Hello Physics Forums, Many of my google searches made in an attempt to understand relativity landed me on threads here, so I thought this would be a good place to get some clarification on a concept I recently read in the book God? by C. DeSalvo (my http://boulderchurch.net/viewtopic.php?t=362 [Broken] for the curious) which contains this paragraph: I'm wondering what people here, who understand relativity better than me (and probably the book's author), think of these statements. The paragraph and its implications confused me, because if photons *move* through space, then that implies a measure of time (from some perspective, at least). My understanding of relativity is that time is one of four coordinates of spacetime which describe the positions of bodies/events relative to a rest frame of reference. And so it doesn't seem sensible to assign time as an attribute of matter. I initially wrote it off as a result of the author's speculations about the point-of-view of photons (they don't experience space or time, etc.). However, even ignoring any "let's try to apply SR to the non-inertial reference frame of a photon' games, I'm not sure what time would mean in a universe without any massive particles. That is, if photons were the only particles in the universe, which would mean that there were no bodies moving below the speed of light and so no inertial frames of reference associated with any real thing, then would SR have anything to say about that universe? So, to conclude before I start rambling in my confusion, is the statement "Without matter or antimatter, time does not exist" sensible within the model of Einstein's theories of relativity?