Re: This is probably a dumb question, but here goes anyway. What does the physi

Not a dumb question at all... Some people think it can't really be asked (see this link, for example) but I'll give it a try.

It's interesting to try to think about the universe from a photon's perspective, because according to special relativity, the photon will "see" a universe that looks like an infinitely thin pancake: the universe has no depth at all in the direction that the photon is travelling. Instead, everything along that direction is compressed to a single plane. (The universe still extends in the other two spatial dimensions, though.)

Even stranger, there's no longer any passage of time. A photon will leave its emitter and instantly be absorbed by its absorber, with no passage of time at all. This is possible because the emitter and absorber appear to occupy the same point in space (remember, everything's been smooshed together). So far there's no paradox.

But, you might ask, what if the photon has no future absorber? What if it's fated to travel out into space "forever"? This little humdinger has puzzled quite a few physicists -- there's even been an experiment or two where lasers are shot out at particularly empty regions of space, to see if the lack of future absorbers prevents the photons from being emitted in the first place! (These experiments showed no noticable effect). So either the universe is closed (ending in an eventual Big Crunch where everything is absorbed), or the universe is open but there's still enough material out there to absorb every photon, or else something very strange is allowed to happen from the photon's perspective. This is where the paradoxes come in -- if there's no future absorber, what happens to the photon in its own reference frame? As far as I know, this is an open question.