
#1
Mar2311, 04:47 AM

P: 2

The example I have read to show time dilation is that of a light clock on a train. Two mirrors are one metre apart. Light is beamed from the first mirror to the second. An observer on the train sees that it takes the amount of time for light to travel 1m for the light to reach the second mirror. T a second observer at a train station, the light travels more than 1m, as the train, and thus the second mirror, has moved away from its original position between the light leaving the first mirror and hitting the second. As light speed is a constant, more time must pass for the observer on the platform than for the observer on the train in order to cater for the extra distance.
However, when the light rebounds from the second mirror to the first, while the observer on the train still perceives the light to travel 1m, for the observer on the platform, the light travels less than 1m as mirror one has moved closer to the point of the rebound (relative to the observer on the platform). Where is the fault in my logic? It would seem that I am using the same arguments as those who thought that light travelled through "the ether". I can't see my error, however. 



#2
Mar2311, 05:00 AM

Mentor
P: 40,878

Draw yourself a diagram of the light path as seen by the platform observers. (Realize that the line between the two mirrors of the light clock is oriented perpendicular to the direction of travel.) 



#3
Mar2311, 05:23 AM

P: 2





#4
Mar2311, 06:01 AM

Sci Advisor
Thanks
P: 3,853

Time dilation yes, why no contraction?
It doesn't matter whether they are oriented parallel or perpendicular to the motion. It's the round trip time that counts, and this is where you'll see the effect of time dilation (same for all orientations).



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