Quantum mechanical rules for observed observers and the consistency of quantum theory

TL;DR

This paper demonstrates that the rules of unitary quantum mechanics, when considering macroscopic superpositions, resolve recent inconsistencies in quantum theory interpretations, ensuring internal consistency for observers involved in such superpositions.

Contribution

It shows that the standard quantum rules need to be modified for macroscopic superpositions to maintain consistency, addressing recent interpretational issues.

Findings

Observers in macroscopic superpositions cannot reliably predict measurement outcomes.

The Born rule does not apply straightforwardly to observers in superpositions.

Quantum rules, with these modifications, become fully consistent.

Abstract

I argue that the rules of unitary quantum mechanics imply that observers who will themselves be subject to measurements in a linear combination of macroscopic states (``cat" measurements) cannot make reliable predictions on the results of experiments performed after such measurements. This lifts the inconsistency in the interpretation of quantum mechanics recently identified by Frauchiger and Renner. The Born rules for calculating the probability of outcomes and for communicating with other observers do not generally apply for cat-measured observers, nor can they generally be amended to incorporate upcoming cat measurements. Quantum mechanical rules completed with these conditions become fully consistent.