I don't get how it works. Using the classic train example... train with speed v rel. to S --> [========C'=========>S' A----------C-----------B----------S |----L-----| First of all, why do we use light to measure time? Why don't we use time to measure time, if that makes any sense? For instance, suppose at t=0, t'=0. Suppose A shines a light at t=0. Then doesn't S' measure that A shines the light at t'=0, ignoring this confusing light travel stuff? Same with B. What does it matter where the light goes and when it gets there? Now if someone is sitting at C in S, and someone shines a light at A the time it takes to reach C is t=L/c But if someone is sitting at C' in S', he too will measure c for the speed of light. And according to this observer, the light has to travel the same distance L (half the length of the train), so the time it takes to reach C' according to an observer there is t=L/c So both these observers agree with the time at which the light was shone. Same thing with a light from B. So if something is simultaneous in S it should be simultaneous to S'. Can someone point out my fatal flaw?