According to the theory of optics (geometrical or quantum optics), all waves are associated with two kind of velocities (whose absolute magnitudes are the respective speeds), the group velocity and the phase velocity. See site
http://scienceworld.wolfram.com/phys...pVelocity.html
and
http://scienceworld.wolfram.com/phys...eVelocity.html
the product of group velocity and phase velocity is the square of the speed of light in vacuum. Note that for a photon energy is propagated at the group velocity, so that for light the group velocity is the same as the phase velocity but for an electron, its group velocity is less than light, therefore its phase velocity is superluminal. This phase velocity of matter can also explain quantum entanglements, which is also superluminal but does not carry any information of energy.