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Annotated Fact Desk

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Sources

  1. www.bipm.org/en/si-base-units/metre0 helpful
  2. en.wikipedia.org/wiki/Speed_of_light0 helpful

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This speed is exact because it defines the metre

The NIST sentence states a defined value, not a rounded laboratory measurement. Since 1983 the metre has been the distance light travels in vacuum in 1/299792458 of a second, so the speed of light in vacuum is exactly 299792458 metres per second by international agreement. The spaces in “299 792 458” are the SI digit-grouping style; they are the same number as 299,792,458. The BIPM metre page records the same relation, c = 299 792 458 m s−1.

The speed of light in vacuum is 299 792 458 meters per second.

SI Units – Length | NIST

National Institute of Standards and Technology

nist.gov

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