Extended Wigner's friend problem and the internal consistency of standard quantum mechanics

TL;DR

This paper revisits the extended Wigner's friend problem using basic quantum mechanics rules, demonstrating that a consistent and unambiguous account of measurement outcomes is possible within standard quantum theory.

Contribution

It shows that the extended Wigner's friend problem can be coherently explained using standard quantum mechanics, countering claims of inconsistency.

Findings

Standard quantum rules yield consistent measurement outcomes.

The extended Wigner's friend problem is resolvable within existing quantum theory.

No need for modifications to quantum mechanics to address the problem.

Abstract

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 Feynman in the well-known "Feynman Lectures on Physics". Although recent discussions have suggested that the extended Wigner's friend problem cannot consistently be described by quantum theory, we show here that a straightforward application of these standard rules results in a non-ambiguous and consistent account of the measurement outcomes for all agents involved.