The kilogram is the only fundamental metric unit based on a physical artifact, but it’s losing weight and scientists are looking for a natural constant.
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.