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 laser can then be locked to the frequency, and its wavelength can be determined using interferometry. This technique was due to a group at the National Bureau of Standards (NBS) (which later became NIST). They used it in 1972 to measure the speed of light in vacuum with a fractional uncertainty of . Until the early modern period, it was not known whether light travelled instantaneously or at a very fast finite speed. The first extant recorded examination of this subject was in ancient Greece. The ancient Greeks, Muslim scholars, and classical European scientists long debated this until Rømer provided