redefinition of SI base units
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| redefinition of SI base units | |
|---|---|
| Name | Redefinition of SI base units |
| Date | 2019 |
| Location | International |
| Participants | International Bureau of Weights and Measures, International Committee for Weights and Measures, General Conference on Weights and Measures |
| Outcome | Constants-based SI |
redefinition of SI base units
The redefinition of SI base units converted the metre, kilogram, second, ampere, kelvin, mole and candela from artifact- or experiment-based standards to definitions tied to fundamental constants, affecting metrology, physics, chemistry, engineering, philosophy of science and international trade. The effort culminated in changes adopted at a plenary of the General Conference on Weights and Measures in 2018 and implemented in 2019, reflecting decades of work by institutions such as the International Bureau of Weights and Measures, the International Committee for Weights and Measures, the National Institute of Standards and Technology, and national metrology institutes including Physikalisch-Technische Bundesanstalt, National Physical Laboratory (UK), Laboratoire national de métrologie et d'essais, BIPM laboratories. This article outlines motivations, history, methods, implementation and continuing debates.
Background and motivation
The shift toward constants-based definitions drew on earlier efforts exemplified by the Metre Convention (1875), the creation of the International Bureau of Weights and Measures (BIPM), and landmark projects like the Avogadro Project and the development of the Kibble balance (formerly watt balance) at institutions such as NPL and NIST. Concerns motivating change included reliance on the International Prototype Kilogram, vulnerability of physical artifacts at the Bureau International des Poids et Mesures vault, and demands from communities including photonics, quantum metrology, thermodynamics and chemical metrology for reproducible, globally accessible standards. Influential individuals and groups included Lord Kelvin, Max Planck, Albert Einstein, Willard Libby, Richard Feynman, committees like the Consultative Committee for Units, and bodies such as the International Organization for Standardization.
Historical evolution of the SI base units
The metre’s definition evolved from the meridian arc survey efforts of Jean-Baptiste Delambre and Pierre Méchain through the platinum-iridium bar adopted under the Metre Convention, to the 1960s laser interferometry work at BIPM and the 1983 definition via the constant speed of light c. The second moved from astronomical definitions tied to Greenwich Observatory and the Ephemeris Second to the atomic standard based on the caesium-133 hyperfine transition developed at National Physical Laboratory (UK), NIST, and researchers such as Isidor Rabi. The kilogram’s trajectory from the International Prototype of the Kilogram to the Avogadro and Kibble approaches involved collaborations across BIPM, Naval Research Laboratory, National Metrology Institute of Japan and the International Avogadro Coordination. The ampere’s and kelvin’s redefinitions reflected advances in quantum Hall effect research by Klaus von Klitzing and John B. Goodenough-era thermometry, while the mole’s link to Avogadro’s constant drew on work by Amedeo Avogadro and modern chemical metrologists at IUPAC. The candela’s photometric basis connects to standards at CIE (International Commission on Illumination) and radiometry teams at PTB.
Principles and methods of redefinition
Redefinition followed principles set by the CGPM (General Conference on Weights and Measures), aiming for stability, universality, and realizability. Methods included fixing exact numerical values to constants such as the Planck constant, the elementary charge, the Boltzmann constant, Avogadro constant, and retaining established constants like speed of light c. Experimental techniques encompassed the Kibble balance for mass, X-ray crystal density (XRCD) and the Avogadro project for Avogadro constant determination, atomic clocks using caesium-133 and optical lattice clocks at NIST, PTB and NPL, quantum electrical standards harnessing the Josephson effect and quantum Hall effect, and primary thermometry methods like acoustic gas thermometry developed at institutions including LNE and NMIJ.
2019 redefinition and implementations
At the 26th CGPM session in 2018 member states ratified fixed numerical values for five constants, and the definitions took effect on 20 May 2019, World Metrology Day. The Planck constant h, elementary charge e, Boltzmann constant k, and Avogadro constant NA were fixed, leaving the metre and second definitions intact via c and the caesium hyperfine transition. Implementations involved disseminating realizations through national metrology institutes like NIST, PTB, NPL, LNE, CENAM, INRIM, VSL and coordination by BIPM. Industries including semiconductor manufacturing firms, pharmaceutical companies, aerospace contractors such as Boeing and Airbus, and financial infrastructures for trade adapted calibration chains to new realizations.
Impact on measurement practice and industry
The constants-based SI improved long-term stability for high-precision sectors such as quantum computing research at IBM and Google, nanotechnology at IMEC, metrology services for telecommunications and space agencies like ESA and NASA, and chemical metrology in pharmaceutical and petrochemical industries. Traceability chains now link to quantum standards and primary realizations at national labs, affecting calibration services, standards provided by ISO, and conformity assessment bodies. The change has influenced instrument manufacturers including Keysight Technologies, Thermo Fisher Scientific, and standards providers like UL, and supported innovation in technologies such as optical clocks, single-electron pumps, and Kibble balances at research centers including MIT and Caltech.
Controversies, challenges, and debate
Debate accompanied the process: historians and curators at institutions tied to the International Prototype of the Kilogram voiced cultural concerns, some researchers questioned residual uncertainties from XRCD silicon spheres produced by IMEC collaborators, while metrologists discussed practical realizability and dissemination when primary devices remain expensive or complex. Tensions involved resource allocation at national labs like NIST and PTB, standards harmonization across regions such as the European Union and United States, and outreach to industries reliant on legacy artifact-based calibration. Philosophers and scientists including proponents linked to Karl Popper-inspired perspectives and institutional stakeholders such as IUPAP engaged in discussions about the conceptual foundations of units and the role of fixed constants.
Future developments and ongoing research
Ongoing work addresses improved realizations via optical frequency standards at PTB, NIST, SYRTE, and NMIJ, advances in single-electron current sources pursued at NIST and PTB, enhanced Kibble balances at NPL and METAS, and quantum triangle experiments linking Josephson, quantum Hall and single-electron devices studied in collaborations including EURAMET and the Joint Committee for Guides in Metrology. Future debates may involve additional practical conventions by the CCU and further refinement of uncertainty budgets across international comparisons organized by CIPM. Research communities at universities such as Oxford University, Cambridge University, ETH Zurich, University of Tokyo, and Harvard University continue to drive innovations that will shape realizations and industrial uptake.