ABSTRACT
In this chapter, the authors turn their attention to microscopic theories of interacting Bose systems, that class of theories which begin with a given law of interaction between the particles, and derive the various properties of the system therefrom. Microscopic theory does not provide a quantitative description of liquid He II. Indeed, there exists a satisfactory microscopic theory of an interacting boson system in only two limiting cases, those of: Weak repulsive interactions between the bosons; and a dilute gas with arbitrary repulsive interaction between the particles. One sees how the macroscopic occupation of a single quantum state dictates the structure of the microscopic description of system behavior. The Hamiltonian serves as a starting Hamiltonian in authors consideration of microscopic theory; they show that the existence of macroscopic occupation of a single quantum state leads to an effective Hamiltonian which looks rather different.
