Miss Dada  🏳️‍⚧️

Why are the kilogram, ampere, kelvin, and mole still considered base units and not derived? Given their current definitions, they're purely defined in terms of other units and cannot be independently measured, right?

It's no different to how the metre is defined by using the speed of light - it's taking a fixed value for a physical constant and then deriving the value of the base unit.

Right, but the Planck constant isn't something you can independently measure. The kg is now defined by giving it an exact value

But that argument also applies to the speed of light doesn't it? Can't measure how fast something is without a distance. I think the phrase 'base' at this point mainly means they're the units to use when doing dimensional analysis and not some kind of fundamental value.

> But that argument also applies to the speed of light doesn't it? Can't measure how fast something is without a distance. That's sorta the point. Distance is determined by how fast light travels. You need a defined second and measure how far light travels in a fixed time.

> not some kind of fundamental value. This just makes the base/derived split weird though. The name implies (and it used to mean AFAIK) a unit that was fundamentally required to describe the universe. If its definition is in entirely terms of other units, that's not the case

Granted I also don't fully understand how the modern definition of the kg even works since it seems like it defines it in terms of the mass of a photon but photons have no mass so ¯\_(ツ)_/¯