Given
I wonder why are there two photons entering in a) pair production? Isn't it one photon resulting in $e^+e^-$ ?
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It is not possible to satisfy both energy and momentum conservation with only one photon.
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1$\begingroup$ 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. $\endgroup$ Aug 16 '19 at 12:36
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$\begingroup$ 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 $\endgroup$– BenAug 16 '19 at 12:37
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$\begingroup$ @Ben "why is a photon resulting in e-e+ in general?" That probably deserves a fresh question (if it doesn't exist already). $\endgroup$– PM 2RingAug 16 '19 at 12:50