Einstein K Einstein and the rebirth of the ether o s t r o Although Einstein is widely credited with abolishing the ether concept, he and the ether actually introduced a new relativistic ether in 1916, developing the idea in his later works. • How Einstein came to reject the 19th century ether • Three relativistic ether models developed by Einstein • Einstein’s treatment of space-time as a material entity—a “new ether” Einstein, as Kostro shows in great detail, acknowledged only three kinds of ether. But what Einstein called “ether” is no longer a rarified material medium that permeates all space, but rather the much more abstract geometrodynamic constituent of spacetime which determines the inertio-gravitational behavior of matter… We must be indebted to Professor Kostro for having devoted himself to study in such detail this facet of Einstein’s work and for drawing our atten- E tion to this generally unknown chapter in the scientific biography of the man whom the periodi- i n cal Time recently named “Person of the Century.” s t e —from the Foreword by Prof. Max Jammer in a n d About the Author th e e Ludwik Kostro studied physics and philosophy at the “Sapienza” University and the t h Gregorian University in Rome from 1963 to 1970. In 1975 he joined the University of e r Gda_sk, until 1994 as a Lecturer and Assistant Professor in the Physics Institute and from 1994 onward as a Full Professor in the Institute of Philosophy and Sociology, of which he served as Director. He is presently Director of the Department for Logic, Methodology and Philosophy of Science at the same University. Since 1988 he has been a member of the Editorial Board of the journal Physics Essays (Ottawa, Canada). He is a member of the board of the Interdivisional Group of History of Physics at the European Society of Physics, and serves on the Scientific Committee of the Interna- tional Conferences on Physical Interpretations of Relativity Theory held every two years at Imperial College in London and sponsored by the British Society for Philosophy of Science. Since 1986 he has been Secretary of the Department of Mathematics, Physics and Chemistry at the Gdansk Scientific Society. He is the author of 79 scientific papers in physics and philosophy, as well as several books, e.g., Eros, Sex and Abortion in the Critical Catholicism (Scientia, 1999). He has been awarded a number of major prizes. The French Goverment decorated him with the Les Palmes Académiques medal. A p Ludwik Kostro e ir o n Einstein and the Ether Ludwik Kostro Apeiron Montreal Published by Apeiron 4405, rue St-Dominique Montreal, Quebec H2W 2B2 Canada http://redshift.vif.com © Ludwik Kostro First Published 2000 Canadian Cataloguing in Publication Data Kostro, Ludwik Einstein and the ether Includes bibliographical references and index. ISBN 0-9683689-4-8 1. Ether (Space)--History--20th century. 2. General relativity (Physics) 3. Einstein, Albert, 1879-1955. I. Title. QC177.K68 2000 530.1 C00-901214-1 Cover design by Dominic Turgeon Table of Contents Foreword.......................................................................................................................i Abbreviations..............................................................................................................v Introduction.................................................................................................................1 Chapter 1 Einstein’s views on the ether before 1905............................................................11 1.1 First notions of electromagnetism and the ether........................................11 1.2 Einstein’s youthful “scientific work” on the ether and magnetic field....................................................................................................................12 1.3. Einstein designs experiments to confirm the Earth’s motion through the ether.............................................................................................15 1.4. Einstein’s first doubts about the existence of the ether and the electrodynamics of his time...........................................................................16 1.5 Conceptual premises for doubts concerning the ether..............................17 Chapter 2 Einstein denies the existence of the ether (1905-1916)......................................27 2.1. Works published before the Special Relativity Theory..............................27 2.2 The ether becomes superfluous....................................................................29 2.6 Beginnings of the General Theory of Relativity.........................................33 2.7 Einstein finds a new argument against the ether........................................35 2.8 Other works in which Einstein rejects the ether........................................38 2.9 Origin of the dispute between Einstein and Lenard..................................40 2.10 Minkowski’s four-dimensional world...........................................................43 2.11 Einstein on the path to the new ether..........................................................44 2.12 Summary...........................................................................................................61 Chapter 3 Einstein introduces his new concept of the ether (1916-1924).........................63 3.1 Correspondence with Lorentz, polemic with Lenard................................63 3.2 The new ether concept in the “Morgan Manuscript”....................................77 3.3 The anti-Einstein campaign over the ether.................................................79 3.4 Preparations for an extensive presentation of the new ether concept..............................................................................................................84 3.5 The Einstein–Lenard debate in Bad Nauheim...........................................85 3.6 Lenard’s reaction to Einstein’s response.....................................................88 3.7 Weyl replies to Lenard’s objections..............................................................88 3.8 Einstein’s inaugural lecture in Leiden...........................................................91 3.9 Eddington’s relativistic ether.........................................................................98 3.10 Weyl’s improved version of the relativistic ether....................................100 3.11 Kaluza’s pentadimensional world..............................................................100 3.12 Einstein’s second major work on the new ether.....................................101 3.13 Evolution of Einstein’s epistemological views.........................................106 Chapter 4 Development of Einstein’s ether concept (1925-1955)...................................115 4.1 Einstein’s first attempt to solve the unification problem.......................116 4.2 The Kaluza-Klein pentadimensional continuum.....................................117 4.3 Space-time continuum with teleparallelism...............................................117 4.4 Four-dimensional space-time with pentavectors.....................................129 4.5 Anti-Einstein campaign. Einstein leaves Europe.....................................132 4.6 Elementary particles as “portions” of space.............................................137 4.7 History of ether continued in relativity theory.........................................140 4.8 Material nature of the space-time continuum...........................................143 4.9 New attempt to improve Kaluza’s theory.................................................144 4.10 Einstein finally rejects Kaluza’s theory......................................................144 4.11 The theory of bivector fields.......................................................................145 4.12 A new attempt to generalise General Relativity.......................................146 4.13 Asymmetric field—return to the 1925 idea..............................................147 4.14 Changes in Einstein’s views on physical space.........................................148 4.15 Did Einstein stop using the term “ether” after 1938?............................149 4.16 New editions of Einstein’s works on the new ether................................152 4.17 Did the idea of a relativistic ether survive?...............................................153 Chapter 5 Physical meaning of Einstein’s relativistic ether................................................157 5.1 Einstein stresses the model-like nature of physical cognition...............157 5.2 Einstein’s space-time models and contemporary physics.......................158 5.3. Three models of Einstein’s relativistic ether.............................................166 5.4. Essential attributes of Einstein’s ether......................................................168 5.5 “Physical space,” “ether,” “field”: are they synonymous?......................183 5.6 Should the expressions “new ether” and “relativistic ether” be used today?.....................................................................................................185 Appendix Original Quotations...............................................................................................189 a: Introduction........................................................................................................189 b: Chapter 1.............................................................................................................190 c: Chapter 2.............................................................................................................192 d: Chapter 3.............................................................................................................198 e: Chapter 4.............................................................................................................214 f: Chapter 5..............................................................................................................221 Bibliography................................................................................................................227 Index of Proper Names.............................................................................................239 F OREWORD T he July 2000 issue of the American Journal of Physics contains a seven-page-long review essay on three recently published books about Einstein. The reviewer, Jeremy Bernstein, himself the author of an Einstein biography, begins his essay with the question of whether too much has been written about Einstein, and he concludes it with the statement: “We seem to discover new riches in Einstein’s science each year and new oddities about his life. Has too much been written about him? Not yet.” Professor Ludwik Kostro’s present study of “Einstein and the Ether” corroborates this conclusion. It challenges the widespread view that Einstein in his theory of relativity abolished, once and for all, the concept of the ether from modern physical theorising. True, in his famous 1905 seminal paper “On the Electrodynamics of Moving Bodies” Einstein declared: “The introduction of a ‘luminiferous ether’ will prove to be superfluous inasmuch as the view here to be developed will not require an ‘absolutely stationary space’ provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.” But, as Kostro points out, what Einstein regarded as dispensable was the notion of an ether that has been postulated by Maxwell and his followers as well as by Poincaré to serve as the medium that explains the propagation of electromagnetic waves through space. Since such a medium defines an absolute or preferential reference system Einstein could have dismissed it not only as superfluous but even as incompatible with his special theory of relativity. However, half a year after having constructed his general theory of relativity, Einstein readmitted the notion of an ether; for in a letter to Lorentz, a strong defender of this concept, Einstein wrote in June 1916: “...the general theory of relativity is nearer to an ether hypothesis than is the special relativity theory. But this new ether would not violate the principle of relativity, because its state, g = aether, would not be that of μν a rigid body in an independent state of motion, but its state of motion would be a function of position determined via the material processes.” Ludwik Kostro, Einstein and the Ether (Montreal: Apeiron, 2000). i ii Einstein and the Ether For similar reasons also Hermann Weyl, in the 1919 German edition of his treatise on relativity Raum–Zeit–Materie suggested that, because the coefficients of the fundamental metrical tensor determine which world- points interact with another or constitute a Wirkungszusammenhang, the term “gravitational field” should be replaced by “ether.” If furthermore we recall that in 1951 also Paul Dirac in his paper “Is there an aether?” published in Nature, volume 168, declared, though for quantum-field theoretical reasons, that “we can now see that we may very well have an aether, subject to quantum mechanics and conforming to relativity,” it is clear that, contrary to the widely accepted view, the concept of ether is far from defunct. The generally accepted contention that the famous Michelson-Morley experiment dealt the coup de grâce to the ether conception had been challenged as early as December 1911 when William Magie, in an address to the American Physics Society, declared: “The principle of relativity accounts for the negative result of the Michelson and Morley experiment, but without an aether how do we account for the interference phenomena which made that experiment possible?” (Science, volume 35, 1912) But what is the secret of the ether’s longevity? Einstein himself provided the answer when he wrote in the book The Evolution of Physics, co-authored in 1951 with Leopold Infeld: “This word ether has changed its meaning many times in the development of science.” Einstein was certainly right. It is probably no exaggeration to say that no other term in the vocabulary of physical theorising has suffered so many semantic changes. In fact, already at the very beginning of the history of this word, when the term aither, a derivation of the Sanskrit aidh denoting an intensely burning fire, was used in the mythopoetic language of the ancient Greeks, Homer used it as a feminine, Hesiod as a masculine noun; this difference in grammatical gender shows that different conceptions had been associated with that term. For Plato and Aristotle it denoted the supramundane fifth element which later commentators identified with the quinta essentia and ascribed to it theological connotations. For the Stoics it was the pneuma, the medium for the interactions in physical processes but also the source of life. Beginning with Descartes, followed by Newton and Boerhaave, the ether became an integral part of the mechanical philosophy and reached its climax in the different ether theories of the nineteenth century. For Oliver Lodge, for example, one of the most vociferous advocates of the ether, it served as the medium not only for the propagation of electromagnetic waves but Foreword iii also for the transmission of thoughts in extrasensory or telepathic phenomena. The profusion of different ether conceptions is so vast that all the books that have been written on this subject, such as those by E. M. Lémeray (1922), E. T. Whittaker (1951), K. F. Schaffner (1969), L. S. Swenson (1972), G. N. Cantor and M.J.S. Hodge (1981), present only a small part of the story. It would be wrong to assume that the different ether conceptions evolved sequentially one from the other. O. Moon, the author of a book on Fresnel and his ether, reports that in the middle of the nineteenth century fourteen disparate ether concepts had been in use at one and the same time. Einstein, as Kostro shows in great detail, acknowledged only three kinds of ether. But what Einstein called “ether” is no longer a rarefied material medium that permeates all space, but rather the much more abstract geometrodynamic constituent of spacetime which determines the inertio-gravitational behaviour of matter. In order to understand this point let us recall that before Einstein space and time had played the role of merely a passive background in which events take place, but Einstein’s theories transformed them into active participants in the dynamics of the cosmos. We should also recall that Einstein created not only the special and general theory of relativity: during the last three decades of his life he was preoccupied, albeit without success, with establishing a third theory, a unified theory that unites the gravitational with the electromagnetic forces. With each of these three theories he associated an “ether” in the above-mentioned sense. The distinction between the three kinds of “ether” finds its mathematical expression in the different properties of the corresponding gravitational potentials g of the fundamental metric μν tensor: the ether of the special theory of relativity is characterised by the condition that g = η , where the latter is the Minkowski metric, the μν μν ether of the general theory by g = g of the Riemann metric, and the μν μν ether of the unified theory by the fact that g ≠ g . μν νμ The term “ether” is unique in the history of physics not only because of the so many different meanings in which it has been used but also because it is the only term that has been eliminated and subsequently reinstated, though with a different connotation, by one and the same physicist. Although Einstein repeatedly emphasised the difference between the two usages of this term, his homonymous application of the term was not only exploited by his opponents, like Lenard, as an argument against the theory of relativity, but was also criticised even by iv Einstein and the Ether his staunch supporters. Max von Laue, for example, the author of the earliest textbook on relativity (1911), wrote in its fifth edition (1952) that the term “ether for the set of the g (or Führungsfeld) “should be avoided μν in order not to revive the idea of a material ether.” Similarly, J. L. Synge, in his well-known treatise on the special theory declared that “it is best to avoid that dangerous word in relativity.” How Einstein’s enemies used this homonymy in the early twenties in order to discredit his theory has been vividly recorded by Philipp Frank in his Einstein biography, where he quotes them as having said: “For a long time efforts were made to convince us of the sensational fact that the ether had been got rid of, and now Einstein himself reintroduces it; this man is not to be taken seriously, he contradicts himself constantly.” Although Einstein’s triplication of the ether has lost today its scientific actuality, his theory of relativity has altered for ever man’s conceptions of space, time, and motion. Even though his ideas about the ether can no longer be upheld, they certainly deserve our attention for historical reasons at least and, in particular, for a deeper understanding of Einstein’s own conception of his general theory of relativity. For it was not by accident that in 1916 Einstein reintroduced the ether which he had previously discarded. At that time he abandoned Mach’s positivistic philosophy in spite of the fact that, when constructing his general theory, he had been fascinated by Mach’s view that inertial resistance counteracts not as an acceleration relative to absolute space but as an acceleration with respect to the masses of the other bodies in the universe. As J. Earman, M. Friedman, J. Norton, J. Stachel and R. Torretti in their recent historical studies of Einstein’s work on general relativity have made abundantly clear, Einstein’s conception of the nature of spacetime is intimately connected with the now much discussed “hole argument” and the “point-coincidence argument.” Although Kostro does not discuss these recent studies, his presentation of Einstein’s arguments for the revival of the notion of ether, as “relative ether” or “total ether”, is a useful contribution to the history of the ontological status of spacetime in modern physics. We are indebted to Professor Kostro for having devoted himself to study in such detail this facet of Einstein’s work and for drawing our attention to this generally unknown chapter in the scientific biography of the man whom the periodical Time recently named “Person of the Century.” Max Jammer Abbreviations AdP Annalen der Physik AHESc Archive for History of Exact Sciences AoM Annals of Mathematics Asc Annals of Science ASPN Archives des sciences physiques et naturelles EA Albert Einstein Archive in the Princeton University Library FPh Forum Philosophicum JFI Journal of Franklin Institute JR Jahrbuch der Radioaktivität MA Mathematische Annalen NRRSL Notes and Records of the Royal Society of London PF Postępy fizyki (Progress in Physics) PhB Physikalische Blätter PhR Physical Review PhZ Physikalische Zeitschrift SPAW Sitzungsberichte der Preussischen Akademie der Wissenschaften SPAW(pmK) Sitzungsberichte der Preussischen Akademie der Wissenschaften, phys.- math. Klasse VDPG Verhandlungen der deutschen physikalischen Gesellschaft VgM Vierteljahresschrift für gerichtliche Medizin VNGZ Vierteljahresschrift der naturforschenden Gesellschaft, Zürich VSNG Verhandlungen der schweizerischen naturforschenden Gesellschaft ZftP Zeitschrift für technische Physik ZMPh Zeitschrift für Mathematik und Physik