## Weighty matters

How much does a kilogram weigh? Well, it depends. Weight is merely a measure of mass―the force exerted by gravity―and it turns out that can vary, even here on Earth. So it’s best to go straight to the standard reference: you’ll find it in a heavily guarded vault just outside Paris, at the International Bureau of Weights and Measures in Sèvres. Inside, you’ll see a platinum and iridium cylinder that represents, as best we can determine, what a kilogram actually weighs. Except that it doesn’t.

That cylinder has been there now for 126 years, and like any object, it’s been shedding mass all that time. Since 1889, it’s shed 60 micrograms.

Thankfully, we’ve got our best people working on it, and they’re called metrologists. For them, measurement is pure passion, and they’ve been unhappy with the flagging performance of the cylinder—“le Grand K”—for decades. It’s the only remaining Système International d’Unités reference unit based on a physical object, and since the 1970s, teams have been collecting data, building a case for a new, mathematically expressed kilogram.

Come 2018, the kilo will come courtesy of Planck’s constant, which relates a particle’s energy to its frequency. Then, we turn to Einstein and apply his exalted equation—E=mc^{2}—to link them both to its mass. But the metrologists’ quest down the decades has been to first establish the base Planck’s value, as best they can, from an old, decaying lump of metal.

In October, they reported in Nature that they finally had the numbers—arrived at through two different methods—sufficiently accurate and agreeable to consign le Grand K to the museum. The new formula won’t make the definition of a kilogram any more precise, but at least it’ll stay constant, and you’ll be able to work out your weights anywhere on the planet without having to go to Paris to check them. Once the kilo has been tamed, the metric system’s seven base units—mass (kilogram), distance (metre), time (seconds), electric current (ampere), temperature (Kelvin), substance (mole) and luminosity (candela)—will all be based on universal constants of nature.

There is, believe it or not, an International Committee for Weights and Measures (CIPM), which presides over such, er… weighty matters, and it decided by draft resolution to revamp a whole suite of other standard units as well by 2018. The catch was that the new kilogram constant had to be derived by at least three separate methods, and two of them had to agree within tight parameters.

Perhaps only a metrologist could describe this, as did David Newell, a physicist at the US National Institute of Standards and Technology (NIST), as “an exciting time”. He painted a picture of electric tension as factions in the CIPM locked calipers over increments. Some philistines suggested simply averaging measurements from the two different calculation methods. “I think every metrologist worried, ‘What if they never converge?’” Davis told *Nature*. We should be grateful there are trained professionals paid to fret about this while we sleep oblivious.

So how did they do it? One team at the German National Metrology Institute (PTB) in Braunschweig, known as the Avogadro Project, counted all the atoms in two silicon-28 spheres that each weighed the same as le Grand K. That tally gave them a value for Avogadro’s constant, a formula that relates molar mass (the amount of substance as measured by one mole) proportionally to mass. Once they had that, it was a relatively easy case of converting the Avogadro’s value into a Planck’s constant value.

Meanwhile, a separate team at Ottawa’s Measurement Science and Standards laboratory purchased a watt balance—a meter that measures an object’s weight by the strength of an electromagnetic force fed through it. They weighed a test mass calibrated to le Grand K, and derived their Planck’s constant from those results. But then a bombshell: they didn’t match the Avogadro Project’s results. Newell, who chairs the Committee on Data for Science and Technology group on fundamental constants, described a trudge back to a certain drawing board: “We brought in a whole new research team. We went over every component, went through every system.” The anomaly was never solved, but late in 2014, everything, as sometimes happens in the world of physics, suddenly coalesced: the teams achieved a match.

Meanwhile, relative uncertainties had been pared down to the exacting levels demanded by the CIPM. We have a new kilogram—down to the nearest 12 parts per billion.

Or do we? For the Avogadro team, it seems, Planck’s constant is anything but. They cannot live under such a thunderous cloud of uncertainty. They mean to use every month until a July 2017 CIPM deadline to pin it down still further. In a relentless quest for exactitude, they mean to buy new spheres from Russia for experiments that will probe ever deeper into the nanoscopic depths of metrological arcana. Are they mad? What if the results divaricate again? They could unwittingly destroy the fragile balance, so laboriously construed, that is the kilogram. “Then we would be in trouble,” allowed Joachim Ullrich, president of the PTB.

Taking this new wizard’s broomstick to the kilogram could upset the very fundament of modern civilisation: grocers everywhere would be oversold into insolvency. Weightwatchers, disoriented, would hurl down their medicine balls in frustration. Merely exhilarating for now, bungy jumping would be downright reckless. Nobody would cross a bridge. High rises would ring to the alarms of overloaded elevators.

After all, as John Quincy Adams told the US Congress in 1821, “Weights and measures may be ranked among the necessaries of life to every individual of human society… The knowledge of them, as in established use, is among the first elements of education, and is often learned by those who learn nothing else, not even to read and write.”

In metrologists, it seems, we can only trust…