Hi there. I've been reading Brian Greene's book "An Elegant Universe" for the sake of curiosity and one of his examples to demonstrate special relativity really confuses me about the entire concept. To start, this is my current understanding of SR, so please correct me if I am wrong: -The faster an object is moving through space, the slower it moves through time. -Everything is essentially moving at the speed of light through the four dimensions of space time. Greene's example is about two astronauts floating through black space with the only means to compare their movement is to each other - each can claim that they are stationary. That's pretty easy to understand - they don't know who's moving, it's just a matter of their perception. However, he later mentions that each has a valid claim that other's clock is running slower. This I don't understand. Shouldn't this also be a way to determine who's moving? Even if they both can say that the other person looks like they are moving, only one is actually moving; therefor, only their clock should slow down since their time is slowing down because of their flow through space. I would really appreciate some clarification as this subject interests me greatly. I am starting college next year in hopes of going into physics, however, it's troubling to know that after reading the first chapter of Greene's book I still can't fully conceptualize SR.