Editing Kilogram (section)

== Redefinition based on fundamental constants ==
[[File:Unit relations in the new SI.svg|thumb|upright=1.35|The [[International System of Units|SI system]] after the 2019 redefinition: the kilogram is now fixed in terms of the [[second]], the [[speed of light]] and the [[Planck constant]]; furthermore the [[ampere]] no longer depends on the kilogram]]
[[File:Watt balance, large view.jpg|thumb|upright=1.35|A [[Kibble balance]], which was originally used to measure the [[Planck constant]] in terms of the IPK, can now be used to calibrate secondary standard weights for practical use.]]
{{main|2019 revision of the SI}}

The replacement of the [[International Prototype of the Kilogram]] (IPK) as the primary standard was motivated by evidence accumulated over a long period of time that the mass of the IPK and its replicas had been changing; the IPK had diverged from its replicas by approximately 50&nbsp;micrograms since their manufacture late in the 19th century. This led to [[Alternative approaches to redefining the kilogram|several competing efforts]] to develop measurement technology precise enough to warrant replacing the kilogram artefact with a definition based directly on physical fundamental constants.<ref name="vox">{{cite news |last1=Resnick |first1=Brian |title=The new kilogram just debuted. It's a massive achievement. |url=https://www.vox.com/science-and-health/2019/5/17/18627757/kilogram-redefined-world-metrology-day-explained |access-date=May 23, 2019 |publisher=vox.com |date=May 20, 2019}}</ref>

The [[International Committee for Weights and Measures]] (CIPM) approved a [[2019 revision of the SI|revision]] in November 2018 that defines the kilogram by defining the [[Planck constant]] to be exactly {{val|6.62607015|e=−34|u=kg⋅m<sup>2</sup>⋅s<sup>−1</sup>}}, effectively defining the kilogram in terms of the second and the metre. The new definition took effect on 20 May 2019.<ref name="vox"/><ref name=draft-resolution-A /><ref>
{{cite news
 |url= https://www.bbc.com/news/science-environment-46143399
 |title= Kilogram gets a new definition
 |author= Pallab Ghosh
 |date= November 16, 2018
 |journal= BBC News
 |access-date= November 16, 2018
}}</ref>

Prior to the redefinition, the kilogram and several other SI units based on the kilogram were defined by a man-made metal artifact: the ''[[Kilogramme des Archives]]'' from 1799 to 1889, and the IPK from 1889 to 2019.<ref name="vox"/>

In 1960, the [[metre]], previously similarly having been defined with reference to a single platinum-iridium bar with two marks on it, was redefined in terms of an invariant physical constant (the wavelength of a particular emission of light emitted by [[krypton]],<ref>{{SIbrochure8th|page=112}}</ref> and later the [[speed of light]]) so that the standard can be independently reproduced in different laboratories by following a written specification.

At the 94th Meeting of the CIPM in 2005, it was recommended that the same be done with the kilogram.<ref>
{{cite conference
 |url=https://www.bipm.org/cc/CIPM/Allowed/94/CIPM-Recom1CI-2005-EN.pdf
 |conference=94th meeting of the International Committee for Weights and Measures
 |title=Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants
 |date=October 2005
 |page=233
 |access-date=February 7, 2018
 |url-status=live
 |archive-url=https://web.archive.org/web/20070630011658/https://www.bipm.org/cc/CIPM/Allowed/94/CIPM-Recom1CI-2005-EN.pdf
 |archive-date=June 30, 2007
}}</ref><!-- Original URL: http://www.bipm.org/utils/en/pdf/CIPM2005-EN.pdf -->

In October 2010, the CIPM voted to submit a resolution for consideration at the [[General Conference on Weights and Measures]] (CGPM), to "take note of an intention" that the kilogram be defined in terms of the [[Planck constant]], {{math|''h''}} (which has dimensions of energy times time, thus mass × length{{sup|2}} / time) together with other physical constants.<ref>{{cite journal|url=https://www.nist.gov/pml/wmd/20101026_si.cfm |title=NIST Backs Proposal for a Revamped System of Measurement Units |journal=NIST |date=October 26, 2010 |publisher=Nist.gov |access-date=April 3, 2011}}</ref><ref name="draft">
{{cite web
 |url = http://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf
 |title = Draft Chapter 2 for SI Brochure, following redefinitions of the base units
 |author = Ian Mills
 |publisher = CCU
 |date = September 29, 2010
 |access-date =January 1, 2011
}}</ref> This resolution was accepted by the 24th conference of the CGPM<ref>
{{cite conference
 |url= http://www.bipm.org/utils/en/pdf/24_CGPM_Resolution_1.pdf
 |title= Resolution 1 – On the possible future revision of the International System of Units, the SI
 |conference= 24th meeting of the General Conference on Weights and Measures
 |location = Sèvres, France
 |date = October 17–21, 2011
 |access-date =October 25, 2011
}}</ref> in October 2011 and further discussed at the 25th conference in 2014.<ref name=":0">{{Cite web|url=http://www.bipm.org/en/CGPM/db/25/1/|title=BIPM – Resolution 1 of the 25th CGPM|website=www.bipm.org|access-date=March 27, 2017}}</ref><ref>
{{cite press release
 | url = http://www.bipm.org/utils/en/pdf/Press_release_resolution_1_CGPM.pdf
 | title = General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram.
 | publisher = [[General Conference on Weights and Measures]]
 | location = Sèvres, France
 | date = October 23, 2011
 | access-date = October 25, 2011
}}</ref> Although the Committee recognised that significant progress had been made, they concluded that the data did not yet appear sufficiently robust to adopt the revised definition, and that work should continue to enable the adoption at the 26th meeting, scheduled for 2018.<ref name=":0" /> Such a definition would theoretically permit any apparatus that was capable of delineating the kilogram in terms of the Planck constant to be used as long as it possessed sufficient precision, accuracy and stability. The [[Kibble balance]] is one way to do this.<ref>{{cite journal |doi=10.1088/0026-1394/53/5/A46 |title=The watt or Kibble balance: A technique for implementing the new SI definition of the unit of mass |journal=Metrologia |volume=53 |issue=5 |pages=A46–A74 |year=2016 |last1=Robinson |first1=Ian A. |last2=Schlamminger |first2=Stephan |pmid=35023879 |pmc=8752041 |bibcode=2016Metro..53A..46R |doi-access=free }}</ref>

As part of this project, a variety of very [[Alternative approaches to redefining the kilogram|different technologies and approaches]] were considered and explored over many years. Some of these approaches were based on equipment and procedures that would enable the reproducible production of new, kilogram-mass prototypes on demand (albeit with extraordinary effort) using measurement techniques and material properties that are ultimately based on, or traceable to, physical constants. Others were based on devices that measured either the acceleration or weight of hand-tuned kilogram test masses and that expressed their magnitudes in electrical terms via special components that permit traceability to physical constants. All approaches depend on converting a weight measurement to a mass and therefore require precise measurement of the strength of gravity in laboratories ([[gravimetry]]). All approaches would have precisely fixed one or more constants of nature at a defined value.{{Citation needed|date=July 2023}}