If the electron e − and the positron e + both have rest energy 0.51 MeV, nd the minimum energy a particle of light γ (a photon) can have in the reaction e + + e − = γ + γ. Why can't there be a reaction producing only one photon, e + + e − = γ? Assume the center-of-mass frame.

For Total energy and momentum to be conserved, the minimum energy of the photons released is equal to twice the rest mass energy of an electron that is  

The annihilation of electron -positron cannot produce a single photon. It is prohibited by the law of conservation of energy and momentum.

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