its been years since I took a physics course so I might have what amounts to a really dumb question. However, I got into a group debate the other day where i was caught in the middle between person A who was claiming that the relativity of velocity did not entail the equal velocity of each frame, relative to each of only two frames and person B who claimed that it did. A spells this out thusly: "... if the value of the velocity is a property of the relationship, then it follows that both have the same velocity (as the relationship is the same for both). If there are two objects in space, a rock, and a ship flying away from the rock (and no other reference point), if velocity is X for the ship, it's X for the rock per your view. You simply can't have it both ways. Either velocity is the relationship (rather than us needing the relationship to measure velocity), then it follow that whatever velocity the ship is moving at away from the rock, the rock is moving at the same velocity away from the ship (and hence time will slow down for the rock just as much as the ship). I'm saying you can't have it both ways." I've been trying to find an explicit answer to this issue that doesn't just get too deep into the twin paradox. Essentially to rephrase, IF I were on a deserted planet in space and I launched into space with my ship, how do I know if I am moving away from the planet at some velocity or if the planet is moving away from me at the same velocity i'm traveling at. Person A was claiming that according to some "bad" interpretations of relativity theory that one would be forced to say that my velocity was the same as this planet's (even if we know that the planet obviously doesn't have the same velocity as the space ship at least per common sense). I do not agree with this view, that is I disagree with person A's point of view on the matter but im not able to clearly say why without falling back on some uneducated vague ideas about the twin paradox. I am far, far from being proficient in physics. If anyone can help a bit here I would appreciate it a lot. I am way over my head on these matters especially as far as the math goes, so plz go easy on the math and any assumed understanding on my part:-) thanx.