De Sitter's Astronomical Proof of the Constancy of Light Speed

By a method proposed in 1910 by Daniel Frost Comstock[1], de Sitter's 1913[2] study of orbits of binary stars showed that time for light to reach Earth from one of a binary pair was independent of whether the star in its orbit was receding or approaching when it emitted the light. De Sitter pointed out that if the velocity of light were constant relative to its source (as in emission theories), the light from the receding star would arrive later, and from the advancing star earlier. Thus, without measuring the speed of light, de Sitter showed that in free space the speed of light relative to its source is not constant. The proof was understood and appreciated as the refutation of emission theories. (Remarkably, advocates of emission theory may still be found. See for example “The de Sitter Effect”. Many others are members of the Natural Philosophy Alliance.) That “constancy of [absolute] speed of light” in de Sitter's proof differed from “constancy of [relative] speed of light” in Einstein's theory seems to have been overlooked; some even hail de Sitter's proof as a confirmation of Einstein's theory which it actually refutes.

For the velocity of light to be determined by the velocity of the object upon which at some future time the light will eventually fall is an obvious violation of the principle of cause and effect, and furthermore the argument presented against light speed being constant relative to its source serves with equal force to refute constancy relative to the observer. So the speed of light through space is independent of the motion of both source and observer. The ill-conceived relativistic definition of simultaneous events defines speed of light to be constant relative to source and coincident observer. In speaking of the speed of light in free space de Sitter had nothing relative to which that speed could be expressed except other light, so we are free to consider relative to what light speed is isotropic: light traveling at the same speed in every direction is the defining characteristic of the unique absolutely stationary frame of reference.

In 1905, nobody had measured and compared unidirectional light speeds with sufficient accuracy to reveal a directional difference. In 1905, that was not yet possible.

[1] Comstock, D.F., “A Neglected Type of Relativity,” Physical Review, February 1910, 30 (2): 267 (1910), online

[2] de Sitter, Willem, “Ein astronomischer Beweis für die Konstanz der Lichgeshwindigkeit," Physik. Zeitschr, 14, 429 (1913). “A proof of the constancy of the velocity of light,” Proceedings of the Royal Netherlands Academy of Arts and Sciences 15 (2): 1297-1298, online in English

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