Abstract This chapter considers the worry that, rather than there being no description of the microworld available, there may be several competing descriptions. The problematic role of measurement in standard quantum mechanics is explained. Three leading approaches for responding to the measurement problem are laid out—spontaneous collapse theories, hidden variable theories, and many-worlds theories. This is a prima facie example of underdetermination: Several incompatible descriptions of the world are consistent with the phenomena. However, it has been argued that there is no real underdetermination here, because the former two approaches reduce to the many-worlds approach. This argument is shown to rest on unresolved questions concerning the empirical adequacy of the many-worlds approach. So at present the underdetermination remains, but it is argued that it presents no real barrier to deriving metaphysical conclusions from quantum mechanics.
Abstract This chapter considers the worry that, rather than there being no description of the microworld available, there may be several competing descriptions. The problematic role of measurement in standard quantum mechanics is explained. Three leading approaches for responding to the measurement problem are laid out—spontaneous collapse theories, hidden variable theories, and many-worlds theories. This is a prima facie example of underdetermination: Several incompatible descriptions of the world are consistent with the phenomena. However, it has been argued that there is no real underdetermination here, because the former two approaches reduce to the many-worlds approach. This argument is shown to rest on unresolved questions concerning the empirical adequacy of the many-worlds approach. So at present the underdetermination remains, but it is argued that it presents no real barrier to deriving metaphysical conclusions from quantum mechanics.
Quantum physics confronts contemporary metaphysicians with a fascinating and frustrating puzzle. It appears to be telling us something novel and surprising about the nature of the physical world, but it is hard to pin down exactly what it tells us. Hidden variable theories constitute a second major class of solutions to the measurement problem. They differ from the standard theory and spontaneous collapse theories in eschewing collapse altogether; according to hidden variable theories, the wavefunction always evolves according to the Schrodinger equation. The simplest and most radical response to the measurement problem is to assert that standard quantum mechanics without wavefunction collapse constitutes a complete physical theory. The many worlds theory is more flexible in this regard; according to the standard contemporary version, no presupposition is made concerning precisely which physical properties form the basis of measurement results. There are several approaches to quantum mechanics, any or none of which might yield an acceptable physical theory.
Abstract The book starts with a description of classical mechanics then discusses the quantum phenomena that require us to give up our commonsense classical intuitions. We consider the physical and conceptual arguments that led to the standard von Neumann-Dirac formulation of quantum mechanics and how the standard theory explains quantum phenomena. This includes a discussion of how the theory’s two dynamical laws work with the standard interpretation of states to explain determinate measurement records, quantum statistics, interference effects, entanglement, decoherence, and quantum nonlocality. A careful understanding of how the standard theory works ultimately leads to the quantum measurement problem. We consider how the measurement problem threatens the logical consistency of the standard theory then turn to a discussion of the main proposals for resolving it. This includes collapse formulations of quantum mechanics like Wigner’s extension of the standard theory and the GRW approach and no-collapse formulations like pure wave mechanics, the various many-worlds theories, and Bohmian mechanics. In discussing alternative formulations of quantum mechanics we pay particular attention to the explanatory role played by each theory’s empirical ontology and associated metaphysical commitments and the conceptual trade-offs between theoretical options.
Abstract The foundations of quantum mechanics are so contested that it yields few definitive answers to metaphysical questions. So the best we can do is to lay out the various possibilities. The six possibilities considered are as follows: Bohm’s hidden variable theory, particle-only hidden variable theories, the GRW spontaneous collapse theory, spontaneous collapse theories with added three-dimensional ontology, the “flashy” spontaneous collapse theory, and the many-worlds theory. The metaphysical consequences of each of these theories are summarized. Even if quantum mechanics does not settle many ontological questions, it shifts the debate in interesting and fruitful ways. Furthermore, the metaphysical consequences may in some cases reflect back on the tenability of the theories themselves.
The quantum measurement problem remains a central unresolved issue in theoretical physics, arising from the incompatibility between unitary quantum dynamics and the emergence of definite measurement outcomes. Closely related is the question of the physical status of the wavefunction within contemporary quantum theory. This review critically surveys leading approaches to the measurement problem, focusing on their formal consistency, physical assumptions, and empirical implications. We examine the standard measurement framework, the role of environment-induced decoherence and quantum Darwinism, and the restrictions imposed by no-go theorems such as Bell, Kochen–Specker, and Pusey–Barrett–Rudolph. Interpretations including Everettian many-worlds, Bohmian mechanics, spontaneous collapse models, relational quantum mechanics, QBist approaches, and Copenhagen-type frameworks are assessed in terms of their ability to account for classical definiteness without sacrificing empirical adequacy. We also review experimental efforts probing macroscopic superpositions and collapse dynamics. The analysis indicates that, despite its predictive success, quantum theory lacks a complete physical account of outcome selection. This suggests that the measurement problem reflects a fundamental structural limitation of current theory, motivating either new physical principles or a redefinition of the wavefunction’s ontological role.
The collapse postulate in quantum mechanics is problematic due to the measurement problem. The problem lies not with collapse per se, but with the appeal to measurement; a theory that could underwrite the collapse process without ineliminable reference to measurement would constitute a solution to the measurement problem. This is the strategy pursued by dynamical (or spontaneous) collapse theories. But dynamical collapse theories face a number of challenges. First, they make different empirical predictions from standard quantum mechanics, and hence are potentially empirically refutable. Second, there are difficulties reconciling the dynamical collapse mechanism with special relativity. Third, the post-collapse state is not the same as the post-measurement state of standard quantum mechanics, raising the possibility that dynamical collapse theories do not solve the measurement problem after all. Assuming that these challenges can be met, dynamical collapse theories can lay claim to being serious contenders for the correct description of the quantum world. And the description they provide has a number of interesting consequences. First, it makes indeterminism an irreducible fact about the physical world. It has been argued that this has important consequences for the foundations of statistical mechanics, for free will, and for consciousness. Second, many dynamical collapse theories can be taken to imply that the quantum wave function is fundamental, giving rise to a new kind of vagueness, since a wave can be fuzzy around the edges in a way that a particle cannot. Furthermore, the quantum wave function is defined over a high-dimensional space, not the three-dimensional space of experience, suggesting to some that three-dimensionality is an illusion.
Metaphysical underdetermination arises when we are not able to decide, by purely theoretical criteria, between competing interpretations of scientific theories with different metaphysical commitments. This is the case in which non-relativistic quantum mechanics (QM) founds itself in. Among several available interpretations, there is the interpretation which states that the interaction with the conscious mind of a human observer causes a change in the dynamics of quantum objects undergoing from indefinite to definite states. This is, in a nutshell, the received view of the consciousness causes the collapse hypothesis (CCCH), which is the basis for a set of interpretations known as subjectivistic interpretations of QM. Here we propose a further distinction between three levels of metaphysical underdetermination within the discussion related to the interpretations of QM: the first level, described above, concerns the general framework of interpretations of QM (where CCCH is a option among several others, and which is not addressed here); the second one emerges within a particular set of interpretations (namely, CCCH), and the third one from two possible approaches to a particular branch of this set of interpretations. We suggest breaking the last two using metaphysical arguments. As a general result, we have been able to rule out the dualist approach of CCCH (though we cannot rule it out empirically) and to realize that the only viable phenomenological approach to CCCH is the eidetic one.