  We construct a low-energy effective Lagrangian describing zero-temperature
supersolids. Galilean invariance imposes strict constraints on the form of the
effective Lagrangian. We identify a topological term in the Lagrangian that
couples superfluid and crystalline modes. For small superfluid fractions this
interaction term is dominant in problems involving defects. As an illustration,
we compute the differential cross section of scatterings of low-energy
transverse elastic phonons by a superfluid vortex. The result is
model-independent.
