Missing Micrograms Set a Standard

“Your guess is as good as mine,” said Dr. Terry Quinn, emeritus director of the International Bureau of Weights and Measures in this town on the fringes of Paris. It is here that the kilogram — the universal standard against which all other kilograms are measured — resides in controlled conditions set out in 1889, in an underground vault that can be opened only with three different keys possessed by three different people. The change, discovered when the prototype was compared with its official copies, amounts only to some 50 micrograms, equal to the mass of a smallish grain of sand. But it shows that the prototype has fallen down on its primary job, to be a beacon of stability in a world of uncertainty. And it means, scientists say, that it is time to find a new way to calculate the kilogram, which currently enjoys a delightfully frustrating definition: “a unit of mass equal to the mass of the international prototype of the kilogram.” The idea would be to base the future kilogram on a fundamental physical constant, not an inconstant object, said Dr. Peter J. Mohr, a theoretical physicist at the National Institute of Standards and Technology in Gaithersburg, Md. “We want to have something that’s not changing, so that we can have a stable system of measurement,” he said. The kilogram is the last base unit of measurement to be expressed in terms of a manufactured artifact. (Its cousin, the international prototype of the meter, was retired from active duty in 1960, when scientists redefined the meter. They redefined it again in 1983; a meter is now officially “the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second,” for those who would like to try it at home.) Scientists now have similarly bold plans for the kilogram, and indeed for several other base units of measure. A draft resolution to be considered at the General Conference of Weights and Measures in October includes new and improved definitions for the ampere, the mole and the candela. “This would be the biggest change in metrology since the metric system was introduced during the French Revolution,” Dr. Quinn said. Which is all very exciting and very revolutionary. But it is easier said than done. The proposed new kilogram definition is based on a physical quantity known as Planck’s constant — a constant beloved by quantum physicists but not yet as precisely expressed as it might be. Half a dozen teams around the world have been toiling for years to measure Planck’s constant to an acceptably low degree of uncertainty. A resolution could take 5 to 10 more years, or maybe not, said Prof. Michael Kühne, the current director of the measures bureau. “While everyone hopes the experiments will yield excellent results, I don’t have a crystal ball.” None of this is meant to denigrate the ur-kilogram, still resting in its safe, beneath three cheese-plate-style bell jars. Until a new definition takes effect, the prototype remains the Platonic ideal — so precious that it is has been removed from the safe only three times in its life (to be measured against all the copies), so singular that the French call it Le Grand K, and so iconic that writers of scientific papers sometimes designate it simply by the Gothic letter K. “Despite all its shortcomings, the reason it hasn’t been redefined before now is that nobody has come up with something better,” said Professor Kühne, who has a fetching model of the prototype in his office. (He also has one of the keys to the safe, which he keeps in a different safe. The second key is held by the president of the International Committee for Weights and Measures; the third is in the French National Archives.) There are about 100 working copies of the international prototype in countries around the world. These are periodically brought back to Sèvres to be compared with the original. This is a fraught and delicate undertaking. Because of a legendarily horrifying incident in which one of the national kilograms was wrested from its casings by a customs agent and exposed to a hostile environment teeming with airborne detritus, not to mention the agent’s organic matter, countries are advised to ship their kilograms in diplomatic pouches. Dr. Mohr and his colleague Dr. David Newell, who was recently charged with escorting the American kilogram to Sèvres, opted for a backpack and an official Do Not Touch Our Kilogram letter from the standards and technology institute. They made it through, after some harrowing moments. “At one point, there were about a dozen people standing around the screen looking at it,” Dr. Mohr said of the kilogram. “Of course, it’s platinum, so the X-rays can’t go through it, and you can’t see inside it.” The most obvious argument for the prototype’s eventual obsolescence is the tautological underpinning of its existence, which calls to mind the question, “How long is a piece of string?” Because a kilogram is defined as whatever the mass of the prototype is, it does not, for definitional purposes, matter if the prototype loses mass, or indeed packs on the micrograms by spending all its free time gorging on éclairs: it is still a kilogram. Like Norma Desmond in “Sunset Boulevard,” the prototype could plausibly argue that it has not gotten lighter — the other kilograms have gotten heavier. (That hypothesis is theoretically possible — these questions are all relative — but highly unlikely, scientists say.) The new definition should render unnecessary such frustrating intellectual exercises. Even still, it is a little sad to contemplate the demotion of the prototype, which has served so valiantly for so many years and which now seems destined to spend its retirement on a shelf somewhere, its glory days behind it.