Scientists Are Racing to Redefine the Kilogram

Just when you sort of got used to it in the first place.

In Paris, behind a vault door that requires three keys to unlock it, is the most precious lump of platinum-iridium in the world. For the casual observer, it’s tough to assign a monetary value to the lump, but for a group of scientists, it’s a practically priceless heap of metal that gives gold a run for its value.

This lump, after all, does the incredibly important job of being the standard for the kilogram. But an international group of researchers is hard at work to redefine the kilogram and make this prized item obsolete.

“The kilogram artifact is actually a really wonderful thing,” says Alan Steele, the chief metrologist at the National Research Council of Canada. “But it’s not fundamental, it’s not basic — it’s just a thing, and if something happened to that thing we’d be in real trouble.”

Steele is part of a group that is crusading to redefine the kilogram. Its weight wouldn’t change — that sucker is still sitting pretty at about 2.2 pounds — but Steele and his colleagues think it’s high time for a mass update, one that is based on a “fundamental constant” — an inherent universal property, similar to something like the speed of light — instead.

Before you think the kilogram crusade is the work of a fringe group of mad scientists, it’s not. The update requires a “scientific tour de force,” says Steele, because defining the unit based on fundamental constants requires three experiments, in two different methods, in multiple countries. It is incredibly difficult to get consensus across all three experiments in both systems. “These are just really beautiful experiments, with great, easy to explain concepts,” he says, “but man, is the devil in the details.”

Figuring out how to measure the kilogram requires defining the fundamental constant — known among physicists as “Planck’s constant” — within an uncertainty of 50 parts per billion. In English? That’s pretty, pretty, pretty exact. Some chemistry labs are measuring this by calculating the number of atoms in a kilogram silicon sphere. But others are using a physics approach, using a device called a Kibble balance, that uses energy and electrical constants to perform the measurement. The trick is that the chemistry and physics approaches to define Planck’s constant have to match — and making the systems equivalent has never happened before.

Redefining our units of measurement is “an event that happens probably once in our lifetime,” says Stephan Schlamminger, a physicist at the National Institute of Standards and Technology. His team has gotten a measurement of Planck’s constant with an error of 34 parts per billion (in science, the smaller the error, the better, so 34 parts per billion is definitely a good sign compared to the industry accepted maximum of 50 parts per billion). So far there is an international consensus to the number, and Schlamminger hopes to get closer to perfection with an error of 20 parts per billion by July 1, 2017, when the International Bureau of Weights and Measures will collect all the measurements to be incorporated into the international average. Once all this occurs, the final number will be debated and voted on in November 2018.

“For the end user, the transition will be seamless; you won’t know the difference,” says Schlamminger about the change. But for the scientific community, redefining these units will be invaluable, turning that lump in Paris into a symbol of a new understanding of fundamental measurements of the universe — and that’s a bragging right most lumps just don’t have.

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