Volumes of Space as Subsystems

As a novel approach with possible relevance to semiclassical gravity, we propose to define regions of space as quantum subsystems. After recalling how to divide a generic quantum system into ``parts'', we apply this idea to a free scalar field in Minkowski space and we compare two different localization schemes. The first scheme is the standard one, induced by the local relativistic fields; the alternative scheme that we consider is the one induced by the Newton-Wigner operators. If degrees of freedom are divided according to the latter, the Hamiltonian of the field exhibits a certain amount of non-locality. Moreover, when a region of space is cut off from the rest according to the Newton-Wigner scheme, the geometric entropy is finite and exhibits a sensible thermodynamic behaviour.