Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Physical Review A

Macroscopic Wigner's friend paradoxes: Consistency with weak macroscopic realism

Ria Rushin Joseph, Manushan Thenabadu, Channa Hatharasinghe, Jesse Fulton, Run Yan Teh, P. D. Drummond, M. D. Reid

Published Aug 13, 2024• 3 citations
Publisher

Abstract

Wigner's friend paradoxes highlight contradictions between measurements made by friends inside a laboratory and superobservers outside the laboratory, who have access to an entangled state of the measurement apparatus. Where there are two separated laboratories, the contradictions lead to no-go theorems for observer-independent facts, thus challenging concepts of objectivity. Here, we examine the paradoxes from the perspective of establishing consistency with macroscopic realism. We present versions of the Brukner-Wigner's-friend and Frauchiger-Renner paradoxes in which the spin-$1/2$ systems measured by the friends correspond to two macroscopically distinct states. The local unitary operations ${U}_{{\ensuremath{\theta}}_{j}}$ that determine the measurement settings ${\ensuremath{\theta}}_{j}$ at each laboratory are carried out using nonlinear interactions, which are tailored to ensure that measurements need only distinguish between the macroscopically distinct states. The macroscopic paradoxes are perplexing, seemingly suggesting there is no objectivity in a macroscopic limit. However, we demonstrate consistency with a contextual weak form of macroscopic realism (wMR): The premise wMR asserts that each system $j$ can be considered to have a definite spin outcome ${\ensuremath{\lambda}}_{{\ensuremath{\theta}}_{j}}$, at the time after the system has undergone the local unitary rotation ${U}_{{\ensuremath{\theta}}_{j}}$ to prepare it in a suitable measurement basis. We further show that the paradoxical outcomes imply failure of deterministic macroscopic local realism and arise when there are unitary interactions ${U}_{{\ensuremath{\theta}}_{j}}$ occurring due to a change of measurement setting at both laboratories, with respect to the state prepared by each friend. In models which validate wMR, there is a breakdown of a subset of the assumptions that constitute the Bell-locality premise. A similar interpretation involving a contextual weak form of local realism exists for the original paradoxes.

DOI
10.1103/physreva.110.022219

Related papers

Wigner's Friend paradoxes: consistency with weak-contextual and weak-macroscopic realism models

Ria Rushin Joseph, Manushan Thenabadu, Channa Hatharasinghe +4 more

Wigner's friend paradoxes highlight contradictions between measurements made by Friends inside a laboratory and superobservers outside a laboratory, who have access to an entangled state of the measurement apparatus. The contradictions lead to no-go theorems for observer-independent facts, thus challenging concepts of objectivity. Here, we examine the paradoxes from the perspective of establishing consistency with macroscopic realism. We present versions of the Brukner-Wigner-friend and Frauchiger-Renner paradoxes in which the spin-$1/2$ system measured by the Friends corresponds to two macroscopically distinct states. The local unitary operations $U_θ$ that determine the measurement setting $θ$ are carried out using nonlinear interactions, thereby ensuring measurements need only distinguish between the macroscopically distinct states. The macroscopic paradoxes are perplexing, seemingly suggesting there is no objectivity in a macroscopic limit. However, we demonstrate consistency with a contextual weak form of macroscopic realism (wMR): The premise wMR asserts that the system can be considered to have a definite spin outcome $λ_θ$, at the time after the system has undergone the unitary rotation $U_θ$ to prepare it in a suitable pointer basis. We further show that the paradoxical outcomes imply failure of deterministic macroscopic local realism, and arise when there are unitary interactions $U_θ$ occurring due to a change of measurement setting at both sites, with respect to the state prepared by each Friend. In models which validate wMR, there is a breakdown of a subset of the assumptions that constitute the Bell-Locality premise. A similar interpretation involving a weak contextual form of realism exists for the original paradoxes.

arXiv:2211.02877•Nov 5, 2022•1 citation

Alternative Einstein-Podolsky-Rosen argument based on premises not falsified by Bell's theorem: weak macroscopic realism and weak local realism

Jesse Fulton, Run Yan Teh, M. D. Reid

The Einstein-Podolsky-Rosen (EPR) paradox gives an argument for the incompleteness of quantum mechanics based on a plausible criterion for reality, as well as the assumption of locality. A general view is that the argument is compromised, because EPR's premises are falsified by Greenberger-Horne-Zeilinger (GHZ) and Bell experiments. In this paper, we present an EPR argument based on premises not falsified by these experiments. First, we consider macroscopic EPR-Bohm, Bell, and GHZ experiments using spins ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}}_{\ensuremath{\theta}}$ defined by two macroscopically distinct states. The analyzers that realize the unitary operations ${U}_{\ensuremath{\theta}}$ determining the measurement settings $\ensuremath{\theta}$ are nonlinear devices creating macroscopic superposition states. We note two definitions of macroscopic realism (MR). For a system with two macroscopically distinct states available to it, MR posits a predetermined outcome for the measurement ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}}_{\ensuremath{\theta}}$ distinguishing between the states. Deterministic macroscopic realism assumes MR for the system defined prior to the interaction ${U}_{\ensuremath{\theta}}$ being carried out but is falsifiable by the macroscopic Bell and GHZ proposals. Motivated by arguments to uphold MR (without resorting to decoherence), as well as no-signaling and the properties of a meter, we define a set of premises referred to as weak macroscopic realism (wMR). Weak macroscopic realism (wMR) posits MR for the system after ${U}_{\ensuremath{\theta}}$, at the time ${t}_{f}$---when the system is prepared with respect to the measurement basis, ready for a ``pointer'' measurement and readout. For this system, wMR posits that the outcome of ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}}_{\ensuremath{\theta}}$ is determined and not changed by interactions that might subsequently occur at a remote system $B$. The premise wMR also posits a weaker version of EPR's criterion for reality. Importantly, we show that the GHZ and Bell predictions are consistent with wMR. Yet, an EPR paradox arises for a macroscopic EPR-Bohm state, based on wMR. As considered by Schr\"odinger, it is possible to measure two complementary spins of system $A$ simultaneously, ``one by direct, the other by indirect measurement'': If we assume wMR, then once both settings are fixed, the outcomes of the two spins are both determined. We revisit the original EPR paradox and find a similar result: An EPR argument can be based on a set of premises, weak local realism, which we show are not falsifiable by GHZ or Bell experiments.

Physical Review A•Aug 13, 2024•2 citations

An Einstein-Podolsky-Rosen argument based on weak forms of local realism not falsifiable by GHZ or Bell experiments

Fulton, Jesse, Run Yan Teh, M. D. Reid

The Einstein-Podolsky-Rosen (EPR) paradox gives an argument for the incompleteness of quantum mechanics based on the premises of local realism. A general view is that the argument is compromised, because EPR's premises are falsified by Greenberger-Horne-Zeilinger (GHZ) and Bell experiments. In this paper, we present an EPR argument based on premises not falsifiable by these experiments. We propose macroscopic EPR and GHZ experiments using spins $S_θ$ defined by two macroscopically distinct states. The analyzers that realize the unitary operations $U_θ$ determining the measurement settings $θ$ are devices that create macroscopic superposition states. For a system with two macroscopically distinct states available, macroscopic realism (MR) posits a predetermined outcome for a measurement $S_θ$ distinguishing between the states. Deterministic macroscopic realism (dMR) posits MR for the system prior to the interaction $U_θ$. Weak macroscopic realism (wMR) posits MR for the system after $U_θ$, at the time $t_f$ (when the system is prepared for a final "pointer" measurement), the outcome of $S_θ$ not being changed by interactions that might occur at a remote system $B$. The premise also posits that if the outcome for $S_θ^A$ of a system $A$ can be predicted by a pointer measurement on a system $B$ defined after the interaction fixing the setting at $B$, then the outcome for $S_θ^A$ is determined at this time. The GHZ predictions negate dMR but are consistent with wMR. Yet, an EPR paradox arises based on wMR for the set-up proposed by Schrödinger, where one measures two complementary spins simultaneously, "one by direct, the other by indirect" measurement. We revisit the original EPR paradox and find similarly that an EPR argument can be based on a weak form of local realism not falsifiable by GHZ or Bell tests.

arXiv:2208.01225•Aug 2, 2022•0 citations

Wigner's Friend Paradox: Quantum Measurement, Observation, and the Problem of Objectivity

Mirza Adnan Mohtashim

This paper offers a comprehensive analysis of Wigner’s Friend paradox (1961), a foundational thought experiment in quantum mechanics that highlights the tension between the internal perspective of a measuring observer (who experiences a definite outcome) and the external perspective of a super-observer who treats the entire laboratory as a quantum system in superposition. It traces the historical lineage from Schrödinger’s cat and von Neumann’s measurement theory, provides a formal mathematical description of the state evolution and logical structure of the paradox, examines implications for major interpretations of quantum mechanics (Copenhagen, Everettian many-worlds, QBism, relational quantum mechanics, and objective collapse theories), and discusses modern extensions such as the Frauchiger–Renner paradox. The work explores profound philosophical and epistemological consequences for the nature of observation, objectivity, intersubjectivity, and the role of consciousness in quantum theory (2026).

•May 29, 2026•0 citations

On the Objectivity of Measurement Outcomes

Elias Okon

Recent arguments, involving entangled systems shared by sets of Wigner's friend arrangements, allegedly show that the assumption that the experiments performed by the friends yield definite outcomes, is incompatible with quantum predictions. From this, it is concluded that the results of measurements cannot be thought of as being actual or objective. Here, I show that these arguments depend upon a mistaken assumption, regarding the ("mixed") correlations between the results of the friends and those of "the Wigners", which leads to invalid predictions. It is not, then, that the assumption of definite outcomes leads to trouble, but that the results derived with such an assumption are contrasted with faulty predictions. Next, I explore the more famous no-go theorems by Frauchiger and Renner and by Brukner, on which these recent arguments are motivated. Regarding the first, I show that, although it is cast in a different form, it also rests on the mistaken assumption regarding "mixed" correlations--rendering it invalid. Regarding Brukner's theorem, I point out that it depends on the quite unattractive assumption of locality, a fact that fully diminishes its force. Throughout, I illustrate my claims with explicit calculations within pilot-wave theory.

arXiv:2110.07743•The British Journal for the Philosophy of Science•Jan 21, 2022•3 citations

Wigner's Friend Has Already Measured: The K-Functional Resolution of Observer-Dependent State Assignment

J. Rosa

Wigner's friend scenarios expose a tension in quantum mechanics: an observer inside a laboratory obtains a definite measurement outcome, while an observer outside describes the laboratory as being in a superposition. The Frauchiger-Renner (2018) extension produces logical contradictions when agents use quantum theory to reason about each other. We show that the recursion-consistency functional K resolves these contradictions without modifying quantum mechanics. The resolution rests on a single physical fact: the friend's measurement creates ~10^20 environmental records inside the laboratory, driving K_rec to its minimum in ~10^-30 seconds. Wigner's description of the laboratory as a superposition is not wrong but incomplete: it omits the internal records that have already selected a definite outcome. The superposition description is operationally correct for Wigner (he cannot access the internal records) but ontologically incorrect (the records exist whether he accesses them or not). This distinction dissolves the Frauchiger-Renner contradiction, resolves the quantum eraser and delayed choice experiments as corollaries, and commits the framework to an ontic interpretation of the quantum state consistent with the PBR theorem.

•Apr 6, 2026•0 citations
Wigner's friend paradoxes highlight contradictions between measurements made by friends inside a laboratory and superobservers outside the laboratory, who have access to an entangled state of the measurement apparatus. Where there are two separated laboratories, the contradictions lead to no-go theorems for observer-independent facts, thus challenging concepts of objectivity. Here, we examine the paradoxes from the perspective of establishing consistency with macroscopic realism. We present versions of the Brukner-Wigner's-friend and Frauchiger-Renner paradoxes in which the spin-1/21/21/2 systems measured by the friends correspond to two macroscopically distinct states. The local unitary operations U\ensuremathθj{U}_{{\ensuremath{\theta}}_{j}}U\ensuremathθj​​ that determine the measurement settings \ensuremathθj{\ensuremath{\theta}}_{j}\ensuremathθj​ at each laboratory are carried out using nonlinear interactions, which are tailored to ensure that measurements need only distinguish between the macroscopically distinct states. The macroscopic paradoxes are perplexing, seemingly suggesting there is no objectivity in a macroscopic limit. However, we demonstrate consistency with a contextual weak form of macroscopic realism (wMR): The premise wMR asserts that each system jjj can be considered to have a definite spin outcome \ensuremathλ\ensuremathθj{\ensuremath{\lambda}}_{{\ensuremath{\theta}}_{j}}\ensuremathλ\ensuremathθj​​, at the time after the system has undergone the local unitary rotation U\ensuremathθj{U}_{{\ensuremath{\theta}}_{j}}U\ensuremathθj​​ to prepare it in a suitable measurement basis. We further show that the paradoxical outcomes imply failure of deterministic macroscopic local realism and arise when there are unitary interactions U\ensuremathθj{U}_{{\ensuremath{\theta}}_{j}}U\ensuremathθj​​ occurring due to a change of measurement setting at both laboratories, with respect to the state prepared by each friend. In models which validate wMR, there is a breakdown of a subset of the assumptions that constitute the Bell-locality premise. A similar interpretation involving a contextual weak form of local realism exists for the original paradoxes.
Wigner's friend paradoxes highlight contradictions between measurements made by Friends inside a laboratory and superobservers outside a laboratory, who have access to an entangled state of the measurement apparatus. The contradictions lead to no-go theorems for observer-independent facts, thus challenging concepts of objectivity. Here, we examine the paradoxes from the perspective of establishing consistency with macroscopic realism. We present versions of the Brukner-Wigner-friend and Frauchiger-Renner paradoxes in which the spin-1/21/21/2 system measured by the Friends corresponds to two macroscopically distinct states. The local unitary operations UθU_θUθ​ that determine the measurement setting θθθ are carried out using nonlinear interactions, thereby ensuring measurements need only distinguish between the macroscopically distinct states. The macroscopic paradoxes are perplexing, seemingly suggesting there is no objectivity in a macroscopic limit. However, we demonstrate consistency with a contextual weak form of macroscopic realism (wMR): The premise wMR asserts that the system can be considered to have a definite spin outcome λθλ_θλθ​, at the time after the system has undergone the unitary rotation UθU_θUθ​ to prepare it in a suitable pointer basis. We further show that the paradoxical outcomes imply failure of deterministic macroscopic local realism, and arise when there are unitary interactions UθU_θUθ​ occurring due to a change of measurement setting at both sites, with respect to the state prepared by each Friend. In models which validate wMR, there is a breakdown of a subset of the assumptions that constitute the Bell-Locality premise. A similar interpretation involving a weak contextual form of realism exists for the original paradoxes.
The Einstein-Podolsky-Rosen (EPR) paradox gives an argument for the incompleteness of quantum mechanics based on a plausible criterion for reality, as well as the assumption of locality. A general view is that the argument is compromised, because EPR's premises are falsified by Greenberger-Horne-Zeilinger (GHZ) and Bell experiments. In this paper, we present an EPR argument based on premises not falsified by these experiments. First, we consider macroscopic EPR-Bohm, Bell, and GHZ experiments using spins ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}}_{\ensuremath{\theta}}$ defined by two macroscopically distinct states. The analyzers that realize the unitary operations ${U}_{\ensuremath{\theta}}$ determining the measurement settings $\ensuremath{\theta}$ are nonlinear devices creating macroscopic superposition states. We note two definitions of macroscopic realism (MR). For a system with two macroscopically distinct states available to it, MR posits a predetermined outcome for the measurement ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}}_{\ensuremath{\theta}}$ distinguishing between the states. Deterministic macroscopic realism assumes MR for the system defined prior to the interaction ${U}_{\ensuremath{\theta}}$ being carried out but is falsifiable by the macroscopic Bell and GHZ proposals. Motivated by arguments to uphold MR (without resorting to decoherence), as well as no-signaling and the properties of a meter, we define a set of premises referred to as weak macroscopic realism (wMR). Weak macroscopic realism (wMR) posits MR for the system after ${U}_{\ensuremath{\theta}}$, at the time tf{t}_{f}tf​---when the system is prepared with respect to the measurement basis, ready for a ``pointer'' measurement and readout. For this system, wMR posits that the outcome of ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}}_{\ensuremath{\theta}}$ is determined and not changed by interactions that might subsequently occur at a remote system BBB. The premise wMR also posits a weaker version of EPR's criterion for reality. Importantly, we show that the GHZ and Bell predictions are consistent with wMR. Yet, an EPR paradox arises for a macroscopic EPR-Bohm state, based on wMR. As considered by Schr\"odinger, it is possible to measure two complementary spins of system AAA simultaneously, ``one by direct, the other by indirect measurement'': If we assume wMR, then once both settings are fixed, the outcomes of the two spins are both determined. We revisit the original EPR paradox and find a similar result: An EPR argument can be based on a set of premises, weak local realism, which we show are not falsifiable by GHZ or Bell experiments.
The Einstein-Podolsky-Rosen (EPR) paradox gives an argument for the incompleteness of quantum mechanics based on the premises of local realism. A general view is that the argument is compromised, because EPR's premises are falsified by Greenberger-Horne-Zeilinger (GHZ) and Bell experiments. In this paper, we present an EPR argument based on premises not falsifiable by these experiments. We propose macroscopic EPR and GHZ experiments using spins SθS_θSθ​ defined by two macroscopically distinct states. The analyzers that realize the unitary operations UθU_θUθ​ determining the measurement settings θθθ are devices that create macroscopic superposition states. For a system with two macroscopically distinct states available, macroscopic realism (MR) posits a predetermined outcome for a measurement SθS_θSθ​ distinguishing between the states. Deterministic macroscopic realism (dMR) posits MR for the system prior to the interaction UθU_θUθ​. Weak macroscopic realism (wMR) posits MR for the system after UθU_θUθ​, at the time tft_ftf​ (when the system is prepared for a final "pointer" measurement), the outcome of SθS_θSθ​ not being changed by interactions that might occur at a remote system BBB. The premise also posits that if the outcome for SθAS_θ^ASθA​ of a system AAA can be predicted by a pointer measurement on a system BBB defined after the interaction fixing the setting at BBB, then the outcome for SθAS_θ^ASθA​ is determined at this time. The GHZ predictions negate dMR but are consistent with wMR. Yet, an EPR paradox arises based on wMR for the set-up proposed by Schrödinger, where one measures two complementary spins simultaneously, "one by direct, the other by indirect" measurement. We revisit the original EPR paradox and find similarly that an EPR argument can be based on a weak form of local realism not falsifiable by GHZ or Bell tests.
This paper offers a comprehensive analysis of Wigner’s Friend paradox (1961), a foundational thought experiment in quantum mechanics that highlights the tension between the internal perspective of a measuring observer (who experiences a definite outcome) and the external perspective of a super-observer who treats the entire laboratory as a quantum system in superposition. It traces the historical lineage from Schrödinger’s cat and von Neumann’s measurement theory, provides a formal mathematical description of the state evolution and logical structure of the paradox, examines implications for major interpretations of quantum mechanics (Copenhagen, Everettian many-worlds, QBism, relational quantum mechanics, and objective collapse theories), and discusses modern extensions such as the Frauchiger–Renner paradox. The work explores profound philosophical and epistemological consequences for the nature of observation, objectivity, intersubjectivity, and the role of consciousness in quantum theory (2026).
Recent arguments, involving entangled systems shared by sets of Wigner's friend arrangements, allegedly show that the assumption that the experiments performed by the friends yield definite outcomes, is incompatible with quantum predictions. From this, it is concluded that the results of measurements cannot be thought of as being actual or objective. Here, I show that these arguments depend upon a mistaken assumption, regarding the ("mixed") correlations between the results of the friends and those of "the Wigners", which leads to invalid predictions. It is not, then, that the assumption of definite outcomes leads to trouble, but that the results derived with such an assumption are contrasted with faulty predictions. Next, I explore the more famous no-go theorems by Frauchiger and Renner and by Brukner, on which these recent arguments are motivated. Regarding the first, I show that, although it is cast in a different form, it also rests on the mistaken assumption regarding "mixed" correlations--rendering it invalid. Regarding Brukner's theorem, I point out that it depends on the quite unattractive assumption of locality, a fact that fully diminishes its force. Throughout, I illustrate my claims with explicit calculations within pilot-wave theory.
Wigner's friend scenarios expose a tension in quantum mechanics: an observer inside a laboratory obtains a definite measurement outcome, while an observer outside describes the laboratory as being in a superposition. The Frauchiger-Renner (2018) extension produces logical contradictions when agents use quantum theory to reason about each other. We show that the recursion-consistency functional K resolves these contradictions without modifying quantum mechanics. The resolution rests on a single physical fact: the friend's measurement creates ~10^20 environmental records inside the laboratory, driving K_rec to its minimum in ~10^-30 seconds. Wigner's description of the laboratory as a superposition is not wrong but incomplete: it omits the internal records that have already selected a definite outcome. The superposition description is operationally correct for Wigner (he cannot access the internal records) but ontologically incorrect (the records exist whether he accesses them or not). This distinction dissolves the Frauchiger-Renner contradiction, resolves the quantum eraser and delayed choice experiments as corollaries, and commits the framework to an ontic interpretation of the quantum state consistent with the PBR theorem.