The 2019 SI Redefinition: How the Kilogram Lost Its Cylinder

For 130 years the kilogram was whatever a single platinum-iridium cylinder kept in a vault at Sèvres, near Paris, happened to weigh. The International Prototype Kilogram was approved by the first General Conference on Weights and Measures in 1889, and every scale in the world was ultimately calibrated against it through a chain of national copies. The trouble was that the copies and the original slowly drifted apart by tens of micrograms, and nobody could say which of them was changing.

The 26th General Conference, meeting in Versailles on 16 November 2018, voted unanimously to end the dependence on artefacts. From 20 May 2019, World Metrology Day, the SI has been defined by seven constants with exact values: the caesium hyperfine frequency of 9,192,631,770 Hz, the speed of light of 299,792,458 m/s, the Planck constant of 6.62607015 × 10⁻³⁴ J·s, the elementary charge of 1.602176634 × 10⁻¹⁹ C, the Boltzmann constant of 1.380649 × 10⁻²³ J/K, the Avogadro constant of 6.02214076 × 10²³ per mole and the luminous efficacy of 683 lm/W.

Four base units changed definition: the kilogram now follows from the Planck constant, the ampere from the elementary charge, the kelvin from the Boltzmann constant and the mole from the Avogadro constant. The second, metre and candela were reworded but in substance unchanged. The values were chosen so that nothing measurably shifted on the day: a kilogram of flour weighed the same on 21 May as on 19 May.

What changed is who can realise the units. Any well-equipped laboratory can now build a Kibble balance, which weighs a mass against electromagnetic force, or count atoms in a silicon sphere, and derive the kilogram without visiting France. The prototype still exists, but it is now simply a very well-characterised lump of metal.