Abstract
It is still commonplace to see Bohr as a positivistically inspired instrumentalist who treated the formulae of QT (Quantum Theory) as empty of any real content, as mere symbolic instruments for predicting the values of a range of empirical quantities, such as scattering cross-sections and spectral line densities. In particular, it is usual to contrast Bohr’s attitude with a‘realist’ approach which takes the formulae of QT as not only meaningful but also literally true descriptions of the way things really are. Thus Feyerabend writes in his exposition of Bohr’s views:
At the same time we must deny the universal validity of the superposition principle [which here does duty for all the quantum theoretic formalism] and must admit that it is but a (very useful) instrument of prediction. (Feyerabend, 1981, 258)1
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Notes
Feyerabend continues this theme in Section 6 of the same article.
See Hesse 1961 for a discussion of this view.
Sophisticated versions of instrumentalism, such as Poincaré’s, were able to cope with the historical fact that what started out as purely theoretical entities with no direct empirical basis (atoms, for example) later became observable. Because of such cases, one must allow that propositions which were meaningless at one time can become meaningful later (see Krips, 1986).
By contrast, Heisenberg derived them from the quantum theoretic formalism (Heisenberg, 1930, 13–19).
The association between energy and momentum on the one hand, and the wave picture on the other, is made via the fundamental de Broglie and Planck relations: p = hλ and E = hv. These two relations, which are abductions from the well known diffraction effects exhibited by a range of quantum systems and the photoelectric effect, draw connections between the momentum p and energy E of a quantum system on the one hand and a corresponding wave-length λ and frequency v on the other.
The discovery of such implicit relationalism has not been restricted to quantum theory and relativity. People sometimes talk as if a surface really had a single colour which is distorted or appears other than it really is in certain light conditions. But such talk, we now recognize, is misleading. On the basis of current optical theory, we recognize that the emission spectrum of a surface is the basis of its colour. But the emission spectrum is a response to and depends upon the characteristics of the ambient light; and so the colour of a surface emerges as a relation between the surface and the spectrum of the ambient light. In other words, it makes no sense to talk of colour except as a particular response to ambient light conditions. Consequently, the surface’s colour really is different in different light conditions, and the dependence of the colour upon the light conditions emerges as logical in nature. But this dependence is causal as well as logical, because changes in the ambient light conditions actually bring about different colour responses from the surface. By contrast, the dependence which we have considered so far (namely the dependence of the magnitude of temporal intervals upon reference frames and of degrees of fit upon experimental conditions) are purely logical relations — there is no question of causal influences at work as well. The fact that causal relations may also be logical is discussed in Hanson (1958), Chap. 3.
In other words, Einstein’s primary concern (unlike that of the instrumentalist) is with the truth underlying the appearances rather than the appearances as such. Unfortunately, this realist aspect of his work (and that of Bohr) is not always clear, especially in Einstein’s earlier writings.
There is some controversy over how exactly to interpret this probability. One way is as the conditional probability of measuring Q to have value q in S at t given that a Q measurement takes place on S at t. For a discussion of these and other issues, see Krips, (1987).
I take for granted an objective account of the relevant probabilities, so that they can be classified as physical quantities in their own right. For one such account of probabilities, see Mellor (1971).
Part of the reason for this dismissal of probabilities and propensities from classical causal narratives lies in Enlightenment philosophical criticism, both formal and informal, of the medieval ontology of dormative virtues and their ilk.
Some readers may recognize this equation only when written in less general form in terms of the ‘state-vector’ or ‘state-function’ rather than the density operator. I have preferred the more general density operator formalism for the rhetorical reason that its abstractness emphasizes the conventional nature of the ‘state representations’ of QT. The density operator formalism also has various formal advantages which I discuss at length in Krips (1987), especially Chap. 4.
I discuss this interpretation of the quantum formalism in Krips (1987), Chap. 3; see also Jammer (1974), 44, where it is argued that Heisenberg adopted this interpretation in his later work.
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Krips, H. (1994). A Critique of Bohr’s Local Realism. 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_12
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DOI: https://doi.org/10.1007/978-94-015-8106-6_12
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