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Introduction to QED

Introduction to QED. Quantum Electrodynamics Part III. Double Slit Diffraction. Interference occurs and probability is between 0% and 4%. When Detectors are Added. Interference disappears and probability is always 2%. When Detectors are Fallible. Electron – Photon Interactions.

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Introduction to QED

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  1. Introduction toQED Quantum Electrodynamics Part III

  2. Double Slit Diffraction Interference occurs and probability is between 0% and 4%

  3. When Detectors are Added Interference disappears and probability is always 2%

  4. When Detectors are Fallible

  5. 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.

  6. 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.

  7. Objects Moving from Place to Place

  8. Light’s 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). • Photon’s highest probability occurs at speed c.

  9. Light Diagrams

  10. Electron Diagrams

  11. 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

  12. Coupling

  13. Simultaneous Electron Movements

  14. More Possibilities

  15. Photons Moving at Multiple Speeds

  16. Two Photons Exchanged

  17. 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.

  18. 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.

  19. Photon Scattering

  20. 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.

  21. Hydrogen Atom

  22. Partial Reflection

  23. Partial Transmission

  24. 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.”

  25. 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.

  26. Two Photons Moving

  27. When the Points Converge

  28. Electrons and the Exclusion Principle

  29. 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.

  30. An Alternative Possibility

  31. Dirac’s Basic Diagram

  32. More Complex Possibilities

  33. Diagrams: Feynman, Richard P. QED: The Strange Theory of Light and Matter. Princeton University Press. Princeton, NJ, 1988.

  34. Questions?

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