page_title: History of the metre
section_title: Speed of light standard
hierarchical_section_title: History of the metre / Speed of light standard
caption_attribution_description: Helium-Neon laser demonstration at the Kastler-Brossel Laboratory at Paris VI: Pierre et Marie Curie. The glowing ray in the middle is a discharge tube (akin to that of a neon light), it is not the laser beam. The laser beam crosses the air and marks a red point on the screen to the right.
context_page_description: The history of the metre starts with the scientific revolution that began with Nicolaus Copernicus's work in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision in use, they also preferred a decimal system to ease their calculations.
  With the French Revolution came a desire to replace many features of the Ancien Régime, including the traditional units of measure. As a base unit of length, many scientists had favoured the seconds pendulum one century earlier, but this was rejected as it had been discovered that it varied from place to place with local gravity and that it could complement meridian arc measurements in determining the figure of the Earth. A new unit of length, the metre was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth's flattening of 1/334.
  For practical purposes however, the standard metre was made available in the form of a platinum bar held in Paris.
context_section_description: The krypton-86 discharge lamp operating at the triple point of nitrogen (63.14 K, −210.01 °C) was the state-of-the-art light source for interferometry in 1960, but it was soon to be superseded by a new invention: the laser, of which the first working version was constructed in the same year as the redefinition of the metre. Laser light is usually highly monochromatic, and is also coherent (all the light has the same phase, unlike the light from a discharge lamp), both of which are advantageous for interferometry.
  The shortcomings of the krypton standard were demonstrated by the measurement of the wavelength of the light from a methane-stabilised helium–neon laser (λ ≈ 3.39 μm). The krypton line was found to be asymmetrical, so different wavelengths could be found for the laser light depending on which point on the krypton line was taken for reference. The asymmetry also affected the precision to which the wavelengths could be measured.
  Developments in electronics also made it possible for the first time to measure the frequency of light in or near the visible region of the spectrum, instead of inferring the frequency from the wavelength and the speed of light. Although visible and infrared frequencies were still too high to be directly measured, it was possible to construct a "chain" of laser frequencies that, by suitable multiplication, differ from each other by only a directly measurable frequency in the microwave region. The frequency of the light from the methane-stabilised laser was found to be 88.376 181 627(50) THz.
  Independent measurements of frequency and wavelength are, in effect, a measurement of the speed of light (c = fλ), and the results from the methane-stabilised laser gave the value for the speed of light with an uncertainty almost 100 times lower than previous measurements in the microwave region. Or, somewhat inconveniently, the results gave two values for the speed of light, depending on which point on the krypton line was chosen to define the metre. This ambiguity was resolved in 1975, when the 15th CGPM approved a conventional value of the speed of light as exactly 299 792 458 m s⁻¹.
  Nevertheless, the infrared light from a methane-stabilised laser was inconvenient for use in practical interferometry. It was not until 1983 that the chain of frequency measurements reached the 633 nm line of the helium–neon laser, stabilised using molecular iodine. That same year, the 17th CGPM adopted a definition of the metre, in terms of the 1975 conventional value for the speed of light:
  The metre is the length of the path travelled by light in vacuum during a time interval of ​¹⁄₂₉₉ ₇₉₂ ₄₅₈ of a second.
  This definition was reworded in 2019:
  The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s⁻¹, where the second is defined in terms of the caesium frequency ΔνCs.
  The concept of defining a unit of length in terms of a time received some comment. In both cases, the practical issue is that time can be measured more accurately than length (one part in 10¹³ for a second using a caesium clock as opposed to four parts in 10⁹ for the metre in 1983). The definition in terms of the speed of light also means that the metre can be realised using any light source of known frequency, rather than defining a "preferred" source in advance. Given that there are more than 22,000 lines in the visible spectrum of iodine, any of which could be potentially used to stabilise a laser source, the advantages of flexibility are obvious.
caption: A helium–neon laser at the Kastler-Brossel Laboratory at Univ. Paris 6
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