Why is Pair Annihilation possible in vacuum but pair production needs a vacuum or a 3rd material to conserve energy and momentum at the same time.
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With annihilation two Photons are created, so they could take the energy and momentum. With pair production from one photon you have two particles with momentums which can not sum up to the momentum of the photon. So you have to have something to take the recoil.See https://en.wikipedia.org/wiki/Pair_production
