Nonlocal action in Everettian Quantum Mechanics

TL;DR

This paper challenges the view that Everettian quantum mechanics is local by arguing that global state changes due to local actions should also be considered nonlocal, impacting the interpretation of quantum nonlocality.

Contribution

It introduces a novel argument that global state changes in EQM are nonlocal actions, questioning the common local interpretation of the theory.

Findings

Global state changes in EQM are nonlocal actions.

The intrinsic-extrinsic distinction is inadequate for analyzing nonlocality.

Global states are essential explanatory mechanisms in EQM.

Abstract

According to a common view, Everettian quantum mechanics (EQM) is a local theory because it avoids nonlocal action at a distance, and this is an important point in EQM's favor. Unlike collapse theories, EQM does not allow an action on one system to change the reduced density matrix (RDM) of a remote entangled system - a clear case of nonlocal action. However, EQM does allow an action on one system to change the global state of the system and its remote entangled partners. We argue that such changes should also count as nonlocal actions, meaning EQM is not local after all. First, we consider an argument to the contrary, which deems such global changes to be mere extrinsic changes, whereas nonlocal action requires intrinsic changes to the remote system. We respond that the intrinsic-extrinsic distinction is problematic and cannot hold the weight of this argument. We then try to clarify when actions that change global states count as nonlocal actions. We argue that it is when the global states are essential explanatory mechanisms of the theory. In EQM, the global state is needed to explain why, in an anti-correlated Bell state, Alice's measuring spin-up ensures that she encounters only the branch where Bob measures spin-down.