The revision of the SI—the result of three decades of progress in metrology

TLDR

The SI was revised on 16 November 2018 at the General Conference on Weights and Measures. The new SI definitions leave realization methods open, enabling future development of more accurate measurement techniques. The revision links base units to defined constants, updates the physical concepts of the kilogram, ampere, kelvin, and mole—eliminating the artefact‑based kilogram—and connects atomic‑scale constants to macroscopic measurements.

Abstract

On 16 November 2018 a revision of the International System of Units (the SI) was agreed by the General Conference on Weights and Measures. The definitions of the base units were presented in a new format that highlighted the link between each unit and a defined value of an associated constant. The physical concepts underlying the definitions of the kilogram, the ampere, the kelvin and the mole have been changed. The new definition of the kilogram is of particular importance because it eliminated the last definition referring to an artefact. In this way, the new definitions use the rules of nature to create the rules of measurement and tie measurements at the atomic and quantum scales to those at the macroscopic level. The new definitions do not prescribe particular realization methods and hence will allow the development of new and more accurate measurement techniques.