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This book presents the most comprehensive study yet of a problem that has puzzled physicists and philosophers since the 1930s. The standard theory of quantum mechanics is in one sense the most successful physical theory ever, predicting the behaviour of the basic constituents of all physical things; no other theory has ever made such accurate empirical predictions. However, if one tries to understand the theory as providing a complete and accurate framework for the description of the behaviour of all physical interactions, it becomes evident that the theory is ambiguous, or even logically inconsistent. The most notable attempt to formulate the theory so as to deal with this problem, the quantum measurement problem, was initiated by Hugh Everett III in the 1950s. This book gives a careful and challenging examination and evaluation of the work of Everett and those who have followed him. The informal approach, minimizing technicality, makes the book accessible and illuminating for philosophers and physicists alike.

Quantum theory was developed in response to a welter of new experimental phenomena, yet appeared to depict a world so esoteric as to be literally unimaginable. Interpretation of the theory became feasible only after von Neumann's theoretical unification, but von Neumann's own interpretation astonishingly implied that in measurement something happens that violates Schroedinger's equation, the theory's cornerstone. This book argues first of all that the phenomena themselves, without theoretical motives, suffice to eliminate ’common cause’ models, thus requiring a radical departure from classical physics models. The measurement process, however, has an adequate description of itself as a quantum‐mechanical process, so that the theory can be seen as complete in a relevant sense. But the question of interpretation, ‘How could the world possibly be the way this theory says it is?’, is not thereby answered. In response to that question it is argued that the theory admits a plurality of interpretations, each of which helps to understand the theory further, but also advocates one particular interpretation (the Copenhagen Variant of the Modal Interpretation). That interpretation is then applied to such topics as the Einstein–Podolsky–Rosen paradox and the problem of ’identical’ particles, quantum statistics, identity, and individuation.

This book is a conceptual analysis of quantum information theory and the questions it raises for our understanding of the quantum world. Beginning from a detailed analysis of the concepts of information in the everyday and classical (Shannon) information-theory settings, an ontologically deflationary account of the nature of quantum information is developed. The account provided sheds light on the nature of nonlocality and information flow in the presence of entanglement and, in particular, dissolves puzzles surrounding the process of quantum teleportation. In addition, it permits a clear view of what the ontological and methodological lessons provided by quantum information theory are; lessons which bear on the gripping question of what role a concept like information has to play in fundamental physics. With a clear grasp of the concept of information in hand, attention turns to the pressing question whether advances in quantum information theory pave the way for the resolution of the traditional conceptual problems of quantum mechanics: the deep problems which loom over measurement, nonlocality and the general nature of quantum ontology. A number of common pitfalls are marked-out to be avoided before some concrete proposals are analysed in detail, including the radical quantum Bayesian programme of Caves, Fuchs and Schack. One central moral which is drawn is that, for all the interest that the quantum information-inspired approaches hold, no cheap resolutions to the traditional problems of quantum mechanics are to be had.

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.

The fascinating discoveries of the new fields of quantum information, quantum computation, and quantum cryptography are brought to life in this book in a way that is accessible and interesting to a wide range of readers, not just the experts. From a modern perspective, the characteristic feature of quantum mechanics is the existence of strangely counterintuitive correlations between distant events, which can be exploited in feats like quantum teleportation, unbreakable cryptographic schemes, and computers with enormously enhanced computing power. Schrödinger coined the term “entanglement” to describe these bizarre correlations, which show up in the random outcomes of different measurements on separated quantum systems. Bananaworld – an imaginary island with entangled bananas – is used to discuss sophisticated quantum phenomena without the mathematical machinery of quantum mechanics. As far as the conceptual problems of the theory that philosophers worry about are concerned, one might as well talk about bananas rather than quantum states. Nevertheless, the connection with quantum correlations is fully explained in sections written for the non-physicist reader with a serious interest in understanding the mysteries of the quantum world. The result is a subversive but entertaining book, with the novel thesis that quantum mechanics is about the structure of information, and what we have discovered is that the possibilities for representing, manipulating, and communicating information are different than we thought.