I understand about doppler effect.
What I'm questioning is that since speed is relative, somebody observing both A & B, could actually conclude that it was A that was speeding away from B and then returning...or that both A & B were speeding away from each other then back. With other words, A...
Quoting from Wikipedia:
"Time dilation would make it possible for passengers in a fast-moving vehicle to travel further into the future while aging very little, in that their great speed slows down the rate of passage of on-board time. That is, the ship's clock (and according to relativity...