I have a time question concerning relativity. I read that time slows down on a moving object from my perspective. This is so based on a lightbeam going up and down on the moving object. In my persepctive the light path is longer. In his perspective it is shorter. Thus, in my perspective his time slows down. The question is as follows: Why is a lightbeam going up and down the sole derterminor. If we use a lightbeam going from the front of the moving object to the back of the moving object to tell time, we will have the opposite effect. In my perspective the light path is shorter. In his perspective it is longer. We can thus say that in my perspective his time goes faster. This is my question. Any answers?