Sci­en­tists may change what a kilo­gram is

The Denver Post - - NEWS - By Sarah Ka­plan

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.

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