The metaphysics of D-CTCs: On the underlying assumptions of Deutsch׳s quantum solution to the paradoxes of time travel

I argue that Deutsch׳s model for the behavior of systems traveling around closed timelike curves (CTCs) relies implicitly on a substantive metaphysical assumption. Deutsch is employing a version of quantum theory with a significantly supplemented ontology of parallel existent worlds, which differ in kind from the many worlds of the Everett interpretation. Standard Everett does not support the existence of multiple identical copies of the world, which the D-CTC model requires. This has been obscured because he often refers to the branching structure of Everett as a “multiverse”, and describes quantum interference by reference to parallel interacting definite worlds. But he admits that this is only an approximation to Everett. The D-CTC model, however, relies crucially on the existence of a multiverse of parallel interacting worlds. Since his model is supplemented by structures that go significantly beyond quantum theory, and play an ineliminable role in its predictions and explanations, it does not represent a quantum solution to the paradoxes of time travel.     

Quantum probability and many worlds

We discuss the meaning of probabilities in the many worlds interpretation of quantum mechanics. We start by presenting very briefly the many worlds theory, how the problem of probability arises, and some unsuccessful attempts to solve it in the past. Then we criticize a recent attempt by Deutsch to derive the quantum mechanical probabilities from the non-probabilistic parts of quantum mechanics and classical decision theory. We further argue that the Born probability does not make sense even as an additional probability rule in the many worlds theory. Our conclusion is that the many worlds theory fails to account for the probabilistic statements of standard (collapse) quantum mechanics.     

Everettian quantum mechanics and physical probability: Against the principle of “State Supervenience”

Everettian quantum mechanics faces the challenge of how to make sense of probability and probabilistic reasoning in a setting where there is typically no unique outcome of measurements. Wallace has built on a proof by Deutsch to argue that a notion of probability can be recovered in the many worlds setting. In particular, Wallace argues that a rational agent has to assign probabilities in accordance with the Born rule. This argument relies on a rationality constraint that Wallace calls state supervenience. I argue that state supervenience is not defensible as a rationality constraint for Everettian agents unless we already invoke probabilistic notions.     

The best of many worlds,or, is quantum decoherence the manifestation of a disposition?

In this paper I investigate whether the phenomenon of quantum decoherence, the vanishing of interference and detectable entanglement on quantum systems in virtue of interactions with the environment, can be understood as the manifestation of a disposition. I will highlight the advantages of this approach as a realist interpretation of the quantum formalism, and demonstrate how such an approach can benefit from advances in the metaphysics of dispositions. I will also confront some commonalities with and differences to the many worlds interpretation, and address the difficulties induced by quantum non-locality. I conclude that there are ways to deal with these issues and that the proposal hence is an avenue worth pursuing.     

Quantum mechanics,time and ontology

Against what is commonly accepted in many contexts, it has been recently suggested that both deterministic and indeterministic quantum theories are not time-reversal invariant, and thus time is handed in a quantum world. In this paper, I analyze these arguments and evaluate possible reactions to them. In the context of deterministic theories, first I show that this conclusion depends on the controversial assumption that the wave-function is a physically real scalar field in configuration space. Then I argue that answers which restore invariance by assuming the wave-function is a ray in Hilbert space fall short. Instead, I propose that one should deny that the wave-function represents physical systems, along the lines proposed by the so-called primitive ontology approach. Moreover, in the context of indeterministic theories, I argue that time-reversal invariance can be restored suitably redefining its meaning.     

Computation, individuation, and the received view on representation

The ‘received view’ about computation is that all computations must involve representational content. Egan and Piccinini argue against the received view. In this paper, I focus on Egan’s arguments, claiming that they fall short of establishing that computations do not involve representational content. I provide positive arguments explaining why computation has to involve representational content, and how that representational content may be of any type (distal, broad, etc.). I also argue (contra Egan and Fodor) that there is no need for computational psychology to be individualistic. Finally, I draw out a number of consequences for computational individuation, proposing necessary conditions on computational identity and necessary and sufficient conditions on computational I/O equivalence of physical systems.     

Effective Field Theories,Reductionism and Scientific Explanation

Effective field theories have been a very popular tool in quantum physics for almost two decades. And there are good reasons for this. I will argue that effective field theories share many of the advantages of both fundamental theories and phenomenological models, while avoiding their respective shortcomings. They are, for example, flexible enough to cover a wide range of phenomena, and concrete enough to provide a detailed story of the specific mechanisms at work at a given energy scale. So will all of physics eventually converge on effective field theories? This paper argues that good scientific research can be characterised by a fruitful interaction between fundamental theories, phenomenological models and effective field theories. All of them have their appropriate functions in the research process, and all of them are indispensable. They complement each other and hang together in a coherent way which I shall characterise in some detail. To illustrate all this I will present a case study from nuclear and particle physics. The resulting view about scientific theorising is inherently pluralistic, and has implications for the debates about reductionism and scientific explanation.     

Explanation,understanding, and unrealistic models

How can false models be explanatory? And how can they help us to understand the way the world works? Sometimes scientists have little hope of building models that approximate the world they observe. Even in such cases, I argue, the models they build can have explanatory import. The basic idea is that scientists provide causal explanations of why the regularity entailed by an abstract and idealized model fails to obtain. They do so by relaxing some of its unrealistic assumptions. This method of ‘explanation by relaxation’ captures the explanatory import of some important models in economics. I contrast this method with the accounts that Daniel Hausman and Nancy Cartwright have provided of explanation in economics. Their accounts are unsatisfactory because they require that the economic model regularities obtain, which is rarely the case. I go on to argue that counterfactual regularities play a central role in achieving ‘understanding by relaxation.’ This has a surprising implication for the relation between explanation and understanding: Achieving scientific understanding does not require the ability to explain observed regularities.     

Circularity,indispensability, and mathematical explanation in science

In this paper I consider the objection that the Enhanced Indispensability Argument (EIA) is circular and hence fails to support mathematical platonism. The objection is that the explanandum in any mathematical explanation of a physical phenomenon is itself identified using mathematical concepts. Hence the explanandum is only genuine if the truth of some mathematical theory is already presupposed. I argue that this objection deserves to be taken seriously, that it does sometimes undermine support for EIA, but that there is no reason to think that circularity is an unavoidable feature of mathematical explanation in science.     

Have we lost spacetime on the way? Narrowing the gap between general relativity and quantum gravity

Important features of space and time are taken to be missing in quantum gravity, allegedly requiring an explanation of the emergence of spacetime from non-spatio-temporal theories. In this paper, we argue that the explanatory gap between general relativity and non-spatio-temporal quantum gravity theories might significantly be reduced with two moves. First, we point out that spacetime is already partially missing in the context of general relativity when understood from a dynamical perspective. Second, we argue that most approaches to quantum gravity already start with an in-built distinction between structures to which the asymmetry between space and time can be traced back.     

Giordano Bruno and the heresy of many worlds

This paper analyses the importance of Giordano Bruno's belief in many worlds, including the Moon, the planets and the stars, in the context of his trial by the Inquisitions in Venice and Rome. Historians have claimed that this belief was not heretical and therefore was not a major factor in Bruno's trial or execution. On the contrary, by examining neglected treatises on theology, heresies and Catholic canon law, I show that the belief in many worlds was formally heretical. Multiple Christian authorities denounced it. A systematic analysis of the extant primary sources shows that Bruno's belief in many worlds was, surprisingly, of primary importance in his trial and execution. The evidence includes recent and newly discovered primary sources.     

Uncomfortable bedfellows: Objective quantum Bayesianism and the von Neumann–Lüders projection postulate

In this paper I critically evaluate the justification of the von Neumann–Lüders projection postulate for state changes in projective measurement contexts from the objective quantum Bayesian perspective. I point out that the justification provided so far for the von Neumann–Lüders projection postulate is insufficient. I argue that the best way to correct this problem is to make an assumption, Benign Realism, which is contradictory to the objective quantum Bayesian interpretation of quantum states.     

Did Samuel Clarke really disavow action at a distance in his correspondence with Leibniz?: Newton,Clarke, and Bentley on gravitation and action at a distance

In this paper I ague against John Henry's claim that Newton embraced unmediated action at a distance as an explanation of gravity (Henry, 1994, 1999, 2011, 2014). In particular, I take issue with his apparent suggestion that the fact, as he sees it, that two of Newton's prominent followers, namely, Richard Bentley and Samuel Clarke, embraced unmediated action at a distance as an explanation of gravity provides significant supporting evidence that Newton did as well (see Henry, 1994 and 1999). Instead, I argue that while Bentley did ultimately defend the notion of unmediated action at a distance as an explanation of gravity, Newton himself accepted that notion neither in his correspondence with Bentley, as Henry has maintained, nor in any of his later works. I also provide evidence that suggests that Newton did, in fact, accept both the principle of local causation and the passivity of matter. Finally, I argue that whatever the case may be with respect to Newton on the matter, it is clear from his correspondence with Leibniz, as well as from his Boyle lectures, that contrary to what Henry has maintained, Clarke was a stalwart opponent of unmediated action at a distance due to his strong commitment to both the principle of local causation and the passivity of matter.     

The problem of confirmation in the Everett interpretation

I argue that the Oxford school Everett interpretation is internally incoherent, because we cannot claim that in an Everettian universe the kinds of reasoning we have used to arrive at our beliefs about quantum mechanics would lead us to form true beliefs. I show that in an Everettian context, the experimental evidence that we have available could not provide empirical confirmation for quantum mechanics, and moreover that we would not even be able to establish reference to the theoretical entities of quantum mechanics. I then consider a range of existing Everettian approaches to the probability problem and show that they do not succeed in overcoming this incoherence.     

A quantum computer only needs one universe

The nature of quantum computation is discussed. It is argued that, in terms of the amount of information manipulated in a given time, quantum and classical computation are equally efficient. Quantum superposition does not permit quantum computers to “perform many computations simultaneously” except in a highly qualified and to some extent misleading sense. Quantum computation is therefore not well described by interpretations of quantum mechanics which invoke the concept of vast numbers of parallel universes. Rather, entanglement makes available types of computation processes which, while not exponentially larger than classical ones, are unavailable to classical systems. The essence of quantum computation is that it uses entanglement to generate and manipulate a physical representation of the correlations between logical entities, without the need to completely represent the logical entities themselves.     

Newton’s “satis est”: A new explanatory role for laws

In this paper I argue that Newton’s stance on explanation in physics was enabled by his overall methodology and that it neither committed him to embrace action at a distance nor to set aside philosophical and metaphysical questions. Rather his methodology allowed him to embrace a non-causal, yet non-inferior, kind of explanation. I suggest that Newton holds that the theory developed in the Principia provides a genuine explanation, namely a law-based one, but that we also lack something explanatory, namely a causal account of the explanandum. Finally, I argue that examining what it takes to have law-based explanation in the face of agnosticism about the causal process makes it possible to recast the debate over action at a distance between Leibniz and Newton as empirically and methodologically motivated on both sides.     

Principle or constructive relativity

Appealing to Albert Einstein's distinction between principle and constructive theories, Harvey Brown has argued for an interpretation of the theory of relativity as a dynamic and constructive theory. Brown's view has been challenged by Michel Janssen and in this paper I investigate their dispute. I argue that their disagreement appears larger than it actually is due to the two frameworks used by Brown and Janssen to express their respective views: Brown's appeal to Einstein's principle–constructive distinction and Janssen's framing of the disagreement as one over the question whether relativity provides a kinematic or a dynamic constraint. I appeal to a distinction between types of theories drawn by H. A. Lorentz two decades before Einstein's distinction to argue that Einstein's distinction represents a false dichotomy. I argue further that the disagreement concerning the kinematics–dynamics distinction is a disagreement about labels but not about substance. There remains a genuine disagreement over the explanatory role of spacetime geometry and here I agree with Brown arguing that Janssen sees a pressing need for an explanation of Lorentz invariance where no further explanation is needed.     

Measurements,disturbances and the quantum three box paradox

A quantum pre- and post-selection paradox involves making measurements at two separate times on a quantum system, and making inferences about the state of the system at an intermediate time, conditional upon the observed outcomes. The inferences lead to predictions about the results of measurements performed at the intermediate time, which have been well confirmed experimentally, but which nevertheless seem paradoxical when inferences about different intermediate measurements are combined. The three box paradox is the paradigm example of such an effect, where a ball is placed in one of three boxes and is shuffled between the boxes in between two measurements of its location. By conditionalising on the outcomes of those measurements, it is inferred that between the two measurements the ball would have been found with certainty in Box 1 and with certainty in Box 2, if either box been opened on their own. Despite experimental confirmation of the predictions, and much discussion, it has remained unclear what exactly is supposed to be paradoxical or what specifically is supposed to be quantum, about these effects. In this paper I identify precisely the conditions under which the quantum three box paradox occurs, and show that these conditions are the same as arise in the derivation of the Leggett–Garg Inequality, which is supposed to demonstrate the incompatibility of quantum theory with macroscopic realism. I will argue that, as in Leggett–Garg Inequality violations, the source of the effect actually lies in the disturbance introduced by the intermediate measurement, and that the quantum nature of the effect is that no classical model of measurement disturbance can reproduce the paradox.     

Worlds in the Everett interpretation

This is a discussion of how we can understand the world-view given to us by the Everett interpretation of quantum mechanics, and in particular the rôle played by the concept of ‘world’. The view presented is that we are entitled to use ‘many-worlds’ terminology even if the theory does not specify the worlds in the formalism; this is defended by means of an extensive analogy with the concept of an ‘instant’ or moment of time in relativity, with the lack of a preferred foliation of spacetime being compared with the lack of a preferred basis in quantum theory. Implications for identity of worlds over time, and for relativistic quantum mechanics, are discussed.