The speed of light (in the vacuum) is a function of the permeability and permittivity of the vacuum. In other mediums the phase velocity will be different. It is assumed (by me) that the speed of a gravitational wave does not change depending on the medium i.e. a gravitational wave would not slow down while passing through the sun. This would cause diffraction and presumably dispersion of the wave. Why then is the speed of a gravitational wave limited by the speed of light in a vacuum? How did Einstein come to this conclusion?