I've seen an experiment with a light clock that explains time dilation. A photon is fired and gets reflected off of 2 parallel mirrors. It keeps bouncing back and forth like that, with each bounce qualifying as one tick of the clock. And it looks like that from a stationary reference frame. Now if the clock is moving to the right at some highs speed relative to us, the photon would need to travel in a triangular path. Therefor it takes longer for the clock to tick considering that the speed of light or a photon is constant and equal "c" for all reference frames. I'm sure you all know what the diagram of this experiment looks like: http://spiff.rit.edu/classes/phys200/lectures/dilation/red_time_a.gif Now here's what I don't understand. Imagine the same scenario, now with a person in the same reference frame as the clock inside a space ship. So now we have that person and our light clock moving to the right at some high speed. The person looks at the clock, a photon is fired straight up from the bottom mirror to the top mirror so it's just bouncing back and forth infinitely. When we look at the photon being fired, wouldn't the photon just travel straight up and down a few times (non triangular path) and then just slam into the back of the spaceship (because the space ship is moving towards it). That's what I don't understand. Because from what I've read, the photon is moving to the right with the entire space ship. How does it get that velocity to the right when the speed of electromagnetic waves is not dependant of the speed of the source. Why wouldn't the photon slam into the back of the spaceship? Thanks in advance.