Microcausality, one of the general principles of relativistic quantum field theory, says that observables localized in causally disjoint regions of spacetime should commute with each other. This principle is meant to ensure that measurements performed in causally disjoint regions cannot influence each other, but Sorkin's paradox shows that microcausality by itself is not enough. This article reviews Sorkin's paradox and uses it as a reminder that some of the things we formally designate as "observables" may not actually be measurable by any physical process.
