The extended Wigner's friend, many- and single-worlds and reasoning from observation

TL;DR

This paper critically examines the extended Wigner's friend scenario, questioning assumptions about isolated systems, and discusses implications for single- and many-worlds interpretations of quantum mechanics.

Contribution

It challenges the validity of subjective collapse interpretations and clarifies conditions under which single-world interpretations remain consistent.

Findings

Questionable validity of isolated system approximation in measurement processes

Subjective collapse interpretations are incompatible with certain thought-experiments

Single-world interpretations can be valid if outcomes are not quantum-erased

Abstract

The concept of an isolated system, and Frauchiger and Renner's extended `Wigner's friend' scenario are discussed. It is argued that: (i) it is questionable whether the approximation of the isolated system is valid when measurement-like processes are involved; (ii) one may infer, from Frauchiger and Renner's thought-experiment, and similar thought-experiments, that any interpretation of quantum theory involving *subjective collapse* fails; (iii) this does not distinguish single-world from many-world (relative-state) interpretations of quantum theory; (iv) reasoning from observations has to take into account the possible quantum-erasure of those observations if it is to be valid reasoning; (v) a single-world interpretation is valid if certain kinds of outcome are not quantum-erased in the future.