The view from a Wigner bubble

TL;DR

This paper explores the implications of a no-go theorem challenging local realism in quantum mechanics, emphasizing the role of pragmatic truth and agent perspectives within QBism to address the measurement problem.

Contribution

It offers a perspective on the Wigner's friend scenario from a QBist viewpoint, connecting metaphysical assumptions with theories of truth and pragmatic elements of reality.

Findings

The no-go theorem constrains realist interpretations of quantum mechanics.

Quantum events are not invariant across different agent perspectives.

A pragmatic approach to the measurement problem emphasizes invariant elements valued by rational agents.

Abstract

In a recent no-go theorem [Bong et al, Nature Physics (2020)], we proved that the predictions of unitary quantum mechanics for an extended Wigner's friend scenario are incompatible with any theory satisfying three metaphysical assumptions, the conjunction of which we call "Local Friendliness": Absoluteness of Observed Events, Locality and No-Superdeterminism. In this paper (based on an invited talk for the QBism jubilee at the 2019 Vaxjo conference) I discuss the implications of this theorem for QBism, as seen from the perspective of experimental metaphysics. I argue that the key distinction between QBism and realist interpretations of quantum mechanics is best understood in terms of their adherence to different theories of truth: the pragmatist versus the correspondence theories. I argue that a productive pathway to resolve the measurement problem within a pragmatist view involves taking seriously the perspective of quantum betting agents, even those in what I call a "Wigner bubble". The notion of reality afforded by QBism, I propose, will correspond to the invariant elements of any theory that has pragmatic value to all rational agents -- that is, the elements that are invariant upon changes of agent perspectives. The classical notion of `event' is not among those invariants, even when those events are observed by some agent. Neither are quantum states. Nevertheless, I argue that far from solipsism, a personalist view of quantum states is an expression of its precise opposite: Copernicanism.