SI units are now defined directly in terms of physical laws (no prototypes needed).

https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...

It’s an SI unit and directly defined through the speed of light which is a universal constant number.

While you need „second“ to define the „meter“, the former is not derived from other units but from an observation of nuclear reactions in Caesium.

Of course, once you get to the second, we are now just measuring something, and our unit is based on counting a thing. But how do you measure something that can't be counted?