I had this thought for a couple of days now but couldnt solve it so I wonder if anyone could help. Imagine person A & B in space. Where person A is standing still in a certain point in space. Person B passes by person A, let's say he is just moving in a straight line (from our point of view as an outside observer). Is there any chance you can now say who is the person really moving (if we where either person A or B)? Some "absolute movement"? The problem for me is, let's say I'm person A, I measure the speed of person B and get the velocity, let's say, v1 Now person B measures the speed of person A who will measure the speed (v2), which is equivalent to the speed person A is approaching then later on receding. v2=v1 in our case right? However, how can person B determine that he is moving from just measuring the speed against person A? My mind says, for person B it can look like just as person A is moving and person B is stationary, right? They are both seeing each other approaching and receding. Same thing if I drive next to a car at the same speed, it will look like the environment is moving while me and the other car is stationary (which isn't the case, since we know we are moving relative the "outside world"). I know speed is relative, and I'm not familiar to any math that has to do with general or special relativity, yet. I hope you understand my question. Best Regards, Curious student.