So finally I can understand the time dilation principle. Correct me if I am wrong but here is how I understand it: If 2 twins on earth conducted an experiment of time dilation, twin 2 hops into a rocket ship and flies around the earth at a high rate of speed for years. When they meet up again the twin 2 appears younger than twin 1 on earth, because of the time dilation. So during this time twin 2 is flying, twin 1 can watch his brother age slower. All his activities on board are slower but to twin 2 everything is normal. What if you took twin 2's perspective... Twin 2 feels normal and he looks back to earth to see his brother. To twin 2 his brother (twin 1) is moving at a high rate of speed because relative to twin 2, in the ship, he is not moving. So, would twin 2 see his brother twin 1 age slower because of time dilation relative to twin 2 in the ship? This is why I get stumped. If twin 2 and twin 1 meet up and supposedly twin 2 is younger than twin 1, then twin 2 must be seeing his brother twin 1 age faster even though relative to twin 2 his brother should experience the time dilation. Maybe an even deeper question is where is the absolute perspective that decides who will age slower. because if you are on another planet, the rocket ship may actually appear to be going slower than the earth which would say that twin 1 should be younger than twin 2. edit: I read a little bit of similar posts, sorry for the repost but i did read that the guy twin who is accelerating back to earth will age less because of that acceleration. but my question is again....only to the observer the guy is accelerating to him, whether he is on earth or in the ship, who decides who will age?