I'm sure this question has been asked thousands of times before, but I can't see how nature determines which twin accelerates (or is subject to a gravitational field) and which one doesn't. People say one twin will feel the acceleration and the other won't, but suppose neither twin had an accelerometer present (internal or otherwise). All they can tell is that the seperation between twins was increasing at a constant rate, and then changes at the moment of the acceleration. The twin in a spaceship sees the Earth moving away and then turn around, while the twin on Earth sees the spaceship moving away and then turn around. If we use the gravitational time dilation explanation, it would seem to still be completely arbitrary who we treat as being in an inertial frame, and the gravitational field would seem to each observer to simply be acting on the opposite observer with an opposite direction. I feel I understand everything else about what's going on here with SR, except that this seems completely arbitrary, and even scarier, each one actually should be younger than the other one because each one has seen the other one switch frames (which is truely impossible). If someone can explain this it'll seriously help alleviate my headache, and probably save me days of worrying about it.