In particleantiparticle annihilation or pair production, how must you have to photons released to save momentum? I get momentum conservation, but how, mathematically, does it work out that you must have two photons? Someone please show me!

In particleantiparticle annihilation or pair production, how must you have to photons released to save momentum? I get momentum conservation, but how, mathematically, does it work out that you must have two photons? Someone please show me!
You need to have both energy and momentum conservation. The easiest way to see that two photons will be needed is to look at the process in the frame where the particle and antiparticle are coming directly toward each other. The is the centerofmomentum frame, and the momenta of the two particles sum to a total momentum of zero in that frame. There is no way you can have a single photon come out with nonzero energy and zero momentum. Hence there must be at least two photons.
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