Sagnac, Georges M. M.

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(b. Périgueux, France, 14 October 1869; d. Meudon, France, 26 February (1928)


Sagnac studied mainly the radiation produced by X rays and the optics of interference. He is remembered today primarily for his design of a rotating interferometer and for the experimental results it provided. He was very interested in the theoretical analysis of optics of moving systems; his approach was classical, and he interpreted the results of his optical experiments as contradcting Einstein’s theory of relativity.

Sagnac came from an old bourgeois family. His father was a lawyer who directed the Assurances Générales, and his mother was the daughter of a notary. From 1889 to 1894 Sagnac studied at the École Normale Supérieure and at the University of Paris. In 1894 he received his agrégé and became an assistant in physics at the university. In 1900 he was awarded the docteur ès sciences degree and became professor of physics at the University of Lille. From 1911 he was professor of physics at the University of Paris. He was twice a candidate for the physics section of the Academy, and he received the Pierson-Perrin, the Wilde, and the Lacaze prizes.

Sagnac’s earliest research (and the subject of his dissertation) was motivated by the newly observed X rays.1 From 1896, the year following Röntgen’s discovery, to 1900 Sagnac studied the radiations produced by X rays. He observed that secondary radiation is produced when X rays fall on heavier metals. He showed that this radiation contained secondary X rays (of lower frequency than the original X rays) and negatively charged rays. He suggested that it was through the action of the negative rays that X rays produce ionization in air.

Sagnac next worked on the propagation of light in moving systems (he was particularly interested in the Fizeau effect) and with the optics of interference. From about 1910 he was especially concerned with designing and using a rotating interferometer. In this instrument all the components—light source, mirrors, and photographic plate—are on a disk that can be rotated at various velocities. Light travels around the disk along a polygonal circuit determined by successive reflections from four mirrors placed around it. Light from the source is split into two beams that travel around the disk in opposite directions and, on recombination, produce interference fringes on the photographic plate. In 1913 Sagnac measured the shift in the position of the fringes when the direction of rotation was reversed.

Sagnac interpreted the shift in the position of the interference fringes in terms of an “ether wind” that gave the light beams traveling in opposite directions different velocities. He was convinced that the phenomenon observed with the rotating interferometer demonstrated the existence of an immobile ether. Sagnac later tried to develop a theory of electrodynamics that would retain classical ideas of space, time, and ether by analyzing the propagation of energy statistically and separately from the propagation of motion. He retained a lifelong dislike for relativity, and in 1923 he interpreted some observations of stellar color shift as being due to an ether wind, rather than as supporting the general theory of relativity.

The results of Sagnac’s interferometer experiments were used by some scientists in France as an argument against the theory of relativity. As late as 1937 Dufour and Prunier repeated the experiment in a modified form for that purpose.2 The experiment does not contradict relativity theory in any way, however, and in 1921 and 1937 Langevin responded with an explanation of how it should be interpreted.3

As Langevin explained it, the experiment can be understood equally well in terms of both classical and relativistic theories (the phenomenon involves only a first-order effect in v/c). The fringe shift is produced by the difference in optical path for light beams traveling in opposite directions around the rotating disk. In fact, the experiment provides a neat way to demonstrate the rotation of a system without reference to anything external to it. As Pauli observed, “It is essentially the optical analogue of the Foucault pendulum.”4 In terms of general relativity the experiment can be understood by thinking of the light in the coordinate system of the disk as being subjected to a gravitational field directed away from the center.

An analysis of the rotating interferometer experiment existed in Germany even before Sagnac performed it. Max von Laue discussed the theory of the experiment in 1911 in response to Michelson’s suggestion that it be used to demonstrate the earth’s motion with respect to the ether5. He showed that both relativity theory and ether theory predicted the same result for the experiment. Laue discussed Sagnac’, s experiment explicitly in 1919 and the similar experiment of F. Harress in 19206.


1.De L’optique des rayons de Röntgen et des rayons secondaires qui en dérivent (Paris, 1900).

2. A. Dufour and F. Prunier, “Sur I’observation du phénomène de Sagnac par un observateur non entraîné”, in Comptes rendus hebdomadaires des séances de I’Académie des sciences. 204 (1937), 1925–1927.

3. Paul Langevin, “Sur la théorie de relativité et I’expérience de M. Langevin, “Sur I’expérienc de M. Sagnac”, in his Oeuvres scientifiques (Paris, 1950). 467–472.

4. W. Pauli. Theory of Relativity (New York, 1958), 19.

5. Max von Laue, “Über einem Versuch zur Optik der bewegten Körper”, in his Gesammelte Schriften and Vorträge. I (Brunswick, 1961), 154–161.

6. Max von Laue, Das Relativtätsprinzip, 3rd ed. (Leipzig, 1919); and “Zum Versuch von F. Harress”, in his Gesammelte Schrifiten. I. 526–541.


I. Original Works. Sagnac’s papers include “Sur la transformation des rayons X par la matière”, in Éclairage électrique, 18 (1899), 41–48; “Théorie nouvelle des phénomènes optiques d’entraînement de I’éther par la matière”, in Comptes rendus hebdomadaires des séamces de I’Académie des sciences. 129 (1899), 818–821; “Electrisation negative des rayons secondaries issus de la transformation des rayons X”, in Journal de physique théorique et appliquée, 4th ser., 1 (1902), 13–21. with P. Curie: “Sur la progagation de la lumière dans un système en translation et sur I’aberration des étoiles”, in Comptes rendus hebdomadaires des séances de I’Académie des sciences, 141 (1905), 1220–1223; “Sur les interférences de deux faisceaux superposés en sens inverses le long d’un circuit optique de grandes dimensions”, ibid., 150 (1910). 1302–1305; “Les systèmes optiques en mouvement et la translation de la terre”, ibid., 152 (1911), 310–313; “L’éther lumineux demontré par I’effet du vent relatif d’éther dans un interféromètre en rotation uniforme”, ibid., 157 (1913), 708–710; “Effet tourbillonnaire optique. La circulation de I’éther lumineux dans un interférographe tournant”, in Journal de Physique théorique et appliquée, 5th ser., 4 (1914), 177–195; “Les deux mécaniques simultanées et leurs liaisons réelies”, in Comptes rendus hebdomadaires des sénces de I’Académie des sciences,171 (1920), 99–102; and “Sur le spectre variable périodique des étoiles doubles; Incompatibilité des phénomènes observés avec la théorie de la relativité générale”, ibid., 176 (1923), 161–173.

II. Secondary Literature. Sagnac’s scientific work was summarized in connection with the Academy’s award of the Pierson-Perrin Prize in Comptes rendus hebdomadaires des séances de I’Académie des sciences, 169 (1919), 1227–1232. See also Sagnac’s Notice sur les titres et travaux scientifiques de M. Georges Sagnac (Paris, 1920). For discussions of Sagnac’s rotating interferometer experiment, see the works by Langevin, Pauli, and Laue listed in the notes and André Metz. “Les problèmes relatifs à la rotation dans la théorie de la relativité”, in Journal de physique et le radium, 8th ser., 13 (1952), 224–238.

Sigalia Dostrovsky