Weight, what? A new definition of a kilogram

Apparently, our species took a giant step forward on Friday and nobody realized it, at least until today when the Washington Post reported on the work of John Pratt, the chief of quantum measurement at the National Institute of Standards and Technology, which oversees weights and measures in the United States.

He and his colleagues published a new measurement standard of weight on Friday.

Hard to believe that this page-turner study, described in the abstract, didn’t get the world’s attention.

A detailed analysis of these measurements and their uncertainties has led to the value h = 6.626 069 79(30) × 10−34 J s. The relative standard uncertainty is 45 × 10−9. This result is 141 × 10−9 fractionally higher than h90. Here h90 is the conventional value of the Planck constant given by $h_{90}\equiv 4 /( K_{{\rm J\mbox{-}90}}^2R_{{\rm K\mbox{-}90}})$ , where KJ-90 and RK-90 denote the conventional values of the Josephson and von Klitzing constants, respectively.

Bottom line? A kilogram on Thursday wasn’t what a kilogram is today.

“It’s not obvious that it’s a big deal, but it’s a big deal,” Pratt tells the Washington Post. “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.”

There is a cylinder in France. It is a kilogram. More precisely, it is the kilogram which defines what a kilogram is.

And it’s been losing weight mass for reasons that are unclear, perhaps impurities in the metal. Who knows?

But it threw the world into disarray, even if you didn’t realize it.

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

It took 16 months for Pratt and his team to come up with a new way of determining the weight of a kilogram.

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.

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

Weigh to go, humanity!

I have been corrected that the kilogram is a measurement of mass, not weight. Change all puns accordingly. I’m sure they’ll still work.