 The speed of light
is related to the vacuum permittivity "ε"0 and the vacuum permeability "μ"0.Therefore, not only the phase velocity, group velocity, and energy flow velocity of electromagnetic waves but also the velocity of a photon can be faster than "c" in a special material has the constant permittivity or permeability whose value is less than that in vacuum.Special relativity postulates that the speed of light in vacuum is invariant in inertial frames.