Bremsstrahlung is the radiation produced when a charged particle decelerates. Cherenkov radiation is the optical equivalent of a sonic boom, seen when a particle breaks the light barrier in a medium. It is usually presumed that a superluminal particle in vacuo would produce Cherenkov radiation or something similar, such as electron-positron pairs. In some models (but not in traditional tachyon theory) this energy loss will decelerate the superluminal particle. These forms of radiation thus provide on the one hand a possible signature of superluminal motion, and on the other (at least in certain models) a constraint: the neutrinos must not radiate away all their energy before they can be observed.