okay, i have to ask a stupid question. a short while ago, there was a thread discussing photon - photon collisions. being an amateur, i had always understood that two photons could not collide, and i stuck in a comment to that effect, indicating that since photons, once they are emitted, only exist as a probability function, and have no exact location until they are absorbed. i was corrected, and told that the wiki article supporting what i had said was wrong, and pointed at some papers referring to current experiments with photon-photon collisions. so, apparently, photons can collide with each other, but i simply cannot understand how that corresponds with what i have been taught, regarding the photon probability function and no precise location of a photon. for photons to collide, they must have some precise location, but to have a location, the probability function must be collapsed, which seems in conflict with the basic premise of QED. can someone please try to provide some explanation of this for me? what am i misunderstanding? thanks.