Why are there two photons in pair production Feynman diagram?

Question

Given

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

score 3 · Accepted Answer · answered Aug 16 '19 at 12:30

3

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

answered Aug 16 '19 at 12:30

mike stone

1

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. – infinitezero 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). – PM 2Ring Aug 16 '19 at 12:50

1 Answers1