I'm trying to understand the relativity train thought experiment. One of the key concepts seems to be that the clocks at the front and back of the train are "synchronized" such that the clock at the front of the train is running slightly behind the clock at the back of the train (as seen by an observer outside the train). As I understand it, this is necessary in order for a beam of light shined toward the front of the train, and a beam of light simultaneously shined toward the back of the train to arrive at their respective destinations simultaneously (as seen by an observer on the train).(adsbygoogle = window.adsbygoogle || []).push({});

I've also read that if the two clocks are synchronized together at the middle of the train, the clock that is then taken to the front of the train will lose some time (due to its having run slower as it traveled at the combined speed of the train plus that of moving the clock forward) while the clock at the back of the train will gain some time (due to its having run faster as it traveled at the reduced speed of the train minus that of moving the clock rearward)--which results in the forward clock running slightly behind the after clock.

So far, so good.

But what if a different method is used to synchronize the two clocks, say a mechanical rod extending forward and backwards to both clocks, which is used to start both clocks running at the same time. If the train is stopped, when the two clocks are brought together at the middle of the train, they'll show the same time. But if the train is moving at near the speed of light, wouldn't they show different times? And wouldn't this violate one of the postulates of special relativity?

I'm sure I'm missing something, and I'd appreciate your enlightening me.

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# Relativity train and clock synchronization

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