How is the microscopic ontology of quantum mechanics to be understood according to the Everett interpretation? And how, in particular, are we to understand concepts like spatial locality in Everettian quantum mechanics? Building on joint work with Chris Timpson, the chapter develops a general approach to these questions (‘spacetime state realism’) and apply it to questions of locality and entanglement. After a digression on the general metaphysical problem of how we should think about ontology, the chapter compares this approach with others in the literature.
How is the microscopic ontology of quantum mechanics to be understood according to the Everett interpretation? And how, in particular, are we to understand concepts like spatial locality in Everettian quantum mechanics? Building on joint work with Chris Timpson, the chapter develops a general approach to these questions (‘spacetime state realism’) and apply it to questions of locality and entanglement. After a digression on the general metaphysical problem of how we should think about ontology, the chapter compares this approach with others in the literature.
The aim of this volume is to develop and evaluate the suggestion that if one adopts the interpretation of quantum mechanics initially proposed by Hugh Everett, today frequently referred to as the “many-worlds interpretation,” then one can avoid the kind of nonlocality that is often thought to be a consequence of quantum entanglement according to other approaches, and thus permit the compatibility of these two pillars of modern physics: quantum mechanics and special relativity. Although this is an idea that seems to have been recognized by Everett and is sometimes touted as a reason to prefer his approach to interpreting quantum mechanics over others, the precise way in which Everett’s interpretation achieves this locality, indeed whether it even does achieve locality, has been debated, including among the interpretation’s proponents. The aim of this introductory chapter is to provide readers with a background of this debate concerning the locality of Everettian quantum mechanics. We begin with a brief historical introduction to Everett’s proposed interpretation of quantum mechanics and its later development into the many-worlds interpretation. The following section discusses how the by now well-established phenomenon of quantum entanglement seems to imply what Albert Einstein, in a 1947 letter to Max Born, called “spooky action at a distance,” and a tension with special relativity. We then discuss why one might think that the many-worlds interpretation allows one to avoid nonlocality in this sense. The next sections bring the reader up to date with how the locality of Everettian quantum mechanics has been debated both by critics and proponents of the interpretation. As we will see, the desire to understand Everettian quantum mechanics as an interpretation that avoids nonlocality has opened up some interesting debates concerning both what it means to say an interpretation of quantum mechanics is local and over the metaphysical and mathematical interpretation of Everettian quantum mechanics itself. This introductory chapter concludes with a summary of the contributions to the present volume in the context of this ongoing debate.
Abstract What does realism about the quantum state imply? What follows when quantum theory is applied without restriction, if need be, to the whole universe? These are the questions which are debated in this book. All the chapters start from the point of realism, and based on the need, or the aspiration, for a theory that unites micro- and macroworlds, at least in principle. But the further claim argued by some is that if you allow the Schrödinger equation unrestricted application, supposing the quantum state to be something physically real, then this universe is one of countlessly many others, constantly branching in time, all of which are real. The result is the many worlds theory, also known as the Everett interpretation of quantum mechanics. The contrary claim sees this picture of many worlds as in no sense inherent in quantum mechanics, even when the latter is allowed unrestricted scope and even given that the quantum state itself is something physically real. For this picture of branching worlds fails to make physical sense, let alone common sense, even on its own terms. The status of these worlds, what they are made of, is never adequately explained. Ordinary ideas about time and identity over time become hopelessly compromised. The concept of probability itself is brought into question. This picture of many branching worlds is inchoate, it is a vision, an error. There are realist alternatives to many worlds, some even that preserve the Schrödinger equation unchanged. Twenty chapters, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They focus first on the question of ontology, the existence of worlds, second on the interpretation of probability, and third on alternatives or additions to many worlds. The introduction offers a helpful guide to the arguments for the Everett interpretation, particularly as they have been formulated in the last two decades.
According to scientific realism, science gives us an approximately true description of what the world is like. But what does it mean? In this chapter, we focus on the ontological and metaphysical aspects of this discussion. That is, we are concerned with the following questions: what there is, according to our best scientific theories? And how are these things that theories say exist? We start from the assumption that a genuine scientific realism must deal with these issues. Taking non-relativistic quantum mechanics as a case study, we discuss some of the challenges currently faced by a genuinely realistic stance. We argue, first, that in the ontological aspect, realists are in the same boat as science, that is, without sufficient epistemic justification to adopt the belief in the entities postulated by a single quantum theory, given that experience (currently) is not able to decide between rival theories; in the metaphysical aspect, realists also encounter problems with theory choice, given the possibility of associating more than one metaphysical theory with the description of the entities postulated by each quantum theory. The same goes for structuralist alternatives, since, even if we accept that scientific theories are committed to the existence of structures, but not objects, we still do not know what structures are, metaphysically. Finally, we move forward in the discussion with the meta-Popperian method, which serves to reduce the metaphysical alternatives associated with the ontological postulates of scientific theories. We may not know what the correct alternative is, but we have been able to progressively check which metaphysical alternatives are incompatible with each scientific theory.
Everett's relative-state construction in quantum theory has never been satisfactorily expressed in the Heisenberg picture. What one might have expected to be a straightforward process was impeded by conceptual and technical problems that we solve here. The result is a construction which, unlike Everett's one in the Schrödinger picture, makes manifest the locality of Everettian multiplicity, its inherently approximative nature and its origin in certain kinds of entanglement and locally inaccessible information. (By Everettian , we are referring not only to Everett's own work, but also to versions of quantum theory that elaborate and refine his. The notion of relative states first appeared in Everett (Everett 1973 In The many worlds interpretation of quantum mechanics (eds BS DeWitt, N Graham)). We are proposing a formalism for relative states that is more detailed and more illuminating than Everett's.) Our construction also allows us to give a more precise definition of an Everett ‘universe’, under which it is fully quantum, not quasi-classical, and we compare the Everettian decomposition of a quantum state with the foliation of a space–time.
Abstract Metaphysicians should pay attention to quantum mechanics, but not because it provides definitive answers to many metaphysical questions. The theory itself is remarkably silent on the nature of the physical world, and the various interpretations of the theory on offer present conflicting ontological pictures. Rather, quantum mechanics is essential to the metaphysician because it reshapes standard metaphysical debates and opens up unforeseen new metaphysical possibilities. Even if quantum mechanics provides few clear answers, there are good reasons to think that any adequate understanding of the quantum world will result in a radical reshaping of our classical worldview in some way or other. Whatever the world is like at the atomic scale, it is almost certainly not the swarm of particles pushed around by forces that is often presupposed. This book guides readers through the theory of quantum mechanics and its implications for metaphysics in a clear and accessible way. The theory and its various interpretations are presented with a minimum of technicality. The consequences of these interpretations for metaphysical debates concerning realism, indeterminacy, causation, determinism, holism, and individuality (among other topics) are explored in detail, stressing the novel form that the debates take given the empirical facts in the quantum domain. While quantum mechanics may not deliver unconditional pronouncements on these issues, the range of possibilities consistent with our knowledge of the empirical world is relatively small—and each possibility is metaphysically revisionary in some way.
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.