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According to QBism, quantum states, unitary evolutions, and measurement operators are all understood as personal judgments of the agent using the formalism. Meanwhile, quantum measurement outcomes are understood as the personal experiences…

Quantum Physics · Physics 2022-10-19 John B. DeBrota , Christopher A. Fuchs , Ruediger Schack

Wigner's friend experiment and its modern extensions display the ambiguity of the quantum mechanical description regarding the assignment of quantum states. While the friend applies the state-update rule to the system upon observing an…

Quantum Physics · Physics 2025-10-22 Flavio Del Santo , Gonzalo Manzano , Caslav Brukner

Exploiting the tension between the two dynamics of quantum theory (QT) in the Wigner's Friend thought experiment, we point out that the standard QT leads to inconsistency in observed probabilities of measurement outcomes between two…

Quantum Physics · Physics 2023-03-07 Shubhayan Sarkar , Debashis Saha

The Wigner's Friend (WF) thought experiment concerns quantum measurements by a 'superobserver' of an observer measuring a quantum system. Variations on the setup and its extended versions have seen a resurgence in recent years, in light of…

Quantum Physics · Physics 2025-07-30 Tom Rivlin , Sophie Engineer , Veronika Baumann

In 1962, Eugene P. Wigner introduced a thought experiment that highlighted the incompatibility in quantum theory between unitary evolution and wave function reduction in a measurement. This work resulted in a class of thought experiments…

Quantum Physics · Physics 2021-05-19 Vinicius P. Rossi , Diogo O. Soares-Pinto

In a joint paper Jeff Bub and Itamar Pitowski argued that the quantum state represents `the credence function of a rational agent [...] who is updating probabilities on the basis of events that occur'. In the famous thought experiment…

Quantum Physics · Physics 2019-02-01 Veronika Baumann , Časlav Brukner

This paper reviews and suggests a resolution of the problem of definite outcomes of measurement. This problem, also known as "Schrodinger's cat," has long posed an apparent paradox because the state resulting from a measurement appears to…

Quantum Physics · Physics 2022-06-14 Art Hobson

The concept of an isolated system, and Frauchiger and Renner's extended `Wigner's friend' scenario are discussed. It is argued that: (i) it is questionable whether the approximation of the isolated system is valid when measurement-like…

Quantum Physics · Physics 2026-05-12 Andrew Steane

Wigner's thought experiment illustrates quantum theory's measurement problem by considering an observer who measures a quantum system inside a sealed lab, modeled unitarily by an outsider. Recent extensions of this thought experiment,…

Quantum Physics · Physics 2026-05-15 Laurens Walleghem

The description of Wigner-friend scenarios -- in which external agents describe a closed laboratory containing a friend making a measurement -- remains problematic due to the ambiguous nature of quantum measurements. One option is to…

Quantum Physics · Physics 2023-10-16 J. Allam , A. Matzkin

The extended Wigner's friend problem deals with two Observers each measuring a sealed laboratory in which a friend is making a quantum measurement. We investigate this problem by relying on the basic rules of quantum mechanics as exposed by…

Quantum Physics · Physics 2021-10-19 D. Sokolovski , A. Matzkin

The notorious Wigner's friend thought experiment (and modifications thereof) has in recent years received renewed interest especially due to new arguments that force us to question some of the fundamental assumptions of quantum theory. In…

Quantum Physics · Physics 2021-05-18 Philippe Allard Guérin , Veronika Baumann , Flavio Del Santo , Časlav Brukner

I present a simple variant of the Schr\"odinger cat meets Wigner's friend thought experiment. If you are shocked by it, you have not understood quantum physics (no words are missing from this sentence).

Quantum Physics · Physics 2016-03-16 Vlatko Vedral

We present a discussion of the extended Wigner's friend thought experiment proposed by Frauchiger and Renner in [1]. We show by using various arguments, including textbook quantum mechanics and the ontological approach of Contexts, Systems,…

Quantum Physics · Physics 2023-01-10 Maxime Federico , Philippe Grangier

An experiment by Proietti {\it et al} purporting to instantiate the `Wigner's Friend' thought experiment is discussed. It is pointed out that the stated implications of the experiment regarding the alleged irreconcilability of facts…

Quantum Physics · Physics 2023-03-30 R. E. Kastner

The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics, the objectivity of observations is not so clear, most dramatically…

Extended Wigner's friend no-go theorems provide a modern lens for investigating the measurement problem, by making precise the challenges that arise when one attempts to model agents as dynamical quantum systems. Most such no-go theorems…

Quantum Physics · Physics 2025-11-18 Laurens Walleghem , Rafael Wagner , Yìlè Yīng , David Schmid

It is shown that the absence of an objective existence of the results of quantum measurements cannot be proved by known experiments. There are also general arguments confirming this conclusion.

Quantum Physics · Physics 2019-05-28 Alexander Belinsky

The Schrodinger equation is incomplete, inherently unable to explain the collapse of the wavefunction caused by measurement; a fundamental issue known as the quantum measurement problem. Quantum mechanics is generally constrained by the…

Quantum Physics · Physics 2024-12-19 Kyoung Yeon Kim

In this presentation, I argue that weak measurements empirically support the notion of quantum superpositions as statistical alternatives. In short, weak measurements show that Schroedinger's cat is already dead or alive before the…

Quantum Physics · Physics 2009-11-03 Holger F. Hofmann