You are standing on one. The two planets were closing in on a speed of 1x. You just accelerated one of the 2 planets towards the other with a super powerful rocket engine, and then sat on one of them. Now they are closing in on each other at speed 10x. The universe is otherwhise empty. The problem is, your memory got wiped after you did this. How would you determine which planet is moving faster? They are the same size and weight. No atmosphere. You are standing on one. Since there is no star to light the thing up, let's say we installed lights on those planets. It must appear if you are sitting on either one of them, that the other planet is coming towards you. Can we state here that neither entity has a absolue speed, and it is impossible to determine which one is moving? Maybe something with einstein's equations? Since the one that is moving 10x the speed of the other has time slowing down relative to the other? (you can hop between the two while this is going on). Any thoughts?