Re: When two particles collide
A photon has mass; what it doesn't have is any rest mass, but since they never stop moving, this is no handicap. Since energy =k X mass,(where, as every schoolboy knows, k is cee squared) to have any energy it's forced to create a small quantity of gravity, and de diverted by large quantities of it.
Gravity operates in three dimensions. We know this because it drops off as the square of the distance from the centre of gravity of the generating mass ( unless you are inside a hollow body in which case– oh, never mind) Which demonstrates that a constant quantity is spreading out over an ever larger area, not volume.
Einstein's theory of special relativity is just that; a theory. It is not Revealed Truth, and is almost certainly wrong. In fact, if quantum entanglement can be proved it kicks out one of the basic premises; that simultaneity has no meaning, and any frame of reference is as valid as any other. What it is is better than any competing model at describing the universe as we see it, and whatever theory supersedes it will definitely contain elements of it, or even be a mildly corrected version of the original.