How Units Are Defined
Pick one of the 7 SI base units and meet the constant of nature that defines it — since 2019, even the kilogram is just Planck's constant.
The Kilogram is defined by Planck constant.
Since the 2019 SI redefinition, all 7 base units are anchored to the exact values of 7 constants of nature — no physical artifacts required.
Pick a base unit
Kilogram
Measures: Mass
The kilogram is defined by fixing the Planck constant h at exactly 6.626 070 15 × 10⁻³⁴ joule-seconds. With the metre and second already fixed, this pins the kilogram with no physical object — realised by a Kibble balance.
The kilogram was the mass of the International Prototype Kilogram, a single platinum-iridium cylinder kept near Paris since 1889 — and it was slowly drifting from its copies.
This unit was materially redefined in 2019 — its old physical reference was retired in favour of a fixed constant of nature.
Every measurement you have ever made rests on seven base units: the second, metre, kilogram, ampere, kelvin, mole, and candela. For most of history these were tied to physical things — a brass bar, a platinum cylinder, a swing of a pendulum. But on 20 May 2019 the world changed how it defines them: each base unit is now pinned to the exact, unchanging value of a constant of nature. The kilogram is fixed by Planck's constant h, the second by the cesium atom, the metre by the speed of light. No object can be lost, scratched, or drift — the units are written into the fabric of the universe itself.
Sources
- SI Brochure, 9th ed. (BIPM, 2019) — the 7 defining constants
- bipm.org — SI Brochure
- nist.gov — SI redefinition
- en.wikipedia.org — 2019 revision of the SI
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