  Virtual phonons of a quantum liquid scatter off impurities and mediate a
long-range interaction, analogous to the Casimir effect. In one dimension the
effect is universal and the induced interaction decays as $1/r^3$, much slower
than the van der Waals interaction $\sim1/r^6$, where $r$ is the impurity
separation. The sign of the effect is characterized by the product of
impurity-phonon scattering amplitudes, which take a universal form and have
been seen to vanish for several integrable impurity models. Thus, if the
impurity parameters can be independently tuned to lie on opposite sides of such
integrable points, one can observe an attractive interaction turned into a
repulsive one.
