Criticism of the single-particle-green's-function approach to the excitation spectrum of superfluid liquid helium

The experimentally observed spectrum for superfluid liquid helium has properties which are characteristic of a sound spectrum but in disagreement with the predictions of a naive use of the single-particle-Green's-function approach. It is argued that the dynamical origins of the spectrum are closely related to those which give rise to the sound propagation in normal liquids, and therefore that the helium spectrum should be sought in structures and formalisms that would be used to study sound in normal liquids.