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?

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Answer 1

Clocks generally work due to something that *oscillates*. A lot of systems have a natural frequency at which they tend to oscillate, like ringing a bell. The trick is to make them do so exactly the same way over long periods of time.

For example, a grandfather clock has a pendulum that swings back and forth and “wants” to go at a specific rate based on the length of the pendulum. But a lot of factors (friction, temperature, etc.) can make this system not behave perfectly. Pretty darned good, but there’s lots of room for improvement. Mechanical watches use an escapement mechanism instead of a pendulum since it’s rather impractical to put a pendulum in a wristwatch.

In many digital clocks, the oscillation is based upon a thin quartz crystal that expands/contracts a little when exposed to a voltage, and creates a pulse of current when it contracts. If you excite it with a changing voltage close to the frequency it “wants” to naturally oscillate at, it will do so with great accuracy. The combination of the quartz crystal and the circuit it is in creates an oscillating system.

These crystal based electronic clocks are a big improvement over mechanical clocks, but there are still various factors that limit how precise they will be over long periods of time.

An atomic clock takes this idea to another level. Instead of using something mechanical or electronic, atoms that can be in one of two energy states are used. They have a frequency at which such transitions naturally happen when excitation energy is supplied. If you bombard these atoms with microwave radiation that is at a frequency close to their natural oscillation frequency and then tune the microwave frequency to maximize the number of atoms changing state, the microwave frequency you end up with will precisely match the rate at which they naturally “want” to oscillate.

Making a setup like this is not trivial, but mostly it simply won’t work at all unless you get things right. Once it works, it tends to behave quite dependably and provides an extremely precise time reference.