Introduction to QED

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Introduction toQED

• Quantum Electrodynamics

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Introduction

• Created in 1929 by a number of scientists to
  describe the interaction of light and matter
• Melding of Maxwells theory and quantum mechanics
• Attempts to describe behavior of electrons
• Paul Dirac made relativistic adjustments
• 1948 Richard Feynman, Julian Schwinger and
  Sin-Itiro Tomonaga calculated the correction due
  to light

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Significance

• Describes all phenomenon but gravity and
  radioactivity
• QED is the theory behind chemistry and governs
  properties of chemicals
• Has survived over 50 years of testing

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Basics

• Describes what happens, not why
• Light behaves like particles, not waves
• Only probability can be calculated
• Little arrows (probability amplitudes)

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General Principle of Quantum Theory

• The probability that a particular event occurs
  is the square of a final arrow (probability
  amplitude) that is found by drawing an arrow for
  each way the event could happen, and then
  combining (adding) the arrows

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Glass Thickness
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Arrow Lengths
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Adding Arrows
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Determining Direction
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Partial Reflection
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As the glass gets thicker
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Extremes
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For varying frequencies
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Light Propagation

• A photon has nearly equal chances of going on any
  path.
• Therefore, all the arrows are nearly the same
  length.
• This difference is negligible.

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Mirrors and Angle of Incidence
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Equal Chances
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Arrows have equal lengths, but different
directions.
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The middle contributes more.
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• Where the time is least is also where the
  time for the nearby paths is nearly the same
  thats where the little arrows point in nearly
  the same direction and add up to a substantial
  length thats where the probability of a photon
  reflecting off a mirror is determined. And thats
  why, in approximation, we can get away with the
  crude picture of the world that says light only
  goes where the time is least.
• -Richard Feynman.
• Time is least where the angle of incidence
  equals the angle of reflection.

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The Edges of the Mirror
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Cutting out Pieces
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Diffraction Grating
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Light through Multiple Media
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The Lifeguard
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Mirage
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Light travels in straight lines?
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Light does not move in a straight path, but
rather uses a core of nearby space. (neighboring
paths)
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Restricting the Paths(Single Slit Diffraction)
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Uncertainty Principle?

• If the paths are too restricted, the light
  spreads out.
• There is no need for the uncertainty principle.

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Light traveling through many paths.
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The Focusing Lens
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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?