Constant (speed of light)

light travelling between Earth and Moon

A beam of light travelling between Earth and Moon in the time it takes a light pulse to move between them: 1.255 seconds


O.C. Rømer
O.C. Rømer
The unit meter (m) is defined by fixing the speed of light c in vacuum to be equal to 299,792,458 m·s-1, where second is defined by the atomic clock.

History

In 1676, the Danish astronomer Ole Christensen Rømer (1644 - 1710) was the first to measure the speed of light. The result 225,000 km·s-1 was about 25% too small.

The Prussian physicist Albert Abraham Michelson (1852 - 1931) used an interferometer and came to the - for those days - remarkable conclusion that the speed of light in vacuum was the same in all directions, namely 299,774±11 km·s-1. This formed the basis for the theory of relativity of the German physicist Albert Einstein (1879 - 1955).

Related concepts

  • base units
  • constant (atomic clock)
  • constant (Avogadro)
  • constant (Boltzmann)
  • constant (electron charge)
  • constant (Planck)
  • length
  • meter

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