Introduction to QED

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Introduction toQED

• Quantum Electrodynamics
• Part III

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Double Slit Diffraction

• Interference occurs and probability is between 0
  and 4

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When Detectors are Added

• Interference disappears and probability is always
  2

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When Detectors are Fallible
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Electron Photon Interactions

• Electrons behave like photons.
• - On a large scale, they appear to move in
• straight lines.
• - On a small scale, they can move
• anywhere.
• - Interference becomes important.

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Electron Photon Interactions

• Three basic actions
• - A photon goes from place to place.
• - An electron goes from place to place.
• - An electron emits or absorbs a photon.

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Objects Moving from Place to Place
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Lights Probability of Movement

• Probability is dependent on only the distance the
  photon must travel and the time it takes to do so
  (difference of squares).
• Photon moves at c when distance and time
  components are equal (at a 45 angle).
• Photons highest probability occurs at speed c.

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Light Diagrams
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Electron Diagrams
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Coupling

• An electron has a specific probability to emit or
  absorb a photon.
• Since probability for either to happen is equal,
  they will be regarded as the same event and be
  referred to as coupling.
• The probability of coupling is the constant j
  (junction number), which is related to the
  electrons charge. j -0.1

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Coupling
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Simultaneous Electron Movements
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More Possibilities
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Photons Moving at Multiple Speeds
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Two Photons Exchanged
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Diminishing Contribution

• The more couplings that are required, the more
  times the probability must be multiplied by j.
• Since j is less than one, the probability of
  occurrence decreases with every required coupling.

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Photon Scattering

• Defined as when an electron absorbs and emits a
  photon, but not necessarily in that order
• A photon can disintegrate into an electron -
  positron pair.
• An electron and positron can annihilate and
  become a photon or two.

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Photon Scattering
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Electrons in an Atom

• Electrons are kept in orbit by exchanging
  photons with the protons in the nucleus.
• Scattering of photons by electrons in atoms is
  the cause of numerous optical phenomena.

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Hydrogen Atom
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Partial Reflection
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Partial Transmission
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Index of Refraction

• The additional turning of the probability
  amplitude causes the photon to appear to be
  moving more slowly through the material.
• The more opaque the material, the more turning
  occurs and the more slowly the light appears to
  travel.
• The amount of turning by the final arrow caused
  by the electrons in a material is called the
  index of refraction.

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Stimulated Emission

• Photons tend to get into the same state.
• The chance that an atom emits a photon is
  enhanced if some photons are already present.
• Principle used in lasers.

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Two Photons Moving
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When the Points Converge
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Electrons and the Exclusion Principle
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Magnetic Moment of an Electron

• Represents the response of an electron to an
  external magnetic field
• The number changes over time as more
  possibilities for an electron to absorb a photon
  are calculated.

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An Alternative Possibility
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Diracs Basic Diagram
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More Complex Possibilities
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• Diagrams
• Feynman, Richard P. QED The Strange Theory of
  Light and Matter. Princeton University Press.
  Princeton, NJ, 1988.

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Questions?