I outline a neo-Bohrian interpretation of quantum mechanics -- a view of quantum mechanics that accords with the core insights in Bohr's thinking, with a twist that justifies the prefix `neo.' In a second part of the paper, I show how von Neumann's work on infinite direct products provides a theoretical framework that deflates the measurement problem and justifies Bohr's insistence on the primacy of classical concepts.
Comments
15 pages. Some references added to this version, as well as some clarifying changes to section 2. Several technical edits suggested by Krzysztof Sienicki in arXiv: 2512.24198 have been incorporated into this version
I outline a neo-Bohrian interpretation of quantum mechanics -- a view of quantum mechanics that accords with the core insights in Bohr's thinking, with a twist that justifies the prefix `neo.' In a second part of the paper, I show how von Neumann's work on infinite direct products provides a theoretical framework that deflates the measurement problem and justifies Bohr's insistence on the primacy of classical concepts.
In a recent preprint [1] Jeffrey Bub presents a discussion of neo-Bohrian interpretations of quantum mechanics, and also of von Neumann's work on infinite tensor products [2]. He rightfully writes that this work provides a theoretical framework that deflates the measurement problem and justifies Bohr's insistence on the primacy of classical concepts. But then he rejects these ideas, on the basis that the infinity limit is "never reached for any real system composed of a finite number of elementary systems". In this note we present opposite views on two major points: first, admitting mathematical infinities in a physical theory is not a problem, if properly done; second, the critics of [3,4,5] comes with a major misunderstanding of these papers: they don't ask about "the significance of the transition from classical to quantum mechanics", but they start from a physical ontology where classical and quantum physics need each other from the beginning. This is because they postulate that a microscopic physical object (or degree of freedom) always appears as a quantum system, within a classical context. Here we argue why this (neo-Bohrian) position makes sense.
Abstract The Bohr and von Neumann views on the measurement process in quantum mechanics have been interpreted for a long time in somewhat controversial terms, often leading to misconceptions. On the basis of some textual analysis, I would like to show that—contrary to a widespread opinion—their views should be taken less inconsistent, and much closer to each other, than usually thought. As a consequence, I claim that Bohr and von Neumann are conceptually on the same side on the issue of the universality of quantum mechanics: hopefully, this might contribute to a more accurate history of the measurement problem in quantum mechanics.
I distinguish two senses in which one can take a given physical theory to be `complete'. On the first, a complete physical theory is one that, in principle, completely describes physical reality. On the second, a complete physical theory is one that provides all of the conceptual resources one needs to describe any (in general probabilistic) physical phenomenon to any level of detail one likes, in principle. I argue that while the (neo-)Everettian approach to interpreting quantum mechanics aims to show that it is complete in the first sense, the (neo-)Bohrian approach begins from an understanding of quantum mechanics as being complete in the second sense. I then discuss some of the essential differences between how classical and quantum theory describe phenomena, and the way in which the quantum description can be thought of as a ``natural generalisation'' (to use Bohr's phrase) of the classical description. Finally, I elaborate upon the two visions of physics from which one can motivate the first and the second sense of completeness, respectively: \emph{metaphysical realism}, on the one hand, and what I will call \emph{methodological realism}, on the other -- and discuss what one can say about the significance of the differences between quantum and classical description from each of these points of view. I suggest that there is a sense in which the views of (neo-)Everett and the views of (neo-)Bohr can be understood to be mutually supporting positions, from their respective perspectives, even though they are diametrically opposed.
For Bohr quantum mechanics is complete. It expresses all that can be said about the atomic world, which he regards as a newly discovered and separate ontological component of reality, operating at the Planck scale, and describable only with a separate physics, namely, quantum mechanics. The classical world, in contrast, represents its binary and symbiotic counterpart, which is also ontologically substantive, but operates according to 'classical concepts'. The different physics of each component indicates a different ontology, the classical one is accessible to us, as we are classical objects, where the Planck scale does not apply, while the other is not. The measurement problem cannot therefore be solved through the discovery of hidden variables as these are classical in nature and therefore ineffective in explaining the quantum world. The measurement problem is therefore not due to epistemological limitations but emerges directly from a 'two worlds' dualism. Bohr's Copenhagen interpretation has therefore been erroneously misunderstood as a positivist retreat rather than as a view of reality proposing an ontological structure which is dualistic. l then propose similarities between Bohr and a ‘two worlds’ view of Kant, where noumena is an ontology which is inherently unknowable as it is devoid of a priori agents, much as the quantum world is unknowable as it is devoid of Bohr's classical concepts. However, just as classical concepts in Bohr are crucial in constituting classical reality, a priori intuitions and concepts in Kant are crucial in constituting phenomena, which are real ontological objects and not just appearances. The noumenal and quantum realms are therefore both substantive, but impossible to investigate. The phenomenal and classical worlds are also substantive. They are where objects and therefore meaning condenses. They are therefore open to our investigations.
We begin by discussing ``What exists?'', i.e. ontology, in Classical Physics which provided a description of physical phenomena at the macroscopic level. The microworld however necessitates a introduction of Quantum ideas for its understanding. It is almost certain that the world is quantum mechanical at both microscopic as well as at macroscopic level. The problem of ontology of a Quantum world is a difficult one. It also depends on which interpretation is used. We first discuss some interpretations in which Quantum Mechanics does not provide a complete framework but has to be supplemented by extra ingredients e.g. (i) Copenhagen group of interpretations associated with the names of Niels Bohr, Heisenberg, von-Neumann, and (ii) de-Broglie-Bohm interpretations. We then look at some interpretations in which Quantum mechanics is supposed to provide the entire framework such as (i) Everett-deWitt many world, (ii) quantum histories interpretations. We conclude with some remarks on the rigidity of the formalism of quantum mechanics, which is sharp contrast to it's ontological fluidity.
The spectacular successes of quantum physics have made it a commonplace to assert that we live in a quantum world. This idea seems to imply a kind of "quantum fundamentalism" according to which everything in the universe (if not the universe as a whole) is fundamentally of a quantum nature and ultimately describable in quantum-mechanical terms. Bohr's conception of quantum mechanics has traditionally been seen as opposed to such a view, not least because of his insistence on the necessity of the concepts of classical physics in the account of quantum phenomena. Recently, however, a consensus seems to be emerging among careful commentators on Bohr to the effect that he, after all, did subscribe to some version of quantum fundamentalism. Against this consensus, and by re-examining the historical record, I will defend a variant of the traditional reading of Bohr in which (1) the answer to what an object is (quantum or classical) depends on the experimental context; and (2) in principle, any physical system can be treated quantum-mechanically but not all systems can be treated that way at the same time.