[ETL]

From the website I linked to earlier...

(sorry, you may have to zoom your browser out in order to see the whole animated graphic. Or find and click on the earlier link. For some reason, it appears much smaller there)

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(SNIP)

"I want you to imagine that you’ve got a clock, only instead of having a clock where a gear turns and the hands move, you have a clock where a single photon of light bounces up-and-down between two mirrors. If your clock is at rest, you see the photon bouncing up-and-down, and the seconds pass as normal. But if your clock is moving, and you look on it, how will the seconds pass, now?

A light clock moving close to the speed of light will appear to run slower relative to an observer at rest. Image credit: John D. Norton, via http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_clocks_rods/.

Quite clearly, it takes longer for the bounces to occur if the speed of light is always a constant. If time ran at the same rate for everyone, everywhere and under all conditions, then we’d see the speed of light be arbitrarily fast the faster something moved. And what’s even worse, is if something moved very quickly and then turned on a flashlight in the opposite direction, we’d see that light barely move at all: it’d be almost at rest.

Since light doesn’t do this — or change its speed-in-a-vacuum under any circumstances — we know this naive picture is wrong.

[ETL]

...now, imagine that light clock above were on the moving train I described a couple posts back. A passenger on the train would see the light beam simply move up and down the clock (his/her sense of what a particular unit of time was).

However, the person on the platform watching the train passing by him/her would see the light pulse trace out a triangular path as it moved from the bottom to the top of the clock. And since light travels at the same speed for everyone, regardless of motion, the light pulse, from the platform person’s point of view, sees it take a longer time to move up and down the clock than the person alongside it on the train. Therefore, one’s sense of time depends on their state of motion.