[oldandtired]

the speed of light is based on two specific values of the medium in which the photons are travelling. The permeability and the permittivity, one affects the magnetic the other affects the electric.

Hence by the change of these two values, the speed of light (magnitude) changes. Hence, if we are able to create media in which these values are appropriately specified, we can create media in which the specific speed of light is less than c0 or greater than c0.

It, therefore, may be possible to create a medium in which light will pass through faster than a vacuum.