KILOGRAMS AWEIGH
Saying farewell to physical weight standards
THERE IS A DEPTH TO THIS DECISION THAT GOES BEYOND TIDINESS. IF HUMANKIND CANNOT AGREE ON UNIVERSAL RIGHTS, IT CAN AT LEAST AGREE ON UNIVERSAL AMPERES.
London’s Science Museum is stocked with cabinets of wonder. In its mathematical gallery, though, there is a cabinet of not-needing-towonder. The sets of weights stored in its 71 mahogany drawers like toys in a hyperorganized 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 recognized and accounted for.
The cabinet covers the whole wide world of early19th-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 metre (one 40,000th of the Earth’s circumference) and the kilogram (one 1,000th of the weight of a cubic metre of pure water). Whether it was in weights and measures or the rights of man, such universalism was deeply suspect.
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. 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.
The resulting International System of Units saw the kilogram and the metre returned to scientific and everyday use. In almost all countries — including, since 1963, Britain — the kilogram is the official unit of mass. Only the U.S., Liberia and Myanmar have held out.
But knowing what weighs a kilogram still requires that 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, it 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.
It is the end of an era. 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 metre, 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 the wax from a sperm whale’s head burning at a rate of 120 grains an hour.
All this history and specificity is now washed away. The seven basic units of measurement — the kilogram, second, metre, ampere, candela, Kelvin (temperature) and mole (unit of quantity) — will be defined in terms of constants that should be the same everywhere. Some of these constants play profound roles in physics. Others are arbitrary, like that particular property of cesium atoms that, through its frequency, defines the second. But all of them are universal. 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. 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.
Irish conservative Edmund Burke 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, in favour 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.