1. The problem statement, all variables and given/known data state why p + p_bar --> 1 photon is forbidden 2. Relevant equations 3. The attempt at a solution I have checked all quantum numbers and they are okay. I wondering altough if p_bar has intrinsic parity -1 (p has parity +1)?? According to the soloution, it is forbidden because of energy and momentum conservation. I have not been giving a single word about the production conditions and so forth. HOW can this (E and p) be violated if I choose whatever annihilation condition so that p and E is not violated??!!