  In this paper we study the speed of gravitational waves in a brane world
scenario and show that if the extra dimension is space-like, the speed of the
propagation of such waves is greater in the bulk than that on the brane.
Therefore, the 4D Lorentz invariance is broken in the gravitational sector. A
comparison is also made between the red shift of such waves and those of the
electromagnetic waves on the brane. Such a comparison is essential for
extracting the signature of the extra dimension and thus clarifying the
question of maximal velocity of gravitational waves in the bulk.
