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?

Question

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

Answer 1

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.