Editing Speed of light (section)

=== Increased accuracy of ''c'' and redefinition of the metre and second ===
{{See also|History of the metre}}

In the second half of the 20th century, much progress was made in increasing the accuracy of measurements of the speed of light, first by cavity resonance techniques and later by laser interferometer techniques. These were aided by new, more precise, definitions of the metre and second. In 1950, [[Louis Essen]] determined the speed as {{val|299792.5|3.0|u=km/s}}, using cavity resonance.<ref name="Essen1950"/> This value was adopted by the 12th General Assembly of the Radio-Scientific Union in 1957. In 1960, the [[history of the metre#Krypton standard|metre was redefined]] in terms of the wavelength of a particular spectral line of [[krypton-86]], and, in 1967, the second was redefined in terms of the hyperfine transition frequency of the ground state of [[caesium-133]].<ref name="13thCGPMr1">
{{Cite web
 |year=1967
 |title=Resolution 1 of the 15th CGPM
 |url=https://www.bipm.org/en/CGPM/db/13/1/
 |publisher=[[International Bureau of Weights and Measures|BIPM]]
 |access-date=14 March 2021
 |archive-date=11 April 2021
 |archive-url=https://web.archive.org/web/20210411132806/https://www.bipm.org/en/CGPM/db/13/1/
 |url-status=dead
}}</ref>

In 1972, using the laser interferometer method and the new definitions, a group at the US [[National Institute of Standards and Technology|National Bureau of Standards]] in [[Boulder, Colorado]] determined the speed of light in vacuum to be ''c''&nbsp;=&nbsp;{{val|299792456.2|1.1|u=m/s}}. This was 100 times less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre.<ref name="11thCGPM">
{{Cite web
 |year=1967
 |title=Resolution 6 of the 15th CGPM
 |url=http://www.bipm.org/en/CGPM/db/11/6/
 |publisher=[[International Bureau of Weights and Measures|BIPM]]
 |access-date=13 October 2010
}}</ref><ref name="NIST heterodyne"/> As similar experiments found comparable results for ''c'', the 15th [[General Conference on Weights and Measures]] in 1975 recommended using the value {{val|299792458|u=m/s}} for the speed of light.<ref name="15thCGPM">
{{Cite web
 |year=1975
 |title=Resolution 2 of the 15th CGPM
 |url=http://www.bipm.org/en/CGPM/db/15/2/
 |publisher=BIPM
 |access-date=9 September 2009
}}</ref>