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A No-Go Theorem for Observer-Independent Facts

Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
Institute of Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria
Entropy 2018, 20(5), 350;
Received: 5 April 2018 / Revised: 29 April 2018 / Accepted: 2 May 2018 / Published: 8 May 2018
(This article belongs to the Special Issue Quantum Information and Foundations)
PDF [449 KB, uploaded 8 May 2018]


In his famous thought experiment, Wigner assigns an entangled state to the composite quantum system made up of Wigner’s friend and her observed system. While the two of them have different accounts of the process, each Wigner and his friend can in principle verify his/her respective state assignments by performing an appropriate measurement. As manifested through a click in a detector or a specific position of the pointer, the outcomes of these measurements can be regarded as reflecting directly observable “facts”. Reviewing arXiv:1507.05255, I will derive a no-go theorem for observer-independent facts, which would be common both for Wigner and the friend. I will then analyze this result in the context of a newly-derived theorem arXiv:1604.07422, where Frauchiger and Renner prove that “single-world interpretations of quantum theory cannot be self-consistent”. It is argued that “self-consistency” has the same implications as the assumption that observational statements of different observers can be compared in a single (and hence an observer-independent) theoretical framework. The latter, however, may not be possible, if the statements are to be understood as relational in the sense that their determinacy is relative to an observer. View Full-Text
Keywords: Wigner-friend experiment; no-go theorem; quantum foundations; interpretations of quantum mechanics Wigner-friend experiment; no-go theorem; quantum foundations; interpretations of quantum mechanics

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Brukner, Č. A No-Go Theorem for Observer-Independent Facts. Entropy 2018, 20, 350.

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