Georges M M Sagnac

Georges M. M. Sagnac
Born	(1869-10-14)October 14, 1869 Périgueux, France
Died	February 26, 1928(1928-02-26) (aged 58) Bellevue, Meudon, France
Residence	Meudon, France
Nationality	French
Alma mater	École Normale Supérieure, University of Paris
Known for	Sagnac Effect, Optical ether, X-ray secondary radiation
Scientific career
Fields	Optics, X-ray physics, Interferometry
Institutions	University of Lille, University of Paris (Sorbonne)

Georges Marc Marie Sagnac (14 October 1869 – 26 February 1928) was a French experimental physicist. In 1913 he built a rotating interferometer and showed that the interference of two counter-propagating light beams depends on the rotation of the apparatus — the Sagnac effect, today the operating principle of the ring laser gyroscope and a correction applied routinely in GPS. He announced the result as a direct proof of the luminiferous ether.

Sagnac's case is among the most instructive on this wiki, because his fate was not that of a crank ignored for lack of results. He was a first-rate experimentalist at the centre of French physics: a friend and correspondent of Pierre and Marie Curie, Jean Perrin, Paul Langevin and Henri Poincaré, one of the first in France to investigate X-rays, and the discoverer of effects that anticipated X-ray spectroscopy and the X-ray photoelectric effect. His 1913 measurement has never been challenged — it is reproduced daily in commercial navigation instruments. What was set aside was his interpretation of it, and with it the man himself. He died in 1928 having watched his life's framework declared obsolete, described in his brother's obituary as un vaincu de la vie — one defeated by life.

Biography

Early life and education

Sagnac was born on 14 October 1869 in Périgueux, into a bourgeois family; his father was a lawyer who directed the Assurances Générales and his mother was a notary's daughter. His mother raised him with the help of an uncle who became Mayor of Périgueux. His brother Philippe became a distinguished historian.

He entered the École Normale Supérieure in 1890 already drawn to optics and familiar with Newton's and Huygens's writings on the subject. There he studied the work of the two French masters he took as his models for life, Augustin Fresnel and Hippolyte Fizeau, and began to develop an original theory of the propagation of light as multiple scattering by molecules interspersed in a stationary ether. He took the agrégation in 1893 and became agrégé préparateur in charge of optical experiments in Edmond Bouty's laboratory at the Sorbonne, receiving the docteur ès sciences in 1900.

Poor health dogged him throughout his life, and he spent much of it at home with his mother. His recreations were alpinism — which he turned to scientific use, testing an optical theory of the blue of the sky from mountain summits — and music.

Career

After completing his dissertation Sagnac taught as maître de conférences at the University of Lille until 1904, then returned to the University of Paris, where he was appointed professor and from 1919 held a chair that also required him to teach "celestial physics." He was twice a candidate for the physics section of the Académie des Sciences and was never elected. He received the Pierson-Perrin, Wilde and Lacaze prizes.

He made repeated use of the Académie's plis cachetés — sealed envelopes deposited to establish priority without publication. The last of them, dated 18 August 1913, contains his first account of the effect that bears his name.

Scientific work

X-rays and secondary radiation

Following Röntgen's discovery, Sagnac became one of the first in France to study X-rays. He showed that early claims of X-ray diffraction were optical illusions, and established that when X-rays fall on heavier metals they produce a heterogeneous secondary radiation of lower frequency — anticipating what would later be understood as X-ray fluorescence and X-ray spectroscopy. He further demonstrated that this secondary radiation contains a negatively charged component, inaugurating the study of the X-ray photoelectric effect, and argued that it was through these negative rays that X-rays ionize air.

The work brought him into the centre of French physics. His demonstration that Jean Perrin's "metal effect" was really due to secondary radiation prompted Perrin's rueful complaint in a letter to Langevin: "Sagnac has just demolished one third of my dissertation." Pierre Curie was his closest friend and his collaborator on one occasion; Curie's accidental death in 1906 affected him for the rest of his life. Marie Curie recorded that they had traded ideas around the time of the discovery of radioactivity.

The optics of moving bodies

Sagnac's guiding conviction was that light propagates through a stationary ether, and that the optical effects of motion follow from a single principle he formulated as a relation between the phase shift around a light circuit and the circulation of the relative ether flow. He gave an ingenious account of the Fresnel drag on this basis, and by 1908 had generalised his principle to the form δt = −∫v(r)·dr/c², an expression whose structure he drew from the circulation theorems of his friend Vilhelm Bjerknes.

His experiments of 1910 and 1913 were designed to test the rotational character of that ether flow.

The 1913 experiment

Sagnac mounted a light source, four mirrors and a photographic plate on a single disk that could be spun. Light from the source was divided into two beams which traversed the polygonal circuit in opposite directions before recombining to form interference fringes. Reversing the direction of rotation shifted the fringes, by an amount proportional to the angular velocity and to the area enclosed.

He published the result in the Comptes rendus in 1913 under titles that left no doubt what he took it to mean: "L'éther lumineux démontré par l'effet du vent relatif d'éther dans un interféromètre en rotation uniforme" (The luminiferous ether demonstrated by the effect of the relative ether wind in a uniformly rotating interferometer), and "La preuve de la réalité de l'éther lumineux par l'expérience de l'interférographe tournant" (The proof of the reality of the luminiferous ether by the rotating interferograph experiment). A fuller account, "Effet tourbillonnaire optique," followed in 1914.

The reception among those competent to judge the experiment itself was strong. Reviewing recent advances in optics in 1914, the Marseilles professor Louis Houllevigue wrote:

M. Sagnac seems to have taken all the cautionary measures we might imagine in order to shelter himself from errors. If he has not omitted anything, the result of his experiment is one of the most important that optics has registered since Fresnel.

For a fuller treatment of the effect and the century of argument that followed, see Sagnac Effect.

Sagnac and relativity

The dispute

The orthodox response, given its standard form by Paul Langevin in 1921 and 1937, is that the rotating frame is non-inertial and that special relativity predicts the effect exactly; Max von Laue had shown as early as 1911 that stationary-ether theory and relativity yield the same first-order result. Since both frameworks reproduce a first-order effect, the experiment does not by itself decide between them — which is why the argument has never closed. Sagnac himself never accepted the relativistic reading, and continued to hold that the fringe shift exhibited a real ether wind.

Einstein in Paris, 1922

When Einstein visited Paris in the spring of 1922 and reported on relativity at the Collège de France, the sessions were followed by two rounds of discussion with French physicists and mathematicians. Sagnac spoke in the first round. Einstein did not engage with him. The historian Olivier Darrigol records that other experts in the optics of moving bodies and in relativity likewise ignored Sagnac's theory, noting in mitigation that Sagnac sketched it only briefly and made little effort to be understood.

The withdrawn praise

The clearest documented trace of how the ground shifted beneath Sagnac is found in the reports of the chemist Daniel Berthelot. In an earlier assessment Berthelot had written of Sagnac's work:

This penetrating synthesis, based on a subtle analysis of phenomena and on a series of ingenious new experiments, reveals a great effort of thought which, beyond mathematical formulas, endeavors to reach the reality of physical phenomena.

When Berthelot came to report on Sagnac's candidature for a vacant seat at the Académie des Sciences in 1923, he dropped most of his description of Sagnac's theory, and the passage of praise with it. Darrigol's reading is direct: "Perhaps he then judged that the winds had turned in favor of relativity theory." Sagnac was not elected.

The episode is worth dwelling on, because it is the sociology of a paradigm change caught in the act, in the archives, with names and dates attached. Nothing in Sagnac's experiments had been refuted between the two reports. What changed was which way it was prudent to be facing.

The final work

From 1919 Sagnac published a series of Comptes rendus notes developing what he called an "absolute mechanics of undulations," distinguishing "outer laws" governing the time-averaged energy of radiation, which obeyed Galilean relativity, from "inner laws" governing the details of wave propagation in the ether, which depended on motion through it. Wave groups travelled at c with respect to their source, in the manner of an emission theory, while the elementary sine waves did not.

Required from his 1919 chair to teach celestial physics, he turned the duty into a search for astronomical evidence. In 1922 he took up an anomaly in the Doppler shift of certain double stars, in which the extrema of the shift failed to coincide with the corresponding phases of orbital motion, and explained it by the differing retardation of light emitted when the source approaches and recedes. He reported quantitative agreement. Every one of his remaining publications, through the last in August 1924, concerned double stars and their phase anomalies, and each was intended to verify his theory and to disprove general relativity.

Historians have judged these final papers harshly — Darrigol calls the campaign quixotic and notes that Sagnac, after the war, indulged in speculation that the younger Sagnac would have been the first to criticise. Whether that decline is best read as intellectual or as medical is not settled: his health collapsed in 1924, and by 1926 he had applied for retirement.

Death and afterward

Sagnac died on 26 February 1928 at Bellevue, in Meudon, aged fifty-nine — "suddenly" according to his brother Philippe, "after a long illness" according to Bénard. Philippe Sagnac's obituary asked how a scientific life that "started so happily and went on so brilliantly" could "finish so early," and concluded that for some years his brother had no longer been himself, but un vaincu de la vie.

He lived just long enough to watch relativity and quantum theory dismantle the classical optical worldview that had sustained his entire working life. The effect survived him. Darrigol, who is no partisan of the ether, ends his study of Sagnac with a line this wiki can endorse without qualification: "A lot more of his life is worth remembering."

Legacy

The Sagnac effect became a textbook classic and an obligatory exercise for relativists, who felt compelled to account for it in both special and general relativity. Béla Pogány's improved repetition of the Harress experiment in 1926 was named the "Harress-Sagnac" experiment; variants have been performed continuously from the interwar period to the present. When laser technology arrived, the effect became the basis of the ring laser gyroscope and the fibre-optic gyroscope, and it is corrected for in GPS as a matter of routine engineering.

Among the researchers collected on this wiki, Sagnac's own reading of his experiment has never been abandoned. Franco Selleri, Alphonsus G Kelly, John-Erik Persson, Ruyong Wang, Howard C Hayden and others have continued to argue that the effect points to a preferred frame and an anisotropic one-way speed of light. Darrigol, surveying that literature, dismisses it as "the anti-relativist sect"; the papers themselves are gathered at Sagnac Effect, and readers may judge for themselves whether a century of continued experimental work — including Wang's demonstration of a linear Sagnac effect in Physics Letters A — deserves that description.

His name is also carried by the Sagnac Award, presented within this community not for results or new ideas but, as Peter Marquardt said in conferring it on Halton C Arp in 2011, to honour "Freedom and Courage."

Selected publications

• "L'éther lumineux démontré par l'effet du vent relatif d'éther dans un interféromètre en rotation uniforme," C. R. Acad. Sci. Paris 157 (1913), pp. 708–710.
• "La preuve de la réalité de l'éther lumineux par l'expérience de l'interférographe tournant," C. R. Acad. Sci. Paris 157 (1913), pp. 1410–1413.
• "Effet tourbillonnaire optique. La circulation de l'éther lumineux dans un interférographe tournant," J. Phys. Theor. Appl. 4 (1914), pp. 177–195.
• "Éther et mécanique absolue des ondulations," C. R. Acad. Sci. Paris 169 (1919), pp. 469–471, 529–531.
• Notice sur les Titres et Travaux Scientifiques, Gauthier-Villars, Paris (1920).

Related papers on this wiki

• 1926 - Repeating the Harress-Sagnac Experiment (English Translation) — Béla Pogány, trans. Walter Rella
• 1965 - Georges Sagnac and the Discovery of the Ether — John E Chappell
• 1991 - On a Recent Mininterpretation of Sagnac's Experiment — Howard C Hayden
• 1997 - Sagnac's Experiment Misdescribed — Robert B Driscoll
• 2002 - The Interpretation of the Sagnac Experiment — Domina Eberle Spencer

See also

• Sagnac Effect — the effect, the century of argument, and 35 papers on this wiki
• GPS — where the Sagnac correction is applied operationally
• Relativity

External links

• Olivier Darrigol, "Georges Sagnac: A life for optics", C. R. Physique 15 (2014), pp. 789-840 — the definitive scholarly biography, open access
• "The Sagnac effect and its interpretation by Paul Langevin", C. R. Physique (2017)