It's Official: The Kilogram (As We've Always Known It) Is Dead

There's a new standard in town.

It’s hard to say goodbye, but when it’s time, it’s time. On Friday, an international group of scientists did just that with the kilogram. After 119 years of serving loyally as the world’s standard for the kilogram, the International Prototype Kilogram was officially retired in favor of a more universal standard measure of mass. The IPK — affectionately known as Le Grand K — is housed in a vacuum-sealed enclosure in Paris, and since 1889, it’s been the world’s standard measure of the kilogram. Officials from all over the globe would regularly come to measure their copies against it to make sure a kilogram was still a kilogram. But that’s all changing, with the help of some sophisticated physics.

On Friday, at the General Conference on Weights and Measures in Versailles, France, representatives from 58 member countries voted unanimously to establish new standards for the kilogram. In an effort to take the pressure off Le Grand K, which has actually been slowly losing mass over the years, scientists devised a new way to actually measure the amount of matter in an object. Therefore, instead of being defined in terms of a lump of platinum and iridium in Paris, the kilogram will now be defined in terms of a universal constant, rather than in reference to a human-made artifact.

Image from page 20 of "The Ontario high school physics" (1917)
This image from a 1917 high school physics textbook shows the IPK. Over 100 years later, it still looks the same, but it's become obsolete.

The IPK’s retirement is part of a long-term project by the international scientific community to get rid of measurement artifacts and base measurements on abstract universal measurements rather than objects. For instance, in 1983 the platinum bar with two marks on it signifying an official meter was retired in favor of a new measurement: the distance a light beam travels in 1/299,792,458 seconds. This may sound like a really complicated way to define measurements, but it’s the only way to ensure that anyone around the world can know that they’re getting the correct deal in international trade or measuring accurately in a scientific experiment. Imagine a team trying to collaborate to build rockets if we couldn’t agree on what a meter or kilogram was. It probably wouldn’t work.

So instead of the IPK, the new standard will be determined by a machine known as a Kibble balance, which is described in the video at the top of this article. Whereas gravity usually tells us the mass of an object on a scale, the Kibble balance uses electromagnetism to translate mass to electrical current. In this way, universal measures of mass will no longer depend on an old artifact, but will be verifiable and repeatable with a machine.

The IPK will still be kept around as kind of an emeritus professor, but its responsibilities will be surrendered to the Kibble balance.

Three other units got redefined at the conference, too: the ampere (electrical current), the mole (used for measuring numbers of particles), and the kelvin (temperature).