, and quantum mechanics

Einstein, light and quantum mechanics Folklore

Einstein proved that nothing can go faster than light

Einstein didn’t believe in quantum mechanics

Fact Many things go or appear to go faster than

light (in some circumstances) Einstein got his Nobel Prize for quantum

mechanics

things that go faster than light the expansion of the universe

objects cannot go faster than light across space space can expand as fast as it wants

optical illusions jets emitted by many radio galaxies and quasars the “lighthouse beam” from a pulsar

many particles in material the limit is the speed of light in a vacuum in material, e.g. glass, water, light slows down, but

particles don’t

Čerenkov radiation

Čerenkov radiation is emitted when a particle is moving faster than the speed of light in the material it’s travelling through – analogous to sonic boom

Using Čerenkov radiation

Detecting Čerenkov radiation

light in

electric charge out

Einstein and the nature of light Einstein said that the speed of light in a

vacuum should appear the same to all observers this is based on Maxwell’s

theory of electromagnetism in Maxwell’s theory light is

a wave Therefore special relativity is

a consequence of the wavepicture of light

Einstein and the nature of light But Einstein got his Nobel prize for his

explanation of the photoelectric effect “the bold, not to say the reckless,

hypothesis of an electro-magnetic light corpuscle” [Millikan]

Therefore Einstein’s Nobel prize is a consequence of the particle theory of light

The photoelectric effect Light striking the surface of some materials

causes emission of electrons but only if frequency is high enough higher frequency → higher energy electrons greater amplitude → more electrons

but only if frequency is high enough This is easily explained if we think of light

as consisting of “photons” with energy related to their frequency theory used by Max Planck to explain radiation

from hot objects not widely believed at the time

Quantum doubts Planck (who invented it)

“One should not hold against him too much that in his speculations he might have occasionally overshot the goal, as for example in his hypothesis of the quanta of light.” [A reference letter for Einstein!]

Millikan (who measured it) “This hypothesis may well be called reckless…it flies in

the face of the thoroughly established facts of interference” [His paper on the measurement of h]

“…this work resulted, contrary to my own expectation, in the first direct experimental proof…” [Nobel lecture]

Einstein and light Einstein developed the theory of

relativity by taking Maxwell’s wave theory of light and developing its logical consequences

Einstein developed the theory of the photoelectric effect by taking Planck’s theory of light and developing its logical consequences

It is a measure of his genius that he was able to do both!

Max von Laue,

physics, 1914, X-ray diffraction

Robert Millikan, physics, 1923,

e and h

Max Planck, physics, 1918, quanta

Albert Einstein, physics,

1921, photoelectric

effect

Walther Nernst,

chemistry, 1920,

thermo-chemistry