I believe I understand the basics concerning the twin paradox. One twin leaves Earth on a near light speed round trip to a local star. On his return, he is younger than his twin due to the fact that time progresses more slowly the faster an object moves. Here are my points of confusion: If movement is relative and there is no absolute frame of reference for motion, why would time proceed at different rates for the twins. I read that it is due to the force of acceleration that the twin on the spacecraft feels. However, the twin on Earth would also feel acceleration in the form of gravity. I am sure there is more to this. Would someone mind filling in my gaps of understanding on this issue? Thanks.