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arXiv:2506.00112

Copenhagenish interpretations of quantum mechanics

David Schmid, Yìlè Yīng, Matthew Leifer

Published May 30, 2025• 0 citations
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Abstract

We define a class of Copenhagenish interpretations encompassing modern interpretations that follow the Copenhagen spirit. These interpretations are characterized by four postulates: Observers Observe, Universality, Anti-$ψ$-ontology, and Completeness. We explain why such interpretations are not equivalent to the textbook (or orthodox) interpretation, nor to the view that one should shut up and calculate, nor to strict operationalism. We then discuss what lessons are implied for Copenhagenish interpretations by the measurement problem, the Wigner's friend thought experiment, and the simple variants of the Wigner's friend thought experiment that we term Wigner's enemy, stalkee, and penpal. In particular, we discuss how Copenhagenish interpretations give multiple distinct descriptions of each experiment, where these descriptions are each individually true, yet cannot be combined into any single description. To make such interpretations consistent, then, one requires epistemological constraints forbidding certain perspectives from being combined. We discuss these constraints, their motivations, and some of the challenges they introduce.

DOI
10.48550/arxiv.2506.00112

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We define a class of Copenhagenish interpretations encompassing modern interpretations that follow the Copenhagen spirit. These interpretations are characterized by four postulates: Observers Observe, Universality, Anti-ψψψ-ontology, and Completeness. We explain why such interpretations are not equivalent to the textbook (or orthodox) interpretation, nor to the view that one should shut up and calculate, nor to strict operationalism. We then discuss what lessons are implied for Copenhagenish interpretations by the measurement problem, the Wigner's friend thought experiment, and the simple variants of the Wigner's friend thought experiment that we term Wigner's enemy, stalkee, and penpal. In particular, we discuss how Copenhagenish interpretations give multiple distinct descriptions of each experiment, where these descriptions are each individually true, yet cannot be combined into any single description. To make such interpretations consistent, then, one requires epistemological constraints forbidding certain perspectives from being combined. We discuss these constraints, their motivations, and some of the challenges they introduce.
The Wigner's friend thought experiment was intended to illustrate the difficulty one has in describing an agent as a quantum system when that agent performs a measurement. While it does pose a challenge to the orthodox interpretation of quantum theory, most modern interpretations have no trouble in resolving the difficulty. Recently, a number of extensions of Wigner's ideas have been proposed. We provide a gentle introduction to six such arguments, modifying the specifics of many of them so that they are as simple and unified as possible. In particular, we show that all of the arguments hinge on assumptions about correlations between measurement outcomes that are not accessible to any observer, even in principle. We then provide a critical analysis of each argument, focusing especially on how well one can motivate the required assumptions regarding these inaccessible correlations. Although we argue that some of these assumptions are not entirely well-motivated, all of the arguments do shed light on the nature of quantum theory, especially when concerning the description of agents and their measurements.
A recent paper "Single-world interpretations of quantum theory cannot be self-consistent" [arXiv:1604.07422] by D. Frauchiger and R. Renner has attracted a considerable interest of a broader physics audience and shortly elicited a number of replies. In spite of the objections that ensued, we find that significant part of the controversial initial claim has not been refuted - on the contrary, arguments presented both in the initial paper and in the replies seem to leave no longer room for the basic Copenhagen interpretation of quantum mechanics. We revisit the controversy trying to pinpoint the exact phenomenon that lies in its core, pointing out to some aspects of the problem that seem to have been mostly overlooked. Taking all conclusions into account, we here propose novel ways to approach the ostensible paradox, in which the Copenhagen interpretation is naturally preserved. Whereas arguments presented in Frauchiger and Renner paper and the replies that followed revolve over whether Wigner's friend would perceive only one or both of the possible outcomes, we offer the third, pretty obvious solution - namely, that Wigner's friend, in the well known setup, would subjectively perceive {\it none} of the potentially conflicting outcomes. As a side note, we also argue that, contrary to the often stated beliefs, hypothetical real experimental realization of Wigner's friend thought experiment actually would not provide us with any new insight into the quantum mechanics.
This paper presents a critical review of the Wigner’s Friend thought experiment and its modern extensions within quantum mechanics. The study explores observer-dependent reality, measurement theory, quantum state assignment, and interpretational challenges arising from nested observers in quantum systems. Particular attention is given to the Frauchiger–Renner argument and its implications for the consistency of single-world interpretations of quantum mechanics. The paper discusses conceptual tensions between unitarity, observer perspectives, and objective reality, while examining possible resolutions proposed in contemporary quantum foundations research. This work is intended as an independent scholarly review aimed at contributing to discussions in quantum foundations and quantum information theory.
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).
I argue here that progress in understanding the lessons of quantum physics has been hindered by the tendency to cast Niels Bohr as a villain. Building on the work of Favrholdt, Faye, and Howard, I present a more accurate view of Bohr's proposal for the "epistemological lesson" of quantum physics. I then argue that several interpretive programs -- often presented as alternatives to Copenhagen -- are, after substantial conceptual work, arriving at a view that is notably similar to Bohr's. Using Maudlin’s formulation of the measurement problem as a foil, I argue that it relies on a notion of wave-function completeness that is inappropriate for a genuinely indeterministic theory, and one that Bohr consistently rejected. On Bohr's view, quantum theory permits concrete agents, situated within specific experimental contexts, to make objective descriptions, while refusing the demand for a single, context-free "description from nowhere".
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.