The quantum theory of Bohr has roots in the theories of Rutherford and J. J. Thomson on the one hand, and that of Nicholson on the other. We note that Bohr neither presented the theories of Rutherford and Thomson faithfully, nor did he refer to the theory of Nicholson in its own terms. The contrasting attitudes towards these antecedent theories is telling and reveals the philosophical disposition of Bohr. We argue that Bohr intentionally avoided the concept of model as inappropriate for describing his proposed theory. Bohr had no problem in referring to the works of others as 'models', thus separating his theory from previous theories. He was interested in uncovering 'a little piece of reality'.
The quantum theory of Bohr has roots in the theories of Rutherford and J. J. Thomson on the one hand, and that of Nicholson on the other. We note that Bohr neither presented the theories of Rutherford and Thomson faithfully, nor did he refer to the theory of Nicholson in its own terms. The contrasting attitudes towards these antecedent theories is telling and reveals the philosophical disposition of Bohr. We argue that Bohr intentionally avoided the concept of model as inappropriate for describing his proposed theory. Bohr had no problem in referring to the works of others as 'models', thus separating his theory from previous theories. He was interested in uncovering 'a little piece of reality'.
Bohr’s atomic model, one of the very few pieces of physics known to the general public, turned a hundred in 2013: a very good reason to revisit Bohr’s original publications in the Philosophical Magazine, in which he introduced this model. It is indeed rewarding to (re-)discover what ideas and concepts stood behind it, to see not only ‘orbits’, but also ‘rings’ and ‘flat ellipses’ as electron trajectories at work, and, in particular, to admire Bohr’s strong belief in the importance of Planck’s law.
Something is rotten in the state of Denmark. Shakespeare , Hamlet, Act 1 Scene 4 The rise and fall of the old quantum theory The principles of the old quantum theory were first articulated by Niels Bohr in 1913 in a three-part paper titled “On the Constitution of Atoms and Molecules.” Bohr had adopted Ernest Rutherford's model of the atom, according to which the mass of the atom is concentrated in a small central nucleus, while the electrons orbit the nucleus in planetary trajectories. The key challenge facing Rutherford's model was that it was unstable: according to classical electrodynamics, the electron, which is a moving charged body, should radiate energy and so eventually collapse into the nucleus. Bohr's solution was to incorporate Max Planck's theory of radiation, according to which “the energy radiation from an atomic system does not take place in the continuous way assumed in ordinary electrodynamics, but that it, on the contrary, takes place in distinctly separated emissions” (Bohr 1913, p. 4; BCW 2, p. 164). In this early paper, and most subsequent accounts, Bohr summarized his quantum theory by means of two assumptions or postulates. According to the first postulate, electrons cannot travel in any path they like around the nucleus, rather atomic systems can only exist in a series of discrete “stationary states,” in which the electron is in a particular allowed stable periodic orbit and is not emitting radiation.
What was Bohr's response to the problem of wave-particle duality? Before considering this question in detail it will be useful to survey briefly Bohr's work in physics between 1911 and 1923. Bohr and the ‘old’ quantum theory In 1911 Bohr completed his doctoral dissertation on the classical electron theory of metals. Developed by Sir J.J. Thomson and Lorentz, the theory explained electrical, magnetic and thermal properties of metals in terms of the motions of free electrons. Although Bohr made no use of the quantum hypothesis in his dissertation, he noted that classical electrodynamics was incapable of explaining Planck's radiation law and entailed the ultraviolet catastrophe. After completing his dissertation Bohr pursued his post-graduate studies under Thomson at Cambridge University. Finding that Thomson was little interested in his work, Bohr went on to Manchester University in March 1912, to work under Ernest Rutherford. Bohr's move to Manchester was very opportune: the quantum hypothesis was just beginning to receive a wider recognition, thanks largely to the first Solvay Conference in 1911, which Rutherford attended; Rutherford's laboratory, moreover, was one of the most active centres in the investigation of radio-activity. In order to explain the results of experiments on the passage of alpha and beta particles through matter Rutherford had revived Hantaro Nagaoka's (1904) planetary model of the atom. On this model an atom consists of a positively charged nucleus which is orbited by negatively charged electrons, the number of which corresponds to the number of charges on the nucleus.
Now that Bohr's philosophy of quantum physics has been surveyed in detail, it is time to provide a general description of his philosophy that illuminates the details. The question which I wish to address, then is: what is the general philosophy, or philosophical point of view, that underlies Bohr's philosophy of physics? Can his philosophy be neatly categorised in terms of any well known philosophical position? Apropos his philosophy of physics in particular, the question of primary interest is: to what extent, if any, is Bohr's philosophy of physics realist? No simple answer to this question can be given: indeed, certain realist and non-realist components are subtly interwoven in his philosophy, and to unravel them we shall need a fairly refined conception of what constitutes a realist interpretation of physics. Realism in the interpretation of physics The basis of realism is the view that statements of a certain class predicate properties of real objects, the truth-values of these statements being determined by reality as it is independently of how it appears to us to be. This notion is captured in the following characterisation of what constitutes the adoption of a realist interpretation of a physical theory.
Now that my account of Bohr's philosophy of physics has been completed, it remains for me to give a general assessment of it. What value should we assign to Bohr's main ideas – the notions of correspondence and complementarity? How does his interpretation of quantum physics fare in comparison with some of its rivals? What should our assessment of the Bohr–Einstein debate be? I shall take up these questions in the reverse order. Einstein or Bohr? The final verdict The Bohr–Einstein debate is primarily a dispute about the philosophical foundations in terms of which the interpretation of quantum mechanics should be based: specifically it is a dispute about the intrinsic-properties theory, i.e. about the scope of a realist interpretation of quantum mechanics. Einstein wished to maintain the two realist theses, the intrinsic-properties theory and the objective-values theory, whereas Bohr wished to hold only the latter. My own philosophical leanings are in the direction of realism. To my mind the notion of a physical reality the existence and constitution of which are independent of human thought and perception is coherent and plausible: what physical objects there are, and what properties they have, is not determined by human thought or perception, or by human practical interests. I have sympathy, therefore, with Einstein's position concerning the indefinability thesis and the question of the completeness of quantum mechanics.