Roberto De Andrade Martins. Searching for the Ether: Leopold Courvoiser’s Attempts to Measure the Absolute Velocity of the Solar System // DIO, vol. 17, december 2011

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Roberto Martins

Searching for the Ether

DIO 17

Historians of science have also neglected those researches,5 although they present the largest set of empirical results that was ever published against the theory of relativity by a professional scientist. Courvoisier exhibited an outstanding theoretical and experimental skill, and his results can be regarded as one of the strangest puzzles in the history of relativity.

Courvoisier and relativity

Courvoisier's earliest involvement with relativity was an outcome of his routine measurements of star positions. In the beginning of the twentieth century, Courvoisier had noticed that the right ascension and declination of fixed stars suffered a small influence when they are observed close to the Sun. As this influence had a period of one year, he called it “annual refraction”. His first work on the subject was published in 1905,6 that is, much earlier than the development of the general theory of relativity. In 1911, after the publication of Einstein’s early thoughts on the gravitational deflection of light rays, Erwin Freundlich recalled that Courvoisier's work had exhibited an effect that was qualitatively similar to the one predicted by Einstein.7 Courvoisier interpreted the effect he had measured as due to refraction of light by a denser medium around the Sun, not as a consequence of relativity. It seems that Courvoisier’s opposition to Einstein's work grew steadily from this time onward and he became one of the most intransigent supporters of ether theory after the theory of general relativity received strong confirmation (the eclipse measurements), in 1919. Courvoisier's main anti-relativistic work, however, is not directly linked to “annual refraction”.8

Courvoisier accepted the existence of a static ether, similar to the medium proposed in the early eighteenth century by Augustin Fresnel. That theory led to the conclusion that there could be no first-order influence of the motion through the ether upon optical experiments performed in the Earth. Besides that, the negative outcome of the Michelson-Morley experiment required an additional hypothesis, and Courvoisier accepted that motion

5 Klaus Hentschel studied some of Courvoisier's works but he did not analyse the researches described in this paper. See Klaus Hentschel, “Freundlich, Erwin, Finlay and testing Einstein’s theory of relativity”, Archive for history of exact sciences, xlvii (1994), 143-201; Klaus Hentschel, The Einstein tower. An intertexture of dynamic construction, relativity theory, and astronomy (Stanford, 1997).

6 Leopold Courvoisier, “Kinemara's Phänomen und die ‘jährliche Refraktion’ der Fixsterne”, Astronomische Nachrichten, clxvii (1905), 81-106.

7 Hentschel, The Einstein tower (reí. 5), 10-11.

8 Klaus Hentschel, The Einstein tower (ref. 5), 11, claimed that Courvoisier derived the speed of the Earth’s motion through the ether from his data on annual refraction, but his data for the computation of the speed of the Earth was taken from completely independent sources, as will be shown in this paper.

Roberto Martins Searching for the Ether DIO 17

through the ether produced a real contraction of all moving bodies, according to the early explanation proposed by Fitzgerald and Lorentz. According to Lorentz, the principle of relativity would hold exactly for any optical or electromagnetic phenomenon, but Courvoisier did not follow Lorentz’s theory in this respect. He directly denied the principle of relativity and attempted to measure the motion of the solar system through the ether using several different techniques.

In 1921 Courvoisier published his first thoughts on the possibility of measuring the absolute velocity of the Earth through the ether.9 According to Courvoisier’s own declaration, his early calculations concerning the motion of the Earth were an outcome of routine work.10 In 1920 the Leyden Observatory published the details of a large series of observations of stars close to the North Pole that had been made between 1862 and 1874. Those measurements used an old method aiming to reduce observational errors: the stars were observed both with the meridian telescope directly pointed to them, and with the telescope pointed to the images of the stars reflected by a mercury mirror. This double assessment allowed corrections for any changes of the local vertical due to geological motions. It occurred to Courvoisier that those determinations could be used to measure the speed of the Earth through the ether.

Courvoisier assumed that the reflection of light by a mirror could undergo some influence of the motion of the mirror through the ether, even when the effect was observed relative to the proper reference system of the mirror. Any observable effect should be of the second order in v/c. It would be impossible to detect such a small effect if the speed of the Earth relative to the ether was about 10~4 c (that is, its orbital velocity), because for usual angle measurements (let us say, 60°) a difference of 10~8 would amount to only 0.002" - an effect that could not be observed. However, Courvoisier assumed that there could exist a much larger speed of the whole solar system relative to the ether, and analyzed the data published by the Leyden Observatory searching for some systematic effect.

He computed the difference z- z' between the direct zenith distance z and the reflected zenith distance z' of the stars listed in the catalogue, attempting to find a systematic effect that varied in a periodic way with the sidereal time of observations. Using a graphical method, he did find such an effect, and then he submitted the data to quantitative analysis. He derived an equation to describe the reflection of light in a moving mirror and

9 Leopold Courvoisier, “Zur Frage der Mitführung des Lichtäthers durch die Erde”, Astronomische Nachrichten, ccxiii (1921), 281-8; idem, “Über astronomische Methoden zur Prüfung der Lichtätherhypothese”, Astronomische Nachrichten, ccxiv (1921), 33-36.

10 Leopold Courvoisier, “Ergebnisse von Beobachtungen und Versuchen zur Bestimmung der ‘absoluten’ Erdbewegung”, Scientia, xlvii (1930), 165-74; French translation: “Résultats d’observations et d’expériences faites pour la détermination du mouvement ‘absolu’ de la Terre”, Scientia (supplément), xlvii (1930), 76-84.