Something that puzzles me about "the speed of light in X" discussions is exactly the confusion over whether the speed of light is "different" in a medium. On the one hand, it seems like it isn't, since no actual photons are moving more slowly. On the other, things like Cherenkov radiation seem to indicate that the apparent speed of light matters for things that you'd expect to be based on the constant speed of light in a vacuum. There's a physicist, Ronald Mallett, who has suggested an experiment to see if the constant speed of light or the apparent speed of light is what matters for frame-dragging (!).
But AFAIK Cherenkov radiation occurs in medium, so there isn't any contradiction to the generally accepted limits, and Ronald Mallet is highly unlikely to be trusted, see the wikipedia entry:
I was under the impression that tachyons (should they exist) would be expected to give off Cherenkov radiation in vacuum, but I'm not sure where I got that impression.
I hadn't actually heard anything about Mallett for a few years, and I guess this is why. :)
That would imply that tachyons interact with the electromagnetic force.
But if they do we would have detected them long ago.
But it's wrong anyway, it's not the moving particle that gives off the Cherenkov radiation, it's the matter that they move near. And there is no matter in a vacuum.
Virtual matter can not give off photons - it would violate conservation of momentum (unless it was in pairs, but that would not work with Cherenkov radiation).
There seems to be one paper about Cherenkov radiation in a vacuum [1]. But I have only read the abstract. So you are probably right, that there's no Cherenkov radiation in a vacuum. Or does anyone has any better knowledge?
[1] "Cherenkov radiation in vacuum and plasma-filled microwave sources in the absence of guiding magnetic fields" by GS Nusinovich
