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| 2019 redefinition of SI | |
|---|---|
| Name | 2019 redefinition of SI |
| Date | 16 November 2018 (CGPM vote), effective 20 May 2019 |
| Location | International Committee for Weights and Measures / General Conference on Weights and Measures meetings, Bureau International des Poids et Mesures |
| Significance | Redefined seven base units by fixing values of physical constants |
| Participants | International Bureau of Weights and Measures, General Conference on Weights and Measures, International Committee for Weights and Measures, national metrology institutes such as National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Physical Laboratory (United Kingdom) |
2019 redefinition of SI
The 2019 redefinition of the International System of Units was a global overhaul that replaced artefact-based definitions with definitions tied to invariant constants of nature. It established exact numerical values for several fundamental constants, thereby redefining the seven base units used worldwide in science, industry, and commerce. The change was the culmination of decades of research led by national metrology institutes and international organizations aiming to improve long-term stability, universality, and precision.
Efforts leading to the redefinition involved sustained work by Bipm-affiliated committees, collaborations among National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, and Laboratoire national de métrologie et d'essais, and inputs from scientific bodies including the International Union of Pure and Applied Physics and the International Union of Pure and Applied Chemistry. Historical triggers included limits of the International Prototype of the Kilogram, concerns raised by comparisons among national prototypes in the Bureau International des Poids et Mesures intercomparisons, and advances in quantum electrical standards exemplified by the Josephson effect and the quantum Hall effect. The transition built on preceding milestones: the 1967 redefinition of the second using the cesium standard, the adoption of the metre based on the speed of light in 1983, and implementations of the avogadro project and watt balance experiments at institutes such as NPL and PTB.
The redefinition fixed exact values for the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), thereby redefining the kilogram, ampere, kelvin, and mole. The kilogram ceased to be defined by the International Prototype of the Kilogram and instead is defined via the value of h, linking mass to electromagnetic and quantum standards such as the Kibble balance (formerly watt balance) and XRCD experiments like the Avogadro project. The ampere was redefined through the fixed value of e, aligning electrical units with the Josephson effect and quantum Hall effect realizations used at National Institute of Standards and Technology and Physikalisch-Technische Bundesanstalten. The kelvin was redefined by fixing k, connecting temperature to energy via statistical mechanics principles developed since work by Ludwig Boltzmann. The mole was redefined by fixing NA as an exact count, decoupling it from the kilogram artifact and aligning it with chemical metrology practices used in laboratories such as the Max Planck Institute for Chemistry and industrial chemical standards bodies.
Linking units to fundamental constants enhanced traceability and reduced dependence on single artifacts, impacting precision measurements in fields ranging from quantum information and particle physics to thermodynamics and analytical chemistry. Metrologists benefited from improved consistency in electrical metrology using quantum standards deployed at institutions including Fermilab and CERN laboratories conducting precision tests. High-precision mass metrology incorporating Kibble balances enabled new comparisons between mass and Planck-scale calibrations relevant to experiments at LIGO and tests of fundamental symmetries pursued at facilities such as TRIUMF and Lawrence Berkeley National Laboratory. Temperature measurements tied to k supported low-temperature research at National Institute of Standards and Technology and cryogenic experiments at Brookhaven National Laboratory. The redefinition also influenced industrial calibration in sectors represented by organizations like International Organization for Standardization and International Electrotechnical Commission.
National metrology institutes coordinated phasing-in protocols through the Bureau International des Poids et Mesures and regional bodies such as the European Association of National Metrology Institutes, Asia Pacific Metrology Programme, and the Inter-American Metrology System. Laboratories performed primary realizations using devices including Kibble balances, dielectric spheres from XRCD projects, and acoustic gas thermometers developed at NPL and PTB. Technical guides, calibration services, and new standard operating procedures were published by institutes such as NIST and PTB and disseminated via conferences hosted by CGPM and workshops at the International Committee for Weights and Measures. Industries relying on legal metrology—banks of scales, semiconductor fabs like TSMC, and pharmaceutical labs—received transition guidance from national institutes and standardization bodies.
The change required diplomatic and technical consensus achieved at sessions of the General Conference on Weights and Measures where member states represented by national delegations voted to approve fixed values proposed by the International Committee for Weights and Measures. Negotiations drew on evidence from interlaboratory comparisons coordinated by the BIPM and expert review panels including representatives from CIPM-affiliated consultative committees. The 26th CGPM session formalized acceptance after prior resolutions and recommendations emanating from conferences and workshops involving stakeholders such as European Commission metrology advisers and national standards agencies.
The redefinition prompted updates to curricula at universities like University of Oxford, Massachusetts Institute of Technology, University of Tokyo, and ETH Zurich, requiring revisions to textbooks authored by authors affiliated with Cambridge University Press and Springer Nature publishing programs. Laboratory training incorporated quantum electrical standards, Kibble balance demonstrations, and uncertainty analysis practices aligned with the Guide to the Expression of Uncertainty in Measurement used worldwide. Metrology practice shifted toward increased reliance on quantum and fundamental-constant realizations at national institutes, regional calibration chains, and commercial calibration services used by industries such as Boeing, Siemens, and Pfizer.