London’s Science Museum is amply stocked with cabinets of wonder. In its mathematical gallery, though, there is a cabinet of not-needing-to-wonder. The sets of weights stored in its 71 mahogany drawers like toys in a hyper-organized toy box provided Lord Castlereagh, Britain’s foreign secretary, with welcome certainty about what weighed what around the world. They had been furnished, at Castlereagh’s command, by British consuls abroad, so that what counted as a libra in Rio de Janeiro could be compared with a funt from St. Petersburg or a pound from Philadelphia; and the discrepancies between them — in ounces, zolotniks and onças — recognized and accounted for.
The cabinet is a doubly worldly object. It covers the whole wide world of early-19th-century trade, and it evinces a worldly acceptance of human disparity and vagary. Not for Castlereagh the idealism of trying to set up a single global system of measurement, as revolutionary France had done with its invention of the meter (one 40,000,000th of the Earth’s circumference) and the kilogram (one 1,000th of the weight of a cubic meter of pure water). Whether it was in weights and measures or the rights of man, such universalism was deeply suspect. That His Majesty’s government should know for sure what the foreigners were talking about was quite sufficient.
In the 200 years since, though, the world has slowly come to prefer highfalutin’ Gallic universalism to pragmatic British interoperability. When scientists measured the same things as tradesmen — lengths, breadths, weights and the like — they could make use of the customary units that came to hand (some of which, like the span, were actually based on the hand). But once they started to measure things beyond the immediate realm of the senses, such as electric charges and magnetic moments, they needed units of their own.
These were developed systematically, and the resulting International System of Units saw the kilogram and the meter, which had gone into brief eclipse after the restoration of the French monarchy, returned to scientific and everyday use. In almost all countries — including, since 1963, Britain — the kilogram is the official unit of mass. Only the United States, Liberia and Myanmar have held out.
But knowing what weighs a kilogram still requires that, like one of Castlereagh’s civil servants, you have some physical basis of comparison. In the case of the kilogram, the ultimate comparator has, for more than a century, been a precisely machined archetype made of platinum and iridium and housed in a vault in Paris, “Le Grand K.”
On Friday, this ended, as the International Committee for Weights and Measures abandoned Le Grand K. From now on, the committee will instead define mass purely in terms of the frequency of a particular resonance in cesium atoms and two universal constants: Planck’s constant, ubiquitous in quantum mechanics, and the speed of light.
This does not just mean that that singular lump of platinum and iridium can finally lay down the burden of being the most precisely calibrated burden there is and move on to a comfortable retirement in a suitable museum. It is the end of an era more generally. The Paris kilogram was the last link between the way that humankind — or at least the scientific and legalistic bits of it — measures the universe and any individual objects within it.
Once there was a specific physical standard for the meter, too — a rod of metal that neither expanded nor contracted. Other units were defined in more general but still specifically earthly ways. The second was one 86,400th of a day — a period of time that astronomers calculated obsessively. More reconditely, the candlepower, a predecessor to the candela, the unit by which the brightness of light is now measured, was defined until the first half of the 20th century in terms of the light produced by a candle made from pure spermaceti — the wax from a sperm whale’s head — burning at a rate of 120 grains an hour. The grain, in its turn, was a measure of mass dating back to what King Offa (an Anglo-Saxon monarch better known for constructing defenses against the marauding Welsh) reckoned a grain of wheat weighed.
All this history and specificity is now washed away. The seven basic units of measurement from which all other units of measurement are derived — the kilogram, the second, the meter, the ampere (electric current), the candela, the Kelvin (temperature) and the mole (the unit of quantity) — will be defined in terms of constants that should be the same everywhere in the universe. Some of these constants play profound roles in physics: As well as Planck’s constant, there is Boltzmann’s constant, which crops up in the laws of thermodynamics, and the speed of light in a vacuum, a quantity so central to relativity that it might as well be called as Einstein’s constant. Others are arbitrary, like that particular property of cesium atoms that, through its frequency, defines the second. But all of them are universal, the same on a planet orbiting Proxima Centauri as they are on Earth. No standards in vaults need be consulted; no sperm whales need give up their wax.
There is a depth to this decision that goes beyond metrological tidiness. If humankind cannot agree on universal rights, it can at least agree on universal amperes, defined only by the behavior of any cesium atom and the charge on any electron. It is hard not to think us a little grander as a result. But also a bit more exposed. In dispensing with the props of specifically earthly measures, we face the universe on its own terms.
Edmund Burke, an Irish conservative of the generation before Castlereagh’s, understood the sublime as the apprehension of nature’s fearful power from a vantage point of personal safety. The feeling some adults experience on the death of their parents — a sense of being, finally and irrevocably, on the front line and of looking eternity, or its absence, in the face — is a form of that sublime. And there is something of the same feeling in abandoning measures that have stories, measures that can be secured in comforting boxes, old measures from and of the human world, in favor of the unyielding, inhuman constants of the uncaring universe.
It is a small change in outlook — you would be hard put to quantify it, whatever units you used. But perhaps you can feel it.
[CORRECTION: A previous version of this story said that a meter was one-forty-thousandth of the earth’s circumference. It is actually one-forty-millionth.]