MOTION SLOWS THE PASSAGE OF TIME
Imagine a clock made from two parallel mirrors with a photon ( particle of light) bouncing between them. If the mirrors are placed the correct distance apart, the photon will take one second to make a round trip between them ( A). If the clock is moving horizontally, however, the photon will trace out two sides of a triangle, travelling a greater distance (B). Since the speed of light is constant, the photon will take longer to bounce between the moving mirrors, and – from our point of view – each second on the moving clock will take longer than on the stationary clock.