Valia Allori

Quantum theory and the de Broglie – Bohm pilot-wave theory are empirically equivalent. In addition to other objections to the pilot-wave theory, many physicists (and some philosophers) take this to be enough to dismiss the pilot-wave theory, as they say it adds nothing to the standard theory. In this short paper I review some objections and replies to the pilot-wave theory. In particular I respond to the empirical equivalence challenge arguing that, given their mutual relationship, there is no reason to expect the two theories to make different predictions, even if they actually might.

This is a relatively short book about the metaphysics of quantum theory, in which I present my view on what we should believe exists in the world if quantum theory is true. I endorse scientific realism, the view that scientific theories can inform us about the nature of reality. The title indicates that the book is about “quantum things,” referencing Lucretius’ De Rerum Natura (On the Nature of Things), arguably the first text on naturalized metaphysics. Also, the subtitle carries a double meaning, relying on the ambiguity of the word fundamentals. The term can refer to the basic principles of a discipline, as in a title like The Fundamentals of Tensor Calculus for Physicists. However, it can also be understood as the fundamental entities of the quantum world. In the first three chapters, I explore the former meaning (the basic principles), offering a brief review of several approaches that examine the connection between physics and metaphysics, and how these have been adapted to address quantum physics. In the later chapters, I turn to the second meaning of the subtitle (what lies at the foundation). There, I propose what I consider to be the best approach for investigating the metaphysics of quantum theory, and offer my answer to the question: What are the fundamental ingredients of the quantum world?

There are several important philosophical problems to which quantum mechanics is often said to have made significant contributions: • Determinism: quantum theory has been taken to refute determinism; • Free Will: in turn, this is thought to open the door to free will; • The mind-body problem: relatedly, it is sometimes said to shed light on consciousness; • Idealism: more radically, quantum theory is assumed to have refuted realism and to have placed the observer at the center of the world; • Reductionism: even granting realism, it has been claimed that quantum theory undermines reductionism. Our main thesis in this paper is that none of this is either necessary or desirable. By adopting the de Broglie–Bohm theory (or Bohmian mechanics), one can straightforwardly account for quantum phenomena without endorsing any of these claims.

Quantum theory and the de Broglie-Bohm pilot-wave theory are empirically equivalent. In addition to other objections to the pilot-wave theory, many physicists (and some philosophers) take this to be enough to dismiss the pilot-wave theory, as they say it adds nothing to the standard theory. In this short paper I review some objections and replies to the pilot-wave theory. In particular I respond to the empirical equivalence challenge arguing that, given their mutual relationship, there is no reason to expect the two theories to make different predictions, even if they actually might.

This is a brief review of the history and development of quantum theories. Starting from the experimental f indings and theoretical results which marked the crisis of the classical framework, I overview the rise of axiomatic quantum mechanics through matrix and wave mechanics. I discuss conceptual problems such as the measurement problem that led scientific realists to explore other, more satisfactory, quantum theories, as well as Bell’s theorem and quantum nonlocality, concluding with a short review of relativistic theories.

In this chapter, I discuss time in nonrelativistic quantum theories. Within an instrumentalist theory like von Neumann’s axiomatic quantum mechanics, I focus on the meaning of time as an observable quantity, on the idea of time quantization, and whether the wavefunction collapse suggests that there is a preferred temporal direction. I explore this last issue within realist quantum theories as well, focusing on time reversal symmetry, and I analyze whether some theories are more hospitable for time travel than others.

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.

In this paper, I argue that Conway and Kochen’s Free Will Theorem (1,2) to the conclusion that quantum mechanics and relativity entail freedom for the particles, does not change the situation in favor of a libertarian position as they would like. In fact, the theorem more or less implicitly assumes that people are free, and thus it begs the question. Moreover, it does not prove neither that if people are free, so are particles, nor that the property people possess when they are said to be free is the same as the one particles possess when they are claimed to be free. I then analyze the Free State Theorem (2), which generalizes the Free Will Theorem without the assumption that people are free, and I show that it does not prove anything about free will, since the notion of freedom for particles is either inconsistent, or it does not concern our common understanding of freedom. In both cases, the Free Will Theorem and the Free State Theorem do not provide any enlightenment on the constraints physics can pose on free will.

Bell’s inequality is an empirical constraint on theories with hidden variables, which Einstein, Podolsky and Rosen argued are needed to explain observed perfect correlations if keeping locality. One way to deal with the empirical violation of Bell’s inequality is by openly embracing nonlocality, in a theory like the pilot-wave theory. Nonetheless, recent proposals have revived the possibility that one can avoid nonlocality by resorting to superdeterministic theories. These are local hidden variables theories which violate statistical independence which is one assumption of Bell’s inequality. In this paper I compare and contrast these two hidden variable strategies: the pilot-wave theory and superdeterminism. I show that even if the former is nonlocal and the other is not, both are contextual. Nonetheless, in contrast with the pilot-wave theory, superdeterminist contextuality makes it impossible to test the theory (which therefore becomes unfalsifiable and unconfirmable) and renders the theory uninformative (measurement results tell us nothing about the system). It is questionable therefore whether a theory with these features is worth its costs.

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 plays the necessary explanatory and predictive roles, while the subjective components essentially guide beliefs. In this way, the paradox is solved.

Among the various proposals for quantum ontology, both wavefunction realists and the primitive ontologists have argued that their approach is to be preferred because it relies on intuitive notions: locality, separability and spatiotemporality. As such, these proposals should be seen as normative frameworks asserting that one should choose the fundamental ontology which preserves these intuitions, even if they disagree about their relative importance: wavefunction realists favor preserving locality and separability, while primitive ontologists advocate for spatiotemporality. In this paper, first I clarify the main tenets of wavefunction realism and the primitive ontology approach, arguing that seeing the latter as favoring constructive explanation makes sense of their requirement of a spatiotemporal ontology. Then I show how the aforementioned intuitive notions cannot all be kept in the quantum domain. Consequently, wavefunction realists rank locality and separability higher than spatiotemporality, while primitive ontologists do the opposite. I conclude that however, the choice of which notions to favor is not as arbitrary as it might seem. In fact, they are not independent: requiring locality and separability can soundly be justified by requiring spatiotemporality, and not the other way around. If so, the primitive ontology approach has a better justification of its intuitions than its rival wavefunction realist framework.

Einstein thought that quantum mechanics was incomplete because it was nonlocal. In this paper I argue instead that quantum theory is incomplete, even if it is nonlocal, and that relativity is incomplete because its minimal spatiotemporal structure cannot naturally accommodate such nonlocality. So, I show that relativistic pilot-wave theories are the rational completion of quantum mechanics as well as relativity: they provide a spatiotemporal ontology of particles, as well as a spatiotemporal structure able to explain quantum correlations.

In this paper, I argue that the many-worlds theory, even if it is arguably the mathematically most straightforward realist reading of quantum formalism, even if it is arguably local and deterministic, is not universally regarded as the best realist quantum theory because it provides a type of explanation that is not universally accepted. Since people disagree about what desiderata a satisfactory physical theory should possess, they also disagree about which explanatory schema one should look for in a theory, and this leads different people to different options.

For scientific realists, quantum mechanics is unsatisfactory because it suffers from the measurement problem. However, there are at least three promising solutions: the pilot-wave theory, the many-worlds theory, and the theory of spontaneous collapse. In this paper I argue that the measurement problem is a false problem for the realist: it was proposed as the last resort to convince the positivists that the theory is not empirically adequate. Instead realists should focus on preserving the reductive explanatory schema that had worked so well in physics before, which requires a theory to have a three-dimensional ontology. Incompleteness argument to this effect have been proposed in the 1920s, but effectively ignored due to the positivistic climate, other unscientific reasons, and theorems which claimed that this project was impossible. When realists re-examined quantum mechanics in the 1950s and later, they happened to focus on the measurement problem. In this paper I speculate on what would have happened if realists instead focused on finding a three-dimensional ontology to complete quantum theory. I show that most paradoxes, puzzles and mysteries connected with quantum mechanics would have never emerge, and that many of what are now considered possible ontological interpretations of the theory would have hardly been taken as viable options.

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.

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.

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.

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.

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.

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.

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.

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.

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 violation of Bell’s inequality has shown that quantum theory and relativity are in tension: reality is nonlocal. Nonetheless, many have argued that GRW-type theories are to be preferred to pilot-wave theories as they are more compatible with relativity: while relativistic pilot-wave theories require a preferred slicing of space-time, foliation-free relativistic GRW-type theories have been proposed. In this paper I discuss various meanings of ‘relativistic invariance,’ and I show how GRW-type theories, while being more relativistic in one sense, are less relativistic in another. If so, the initial claim that GRW-type theories have a greater compatibility with relativity is unwarranted: both type of theories violate relativity, one way or another.

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.