So you guys are probably tired of answering this question but I can't find a resource that properly explains this. Two twins in spaceships in flat, empty space are converging at a rate of 0.8c (for instance). They pass each other (each assuming that he is the one at rest) and as they're flying, they turn to the left to make a circle so that they cover a distance but come back to the same place. Which twin is younger? Why? If the spaceships were made of glass and the twins had supervision, other than this being a bad spin-off of the justice league, each would see the other aging at a slower rate than himself. This is similar to a thread posted recently (which made this question come to mind) but it removes the idea of "acceleration" and other technicalities. If the fact that neither is in an inertial reference frame (because they're turning) is a problem, then change the question from "making a circle" to "they both stop and put it in reverse, backing up to meet one another". I assume there's some way to make it so that the experience of both twins is identical and yet, time dilation dictates that they are both younger than the other. Resolution? Thanks. edit: here is a diagram that I didn't understand (though I looked at it and read the explanation carefully), if that helps at all with your explanation - http://web.comhem.se/~u87325397/Twins.PNG here is the explanation: https://www.physicsforums.com/showpost.php?p=1881599&postcount=3 thanks again.