Variable speed of light with Einstein and stated both speed and direction of light change in a gravity field first published 1923 in English, 1920 in German. Born expressed the magnitude of light speed as the square root of (−"g"/"g") gravity components in general relativity to leave no doubt about his intent. Richard Tolman also agreed with Einstein and expressed the radial speed of light as "dr"/"dt" in a gravity field. where "m" is "MG"/"c" and where natural units are used such that "c" is equal to one. Robert Dicke, in 1957, developed a related VSL theory of gravity. In contrast to Einstein, Dicke
Speed of light precise measurements, in 1975 the speed of light was known to be with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/ of a second. The speed of light in vacuum is usually denoted by a lowercase , for "constant" or the Latin (meaning "swiftness, celerity"). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used for a different constant later shown to equal times the speed of light in vacuum. Historically, the symbol "V" was used as