What makes it the preferred means of measuring time? And I guess, really, how do atomic clocks work?
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The official, formal definition of a second is:
> The second […] is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.
So cesium is the natural choice.
Mostly because it has the right physical properties: it’s easily vaporised (it must be in a gas state at very low temperature), and it has only 1 stable isotope, so no need to refine it.
Today’s episode on podcast Stuff You Should Know actually was about exactly this
[link](https://www.iheart.com/podcast/105-stuff-you-should-know-26940277/)
This is very hard to explain briefly and simply, but the *real* answer is that **cesium only has one electron in its outermost shell.**
This means that it can easily and controllably transition between two hyperfine states. Because all you need to do is shoot radiation with the correct wavelength and the transition will occur. The corresponding wavelength also has a good length (we don’t want e.g. visible light, since it interacts with other matter).
If there are more electrons in that shell, there are too many states in the hyperfine band so that we don’t get a controlled deexcitation. Rather you could get multiple deexcitations of lesser energy (longer wavelengths).
When we emit radiation to the cesium, we need to get the same energy out again (for how the principle of atomic clocks work).
We *could* use other elements with only one electron in the outermost shell, and we *do*, but for practical reasons, cesium is best (it’s readily available and relatively cheap, it’s stable, it’s a solid).
In very general terms, to build an atomic oscillator you want to prioritize a few things.
– You want it to be a gas, to avoid quantum mechanical effects that appear in solid states.
– You want it to be cold, so that the thermal energy of the atoms in motion doesn’t throw anything off.
– You want it to have as few stable isotopes as possible, so that it’s not a mix that could generate different frequencies.
– You want a specific nuclear configuration, specifically having as few electrons as possible in the outermost shell. This maximizes the oscillation that you’re trying to detect.
When taken together cesium is the best choice. It has a single stable isotope; it can be vaporized at extremely low temperatures; and it has just one electron in its outermost shell. It’s also no coincidence that other common elements for clocks are rubidium and strontium; they’re right above cesium on the periodic table and would share a number of characteristics.
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