Particle creation and annihilation

[DiracPool]

I read somewhere that when, say, an electron and positron annihilate, they create two photons which travel in opposite directions in order to conserve linear momentum. Is this true? What about pair production. Do you likewise need two photons to do this? Or, can you do it with one photon provided it has enough energy?

 

[Dale]

I read somewhere that when, say, an electron and positron annihilate, they create two photons which travel in opposite directions in order to conserve linear momentum. Is this true?

Yes, it is true, but “I read somewhere” isn’t a good reference. You should usually explicitly state where you read it.

Do you likewise need two photons to do this? Or, can you do it with one photon provided it has enough energy?

Both energy and momentum must be conserved (as well as charge, spin, etc). So one photon cannot do it.

You need two photons with opposite momentum and each with >511 keV energy, or a photon with >1022 keV energy and an atom. In the latter case the atom can take any leftover momentum.

 

[mathman]

Pair production with one photon takes place when in the presence of a nucleus making the needed momentum, etc. balance.