Under what conditions does a photon turn into a matter/anti-matter particle?
Let's suppose the photon has enough energy to create an electron/anti-electron pair, and it's merrily zooming through space at the speed of light, far from any other influences (no other photons, matter, or gravity near by.)
What determines if this photon turns into matter? Is this governed by some probability equation in QM? Or does the photon need to interact with something else to "trigger" the transformation into matter?