New guy here with a question about Special Relativity and stopping acceleration. Forgive me if I have any of this wrong. I'm generally unable to follow physics for a long time before getting lost (I'm very right-brained--which means I'm artsy, not mathematical--and I've been told my brain can't organize details in a normal fashion), so I may not have my facts straight. I've been reading Brian Greene's The Elegant Universe and, in it, he describes Einstein's special theory of relativity. Einstein claimed that any observer going at a constant velocity can claim everyone else is moving and he's standing still. Take the tried-and-true driving example. A guy driving in a constant westward direction at a constant 50 km/h can, accoring to special relativity, say that everyone else is traveling eastward at 50 km/h and he and his car are standing still. But once he accelerates, decelerates, or changes direction, he loses his right to claim that he's standing still. I hope I'm right so far, but tell me if I've got something confused. Anyway, take someone who is traveling at just under the speed of light. While his mass may be a certain "number" while he's at rest, mass dilation says that his mass will grow exponentially so that he can attain a speed just under light's. Time dilation says that time will slow down for him. The Lorentz contraction states that he'll shrink along the direction of travel. Do I still have this right? This is where I get confused. When he stops accelerating (i.e. he goes at a constant speed now, but doesn't stop moving), do his mass, size, and passage through time return to normal? If special relativity states now that he's on equal footing with anyone who is traveling at a constant velocity (including those who are standing still, if you can call anyone that), then he shouldn't be experiencing any of these dilations, right? Does his mass dilation et al reset to 0? If he decelerates, does that mean that these dilations start increasing again, since he's accelerating in the opposite direction?