Moves Are Afoot to Redefine Measurements; Le Grand K Feels a Wee Bit Lighter
In a vault beneath a 17th-century pavilion on the outskirts of Paris sits a platinum cylinder known as Le Grand K. Since 1889 it has been the international prototype for the kilogram, the standard against which all other kilos are measured. (more after the break)
But over the years, scientists have noticed a problem: Le Grand K has been losing weight. Weigh-ins at the International Bureau of Weights and Measures show that the bar has shed approximately 50 micrograms—roughly equal to a grain of sand.
The problem has vexed scientists who monitor the kilo the way tabloids track the waistlines of Valerie Bertinelli and Kirstie Alley. The stakes, however, are weightier.
"It's a scandal that we've got this kilogram hanging around changing its mass and therefore changing the mass of everything else in the universe!" Bill Phillips, a Nobel Prize winning physicist, exclaimed at a scientific summit in London this week. No one knows for sure what went wrong with Le Grand K, but some theorize it lost weight from being cleaned.
Dr. Phillips and other mandarins of metrology were gathered at Britain's Royal Society to debate an urgent question in the science of measurement—how to re-define the basic unit of mass, as well as other measurements such as the second, ampere, kelvin and mole.
The aim is to tie each to a widely accepted property of nature, rather than to a lump of metal or some other imprecise benchmark. The meter, for instance, was once measured as the distance between two notches on a metal bar. It is now defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
The new definitions are "as big a change as the introduction of the metric system during the French Revolution," says Terry Quinn, a dapper Briton who organized the seminar and once served as director of the International Bureau of Weights and Measures, which ensures world-wide uniformity of measurements. Frequent clashes about the best approach mean the temperature of debate has at times "risen quite high," he added, without specifying by how much.
The new definition of the kilo hinges on first determining an exact value for something called Planck's constant—a fundamental constant of physics, much like the speed of light. Once its value is fixed—which could be years away—scientists will be able to plug Planck's constant into an equation, along with other variables, that will define the kilogram. To calculate the value of Planck's constant, they are using a high-tech laboratory scale known as a watt balance.
Dressed in solar-system ties and Alfred Nobel lapel pins, delegates at the meeting at the Royal Society dissected a clutch of experiments that so far suggest Planck's constant should equal 6.62606896 x 10 to the power of -34 joule seconds. Despite extending to an impressive eight decimal places, the number has been deemed imprecise by some. Others criticize the complexity and expense of using a watt balance.
"This in some ways takes [the kilo] away from the ordinary man...and makes it much less accessible," John Hall, a Nobel Prize winning physicist from Boulder, Colo., warned the gathering.
One scientist from the U.K.'s National Physical Laboratory rose to remind the assembled about the risks of relying on a physical object like Le Grand K to define a measurement. He cited the sad history of the Imperial Yardstick, which was badly damaged by a fire that swept through the Houses of Parliament in 1834, and was only recreated years later. Man-made artifacts, the scientist cautioned, are vulnerable to damage.
Day two of the meeting began at precisely 9 a.m. with a lecture titled: "The Boltzmann Constant From the Speed of Sound in a not Quite Spherical Cavity."
"Thank you," the meeting chairman told the speaker after the 30-minute lecture concluded. "A really clear explanation."
Considering the crowd, other aspects of the meeting seemed inexact. One lecturer admitted to cribbing a definition of stoichiometry from Wikipedia. And when the French astronomer Bernard Guinot asked how much time he had left to conclude his speech, no one seemed to know.
Metrologists are aiming to adopt a framework for redefining the kilo at the General Conference on Weights and Measures, which meets in Paris in October. Much is riding on the outcome: the joule, watt, volt, farad, weber and ohm are only some of the units derived in part from the kilo.
During a tea break at the London meeting, delegates mused on the importance of accuracy. Duncan Burns, a 75-year-old chemistry professor, recalled the time he confused centigrade with Fahrenheit while cooking resins in the laboratory at his first job. "I started a bloody great fire!" he said.
"To the man in the street, these are very fine distinctions," John Everest, an 88-year-old retired mechanical engineer in a tweed suit, said of the meeting's theme. "But they do matter."
Back when Dr. Quinn was director of the International Bureau of Weights and Measures, he held one of three keys needed to open the safe where Le Grand K is stored, under a trio of glass domes resembling a nesting doll. Three different people hold the keys and must be present to open the safe, "like setting off a nuclear warhead," Dr. Quinn chuckles.
Several dozen copies of the original are stored at national laboratories around the globe. Once every 50 years or so, scientists carry the copies by hand to Sèvres, just outside Paris, in little boxes, to compare them to the original. At the most recent summit in 1989, they noticed that the kilos differed by an average of about 50 micrograms. This is often described as Le Grand K losing mass, though to be precise, it's possible that the copies had gained mass.
Putting a stop to such vagary is what the new definitions are all about. But when one journalist asked whether they would help bring stability to an unstable world, Dr. Quinn paused and shook his head: "We have big ambitions, but not as big as that."
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