eli5 what’s the difference between a regular clock and an atomic clock?

Question

eli5 what’s the difference between a regular clock and an atomic clock?

In: 26

Answer 1

All clocks need *something* to move or change at a regular, predictable rate. It’s those movements that we count to measure the passage of time. For example, one property of pendulums is that for a given weight and length, no matter how high the pendulum goes it will always swing in the same period. So, you tune the pendulum so that the period is 1 second, meaning it will swing back and forth once per second – and attach that to a [mechanism](https://upload.wikimedia.org/wikipedia/commons/2/29/Anchor_escapement_animation_217x328px.gif) that turns the gears that turn the clock hands.

The problem with these is that you still need a spring to keep tension and turn the gears, and as the spring loses tension and runs out of energy, it changes the rate at which that the mechanism will move, so the time will creep off. And, pendulums and springs change with things like weather and heat, which also makes your time less accurate. Of course, that’s assuming they were built perfectly to begin with!

Digital clocks use a tiny [quartz crystal oscillator](https://upload.wikimedia.org/wikipedia/commons/thumb/9/91/32768_Hz_quartz_crystal_resonator.jpg/220px-32768_Hz_quartz_crystal_resonator.jpg) to track the time. It vibrates just like a tuning fork. The tines (or forks) of the oscillator are carefully crafted to resonate at a particular frequency when electricity is connected through the crystal due to [piezoelectric effects](https://en.wikipedia.org/wiki/Piezoelectricity). That’s its own ELI5 but the important part is electricity makes the the crystal vibrate, and the size and shape are designed to make it vibrate a very specific frequency. That frequency is *32768 times each second*, which is very fast but that doesn’t matter. What matters is that the frequency is known, and a tiny computer chip can keep track of that and count the clock forward on time.

But, like a pendulum or spring the quartz crystal would need to be *perfect* to keep perfect time. Most aren’t. Even very high quality crystals will change slightly with temperature and pressure and whatnot.

An atomic clock uses cesium atoms as the oscillator instead of a quartz crystal. When you beam microwaves at the cesium atom at just the right frequency, the atom will absorb the microwave and transition into a higher energy state. The atom then emits light back out at *exactly* that frequency, or wavelength. Frequency is the definition of changing over time, so…that’s the oscillation they measure. It’s *super* reliable and a fundamental part of physics so it’s very very very very very accurate.