It's my firt time reading special relativity and this question bug's me . It is said that times runs slower at greater velocities . Let's say I leave home in a space craft and fly for some time at 99% the speed of light measured from earth. It is said that when i come home i will be older than my twin brother . Now let's transfer ourselves in the space craft . The earth now will move with 99% the speed of light from me . So when my brother comes back to me (he remained back on earth ) he should be younger than me . So which one is it ?