A general summary of the experiment is that when the light clock is stationary on the platform it runs as normal, up and down, but when someone is looking at a clock on a moving train, it appears to move slower because the light has further to travel, and thus, time runs slower. What confuses me is that there seems to be a general agreement that it doesn't matter what type clock you use, it will always appear to run slower on a moving train. Take the example of a pendulum clock. Just as the reason for the light clock running slower is because the light has further to travel, what would be the reason for a pendulum clock running (or appearing to run) slower under the same conditions? Or an atomic clock, or a watch, or any type of clock? In my head, it makes sense that the experiment only works for light clocks, I don't see any reason why a normal watch would be slower when moving, I mean, it doesnt have a greater distance to travel just as the light in a light clock does. If I'm going to be honest (and I know this is going to sound stupid) I don't get how the light clock experiment proves that time runs slower for moving objects at all. I mean, it proves that light in a moving light clock appears to travel a greater distance, but i still don't think it proves anything about time. If a blue car travels 100m and a red car travels 50m, if they both have the same speed (just as the speed of light is constant) it is clear that the red car is going to have a shorter journey. That still proves nothing about time. It doesn't show that time somehow runs faster for the red car. It just shows that one had a greater distance to travel, just as the light in the moving light clock had a greater distance to travel. So my questions are: 1. How does the experiment prove that time runs slower for moving objects? 2. Why would other clocks behave the same way (other than simply "because time runs slower")? What is the actual physical reason?