Einstein | American Museum of Natural History

Time seems to follow a universal, ticktock rhythm.

But it doesn't.

In the Special Theory of Relativity, Einstein determined that time is relative—in other words, the rate at which time passes depends on your frame of reference. Just as observers in two different frames of reference don't always agree on how to describe the motion of a bouncing ball, they also don't always agree on when an event happened or how long it took. A second in one reference frame may be longer compared to a second in another reference frame.

The faster a clock moves, the slower time passes according to someone in a different frame of reference. To explain this bewildering result, physicists point to a thought experiment involving a clock that uses light to mark time. Although this "light clock" experiment is a hypothetical one, the same effects are true for any timepiece, from old-fashioned grandfather clocks to atomic clocks, the most accurate time-keeping devices available. Time is relative even for the human body, which is in essence a biological clock. The effect of time slowing down is negligible at speeds of everyday life, but it becomes very pronounced at speeds approaching that of light.

What is a light clock?

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Imagine a clock that consists of a pulse of light and two mirrors, one at the top of the clock and one at the bottom. The clock "ticks" when the pulse reaches the mirror at the top of the clock. and "tocks" at the bottom. The pulse bounces back and forth between the mirrors at a constant rate. When the clock moves, as shown at left, the time between ticks is longer.

How long is a second?

The idea that a second is not always a second is one of the most surprising findings of Einstein's Special Theory of Relativity. Researchers have actually observed this effect, which is only detectable at high speeds. Scientists synchronized two highly accurate atomic clocks and then flew one around the Earth aboard an airplane. When the airborne clock returned to Earth, it was a tiny fraction of a second behind the one that remained on the ground. A thought experiment using a light clock reveals why this is so.

All moving clocks run slow-not just light clocks.

But the effect is insignificant except at speeds approaching that of light. Intriguingly, someone moving will not think that her clock is running slow, because everything in her frame of reference will have slowed down as well. According to a stationary observer in space watching Earth move around the Sun, all of the clocks on our planet are running slow, yet we don't notice anything out of the ordinary.