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Reposs #14: Quantenspringerei: Schrãűdinger Vs. Bohr

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Reposs #14: Quantenspringerei: Schrãűdinger Vs. Bohr RePoSS: Research Publications on Science Studies RePoSS #14: Quantenspringerei: SchrÃűdinger vs. Bohr Helge Kragh February 2011 Centre for Science Studies, University of Aarhus, Denmark Research group: History and philosophy of science Please cite this work as: Helge Kragh (Feb. 2011). Quantenspringerei: Schrödinger vs. Bohr. RePoSS: Research Publications on Science Studies 14. Aarhus: Centre for Science Studies, University of Aarhus. url: http://www.css.au.dk/reposs. Copyright c Helge Kragh, 2011 1 Quantenspringerei: Schrödinger vs. Bohr HELGE KRAGH 1. Introduction: Two giants of quantum physics When quantum mechanics burst on the scene of physics in the fall of 1925, it was sometimes informally referred to as Knabenphysik because of the tender age of pioneers like Werner Heisenberg, Pascual Jordan, Wolfgang Pauli and Paul Dirac all of whom were in their early twenties. But not all the architects of our present quantum understanding of the world were Knaben. Niels Bohr was at the time forty years old, and Erwin Schrödinger two years his junior, and yet both of these two ‚oldies‛ made seminal contributions to quantum mechanics and its interpretation. It is with these contributions, and more generally the scientific and intellectual relations between the two physicists, that the present essay is concerned. The scientific roads of Bohr and Schrödinger crossed at a number of times, and they were at a few occasions engaged in important dialogues concerning the foundation of physics and the meaning of the new quantum mechanics. It is worth emphasizing from the very outset the differences between these two giants of modern physics, who had not only quite different This is an extended and revised version of an invited lecture given to the Erwin Schrödinger Symposium held 13-15 January at the ESI (Erwin Schrödinger International Institute for Mathematical Physics) in Vienna. The official occasion for the symposium was Schrödinger’s death fifty years ago. I am grateful to Wolfgang Reiter and the Organizing Committee for the invitation. Department of Science Studies, University of Aarhus, 8000 Aarhus, Denmark. E- mail: [email protected]. 2 ideas about the nature of quantum physics but also different backgrounds and scientific careers. While Bohr was a successful school-builder who relied closely on interactions with colleagues and assistants, Schrödinger was much more his own. Although he agreed more with some physicists than others, he was basically not a member of any school, and he often worked in relative isolation, either because he wanted to or in some cases because he was forced to by external circumstances.1 Niels Bohr (1885-1962) Erwin Schrödinger (1887-1961) Ever since his stay in England before World War I, first with J. J. Thomson in Cambridge and subsequently (and more fruitfully) with Ernest Rutherford in manchester, Bohr followed a research programme focusing on atomic structure 1 Raman and Forman 1969 (p. 297) describes Schrödinger in the early 1920s as ‚a marginal man, a loner, a member of no school,‛ and argues that this independence was an important reason why he, contrary to other physicists, felt attracted by the ideas of a wave-particle dualism suggested by another ‚marginal man,‛ namely Louis de Broglie in Paris. 3 and quantum theory. Later, in the 1930s, he successfully extended this programme into the realm of nuclear physics. These were the areas of physics on which he concentrated with great determination, and he stayed with them for fifty years. Schrödinger was, in a sense, a more versatile and less disciplined physicist with a broader range of scientific interests, which in his early years covered radioactivity, thermodynamics, statistical physics and colour theory. He had a deep interest in the general theory of relativity and unified field theories, and he also made noteworthy contributions to cosmology and the physics-biology interface. Naturally he considered atomic physics to be of great importance, but he also stressed that it was only part of a broader and more general picture of science and life, such as he confided in a letter to his colleague, the German physicist Wilhelm Wien. In this letter, written while he was struggling to understand the physical consequences of his new wave mechanics, he said: Bohr’s standpoint, that a space-time description is impossible, I reject a limine. Physics does not consist only of atomic research, science does not consist only of physics, and life does not consist only of science. The aim of atomic research is to fit empirical knowledge concerning it into our other thinking. All of this other thinking, so far as it concerns the external world, is active in space and time. If it cannot be fitted into space and time, then it fails in its whole aim and one does not know what purpose it really serves.2 A broader view of the kind indicated in the letter was certainly not foreign to Bohr, who might well have agreed with the formulation in the second sentence, 2 Schrödinger to Wien, 25 August 1926, as translated in Moore 1989, p. 226. 4 but Bohr cultivated the view in a very different fashion. To borrow a term from historian Loren Graham, both were ‚expansionists‛ rather than ‚restrictionists,‛ although Schrödinger’s expansionism was considerably more limited and cautious than Bohr’s.3 Moreover, Bohr would not subscribe to the last sentence, the content of which constituted a major disagreement between the two physicists. In spite of all their differences, both on a scientific and a personal level, Bohr and Schrödinger had much in common. First of all, they both endeavoured to understand nature on a basic level where physics could not be easily separated from philosophy. To put it differently, they were natural philosophers, not merely physicists in the ordinary and more narrow sense. Indeed, one can fairly say that they (together with Einstein and a few others) belonged to the last generation of this proud tradition of sages. It should also be pointed out that in spite of their disagreements they maintained a great deal of respect for one another on both a personal and professional level, which was an important element in and precondition for their dialogue concerning the meaning of quantum physics. For instance, Bohr was full of admiration for Schrödinger’s works on wave mechanics, which he in 1931 nominated for a Nobel Prize. When Schrödinger received the prize two years later, sharing it with Dirac, it was to a large extent the result of Bohr’s recommendation to the Swedish Academy of Science. Schrödinger was first proposed for the prize in 1928, but it was Bohr’s nomination of him (together with Heisenberg) that gave the wanted result. The Bohr-Schrödinger dialogue is an interesting chapter in the history of modern physics, but of course it was only one among several quantum dialogues involving also other physicists, of which the most important were 3 For the notions of expansionism and restrictionism, see Graham 1981. 5 Einstein, Heisenberg, Pauli and Born. Although Bohr and Schrödinger met several times in the 1920s and 1930s, they were not really closely acquainted and they did not exchange a great many letters. Schrödinger was only once in Copenhagen and Bohr never visited Schrödinger either in Berlin or later during his long period in Dublin. As far as I know, the last time they met was during the 8th Solvay Congress in 1948. While their personal contacts were not extensive, the dialogue between them is nonetheless of great importance to historians and philosophers of science concerned with quantum mechanics.4 2. Before quantum mechanics In the historiography of quantum theory is is customary to distinguish between two research programmes, one of which focused on atomic structure and led to the Göttingen quantum mechanics, and another one which was guided by radiation theory and led to wave mechanics. The first of these traditions had its roots in the old quantum theory of Bohr and Sommerfeld; it stressed the discontinuous nature of quantum processes, wanted to eliminate light quanta, and was mainly anchored in experimental and theoretical spectroscopy. On the other hand, the Einstein-de Broglie-Schrödinger programme was in favour of light quanta and a space-time description of atomic processes, it stressed the necessity of differential equations in quantum mechanics, and it wanted to do away with the discontinuous features of quantum theory. Apart from Bohr, the first research programme was followed by Heisenberg, Pauli and Born; while Max von Laue was among those who supported the second one. Whether this simplistic distinction is reasonable or not, I shall not discuss here, but it is important to point out that although Schrödinger belonged to the second of the 4 Apart from the literature cited later in this essay, see also Chevalley 1992 and Harré 1982, which both are of a philosophical nature. Beller 1999 includes on pp. 122-131 an analysis of Bohr’s dialogue with Schrödinger. 6 research traditions, in his younger days he had research interests that crossed the divide. In spite of coming late to atomic theory, by the early 1920s Schrödinger was well acquainted with the Bohr-Sommerfeld tradition and had himself published a couple of important papers on atomic theory and theoretical spectroscopy. The one paper that I want to call attention to – because it was the one that established a direct connection to Bohr – is a paper of early 1921 in which Schrödinger explained the emission spectra of the alkali metals by suggesting that the valence electron in its elliptic motion would sometimes penetrate, or dip into, the inner stable shell of eight electrons in orbits which he called Tauchbahnen.5 This paper, one of the more important ones in the old quantum theory, was squarely in the Bohr-Sommerfeld tradition of atomic structure and naturally attracted the attention of Bohr in Copenhagen.
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