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It's my understanding that light travels at the same speed in all inertial frames of reference. Suppose there's a small train car 5 feet long (at our rest point of view) traveling at a constant speed near light speed somewhere between events A and B, and there's an observer in the front of the train and another observer in the back. Event A is well behind the train and event B is well in front. Would time for the front half of the train have to tick slower for the light from B to reach the train at its normal vacuum speed, while time for the back of the train would have to tick faster for the light from A to reach the rear observer? Every time dilation formula I ever see always compares objects moving side by side, instead of one of the objects being in front of the other. Would the train also shrink more in the front than the back due to contraction effects? Or what if there was just a person traveling without a train? Would the back of the person age faster than the front of him? Am I missing something here?

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  • $\begingroup$ I don't understand the scenario. You state where A and B are, but not how they're related in time, or how they interact with the train (or what part of the scenario confuses you). But regardless, all portions of the train would experience time at the same rate. $\endgroup$ – BowlOfRed Jan 28 '15 at 20:30
  • $\begingroup$ Sorry. I think I meant to imply that A is a star behind the train and B is a star in front of the train. $\endgroup$ – theboombody Jan 28 '15 at 20:43
  • $\begingroup$ Yes, I understand the locations, but not what the observer on the train experiences. Or why you think that time should tick differently in different parts of the train. $\endgroup$ – BowlOfRed Jan 28 '15 at 20:48
  • $\begingroup$ Imagine light is like a goblin running in the direction you are facing. If you're facing forwards as you move, the goblin will appear to be running much, much faster than if you're facing backwards as you move. But since light doesn't go slower or faster, time has to change itself to accommodate the speed. So I figure depending on which direction you face, time will change in a different way. I believe my logic is flawed, but I can't find out where. That's why I posted the question. $\endgroup$ – theboombody Jan 28 '15 at 23:13
  • $\begingroup$ The direction you face has nothing to do with it, only the direction you are moving. The front and back of the train are moving in the same direction at the same speed, so view relativistic effects like dilation the same. $\endgroup$ – BowlOfRed Jan 28 '15 at 23:23

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