John Pratt doesn't want to tamper with the global system of mass. He wants to revolutionize it.
Pratt is 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 at the institute are part of an international effort to redefine the kilogram on the basis of 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.
The basis for the kilogram has changed over the years. In the 1700s, it was defined as the weight of 1 cubic decimeter of water, the equivalent of 1 liter, at 4 degrees Celsius -- the temperature at which water is most dense, before it expands as it freezes. A permanent kilogram was then made to match that weight.
The kilogram is now based "le grand K" -- a small, platinum and iridium cylinder weighing exactly 1 kilogram, crafted in 1889 to serve as the single standard by which all other kilograms are measured. That kilogram is kept in a temperature-controlled, airtight safe deep in the bowels of an elegant French baroque building that serves as headquarters for the International Bureau of Weights and Measures in Paris. It is retrieved on occasion to test the accuracy of the world's other kilograms.
On June 30, Pratt's team got their most precise measurement ever for a constant that he hopes will replace "le grand K."
"It's not obvious that it's a big deal, but it's a big deal," Pratt said. With the 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 bureau's 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, the U.S. institute had to reissue certificates for its kilograms because they were 45 billionths of a kilogram off the French prototype, about the weight of an eyelash. This meant that companies that produce weights based on the U.S. standards had to reissue their own weights, and they were not happy about it. Lawmakers were called. The National Institute of Standards and Technology was accused of incompetence. In the end, it turned out that the problem stemmed from "le grand K," not the U.S. institute.
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 billionths of a kilogram.
"There's a lot that rides on these sorts of things that people take for granted," Pratt said. "Like breathing."
Many scientists say it's long past time to retire "le grand K." Some joke that using 19th-century technology for 21st-century physics is like trying to get to Mars on a rocket powered by a steam engine.
So in 2014, at the quadrennial General Conference on Weights and Measures, 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 and his colleagues at the U.S. institute have spent much of the past few years trying to come up with a precise, accurate number.
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 was finally ready to release its result. Applying 16 months' worth of measurements, it calculated Planck's constant to be 6.626069934 x 10−34 kg∙m2/s.
The number itself is a big deal, but the most important thing about the U.S. measurement is the uncertainty: just 13 parts per billion. This means that the U.S. scientists think their measurement of Planck's constant is within 0.0000013 of a 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 parts per billion. But with the new U.S. measurement, the world now has at least three experiments below 20 parts per billion. Another was conducted by a Canadian team using a Kibble balance, and the third was conducted by an international group that calculates Planck's 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 the 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.
SundayMonday on 07/16/2017