Logic meets Wigner's Friend (and their Friends)

TL;DR

This paper reevaluates Wigner's Friend thought-experiment and its variants, proposing a new formal approach to interpret quantum state multiplicity and observer inclusion consistent with quantum mechanics.

Contribution

It introduces a novel formal framework for understanding quantum state assignments and observer roles in Wigner's Friend scenarios, addressing longstanding paradoxes.

Findings

Proposes a new interpretation of multiplicity of state assignments

Develops a formal implementation for including classical observers

Provides a solution to Wigner Friend-type paradoxes

Abstract

We take a fresh look at Wigner's Friend thought-experiment and some of its more recent variants and extensions, such as the Frauchiger-Renner (FR) Paradox. We discuss various solutions proposed in the literature, focusing on a few questions: What is the correct epistemic interpretation of the multiplicity of state assignments in these scenarios? Under which conditions can one include classical observers into the quantum state descriptions, in a way that is still compatible with traditional Quantum Mechanics? Under which conditions can one system be admitted as an additional 'observer' from the perspective of another background observer? When can the standard axioms of multi-agent Epistemic Logic (that allow "knowledge transfer" between agents) be applied to quantum-physical observers? In the last part of the paper, we propose a new answer to these questions, sketch a particular formal implementation of this answer, and apply it to obtain a principled solution to Wigner Friend-type paradoxes.