  General relativity is extended by promoting the three-dimensional
gravitational Chern-Simons term to four dimensions. This entails choosing an
embedding coordinate v_\mu -- an external quantity, which we fix to be a
non-vanishing constant in its time component. The theory is identical to one in
which the embedding coordinate is itself a dynamical variable, rather than a
fixed, external quantity. Consequently diffeomorphism symmetry breaking is
hidden in the modified theory: the Schwarzschild metric is a solution;
gravitational waves possess two polarizations, each traveling at the velocity
of light; a conserved energy-momentum (pseudo-) tensor can be constructed. The
modification is visible in the intensity of gravitational radiation: the two
polarizations of a gravity wave carry intensities that are suppressed/enchanced
by the extension.
