Collective-excitation conjecture for many-particle dynamics

Consider a many-particle system placed in a heat bath and represent its equilibrium state as a Hilbert-space vector. The authors assume that the many-particle system and its dynamics can be understood as being built from weakly interacting, long-lived collective or elementary excitations, which may be completely different from the elementary building blocks of the bulk system. This assumption permits the introduction of a quasi-free Hamiltonian and creation and annihilation operators for the new excitations. The proposal is used as a physical model for explaining the doubling in the box-canonical ensemble, but no general proof or resolution is supplied.