Why do quantum systems forget their past and reach equilibrium?

Standard quantum mechanics assumes perfect knowledge of a system's state, but real measurements give only finite-precision intervals. By representing quantum states as convex sets of density matrices matching those intervals, this framework proves that any large, complex quantum system will thermalize — its observables converging to microcanonical (thermal equilibrium) values at most late times. The result depends only on a single number, the minimum effective dimension of the state set, not on the system's detailed configuration.