Let us assume that we want to describe the full process of photon emission by electron A and absorption by electron B.(adsbygoogle = window.adsbygoogle || []).push({});

Therefore electron B must be on the forward lightcone of electron A.

In the normal forwards in time description a virtual photon propagates from A to B depositing a certain amount of energy and momentum onto electron B.

But does this process alone also describe the recoil of electron A?

Should one also include the backwards-in-time virtual photon which propagates from B to A depositing an equal amount of negative energy and momentum onto electron A?

One could say that the product of the forward-in-time and backward-in-time propagators give the full probability of photon emission by A and absorption by B.

Is this where the Born rule comes from?

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# Born rule and Feynman propagators

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