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Journal of the Japan Association for Philosophy of Science

What is a Quantum Object?

Hiroshi Ohata

Published Jan 1, 2024• 0 citations
Publisher

Abstract

What is a quantum object? According to individualism, quantum objects are treated as individual objects. Therefore, we can count quantum objects because they are distinct individuals. However, if the individuality of a quantum object is given by its substratum, or primitively, the object would possess a definite position, which contradicts the orthodox interpretation of quantum mechanics. Given this situation, I propose that quantum objects should be regarded as “indefinite multi-located individuals.” A quantum object can exist wholly at multiple locations simultaneously but is probabilistically located with respect to each of those locations. This view is based on the following metaphysical framework: the ontology of thisness bundle theory for quantum objects, and the ideology of property (co-)instantiation with probability.

DOI
10.4288/kisoron.51.1-2_93

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What is a quantum object? According to individualism, quantum objects are treated as individual objects. Therefore, we can count quantum objects because they are distinct individuals. However, if the individuality of a quantum object is given by its substratum, or primitively, the object would possess a definite position, which contradicts the orthodox interpretation of quantum mechanics. Given this situation, I propose that quantum objects should be regarded as “indefinite multi-located individuals.” A quantum object can exist wholly at multiple locations simultaneously but is probabilistically located with respect to each of those locations. This view is based on the following metaphysical framework: the ontology of thisness bundle theory for quantum objects, and the ideology of property (co-)instantiation with probability.
The paper sides with the primitive ontology approach to quantum physics, as exemplified notably in Bohmian mechanics. It argues that the objects of the primitive ontology of quantum physics are structurally individuated by the spatial relations in which they stand. They neither have a primitive thisness (haecceity) nor intrinsic properties (such as mass or charge), but only stand in spatial relations. These relations make it that they are physical objects. Furthermore, in virtue of these relations, they are individuals, being absolutely discernible. Ontic structural realism thus plays a twofold role in the philosophy of quantum physics: it comes in not only at the level of the dynamics (structures of entanglement), but already at the level of the objects (primitive ontology) that instantiate the structures of entanglement, these objects being individuated by spatial relations.
Abstract The issue about the nature of quantum systems usually hinges on the category of individual: the question is whether this category can be applied to understand the peculiarities of quantum behavior. However, some metaphysical subtleties are commonly overlooked in this discussion. What is the difference between the notions of object and of individual? What metaphysical principles are quantum systems supposed to fulfill? What ontological kinds of properties are involved in the quantum domain? These questions should be taken into account when the issue at stake is the ontological nature of quantum systems and, more generally, the structure of the quantum realm. In order to address these and other metaphysical questions, different ontological distinctions that are scarcely taken into account in the debate are brought to the fore in this chapter. In light of these distinctions, the ontological challenges that quantum mechanics poses to the ontological picture presupposed by traditional metaphysics will be discussed in the following chapter.
In the non-reflexive formulation of quantum physics, attention is given to a metaphysical view of quantum objects (q-objects) as non-individuals, characterised as entities to which the standard notion of identity lacks sense. The reference to the nature of q-objects is usually omitted in other interpretations and the most we can find is some vague reference to “to each [quantum] system we associate a Hilbert space”, and nothing more is said about them; we need to infer from the theory their characteristics, and it gives us several alternatives. One of them, to be justified here in broad aspects, is the non-individual view. It is important to enlighten that we are not trying to just find a way to deal with quantum entities but to defend a possible ontology of non-individuals, which can be applied also to ‘particles’ arising from quantum fields. Our account is based on two assumptions: (i) the ‘classical theory of identity’ does not apply to q-objects, and (ii) their non-individuality must be attributed from the start, that is, as a primitive notion and not ‘made a posteriori’ say by assuming symmetry principles. The underlying logic is called non-reflexive because the reflexive law of identity ∀x(x=x)\forall{x{({x = x})}}∀x(x=x) is supposed not to hold for q-objects. Since all the details demand a lot of space, here we only sketch and justify our view. The references provide a more detailed account.
Standard quantum mechanics poses serious challenges to some principles deeply entrenched in traditional metaphysics. In particular, the theory defies the ontological category of individual object through three of its features: contextuality, non-separability, and indistinguishability. The difficulties have prompted the development of non-traditional ontological frameworks intended to account for the quantum domain. Some approaches advocate replacing the classical conception of a realm of objects endowed with properties by an ontology that either dispenses with the category of object altogether or treats it as a derivative notion. The Modal-Hamiltonian Interpretation (MHI) of quantum mechanics and Ontic Structural Realism (OSR) exemplify this position. MHI proposes an ontology in which quantum systems are non-objectual bundles of properties linked by definite physical relations. The OSR defends the ontological primacy of structures, within which objects are construed as nodes of relations. The aim of this article is to compare these two ontological approaches, highlighting both their points of convergence and their divergences, particularly in light of the ontological challenges posed by quantum mechanics. With this aim, the article is structured as follows. Section 2 outlines the ontology of MHI, introducing the distinction between type-properties and case-properties, characterizing quantum systems as non-objectual bundles, and analyzing the role of symmetry groups in providing physical content to the theory and governing actualization. Section 3 turns to OSR, examining its motivations, its distinction between physical and mathematical structures, and the modal character of structures. Section 4 offers a systematic comparison of the two approaches, considering their respective motivations and scope, their shared commitment to an object-free ontology, the role played by symmetry groups within each framework, and their contrasting interpretations of modality. Finally, Section 5 presents some concluding remarks.
Russell’s theory of descriptions depends essentially on the concept of identity, in particular to eliminate definite descriptions by contextual definitions. But quantum objects, according to a possible metaphysical interpretation I shall assume without discussion here, seem not to satisfy the standard theory of identity, and do not have individuality in a sense. Even so, physicists refer informally to both ‘a quantum so-and-so’ and, worst, to ‘the quantum so-and-so’, as if these expressions were something like descriptions. In this article, we sketch a possible theory of ‘quantum (definite) descriptions’ using a weaker concept of indiscernibility (or indistinguishability), and advance some consequences such a theory may have for the discussions on the logical foundations of quantum theory.