Introduction to qed quantum Electrodynamics

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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

  • 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.

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

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.

Light Diagrams

Electron Diagrams


  • 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


Simultaneous Electron Movements

More Possibilities

Photons Moving at Multiple Speeds

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 Reflection

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

When the Points Converge

Electrons and the Exclusion Principle

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.

An Alternative Possibility

Dirac’s Basic Diagram

More Complex Possibilities

  • Diagrams:

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


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