I'm having a hard time wrapping my head around some of the basic concepts. Imagine a spaceship moving at a certain velocity relative to you. To you, his time is slower than yours. He guy in the spaceship doesn't know his time is slowed down. If his time is slower than yours, it means that he thinks your time is faster than normal. But if we turn it around the guy in the spaceship can say that you're moving at a certain velocity and therefore your time is slower than his and that you think his time is faster than normal. So what's going on? Who seems slower to who? And another thing. If mass increases with velocity and you're in a spaceship. Wouldn't you notice the increase of inertia? Or, more likely, if mass is normal to you and your mass seems increased to the observer outside on top of the slowed down time. Would he think you're stumbling around weirdly while everything is alright to you? Such a fun theory.