Abstract
In a joint paper Jeff Bub and Itamar Pitowsky argued that the quantum state represents “the credence function of a rational agent […] who is updating probabilities on the basis of events that occur”. In the famous thought experiment designed by Wigner, Wigner’s friend performs a measurement in an isolated laboratory which in turn is measured by Wigner. Here we consider Wigner’s friend as a rational agent and ask what her “credence function” is. We find experimental situations in which the friend can convince herself that updating the probabilities on the basis of events that happen solely inside her laboratory is not rational and that conditioning needs to be extended to the information that is available outside of her laboratory. Since the latter can be transmitted into her laboratory, we conclude that the friend is entitled to employ Wigner’s perspective on quantum theory when making predictions about the measurements performed on the entire laboratory, in addition to her own perspective, when making predictions about the measurements performed inside the laboratory.
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Acknowledgements
We thank Borivoje Dakic, Fabio Costa, Flavio del Santo, Renato Rennner and Ruediger Schack for useful discussion. We also thank Jeff Bub for the discussions and for the idea of the alternative protocol. We acknowledge the support of the Austrian Science Fund (FWF) through the Doctoral Programme CoQuS (Project no. W1210-N25) and the projects I-2526-N27 and I-2906. This work was funded by a grant from the Foundational Questions Institute (FQXi) Fund. The publication was made possible through the support of a grant from the John Templeton Foundation. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation.
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Baumann, V., Brukner, Č. (2020). Wigner’s Friend as a Rational Agent. In: Hemmo, M., Shenker, O. (eds) Quantum, Probability, Logic. Jerusalem Studies in Philosophy and History of Science. Springer, Cham. https://doi.org/10.1007/978-3-030-34316-3_4
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