The KILOGRAM rose from the ashes of the French Revolution. Now it’s been REBORN

THE REPUBLIC OF SCIENCE ADOPTS A NEW GOD

The summer of the New Kilogram has ended in apparent success, according to a press release from CERN, the famous European Organization for Nuclear Research. Scientists appear to have met the standards set out for a total redefinition of the kilogram — the last fundamental unit in the worldwide Système International (SI) that had been ultimately defined with reference to a single, specific physical item.

This marks, in a sense, the completion of the universalizing dream of the metric system, born in the fires and bloodshed of the French Revolution.

It is the capstone of a vault representing centuries of effort, largely by anonymous persons — an unspeakably beautiful cathedral of measurement.

We will now have a complete system of weights and measures so abstract that it could be re-established, with all the essential quantities remaining the same, by humans or other sentient creatures on another world starting from scratch.

The metric system began as a mix of definitions depending on natural quantities that seemed, within the measurement capabilities of the 18th and 19th centuries, to be universally reproducible. This is a precondition for scientific dialogue between peoples: if you are going to measure things in inches or newtons or degrees Celsius, and talk about it, you have to have strong agreement on what exactly those things are. So the metre was originally defined as one ten-millionth of the distance between the North Pole and the Equator — measured through Paris, bien sûr. The second was just one 86,400th of an Earth day.

In the 20th century, we apes got so good at measurement that the uncertainty and variations in these grubby natural quantities began to be a problem. It turns out, for example, that the eccentricity of the Earth’s orbit makes the length of the solar day vary within almost a full minute on different dates — and even the so-called mean solar day is changing from century to century.

Designers of weights and measures began to search for more fundamental, unchanging definitions of units. “Invariant quantities of nature” is the fancy phrase.

In the SI, the second is now defined with respect to the wavelength of radiation from a cesium atom (in a particular quantum-mechanical state and at a particular temperature). In practice no one measures seconds by borrowing a cup of cesium from a neighbour, but in principle we have a “second” that is the same throughout the universe, and that could be reconstructed independently.

So too with the other units — except for the ugly black sheep of the SI family: the fundamental unit of mass, the kilogram. A kilogram has been universally defined, since 1889, as a mass equal to the kilogram — which is just a cylinder of metal sitting in a vault in a suburb of Paris. The kilogram is our last physical measure based on what the boffins distastefully call an “artefact” — a thing that can be dropped, or stolen, or destroyed.

This is obviously unacceptable, even embarrassing. But how does one identify an “invariant quantity of nature” to serve as a standard of mass without just choosing some other object?

Atoms will not do. If you know physics, you know the answer is going to have to depend on the lumpy quantum nature of the universe.

Einstein taught us that mass and energy are the same thing. Mass-energy has a consistent smallest possible quantity, defined by the Planck constant, h. If we can measure the Planck constant to a great degree of accuracy — and only if we can do that — we can write down a definition of the kilogram that is truly invariant.

To make a long story not nearly as short as a good writer would: this set off a sort of worldwide Planck challenge. The committee that tends the SI demanded independent measurements of the Planck constant agreeing to within 50 parts per billion. I could do another whole column about how insanely difficult and Olympian this criterion is. To take just one example, the labs in which they do these measurements have to worry about, estimate, and account for the gravitational effects of the subterranean water table underneath their particular building.

Canada’s National Research Council, home to an instrument called a “Kibble balance,” played a pivotal role in the process. (The balance was loaned to the NRC by the British lab at which the eponymous Kibble (19382016) worked. It now seems certain that the committee’s test of independent reproducibility has been met, and that the SI will soon be updated with a genuinely invariant definition of mass.

This makes me wonder about the fate of the old standard Kilogram, that little phallic platinum-iridium totem in Paris. No doubt it will keep being used as a practical working standard for mass measurements: it has, after all, descendant copies around the world which serve national agencies for weights and measures. But there has always been a vaguely sacred aura around the One True Kilogram. Journalistic visitors sense that it is the Ark-like heart of a cult of standardization.

Perhaps it should be made the basis of a monument, or even placed on public display in a sort of temple. The true religion, after all, is the one we profess without question. Anyone taking a first-year science course has genuflected unthinkingly at the altar of the SI — and the Kilogram is its fallen god.

The original kilogram is kept under guard, and glass, in France, with descendant copies in various other parts of the world.