A no-go theorem for observer-independent facts

TL;DR

This paper presents a no-go theorem demonstrating the impossibility of observer-independent facts in quantum mechanics, highlighting the relational nature of observational statements and their implications for self-consistency in quantum theory.

Contribution

It introduces a no-go theorem for observer-independent facts and analyzes its relation to the Frauchiger-Renner theorem, emphasizing the relational aspect of quantum observations.

Findings

No-go theorem for observer-independent facts derived

Observer-independent facts cannot be consistently maintained in quantum theory

Relational interpretation challenges the notion of absolute observational facts

Abstract

In his famous thought experiment, Wigner assigns an entangled state to the composite quantum system made up of Wigner's friend and her observed system. While the two of them have different accounts of the process, each Wigner and his friend can in principle verify his/her respective state assignments by performing an appropriate measurement. As manifested through a click in a detector or a specific position of the pointer, the outcomes of these measurements can be regarded as reflecting directly observable "facts". Reviewing arXiv:1507.05255, I will derive a no-go theorem for observer-independent facts, which would be common both for Wigner and the friend. I will then analyze this result in the context of a newly derived theorem in arXiv:1604.07422, where Frauchiger and Renner prove that "single-world interpretations of quantum theory cannot be self-consistent". It is argued that "self-consistency" has the same implications as the assumption that observational statements of different observers can be compared in a single (and hence an observer-independent) theoretical framework. The latter, however, may not be possible, if the statements are to be understood as relational in the sense that their determinacy is relative to an observer.