|MadSci Network: Physics|
In wave propagation there are different kind of velocities like phase velocity another is group velocity. Phase velocity =c/n where c is velocity of light in vacuum and n is refractive index. The group velocity deals the propagation of an envelop (like pulse) through the medium and depends on the rate of change of refractive index of the medium with frequency. Now there is big field of research for manipulating group velocity in certain mediums. There are methods by using medium of certain atomic configuration and shining with proper lasers group velocity can be manipulated ( as has been done in BEC) in the medium. Here I am avoiding more technical terms and details but you can read something on "Coherent Control" in quantum optics text books. The main point is that there are methods for manipulation of group velocity but phase velocity is same c/n and for Cerenkov radiation some charge particle has to move faster than phase velocity not the group velocity in the medium. But if you say that you can carry some charge and can move faster than phase velocity in some medium result will be as you expect Cerenkov radiation from the medium and you will loose energy in charging the medium.
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