Why pair production doesn't occur in vacuum but annihilation occurs?
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Photons, from which pairs are created, have both energy & momentum, neither of which can be zero.
When a pair is created from the photon the total energy of the pair must equal the original photon energy, which is fine.
But also the total momentum of the pair must equal the original photon momentum. Since this process can be viewed from any frame of reference it is always possible to find a frame of reference in which the total momentum of the pair is zero. Since there is no frame of reference in which the photon momentum is zero, you then have a violation of conservation of momentum.
There must always be another particle present which can be manipulated so the total momentum of all particles is conserved before & after pair production.
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When a pair is created from the photon the total energy of the pair must equal the original photon energy, which is fine.
But also the total momentum of the pair must equal the original photon momentum. Since this process can be viewed from any frame of reference it is always possible to find a frame of reference in which the total momentum of the pair is zero. Since there is no frame of reference in which the photon momentum is zero, you then have a violation of conservation of momentum.
There must always be another particle present which can be manipulated so the total momentum of all particles is conserved before & after pair production.
if helped mark it as brainliest.
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