I am confused with a certain aspect of special relativity. Here is my reasoning... Someone in a rocket, travelling at 0.99c ages slower and thus would be younger after returning to earth than his twin brother. I understand the 'light clock' explanation of this. Here's where I'm getting confused.. Surely, the Earth would also be travelling at 0.99c, relative to the rocket. Does that not suggest that a clock on Earth would go slower than that of the rocket's, relative to the rocket. So to the rocket, the earth is going slow in time. To the earth, the rocket is going slow in time. When the rocket comes back, what dictates that the brother in the rocket would have aged less? Regards..