We tell time by measuring a repeating pattern. The earth spins — causing it to be light and dark, which we translate as a day. Pendulums swing back and forth, which we translate as a second. Scientists would call things-with-repeating-patterns oscillators. However, there is a problem.

Researchers have found that the earth speeds up and slows down in unpredictable ways. So, the earth is not a good way to measure the passing of time. The earth is a bad clock.

This is unfortunate, since we need precise clocks for many of the technologies we use,  like GPS. So we need a better clock–a more precise clock, a clock that is stable for a long time.

To make a better clock, it needs three parts: An oscillator to produce a repeating pattern; a counter to measure how often the pattern occurs. And, a part to make sure that the oscillator is creating this pattern correctly—which is called a discriminator.

Deep in a precise clock, or an atomic clock, is an oscillator. In this case, it is a quartz gem that is vibrating—the quartz acts like a piece of jello that wiggles when hit. And, those wiggles are counted to tell time.

To make sure that the quartz is wiggling correctly, atoms are used to check it. Cesium atoms

How?

Well, inside the atoms are electrons. And, electrons live at different levels from the center. Electrons can move up and down these levels, like a ladder, when they get zapped with energy. However, the electrons can’t keep that energy, so they give back a precise amount of energy when they return to their original level. It ends up that that energy given back has a precise oscillation to it. This is compared to the wiggles made by the quartz to see if the quartz is correct.

Sure, there are lots of steps, but it is worth it, since atoms are very precise. They lose their precision every 1.4 million years.

So it seems that atoms take a licking, and this keeps clocks ticking.