NPR recently ran a fascinating piece on the redefinition of the kilogram. Turns out, the kilogram is actually a hunk of metal in France. When somebody needs to know exactly how much is a kilogram, they have to go take the hunk of metal out from beneath its special glass container with its special lifting tool (no touching the kilogram!) and weigh it.
The problem here is that the kilogram can change, most notably through the hunk of metal gradually losing mass. If you are trying to be very precise with your measurements, it’s obviously not ideal if a kilogram means something different now than it will in another 100 years.
So scientists are working to establish a definition of the kilogram based on “a fundamental constant,” a principle of the universe, rather than an “artifact.” And they have succeeded! So the definition of the kilogram will change to this new, more stable definition. How did scientists arrive at this definition, you ask?
researchers spent years creating an elaborate new kind of weighing machine called a Kibble balance
So now, instead of weighing a special hunk of metal to determine the weight of the kilogram, you will weigh any old hunk of metal in a special, complicating weighing machine.
So how did we get away from defining the kilogram with an artifact, again? As it turns out, it’s artifacts all the way down.