http://en.wikipedia.org/wiki/Twin_paradox I read a little bit about this, and am a little bit confused. I always learned about reference frames. When I jump, you can say that I am moving away from the earth, or the earth is moving away from me. Why is this any different for near-light travel? The proposed idea here is that it's "acceleration" which causes this(which I still don't understand why it matters). This raised a thought question in me which I hope someone could answer. What if the travelling twin, first travels away from the stationary twin. After he gets there, the stationary twin does the same travel. Will they arrive the same age? Why or why not? I'll add an edit ahead of time. I have a problem with either answer to my question. Case 1) They do not arrive the same age. This seems absolutely impossible. They both made the same trip, at different times. How could something different have happened for each? Case 2) They do arrive the same age. My problem here is that it implies that given 2 bodies A, and B, A accelerating toward B is different than B accelerating toward A, which goes against every intuition I have. Please address these cases for your answer.