But who is to decide whether you have travelled fast, or if the Earth has travelled fast, when there is no universal grid or reference? Seen from your spaceship, you'd think that you were stationary and that the Earth was the one moving.<br /><br />So if you went away for 10 years at 0.866c, people on earth would see that you were 5 years yonger than those born the same year as you, because they would have seen you travel very fast. But you would see that it was the people on Earth that had aged only 5 years in the 10 years you were away, because as seen from your ship, it was Earth that moved very fast. Or maybe the other way around but still the one in the spaceship would have a different opinion than the earthlings as to who aged normally and who didn't.<br /><br />So who would be right? Would the universe explode because of this paradox? Not likely as the same thing occurs at walking speed, only to a much smaller extent, and the universe doesn't explode...<br /><br />The possibilities:<br /><br />1. The universe in fact does have a global reference or "grid" for speed, so there's no doubt about who was moving fast - this also means that it is possible to become truly stationary relative to space itself. Earthlings see that the astronaut is only 5 years older than he was when he left 10 years ago, while the astronaut can see that everyone on Earth is 10 years older even though he - and all his clocks in the spacecraft - only think that he left 5 years ago). The earthlings are correct and the astronaut is the one that has gained 5 years.<br /><br />2. Parallel realities are forked out at that moment (and in fact at any moment when anything in the universe moves relative to anything else) and both you in the spaceship and the ones on Earth are correct in their observations, and everyone agree about the observations, although the conclusion is different in the different realities (ie. on Earth they see a person coming out of the spacecraft that is only 5 years older than he o <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>