The new definition of the Kilogram
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So back in the day, we would define things like the metre or the kilogram with physical objects. We said that the distance from the equator to the pole was 10,000km, and then we derived the metre from that, created a physical representation of it, and that became the source of truth.
However, that source of truth isn’t that great. It’s subject to being damaged or lost and might read a different value later in its life. Instead, we looked at some universal constants for how we could define the metre. Importantly, we had to be able to reproduce these results later. Distance was easy since the speed of light in a vacuum is constant. We defined a metre as the distance light travels in 1/299 792 458th of a second. We can reproduce this measurement elsewhere to confirm the distance that a metre is.
Mass is harder. We have a physical object that represents a kilogram, but there isn’t an easy way to represent it with universal constants in a way that can be easily reproduced. We had a few ways in the works, such as trying to get a sphere of pure silicon that weighed exactly 1 kilogram and confirming the number of atoms in it, but that was too difficult.
So we ended up creating a device called a Watt Balance (now known as a Kibble Balance after its late inventor). The device measures the electrical current on a balance to get a precise value for the weight that is pulling on it. It’s hard to get into detail here, but check out this Veritasium video for more: https://www.youtube.com/watch?v=Oo0jm1PPRuo
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