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Sensors & Measurement · Recorded milestone

SI redefined on constants

Recorded year2019

The kilogram and other units are fixed to constants of nature.

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Built on

SuperconductivityRecorded · 1911requires
Single-crystal growthRecorded · 1916requires
Atomic clockRecorded · 1955enables
Integrated circuitRecorded · 1958requires
Atomic secondRecorded · 1967enables
Metre defined by lightRecorded · 1983enables

Led to

Second redefined by optical clocksForecast · ~2030enables

Sources & evidence

Additional source

Dataset updated 2026-10-18. This individual record has not been separately re-reviewed.