faster than light?

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faster than light?

• Einstein, light, and quantum mechanics

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Einstein, light and quantum mechanics

• Folklore
• Einstein proved that nothing can go faster than
  light
• Einstein didnt 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

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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 dont

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Cerenkov radiation
Cerenkov radiation is emitted when a particle is
moving faster than the speed of light in the
material its travelling through analogous to
sonic boom
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Using Cerenkov radiation
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Detecting Cerenkov radiation
electric charge out
light in
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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 Maxwells theory of
  electromagnetism
• in Maxwells theory light is a wave
• Therefore special relativity isa consequence of
  the wavepicture of light

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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 Einsteins Nobel prize is a consequence
  of the particle theory of light

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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

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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 recklessit
  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

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Einstein and light

• Einstein developed the theory of relativity by
  taking Maxwells wave theory of light and
  developing its logical consequences
• Einstein developed the theory of the
  photoelectric effect by taking Plancks theory of
  light and developing its logical consequences
• It is a measure of his genius that he was able to
  do both!

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