ABSTRACT
In the preceding chapters, we have been mostly concerned with the ground state, |0〉, of a Bose liquid; we have discussed at length its structure, its excitation spectrum, and its response to a weak external probe. We have seen that all these properties were deeply influenced by the existence of a condensate, consisting in the n o particles which occupy the single state k = 0. Such a condensate is clearly at rest; it possesses a uniform density and carries no current.
