# How positron and electron annihilate forming photons? [duplicate]

Electron is a particle with momentum $p$ and it spins up. Positron is its antiparticle having momentum $-p$ and it spins down. "A positron is an electron travelling backwards in time" said by Feynman. Therefore how an electron and positron annihilate producing photons since total momentum is $p-p=0$. Therefore how they produce photon with momentum?