Vacuum Cherenkov Radiation
See also: Tachyon and Modern searches for Lorentz violationVacuum Cherenkov radiation is the conjectured phenomenon which refers to the Cherenkov radiation of a charged particle propagating in the physical vacuum.
The classical (non-quantum) theory of relativity clearly forbids any superluminal phenomena including this one because a particle with non-zero rest mass can reach speed of light only at infinite energy (besides, the nontrivial vacuum itself would create a preferred frame of reference, in violation of one of the relativistic postulates). However, according to modern views coming from the quantum theory, physical vacuum is a nontrivial medium which affects the particles propagating through, and the magnitude of the effect increases with the energies of the particles. As a result, an actual speed of a photon becomes energy-dependent and thus can be less than the fundamental constant of speed of light c=299,792,458 metres per second, such that sufficiently fast particles can overcome it and start emitting Cherenkov radiation.
Other possibility arises in some Lorentz-violating theories when a speed of a propagating particle becomes higher than c which turns this particle into the tachyon. The tachyon with an electric charge would lose energy as Cherenkov radiation—just as ordinary charged particles do when they exceed the local speed of light in a medium. A charged tachyon traveling in a vacuum therefore undergoes a constant proper time acceleration and, by necessity, its worldline forms a hyperbola in space-time.
Read more about this topic: Cherenkov Radiation
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