Pair production

[Ratzinger]

In pair production, an nucleus is needed to take away some momentum from the photon so momentum conservation is respected. correct?

 

[jtbell]

Right. Or at least a heavy charged particle. It's just that a nucleus is the most common such beast.

 

[mathman]

There is an alternative way for pair production to take place - it happened right after the big bang. Specifically, a pair of photons can interact to get a particle-antiparticle pair.