Introduction to QED Quantum Electrodynamics Part III
Double Slit Diffraction Interference occurs and probability is between 0% and 4%
Interference disappears and probability is always 2%
When Detectors are Fallible
Electron – Photon Interactions Electrons behave like photons. straight lines. anywhere. - Interference becomes important.
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.
Objects Moving from Place to Place
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). Photon’s highest probability occurs at speed c.
Light Diagrams
Electron Diagrams
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 electron’s charge. j -0.1
Coupling
Simultaneous Electron Movements
More Possibilities
Two Photons Exchanged
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.
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.
Photon Scattering
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.
Hydrogen Atom
Partial Transmission
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.”
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.
Two Photons Moving
Electrons and the Exclusion Principle
Magnetic Moment of an Electron The number changes over time as more possibilities for an electron to absorb a photon are calculated.
An Alternative Possibility
Dirac’s Basic Diagram
Diagrams: Feynman, Richard P. QED: The Strange Theory of Light and Matter. Princeton University Press. Princeton, NJ, 1988.
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