Transcript faster than light? - University of Sheffield

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