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Records tagged “speed-of-light”

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  1. Evidence-backedEditorial seedFactualenas of How the metre is defined: the speed of light since 1983, reworded in the 2019 SISince 1983 the metre has been defined through the speed of light in vacuum, which makes c exactly 299 792 458 m/s. The 2018 CGPM reworded the definition to fix c explicitly as a defining constant; the revised SI took effect on 20 May 2019.