Editing Speed of light (section)

=== Defined as an explicit constant ===
In 1983 the 17th meeting of the General Conference on Weights and Measures (CGPM) found that wavelengths from frequency measurements and a given value for the speed of light are more [[reproducibility|reproducible]] than the previous standard. They kept the 1967 definition of second, so the [[caesium]] [[Hyperfine structure#Use in defining the SI second and meter|hyperfine frequency]] would now determine both the second and the metre. To do this, they redefined the metre as "the length of the path traveled by light in vacuum during a time interval of 1/{{val|299792458}} of a second".<ref name=Resolution_1>
{{Cite web
 |year=1983
 |title=Resolution 1 of the 17th CGPM
 |url=https://www.bipm.org/en/committees/cg/cgpm/17-1983/resolution-1
 |publisher=BIPM
 |access-date=23 August 2009
}}</ref>

As a result of this definition, the value of the speed of light in vacuum is exactly {{val|299792458|u=m/s}}<ref name=Wheeler>
{{Cite book
 |last1=Taylor |first1=E. F. |author-link1=Edwin F. Taylor |last2=Wheeler |first2=J. A. |author-link2=John Archibald Wheeler
 |year=1992
 |title=Spacetime Physics: Introduction to Special Relativity
 |url=https://books.google.com/books?id=PDA8YcvMc_QC&pg=PA59
 |edition=2
 |publisher=Macmillan
 |isbn=978-0-7167-2327-1
 |page=59
}}</ref><ref name=timeline>
{{Cite web
 |last=Penzes |first=W. B.
 |year=2009
 |title=Time Line for the Definition of the Meter
 |url=https://www.nist.gov/pml/div683/upload/museum-timeline.pdf
 |publisher=[[National Institute of Standards and Technology|NIST]]
 |access-date=11 January 2010
}}</ref> and has become a defined constant in the SI system of units.<ref name="fixes"/> Improved experimental techniques that, prior to 1983, would have measured the speed of light no longer affect the known value of the speed of light in SI units, but instead allow a more precise realization of the metre by more accurately measuring the wavelength of krypton-86 and other light sources.<ref name=Adams>
{{Cite book
 |last=Adams |first=S.
 |year=1997
 |title=Relativity: An Introduction to Space–Time Physics
 |url=https://books.google.com/books?id=1RV0AysEN4oC&pg=PA140
 |page=140
 |publisher=CRC Press
 |isbn=978-0-7484-0621-0
 |quote=One peculiar consequence of this system of definitions is that any future refinement in our ability to measure&nbsp;''c'' will not change the speed of light (which is a defined number), but will change the length of the meter!
}}</ref><ref name=W_Rindler>
{{Cite book
 |last=Rindler |first=W.
 |year=2006
 |title=Relativity: Special, General, and Cosmological
 |url=https://books.google.com/books?id=MuuaG5HXOGEC&pg=PT41
 |page=41
 |edition=2
 |publisher=Oxford University Press
 |isbn=978-0-19-856731-8
 |quote=Note that [...] improvements in experimental accuracy will modify the meter relative to atomic wavelengths, but not the value of the speed of light!
}}</ref>

In 2011, the CGPM stated its intention to redefine all seven SI base units using what it calls "the explicit-constant formulation", where each "unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant", as was done for the speed of light. It proposed a new, but completely equivalent, wording of the metre's definition: "The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly {{val|299792458}} when it is expressed in the SI unit {{nowrap|m s<sup>−1</sup>}}."<ref>{{Cite web |url=http://www.bipm.org/en/si/new_si/explicit_constant.html |title=The "explicit-constant" formulation |archive-url=https://web.archive.org/web/20140811195806/http://www.bipm.org/en/si/new_si/explicit_constant.html |archive-date=11 August 2014 |website=BIPM |date=2011}}</ref> This was one of the changes that was incorporated in the [[2019 revision of the SI]], also termed the ''New SI''.<ref>See, for example:
* {{Cite web |last=Conover |first=Emily |author-link=Emily Conover |date=2 November 2016 |title=Units of measure are getting a fundamental upgrade |url=https://www.sciencenews.org/article/units-measure-are-getting-fundamental-upgrade |access-date=6 February 2022 |website=[[Science News]] |language=en-US}}
* {{Cite journal |last1=Knotts |first1=Sandra |last2=Mohr |first2=Peter J. |last3=Phillips |first3=William D. |date=January 2017 |title=An Introduction to the New SI |url=http://scitation.aip.org/content/aapt/journal/tpt/55/1/10.1119/1.4972491 |journal=[[The Physics Teacher]] |language=en |volume=55 |issue=1 |pages=16–21 |doi=10.1119/1.4972491 |bibcode=2017PhTea..55...16K |s2cid=117581000 |issn=0031-921X}}
* {{Cite journal |date=11 May 2018 |title=SI Redefinition |url=https://www.nist.gov/si-redefinition |access-date=6 February 2022 |journal=[[National Institute of Standards and Technology]] |language=en}}
</ref>