Making sense of relativistic Wigner friend scenarios: a problem for unitary accounts of quantum measurements ?

TL;DR

This paper examines the challenges of reconciling quantum measurement scenarios involving observers with relativistic effects, revealing inconsistencies in current unitary models and questioning their fundamental adequacy.

Contribution

It demonstrates that relativistic Wigner-friend scenarios expose fundamental issues in unitary accounts of quantum measurements, especially regarding frame-dependent state updates.

Findings

Frame-dependence leads to inconsistent measurement outcomes.

Relativistic effects complicate the unitary description of measurements.

Current models may be fundamentally inadequate for complex agents.

Abstract

Wigner-friend scenarios -- in which external agents describe a closed laboratory containing a friend making a measurement -- highlight the difficulties inherent to quantum theory when accounting for measurements. In non-relativistic scenarios, the difficulty is to accommodate unitary evolution for a closed system with a definite outcome obtained by the friend. In relativistic scenarios the tensions between quantum theory and relativity induce additional constraints. A generic property of relativistic scenarios is the frame-dependence of state update upon a measurement. Based on a definite example, we will show that this property leads to inconsistent accounts for outcomes obtained in different reference frames. We will further argue that these results point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations made on simple wavefunctions.