A proof that no-signalling implies microcausality in quantum field theory

Antoine Soulas

We study some logical interrelationships between fundamental properties in (relativistic) quantum theories. An operational no-signalling condition is first introduced in the context of quantum mechanics, where we prove its equivalence to an apparently weaker version restricted to ideal measurements, and to a property of factorization of the evolution unitary operator. We then translate this condition in quantum field theory and prove that it implies both microcausality and the spin-statistics theorem, in the ideal case of pointwise measurements implemented in the projection postulate sense. This provides an argument (often invoked but apparently missing in the literature) to see microcausality as a necessary condition for the compatibility of spacelike separated operations.

V.Ya. Fainberg

Charis Anastopoulos, Ntina Savvidou

Detection times are routinely recorded in quantum experiments. We show that, under natural covariance and consistency requirements, the observables describing these times cannot be algebraically localized in any bounded spacetime region, even when the records themselves are localized. This challenges the identification of algebraic localization with the physical localization of measurements and shows why microcausality alone does not establish non-signalling for measurements in quantum field theory.

N. N. Bogolyubov

Christopher Thron

Richard Healey

Bell appealed to the theory of relativity in formulating his principle of local causality. But he maintained that quantum field theories do not conform to that principle, even when their field equations are relativistically covariant and their observable algebras satisfy a relativistically motivated microcausality condition. A pragmatist view of quantum theory and an interventionist approach to causation prompt the reevaluation of local causality and microcausality. Local causality cannot be understood as a reasonable requirement on relativistic quantum field theories: it is unmotivated even if applicable to them. But microcausality emerges as a sufficient condition for the consistent application of a relativistic quantum field theory.