Implications of Local Friendliness violation for quantum causality

TL;DR

This paper reformulates the Local Friendliness no-go theorem using causal principles, revealing it imposes stricter constraints on quantum reality than Bell's theorem and suggesting a need to extend relativity to events.

Contribution

It offers a new causal formulation of the Local Friendliness theorem, highlighting its stronger implications and proposing an extension of relativity to reconcile quantum causality.

Findings

Local Friendliness theorem imposes stricter bounds than Bell's theorem.

Quantum causal models struggle to reconcile with the Local Friendliness constraints.

Extending relativity to events may be necessary for consistent quantum causality.

Abstract

We provide a new formulation of the Local Friendliness no-go theorem of Bong et al [Nat. Phys. 16, 1199 (2020)] from fundamental causal principles, providing another perspective on how it puts strictly stronger bounds on quantum reality than Bell's theorem. In particular, quantum causal models have been proposed as a way to maintain a peaceful coexistence between quantum mechanics and relativistic causality, while respecting Leibniz's methodological principle. This works for Bell's theorem but does not work for the Local Friendliness no-go theorem, which considers an extended Wigner's Friend scenario. More radical conceptual renewal is required; we suggest that cleaving to Leibniz's principle requires extending relativity to events themselves.