Light travels at a fixed speed relative to the observer. If you shine it from a train it will move away from you at the speed of light. I am not standing on tha

Light travels at a fixed speed relative to the observer. If you shine it from a train it will move away from you at the speed of light. I am not standing on that train, it still moves away from me at the speed of light. This is where time dilation comes from. The classic example is a clock of light ticking between two mirrors. If you are moving with that clock it is ticking at the speed of light. If I am in a frame of reference where the clock is moving perpendicular to the light tick's path, the light will have a greater distance to cover. [1] Since the speed is fixed to me, this tick is slower. 1. https://upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Time-dilation-002-mod.svg/1280px-Time-dilation-002-mod.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Ti...