The kg is dead, long live the kg

On November 16th, representatives from nearly 60 nations will be meeting in Versailles, France, to vote to change the definition of a kilogram. Not only that, they will also be changing the fundamental unit of temperature, the kelvin, the unit for amount of substance, the mole, and the unit for electric current, the ampere. That is four of the seven base S.I. units in one day! And after that, all S.I. units will be based on fundamental constants of nature, and not physical artifacts. The kilogram is the last base SI unit to be defined by a physical object.

Since 1799, one kilogram has been defined as exactly the mass of a single metal cylinder stored in Paris. It was swapped out once in 1889. But this International Prototype Kilogram (or Big K as it's affectionately known) has problems! I mean, weighing it with in-theory identical cylinders, scientists have found that their masses are diverging. So it doesn't even seem to maintain its mass. Plus, it's really hard to get access to Big K, and that makes using this definition really difficult.

So how do you create a mass standard that will never change, and also be available to everyone, everywhere? With the solution, you set Planck's constant to have a fixed, exact value. Now I know that sounds a little strange, so bear with me for a moment. I mean, Planck's constant is best known for relating the frequency of a photon, a particle of light, to its energy. But we also know that energy and mass are related through E = mc², so, hopefully, you can see how Planck's constant is involved in mass.

But the problem, as it stands today as I'm recording this video, is that Planck's constant has some uncertainty. I mean, we know the value of Planck's constant to a large number of decimal places, but those last couple of digits... They're actually uncertain. What is certain is the mass of that platinum-iridium cylinder stored in a climate-controlled vault in a basement in Paris—it is exactly one kilogram. No uncertainty.

So the solution is to flip this on its head—set Planck's constant to have an exact fixed value, and then that cylinder in Paris will no longer be exactly 1 kilogram. I mean, it'll be a kilogram, but not exactly. The thing that is now exact is Planck's constant, which determines how big a kilogram is. But if you're gonna fix the value of Planck's constant, well, you better get that value right, so that it's consistent with all of our current measurements and all of the masses that exist in the world right now.

For the last several years, scientists around the world have used multiple different techniques to try to measure Planck's constant as accurately as they possibly can. One of the major methods was using a watt balance, where essentially they balance the weight of a kilogram with the force from an electromagnet. If you want more detail, you should check out my video on that topic. Scientists also created arguably the roundest object in the world made of one type of silicon atoms. These methods have been complementary because now they're able to compare all of their different findings from physics and from this more chemistry method of Avogadro's constant and determine what Planck's constant really should be.

So if the vote goes well, the future definition of Planck's constant will be that it is exactly this number. Planck's constant is fixed. That cylinder in Paris is no longer exactly equal to a kilogram. But you can't redefine the kilogram in isolation, because other base S.I. units depend on it. Take the mole, for example. Currently, the mole is defined as the amount of substance that contains the same number of particles as there are atoms in 12 grams of carbon-12—that's Avogadro's constant and it depends on what 12 grams is, which depends on what a kilogram is.

So again, Avogadro's constant currently has some uncertainty, but after the vote, the plan is to fix Avogadro's constant to be exactly this number in such a way that it is internally consistent with the new definition of Planck's constant. There's a direct relationship between Avogadro's constant and Planck's constant. Likewise, the ampere will no longer depend on the kilogram. Instead, it will be defined based on this newly fixed value for the charge on an electron; and the Kelvin will be based on the newly fixed Boltzmann constant, which relates the temperature of a gas to the average kinetic energy of the molecules, and this will be its exact value with no uncertainties.

Now, will these new definitions change anything? Well, for most people, no. I mean, your food is still going the same way, as are you. And temperature is still gonna work the same way. You know, everything basically stays the same, and that is as it should be. The point of this definition change is not to shake things up, but to keep things consistent and reliable forever. All we're doing is removing the dependence on a physical object, which theoretically, at least, makes it possible for anyone, anywhere to make incredibly precise measurements.

Now, I should point out that a volt will actually change by about one part in ten million, and resistance will change by a little bit less than that. And that's because back in 1990, the electrical metrologists decided to stop updating their value of effectively Planck's constant and just keep the one they had in 1990, and there was a benefit to that. They didn't have to update their definitions or their instruments, but now that we've realized that Planck's constant is actually slightly different than the 1990 value because of better measurement techniques. Well, now the electrical metrologists will have to change, but that's a very tiny change for a very tiny number of people. I think they'll be fine.

You know, I've been trying to ask myself the question, why am I so interested in this topic? I mean, I made like four videos on it, and the reason is, you know, to me, the world and the universe is a big complicated place. And when we're actually able to ascribe numbers to it, it's like we are wrestling some sort of order out of the chaos that is our universe, and that is the beginning of our understanding of the way things work. You know, measurements are the foundation of science; they allow us to make observations.

I think it's no surprise that, you know, Kepler was really able to figure out what was going on with the planets—that they were actually moving in elliptical orbits—once Tycho Brahe had made the most accurate measurements of their positions that people had ever made. I mean, I think that's no coincidence. And if you look at the discovery of the Higgs boson at CERN or the detection of gravitational waves, these are, in my view, the pinnacle of human achievement. I think there are orders of magnitude greater than the achievements that we make in literature, and art, and fashion; and I don't say that to disparage those disciplines. I know that they're hard; I know they take a lot of human brainpower, and I'm not saying scientists are smarter, but the tools that scientists work with and the system in which they work is what allows them to make such great leaps because science builds on itself in almost, you know, an exponentially improving way.

And that, to me, is why this is so important, is because it allows us to take our measurements to the next level. No longer are we bound to physical objects. I mean, face it, up until now, we've essentially been doing a glorified version of Indiana Jones. Now, we are taking that next leap to the abstraction that all of our units are based on the way nature is and the way the universe is. We're no longer tied to physical objects.

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