hi there I am confused by the following setting on special relativity: Suppose there are 4 people, say A, B, C and D, in the deepest vacuum - essentially no gravity or any force at all. Suppose further that all move in one-dimension with constant velocities - A moves towards west with 0.8c relative to D, B towards east at 0.7c relative to D and C towards east at 0.9c relative to D. Everyone is moving with various speeds relative to D. From D's point of view, all A, B and C have time dilation, i.e., A, B and C will be younger after some time of travelling. But from A's point of view, the aging difference will not be the same as D and this discrepancy applies to each point of view - A or B or C. So who in reality, has the greatest time dilation? Or who is the youngest if they started with exactly the same age? As far as I know this is a paradox. I asked one of my physics teachers and he couldn't give me a satisfactory answer. I have certain amount of knowledge in Special Relativity, including mathematical analysis. So any kind of mathematical reasoning is welcome. I think there is no General Relativity going on here according to my settings and assumption. If my assumptions and settings are impractical, just point me out. Thanks.