Two Spaceships Show Time Dilation

The best way to show how time dilation works is using something called a light clock. The clocks on our walls use gears that rotate the hands slightly every second, minute, and hour. A light clock uses a pulse of light fired at a mirror some distance away, and measures how long that pulse of light takes to get back to the source. So if a pulse of light is fired at a mirror 1.5×108 meters away, the pulse will return to the source one second later (as seen by the source).

But relativity is about objects moving relative to other objects, so instead of watching the light pulse while stationary, let’s imagine the light source and mirror moving passed us near the speed of light. What will happen?

Well, according to the Second Postulate of Relativity, the speed of light is constant no matter what frame of reference is chosen. So in the frame of the light source, the pulse will travel straight out to the mirror and come straight back one second later. BUT, if the light source and mirror are moving passed an observer, they will see the pulse traveling along a diagonal path to the mirror, and along a diagonal path back to the source. Since the light travels a farther distance at the same speed, it will take longer for the light clock to tick! Moving clocks tick slower!

The video describes the classic light clock example. Once you see moving light clocks in action, it’s not hard to understand why moving clocks seem to tick slower. But how much slower? Using some complex trigonometry, physicists can actually calculate what they call the time dilation factor (or the Greek letter gamma). This allows physicists to do calculations in situations where speeds are high enough to alter time, distances, even energies!