# Scientists Are About To Change What a Kilogram Is. That’s Massive!

If Jon Pratt were an international criminal mastermind, he would fly to Paris, don an all-black suit and ski mask and sneak into the elegant French baroque building that serves as headquarters for the Bureau International des Poids et Mesures.

His mission: “To set the whole world’s system of mass into disarray,” Pratt said. “This is my dastardly plan.”

In this hypothetical scenario, Pratt slips past the security guards, disables the alarm system and picks the lock on a temperature-controlled, airtight safe deep in the bowels of the BIPM. Inside, he finds his target: a small platinum and iridium cylinder weighing exactly one kilogram. It’s *the*kilogram, crafted in 1889 to serve as the single standard by which all other kilograms are measured. People call it “le grand K.”

“I’d take out a nail file, and I’d scratch a little bit off,” Pratt said. Then he’d slip back into the night. “And the next time they take the thing out” (to test the accuracy of the world’s other kilograms) “everything else will be wrong.”

But Pratt is not a criminal mastermind. He’s a public servant, the chief of quantum measurement at the National Institute of Standards and Technology, which oversees weights and measures in the United States. And he doesn’t want to tamper with the global system of mass. He wants to revolutionize it.

Pratt and his colleagues at NIST are part of an international effort to redefine the kilogram based on a fundamental universal constant — a physical quantity in nature, like the speed of light or the electric charge of a proton, that never changes regardless of when and where you are. And on Friday, the NIST team got their most precise measurement ever for this constant.

“It’s not obvious that it’s a big deal, but it’s a big deal,” Pratt said. With this new measurement, “we could switch from a 19th-century definition of mass to a more 21st- or 22nd-century definition of mass. We could get it based on an idea more than an object. And that’s just beautiful, and I’m proud of our species for getting to this place.”

Here’s the problem with the current standard kilogram: It’s losing weight. It now is ever-so-slightly lighter than the once-identical “witness” cylinders stored in labs around the world. Scientists don’t know whether the BIPM prototype is losing mass, perhaps because of loss of impurities in the metals, or if the witnesses are gaining mass by accumulating contaminants.

Either way, the whole thing is a “huge inconvenience,” Pratt said. Several years ago, NIST had to reissue certificates for its kilograms because they were 45 micrograms off the French prototype — about the weight of an eyelash. This meant that companies that produce weights based on the NIST standards had to reissue their own weights, and they were not happy about it. Lawmakers were called. NIST was accused of being incompetent. In the end, it turned out that the problem stemmed from le grand K, not NIST.

If that seems like a lot of uproar over an infinitesimal change in the mass of an object, consider this: The effectiveness of filters on diesel engines is determined by measuring the mass of the soot they capture — in micrograms.

“There’s a lot that rides on these sorts of things that people take for granted,” Pratt said. “Like breathing.”

Scientists agree it’s long past time to retire le grand K. Using this 19th-century technology for 21st-century physics is like trying to get to Mars on a rocket powered by a steam engine. It just isn’t going to work.

So in 2014, at the quadrennial General Conference on Weights and Measures (yep, that’s a thing), the scientific community resolved to redefine the kilogram based on Planck’s constant, a value from quantum mechanics that describes the packets energy comes in. If physicists could get a good enough measure of Planck’s constant, the committee would calculate a kilogram from that value.

“But it’s a very difficult constant to measure,” Pratt said. He would know: He and his colleagues at NIST have spent much of the past few years trying to come up with a number accurate and precise enough to please the finicky physics community.

They’re using a tool called a Kibble balance. Instead of balancing the scale with weights, Pratt and his colleagues use electromagnetism. An electrical current is sent through a coiled wire, generating a magnetic field that creates the upward force needed to balance the scale. Scientists can figure out the strength of that field by pulling on the coil. If you know the voltage, the current and the velocity at which the coil was pulled, you can calculate the Planck constant with extreme precision.

On June 30, the day before the deadline to submit a value to the weights and measures committee, the team at NIST was finally ready to release its result. Based on 16 months’ worth of measurements, it calculated Planck’s constant to be 6.626069934 x 10^{−34} kg∙m^{2}/s.

Don’t be alarmed by this small, strange number. The most important thing about the NIST measurement isn’t so much the number (though that’s also a big deal) as the uncertainty: just 13 parts per billion. This means that the NIST scientists think their measurement of Planck’s constant is within** **0.0000013 percent of the correct number.

When the International Committee for Weights and Measures announced that it would reconsider the kilogram definition, it said it would require three measurements with uncertainties below 50 parts per billion, and one below 20 ppb. But with the new NIST measurement, the world now has at least three experiments below 20 ppb — another was conducted by a Canadian team using a Kibble balance, the third by an international group that calculates the Planck constant based on the number of atoms in a sphere of pure silicon.

The weights and measures committee will meet this month to establish a global value for Planck’s constant by averaging the values calculated at NIST and other labs. And in 2018, at the next General Conference on Weights and Measures, the scientific community will draft a resolution to redefine kilogram based on this constant.

“I can’t stress enough how impressed I am at humanity for being able to pull this stuff off,” Pratt said.

We’re lucky Pratt decided to use his powers for good instead of evil. These are weighty matters.

As reported in The Washington Post.