Quantum hydrodynamics and the theory of liquid helium

Abstract

The Clebsch formula, u = –∇ ϕ – χ ∇ ψ , for the fluid velocity allows the classical hydro-dynamical equations, including vorticity, to be derived from a variational principle, and put into canonical form. The standard quantization procedure of the theory of fields then gives a set of field operators satisfying the commutation relations obtained (starting from different premises) by Landau (1941). The Hamiltonian contains terms corresponding to the excitation of the ‘roton’ states of Landau’s theory, with an energy spectrum (allowing for the atomicity of real liquids by a ‘cut off’ in the Fourier analysis of the field variables) of the form E = ∆ + p 2 /2 μ . The observed variations of specific heat and second-sound velocity in liquid helium II may be interpreted to give values of ∆ in good agreement with the theory, with an apparent variation of μ with p , perhaps attributable to roton-roton and phonon-roton interactions.