Eye to Eye With Einstein Lecture II Einstein and the Photon Concept Professor Henry Greenside Duke U

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Eye to Eye With Einstein Lecture IIEinstein and
the Photon ConceptProfessor Henry
GreensideDuke University
Albert Einstein 1879-1955
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Three Talks About Einsteins 1905
DiscoveriesImplications For Our 21st World

• Today Relativity and Emc2.
• October 9 What is light? Einsteins concept of
  the photon and a glimpse of the quantum world.
• October 23 What is matter made of? Einsteins
  idea to use Brownian motion to establish the
  existence and properties of atoms.

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Places for More Information

• PBS TV show NOVA this Tuesday, October 11, at 8
  pm Einsteins Great Idea
• www.pbs.org/wgbh/nova/einstein/
• Web page www.phy.duke.edu/hsg/einstein/
• Email hsg_at_phy.duke.edu Feel free to email me
  with any questions related to my talks.

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The ThemeIs Nature Continuous or Discrete?
A key question for Einstein was philosophical
Was nature discrete, made of lumps and
particles? Or was nature continuous? If
continuous, how to reconcile interactions of
lumpy stuff with continuous stuff?
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Properties of Light
Light has a direction, color (energy), and
orientation (polarization)
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The Wave Picture of Light Reflection,
Refraction, Interference, Diffraction, Pressure
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What is a Wave? Slinky Demo.
Transverse wave
Longitudinal wave

• Conclusions about waves
• Strength of wave can be arbitrarily weak.
• Period of wave can be arbitrarily long.
• Multiple waves can occur at same time and can
  interfere, i.e., combine their effects to
  produce a net wave.

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The Spectrum of Light
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Thomas Young Showed That Light Was a Wave
The nature of light is a subject of no material
importance to the concerns of life or to the
practice of the arts, but it is in many other
respects extremely interesting.
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Light Particles or Waves? Youngs Double-Slit
Experiment!
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Interference of Coherent Waves in the
Double-Slit Experiment
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Interference Demos

• Look at light through crack between fingers.
• Laser beam through narrow slot.

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Trouble in Paradise I Discrete Spectra of Light
Where does this discrete behavior come from? If
from atoms, how does light interact with atoms to
produce this behavior? Demo with gas discharge
tubes
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Trouble in Paradise II Fluorescent Minerals
Emitted light does not depend on intensity of
external light. Color emitted always has longer
wavelength that external light.
Ultraviolet light source
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Trouble in Paradise III The Photoelectric Effect
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Photoelectric What Happens If Light is a Wave?

• Number of electrons released (the electrical
  current) should increase with increasing
  intensity of the light but not depend on the
  color (wavelength) of the light.
• Because a wave is spatially spread out, it will
  take some time for an electron to receive enough
  energy to get knocked out of the metal.

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Photoelectric EffectWhat Was Seen
Experimentally?

• No matter how bright the light, no electrons were
  emitted unless the light had a sufficiently short
  wavelength.
• The electrons were emitted essentially instantly
  from the metal when the light was turned on.
• Demo of UV light but not bright light discharging
  an electroscope

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Einsteins Solution Light Comes in Lumps
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Geiger Counter Shows Light Made of Discrete Lumps
Use Co60 radioactive source which emits light in
form of gamma rays.
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A Disbeliever Who Won a Nobel Prize Confirming
Einsteins Photon Hypothesis
Robert Millikan 1868-1953
bold, not to say reckless, hypothesis of an
electro-magnetic light corpuscle of energy hn,
which flies in the face of thoroughly
established facts of interference. 1916
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Light Passing Through a Beam-Splitter
This partial transmission has straightforward
explanation with a wave-theory of light, but is
difficult to understand in limit of extremely low
light intensity when only one photon is detected
at a time the direction seems to be chosen
randomly with 50-50 probability (for 50 beam
splitter).
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Movie of Two-Slit Experiment for Electrons
Akira Tonomura Advanced Research Laboratory,
Hitachi, Japan http//www.pnas.org/content/vol0/is
sue2005/images/data/0504720102/DC1/04720Movie1.mpg
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Einstein Was Unhappy with Quantum Phenomena
"God does not play dice with the universe."
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Application of Photons Solar Cells
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Application of Photons Charge-Coupled Diode
(CCD) Heart of Modern Photography
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A Physicists Appreciation of Eyes Almost
Perfect Efficiency in Capturing Photons
5 photons lead to perception!
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A Physicists Appreciation of Plants
Molecular chlorophyll antenna can capture single
photons!
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Photon Concept Needed to Invent and Understand
Lasers

• Communication via optical fibers.
• Control of chemical reactions.
• Manufacturing.
• Medicine.
• Note In 1920s, Einstein made theoretical
  discovery which led in 1950s to invention of the
  laser.

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Conclusion of Talk II

• Photon concept was invented by Einstein in 1905
  to solve a philosophical problem, continuum or
  discrete, and also specific scientific puzzles.
• Light and particles all act the same way they
  have wave-like and particle-like properties.
  Quantum mechanics is a remarkable theory that
  explains under what conditions these different
  properties occur.
• Photons have turned out to be a truly fundamental
  particle of Nature, charged particles affect each
  other by exchanging photons.
• Nature is inherently random. Many brilliant
  experiments over last several decades have only
  confirmed this fact, Einstein is so far wrong
  that God does not play dice.

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Two-Slit Experiment at Low Intensities
Expected interference behavior for waves
What is actually observed at low light intensities
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Two-slit Experiment With Electrons
Akira Tonomura http//www.pnas.org/content/vol0/is
sue2005/images/data/0504720102/DC1/04720Movie1.mpg
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Maxwell Equations Unified All Electrical and
Magnetic Phenomena, Predicted Light Waves
James Clerk Maxwell 1831-1879
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Maxwells Light Wave
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Electron Microscopes
V100 keV means l.004 nm
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The Schrodinger Equation