Radiation of superluminal sources in vacuum

Vavilov–Cherenkov radiation is emitted by a charged body uniformly moving in a medium when the body velocity exceeds that of light in the medium. Therefore, it was believed that Vavilov–Cherenkov radiation is impossible in vacuum, because the velocity of any material body cannot exceed the light velocity in vacuum. However, it is possible to realize distributions of charges and currents which propagate with any given velocity. Such a superluminal distribution can be used as a source of Vavilov–Cherenkov radiation in vacuum.