Sagnac Effect - History

History

Early suggestions to build a giant ring interferometer to measure the rotation of the Earth were made by Oliver Lodge in 1897, and then by Albert Abraham Michelson in 1904. They hoped that with such an interferometer, it would be possible to decide between the idea of a stationary aether, and an aether which is completely dragged by the Earth. That is, if the hypothetical aether were carried along by the Earth (or by the interferometer) the result would be negative, while a stationary aether would give a positive result. Max von Laue in 1911 continued the theoretical work of Michelson, and also incorporated special relativity in his calculations. He predicted a positive result (to first order in v/c) for both special relativity and for the stationary aether, because in those theories the speed of light is independent of the velocity of the source, and thus the propagation time for the counter-propagating rays is not the same when viewed from inertial frames of reference; only complete-aether-drag models would give a negative result. While Laue confined his investigations on inertial frames, Paul Langevin (1921/35) and others described the effect when viewed from rotating reference frames (in both special and general relativity, see Born coordinates).

In practice, the first interferometry experiment aimed at observing the correlation of angular velocity and phase-shift was performed by the French scientist Georges Sagnac in 1913. Its purpose was to detect "the effect of the relative motion of the ether". Sagnac believed that his results constituted proof of the existence of a stationary aether. However, as explained above, two years earlier, Max von Laue already shown that this effect is consistent with special relativity. An experiment conducted in 1911 by Franz Harress, aimed at making measurements of the Fresnel drag of light propagating through moving glass, was in 1920 recognized by Laue as actually constituting a Sagnac experiment. Not aware of the Sagnac effect, Harress had realized the presence of an "unexpected bias" in his measurements, but was unable to explain its cause.

In 1926, an ambitious ring interferometry experiment was set up by Albert Michelson and Henry Gale. The aim was to find out whether the rotation of the Earth has an effect on the propagation of light in the vicinity of the Earth. The Michelson–Gale–Pearson experiment was a very large ring interferometer, (a perimeter of 1.9 kilometer), large enough to detect the angular velocity of the Earth. The outcome of the experiment was that the angular velocity of the Earth as measured by astronomy was confirmed to within measuring accuracy. The ring interferometer of the Michelson-Gale experiment was not calibrated by comparison with an outside reference (which was not possible, because the setup was fixed to the Earth). From its design it could be deduced where the central interference fringe ought to be if there would be zero shift. The measured shift was 230 parts in 1000, with an accuracy of 5 parts in 1000. The predicted shift was 237 parts in 1000.