  Locality is a central notion in modern physics, but different disciplines
understand it in different ways. Quantum field theory focusses on relativistic
locality, enforced by microcausality, while quantum information theory focuses
on subsystem locality, which regulates how information and causal influences
propagate in a system, with no direct reference to spacetime notions. Here we
investigate how microcausality and subsystem locality are related. The question
is relevant for understanding whether it is possible to formulate quantum field
theory in quantum information language, and has bearing on the recent
discussions on low-energy tests of quantum gravity. We present a first result
in this direction: in the quantum dynamics of a massive scalar quantum field
coupled to two localised systems, microcausality implies subsystem locality in
a physically relevant approximation.
