Why are there two photons in pair production Feynman diagram? [duplicate]

Given

I wonder why are there two photons entering in a) pair production? Isn't it one photon resulting in $$e^+e^-$$ ?

1 Answer

It is not possible to satisfy both energy and momentum conservation with only one photon.

• This is easily seen: For the reverse process, pair annihilation, you can go to the rest frame where the momentum is zero. However, a real photon always moves at the speed of light, there is no rest frame. Aug 16 '19 at 12:36
• Right, I remember! :) But another question that arises: When I need a photon that takes a recoil: How/why do the photons interact with each other? Btw, why is a photon resulting in e-e+ in general? :D
– Ben
Aug 16 '19 at 12:37
• @Ben "why is a photon resulting in e-e+ in general?" That probably deserves a fresh question (if it doesn't exist already). Aug 16 '19 at 12:50