.i. b- . ~ . 174 VASCO RONCHI La storia della scienza, con numerosi episodi del genere. d!mostra con tutta evidenza quanto sia dannoso per iI progresso it formarsi di compartimenti stagni, che impediscono 10 scambio delle Idee e, rendendo dlfficile la critica, procurano lunga vita agli errori. Vasco RONCHI Firenze, /stituto Nazionale di Ottica, Arcetri A~ c.~tvts l~{~\-",G..{' d \ \iht. rr. ~ ~ \~J (\'e"",pj - vo \. 11S- 1'10 ( \'\('S) c··\) Georges Sagnac and the Discovery of the Ether(1) \,~ J'.o~" ('h"'rr e \\ Most historians and philosophers of science today would probably affirm their belief that their field of study is its own justification. Without attempting to defend the extreme opposite view that all scholarship must be tied to immediate practical needs, I would like to suggest that any discipline is falling short of its true potential unless it recognizes and seeks to accomplish tasks related to the needs of the larger community of scholars, and indeed to the needs of humanity as a whole. History and philosophy of science is in a unique position to perform an extremely valuable service in this regard. It can help science revitalize its theoretical approach, by re-emphasizing the interpretation of evidence and equations from the point of view of the natural philosopher, as was carried out in the heroic era of early modern c science :.. Today, practicing scientists are very largely unconcerned with the history of their own disciplines, or at least contemptuous of opinions held in past eras; and in addition they largely ignore the careful logical analysis of the metaphysical framework which gives structure to their own working paradigm (2). They iail to realize that metaphysics plays at least as great a part as measurements and mathematical symbols, in guiding the advance of science or natural p.hilosophy. : But not only do we have, at present, a group of scientists not well informed or deeply concerned, on the whole, in history and philosophy; also, to make matters worse, most philosophers are not well-informed on the specifics of experimental evidence, so that (1) An earlier version of this paper was delivered at the 8th Annual Meeting of the Midwest Junto of the History of Science Society, Still· water, Oklahoma, on 2 April, 1965. ) (2) The importance of the c metaphysical framework :. is stressed by Joseph AGASSI in his c The Nature of Scientific Problems and Their Roots in Metaphysics :t, in Mario BUlfGBL (ed.), The Critical Approach to Science and PhllOlOplall, 1964, p. 189-211. The c paradigm ., which I take to mean a metaphysical framework plus various habitual, organiza­ ~loDa", and instrumental appnrtenances, is discussed by Thomas KUHN 1il hiS The Structure 01 Sclenll/fc Reuolullom. 1962. I 178 JOHN E. CHAPPELL Jr. their philosophizing. even when intended to be about science, remains on a remote and artificial level. Add to this the very often prejudice of many historians for accumulating fact without concern for scientific or philosophical issues per sc, and the result is one of the most dangerous and unfortunate examples of over. fragmentation of intellectual endeavor facing twentieth-century mnn. It is in an attempt to bridge the gaps between history, philosophy, and science that I write this paper. A hit of geography is involved also. The novelty of the attempt can be read in tbe title: as tbe result of historical investigation and philosophical analysis, I am led to affirm the undoubted existence of a lumini. ferous ether, contrary to the current beliefs of the great majorit~ of physicists. Experimental evidence is not always what it seems, particularly to those scientists who unduly ignore rigorous logical analysis. For instance, in 1932, the report of the Kennedy-Thorndike inter­ ferometer experiment boldly. announced « Experimental Establish. ment of the Relativity of Time:. (3). In 1937, Herbert Ives unleashed a brief but devastating argument which delllonstrated that the relativity of time (if such a thing exists) has nothing to do with the r('sults of this cxpC'riment, and that the main signilicancf they have is to disprove the Lorentz-Fitzgerald contraction hypo­ thesis. Ives' argument fell short of a definitive solution to the problems raised by Kennedy and Thorndike, hut it led him to attempt another very important experiment to try to supply the evidence which Kennedy and Thorndike did not find. When he and Stilwell had completed this experiment in 1938, they still quite properly refrained from claiming that it proved the relativity or time; but adherents to relativity theory do use those results to make such a claim (4). For reasons which are beyond the scope or this paper to expound in detail, I believe that not even Ives and Stilwell made the proper interpretation of their 1938 experiment. (3) Roy KENNEDY and Edward THORNDIKB, c Experimt'ntal Establish. 1m9e3n2t), opf. th4e00R.4e1l8a.tivity of Time >, Physical Review, v. 42 no. 3 (1 No,·, (4) Herhert IVBS, c Graphical Exposition of the l'Ilichelson-Morley Experiment >, Journal of the Optical Society of America, v. 27 no. 5 (MaY 1937), p. 177-180; Hf'rbert IVES nnd G. R. STH.WELL, « Experimental Study of the Rate of a Moving Atomic Clock >, ibid., v. 28 no. 7 (July 1938), p. 215-226. The 1938 IvllS-STILWBLL results were confirmed by __ Htili"r. sRc,h.l a 'M;..A,;NCD!fE;nLhl"E\i'BJo G__ al~n_d Louis E"lI" .... _. _ W~I.T_~T.JBN, c Experimental Verification of p,.. .. ,.... .. ....... _. ___ ..... JIo GEOBGES SAGNAC AND THE DISCOVEBY OF THE ETHER 177 They did not because they were still handicapped by the Maxwell theory of light, which declares that the velocity of light is inde­ pendent of the velocity of the source. The Ritz emission theory, which adds the velocity of the source to that of the light. was introduced in 1908 but was quickly discarded. Not only did it disagree with Maxwell's theory, but it apparently was faced with an insurmountable obstacle in the form of evidence from the opposite limbs of the rotating sun and from binary stars, adduced by Tolman and by de Sitter respectively (5). But the fact that evidence is not always what it seems is again strikingly demonstrated in this case; for now in 1965, J. G. Fox makes clear that the Tolman and de Sitter evidence is no longer considered an adequate refutation of the Ritz theory (6). Fox introduces other evidence which he believes docs constitute such a refutation, but admits that it makes a rather sketchy case. Once again, full discussion of all the issues involved in this problem would be beyond the scope of this paper. I have mentioned the problem of relativity of time and the problem of the Ritz emission theory because they do bear importantly on the issues raised by Georges Sagnac anfl other investigators of the problem of ether e drift, and because I do not think a complete understandin~ of Sagnac's results is possible without realizing that the ~ilz approach to electromagnetic theory is essentially correct and represents the main path for the future development of physics. But for those whose present beliefs are strongly at odds with these opinions of , ..... ...\ I · r~h~ji ~.·l.i i ; '. J"t', I ~ ! (5) Richard TOLMAN. « The Second Postulate of Helatlvity >, Physical ReuieLQ,v. 31 no. 1 (July 1910). p. '2~-40; Willem DB SITl'BR, « A Proof of the Constancy of the Velocity of Light >, ProceedinUs of the Section of Sciences of KOllillkliike Akademie van Weteruchappen te Amsterdam, \'. 15 pi. 2 (1913). p. 1297-1298; Willem DB SITl'BR, c On the Constancy of the Velof'ity of Light :., ibid., v. 16 pt. 1 (1913), p. 395-396. On Hitz's Ihc'ory see Alfred O')lAHILLY, Electromagnetics, 1938. J ((i).J. G. Fox, , Revista Based de la upon the Hertzian Real Academia de It'nClas Exaclas, Fisicas II Naturales de Madrid, v. 57 no. 4 (1963), D. 7?'7_'7.u iJ I!' 178 IOHN E. CHAPPELL Ir. mine, it will be sufficient to read tbe following discussion with an open mind, to become convinced that drastic changes of one sort or another must now be made in our accepted ideas about light velocity and the ether. ••• In the case of the Sagnac interferometer experiment, we do not have evidence which needs new interpretation, so much as we have evidence which needs to shed a harmful cloak of re-interpre­ tation which was placed over it by theoreticians who did not fully understand what the experimenter had done. We need to return to the interpretation of Sagnac, who justly claimed that he had discovered the existence of a luminiferous ether. Ether as discovered by Sagnac need not be considered to be an imponderable elastic solid, as it was considered to be through most of the nineteenth century, after light came to be thought of as a c transverse wave J. Ether is simply a medium through which light travels, with respect to which it keeps a fixed velocity. The ether exists in, but is distinct from, any other body or medium which will transmit a light wave. The ether may be of variable density. The ether is not necessarily itself at rest, in a given coordinate system, but may move en masse with a given velocity. I believe that the ether consists of photons, i. e. of radiation particles, mostly of the lower frequencies. Ligbt passing through this ether bounces off the photons it meets ond thus travels in u zig-zag course. This view enables us to accept a constant point­ to-point velocity for all photons, at the same time that we recognize the possibility that such photons may vary in absolute speed along their respective zig-tag paths, which may be of unequal length. The variation in absolute speed results in variation in the energy received (Doppler effect). Again, I do not wish so much to explain or to confirm this view of the ether in great detail, as to suggest to the reader that a plausible conceptualization of the ether may indeed be possible; for no doubt he has read or bas heard that all attempts to posit the existence of an ether have proved to be failures. As will be made clear below, not only Georges Sagnae but also Albert Michelson,perhaps the most skillful experimenter in the history of optics, proved by experiment that there is a luminiferous GEORGES SAGNAC AND THE DlSCOVERY OF THE ETBER n. ether. In view of the widespread misconception that Michelson is responsible for proving that there Is no luminiferous ether (In the Michelson-Morley experiment of 1887), this is a very important point to make. But let me proceed with the historical facts. :. The famolls « negative evidence , experiment conducted by Michelson and Morley proved merely this: that in the latitude of Europe and the United Slates, one cannot prove Iran&lational motion of the earth with respect to an ether, by means of a cross­ type interferometer. This result led to a crisis in the minds of many physicists of the day, and later became a strong argument in favor of the special theory of relativity, even though Einstein apparently used it only indirectly in his own mental operations leading to the theory (7). Strangely enough, Michelson himself was not particularly interested in the negative result of 1887. Although he did perform another experiment in 1897, using a vertically-arranged rectan­ gular interferometer to test possible variation of ether drift with altitude, again with negative results (81, he left to E.W. Morley, D'.C. Miller, and others the work of repeating and refining the original test with the cross-type interferometer. In fact, Michelson felt that the chief value of the 1887 work was in perfecting the interfero­ meter for use in later experiments of a different nature (9). He was much more impressed with the positive results he and Morley obtained in their less famous collaboration of 1886, a repeat of the Fizeau experiment of 1851 on the behavior of light in movtng media (10). Furthermore, Michelson reacted unfavorably to the interpretation of h~s negative results which was embodied! in (7) Albert MICHELSON and Edward MORLEY, « On the Relative Motion of the Earth and the Luminiferous Ether -, American Journal of Science (ser. 3), v. 34 no. 203 (NoY. 1887), p. 333-345. See also Robert SHANKLAND, . c The Mkhelson-Morley Experiment J. Scientific Amerlcan, v. 211 no. 5 (Dec. 1964), p. 107-114. ; (8) Albert MICHBLSON, c Relative Motion of the Ear1h and Ihe Elher _, American Journal 01 Sclencf.' , (,er. 4), v. 3 no. 18 (Sune 1897). p. 475~79. (9) Bernard JAFPE, Michet.on and the Speed of Light, 1960, p. 89ttO. (10) Albert MICHELSON and Edward MORLEY, c InOuence of Motion or the Medium on the Velocity of Light -, American J