ABSTRACT
This chapter considers what the elementary excitation spectrum is like for the only Bose liquid found in nature, He II. If one assumes the Landau theory to be correct, one can work backwards from the experimental data to a form of the quasi-particle spectrum consistent with that data. Neutron scattering experiments furnish us with data for the quasi-particle spectrum over essentially the entire range of momenta for which the quasi-particles are well-defined. Pitaevskii has carried out an elegant field-theoretic analysis of such an endpoint to the quasi-particle spectrum. The first detailed calculation of the excitation spectrum was carried out by Feynman. The idea that a single quasi-particle excitation provides the dominant contribution to the density-fluctuation excitation spectrum is thus seen to be a good first approximation for the description of the excitations throughout the entire momentum range of interest. Backflow must correspond in some sense to taking into account the effect of such multi-particle configurations on the quasi-particle excitation spectrum.
