Richard Healey

The philosophy of quantum mechanics : an interactive interpretation

Preface Introduction 1. Overview 2. Dynamical states 3. Measurement and quantum states 4. Coupled systems 5. Metaphysical aspects 6. Alternatives compared 7. Open questions Appendix Selected bibliography Index.

Quantum Theory: A Pragmatist Approach

While its applications have made quantum theory arguably the most successful theory in physics, its interpretation continues to be the subject of lively debate within the community of physicists and philosophers concerned with conceptual foundations. This situation poses a problem for a pragmatist for whom meaning derives from use. While disputes about how to use quantum theory have arisen from time to time, they have typically been quickly resolved, and consensus reached, within the relevant scientific sub-community. Yet, rival accounts of the meaning of quantum theory continue to proliferate.1 In this article, I offer a diagnosis of this situation and outline a pragmatist solution to the problem it poses, leaving further details for subsequent articles. 1 Introduction 2 The Objectivity of Quantum Probabilities   2.1 Quantum probabilities are objective   2.2 Quantum probabilities do not represent physical reality 3 How Quantum Theory Limits Description of Physical Reality 4 The Relational Nature of Quantum States   4.1 Rovellis relationism   4.2 Quantum Bayesian relationism   4.3 Reference-frame relationism   4.4 Agent-situation relationism and wave-collapse   4.5 Why quantum probabilities are not Lewisian chances 5 The Objectivity of Physical Description in Quantum Theory   5.1 Why violations of Bell inequalities involve no physical non-locality   5.2 Objectivity, inter-subjectivity, and Wigners friend 6 Conclusion 1 Introduction 2 The Objectivity of Quantum Probabilities   2.1 Quantum probabilities are objective   2.2 Quantum probabilities do not represent physical reality   2.1 Quantum probabilities are objective   2.2 Quantum probabilities do not represent physical reality 3 How Quantum Theory Limits Description of Physical Reality 4 The Relational Nature of Quantum States   4.1 Rovellis relationism   4.2 Quantum Bayesian relationism   4.3 Reference-frame relationism   4.4 Agent-situation relationism and wave-collapse   4.5 Why quantum probabilities are not Lewisian chances   4.1 Rovellis relationism   4.2 Quantum Bayesian relationism   4.3 Reference-frame relationism   4.4 Agent-situation relationism and wave-collapse   4.5 Why quantum probabilities are not Lewisian chances 5 The Objectivity of Physical Description in Quantum Theory   5.1 Why violations of Bell inequalities involve no physical non-locality   5.2 Objectivity, inter-subjectivity, and Wigners friend   5.1 Why violations of Bell inequalities involve no physical non-locality   5.2 Objectivity, inter-subjectivity, and Wigners friend 6 Conclusion

On the Reality of Gauge Potentials

Classically, a gauge potential was merely a convenient device for generating a corresponding gauge field. Quantum-mechanically, a gauge potential lays claim to independent status as a further feature of the physical situation. But whether this is a local or a global feature is not made any clearer by the variety of mathematical structures used to represent it. I argue that in the theory of electromagnetism (or a non-Abelian generalization) that describes quantum particles subject to a classical interaction, the gauge potential is best understood as a feature of the physical situation whose global character is most naturally represented by the holonomies of closed curves in space-time.

Nonlocality and the Aharonov-Bohm Effect

At first sight the Aharonov-Bohm effect appears nonlocal, though not in the way EPR/Bell correlations are generally acknowledged to be nonlocal. This paper applies an analysis of nonlocality to the Aharonov-Bohm effect to show that its peculiarities may be blamed either on a failure of a principle of local action or on a failure of a principle of separability. Different interpretations of quantum mechanics disagree on how blame should be allocated. The parallel between the Aharonov-Bohm effect and violations of Bell inequalities turns out to be so close that a balanced assessment of the nature and significance of quantum nonlocality requires a detailed study of both effects.

Can Physics Coherently Deny the Reality of Time

The conceptual and technical difficulties involved in creating quantum theory of gravity have led some physicists to question, and even in some cases to deny, the reality of time. More surprisingly, this denial has found a sympathetic audience among certain philosophers of physics. What should we make of these wild ideas? Does it even make sense to deny the reality of time? In fact physical science has been chipping away at common sense aspects of time ever since its inception. Section 1 offers a brief survey of the demolition process. Section 2 distinguishes a tempered from an extreme-y l radical form that a denial of time might take, and argues that extreme radicalism is empirically self-refuting. Section 3 begins an investigation of the prospects for tempered radicalism in a timeless theory of quantum gravity.

Nonseparable processes and causal explanation

Abstract If physical reality is nonseparable, as quantum mechanics suggests, then it may contain processes of a quite novel kind. Such nonseparable processes could connect spacelike separated events without violating relativity theory or any defensible locality condition. Appeal to nonseparable processes could ground theoretical explanations of such otherwise puzzling phenomena as the two-slit experiment, and EPR-type correlations. We find such phenomena puzzling because they threaten cherished conceptions of how causes operate to produce their effects. But nonseparable processes offer us an alternative deal of natural order, conformity to which makes such phenomena seem quite normal and not at all unexpected. Attempts to answer the further question, as to whether an appeal to a nonseparable process provides a genuine causal explanation, have something to teach us about our concept of causation, but do not threaten to undermine the value of the explanation itself.

Dissipating the quantum measurement problem

The integration of recent work on decoherence into a so-called “modal” interpretation offers a promising new approach to the measurement problem in quantum mechanics. In this paper I explain and develop this approach in the context of the interactive interpretation presented in Healey (1989). I begin by questioning a number of assumptions which are standardly made in setting up the measurement problem, and I conclude that no satisfactory solution can afford to ignore the influence of the environment. Further, I argue that there are good reasons to believe that on a “modal” interpretation environmental interactions rapidly ensure that a quantummechanically describable apparatus indeed records a definite result following a measurement interaction.

Causation, Robustness, and EPR

In his recent work, Michael Redhead (1986, 1987, 1989, 1990) has introduced a condition he calls robustness which, he argues, a relation must satisfy in order to be causal. He has used this condition to argue further that EPR-type correlations are neither the result of a direct causal connection between the correlated events, nor the result of a common cause associated with the source of the particle pairs which feature in these events. Andrew Elby (1992) has used this same condition as a premise in an independent argument for the conclusion that EPR-type correlations cannot be causally explained (except, perhaps, by a nonlocal hidden variable theory). I wish to argue here that robustness is itself too fragile a notion to support such conclusions.

Quantum Analogies: A Reply to Maudlin

Quantum mechanics predicted the Aharonov-Bohm effect and violations of Bell inequalities before either phenomenon was experimentally verified. It is now commonly taken to explain both phenomena. Maudlin has pointed out significant disanalogies between these phenomena. But he has failed to appreciate the striking analogy that emerges when one examines the structure of their quantum mechanical explanations. The fact that each may be explained quantum mechanically in terms of a locally-acting, but nonseparable process suggests that the lesson of quantum nonlocality may be that while there is no action at a distance, the world is nonseparable.

Why error-prone quantum measurements have outcomes

To solve the quantum measurement problem it is necessary to construct quantum mechanical models of measurement interactions to show why properly conducted measurements always yield definite outcomes. The main barrier to a solution has been the interpretive principle that a quantum system has a definite value for an observable only if it may be described by a quantum eigenstate of the corresponding operator. I have recently proposed a solution to the measurement problem based on alternative interpretive principles. The present paper defends this proposal against recent criticisms which seek to show that it fails to solve the problem unless quantum measurements meet highly idealized conditions which no actual measurement could hope to meet. Several models of error-prone measurements are shown to lead to definite outcomes, and a general defense of the appropriateness of these models is sketched.