11. The Last Artefact — The Day the Kilogram Stopped Being an Object

A lump of metal in a vault outside Paris

Until 2019 the kilogram was a cylinder of platinum-iridium held at the International Bureau of Weights and Measures at Sèvres. It was the kilogram by definition: whatever it weighed was one kilogram, and a scratch would have redefined the unit rather than damaged it. Official copies went to member states and were periodically brought back to be compared against the prototype, which is how the problem was found. Across the twentieth century the copies and the prototype drifted apart by some tens of micrograms, a divergence documented in the 2015 Metrologia report on the calibration campaign. Nobody could say which had changed, because there was no reference outside the family to appeal to. A unit defined by an object is a unit that can be dropped, and it is the last kind of standard any system should want.

Fixing a constant instead of keeping a thing

On 20 May 2019 the revised SI took effect, and the kilogram was redefined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule seconds. The ampere, the kelvin and the mole were redefined in the same revision, by fixing the elementary charge, the Boltzmann constant and the Avogadro constant respectively. After that date every base unit in the SI is defined by a constant of nature or an atomic property, and no artefact anywhere is the standard for anything. In practice a kilogram is now realised with a Kibble balance, weighing a mass against an electromagnetic force measurable in terms of Planck's constant, or by counting atoms in a pure silicon sphere. The prototype at Sèvres still exists and is still measured; it has no authority any more, and its mass is now a quantity with an uncertainty like anything else. The switch was engineered so no kilogram changed value on the day, which is why nothing appeared to happen.

What the chain of these chapters was for

Read together, the measurements in this book describe a single movement: from objects, to surveys, to constants. The size of the Earth came from a shadow, the metre from a survey containing a hidden error, the second from an atom, and finally the kilogram from a number that cannot be scratched or lost. Each step removed a dependency on something that could quietly change while nobody watched, which is the only real definition of progress in metrology.

Example: A recipe calling for "a cup" depends on your cupboard; one calling for 240 millilitres does not care whose kitchen it is in.

Verdict. Since 20 May 2019 no SI base unit depends on a physical object, the kilogram having been redefined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ J·s.

Overturned if: independent Kibble balance and silicon-sphere realisations of the kilogram disagree beyond their stated uncertainties.

Sources. 2015 · "Calibration campaign against the international prototype of the kilogram," Metrologia 52(2):310 — IOPscience · 2019 · "2019 revision of the SI" — Wikipedia