Redefinition of the Metre in 1983

A change in the meaning of the term speed of light as used in the SI system of units occurred in 1983. This article explores the background and ramifications of this change in definition at greater length than seems desirable in the article Speed of light.
In 1983 the 17th Conférence Générale des Poids et Mesures defined the metre to be the length of the path traveled by light in vacuum during a time interval of <big>/</big><sub>299,792,458</sub> of a second.
The reasons for using this definition are stated in Resolution 1. A popular account of the decision from that time is provided in the .
The effect of this definition is to redefine the term speed of light in vacuum as a conversion factor with the exact value 299,792,458 m/s. The value of 299,792,458 m/s is approximately the measured value of the speed of light based upon the pre-1983 definition of the metre, and was selected in part to result in minimal dislocation of standards. According to NIST:
and no longer something to be measured.
Improved experimental techniques do not affect the conversion factor speed of light, but do result in a better realization of the metre. Because the second is defined in terms of atomic transitions that can be measured accurately, the new definition, being a ratio of measured times, allows for a definition of the metre with greater accuracy in practical measurement than one based on a ratio of lengths determined using a fringe count of interference patterns. Practical realizations of the metre use recommended wavelengths of visible light in a laboratory vacuum with corrections being applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum.
Increased accuracy and redefinition of the metre
In the second half of the 20th century much progress was made in increasing the accuracy of measurements of the speed of light, first by cavity resonance techniques and later by laser interferometer techniques. In 1972, using the latter method, a team at the US National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado determined the speed of light to be c = . Almost all the uncertainty in this measurement of the speed of light was due to uncertainty in the length of the metre.
Since 1960, the metre had been defined as a given number of wavelengths of the light of one of the spectral lines of krypton, but it turned out that the profile of the chosen spectral line was not perfectly symmetrical. By way of contrast, some He-Ne lasers have a coherence length of about 75 km, making such sources very attractive for measurement of long distances. It was apparent that the krypton discharge was not the best source for determining the metre. It was also apparent that laser technology was rapidly evolving and that new and better sources were being devised. It was desirable to have a standard that could keep abreast of such developments, and allow adoption of new sources that were more precise or more practical as they evolved.
The introduction of many sources required a methodology for comparison. The wavelength of the sources could be compared, but using interferometry to measure wavelengths was subject to some serious errors. On the other hand the measurement of frequencies had become very good. As all believed that frequency and wavelength were related by the speed of propagation, it was evident that a comparison of the frequencies of sources was tantamount to a comparison of wavelengths, but more accurate. The only issue was to insure that the speed of propagation was the same in all such comparisons, which led to the adoption of the speed of light in vacuum as the standard of speed of propagation.
In 1975, considering that similar measurements of c agreed with each other and their uncertainty was comparable to that in the realization of the metre, the 15th Conférence Générale des Poids et Mesures (CGPM) recommended using for "the speed of propagation of electromagnetic waves in vacuum". The effect of this definition gives the speed of light the exact value . As a result, in the SI system of units the speed of light is now a defined constant. Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realisation of the metre.
With this definition, the remaining uncertainties in the realization of the metre are now (i) the uncertainties in the radiations, now tabulated and kept current in the mis en pratique; (ii) the uncertainties in the realization of "vacuum" (or, equivalently, uncertainties in the corrections implemented to account for the medium employed) and (iii) the uncertainty in interferometry in establishing a wavelength. This last is no better than use of a better source and the old definition of the metre as a number of wavelengths of a specific transition. However, the flexibility in choice of source and the ability to let the standard evolve with improved precision of the sources are the determining factors in making the change in definition.

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