Eli5 – How is the kilogram defined using the second and the Plank constant?

Question

The kilogram used to be defined by a specific chunk of platinum-iridium.

But now it’s defined using the second and the Plank constant, somehow.

Assuming you already have a very accurate measurement of the second, and the Plank constant, how do you combine those two things to get the kilogram?

In: 1

Answer 1

So the Planck constant has, as part of it’s formula, a mass component because it involves the relationship between mass and energy.

Obviously, it’s a constant so it doesn’t change, but the unit that we use for the mass part can change what the number looks like — any calculation involving mass will have very different numbers if using a gram, an ounce, or a ton.

So what they did was express the definition of the Planck constant to be in kilograms. The other key parts of the equation are the second and the meter — and both of those are defined using universal constants as well.

The end result of this is that since neither the values for the Planck constant, the second, and the meter are consistent and can never change, that means the kilogram is too, because if you know the other three values, you can calculate what the mass of a kilogram is.