We can understand a lot about relativity by using a device called a light clock to define our basic unit of time. This device holds a flashbulb, mirror, and photocell. The “tick-tock” consists of the flashbulb going off, the light moving up and bouncing off the mirror, and then the light traveling down to the photocell. When it arrives, the photocell triggers the flashbulb, and the whole process is repeated.

This may seem like a strange kind of clock, but look at it this way: By adjusting the position of the mirror, we can make the light clock tick at the same rate as any other clock we want to use. For the record, I always picture a tall grandfather clock next to my light clock, but you can picture any kind of clock you like. Thus, any result we derive for a light clock will apply to any other kind of clock as well.

Now imagine an experiment. We have two identical light clocks — one on the ground and the other on a moving railroad car. Imagine that we set things up so that both flashbulbs go off as the moving clock passes the one on the ground. What will an observer see?

As far as the ground observer is concerned, the clock on the ground is behaving normally — the light travels up to the mirror and back and the clock ticks along. When the ground observer looks at the moving clock, however, she sees something very different. In the time it takes the light to move up to the mirror, the train has moved a certain distance.

Consequently, the ground observer sees the light in the moving clock traveling on a slanting path. The same thing happens as the light comes back to the photocell, so the net result is that the ground observer sees the light in the moving clock traveling in a sawtooth pattern.

In other words, as far as the observer on the ground is concerned, the light in the moving clock travels a longer distance than the light in the clock on the ground. If light travels at the same speed in both frames of reference, then it will take the light in the moving clock longer to finish its path than it takes the light in the clock on the ground to do the same thing. As far as the observer on the ground is concerned, the moving clock ticks more slowly. — J. T.