Cerenkov radiation is radiation emitted when a charged particle travels through a medium at a speed greater than the velocity of light in the medium. This occurs when the refractive index of the medium is high. Cerenkov radiation is like the bow wave of a boat, or the shock wave of a supersonic airplane. Photons bunch up in front of the tachyon and they're radiated away at an angle determined by the speed of the tachyon. Research Cerenkov Radiation