Abstract

A recent paper by Dieks and Nienhuis implies that Sagnac's famous experiment in 1913 cannot be explained by classical physics and needs the Special Theory of Relativity to come to the rescue. It is shown that the conclusions in that paper are both physically and historically erroneous, and that the experiment proves, as do contemporary measurements using satellites, that the speed of light is not constant in the sense used by Einstein in his 1905 paper. The latter measurements also refute the GRT explanation of Sagnac's experiment, since a time difference remains when no area is enclosed.