Before people start bashing me for not reading up on other threads, I did try and perhaps I am not physics-savvy enough to understand but I just found the answers to be contradictory and confusing. So, here are two scenarios that confuse me: Scenario 1: Two twins start off on earth. One experiences acceleration, leaves earth, orbits earth at .8c, returns to find other twin much older. Scenario 2: Two twins start off in different ships orbiting Earth at the same high constant velocity - one goes to earth at time t, the other goes to earth at time t+2 (light years). The second twin finds the first twin to be much older. My questions: 1) What part does acceleration play in this beyond giving them separate frames of reference? 2) AFAIK it is the high relative velocity which causes time dilation not the acceleration. Given that either twin could see the other as travelling at the higher velocity, how does the acceleration required to shift frames of reference affect the problem? 3) Would each of them see the same thing looking at the other (once each of them reaches constant velocity - albeit vastly different velocities) given the symmetrical nature of relativity? How does this change when one of them goes to visit the other?