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Wigner's Friend and Bell's Field Beables

Jeffrey Alan Barrett

Published Dec 22, 2011• 1 citation
OpenAlex

Abstract

A field-theoretic version of Wigner’s friend (1961) illustrates how the quantum measurement problem arises for field theory. Similarly, considering spacelike separate measurements of entangled fields by ob-servers akin to Wigner’s friend shows the sense in which relativistic constraints make the measurement problem particularly difficult to resolve in the context of a relativistic field theory. We will consider proposals by Wigner (1961), Bloch (1967), Helwig and Kraus (1970), and Bell (1984) for resolving the measurement problem for quantum field theory. We will conclude by considering the possibility of giving up rich dynamical explanation in the context of a many-maps formu-lation of relativistic quantum field theory. 1. Wigner’s Friend When Eugene Wigner wrote his now famous paper (1961) on the dynam-ical role of observers in quantum mechanics, quantum field theory was flour-

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Abstract We use the Wigner’s friend story to characterize the quantum measurement problem. On the standard formulation of quantum mechanics, whether a physical system is measured determines which of the theory's two dynamical laws obtains. For this reason, the logical consistency of the theory depends on one specifying strictly disjoint conditions for when when each law obtains, which means that one needs to say precisely what constitutes a measurement. But since the term measurement occurs in the standard theory as an undefined primitive term, the theory is at best incomplete. We see precisely how this conceptual incompleteness threatens the logical inconsistency of the theory and why, on even the most charitable reading, it entails that the theory is empirical incomplete. We end by considering why its empirically incompleteness is extremely difficult to test.

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In this paper, I attempt a personal account of my understanding of the measurement problem in quantum mechanics, which has been largely in the tradition of the Copenhagen interpretation.I assume that (i) the quantum state is a representation of knowledge of a (real or hypothetical) observer relative to her experimental capabilities; (ii) measurements have definite outcomes in the sense that only one outcome occurs; (iii) quantum theory is universal and the irreversibility of the measurement process is only "for all practical purposes".These assumptions are analyzed within quantum theory and their consistency is tested in Deutsch's version of the Wigner's friend gedanken experiment, where the friend reveals to Wigner whether she observes a definite outcome without revealing which outcome she observes.The view that holds the coexistence of the "facts of the world" common both for Wigner and his friend runs into the problem of the hidden variable program.The solution lies in understanding that "facts" can only exist relative to the observer.

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Nonrelativistic quantum mechanics is conceptually and mathematically simpler, and does, after all, work very well in a low-energy regime. Any solution of the measurement problem, or any account of the ontology of quantum theory, must yield sensible results in this regime, and so nonrelativistic quantum mechanics can serve as a testing ground for the viability of interpretations. On any approach, empirical adequacy requires recovery of at least an approximation to the quantum probabilities for outcomes of experiments, insofar as these have been verified by experiment. A quantum-mechanical theory is what results when one subjects a classical theory of a system with finitely many degrees of freedom to quantization. The task of extending additional beables theories to the context of quantum field theories remains a work in progress. A dynamical collapse theory modifies the evolution of the quantum state. Standard quantum theory predicts correlations between outcomes of spacelike separated experiments that violate Bell inequalities.

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A field-theoretic version of Wigner’s friend (1961) illustrates how the quantum measurement problem arises for field theory. Similarly, considering spacelike separate measurements of entangled fields by ob-servers akin to Wigner’s friend shows the sense in which relativistic constraints make the measurement problem particularly difficult to resolve in the context of a relativistic field theory. We will consider proposals by Wigner (1961), Bloch (1967), Helwig and Kraus (1970), and Bell (1984) for resolving the measurement problem for quantum field theory. We will conclude by considering the possibility of giving up rich dynamical explanation in the context of a many-maps formu-lation of relativistic quantum field theory. 1. Wigner’s Friend When Eugene Wigner wrote his now famous paper (1961) on the dynam-ical role of observers in quantum mechanics, quantum field theory was flour-
Wigner-friend scenarios -- in which external agents describe a closed laboratory containing a friend making a measurement -- highlight the difficulties inherent to quantum theory when accounting for measurements. In non-relativistic scenarios, the difficulty is to accommodate unitary evolution for a closed system with a definite outcome obtained by the friend. In relativistic scenarios the tensions between quantum theory and relativity induce additional constraints. A generic property of relativistic scenarios is the frame-dependence of state update upon a measurement. Based on a definite example, we will show that this property leads to inconsistent accounts for outcomes obtained in different reference frames. We will further argue that these results point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations made on simple wavefunctions.
The measurement problem is seen as an ambiguity of quantum mechanics, or, beyond that, as a contradiction within the theory: Quantum mechanics offers two conflicting descriptions of the Wigner's-friend experiment. As we argue in this note there are, however, obstacles from within quantum mechanics and regarding our perspective onto doing physics towards fully describing a measurement. We conclude that the ability to exhaustively describe a measurement is an assumption necessary for the common framing of the measurement problem and ensuing suggested solutions.
Wigner’s friend scenarios—in which external agents describe a closed laboratory containing a friend making a measurement—highlight the difficulties of quantum theory when accounting for measurements. The problem is to accommodate for unitary evolution from the point of view of the external agent with the measurements or other operations carried out by the friend. Here, we show in the context of a relativistic thought experiment that an operation that may be accounted for unitarily in a given reference frame cannot be described unitarily in a different reference frame. This result, based on the frame dependence of the state update in relativistic contexts, could point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations.
Abstract We use the Wigner’s friend story to characterize the quantum measurement problem. On the standard formulation of quantum mechanics, whether a physical system is measured determines which of the theory's two dynamical laws obtains. For this reason, the logical consistency of the theory depends on one specifying strictly disjoint conditions for when when each law obtains, which means that one needs to say precisely what constitutes a measurement. But since the term measurement occurs in the standard theory as an undefined primitive term, the theory is at best incomplete. We see precisely how this conceptual incompleteness threatens the logical inconsistency of the theory and why, on even the most charitable reading, it entails that the theory is empirical incomplete. We end by considering why its empirically incompleteness is extremely difficult to test.
In this paper, I attempt a personal account of my understanding of the measurement problem in quantum mechanics, which has been largely in the tradition of the Copenhagen interpretation.I assume that (i) the quantum state is a representation of knowledge of a (real or hypothetical) observer relative to her experimental capabilities; (ii) measurements have definite outcomes in the sense that only one outcome occurs; (iii) quantum theory is universal and the irreversibility of the measurement process is only "for all practical purposes".These assumptions are analyzed within quantum theory and their consistency is tested in Deutsch's version of the Wigner's friend gedanken experiment, where the friend reveals to Wigner whether she observes a definite outcome without revealing which outcome she observes.The view that holds the coexistence of the "facts of the world" common both for Wigner and his friend runs into the problem of the hidden variable program.The solution lies in understanding that "facts" can only exist relative to the observer.
Nonrelativistic quantum mechanics is conceptually and mathematically simpler, and does, after all, work very well in a low-energy regime. Any solution of the measurement problem, or any account of the ontology of quantum theory, must yield sensible results in this regime, and so nonrelativistic quantum mechanics can serve as a testing ground for the viability of interpretations. On any approach, empirical adequacy requires recovery of at least an approximation to the quantum probabilities for outcomes of experiments, insofar as these have been verified by experiment. A quantum-mechanical theory is what results when one subjects a classical theory of a system with finitely many degrees of freedom to quantization. The task of extending additional beables theories to the context of quantum field theories remains a work in progress. A dynamical collapse theory modifies the evolution of the quantum state. Standard quantum theory predicts correlations between outcomes of spacelike separated experiments that violate Bell inequalities.