As to proving the existence of tachyons, one basically has to discover a particle interaction which can *only* be explained by the presence of one or more tachyons. Some theoreticians argue that if tachyons exist, the universe could be filled with them but they interact so weakly with ordinary matter that we can't detect them. Physicists have searched through some experimental records and so far none of the high-energy accelerator labs have detected an interaction which can *only* be explained by tachyons. This means that tachyons must be far more weakly interacting than neutrinos. If they do exist, tachyons would be extremely difficult to utilize under our current understanding of physics.