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

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.

70 pages, LaTeX2e, no figures. This work is a substantial extension of the framework of arXiv:1611.06201, arXiv:1804.10174, arXiv:2312.13246. The above deals with the finite outcomes. In contrast, arXiv:1909.02854 deals with the infinite outcomes

arXiv:2509.07828 [quant-ph]

(or arXiv:2509.07828v4 [quant-ph] for this version)