Pragmatism, Bohr, and the Copenhagen Interpretation of Quantum Mechanics

Abstract

In this article, we argue that although Bohr's version of the Copenhagen interpretation is in line with several key elements of logical positivism, pragmatism is the closest approximation to a classification of the Copenhagen interpretation, whether or not pragmatists directly influenced the key figures of the interpretation. Pragmatism already encompasses important elements of operationalism and logical positivism, especially the liberalized Carnapian reading of logical positivism. We suggest that some elements of the Copenhagen interpretation, which are in line with logical positivism, are also supported by pragmatism. Some of these elements are empirical realism, fallibilism, holism, and instrumentalism. However, pragmatism goes beyond logical positivism in espousing some other key elements of the Copenhagen interpretation, though imperfectly, such as the correspondence principle, complementarity, and indeterminism.

Acknowledgements

This article would not have been possible without the tolerance and assistance of the editor of this journal. Also, we wish to express our gratitude to the referees of the journal for their detailed comments, which helped us to clarify and improve several aspects of the manuscript. Finally, the first author would like to thank Phil Dowe of the School of History, Philosophy, Religion and Classics, University of Queensland, for his support and supervision.

Notes

[1] In doing so, we follow Murdoch's (1987) reading of Bohr's notion of complementarity, defending the idea that Bohr's theory of meaning regarding the ultimate version of complementarity is based on the pragmatic maxim, which itself should be seen within the framework of the pragmatic theory of meaning.

[2] The later Carnap's empiricism has some important commonalities with Bohr's philosophy. Bohr's philosophy distinguishes between the symbolic language of the quantum formalism, which counts as a tool for prediction, and observation sentences that are visualizable in space and time and refer to the so-called individual phenomena. This is similar to the distinction that Carnap draws between the linguistic framework, which lacks empirical content, and observation sentences within it whose truth or falsity must be verified empirically.

[3] Bohr's correspondence rule has found another reading among some philosophers of science, referred to as the ‘generalized (or general) correspondence principle’. Post's (1971) account of the principle is as follows: ‘any acceptable new theory L should account for the success of its predecessor S by “degenerating” into that theory under those conditions under which S has been well confirmed by tests’ (Post 1971, 228). However, there is no consensus among philosophers as to whether such a reading can apply to the quantum–classical relation. For example, Post himself believes that the generalized correspondence principle does not apply to the quantum–classical relation. He maintains that ‘paradoxically, the only counterexample we have been able to find to the General Correspondence Principle is the paradigm example of the relation of quantum mechanics to classical mechanics’ (Post 1971, 233). Radder (1991, 215) rejects this claim, arguing that the generalized correspondence principle does apply to the case of the quantum–classical relation. He sees Bohr's correspondence principle as an instance of the generalized correspondence principle. If Radder is correct, the principle will provide, at least to the limited extent, a basis for a general account of theory change as a rational process through ‘firstly the recognition and rejection of one of the presuppositions of the old theory, and secondly its replacement by a new hypothesis and a new parameter which, when given a limiting value, enables the laws of the new theory’ (Murdoch 1987, 243). It is plausible that Bohr, in establishing his correspondence rule, had the above understanding of the general account of theory change in physics. If this is the case, then it can be said that Bohr's philosophy is in line with Peirce's fallibilism.

[4] If s is the particle's spin angular momentum and ℓ its orbital angular momentum vector, then the total angular momentum j is j = s + ℓ.

[5] Faye (1991, 200) has a more or less similar idea. He calls Bohr's position on realism ‘objective antirealism’, which just as realism operates with a notion of objectivity, and yet, unlike realism, maintains that only decidable statements possess ‘investigation-independent truth values’, not undecidable ones.

[6] However, instrumentalism as embraced by pragmatists differs from the instrumentalism in Carnapian empiricism in one major sense: whereas pragmatists do not draw any difference between analytic and synthetic sentences with respect to their roles as instruments, for the later Carnap only linguistic frameworks whose choice is a matter of practical needs are considered as instruments. The Carnapian version of instrumentalism does not apply to observation sentences whose truth or falseness must be verified empirically. In this respect, Bohr's instrumentalism is closer to Carnapian empiricism than to pragmatism. For example, Bohr's acceptance of Born's statistical interpretation of the wave function stems from his belief that the ψ function has only a symbolic meaning and does not refer to anything real in the world.

[7] This is why Peirce's notion of ‘synechism’ is not in line with Bohr's philosophy: in synechism, everything is regarded as continuous.