The kilogram, as my international readers will know is the official unit of mass in the metric system—or more precisely, the International System of Units (SI). But less well known is that until today, the kilogram has been defined not as some universal value, but as the mass of a physical object: a chunk of platinum-iridium alloy in a triple bell jar in a secure vault in Paris.

Last week, however, the International Bureau of Weights and Measures (BIPM—it’s French) redefined the kilogram and three others of the seven “base units” from which all other units of measure are derived to make them more reliable and more universal. The new definitions come into effect on May 20. The BBC has a nice analysis of the kilogram in particular here.

The problem with the kilogram is that the *official* kilogram (pictured above) has been losing weight. As hard as platinum and iridium are, it’s lost atoms over the years when they take it out to clean it—about 50 micrograms, we *think*. And because this cylinder is the *definition* of a kilogram, the unit itself has been changing.

This affects us in America, too. American units are defined by law in terms of SI or metric units. That means when the kilogram changes, the pound *legally* changes (technically the slug, but it’s basically the pound), as does every other unit that’s defined in terms of pounds, including some that you wouldn’t expect. The psi, the horsepower, the kilowatt-hour, the volt, and even the calorie have hidden kilograms in them, so they have been legally changing, too. Only by 0.000005%, but still.

That’s why the BIPM decided they needed to change the definition of the kilogram to make it completely fixed, by tying it to one of the constants of nature. They chose Planck’s constant. This is a number that gets used a lot in particle physics. It happens to be 4π times the smallest angular momentum a particle can have, and it also shows up in Heisenberg’s uncertainty principle. The BIPM redefined the kilogram not to a direct measurement of mass, but *such that* Planck’s constant is exactly 6.62607015×10^{-34} J×s.

Joules (J) have kilograms in them, and you can measure Planck’s constant with something called a Kibble balance (better known as a watt balance), which balances a weight against the force of an electromagnet. It’s not as intuitive unless you’re a particle physicist, but it’s a *lot* more reliable, especially when you need things to be precise (and some things *do* need to be) to 0.000001%. And better yet, anyone with the right equipment can measure the kilogram for themselves. They don’t have to go measure against a reference weight in a vault somewhere. This redefinition democratizes the kilograms.

The BIPM redefined some of the other units too, to fix them to other physical constants. The ampere, the unit of electric current, used to be defined in terms of the magnetic forces around a flowing current—not very intuitive when it just measures flowing electrons. Now, they’ve defined it as a certain number of electrons per second—or rather, that one electron per second is exactly 1.602176634×10^{-19} amperes.

The kelvin, the unit of temperature, used to be defined be the triple point of water, a thermodynamic point very close to the freezing point. That was reasonable, but the purity and isotopic composition of water can affect the triple point, and it could only be measured so precisely. Now, they’ve defined it so that Boltzmann’s constant, a number used to compute the average energy of molecules, is exactly 1.380639×10^{-23} J/K.

And finally, we have the mole, (not the animal) which is a measure of the number of atoms or molecules in a given sample. You see it a lot in chemistry. It used to be defined as the number of atoms in 12 grams of carbon-12, also known as Avogadro’s constant. Again, isotopes are the problem. It had to be pure carbon-12 because atomic weights vary slightly even after accounting for the number of protons and neutrons because of their different nuclear energies. So now, they just picked a number that was as close as they could get to the current measurements: 6.02214076×10^{23} atoms or molecules.

Most people will never notice this redefinition in their daily lives. Some units will change a tiny bit, but it will be too small for most measurement devices to even register it. However, in high-precision physics, where measurements sometimes need to be made to one part in a trillion, or even to one part in a sextillion, it’s very important to have your units defined *exactly* so there is no ambiguity. This redefinition solves a longstanding problem in that regard and sets up a system of measurement that can truly be permanent, and that even aliens on the other side of the universe could use just as well. The metric system has well and truly been brought into the twenty-first century.