From locality to factorizability: a novel escape from Bell's theorem

TL;DR

This paper challenges the common derivation of factorizability from local causality in Bell's theorem, showing an extra independence assumption is needed, which allows for models that violate Bell's inequality without abandoning local causality.

Contribution

It demonstrates that deriving factorizability from local causality requires an additional independence assumption, revealing a new perspective on Bell's theorem.

Findings

Factorizability cannot be derived solely from local causality without extra assumptions.

Models satisfying local causality and settings independence can violate Bell's inequality.

An extra independence assumption about measurement apparatus states is necessary for the traditional derivation.

Abstract

While initial versions of Bell's theorem captured the notion of locality with the assumption of factorizability, in later presentations, Bell argued that factorizability could be derived from the more fundamental principle of local causality. Here we show that, contrary to what is commonly assumed, in order to derive factorizability from the principle of local causality, a non-trivial assumption, similar but strictly independent of settings independence, is required. Loosely speaking, such an extra assumption demands independence between the states of the measurement apparatuses. We conclude that it is possible to construct a model, satisfying both the principle of local causality and settings independence, but that, in virtue of violating this additional assumption--and thus factorizability--is able to break Bell's inequality.