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Lundi, 10 Octobre 2011 12:30

The Search for a More Perfect Kilogram

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The perfect kilogram is getting lighter. Can science find a better measure?
Photo: Christopher Griffith; kilogram models by Jim Zivic

The official US kilogram — the physical prototype against which all weights in the United States are calibrated — cannot be touched by human hands except in rare circumstances. Sealed beneath a bell jar and locked behind three heavy doors in a laboratory 60 feet under the headquarters of the National Institute of Standards and Technology 20 miles outside Washington, DC, the shiny metal cylinder is, in many ways, better protected than the president.

“Everything is a potential contaminant,” says Patrick Abbott, a NIST physicist responsible for maintaining it. “There are hydrocarbons on people. There’s water in the air.”

The American prototype is one of some four dozen such national standards around the world, and each of those, in turn, is accountable to an even higher authority: a regal artifact called the international prototype kilogram. Familiarly known as Le Grand K and held in a vault just outside of Paris under three bell jars, it dates back to the 1880s, when it was forged by the British metallurgist George Matthey from an alloy of nine-tenths platinum and one-tenth iridium. As a metric unit, the kilogram is “equal to the mass of the international prototype,” according to the official definition. In other words, as metrologists like to point out, it has the remarkable property of never gaining or losing mass. By definition, any physical change to it alters the mass of everything in the cosmos.

Aside from a yearly ceremonial peek inside its vault, which can be unlocked only with three keys held by three different officials, the prototype goes unmolested for decades. Yet every 40 years or so, protocol requires that it be washed with alcohol, dried with a chamois cloth, given a steam bath, allowed to air dry, and then weighed against the freshly scrubbed national standards, all transported to France. It is also compared to six témoins (witnesses), nominally identical cylinders that are stored in the vault alongside the prototype. The instruments used to make these comparisons are phenomenally precise, capable of measuring differences of 0.0000001 percent, or one part in 1 billion. But comparisons since the 1940s have revealed a troublesome drift. Relative to the témoins and to the national standards, Le Grand K has been losing weight — or, by the definition of mass under the metric system, the rest of the universe has been getting fatter. The most recent comparison, in 1988, found a discrepancy as large as five-hundredths of a milligram, a bit less than the weight of a dust speck, between Le Grand K and its official underlings.

This state of affairs is intolerable to the guardians of weights and measures. “Something must be done,” says Terry Quinn, director emeritus of the International Bureau of Weights and Measures, the governing body of the metric system. Since the early 1990s, Quinn has campaigned to redefine the kilogram based not on a physical prototype but on a constant of nature, something hardwired into the circuitry of the universe. In fact, of the seven fundamental metric units — the kilogram, meter, second, ampere, kelvin, mole, and candela — only the kilogram is still dependent on a physical artifact. (The meter, for example, was redefined 30 years ago as the distance traveled by light in a given fraction of a second.)

Two different approaches to linking the kilogram to a fundamental constant are in the works, but both have proven far more complicated than in the case of the meter. Borrowing tricks from quantum mechanics and techniques used to manufacture atomic bombs, the competing initiatives are finally on the verge of delivering the kind of precision necessary to displace Le Grand K. In anticipation of that achievement, the General Conference on Weights and Measures will vote this month on a proposal to redefine the kilogram based not on a physical artifact but on a fundamental constant. Approval requires a majority of the 55 member states assembled in Paris to vote for the proposal.

The outcome of the ballot is anything but certain. Many metrologists accustomed to venerating the platinum-iridium cylinder are wary of change. “The best thing is to wait,” Abbott says. But as the technologies needed to realize the two competing definitions have matured, Quinn has gained the support of influential scientists such as physicist Barry Taylor of NIST and Nobel Prize-winning physicist Bill Phillips. If the idea of a fundamental constant wins approval, Le Grand K will be on its way to becoming nothing more than a $56,000 hunk of metal.

No one can say for sure why the prototype and its brethren are drifting apart. One rather obvious possibility, suggested by Taylor, is that the national prototypes and even the témoins have been used more often than Le Grand K, which has been handled only three times since 1889. The handling could subtly contaminate the surface. A more exotic theory posits that slight variations in Matthey’s alloy lead to different rates of outgassing, the technical term for the gradual escape of gases trapped in the metal. Whatever the explanation, the divergence is problematic, and not only for theoretical reasons. In fields ranging from particle physics to global commerce, the erratic behavior of the master kilogram shows that a system of measurement based on a physical artifact can’t be trusted. “This is simply not a satisfactory situation,” Quinn says. “You have an object made with the technology of the 19th century upon which a very large proportion of modern measurements are based — not just mass, but electrical measurements and measurements of force and heat and light.” The metric energy unit known as the joule, for example, is defined in terms of the work needed to move a 1-kilogram mass a given distance over a given time period. And the luminosity of light, or candela, is measured in terms of power, designated in watts, or joules per second. In other words, if the kilogram is unreliable, the joule and the candela become unreliable as well. Nobody at the grocery store is fretting over whether a kilo of bananas is a speck of dust lighter or heavier than in their great-grandparents’ era, but the change could eventually matter enormously to engineers optimizing computers and fiber-optic networks.

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