Time dilation-- V relative to what? I'm new to the concept of relativity, and only know what I've read in the few books I have or information that has been exchanged by my peers. I understand that as you approach c, you experience less time. I feel that I finally understand the "experiencing less time" part. I also know that velocity is relative. Anyone anywhere can declare themselves at rest (even if they are experiencing a net force, is this correct?). Now that I've learned both of these concepts, they don't seem to make much sense together. Something experiences less time when it has a velocity close to c relative to what? What if two people, A and B, were both wearing watches; they are in deep space. A is traveling at .9c and B is stationary. Obviously, A's watch is going to be slow compared to B's. Is this the only way to prove that A was, in fact, the one at a higher velocity? But why can't he say that he was at rest, and therefore believe that B's watch MUST be the one experiencing less time, only to be proved wrong when they met again?