On the significance of Wigner's Friend in contexts beyond quantum foundations

Caroline L. Jones; Markus P. Mueller

doi:10.22331/q-2026-06-30-2147

On the significance of Wigner’s Friend in contexts beyond quantum foundations

Caroline L. Jones^1,2 and Markus P. Mueller^1,2,3

¹Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria
²Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
³Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON N2L 2Y5, Canada

Published:	2026-06-30, volume 10, page 2147
Editor:	Paul Skrzypczyk
Eprint:	arXiv:2402.08727v4
Doi:	https://doi.org/10.22331/q-2026-06-30-2147
Citation:	Quantum 10, 2147 (2026).

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Abstract

There has been a surge of recent interest in the Wigner's Friend paradox, sparking several novel thought experiments and no-go theorems. The main narrative has been that Wigner's Friend highlights a counterintuitive feature that is unique to quantum theory, and which is closely related to the quantum measurement problem. Here, we challenge this view. We argue that the gist of the Wigner's Friend paradox can be reproduced without assuming quantum physics, and that it underlies a much broader class of enigmas in the foundations of physics and philosophy. To show this, we first consider several recently proposed Extended Wigner's Friend scenarios, and demonstrate that some of their implications for the absoluteness of observations can be reproduced by classical thought experiments that involve the duplication of agents. Crucially, some of these classical scenarios are technologically much easier to implement than their quantum counterparts. Then, we argue that the essential structural ingredient of all these scenarios is a feature that we call "Restriction A": that a physical theory cannot give us a probabilistic description of the observations of all agents. Finally, we argue that this difficulty is at the core of other puzzles in the foundations of physics and philosophy, and demonstrate this explicitly for cosmology's Boltzmann brain problem. Our analysis suggests that Wigner's Friend should be studied in a larger context, addressing a frontier of human knowledge beyond quantum foundations: to obtain reliable predictions for experiments in which these predictions can be privately but not intersubjectively verified.

Featured image: Sketch of the setup of Thought Experiment 2; Wigner and his friend, Freya, perform a duplication experiment, following which they place bets on the outcome of a coin toss. This leads to the formulation of a classical no-go theorem, demonstrating an instance of Restriction A for classical physics.

Popular summary

The Wigner’s Friend thought experiment poses a conceptual puzzle for our understanding of the nature of reality, suggesting an apparent contradiction between the ways that two agents describe events when using quantum theory. The recent Extended Wigner’s Friend “no-go theorems” make this problem more explicit, by laying out sets of intuitive metaphysical assumptions, the conjunction of which are in contradiction with quantum physics.
We argue that analogous problems and inconsistencies arise even in classical physics, when we consider scenarios that posit similarly fine-grained control over thinking agents. We draw comparisons between Extended Wigner’s Friend scenarios and literature across the foundations of physics and philosophy – including epistemology’s ‘Sleeping Beauty problem’, cosmology’s ‘Boltzmann Brain problem’, and Parfit’s teletransportation thought experiments. This leads us to present a “classical no-go theorem”, composed of three intuitive assumptions that stand in contradiction with classical probability theory. Our analysis suggests a common structural core of these seemingly disconnected puzzles, leading us to define a methodological restriction (“Restriction A”) of our physical theories: that they cannot, for all scenarios, combine the predictions of all agents under a single joint probability distribution.

► BibTeX data

@article{Jones2026significanceof,
  doi = {10.22331/q-2026-06-30-2147},
  url = {https://doi.org/10.22331/q-2026-06-30-2147},
  title = {On the significance of {W}igner's {F}riend in contexts beyond quantum foundations},
  author = {Jones, Caroline L. and Mueller, Markus P.},
  journal = {{Quantum}},
  issn = {2521-327X},
  publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}},
  volume = {10},
  pages = {2147},
  month = jun,
  year = {2026}
}

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[2] Laurens Walleghem, Rui Soares Barbosa, Matthew F. Pusey, and Stefan Weigert, "A refined Frauchiger─Renner paradox based on strong contextuality", Quantum 10, 2116 (2026).

[3] Ladina Hausmann and Renato Renner, "The firewall paradox is Wigner's friend paradox", arXiv:2504.03835, (2025).

[4] Flavio Del Santo and Nicolas Gisin, "Which features of quantum physics are not fundamentally quantum but are due to indeterminism?", Quantum 9, 1686 (2025).

[5] Laurens Walleghem, "Wigner's friend's black hole adventure: an argument for complementarity?", arXiv:2507.05369, (2025).

[6] Gino Elia, Jennifer Carter, and Robert Crease, "Intersubjective Agreement about Quantum States Is Unnecessary in QBism", arXiv:2508.16683, (2025).

[7] Krzysztof Sienicki, "Algorithmic Idealism I: Reconceptualizing Reality Through Information and Experience", arXiv:2412.20485, (2024).

[8] Tom Rivlin, Sophie Engineer, and Veronika Baumann, "Emergence of Classicality in Wigner's Friend Scenarios", arXiv:2507.21221, (2025).

[9] Philipp Berghofer, "Quantum Probabilities Are Objective Degrees of Epistemic Justification", arXiv:2410.19175, (2024).

[10] Laurens Walleghem, "Stunned by Sleeping Beauty: How Prince Probability updates his forecast upon their fateful encounter", arXiv:2408.06797, (2024).

[11] David W. Ring, "A Consistent Approach to Modeling Quantum Observers", Entropy 27 3, 302 (2025).

[12] Julio C. F. Silva, B. F. Rizzuti, and Cristhiano Duarte, "Agreement and Compatibility in Wigner's Friend Paradox", arXiv:2605.27424, (2026).

The above citations are from SAO/NASA ADS (last updated successfully 2026-07-20 13:02:39). The list may be incomplete as not all publishers provide suitable and complete citation data.

On Crossref's cited-by service no data on citing works was found (last attempt 2026-07-20 13:02:38).

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