Quantum electrodynamics
Propagators

Finally, one has to compute P (A to B) and E (C to D) corresponding to the probability amplitudes for the photon and the electron respectively. These are essentially the solutions of the Dirac equation, which describe the behavior of the electron's probability amplitude and the Maxwell's equations, which describes the behavior of the photon's probability amplitude. These are called Feynman propagators. The translation to a notation commonly used in the standard literature is as follows:

where a shorthand symbol such as x A {\displaystyle x_{A}} stands for the four real numbers that give the time and position in three dimensions of the point labeled A.
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