Implications of Local Friendliness violation for quantum causality

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 Bells 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 Leibnizs methodological principle. This works for Bells theorem but does not work for the Local Friendliness no-go theorem, which considers an extended Wigners Friend scenario. More radical conceptual renewal is required; we suggest that cleaving to Leibnizs principle requires extending relativity to events themselves.