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arXiv:2509.07828

An analysis of Wigner's friend in the framework of quantum mechanics based on the principle of typicality

Kohtaro Tadaki

Published Sep 9, 2025• 0 citations
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Abstract

The notion of probability plays a crucial role in quantum mechanics. It appears in quantum mechanics as the Born rule. In modern mathematics which describes quantum mechanics, however, probability theory means nothing other than measure theory, and therefore any operational characterization of the notion of probability is still missing in quantum mechanics. In our former works [K. Tadaki, arXiv:1804.10174], based on the toolkit of algorithmic randomness, we presented a refinement of the Born rule, called the principle of typicality, for specifying the property of results of measurements in an operational way. The Wigner's friend paradox is a Gedankenexperiment regarding when and where the reduction of the state vector occurs in a chain of the measurements by several observers where the state of the consciousness of each observer is measured by the subsequent observer. In this paper, we extend the framework of the principle of typicality so that it can be applicable to situations where apparatuses perform measurements over other apparatuses. We then make an analysis of the Wigner's friend paradox within this extended framework of quantum mechanics based on the principle of typicality. We draw common sense conclusions about it. Deutsch's thought experiment is a variant of the Wigner's friend paradox, which can, in principle, verify the effect of the consciousness of observer on the reduction of the state vector. We make an analysis of it comprehensively within the extended framework. We then make a prediction which is testable in principle. In our extended framework, we can analyze still more complicated situations. As such an example, we introduce a combination of the above two, named the Wigner-Deutsch collaboration, and perform a thorough analysis of it.

DOI
10.48550/arxiv.2509.07828

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The notion of probability plays a crucial role in quantum mechanics. It appears in quantum mechanics as the Born rule. In modern mathematics which describes quantum mechanics, however, probability theory means nothing other than measure theory, and therefore any operational characterization of the notion of probability is still missing in quantum mechanics. In our former works [K. Tadaki, arXiv:1804.10174], based on the toolkit of algorithmic randomness, we presented a refinement of the Born rule, called the principle of typicality, for specifying the property of results of measurements in an operational way. The Wigner's friend paradox is a Gedankenexperiment regarding when and where the reduction of the state vector occurs in a chain of the measurements by several observers where the state of the consciousness of each observer is measured by the subsequent observer. In this paper, we extend the framework of the principle of typicality so that it can be applicable to situations where apparatuses perform measurements over other apparatuses. We then make an analysis of the Wigner's friend paradox within this extended framework of quantum mechanics based on the principle of typicality. We draw common sense conclusions about it. Deutsch's thought experiment is a variant of the Wigner's friend paradox, which can, in principle, verify the effect of the consciousness of observer on the reduction of the state vector. We make an analysis of it comprehensively within the extended framework. We then make a prediction which is testable in principle. In our extended framework, we can analyze still more complicated situations. As such an example, we introduce a combination of the above two, named the Wigner-Deutsch collaboration, and perform a thorough analysis of it.
The notion of probability plays a crucial role in quantum mechanics. It appears in quantum mechanics as the Born rule. In modern mathematics which describes quantum mechanics, however, probability theory means nothing other than measure theory, and therefore any operational characterization of the notion of probability is still missing in quantum mechanics. In our former works [K. Tadaki, arXiv:1804.10174], based on the toolkit of algorithmic randomness, we presented a refinement of the Born rule, called the principle of typicality, for specifying the property of results of measurements in an operational way. In this paper, we make an application of our framework to the argument of local realism versus quantum mechanics for refining it, in order to demonstrate how properly our framework works in practical problems in quantum mechanics.
The notion of probability plays a crucial role in quantum mechanics. It appears in quantum mechanics as the Born rule. In modern mathematics which describes quantum mechanics, however, probability theory means nothing other than measure theory, and therefore any operational characterization of the notion of probability is still missing in quantum mechanics. In our former works [K. Tadaki, arXiv:1804.10174], based on the toolkit of algorithmic randomness, we presented a refinement of the Born rule, called the principle of typicality, for specifying the property of results of measurements in an operational way. In this paper, we make an application of our framework to the argument of Bell's inequality versus quantum mechanics for refining it, in order to demonstrate how properly our framework works in practical problems in quantum mechanics.
The notion of probability plays a crucial role in quantum mechanics. It appears in quantum mechanics as the Born rule. In modern mathematics which describes quantum mechanics, however, probability theory means nothing other than measure theory, and therefore any operational characterization of the notion of probability is still missing in quantum mechanics. In this paper, based on the toolkit of algorithmic randomness, we present a refinement of the Born rule, as an alternative rule to it, for specifying the property of the results of quantum measurements in an operational way. Algorithmic randomness is a field of mathematics which enables us to consider the randomness of an individual infinite sequence. We then present an operational refinement of the Born rule for mixed states, as an alternative rule to it, based on algorithmic randomness. In particular, we give a precise definition for the notion of mixed state. We then show that all of the refined rules of the Born rule for both pure states and mixed states can be derived from a single postulate, called the principle of typicality, in a unified manner. We do this from the point of view of the many-worlds interpretation of quantum mechanics. Finally, we make an application of our framework to the BB84 quantum key distribution protocol in order to demonstrate how properly our framework works in practical problems in quantum mechanics, based on the principle of typicality.
Abstract In this paper, we analyze the thought experiment of "Wigner's friend" and point out that new understanding should be made to Born's rule and measurement process: Born's rule is no longer seen as a rule based on the history of the quantum system's, and the measurement results are no longer directly related to the state of the measured object before the measurement. Inspired by Everett III and H. Zurek's views, we believe that Born's rule reflects the coordination between different parts of the measured results in quantum entanglement systems, so it has nothing to do with the history of particles themselves but rather with the history records. A new interpretation of quantum mechanics is proposed, which is an objective interpretation. Under this interpretation, micro-particles can also be assigned definite states before being observed. Based on this interpretation, Wigner's friend-like scenarios or paradoxes extending Wigner's friend-like scenarios can be effectively explained. The new interpretation brings brand-new perspectives to many quantum phenomena. We also point out potential applications of the new interpretation in the future.
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.
Abstract In this paper, we analyze the thought experiment of "Wigner's friend" and point out that new understanding should be made to Born's rule and measurement process: Born's rule is no longer seen as a rule based on the history of the quantum system's, and the measurement results are no longer directly related to the state of the measured object before the measurement. Inspired by Everett III and H. Zurek's views, we believe that Born's rule reflects the coordination between different parts of the measured results in quantum entanglement systems, so it has nothing to do with the history of particles themselves but rather with the history records. A new interpretation of quantum mechanics is proposed, which is an objective interpretation. Under this interpretation, micro-particles can also be assigned definite states before being observed. Based on this interpretation, Wigner's friend-like scenarios or paradoxes extending Wigner's friend-like scenarios can be effectively explained. The new interpretation brings brand-new perspectives to many quantum phenomena. We also point out potential applications of the new interpretation in the future.