A hypothetical timekeeping device, useful in thinking about relativity and time dilation. It is a set of two connected parallel mirrors, in between which a single photon is bouncing eternally. A single "tick" of this clock occurs when the photon makes a full round-trip between the two mirrors. If the mirrors are one meter apart, a single "tick" takes 9.9×10-9 (we'll call this ttick) seconds-- unless, as we shall see, the clock is moving relative to an observer.

If the clock has some velocity perpendicular to the motion of the photon, the photon's path will be diagonal, rather than the straight up-and-down motion it had when the clock was at rest. The distance that it has to travel with some velocity is equal to the square root of (vertical distance traveled in the space of ttick squared plus distance between mirrors squared), by the Pythagorean theorem. Since light always moves at the same speed, and a greater distance was traveled, we have no choice but to conclude that ttick is greater with a moving clock than with one at rest. Of course, like most relativistic phenomena, this effect is only noticeable at very high velocities, but it is still a relevant effect.

A common assumption here is "Well, so that works with one of these crazy light clocks, but not with this brand-new Rolex, right?" But this is incorrect. (The following demonstration is borrowed from The Elegant Universe by Brian Greene.) Let's place both the light clock and your Rolex in a train moving at constant velocity. If all the windows are closed, then (according to relativity) it should be impossible to detect whether the train is moving at all. But if your assumption is true, then the light clock will slow down while the Rolex does not, which would shatter the illusion of being stationary. Thus, any timekeeping device will slow down when moving.

Light clocks can also be used to demonstrate that length contracts in the direction of motion... maybe later.

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