Hi. Can pair production happen in vacuum? I had this question for a test. And the answer is: no, it cannot because it would violate the conservation of momentum, since you can't have a momentum zero photon. But i seem to recall that pair production happen in space (?). Due to Heisenberg's uncertainty relation, there is a constant energy flux, which allows for pair production. The pairs are produced, and they annihilate each other again within a certain time frame. And if it happens near a black hole, one of the pair may pass beyond the event horizon of the black hole, thus making it appear as if the black hole is radiating. I think i got this info quite some time ago when i was reading on Stephen Hawking. Not sure if i remember it correctly. Any comments?