The continuity of the meter - the redefinition of the meter and the speed of visible-light

Abstract

The product of the frequency and wavelength of the i'h hyperfine com-ponent of the 11-5, R(127) transition of 127h yields a value for the speed of vis-ible red light. This value of c, the most accurate ever measured for visi-ble light, agrees with the value defined in the redefinition of the meter within the 30-error limits of the krypton length standard. The speed of light has intrigued scientists for several centuries and during the short quarter cen-tury of the laser it has not been different. The mea· surement of the speed of light, c, by a group of scientists at the National Bureau of Standards in 1972 [1P reported a value for c from the product of the wavelength and frequency of a 3.39 p.m He-Ne laser which was limited in accuracy by the uncer-tainty in the krypton length standard. This defini-tive measurement for c followed by other confirmative measurements prompted the interna-tional scientific community, through the Comite Consultatif pour la Definition du Metre (CCDM), to a new definition for the meter. The new defini· lpigures in brackets indicate literature references. 11 Generale des Poids et Mesures (CGPM) in October 1983 reads, "The meter is the length of the path travelled by light in vaccum during a time interval of 1/299 792 458 of a second." [2] This definition for the meter fixes the speed of light to be exactly 299 792 458 m/s. With this definition the meter could be realized from the wavelength of any co-herent optical source whose frequency is known, for example, a laser which is stabilized to a narrow atomic or molecular absorption for which the fre-quency is known. The wavelength A would be de-termined from the relation A=clv, where c is the fixed value of the speed of light, and v is the mea-sured frequency of the transition. Since the mea-surement in 1972 there have been four speed of light measurements [3-6]; two at a wavelength of 3.39 p.m and two at a wavelength of9.31 p.m.These measurements have been summarized [7], and the average value for the speed of light is 299 792 458.1 mls with a fractional uncertainty of ±4 X