Question: What things are faster than light?

Susan Cartwright answered on 22 Jun 2015:

As far as we know, nothing travels faster than light in a vacuum.

However:
(1) Light in transparent media like glass or water (or even air) is slowed down by an amount corresponding to the refractive index of the material: for example, the refractive index of water is 4/3, so light in water travels at 3/4 the speed of light in a vacuum. This slowing down does not affect particles other than light photons, so any particle travelling at greater than 3/4 c is travelling faster than the speed of light in water. This would include, for example, electrons with energies >750 keV (1 eV = 1.6×10^-19 J) and protons with energies >1.5 GeV. This is the origin of Cherenkov radiation, which is basically the light equivalent of a sonic boom.
(2) The speed limit applies to objects travelling *through* space. Space itself can expand as fast as it wants. We think that there was a period early in the history of the universe (inflation) when space was expanding much faster than light.
(3) In special relativity, you could in principle have things that travelled faster than light (tachyons, from the Greek for “fast”). However, any such thing would never be able to slow down to below the speed of light – in fact, if its energy was reduced it would go faster. There are no known tachyon particles in nature, and the appearance of a tachyon in your theory is generally treated as evidence that something has gone badly wrong.