Unitary Interactions Do Not Yield Outcomes: Attempting to Model "Wigner's Friend"

TL;DR

This paper critically analyzes an experiment related to Wigner's Friend, arguing that the data do not support the idea of incompatible facts among observers and highlighting that under strict unitarity, measurement outcomes cannot be empirically obtained.

Contribution

It challenges the interpretation of the Wigner's Friend experiment by showing that measurement outcomes are incompatible with strict unitarity and proposes an alternative quantum framework.

Findings

Violation of Bell inequality does not imply inconsistent facts.

Under unitarity, no definite measurement outcomes occur internally.

An alternative quantum formulation explains the experimental data.

Abstract

An experiment by Proietti {\it et al} purporting to instantiate the `Wigner's Friend' thought experiment is discussed. It is pointed out that the stated implications of the experiment regarding the alleged irreconcilability of facts attributed to different observers warrant critical review. In particular, violation of a Clauser-Horne-Shimony inequality by the experimental data actually shows that the attribution of measurement outcomes to the ``Friends'' (modeled by internal photons undergoing unitary interactions) is erroneous. An elementary but often overlooked result regarding improper mixtures is adduced in support of this assessment. A counterexample is provided which refutes the popular notion that quantum theory leads to `relative facts' that never manifest as empirical inconsistencies. It is further noted that under an assumption of unbroken unitarity, no measurement correlation can ever yield an outcome, since all systems remain in improper mixtures, and attributing a definite but unknown outcome contradicts their composite pure state. It is pointed out that there already exists a solution to this conundrum in the form of an alternative formulation of quantum theory, which accounts for the data showing that no outcomes occurred at the interior entangled photon level and also predicts that outcomes can and do occur at the exterior ``super-observer'' level in this type of experiment.