Contextuality with Disturbance and without: Neither Can Violate Substantive Requirements the Other Satisfies

Abstract

Contextuality was originally defined only for consistently connected systems of random variables (those without disturbance/signaling). Contextuality-by-Default theory (CbD) offers an extension of the notion of contextuality to inconsistently connected systems (those with disturbance) by defining it in terms of the systems’ couplings subject to certain constraints. Such extensions are sometimes met with skepticism. We pose the question of whether it is possible to develop a set of substantive requirements (i.e., those addressing a notion itself rather than its presentation form) such that (1) for any consistently connected system, these requirements are satisfied, but (2) they are violated for some inconsistently connected systems. We show that no such set of requirements is possible, not only for CbD but for all possible CbD-like extensions of contextuality. This follows from the fact that any extended contextuality theory T is contextually equivalent to a theory T′ in which all systems are consistently connected. The contextual equivalence means the following: there is a bijective correspondence between the systems in T and T′ such that the corresponding systems in T and T′ are, in a well-defined sense, mere reformulations of each other, and they are contextual or noncontextual together.