ALS in German, signed “E,” one page, 8.5 x 11.25, March 29, 1953. Handwritten explanation relative to the "constancy of the speed of light" and "rotation of the Earth" in experimental proofs of his Special Theory of Relativity, penned below an inquiry sent to him by George Aristotle Solounias. In concluding his letter, Solounias asks: "Dear Professor, Since Michelson-Morley experiment accepts—as I have proved—a newtonian explanation and consequently, it may be considered as proving directly the addition of the velocity of light to that of the source, and since Your Special Theory of Relativity is based essentially on this experiment, contrarily explained by You, the necessity of performing of a direct experiment is imperative. For this aim, I hope that my present proposition will probably help the solution of the problem."
Below, Einstein writes (translated): "The experimental arrangement proposed here is not useful in that the light emitted from an extended light source in opposite directions cannot be brought to a state of interference due to the difference in light paths. Apart from that, the experiment agrees with that carried out by Sagnac, which allows the experimental result to be calculated correctly based on the assumption of the constancy of the speed of light (with reference to the inertial system) (see Laue’s book). Compare it also with the ingenious experiment of Michelson, which needs the rotation of the Earth." He cites the referenced book below: "M. v. Laue, The Theory of Relativity, J. Friedr. Vieweg & Son, Brunswick, 1952." In fine condition, with light intersecting folds. Accompanied by the original mailing envelope.
In his letter to Einstein, Solounias evidently proposed an experiment to determine whether "the velocity of light is independent, or not, of the velocity of the source." He mentions the 'Michelson–Morley experiment,' conducted in 1887, which was an attempt to measure the motion of the Earth relative to the 'luminiferous aether,' a permeating space then thought to carry light waves. The experiment found exactly nothing—the two light beams, as measured by a sensitive interferometer, took precisely the same time to arrive at their destination. This result was the first strong evidence against some aether theories, and initiated a line of research that eventually led to Einstein's development of special relativity in 1905. He also references experiments performed by French physicist Georges Sagnac, who lent his name to the 'Sagnac effect' in 1913: a phenomenon that manifests itself when a beam of light is split and the two beams follow the same path in opposite directions, displaying interference when recombined. Sagnac concluded from his experiments that light propagates at a speed independent of the speed of the source, which is consistent with Einstein’s theory of relativity. German physicist Max von Laue, also referenced here by Einstein, had predicted the Sagnac effect theoretically in 1911.
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