  It is well known that a physical medium that sets a Lorentz frame generates a
Lorentz-breaking gap for a graviton. We examine such generated "mass" terms in
the presence of a fluid medium whose ground state spontaneously breaks spatial
translation invariance in $d = D+1$ spacetime dimensions, and for a solid in $D
= 2$ spatial dimensions. By requiring energy positivity and subluminal
propagation, certain constraints are placed on the equation of state of the
medium. In the case of $D = 2$ spatial dimensions, classical gravity can be
recast as a Chern-Simons gauge theory and motivated by this we recast the
massive theory of gravity in AdS$_3$ as a massive Chern-Simons gauge theory
with an unusual mass term. We find that in the flat space limit the
Chern-Simons theory has a novel gauge invariance that mixes the kinetic and
mass terms, and enables the massive theory with a non-compact internal group to
be free of ghosts and tachyons.
