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The British Journal for the Philosophy of Science

Quantum Theory: A Pragmatist Approach

Richard Healey

Published Mar 13, 2012• 171 citations
Publisher

Abstract

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 Rovelli's 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 Wigner's friend 6 Conclusion

DOI
10.1093/bjps/axr054

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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 Rovelli's 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 Wigner's friend 6 Conclusion
Abstract 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. This essay offers a diagnosis of this situation and outlines a pragmatist solution to the problem it poses, leaving further details for subsequent essays.
Applications of quantum mechanics have led to many successful predictions and explanations of puzzling phenomena, and we now apply quantum mechanics to gain, process, and communicate information in novel ways. We can understand quantum mechanics by understanding how we have applied it. We should not seek agreement on the nature of the world it represents, because this theory does not itself represent the physical world (though its applications do help us to represent it better). When applied to a quantum state, quantum mechanics yields probabiities for physical events: both state and probability are objective--not because they represent elements of phyiscal reality, but because each exerts norrmative authority over the beliefs of anyone who accepts quantum mechanics and applies it relative to a physical situation they may (but need not) occupy. These events may be described by statements that are meaningful in an appropriate environmental context, and quantum mechanics can help one to say when that is. Measurement creates an appropriate context, so here the Born rule indirectly yields probabilities of measurement outcomes. The quantum state of a system does not "collapse" on measurement: a new state must be assigned relative to a physical situation in which information about the outcome is accessible. Understood this way, there is no measurement problem, and violations of Bell inequalities does not demonstrate "spooky" non-local action. Quantum field theories have no physical ontology of their own: a quantum field is a mathematical object in a model whose application helps us to improve and extend our descriptions of the world in other terms. We cannot realise the scenario of Wigner's friend and its recent extensions: but the data that provide overwhelming evidence for quantum mechanics are objective in the same sense as the relative measurement outcomes described in those scenarios.
Applications of quantum mechanics have led to many successful predictions and explanations of puzzling phenomena, and we now apply quantum mechanics to gain, process, and communicate information in novel ways. We can understand quantum mechanics by understanding how we have applied it. We should not seek agreement on the nature of the world it represents, because this theory does not itself represent the physical world (though its applications do help us to represent it better). When applied to a quantum state, quantum mechanics yields probabilities for physical events: both state and probability are objective—not because they represent elements of physical reality, but because each exerts normative authority over the beliefs of anyone who accepts quantum mechanics and applies it relative to a physical situation they may (but need not) occupy. These events may be described by statements (about values of magnitudes) that are meaningful in an appropriate environmental context, and quantum mechanics can help one to say when that is. Measurement creates an appropriate context, so here the Born rule indirectly yields probabilities of measurement outcomes. The quantum state of a system does not “collapse” on measurement: a new state must be assigned relative to a physical situation in which information about the outcome is accessible. Understood this way, there is no measurement problem, and violation of Bell inequalities does not demonstrate “spooky” non-local action. Quantum field theories have no physical ontology of their own: a quantum field is a mathematical object in a model whose application helps to improve and extend our descriptions of the world in other terms. We cannot realise the scenario of Wigner’s friend and its recent extensions: but the data that provide overwhelming evidence for quantum mechanics are objective in the same sense as the relative measurement outcomes described in those scenarios.
Abstract These essays in natural philosophy were all written from a pragmatist point of view developed over the past 15 years. They show what work pragmatism can do in science as well as philosophy. Many essays express a view of quantum theory developed using ideas of contemporary pragmatist philosophers: other essays apply these more widely to science and metaphysics. Quantum models are used not to represent physical systems but to offer good advice on how to represent them. The role of a quantum state is not to describe a system but to yield objective probabilities for alternative outcomes of events involving it. A situated agent could use these probabilities to decide what to do, and one can use them to explain phenomena. They are probabilities for physical events including measurements: pragmatist inferentialism helps one to say what these are, and when they have outcomes, thereby resolving the quantum measurement problem. One can use quantum theory locally to explain non-localized correlations, and why quantum field theory poses no ontological problem. Despite its unrivaled success, quantum theory offers no scientific image of the physical world on which a naturalist could construct a metaphysics. There are structural reasons why the rest of physical theory cannot be reduced to quantum theory. This pragmatist view of quantum theory is not instrumentalist. One essay explains why it should count as realist. Other essays defend pragmatist views of scientific laws and explanations as well as causation, truth, and objectivity.
Quantum theory launched a revolution in twentieth-century physics. But we have yet to appreciate the revolution’s significance for philosophy. Most studies of the conceptual foundations of quantum theory first try to interpret the theory—to say how the world could possibly be the way the theory says it is. But, though fundamental, quantum theory is enormously successful without describing the world in its own terms. When properly applied, models of quantum theory offer good advice on the significance and credibility of claims about the world expressed in other terms. This first of several philosophical lessons of the quantum revolution dissolves the quantum measurement problem. Pragmatist treatments of probability and causation show how quantum theory may be used to explain the non-localized correlations that have been thought to involve ‘spooky’ instantaneous action at a distance. Given environmental decoherence, a pragmatist inferentialist approach to content shows when talk of quantum probabilities is licensed, resolves any residual worries about whether a quantum measurement has a determinate outcome, and solves a dilemma about the ontology of a quantum field theory. This approach to meaning and reference also reveals the nature and limits of objective description in the light of quantum theory. While these pragmatist approaches to probability, causation, explanation, and content may be independently motivated by philosophical argument, their successful application here illustrates their practical importance in helping philosophers come to terms with the quantum revolution.
The paradox of Wigner’s friend challenges the objectivity of quantum theory. A pragmatist interpretation can meet this challenge by judicious appeal to decoherence. Quantum theory provides situated agents with resources for predicting and explaining what happens in the physical world—not conscious observations of it. Even in bizarre Wigner’s friend scenarios, differently situated agents agree on the objective content of physical magnitude statements while, normally, quantum Darwinism permits agents equal observational access to their truth. Quantum theory has nothing to say about conscious experiences. But it does prompt us to reexamine the significance of everyday claims about the physical world.