The decoherence interpretation of quantum measurements is applied to Wigner's-friend experiments. A framework in which all the experimental outcomes arise from unitary evolutions is proposed. Within it, a measurement is not completed until an uncontrolled environment monitorizes the state composed of the system, the apparatus, and the observer. The (apparent) wave-function collapse and the corresponding randomness result from tracing out this environment; it is thus the ultimate responsible party for the emergence of definite outcomes. Two main effects arise from this fact. First, external interference measurements, trademark of Wigner's-friend experiments, modify the memory records of the internal observers; this framework provides a univocal protocol to calculate all these changes. Second, it can be used to build a consistent scenario for the recently proposed extended versions of the Wigner's-friend experiment. In regard to the work of Frauchiger and Renner [Nat. Commun. 9, 3711 (2018)], this framework shows that the agents' claims become consistent if the changes in their memories are properly taken into account. Furthermore, the particular setup discussed by Brukner [Entropy 20, 350 (2018)] cannot be tested against the decoherence framework, because it does not give rise to well-defined outcomes according to this formalism. A variation of this setup, devised to fill this gap, makes it possible to assign joint truth values to the observations made by all the agents. This framework also narrows the requisites for such experiments, making them virtually impossible to apply to conscious (human) beings. Notwithstanding, it also opens the door to future realizations on quantum machines.
The decoherence interpretation of quantum measurements is applied to Wigner's-friend experiments. A framework in which all the experimental outcomes arise from unitary evolutions is proposed. Within it, a measurement is not completed until an uncontrolled environment monitorizes the state composed of the system, the apparatus, and the observer. The (apparent) wave-function collapse and the corresponding randomness result from tracing out this environment; it is thus the ultimate responsible party for the emergence of definite outcomes. Two main effects arise from this fact. First, external interference measurements, trademark of Wigner's-friend experiments, modify the memory records of the internal observers; this framework provides a univocal protocol to calculate all these changes. Second, it can be used to build a consistent scenario for the recently proposed extended versions of the Wigner's-friend experiment. In regard to the work of Frauchiger and Renner [Nat. Commun. 9, 3711 (2018)], this framework shows that the agents' claims become consistent if the changes in their memories are properly taken into account. Furthermore, the particular setup discussed by Brukner [Entropy 20, 350 (2018)] cannot be tested against the decoherence framework, because it does not give rise to well-defined outcomes according to this formalism. A variation of this setup, devised to fill this gap, makes it possible to assign joint truth values to the observations made by all the agents. This framework also narrows the requisites for such experiments, making them virtually impossible to apply to conscious (human) beings. Notwithstanding, it also opens the door to future realizations on quantum machines.
Wigner’s Friend scenarios expose a foundational tension in quantum theory between unitary evolution and the existence of definite measurement outcomes experienced by observers. This paper proposes an Observer-First Interpretation (OFI) in which quantum states are treated as observer-relative informational constructs constrained by evidence accessibility and record boundaries. Within this framework, the friend’s measurement produces a single definite outcome through irreversible informational compression into a stable internal record, while an external observer lacking access to this record may consistently describe the laboratory as unresolved or coherent. Apparent paradoxes arise only when observer-indexed state assignments are illegitimately combined as if they represented simultaneous global facts. By enforcing observer-relative state attribution, the framework resolves Wigner’s Friend and related observer paradoxes without modifying the formalism or empirical predictions of quantum mechanics.
By assuming (i) a universal, observer-independent quantum state, by considering (ii) measurement processes with wave function collapse as part of quantum mechanical time evolution (also within isolated systems), and by (iii) clearly distinguishing the state of a quantum system from the knowledge of conscious observers about this state, we suggest a modified Copenhagen interpretation of quantum mechanics that resolves Wigner's Friend and extended Wigner's Friend paradoxes. The suggested interpretation leads to differences in some outcome probabilities compared to previous analyses which, in principle, make it possible to falsify the suggested modifications or the previous analyses (or both). The fundamental problem of the incompleteness of quantum mechanics in the Copenhagen interpretation regarding the lack of a precise definition and a mechanistic description of the measurement process (including the collapse of the wave function) is not addressed in this study. However, the argumentation regarding the resolution of Wigner's Friend paradoxes also applies to attempts for such a mechanistic description using collapse models with stochastic extensions to the Schrödinger equation or trying to model wave function collapse with unitary time evolution and irreversibility of the measurement process resulting from quantum statistical mechanics.
Wigner’s Friend and the Frauchiger-Renner paradox explore fundamental tensions in quantum mechanics, particularly how measurement, observer perspectives, and entangled states yield contradictory or paradoxical conclusions when extended universally. Conventional treatments prioritize wavefunction collapse or large-scale entanglement, often neglecting the role of partial decoherence. Here, I develop a “Wigner’s Buddy” framework that refocuses these thought experiments on quantum coherence, treating measurement as a continuous, environment-driven process rather than an abrupt collapse. By systematically quantifying off-diagonal terms through measures such as the ℓ1-norm or relative entropy of coherence, and employing techniques that measure coherence such as tomography, interferometry, and ultrafast spectroscopy, I highlight how gradual phase damping reconciles local classicality with residual global coherence. This approach surpasses limitations of fragility in entanglement-based protocols, addresses nested-observer inconsistencies without invoking universal collapse, and accommodates real-world noise in biological systems such as photosynthesis, magnetoreception. Coherence thus bridges theoretical paradoxes and practical phenomena, demonstrating a scalable mechanism by which quantum effects can persist in macroscopic or dissipative environments.In doing so, it aligns Wigner’s Friend and Frauchiger-Renner analyses with a broader spectrum of interpretative frameworks—including Copenhagen, Many-Worlds, relational, Bayesian perspectives, and pilot-wave views—while underscoring the adaptability of quantum mechanics across multiple scales and contexts. Moreover, this perspective resonates with quantum Darwinism, which posits that the environment redundantly encodes pointer states, promoting effective classical objectivity from purely quantum substrates. By clarifying how partial decoherence can stabilize measurement outcomes, the “Wigner’s Buddy” proposal enhances our understanding of quantum theory’s capacity to describe itself without logical contradictions. It also has implications for quantum computing, where maintaining and controlling coherence amid noise is pivotal for scalable information processing. Furthermore, analyzing coherence measures can, in principle, distinguish gradual phase suppression from hypothetical non-unitary collapse if robust empirical tests are devised, though interpretation remains context-dependent. Ultimately, this modification unifies theoretical paradoxes with real-world systems, suggesting that coherence-based approaches can illuminate the transition from micro-level superpositions to macro-level classicality and guide future developments in both foundational studies and emergent quantum technologies.
Extended Wigner’s friend scenarios (EWFSs) model agents themselves as unitarily evolving quantum systems, going beyond the standard use of quantum theory where agents are treated classically. Several no-go results suggest deep challenges in this setting: Frauchiger and Renner argued that quantum agents reasoning using quantum theory would arrive at logical paradoxes, while the local-friendliness theorem raises difficulties for maintaining objective notions of measurement events and causal reasoning in EWFSs. This prompts a key question: Can the ability to make and test scientific predictions and reason consistently still be maintained if quantum theory applies universally, even to agents and their memories? We answer this positively by developing a general quantum circuit framework for EWFSs without assuming absolute measurement outcomes. We formalize Heisenberg cuts in terms of distinct choices of channels in the circuit, labeled by “settings,” and show that Frauchiger-Renner-type paradoxes are fully resolved by making explicit the conditioning on these channels or settings. We provide concrete reasoning rules for quantum agents that ensure logical, probabilistic, and causal consistency. Our framework captures all perspectives in an EWFS within a single causal structure while allowing events to be fundamentally subjective, and we show how objective events and predictions nevertheless emerge in current real-world quantum experiments. Thus, we demonstrate a relational yet operational framework that overcomes key challenges in EWFSs without altering the Born rule, unitarity, or classical logic and probability for outcomes. This establishes a formal platform to analyze and compare different EWFS arguments and extend quantum information methods consistently into the Wigner’s friend domain.
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).
Wigner-friend scenarios -- in which external agents describe a closed laboratory containing a friend making a measurement -- highlight the difficulties inherent to quantum theory when accounting for measurements. In non-relativistic scenarios, the difficulty is to accommodate unitary evolution for a closed system with a definite outcome obtained by the friend. In relativistic scenarios the tensions between quantum theory and relativity induce additional constraints. A generic property of relativistic scenarios is the frame-dependence of state update upon a measurement. Based on a definite example, we will show that this property leads to inconsistent accounts for outcomes obtained in different reference frames. We will further argue that these results point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations made on simple wavefunctions.