ANNALS OF SCIENCE, Vol. 63, No. 4, October 2006, 471Á481 Edward Milne’s Influence on Modern Cosmology THOMAS LEPELTIER Christ Church, University of Oxford, Oxford OX1 1DP, UK Received 25 October 2005. Revised paper accepted 23 March 2006 Summary During the 1930 and 1940s, the small world of cosmologists was buzzing with philosophical and methodological questions. The debate was stirred by Edward Milne’s cosmological model, which was deduced from general principles that had no link with observation. Milne’s approach was to have an important impact on the development of modern cosmology. But this article shows that it is an exaggeration to intimate, as some authors have done recently, that Milne’s rationalism went on to infiltrate the discipline. Contents 1. Introduction. .........................................471 2. Methodological and philosophical questions . ..................473 3. The outcome of the debate .................................476 1. Introduction In a series of articles, Niall Shanks, John Urani, and above all George Gale1 have analysed the debate stirred by Edward Milne’s cosmological model.2 Milne was a physicist we can define, at a philosophical level, as an ‘operationalist’, a ‘rationalist’ and a ‘hypothetico-deductivist’.3 The first term means that Milne considered only the observable entities of a theory to be real; this led him to reject the notions of curved space or space in expansion. The second term means that Milne tried to construct a 1 When we mention these authors without speaking of one in particular, we will use the expression ‘Gale and co.’ 2 George Gale, ‘Rationalist Programmes in Early Modern Cosmology’, The Astronomy Quarterly,8 (1991), 193Á218. George Gale, ‘Philosophical Aspects of the Origin of Modern Cosmology’ in Encyclopedia of Cosmology, edited by Norris S. Herherington (London, 1993), 481Á495. George Gale and John Urani, ‘Philosophical midwifery and the birthpangs of modern cosmology’, American Journal of Physcis, 61 (1993), 66Á73. John Urani and George Gale, ‘E. A. Milne and the Origins of Modern Cosmology: An Essential Presence’, in The Attraction of Gravitation: New Studies in the History of General Relativity (Einstein Studies, vol. 5), edited by John Earman, Michel Janssen and John D. Norton (Boston, 1993), 390Á419. George Gale and Niall Shanks, ‘Methodology and the Birth of Modern Cosmological Inquiry’, Studies in History and Philosophy of Modern Physics, 27 (1996), 279Á296. George Gale and John Urani, ‘Milne, Bondi and the ‘Second Way’ to Cosmology’, in The Expanding Worlds of General Relativity (Einstein Studies, vol. 7), edited by H. Goenner, J. Renn, J. Ritter and T. Sauer (Boston, 1999), 343Á375. George Gale, 2002, ‘Cosmology: Methodological Debates in the 1930s and 1940s’, in The Stanford Encyclopedia of Philosophy (Summer 2002 Edition), edited by Edward N. Zalta (http://plato.stanford.edu/ archives/sum2002/entries/cosmology-30s/), 2002), 1Á18. 3 See, in particular, George Gale and John Urani, 1993 (note 2). Annals of Science ISSN 0003-3790 print/ISSN 1464-505X online # 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/00033790600718578 472 Thomas Lepeltier physics that was not reliant on observation, thus countering the empirical conception in which scientific knowledge begins with observation. Finally, the third term characterized the fact that Milne tried to deduce the laws of physics from general principles in the hope that they would concord with observation. It is from these philosophical and methodological positions that from 1932 onwards, Milne developed a cosmological model to rival the relativistic models.4 While cosmologists had so far applied the laws of physics to the largest object, the Universe, Milne developed a cosmological theory ignoring these laws and relying on a priori hypotheses about the Universe. To put it differently, while the other cosmologists extrapolated the laws of physics to the Universe, Milne deduced these laws from general principles concerning the Universe. This alternative model*which was to inspire Hermann Bondi and Thomas Gold in 1948 in their invention of the steady- state theory5 *did not go unnoticed. It was startling to see a cosmological model, independent of the general theory of relativity, capable of accounting for astronom- ical observations, in particular for the apparent recession of galaxies. However, some researchers judged that Milne’s approach went against scientific method. Milne thus found himself in the middle of an epistemological debate that created a stir in the small world of cosmologists, and even beyond. Because of his atypical position and the extent of his work*Milne sought to reorganize not just cosmology but the whole of physics*his research programme could, according to some commentators, have triggered a scientific revolution if it had been more successful.6 It was not to be the case: Milne’s cosmology had no posterity after his death. This failure does not mean that Milne has not had an influence on modern cosmology. Indeed, as Gale and co. have shown,7 the RobertsonÁWalker metric was developed according to the operational methodology that Milne advocated. However, these authors go further and intimate that Milne played an essential role in the development of modern cosmology and that his rationalism has infiltrated the discipline. For example, they assert that ‘the establishment of the RobertsonÁWalker metric [...], without Milne’s outspoken commitment to a minority philosophical view (i.e. operationalism), would never have come about’.8 Similarly, they claim that the debates sparked by Milne’s work have led to the following outcomes: (1) ‘a pruning of the existential extravagances of ‘‘curved space’’, ‘‘expanding space’’, and [...] conceptual excesses of all sorts, had been accomplished’, as Milne wanted; (2) ‘the hypothetico-deductive methodology [advocated by Milne] had been legitimated’; (3) ‘the rationalist position [of Milne] had been so well grounded that it was available for specific and explicit use by Bondi at the end of the next decade’.9 Calling the approach developed by relativistic models the ‘First Way’, they also write that in ‘the end Milne’s Second Way won legitimacy’.10 4 Edward Arthur Milne, ‘World Structure and the Expansion of the Universe’, Nature, 130 (1932), 9Á 10. 5 Hermann Bondi and Thomas Gold, ‘The Steady-State Theory of the Expanding Universe’, Monthly Notices of the Royal Astronomical Society, 108 (1948), 252Á270. 6 Allen J. Harder, ‘E. A. Milne, Scientific Revolutions and the Growth of Knowledge’, Annals of Science, 31 (1974), 351Á363. 7 In particular, in John Urani and George Gale (note 2). 8 George Gale, 1993 (note 2), 492. 9 George Gale and John Urani, 1993 (note 2), 72. 10 George Gale and Niall Shanks (note 2), 291. Edward Milne’s influence on modern cosmology 473 Milne was without doubt a brilliant astrophysicist, and his cosmological model, at the time it was invented, was recognized as original and sophisticated. Having fallen into oblivion since, its rehabilitation by Gale and co. is welcome. However, as the quotations above show, these authors describe Milne as the victorious champion of the hypothetico-deductive method and of rationalism in physics. This conclusion is exaggerated and leads to confusion. The hypothetico-deductive method is now an integral part of cosmology, but Milne does not seem to bear any responsibility for it. As for his rationalism, it is far from having entered the discipline. The misunder- standing comes, in our opinion, from the fact that the difference between these two methodological positions has not been clearly established. Some physicists can recognize the merits of the hypothetico-deductive method, without necessarily being rationalist or, at least, without adopting physical models that have no link with observation. That is what this article intends to show by going back over the debates sparked by Milne’s approach to cosmology. 2. Methodological and philosophical questions Milne saw his cosmological model as utterly deductive and as deeply different from relativistic cosmology. As he says, there are: two separate ways of attacking the structure of the universe: the one to make use of every available piece of empirical knowledge known to be valid on the small scale; the other to begin with the situation actually presented to us by the totality of things without supposing ourselves to know anything.11 It is this second way that he had chosen, i.e. while relativistic cosmology extrapolated empirical knowledge locally established to devise the structure of the whole, Milne tried to deduce local physics from what he claimed were the inherent principles of that whole. As soon as 1933 (after the first long presentation of his theory12), this approach was sharply criticized by Herbert Dingle.13 The latter thought that ‘its fundamental viewpoint is inconsistent with the general principle of scientific thought’.14 For Dingle, the core of the problem was Milne’s aim to deduce his model from the cosmological principle, which he was still calling ‘the extended principle of relativity’. Dingle argued that this principle is only acceptable if it is interpreted as a hypothesis about the homogeneity of the Universe, that is a revisable hypothesis that depends on observation and not as an axiom.15 For Dingle, scientific method consists in starting from observation and not from a priori hypotheses. All science, the theory of relativity included, is constructed in this way: ‘The spirit of relativity is simply a re-affirmation of Newton’s principle of induction from phenomena. Its fruits*the principle of relativity itself*are the product of countless experiments and observations’.16 In contrast, ‘Milne approaches the problems of physics in precisely the opposite way. He starts, not with phenomena, but with a 11 Edward Arthur Milne, ‘Cosmological Theories’, The Astrophysical Journal, 91 (1940), 129Á158, 133. 12 Edward Arthur Milne, ‘World-Structure and the Expansion of the Universe’, Zeitschrift fu¨ r Astrophysik, 6 (1933), 1Á95. 13 Herbert Dingle, ‘On E. A. Milne’s Theory of World Structure and the Expansion of the Universe’, Zeitschrift fu¨ r Astrophysik, 6 (1933), 167Á179.