  A dynamical model for Landau's original approach to superfluid Helium is
presented, with two velocities but only one mass density. Second sound is an
adiabatic perturbation that involves the temperature and the roton, aka the
notoph. The action incorporates all the conservation laws, including the
equation of continuity. With only 4 canonical variables it has a higher power
of prediction than Landau's later, more complicated model, with its 8 degrees
of freedom. The roton is identified with the massless notoph. This theory gives
a very satisfactory account of second and fourth sounds.
  Second sound is an adiabatic oscillation of the temperature and both vector
fields, with no net material motion. Fourth sound involves the roton, the
temperature and the density. With the experimental confirmation of
gravitational waves the relations between Hydrodynamics and Relativity and
particle physics have become more clear, and urgent. The appearance of the
Newtonian potential in irrotational hydrodynamics comes directly from
Einstein's equations for the metric. The density factor $\rho$ is essential; it
is time to acknowledge the role that it plays in particle theory.
  To complete the 2-vector theory we include the massless roton mode. Although
this mode too is affected by the mass density, it turns out that the wave
function of the unique notoph propagating mode $\mathcal{N}$ satisfies the
normal massless wave equation $\Box\mathcal{N}$ = 0; the roton propagates as a
free particle in the bulk of the superfluid without meeting resistance. In this
circumstance we may have discovered the mechanism that lies behind the flow of
He-II through very thin pores.
