Hello, I apologize in advance for what is probably a boneheaded question, but I'm just a little bit confused. I just started reading The Elegant Universe and in the chapter on the theory of relativity, the author uses an example of a photon-based clock to illustrate how the perception of time changes based on the observer. In the example (which I think I also remember from another book on general relativity), there is a clock made of a photon that travels back and forth between two plates. The clock is moving relative to an observer, and to illustrate the distortion in time, the author states that from the observer's point of view the photon would have to travel sideways to reach the other plate, therefore taking longer to get there than it would in a straight path. What I'm confused about is that just before this example, the author gave another example where a light traveling to both sides of a table on a moving train would get to one side faster than the other because the speed of light is always constant and the side moving along with the train would get to the light faster than the side moving away from the light because the movement of the train has no effect on the light. So what's confusing is why the photon in the clock example would be travelling sideways if the movement of the clock has no effect on it. Once again, I apologize for asking something like this when I barely read anything of the book, but there is no explanation of it and I'm curious about it.