QED Vertex: Examining the Unexplainable G-Factor

[RedX]

The QED vertex has two electrons lines joining with a photon line. However, 4-momentum conservation does not allow this process to occur. So why is this process studied?

Does it really mean anything to calculate the g-factor of an electron for a process that cannot happen? Only the vertex correction contributes to the g-factor right? But if the other end of the photon line has to connect to something else to conserve 4-momentum, then don't you have to have a diagram with the photon self-energy?

 

[Vanadium 50]

You misunderstand what a Feynman diagram is. It's not a description of the sequence of events (this electron is over here, then it emits a photon and moves over here...). It's a mnemonic for writing down the proper terms in doing a field theoretical calculation.

 

[RedX]

Would it make sense to do a field theoretical calculation for the process of incoming electron, outgoing photon, and outgoing electron? The process can't happen because 4-momentum conservation won't allow it.

I understand that such a calculation is useful if at least one of the three participating particles is off-shell, because then that would be a calculation of the basic QED vertex, which can be used to construct processes (such as 2-electron scattering) that actually can happen by 4-momentum conservation.