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SNF Ambizione
Quantum Indeterminacy
A Metaphysics for Spontaneous Collapse Models of Quantum Mechanics
SNF Ambizione Research Grant #208726 (485,300 CHF)
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PI: Cristian Mariani (USI)
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Summary
Quantum mechanics (QM) is at the core of our understanding of nature, for it provides the basis for describing the ultimate constituents of matter and their interactions. As it is well known, however, this theory has some strikingly counterintuitive consequences. Among other things, experiments have revealed that microscopic objects behave as if they can be located at two places at the same time, and have suggested the existence of problematic instantaneous interactions at a distance. Moreover, the textbook approach to this theory, sometimes called Orthodox interpretation, suffers from the notorious measurement problem. Very roughly, this stems from the inability to apply the same dynamical laws to both the physical system under consideration and to the observers and the measurement apparata that we use to describe it. This poses a serious threat to the idea that physics should provide a unified and reductive set of laws to every physical system. For all these reasons, it is no surprise that a great deal of contemporary debates in philosophy of science and metaphysics aims to address the conceptual and ontological consequences of QM (for a comprehensive overview, see Lewis 2016, Quantum Ontology, Oxford University Press). Spontaneous Collapse Models (SCM) are among the main solutions to the measurement problem of Orthodox QM. The core idea of SCM (for a detailed introduction, see Bassi & Ghirardi 2003, Dynamical Reduction Models, Physics Report) is to modify Schroedinger’s dynamical equation of textbook QM by adding a fundamentally stochastic and non-linear element. By doing so, the collapse of the wave function can be described as a physical mechanism without making reference to observers or to experimental apparata. In SCM, collapses happen spontaneously and randomly, with a certain probability rate per unit of time. The rate is such that for microscopic systems like nucleons, the collapse of the wave function is incredibly rare, which explains why microscopic superpositions have empirically detectable effects (such as the interference pattern in a double slit experiment). However, the rate is also such that for macroscopic systems made of a large number of highly entangled particles, the collapse is practically certain to occur, and this is why superposition states have no effect at the macroscopic scale. The goal of my SNF Ambizione project is to provide the first systematic and unified metaphysical framework for SCM. SCM are often considered among the main realist approaches to QM, along with hidden variables theories such as Bohmian Mechanics, and Many-Worlds interpretations such as Everettian QM. While for these latter there have been several systematic attempts to give a more unified metaphysical view, this has not happened in the context of SCM. The proposed research is therefore a unicum in the panorama of contemporary philosophy of physics, for it promises to provide a novel framework for understanding one of the most debated approaches to QM. At the core of my proposal is the notion of quantum indeterminacy (QI), which I will be introducing in the first stage of the project, building upon existing works and upon my previous research on this topic (for an overview, see Calosi and Mariani, 2021, Philosophy Compass). QI is to be distinguished from other kinds of indeterminacy discussed in the philosophical literature. The main, crucial distinction is that QI refers to the objective indefiniteness of values for physical quantities. As such, it cannot be explained away in terms of epistemic or linguistic deficiencies, and it is therefore a kind of ontological, mind-independent indeterminacy. My main working hypothesis is that, once properly understood, QI can allow us to provide a systematic treatment of most of the problematic conceptual puzzles arising from SCM, both in the relativistic and non-relativistic cases. The project will be undertaken with tools from analytic philosophy and naturalistic metaphysics. My main methodological assumption is that metaphysics can be useful for understanding scientific theories, physics prominently. Although this is an assumption I will be making, and therefore I will not defend it further, I strongly believe that the success of the project would provide indirect evidence in its favor. As a matter of fact, if successful the project would show that the interaction between metaphysics and physics is mutually beneficial. The metaphysical hypothesis of QI provides a solid ontological basis for understanding SCM, while SCM in turn will give good motivations for taking the notion of QI seriously.