Why we need SI — and what it would cost to abolish it
A short history of measurement, the French Revolution, and the modern standards regime.
Before the metric system was forced on France in 1795, every region had its own units. An "aune" in Alsace was different from an "aune" in Lyon. The 18th-century astronomer Jean Picard proposed a universal unit based on a pendulum, but it wasn't politically feasible until after the revolution.
Napoleon spread the metric system across Europe. The result was the metre, the gram, and eventually the litre (and second, mole, candela, kelvin, ampere).
The SI system was codified in 1960 by the Bureau International des Poids et Mesures (BIPM). Since 2019, all seven base units are defined by physical constants:
- Kilogram — Planck's constant - Metre — speed of light - Second — hyperfine transition of cesium-133 - Ampere — elementary charge - Kelvin — Boltzmann constant - Mole — Avogadro constant - Candela — luminous efficacy of 540 THz radiation
This means a kilogram in Paris is the same as one in Beijing. The standard doesn't depend on an artefact that could be destroyed. A future civilisation could re-derive the kilogram from the definition, even if every physical reference was lost.
Most countries now use SI for science, industry, and trade. The US is the major outlier.