Question about the "light clock" I'm trying to make sense of Brian Greene's explanations of SR in The Elegant Universe. For those who don't know it, he gives the example of a "light clock" that bounces a single photon between two mirrors and produces a tick after each round-trip. Another light clock moves past it. From the perspective of the stationary clock, the photon in the moving clock is tracing a diagonal pattern (moving forward while moving up and down), and therefore taking longer between ticks. Thus, from the stationary perspective, time is passing more slowly for the moving time clock. My question is, why would the light be constrained by the moving clock? Wouldn't it simply bounce out? It seems to me that the photon would hit the bottom head on, and by the time the new, upward photon reached the top, it would be hitting a section of mirror farther along, and so on, until it had escaped the light clock. This may be a minor point, I don't know, but it's making it very hard for me to grasp relativity. Visual explanations geared for us laypeople seem to hinge on light's being constrainable in a way that it's not.