Valia Allori

This edited collection provides new perspectives on some metaphysical questions arising in quantum mechanics. These questions have been long-standing and are of continued interest to researchers and graduate students working in physics, philosophy of physics and metaphysics. It features contributions from a diverse set of researchers, ranging from senior scholars to junior academics, working in varied fields, from physics to philosophy of physics and metaphysics. The contributors reflect on issues about fundamentality (is quantum theory fundamental? If so, what is its fundamental ontology?), ontological dependence (how do ordinary objects exist even if they are not fundamental?), realism (what kind of realism is compatible with quantum theory?), indeterminacy (can the world itself exhibit ontological indeterminacy?). With contributions from both physicists (including Nobel Prize winner Gerard 't Hooft), science communicators and philosophers.

The year 2005 has been named the World Year of Physics in recognition of the 100th anniversary of Albert Einstein's "Miracle Year," in which he published four landmark papers which had deep and great influence on the last and the current century: quantum theory, general relativity, and statistical mechanics. Despite the enormous importance that Einstein’s discoveries played in these theories, most physicists adopt a version of quantum theory which is incompatible with the idea that motivated Einstein in the first place. This seems to suggest that Einstein was fundamentally incapable of appreciating the `quantum revolution,’ and that his vision of physics as an attempt to reach a complete and comprehensive description of reality was ultimately impossible to obtain. Relativity theory has provided us with a picture of reality in which the world can be though as independent on who observes it, and the same can be said for statistical mechanics. Instead, quantum mechanics seems to suggest that physical objects do not exist `out there’ when someone is not observing them. In this framework, it is often suggested that any kind of causal explanation is impossible in the atomic and subatomic world, and therefore should be abandoned. This is why many think that it is in principle impossible for quantum theory to provide us with a coherent and comprehensive view of the world, in contrast with what happens with relativity and statistical mechanics. Is it really impossible to pursue Einstein’s ideal of physics also in the quantum framework? This book argues that this is not the case: the central idea is that Einstein’s vision of physics is still a live option, and indeed it is the one that best allows obtaining a unitary understanding of our physical theories. One can consider all the three theories mentioned above, suitably modified, as theories that are able to account and explain the world around us without too much departure from the classical framework. ------------------------------------------------------------------------------------------------------------------------------------------------------ La teoria della relatività, la meccanica statistica e la meccanica quantistica hanno profondamente rivoluzionato il nostro modo di concepire spazio, tempo, materia, probabilità e causalità, nonché il rapporto tra universo fisico ed osservatore, nozioni che sono state al centro della discussione filosofica dal mondo greco fino ai nostri giorni. Questo volume, opera di Valia Allori, Mauro Dorato, Federico Laudisa e Nino Zanghì, non solo intende suggerire nuovi metodi di confronto tra fisica e filosofia, ma prova altresì a rendere espliciti i presupposti filosofici che sono presenti nell'interpretazione che i fisici stessi danno del formalismo matematico.

In my dissertation I analyze the structure of fundamental physical theories. I start with an analysis of what an adequate primitive ontology is, discussing the measurement problem in quantum mechanics and theirs solutions. It is commonly said that these theories have little in common. I argue instead that the moral of the measurement problem is that the wave function cannot represent physical objects and a common structure between these solutions can be recognized: each of them is about a clear three-dimensional primitive ontology that evolves according to a law determined by the wave function. The primitive ontology is what matter is made of while the wave function tells the matter how to move. One might think that what is important in the notion of primitive ontology is their three-dimensionality. If so, in a theory like classical electrodynamics electromagnetic fields would be part of the primitive ontology. I argue that, reflecting on what the purpose of a fundamental physical theory is, namely to explain the behavior of objects in three--dimensional space, one can recognize that a fundamental physical theory has a particular architecture. If so, electromagnetic fields play a different role in the theory than the particles and therefore should be considered, like the wave function, as part of the law. Therefore, we can characterize the general structure of a fundamental physical theory as a mathematical structure grounded on a primitive ontology. I explore this idea to better understand theories like classical mechanics and relativity, emphasizing that primitive ontology is crucial in the process of building new theories, being fundamental in identifying the symmetries. Finally, I analyze what it means to explain the word around us in terms of the notion of primitive ontology in the case of regularities of statistical character. Here is where the notion of typicality comes into play: we have explained a phenomenon if the typical histories of the primitive ontology give rise to the statistical regularities we observe.

A common way of characterizing Boltzmann’s explanation of thermodynamics in term of statistical mechanics is with reference to three ingredients: the dynamics, the past hypothesis, and the statistical postulate. In this paper I focus on the statistical postulate, and I have three aims. First, I wish to argue that regarding the statistical postulate as a probability postulate may be too strong: a postulate about typicality would be enough. Second, I wish to show that there is no need to postulate anything, for the typicality postulate can be suitably derived from the dynamics. Finally, I discuss how the attempts to give preference to certain stochastic quantum theories (such as the spontaneous collapse theory) over deterministic alternatives on the basis that they do not need the statistical postulate fail.

The book explores several open questions in the philosophy of statistical mechanics. Each chapter is written by a leading expert in the field. Here is a list of some questions that are addressed in the book: 1) Boltzmann showed how the phenomenological gas laws of thermodynamics can be derived from statistical mechanics. Since classical mechanics is a deterministic theory there are no probabilities in it. Since statistical mechanics is based on classical mechanics, all the probabilities statistical mechanics talks about cannot be fundamental. However, if probabilities are epistemic, how can they play a role, as they seem to do, in laws, explanation, and prediction? 2) Many physicists use the notion of typicality instead of the one of probability when discussing statistical mechanics. What is the connection between the two notions? 3) How can one extend Boltzmann’s analysis to the quantum domain, where some theories are indeterministic? 4) Boltzmann’s explanation fundamentally involves cosmology: for the explanation to go through the Big Bang needs to have had extremely low entropy. Does the fact that the Big Bang was a low entropy state imply that it was, in some sense, “highly improbable” and requires an explanation? 5) What exactly is the connection between statistical and classical mechanics? Is the one of theory reduction or there is no such thing? 6) Statistical mechanics has two main formulation: one due to Botzmann and the other due to Gibbs. What is the connection between the two formulations

This paper aims to investigate the so-called paradox of deterministic probabilities: in a deterministic world, all probabilities should be subjective; however, they also seem to play important explanatory and predictive roles which suggest they are objective. The problem is then to understand what these deterministic probabilities are. Recent proposed solutions of this paradox are the Mentaculus vision, the range account of probability, and a version of frequentism based on typicality. All these approaches aim at defining deterministic objective probabilities as to make them compatible with determinism. In this paper I argue that one can think of the equivalent of subjective and objective deterministic probabilities in terms of typicality. Also, I show that only what I identify with objective probabilities play the necessary explanatory and predictive roles, while the subjective components essentially guide beliefs. In this way, the paradox is solved.

The information-theoretic approach to quantum mechanics, proposed by Bub and Pitowsky, is a realist approach to quantum theory which rejects the “two dogmas” of quantum mechanics: in this theory measurement results are not analysed in terms of something more fundamental, and the quantum state does not represent physical entities. Bub and Pitowsky’s approach has been criticized because their rejection of the first dogma relies on their argument that kinematic explanations are more satisfactory than dynamical ones. However, little attention has been given to the second dogma. If anything, some have discussed the difficulties the informational-theoretical interpretation faces in making sense of the quantum state as epistemic. The aim of this paper is twofold. First, I argue that a realist should reject the second dogma without relying on the alleged explanatory superiority of kinematic explanation over dynamical ones, thereby providing Bub and Pitowsky with a way to avoid the first set of objections to their view. Then I propose a functionalist account of the wavefunction as a non-material entity which does not fall prey of the objections to the epistemic account or the other non-material accounts such as the nomological view, and therefore I supply the proponents of the information-theoretical interpretation with a new tool to overcome the second set of criticisms

This book is an introduction to the foundation of quantum mechanics. As such, this book is perfect: it is the book that my former, physics undergraduate, self would have wanted to read. At the time, like typical physics undergraduates around the globe, I was taught to give up hope of ever understanding what quantum theory claims: at best, the theory is an instrument to predict experimental results. No matter how much we might dislike it, we have to accept it; there is no way out. It was my refusal to give in that led me to become a philosopher of physics. And I was right in being persistent because I later discovered that, as Maudlin clearly explains, there is no reason to be pessimistic: all the quantum mysteries and paradoxes have a solution. Actually, more than one. To have Maudlin’s book at the time would have saved me a lot of time and pain. I am sure it will be a life-saver for many other rightfully puzzled undergraduate physics students wondering if the questions they ask are wrongheaded, as too many of their teachers claim: How can the Schrödinger cat really be dead and alive at the same time? Is reality created by an act of observation or a measurement? How is a measurement not a physical process like any other? Because these are not bad questions at all, and they have straightforward answers: the Schrödinger cat cannot be both alive and dead; reality is not created by observation; a measurement is just a type of interaction between two physical systems. However, the book is also not perfect (becoming, ironically, a superposition). While Maudlin defends his own position on how to make sense of quantum theory, his book is also potentially misleading about the state of the art in terms of the philosophical foundations of the theory. Maudlin omits to mention (or cite, or give reference to) the majority of the literature generated in the last two decades around the problems he discusses. Although this book may be intended as an introductory textbook to the field, I am disappointed that it skirts so much of the work of the past few decades on the foundations of quantum theory. To be fair, there is a habit in philosophy of physics at large of failing to sufficiently acknowledge other work, but this is why the failure to do much to rectify this trend in even a short introductory textbook like this one troubles me as much as it does. Indeed, this can mislead students who are new to the material, in the same way that I felt misled by my undergraduate physics instructors about the nature of quantum theory. This is also puzzling because, as I will discuss below, the lack of contextualization of his view within the actual literature makes Maudlin’s arguments weaker than they could have been.

The scientific realist wants to read the metaphysical picture of reality through our best fundamental physical theories. The traditional way of doing so is in terms of objects, properties, and laws of nature. For instance, there are families of fundamental particles individuated by their properties of mass and charge, which determine how they move around. One could call this view an object-oriented metaphysics grounded on properties. In this paper, I wish to present an alternative view that one can dub a thin object oriented metaphysics grounded on structure: there are fundamental entities with no fundamental properties other than the one(s) needed to specify their nature. I argue that my view has several advantages over the received one. In particular, I compare my proposal to the traditional view in the quantum domain and I argue that my view provides a better fit for both approaches to quantum metaphysics, namely the primitive ontology programme and wave-function realism.

Quantum mechanics is a groundbreaking theory: not only it is extraordinarily empirically adequate but also it is claimed to having shattered the classical paradigm of understanding the observer-observed distinction as well as the part-whole relation. This, together with other quantum features, has been taken to suggest that quantum theory can help us understand the mind-body relation in a unique way, in particular to solve the hard problem of consciousness along the lines of panpsychism. In this paper, after having briefly presented panpsychism, I discuss the main features of quantum theories and the way in which the main quantum theories of consciousness use them to account for conscious experience.

Scientific realists investigate the ontology of the world and explain the observed phenomena by using our best fundamental physical theories. These theories describe the behavior of fundamental objects in terms of their fundamental properties, which determine their behavior. This paper is the natural companion of another paper in which I propose an alternative to this traditional account of metaphysics, according to which fundamental objects have no other fundamental property than the one needed to specify their nature. In that paper I argue that my view fares better than the traditional metaphysics both in the classical and in the quantum domain. In this paper I compare my view to structuralism. After discussing how my proposal shares many motivations with structuralism, I argue in which ways I think it is superior.

Scientific realists usually claim that quantum mechanics can be made compatible with scientific realism by solving the measurement problem, even if there is disagreement about which solution is best. In this paper I argue this is due to having different views about what it means to make quantum theory compatible with scientific realism: ‘relaxed’ realists think it is enough to solve the adequacy problem, ‘modest’ realists believe that there is also a precision problem, while ‘robust’ realists insist that quantum theory still needs to be suitably completed. These attitudes are connected with the type of explanation one favors: while relaxed realists favor principle theories, robust realists prefer constructive theories, and modest realists provide non-constructive dynamical hybrids as long as they preserve locality and separability.

Many agree that the pilot-wave theory is to be understood as a first-order theory, in which the law constrains the velocity of the particles. However, while Dürr, Goldstein and Zanghì maintain that the pilot-wave theory is Galilei invariant, Valentini argues that such a symmetry is mathematical but it has no physical significance. Moreover, some wavefunction realists insist that the pilot-wave theory is not Galilei invariant in any sense. It has been maintained by some that this disagreement originates in the disagreement about ontology, as Valentini, contrary to Dürr, Goldstein and Zanghì, has been taken to endorse wavefunction realism. In this paper I argue that Valentini’s argument is independent of the choice of the ontology of matter: it is based of the notion of natural kinematics for a theory, and the idea that the kinematics should match the dynamics. If so, I also argue that there are several reasons to dispute Valentini’s claim that the kinematical symmetries should constrain dynamical ones.

When discussing quantum ontology, the debate has recently focused on comparing and contrasting wavefunction realism and its rivals. Among them one finds the primitive ontology approach, which is often conflated with the local beables program. In this paper I wish to clarify what I take to be the distinction between the notion of primitive ontology and the one of local beable. I argue that the primitive ontology is the local beable which allows for a dynamical, constructive explanation which preserves symmetries

Spontaneous localization theory is a quantum theory proposed by GianCarlo Ghirardi, together with Alberto Rimini and Tullio Weber in 1986. However, soon it became clear to Ghirardi that his work was more than just one theory: he actually developed a framework, a family of theories in which the wavefunction jumps, but where the ontology of the theory is underdetermined. After acknowledging that the wavefunction did not provide a satisfactory ontology, he assumed that matter was described by a continuous matter density field in three-dimensional space, whose evolution is governed by a stochastic wavefunction evolution. Alternatively, Bell assumed that the wavefunction would govern a spatiotemporal event ontology, dubbed ‘flashes.’ However, not much work has been done with the perhaps most obvious possibility, namely that physical objects are made of particles. This paper has two aims. First to explain the reason why people require spontaneous localization theory to be more than just a theory about the wavefunction. This is done by showing how the problem everyone in the foundation of quantum mechanics take to be the fundamental problem of quantum mechanics, namely the measurement problem, is a red herring. Then, the paper explores the possibility of spontaneous localization theories of particles. I argue that this discussion is not a mere exercise, as spontaneous localization theories of particles may be amenable to a relativistic extension which does not require a foliation, and because in general the peculiar type of indeterminism of spontaneous localization theories may help shedding new light on the nature of the tension between quantum theory and relativity.