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| 2019 redefinition of SI base units | |
|---|---|
| Name | 2019 redefinition of SI base units |
| Date | 16 November 2018 (CGPM vote), effective 20 May 2019 |
| Location | Paris, International Bureau of Weights and Measures |
| Authority | General Conference on Weights and Measures |
| Outcome | Redefinition of the kilogram, ampere, kelvin, mole |
2019 redefinition of SI base units was an international revision that redefined four of the seven SI base units by fixing the numerical values of fundamental constants. The change, approved by the General Conference on Weights and Measures and coordinated by the International Bureau of Weights and Measures (BIPM), moved metrology away from artefact-based standards toward constant-based definitions. The reform was the culmination of decades-long work by national metrology institutes such as National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, and National Physical Laboratory (United Kingdom).
Before 2019, several SI units relied on physical artefacts or empirical realizations such as the international prototype kilogram (IPK) kept at the International Bureau of Weights and Measures in Sèvres, France. Key historical milestones include the creation of the Metre Convention in 1875, the establishment of the International System of Units in 1960, and prior redefinitions like the 1983 metre based on the speed of light fixed by General Conference on Weights and Measures. Influential figures and institutions in the buildup included researchers at Bureau International des Poids et Mesures, scientists such as Kirchhoff-era experimenters, and modern teams at LNE and NIST. International bodies involved encompassed the International Committee for Weights and Measures (CIPM) and regional metrology organizations like EURAMET and APMP.
The 2019 reform fixed exact numerical values for the Planck constant h, the elementary charge e, the Boltzmann constant k, and the Avogadro constant NA, thereby redefining four base units: - Kilogram: defined via the fixed value of the Planck constant h; realization methods include the Kibble balance (formerly watt balance) and the Avogadro project using silicon spheres developed by teams at PTB and NMIJ. - Ampere: defined via the fixed value of the elementary charge e; practical realizations involve single-electron tunnelling devices and quantum electrical standards like the quantum Hall effect and Josephson effect observed in experiments at CERN-affiliated labs and NPL. - Kelvin: defined via the fixed value of the Boltzmann constant k; primary thermometry techniques include acoustic gas thermometry and dielectric-constant gas thermometry used by NIST and LNE. - Mole: defined via the fixed value of the Avogadro constant NA, decoupling the mole from any specific mass artefact and aligning chemical amount with a fixed integer of elementary entities; institutions such as IUPAC and IUPAP engaged in related discussions. The metre, second, and candela remained defined by the speed of light, the caesium hyperfine transition frequency, and luminous efficiency respectively, grounded in laboratory realizations at places like BIPM and NIST.
The rationale combined scientific, practical, and stability arguments: constants-based definitions offer universality, long-term stability, and improved traceability for advanced science and technology. The decision process involved international consultation, measurement campaigns, and coordinated experiments submitted as mise en pratique documents to the CIPM. Key meetings included the 2017 and 2018 sessions of the General Conference on Weights and Measures, advisory input from the International Committee for Weights and Measures, and technical reports by national laboratories such as PTB, NIST, METAS, and CSIRO. Prominent scientists and committees—drawing on work by researchers from Oxford University, University of Tokyo, and École Normale Supérieure—evaluated uncertainty budgets and consensus criteria before the vote.
Implementation required dissemination of new mise en pratique guidance, calibration chain updates, and revisions to national legislation on measurement units by states party to the Metre Convention. Practical impact included improved consistency in high-precision fields such as quantum metrology, semiconductor fabrication, and chemical metrology used by firms like ASML and research centers like Max Planck Society. Metrology services at NPL, PTB, and NIST provided calibration services aligned with the fixed constants. Educational materials for universities such as Imperial College London and Massachusetts Institute of Technology were updated, and industries relying on traceability—pharmaceuticals, aerospace firms like Airbus and Boeing—adjusted quality systems accordingly.
Some controversies centered on perceived loss of historical continuity with the international prototype kilogram, debate over the practicality of realizations for the ampere and kelvin, and concerns from smaller national metrology institutes about resource burdens. Critics included commentators in scientific outlets and voices from certain stakeholder groups in chemistry and precision manufacturing. Disputes over the choice of realization methods—between the Kibble balance and silicon-sphere determinations like those by the International Avogadro Coordination—generated technical debates. Legal and educational challenges arose in aligning statutory definitions across jurisdictions, involving ministries and national standards bodies such as NIST and the European Commission.
The BIPM coordinated experimental comparisons, led dissemination of the fixed-constant framework, and maintained the International System under the oversight of the General Conference on Weights and Measures. Collaboration networks included the International Committee for Weights and Measures, regional metrology organizations like SIM, COOMET, and APMP, and scientific unions such as IUPAC and IUPAP. The BIPM published mise en pratique documents and hosted interlaboratory comparisons with participation from NIST, PTB, NMIJ, LNE, and others to ensure global equivalence of realizations.
The redefinition marked a paradigm shift toward quantum-based metrology and is expected to facilitate advances in quantum technologies at institutions like CERN, IBM Research, and Google Quantum AI. Future developments include improved single-electron devices, enhanced acoustic thermometers, and novel realizations leveraging condensed matter platforms studied at ETH Zurich and TU Delft. Ongoing work by metrology communities and research programs funded by entities such as the European Commission and national research councils will refine uncertainties and expand practical realizations, preserving the SI as a foundation for science and technology. Category:Metrology