Foundations of Physics 50 (12):1859-1874 (2020)

Authors
Rüdiger Schack
Royal Holloway University of London
Abstract
According to QBism, quantum states, unitary evolutions, and measurement operators are all understood as personal judgments of the agent using the formalism. Meanwhile, quantum measurement outcomes are understood as the personal experiences of the same agent. Wigner’s conundrum of the friend, in which two agents ostensibly have different accounts of whether or not there is a measurement outcome, thus poses no paradox for QBism. Indeed the resolution of Wigner’s original thought experiment was central to the development of QBist thinking. The focus of this paper concerns two very instructive modifications to Wigner’s puzzle: One, a recent no-go theorem by Frauchiger and Renner, and the other a thought experiment by Baumann and Brukner. We show that the paradoxical features emphasized in these works disappear once both friend and Wigner are understood as agents on an equal footing with regard to their individual uses of quantum theory. Wigner’s action on his friend then becomes, from the friend’s perspective, an action the friend takes on Wigner. Our analysis rests on a kind of quantum Copernican principle: When two agents take actions on each other, each agent has a dual role as a physical system for the other agent. No user of quantum theory is more privileged than any other. In contrast to the sentiment of Wigner’s original paper, neither agent should be considered as in “suspended animation.” In this light, QBism brings an entirely new perspective to understanding Wigner’s friend thought experiments.
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DOI 10.1007/s10701-020-00369-x
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References found in this work BETA

The Foundations of Statistics.Leonard J. Savage - 1954 - Wiley Publications in Statistics.
The Foundations of Statistics.Leonard J. Savage - 1956 - Philosophy of Science 23 (2):166-166.
Quantum Theory and the Limits of Objectivity.Richard Healey - 2018 - Foundations of Physics 48 (11):1568-1589.
Perspectival Objectivity.Peter W. Evans - 2020 - European Journal for Philosophy of Science 10 (2):1-21.

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Citations of this work BETA

Stable Facts, Relative Facts.Carlo Rovelli & Andrea Di Biagio - 2021 - Foundations of Physics 51 (1):1-13.
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