# Why is cesium used to define a second, as opposed to other atoms that might be more common like Hydrogen or even Oxygen? Also how do we know that’s equal to one second if seconds are arbitrary?

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Also I didn’t know what to flair this sorry. Figured maybe physics tech or math but I wasnt sure.

Edit: apparently it was physics. *shrug*

In: Physics

You might have it a little backwards: the second was defined first, by a long while. Originally, the second was defined based on the rotation of the Earth – 24 hours divided by 60 into minutes, and then again into seconds. But the rotation of the Earth isn’t quite that precise, and so scientists wanted to find a more precise definition of a second, one that could be true anywhere in the universe and one that would be agreed upon by anyone who could be bothered to find that measurement.

Cesium has been used in atomic clocks because the frequency of the microwave that it emits is really, really consistent when evaporated and sent spinning by the mechanism of the atomic clock. Most atoms aren’t this consistent and they’d produce a very wide range, but cesium holds close enough to that one value that the best atomic clocks won’t lose a second over *billions* of years. Scientists figured out how many of those cesium transitions added up the closest to that pre-defined time that we’ve been calling a second (which turns out to be 9,192,631,770), and they re-defined the second to refer back to that value. So the second that we’ve always used is more-or-less the same, only now it’s defined incredibly precisely by a value that will not change in any measurable way, which means that anyone who builds a good atomic clock will have exactly the same time measurements as anyone else. It’s a silly and large number by our typical standards, but it’s reproducible and consistent, which is the key to a good measurement.

Cesium is used because it is incredibly stable. I.e. it oscillates at a very very specific frequency. In this case you just need to count the oscillations.

The commonality of an atom is really irrelevant here for a simple reason

You only need to define a second really once and in a few specific places in the world. It’s not like you need cesium in every watch, and clock on earth.

> **Also how do we know that’s equal to one second if seconds are arbitrary?**

You answered that yourself. It’s arbitrary. We defined it to be that value, period.

We’ve been refering to a length of time equal to 1/60 of a minute which is 1/60 of an hour which is ABOUT 1/24th of a day for a couple thousand years.

We (humans) made up that unit. So when we wanted to do more advanced science, we were actually able to measure a unit of time like that more and more accurately and precisely.

At that point we needed to come up with a universal definition that anyone with the right tools can “set their watch to”. So we observed how long we thought a second should be, counted some cesium oscillations and said great that’s a second.

It’s kind of like saying, why is the colour of the sky called blue. How do we know it’s blue. Because one day we said it is.

We don’t use hydrogen or oxygen, because in their natural state, they don’t emit any radiation. We use cesium because it’s radioactive. The frequency of radiation that cesium produces just happens to have an exact whole number of periods in a second. The previous definition of a second was based on the length of a day, but in the interest of making calculations work a little better and making units a little more precise, a lot of the SI units were given new precise definitions.