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An Old Look at Extended Wigner's Friend Scenarios

Yanis Pianko

Published Jun 11, 2026• 0 citations
OpenAlex

Abstract

We present a general formalism to formulate Bell's theorem, no-hidden variables proofs and extended Wigner's friend scenarios. This common mathematical framework allows to bridge the differences between an old (short-lived) research program (that of modal interpretations) and the more recent literature on perspectival anti-realist interpretations. We find that our unifying mathematical approach gives a clear content to the technical aspect of various no-go theorem, and provides new insights on their conceptual role by allowing comparison between different research programs.

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An Old Look at Extended Wigner Friend Scenarios

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We present a general formalism to formulate Bell's theorem, no-hidden variables proofs and extended Wigner's friend scenarios. This common mathematical framework allows to bridge the differences between an old (short-lived) research program (that of modal interpretations) and the more recent literature on perspectival anti-realist interpretations. We find that our unifying mathematical approach gives a clear content to the technical aspect of various no-go theorem, and provides new insights on their conceptual role by allowing comparison between different research programs.

•Jun 11, 2026•0 citations

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We review a series of recent no-go theorems that extend the methodological and conceptual framework introduced by Bell's theorem to more sophisticated operational scenarios involving independent sources, quantum networks, joint measurements, and multiple observers. Many of these results were originally developed within quantum information science and are scattered across the literature, where their technical and application-driven success has partly obscured their broader foundational significance. We organize them around a common Bell-inspired methodology that extends beyond quantum nonlocality, and emphasize how increasingly rich operational structures change which physical assumptions can be tested.

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A strong no-go theorem and experiment for the Wigner's friend paradox

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How can we address the idea of observers in quantum mechanics? The long-standing Wigner’s friend thought experiment provides a conceptual framework for thinking about this question. It considers the the difficulty of reconciling unitary, evolution of systems and non-unitary state update after a making observations. Following from a recent suggested protocol with two friends that share and entangled state, we prove and investigate, in a photonic experiment, a strong no-go theorem for this Wigner’s friend scenario. Specifically, if quantum mechanics applies at the scale of observers, then there are constraints on the set of assumptions: locality; no-superdeterminism; and absoluteness of observed events. We note that this new theorem places strictly stronger constraints on physical reality than Bell’s theorem.

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Two recent, prominent theorems-the "no-go theorem for observer-independent facts" and the "Local Friendliness no-go theorem"-employ so-called extended Wigner's friend scenarios to try to impose novel, non-trivial constraints on the possible nature of physical reality. While the former is argued to entail that there can be no theory in which the results of Wigner and his friend can both be considered objective, the latter is said to place on reality stronger constraints than the Bell and Kochen-Specker theorems. Here, I conduct a thorough analysis of these theorems and show that they suffer from a list of shortcomings that question their validity and limit their strength. I conclude that the "no-go theorem for observer-independent facts" and the "Local Friendliness no-go theorem" fail to impose significant constraints on the nature of physical reality.

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Testing a New Strong No-Go Theorem for the Wigner’s Friend Scenario

Geoff J. Pryde, Kok-Wei Bong, Aníbal Utreras-Alarcón +5 more

The Wigner’s friend paradox illuminates the quantum measurement problem. We derive—and study, in a series of entangled-photon experiments—a new and robust no-go theorem based on the paradox, with stronger constraints than Bell's theorem.

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Strong versions of Bell’s theorem

Henry Pierce Stapp

Technical aspects of a recently constructed strong version of Bell's theorem are discussed. The theorem assumes neither hidden variables nor factorization, and neither determinism nor counterfactual definiteness. It deals directly with logical connections. Hence its relationship with modal logic needs to be described. It is shown that the proof can be embedded in an orthodox modal logic, and hence its compatibility with modal logic assured, but that this embedding weakens the theorem by introducing as added assumptions the conventionalities of the particular modal logic that is adopted. This weakening is avoided in the recent proof by using directly the set-theoretic conditions entailed by the locality assumption.

Physical Review A•May 1, 1994•10 citations
We present a general formalism to formulate Bell's theorem, no-hidden variables proofs and extended Wigner's friend scenarios. This common mathematical framework allows to bridge the differences between an old (short-lived) research program (that of modal interpretations) and the more recent literature on perspectival anti-realist interpretations. We find that our unifying mathematical approach gives a clear content to the technical aspect of various no-go theorem, and provides new insights on their conceptual role by allowing comparison between different research programs.
We present a general formalism to formulate Bell's theorem, no-hidden variables proofs and extended Wigner's friend scenarios. This common mathematical framework allows to bridge the differences between an old (short-lived) research program (that of modal interpretations) and the more recent literature on perspectival anti-realist interpretations. We find that our unifying mathematical approach gives a clear content to the technical aspect of various no-go theorem, and provides new insights on their conceptual role by allowing comparison between different research programs.
We review a series of recent no-go theorems that extend the methodological and conceptual framework introduced by Bell's theorem to more sophisticated operational scenarios involving independent sources, quantum networks, joint measurements, and multiple observers. Many of these results were originally developed within quantum information science and are scattered across the literature, where their technical and application-driven success has partly obscured their broader foundational significance. We organize them around a common Bell-inspired methodology that extends beyond quantum nonlocality, and emphasize how increasingly rich operational structures change which physical assumptions can be tested.
How can we address the idea of observers in quantum mechanics? The long-standing Wigner’s friend thought experiment provides a conceptual framework for thinking about this question. It considers the the difficulty of reconciling unitary, evolution of systems and non-unitary state update after a making observations. Following from a recent suggested protocol with two friends that share and entangled state, we prove and investigate, in a photonic experiment, a strong no-go theorem for this Wigner’s friend scenario. Specifically, if quantum mechanics applies at the scale of observers, then there are constraints on the set of assumptions: locality; no-superdeterminism; and absoluteness of observed events. We note that this new theorem places strictly stronger constraints on physical reality than Bell’s theorem.
Two recent, prominent theorems-the "no-go theorem for observer-independent facts" and the "Local Friendliness no-go theorem"-employ so-called extended Wigner's friend scenarios to try to impose novel, non-trivial constraints on the possible nature of physical reality. While the former is argued to entail that there can be no theory in which the results of Wigner and his friend can both be considered objective, the latter is said to place on reality stronger constraints than the Bell and Kochen-Specker theorems. Here, I conduct a thorough analysis of these theorems and show that they suffer from a list of shortcomings that question their validity and limit their strength. I conclude that the "no-go theorem for observer-independent facts" and the "Local Friendliness no-go theorem" fail to impose significant constraints on the nature of physical reality.
Technical aspects of a recently constructed strong version of Bell's theorem are discussed. The theorem assumes neither hidden variables nor factorization, and neither determinism nor counterfactual definiteness. It deals directly with logical connections. Hence its relationship with modal logic needs to be described. It is shown that the proof can be embedded in an orthodox modal logic, and hence its compatibility with modal logic assured, but that this embedding weakens the theorem by introducing as added assumptions the conventionalities of the particular modal logic that is adopted. This weakening is avoided in the recent proof by using directly the set-theoretic conditions entailed by the locality assumption.