from frequency measurements and a given value for the speed of light are more reproducible than the previous standard. They kept the 1967 definition of second, so the caesium 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/ of a second." As a result of this definition, the value of the speed of light in vacuum is exactly and has become a defined constant in the SI system of units. 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. 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 when it is expressed in the SI unit ." This was one of the changes that was incorporated in the 2019 redefinition of the SI base units, also termed the New SI. See also Light-second Speed of electricity Speed of gravity Speed of sound Velocity factor Warp factor (fictional) Notes References Further reading Historical references Translated as Modern references External links "Test Light Speed in Mile Long Vacuum Tube." Popular Science Monthly, September 1930, pp. 17–18. Definition of the metre (International Bureau of Weights and Measures, BIPM) Speed of light in vacuum (National Institute of Standards and Technology, NIST) Data Gallery: Michelson Speed of Light (Univariate Location Estimation) (download data gathered by Albert A. Michelson) Subluminal (Java applet by Greg Egan demonstrating group velocity information limits) Light discussion on adding velocities Speed of Light (Sixty Symbols, University of Nottingham Department of Physics [video]) Speed of Light, BBC Radio4 discussion (In Our Time, 30 November 2006) Speed of Light (Live-Counter – Illustrations) Speed of Light – animated