Once a year, three officials bearing three separate keys meet at the bottom of a stairwell at the International Bureau of Weights and Measures, in Sèvres, France. There they unlock a vault to check that a plum-size cylinder of platinum iridium alloy is exactly where it should be. Then they close the vault and leave the cylinder to sit alone, under three concentric bell jars, as it has for most of the past 125 years.
This lonely cylinder is the International Prototype of the Kilogram, known colloquially as Le Grand K, and it is the last remaining physical object to define a unit of measure. It’s a quaint throwback to a time when people compared the ocean’s depth to the span of a man’s outstretched arms and the second to a tiny fraction of a year. Now we fix our rulers to the speed of light and our clocks to a spectral property of cesium. By thus linking measurement to fundamental and unchanging phenomena, scientists have paved the way for GPS satellites, gravity-wave detectors, and many other precision technologies that simply wouldn’t have been possible before.
The trouble posed by the master kilogram is apparent in the many friction-filled steps by which it calibrates other masses. Once every few decades, a scientist plucks the cylinder from its perch with chamois-leather-padded pincers, rubs its surface with a cloth soaked in alcohol and ether, and steam-cleans it. Then he puts the prototype in a precise balance that compares it to the bureau’s official copies, which are in turn compared to copies kept by member countries. And thus the prototype’s mass trickles down to set the standard for the rest of the world.
The system has been far from seamless. When the cylinder was last removed from the vault in 1988, the bureau’s metrologists were disappointed to discover that its mass and those of its official copies had drifted apart by as much as 70 micrograms since 1889. That discrepancy is tiny—comparable to the mass of a small grain of sugar—but it confirmed a troubling instability. All that metrologists can say is that the master kilogram seems to have lost as much as 50 µg over the course of a century relative to its siblings. But the actual drift could be up or down, and it might even be a lot more than 50 µg, because the prototype and its metallurgically identical copies could all be changing as an ensemble.
“It’s a bit ridiculous in this day and age, because it’s not just the mass that depends on the prototype. It’s all energy, all force, all units that are linked in any way to the kilogram,” retired metrologist Terry Quinn explains one gray afternoon at the bureau, about a week before an international committee was set to convene to decide the kilogram’s fate.
A former director of the bureau (often referred to by its French acronym, BIPM), Quinn has been campaigning since the early 1990s to peg the kilogram to an unchanging aspect of nature. Such a change would be a boon to scientists who depend on stable units to perform long-term measurements, Quinn says. It could also have a big impact on electrical engineering, particularly for makers of precision multimeters and other basic tools.
When he began lobbying, Quinn says he thought it would take just a few years to come up with a better standard. But the effort to develop a precise way to measure mass with respect to a constant of nature has turned out to be phenomenally difficult.
The October meeting marked a big turning point for the kilogram. Delegates from the bureau’s then 55 member countries unanimously agreed on a tentative plan to base the kilogram on a fundamental constant of quantum mechanics. Three other core units—the ampere, the mole, and the kelvin—will likely change at the same time.
This coup is largely the result, after decades of work, of steady strides in two challenging strategies for measuring mass. One approach attempts to pin down the exact electromagnetic force needed to balance the gravitational tug on an object. The other harnesses Cold War–era uranium enrichment technology and a host of experimental techniques to count the number of atoms in extremely round balls of ultrapristine silicon.
For years, the two approaches have produced starkly conflicting results. But over the past few months, metrologists have been excited to find glimmers of convergence, and the effort to pin down mass once and for all is beginning to pick up steam.