2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/90669It is argued that in the description of macroscopic systems inside quantum mechanics the study of the dynamics of selected degrees of freedom slowly varying on a suitable time scale, corresponding to relevant observables for the given reduced description, is particularly meaningful. A formalism developing these ideas in the more simple case of a microsystem interacting with a macroscopic system is briefly outlined, together with an application to the field of neutron optics. The obtained reduced description relies on a T-matrix formalism and has the property of complete positivity.12 pages, no figures, contribution to the Proceedings of the Conference on The foundations of quantum mechanics - historical analysis and open questions, 1998 (Lecce, Italy)Quantum Physicstext