Abstract
There was a time, not so very long ago, when Niels Bohr’s influence and stature as a philosopher of physics rivaled his standing as a physicist. But now there are signs of a growing despair — much in evidence during the 1985 Bohr centennial — about our ever being able to make good sense out of his philosophical views.3 I would not beg the question of whether or not Bohr’s philosophy of physics can be given a coherent interpretation, but I think that the despair is premature. What has come unraveled is the illusion of understanding given to us by Bohr’s self-appointed spokespeople in various philosophical camps — the logical positivists are chiefly to blame — who sought vindication for their own views more than an accurate reading of Bohr’s. And this does not imply that understanding is impossible. What is needed at the present juncture is really quite simple. We need to return to Bohr’s own words, filtered through no preconceived philosophical dogmas. We need to apply the critical tools of the historian in order to establish what those words were and how they changed over time. We need to assume, at least provisionally, that Bohr’s words make sense. And we need to apply the synthetic tools of the philosopher in order to reconstruct from Bohr’s words a coherent philosophy of physics. The present paper is intended as a contribution to these efforts.
Every description of natural processes must be based on ideas which have been introduced and defined by the classical theory.
Niels Bohr, 19231
There must be quite definite and clear grounds, why you repeatedly declare that one must interpret observations classically, which lie absolutely in their essence .... It must belong to your deepest conviction and I cannot understand on what you base it.
Erwin Schrödinger to Niels Bohr, 13 October 19352
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Notes
Bohr (1923), 117, quoted in Pais (1991), 196, from the English translation, Bohr (1924), 1.
As quoted in Moore (1989), 313.
For a brief discussion of this attitude of despair, see Howard (1987).
One serious shortcoming of the following analysis, which I hope to correct in the future, is that no effort is made here to place Bohr’s views on the role of classical concepts and complementarity, more generally, in their proper historical context, especially as regards the relevant philosophical context. Much nonsense has been written about alleged philosophical influences on Bohr by thinkers like Søren Kierkegaard and William James. Happily, however, some progress is finally being made toward a more adequate historical understanding of the philosophical context in which Bohr worked. I would recommend, in particular, Chevalley (1991a), (1992a), (1992b), and Faye (1991).
It is preferable to speak only of instrument and object, rather than of observer and observed, in order to avoid confusion about the role of a human observer’s subjective consciousness. Much of the literature on Bohr goes astray in assuming that observation is a relation between a physical object and a conscious subject, an assumption fostered by Bohr’s occasional talk of the “subject-object” relationship, especially where he is seeking psychological analogies to complementarity. But when it comes to observation in physics, Bohr is explicit in insisting, time and time again, that the crucial questions concern the relation between measuring instruments and observed objects, a relation located entirely within the physical realm, and that all talk of “subjects” should be avoided. He says, furthermore: “Since, in philosophical literature, reference is sometimes made to different levels of objectivity or subjectivity or even reality, it may be stressed that the notion of an ultimate subject as well as conceptions like realism and idealism find no place in objective description as we have defined it” (Bohr, 1955, 79). Surely, one can always include the human observer, as another physical system, in the instrumentation, but the important point is that, in Bohr’s view, human consciousness plays no role in elucidating the observational situation in quantum mechanics.
For more on the historical background to debates about separability and the independence of physical systems in the context of the developing quantum theory, see Howard (1990).
Here I have focused almost exclusively on the physical reasons for Bohr’s linking of separability, objectivity, and unambiguous communicability. There is also an interesting historical and philosophical context for this linkage. One important part of that context is the neo-Kantian tradition of Erkenntnistheorie in which Bohr and most of his contemporaries learned their philosophy of science. Catherine Chevalley has made a good start at exploring this tradition as it relates to Bohr; see Chevalley (1991a), (1992a), (1992b); see also Faye (1991). Another part of the context is the turn-of-the-century debate about what was then termed “Das Gesetz der Eindeutigkeit” (the law of “univocity” or “non-ambiguity”); for more on this debate, see Howard (1992).
Does this mean that Bohr’s talk of the size of our instruments in ordinary experiments and of the central place of irreversibility in observation is just a mistake? No. The mistake is ours in supposing that he intended size and irreversibility as necessary criteria for classifying a system as an instrument. My hypothesis is that Bohr meant these criteria to be employed instead in characterizing the closure property necessary for the definition of a quantum mechanical phenomenon. Bohr says, in one essay: “The circumstance that such marks are due to irreversible amplification effects endows the phenomena with a peculiarly closed character pointing directly to the irreversibility in principle of the very notion of observation” (Bohr, 1958c, 98). For a fuller discussion of the concept of a “phenomenon” which plays a central role in Bohr’s philosophy of physics, see Bohr (1949), 237–238, and Howard (1979), 178–204. That Bohr did not see irreversibility as playing a crucial role in the solution of the measurement problem is evident from his remarks in a letter to Pauli of 16 May 1947, where Bohr writes: “Here, I have in mind such considerations about the complementary relationships between thermodynamical and mechanical concepts as I tried to indicate in my old Faraday lecture. Just as such considerations offer a consistent attitude to the well-known paradoxes of irreversibility in thermal phenomena, so it appears to me that, notwithstanding the obvious quantitative relationship between such phenomena and the irreversibility of observations, we may more adequately regard thermodynamical considerations and the essence of the observational problem as different complementary aspects of the description” (Bohr, 1985, 454).
In at least one place, however, Bohr does seem to suggest that the physics of Newton and Maxwell is what he has in mind. A note of 11 February 1930 includes these words: “By classical physical theories we mean the usual mechanics and electrodynamics which have shown in a wonderful way how to explain ordinary phenomena; these theories are tied very closely to our ordinary attitudes to nature” (‘Kvanteteorien og de klassiske fysiske Teorier’, BSM, Niels Bohr Archive, Reel 12, 1; as quoted in Honner (1987), 62). But even this remark is not inconsistent with the interpretation of the notion of “classical concepts“ developed below. 10 As a model of a measurement, the Bohm experiment enjoys at least two advantages over the two-slit experiment. For one thing, only discrete spin observables are involved, in contrast to the continuous position and momentum observables; this avoids inessential mathematical complications. But more importantly, the fact that, at the time of the spin measurements, the decay products may even be separated by a space-like interval, and the fact that no physical interaction takes place between L and R after the decay itself, together imply that any novelties of quantum mechanical observation revealed by consideration of the model cannot be the result of any “dis turbance” of the object by the instrument (of L by R), contrary to the suggestions of many commentators, starting with Heisenberg (see Heisenberg, 1930, 20ff.). For Bohr’s criticism of the disturbance analysis, see, for example, Bohr (1958b), 5. There is, of course, the possibility of a non-local, or superluminal disturbance in such an experimental arrangement; but such disturbances must be excluded if we are to preserve consistency with special relativity.
For a fuller account of such an analysis of the Bohm experiment, see, for example, d’Espagnat (1976), 76–91.
For a more detailed statement and proof of this claim, see Howard (1979), 382–386.
“Entanglement” is Arthur Fine’s translation of Schrödinger’s wonderfully apt expression for non-decomposable joint states, “Verschränkung”; see Fine (1986), 67.
But such a criterion of individuation is not without its problems; see Howard (1989), 248–249.
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Howard, D. (1994). What Makes a Classical Concept Classical?. In: Faye, J., Folse, H.J. (eds) Niels Bohr and Contemporary Philosophy. Boston Studies in the Philosophy of Science, vol 153. Springer, Dordrecht. https://doi.org/10.1007/978-94-015-8106-6_9
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