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In 1983 the 17th CGPM decided to redefine the metre as used in the SI system of units in order to solve problems with obtaining a sufficiently precise realisation of the previous definition of the metre. The definition reads: "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 has the effect that the speed of light now takes an exact value of 299 792 458 metres per second.

History
The speed of light, usually denoted c nowadays, has been subject of speculation or investigation for millennia. However the nature of c, as a universal physical constant, was only established by the development of special relativity in 1905. This left the value of c, as with all physical constants, as a subject of further refinement by measurement.

Since, by 1975, similar laser-based measurements of c agreed with each other with an uncertainty comparable to that of the "realization of the metre", the 15th Conférence Générale des Poids et Mesures recommended using $299,792,458 m/s$ for "the speed of propagation of electromagnetic waves in vacuum". . The recommendation was adopted in 1983 by the 17th CGPM, by their definition of the metre as the length of the path traveled by light in vacuum during a time interval of 1 / 299,792,458 of a second. Further reasons for using this definition are stated in Resolution 1. A popular account of the decision from that time is provided in the New Scientist.

Consequences
The effect of this definition gives the speed of light in vacuum an exact value in metric units, namely 299,792,458 metres/second. This can be regarded as a conversion factor between the second and the metre. 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: “In all of these changes in definition, the goal was not only to improve the precision of the definition, but also to change its actual length as little as possible.” Thus, within the SI system of units, the speed of light is now a defined constant and no longer something to be measured. Improved experimental techniques do not affect the metric value of the speed of light, but do result in a better realization of the metre, i.e. recalibration of the measuring instruments involved. 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.

Rather than measure a time-of-flight, one implementation of this definition is to use a recommended source with established frequency f, and delineate the metre in terms of the wavelength λ of this light as determined using the defined numerical value of c and the relationship. 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


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 = $299,792,456.2 m/s$. 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 the profile of the chosen spectral line was not perfectly symmetrical. This made its wavelength, and hence the length of the metre, ambiguous, because the definition did not specify what point on the line profile it referred to. There were other problems in using the krypton source. For example, one problem is the coherence length of this source of 78 cm, which complicates its use for measuring longer lengths. 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.

Hence the krypton discharge was not the best source for determining the metre. An alternative would be to select a better source, for example the caesium 133 atomic transition selected in 1967 for the standard of time. However, laser technology was rapidly evolving and new and better sources were being devised; a standard was desirable 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. Since frequency and wavelength were related by the speed of propagation, 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 $299,792,456.2 m/s$ for "the speed of propagation of electromagnetic waves in vacuum". In 1983, because the definition of the metre in terms of the Kr86 transition at 6057Å did not allow a sufficiently precise realization of it for all requirements, the 17th CGPM adopted the 1975 recommendation: "the length of the path travelled by light in vacuum during a time interval of $299,792,458.7 m/s$ of a second". The effect of this definition gives the speed of light the exact value $299,792,458 m/s$. 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.

Does the definition preclude some measurements?
The special theory of relativity is based upon a number of postulates concerning properties of the speed of light. In the present SI units system where the speed of light has a defined value of c = 299,792,458 m/s exactly, can these postulates continue to be tested as experimental technique improves? For example, does the definition of c as an exact value mean that any test of whether light is isotropic, dispersionless, etc. is placed beyond test, by definition? The answer is: ‘No. What is placed beyond test is the defined value of c, not any experimental investigations.’

For example, consider the hypothetical observation of anisotropy. In the SI system of units, the anisotropy would take the form of the metre having different lengths in different directions. Of course, a standard of length cannot be allowed to be uncertain in this way, so the effect of this hypothetical observation would be a change in the definition of the metre, to add a directional correction. At the same time, however, the explanation of this anisotropy would be attempted by improvements in theory, and quite possibly the successful explanation would be that the physical phenomenon of the speed of travel of light was anisotropic. The physical phenomenon of the speed of travel of light must be kept separate in our thinking from the numerical value of c in the SI system of units. (This distinction might be a good reason to use the symbol c0 for the SI conversion factor instead of c, as is recommended by the CGPM.)

In sum, tests of the special theory of relativity, for example, are not impeded by an exact definition of c in the SI system of units. The experiments show up as tests of the adequacy of the definition of the metre. Any necessary changes in this definition due to future experimental results will be accompanied eventually by theoretical explanations, and these theories may indeed invoke as yet undiscovered properties of the speed of travel of light.