Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Nonlocal action in Everettian Quantum Mechanics

Mordecai Waegell, Kelvin J. McQueen

TL;DR

The paper argues that Everettian quantum mechanics is not strictly local because local actions can alter the global entangled state in a way that is essential to explaining measurement correlations. It introduces a sufficient condition for nonlocal action: if a theory relies on a global state as an essential explanatory mechanism, actions changing that state count as nonlocal. Through critical examination of the intrinsic-extrinsic distinction and related definitions, the authors contend that reduced density matrices may not reliably mark intrinsic properties, challenging locality arguments that rely on such distinctions. They extend the analysis to various physical theories, showing nonlocal action arises in theories with essential global states (e.g., Bohmian mechanics, collapse theories) but can be avoided in certain many-worlds variants; the work reframes locality in quantum foundations and highlights nuanced pathways toward fully local interpretations.

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.

Nonlocal action in Everettian Quantum Mechanics

TL;DR

The paper argues that Everettian quantum mechanics is not strictly local because local actions can alter the global entangled state in a way that is essential to explaining measurement correlations. It introduces a sufficient condition for nonlocal action: if a theory relies on a global state as an essential explanatory mechanism, actions changing that state count as nonlocal. Through critical examination of the intrinsic-extrinsic distinction and related definitions, the authors contend that reduced density matrices may not reliably mark intrinsic properties, challenging locality arguments that rely on such distinctions. They extend the analysis to various physical theories, showing nonlocal action arises in theories with essential global states (e.g., Bohmian mechanics, collapse theories) but can be avoided in certain many-worlds variants; the work reframes locality in quantum foundations and highlights nuanced pathways toward fully local interpretations.

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.

Paper Structure

This paper contains 12 sections, 5 equations, 1 figure.

Table of Contents

  1. Introduction
  2. The instrincality argument
  3. The basic argument
  4. The Socrates-Xanthippe analogy
  5. The intrinsicality concept
  6. Intrinsic as non-relational
  7. Grounding and contractionist definitions
  8. Global state changes as real changes
  9. The basic idea
  10. The Socrates-Xanthippe analogy revisited
  11. Application to various physical theories
  12. Conclusion

Figures (1)

  • Figure 1: Causality Diagrams. Top: Alice's action affects the global state, (which includes qubit $B$), which is essential to explaining Bob's measurement outcomes. Right: The unfamiliar strange theory. Socrates' execution affects the widowhood relation, which is essential to explaining why Xanthippe weeps. Left: The familiar classical case. The widowhood relation does not explain why Xanthippe weeps, she weeps because she hears of Socrates' death.