Historical Studies in the Physical Sciences, Volume 7

Historical Studies in the Physical Sciences, Volume 7

by Russell McCormmach (Editor)


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ISBN-13: 9780691616971
Publisher: Princeton University Press
Publication date: 03/08/2015
Series: Princeton Legacy Library Series , #7
Pages: 526
Product dimensions: 6.10(w) x 9.10(h) x 3.50(d)

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Historical Studies in the Physical Sciences, Volume 7

By Russell McCormmach


Copyright © 1976 Princeton University Press
All rights reserved.
ISBN: 978-0-691-08169-4


The Ether Problem, the Mechanistic Worldview, and the Origins of the Theory of Relativity



Since the first systematic account by Max von Laue, it has been, and still is, the common practice to introduce the theory of relativity with a survey of the nineteenth century ether problem. By "ether problem" I mean the theoretical and experimental investigations of possible influences of the earth's motion relative to the ether on optical and electromagnetic phenomena. I shall cite a few arbitrarily chosen examples from recent textbooks. Christian Miller begins his book with "a short historical survey of the numerous optical experiments which have been performed in an attempt to detect effects depending on the motion of the apparatus with respect to an absolute space." He says that for Maxwell and his contemporaries "the ether was supposed to represent the absolute system of reference, thus giving a substantial physical meaning to Newton's notion of 'absolute space'." But "the fruitless attempts to find out any influence of the motion of the earth on mechanical, optical, and electromagnetic phenomena gave rise to the conviction among physicists that the principle of relativity was valid for all physical phenomena." W. G. V. Rosser, who aims at filling the gap between advanced textbooks and semi-popular books, gives a detailed account of the aberration of light from the stars and of experiments by Fizeau, Hoek, Airy, Michelson and Morley, and others "to illustrate how the theory of special relativity arose out of classical electromagnetism."

Such a tradition has produced the common understanding that the theory of relativity was formulated as an answer to the ether problem. Scholars holding this view overlook that the ether problem had already received an answer before Einstein's theory in the work of H. A. Lorentz and Henri Poincaré, and that logic would therefore require them to admit Edmund Whittaker's much disputed view that the theory of relativity was formulated by Lorentz and Poincaré. Since Whittaker's book appeared in 1953, many authors, beginning with Max Born, have debated Whittaker's view. Heinrich Lange and G. H. Keswani, agreeing with Whittaker, have asserted that the main results of the theory of relativity were obtained by Poincaré. Charles Scribner Jr., although considering Whittaker's view too extreme, considers Poincaré's work a valuable contribution. Their opponents T. Kahan, Gerald Holton, Stanley Goldberg, Kenneth Schaffner, M. A. Tonnelat, and Arthur I. Miller have insisted on the difference between the Lorentz-Poincaré and Einstein's relativity theory and reject Whittaker's view. O. A. Starosel'skaya-Nikitina, although without referring to Whittaker, has discussed the limitation of Poincaré's scientific thought which prevented him from reaching the theory of relativity. I, too, have briefly discussed the problem.

These discussions have conclusively shown that Lorentz' and Poincaré's theory was not equivalent to the theory of relativity as properly understood, and that Lorentz and Poincaré did not accept the latter theory. But there still remains the question of the origin of the difference between the two theories, that is, the question of the root of Einstein's innovation. In this respect Gerald Holton has contributed the first step forward. He has effectively criticized the traditional view that Einstein put forward his theory chiefly to surmount the difficulty caused by the negative result of the Michelson-Morley ether drift experiment. After a careful investigation he reached the conclusion that "the role of the Michelson experiment in the genesis of Einstein's theory appears to have been so small and indirect that one may speculate that it would have made no difference to Einstein's work if the experiment had never been made at all." In contrast to Einstein, Lorentz, Poincaré, and most other contemporary physicists saw the Michelson-Morley experiment as one of the most urgent problems requiring their theoretical efforts. This difference of attitude toward the experiment between Einstein and the others stems from the difference between the problems which then preoccupied them. The problem that Einsteinviewed as fundamental for physics at that time was different from the central issue of the ether problem which had been discussed by Lorentz, Poincaré, and other contemporary physicists. He saw the situation with a perspective that was quite distinctive. We therefore may say that a fundamental change in the aspect from which problems of physics were viewed was essential for the conception of the theory of relativity. What was the nature of that change? What were the factors that brought about the needed transformation of perspective? The origins of the theory of relativity must be sought in the answers to these questions.

To elucidate the origins of the theory of relativity it is necessary first to consider the actual nature and scope of the ether problem in the nineteenth century. The first part of the present paper is devoted to such a consideration. I do not, however, pretend to give a comprehensive history of the ether problem, but intend only to clarify the nature of the problem with which Lorentz, Poincaré, and others wrestled at the turn of the century. The discussion of the ether problem will help to establish the novelty of Einstein's theory as compared with Lorentz' and Poincaré's theory. The consideration of the novelty of Einstein's approach, especially of his conceptual attitude towards physical problems, requires a reevaluation of the great influence of Mach on Einstein's thought. Differing from the common view, I find Mach's main influence upon Einstein, as far as the genesis of the theory of relativity is concerned, in his devastating criticism of the mechanistic world view. In the last part of the present paper I try to show, by discussing some aspects of Lorentz' and Poincaré's thoughts as well as some remarkable developments in the process by which Einstein's theory became accepted, that a complete emancipation from the mechanistic worldview was of crucial importance for the formation of the theory of relativity.


Histories of the ether problem usually begin with the attempt to explain the aberration of light from the stars and the experiment by François Arago that showed that the refraction of light from the stars was not affected by the motion of the earth. Speaking of these investigations we are unconsciously inclined to believe that they were conducted in an attempt to prove the existence of the ether as an absolute reference system. Such a belief, however, is only a projection into the past of the prejudice of those who have already encountered the theory of relativity. In the early stages of the development of the ether problem, what absorbed the physicists' interest was not the issue of an absolute reference system, but rather the implications of the ether problem for the controversy over the nature of light.

When Arago, in 1810, performed his famous experiment, he designed it on the basis of the emission theory of light, which was then predominant in France. In the wave theory the velocity of propagation of waves is determined exclusively by the properties of the medium, and consequently it has a constant value with respect to the medium irrespective of the motion of the source of light. On the other hand, in the emission theory the velocity of light, in general, depends on the initial velocity with which the light particles are emitted. It therefore must depend on the nature and state of the source of light. Astronomical determinations of light velocity by Ole Rømer and James Bradley had shown that it was constant irrespective of the distance over which the light was propagated. "Some astronomers, however," argued Arago, "doubted that stars having different sizes might emit rays with different velocities...." To test this conjecture it was necessary to determine the velocity of light from various stars with great precision. Arago proposed that this be done by observing the refraction of the light from stars. Some authors had also pointed out that such an experiment would give them a means of investigating the motions of the planets and the sun. Arago thought that "the result [of the experiment] must offer certain data concerning the true nature of light." He used the translatory motion of the earth, "because the motion of our system [the earth] combined with the former [the motion of the whole solar system] would give rise to a sufficiently large inequality [of the light velocities]." It was not his intention to determine the velocity of the earth with respect to an absolute reference system.

Attaching a prism to the front of a meridian circle, Arago observed the deflection by the prism of bundles of light from stars moving toward and receding from the earth. The conclusion which he drew from his observation was not that it was impossible to detect the earth's motion. His conclusions were, first, that light rays are emitted by stars with different velocities, and, second, that, of rays emitted by bodies, only those having velocities within certain limits can be perceived by the human eye. A scientist will generally draw conclusions from a scientific investigation according to the purpose for which he designed the investigation. Arago's conclusions clearly indicate that he intended to solve the currently debated problem of the properties of light particles.

Arago shortly turned to the wave theory of light. In 1815 he acknowledged that Augustin Fresnel's first attempt to explain diffraction by a wave theory was very promising, and he began to encourage andhelp Fresnel. Three years later, in 1818, he encouraged Fresnel to examine if the wave theory could be compatible with the result of his experiment as well as the aberration of light. In response to Arago's suggestion, Fresnel attempted to explain these experimental facts by means of hypotheses of a stationary ether and a drag coefficient of ether within refracting bodies. In the same year, 1818, he finished his Academy Prize paper in which the theory of diffraction was developed to its full extent. His theory of transverse waves was put forward three years later, in 1821. Thus, in the year 1818, the emission theory still reigned and the wave theory was a heresy or, at most, an inferior competitor. Prominent members of the Paris Academy, when they chose the theory of diffraction as the subject of the Academy Prize, expected that a paper on the subject would provide a vindication of the emission theory. Under these unfavorable circumstances Fresnel set out to show the superiority of the wave theory over the emission theory in the explanation of the aberration of light and of Arago's experiment. Referring to Arago's conclusions mentioned above, he stated that the necessity of the hypotheses of the diversity of velocities of light and the limited visibility of light "is not one of the smallest difficulties of the emission theory."

Since the measurement of the velocity of light in a transparent body, which is often cited as the experimentum cruris for the wave theory, was not performed until 1850, the wave theory of light still had not succeeded even in the 1840's. The theory of the aberration of light and Arago's experiment continued to be debated in relation to the legitimacy of the wave theory. When in 1842 Christian Doppler theoretically predicted the effect named after him by discussing the mechanism of propagation of longitudinal waves, he rejected the transverse wave theory of light and stated that, as the difficulties in explaining aberration showed, the assumptions of the transverse wave theory seemed "to contain great inherent improbability." It is, to be sure, upon the hypothesis that light is a transverse wave and not upon the wave theory in general that Doppler cast doubt here. But no one can fail to recognize his strong distrust of Fresnel's theory. His criticism of Fresnel's theory was fully developed in a paper published in the following year, 1843.

In his 1843 paper Doppler classified existing theories of the aberration of light into four groups, each of which, he asserted, had a difficulty of its own. To our eyes all four kinds of explanation seem to be based on the same kinematical principle, but Doppler considered them different from each other because of differences in the physical nature of the motion to which the principle was applied. The four kinds of explanation are: first, the analogy with the phenomenon that rain appears to fall obliquely when we see it from aboard a moving vehicle; second, the consideration of the path of light in the interior of the tube of a telescope, which requires us to tilt the telescope; third, explanation by the combination of the velocities of the earth and light; fourth, William Herschel's explanation that the eyeballs must be rotated forward in the direction of the motion of the earth for the light from a star to reach the center of the retina. Doppler argued that to make the second explanation acceptable one must assume that the ether does not change its position with respect to the solar system. To fill this requirement, however, one must assume that the earth traverses the ether freely without resistance, a hypothesis that is hardly tenable, particularly since it is incompatible with the opaqueness to light of the earth and of other terrestrial bodies. After pointing out the difficulties inherent in the other modes of explanation — immaterial for our present discussion — Doppler concluded that the transverse wave theory, however many facts it might be able to account for, could not be right simply because it clearly contradicted so simple a phenomenon as the aberration of light.

George Gabriel Stokes, too, when he propounded his theory of aberration, directed his criticism to the absurdity of the fundamental hypothesis of the Fresnel theory that the ether moved freely through the earth. But Stokes, contrary to Doppler, believed in the transverse theory of light. His theory of aberration was a part of his efforts to save the transverse wave theory from objections such as Doppler's. Stokes' theory is based on two assumptions: that the ether around the earth moves without the earth having any relative velocity at its surface, and that the motion of the ether is irrotational, that is, that it has a velocity potential. He thus approached the problem hydrodynamically, an approach which came naturally to Stokes who had begun his scientific career as a theoretical hydrodynamicist. On 14 April 1845, four weeks before his theory of aberration was presented at the Cambridge Philosophical Society, Stokes presented to the same society a long memoir on equations of motion for viscous fluids and elastic bodies. Toward the end of this memoir he asserted, on the supposition that solid bodies having small shear elasticity and large plasticity would vanishingly differ from fluids having large viscosity, that the ether, even if it is a fluid, would be able to transmit transverse waves of light. He based his conclusion on the inference that the displacements of the ether particles are small because the wavelength of light is extremely short. It is clear that this argument is intended to solve the then urgent problem of the wave theory of light, that is, the contradiction that whereas the ether, as the medium of transverse light waves, must possess the elasticities of a solid body, it nevertheless exerts no resistance to the motion of the earth. Stokes' attention had been drawn to viscous fluids when, in 1842 or 1843, he learned of James South's experiment that suggested that the air around the plumb of a swinging pendulum moves with it. South's experiment occasioned Stokes to think that fluids might take part in the motion of solid bodies and would naturally have suggested to him the assumption of the "ether drag" in his theory of aberration. If a moving body is to impart motion to the ether, however, there must be tangential stress acting across boundaries within the ether. Such stress would give rise to a shear elasticity and make possible the propagation of transverse waves. Thus, Stokes expected the model of the ether that provided a reasonable explanation of aberration to be, at the same time, the solution to a grave difficulty for the wave theory of light, namely, the enigma of how the fluid ether can transmit transverse waves.

In later years he several times discussed the question of what physical properties should be ascribed to an ether that could satisfy the requirements deriving from his theory of aberration. These discussions ultimately also threw light on the above enigma. He took up the problem in response to James Challis' criticism of his theory of aberration. Refuting Challis' criticism he argued as follows. No stable motion of incompressible fluids has a velocity potential. A fluid that has internal friction, however, can satisfy the requirement of a velocity potential. As he had shown in his earlier paper, a fluid ether having internal friction would behave as an ordinary fluid for the translatory motions of a gross material body and as a solid elastic body for extremely small vibrations. Hence "the astronomical phenomena of the aberration of light should afford an argument in support of the theory of transverse vibrations."


Excerpted from Historical Studies in the Physical Sciences, Volume 7 by Russell McCormmach. Copyright © 1976 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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Table of Contents

  • Frontmatter, pg. i
  • Contents, pg. ix
  • Editor's Foreword, pg. xi
  • The Ether Problem, the Mechanistic Worldview, and the Origins of the Theory of Relativity, pg. 1
  • Einstein’s Early Scientific Collaboration, pg. 83
  • Max Planck’s Philosophy of Nature and His Elaboration of the Special Theory of Relativity, pg. 125
  • The Concept of Particle Creation before and after Quantum Mechanics, pg. 161
  • Chemistry as a Branch of Physics: Laplace's Collaboration with Lavoisier, pg. 193
  • Mayer’s Concept of “Force”: The "Axis" of a New Science of Physics, pg. 277
  • Debates over the Theory of Solution: A Study of Dissent in Physical Chemistry in the English-Speaking World in the Late Nineteenth and Early Twentieth Centuries, pg. 297
  • The Rise of Physics Laboratories in Britain, pg. 405
  • The Establishment of the Royal College of Chemistry: An Investigation of the Social Context of Early-Victorian Chemistry, pg. 437
  • Note on Contributors, pg. 487
  • IN MEMORIAM: Tetu Hirosige (1928-1975), pg. 488

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