1. The problem statement, all variables and given/known data I've been reading through Spacetime Physics by Taylor & Wheeler, but this argument about the invariance of the [itex]y[/itex] coordinate for inertial frames, one moving relative to the other on the [itex]x[/itex] axis, is tripping me up. I'll just write the text word for word: I'm just not understanding this argument at all. There is a way to distinguish between whether the lab clocks were painted by the rocket or not if the lines painted at [itex]y=1[/itex] (rocket frame) weren't at [itex]y=1[/itex] (lab frame)??? Is that the experiment to distinguish the two frames? 2. Relevant equations The laws of physics are the same in any two inertial frames 3. The attempt at a solution Just lost trying to understand how non-invariance of the [itex]y[/itex] coordinate makes the laws of physics different in between the inertial lab and rocket frames.