faster than light?

1. faster than light? Einstein, light, and quantum mechanics

2. 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

3. 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

4. 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

5. Using Čerenkov radiation

6. Detecting Čerenkov radiation electric charge out light in

7. 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 isa consequence of the wavepicture of light

8. 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

9. 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

10. 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]

11. 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!

12. Robert Millikan, physics, 1923, e and h Albert Einstein, physics, 1921, photoelectric effect Max Planck, physics, 1918, quanta Walther Nernst, chemistry, 1920, thermo-chemistry Max von Laue, physics, 1914, X-ray diffraction