Scientists may change what a kilogram is
If John Pratt were an international criminal mastermind, he would fly to Paris, don a black suit and ski mask and sneak into the Bureau International des Poids et Measures.
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 would find his target inside: a small, platinum and iridium cylinder weighing exactly 1 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. On Friday, the NIST team got its 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 onceidentical “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 — about the weight of an eyelash — off the French prototype. 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.