We well know that time is relative according to the Special Theory of Relativity. So, a person on a spaceship close to the speed of light (~c) could go on a trip for some years, return and find out that he is still young, while his friends or relatives are already very old or gone. This happens because the observers on earth see the clockticks on the spaceship go slower. Now, speed is relative, so the observer on the spaceship sees the earth frame of reference move away with the same speed. So he reaches a similar conclusion, that clocks on earth go slower so that he would expect to find his relatives younger than usual. Now this is the contradiction or paradox as we may say: time is relative, ok, but is grey hair relative? Certainly not. Only one of them could be younger than usual. When I was discussing this paradox in 33rd IPHO, Indonesia, some said that the two frames of reference are not equivalent. One accelerates, while the other stays at constant velocity. So the laws of physics are not valid for the observer on the spaceship while it accelerates. I agree. But what happens when the spaceship ceases to accelerate and is in constant velocity? It is an inertial frame of reference as good as the earth is, the physical laws hold. So the observations of the guy on the spaceship are correct and the observations of the persons on the earth are correct as well. Both parties observe that the clocks on the other frame of reference run slower. Isn't this now a paradox? I've thought long about it. If somenone has any idea, I would appreciate their opinion.