A quantum field theory of phonons and boiling in liquids

Phonons and boiling in liquids is investigated from the viewpoint of quantum field theory at finite temperature, starting with atoms interacting with each other as the first principle. The Ward-Takahashi relation associated with the spontaneous breakdown of the translational invariance in the radial direction shows that there is a Goldstone boson, which is a longitudinal phonon. By making use of the cell model, a longitudinal phonon dispersion curve is presented in the random phase approximation. A criterion of boiling is also proposed.