Editing International System of Units (section)

=== 2019 redefinition ===

[[File:Unit relations in the new SI.svg|thumb |right |Reverse dependencies of the SI base units on seven [[physical constant]]s, which are assigned exact numerical values in the [[2019 revision of the SI|2019 redefinition]]. Unlike in the previous definitions, the base units are all derived exclusively from constants of nature. Here, <math>a \rightarrow b</math> means that <math>a</math> is used to define <math>b</math>.]]
{{Main|2019 revision of the SI}}

After the [[history of the metre|metre was redefined]] in 1960, the [[International Prototype of the Kilogram]] (IPK) was the only physical artefact upon which base units (directly the kilogram and indirectly the ampere, mole and candela) depended for their definition, making these units subject to periodic comparisons of national standard kilograms with the IPK.<ref name="NPL kg">{{cite web |title=Redefining the kilogram |url=http://www.npl.co.uk/educate-explore/redefining-the-kilogram/|publisher=UK National Physical Laboratory |access-date=30 November 2014 |url-status=live |archive-url= https://web.archive.org/web/20141227083141/http://www.npl.co.uk/educate-explore/redefining-the-kilogram/ |archive-date= Dec 27, 2014 }}</ref> During the 2nd and 3rd Periodic Verification of National Prototypes of the Kilogram, a significant divergence had occurred between the mass of the IPK and all of its official copies stored around the world: the copies had all noticeably increased in mass with respect to the IPK. During ''extraordinary verifications'' carried out in 2014 preparatory to redefinition of metric standards, continuing divergence was not confirmed. Nonetheless, the residual and irreducible instability of a physical IPK undermined the reliability of the entire metric system to precision measurement from small (atomic) to large (astrophysical) scales.<ref>{{Cite web |date=12 May 2018 |title=A Turning Point for Humanity: Redefining the World's Measurement System |url=https://www.nist.gov/si-redefinition/turning-point-humanity-redefining-worlds-measurement-system |access-date=16 January 2024 |website=NIST |language=en}}</ref>
By avoiding the use of an artefact to define units, all issues with the loss, damage, and change of the artefact are avoided.<ref name=SIBrochure9thEd/>{{rp|page=125}}

A proposal was made that:<ref>{{cite web |title=Appendix 1. Decisions of the CGPM and the CIPM |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-App4-EN.pdf |page=188 |publisher=[[BIPM]] | access-date=27 April 2021}}</ref>
* In addition to the speed of light, four constants of nature – the [[Planck constant]], an [[elementary charge]], the [[Boltzmann constant]], and the [[Avogadro constant]] – be defined to have exact values
* The [[International Prototype of the Kilogram]] be retired
* The current definitions of the kilogram, ampere, kelvin, and mole be revised
* The wording of base unit definitions should change emphasis from explicit unit to explicit constant definitions.

The new definitions were adopted at the 26th CGPM on 16 November 2018, and came into effect on 20 May 2019.<ref>{{cite web |url=http://www.bipm.org/cc/TGFC/Allowed/Minutes/CODATA_Minutes_14-BIPM-public.pdf |title=Report on the Meeting of the CODATA Task Group on Fundamental Constants |date=3–4 November 2014 |publisher=[[BIPM]] | author-first=B. |author-last=Wood |page=7 |quote=[BIPM director Martin] Milton responded to a question about what would happen if ... the CIPM or the CGPM voted not to move forward with the redefinition of the SI. He responded that he felt that by that time the decision to move forward should be seen as a foregone conclusion.}}</ref> The change was adopted by the European Union through Directive (EU) 2019/1258.<ref>{{cite web |url=https://eur-lex.europa.eu/eli/dir/2019/1258/oj |title=Commission Directive (EU) 2019/1258 of 23 July 2019 amending, for the purpose of its adaptation to technical progress, the Annex to Council Directive 80/181/EEC as regards the definitions of SI base units |author=<!--Not stated--> |date=23 July 2019 |website=[[Eur-Lex]] | access-date=28 August 2019}}</ref>

Prior to its redefinition in 2019, the SI was defined through the seven base units from which the derived units were constructed as products of powers of the base units. After the redefinition, the SI is defined by fixing the numerical values of seven defining constants. This has the effect that the distinction between the base units and derived units is, in principle, not needed, since all units, base as well as derived, may be constructed directly from the defining constants. Nevertheless, the distinction is retained because "it is useful and historically well established", and also because the [[ISO/IEC 80000]] series of standards, which define the [[International System of Quantities]] (ISQ), specifies base and derived quantities that necessarily have the corresponding SI units.<ref name="SIBrochure9thEd" />{{rp|page=129}}