"Gravitational wave damping has been detected in an amount that agrees with general relativity to half a percent using the Hulse-Taylor binary pulsar,"
I also have something to say to the first question in comments there, to which the article author answered that he can't answer. The question was "Is the speed of gravity reduced by the medium through which it travels in a analogous manner to the slowing of the speed of light through various media?"
I was lucky enough to read Feynman Lectures recently, so as far as I understood him, the photons themselves don't really slow down in various media. That is, what we see as the result is the slowdown, but only because it's a new light (new photons) on the other side of the media. The photons on the other side are photons which were pushed out of the atoms of the media, that's why it appears that they come out slower. Inside, between the atoms of the media, photons still move at the speed of light.
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
As a slightly-related follow-on, see http://en.wikipedia.org/wiki/Cherenkov_radiation for a nice example of what happens in the case of a charged particle moving through a medium more quickly than the speed of light in that medium.
Comments
The point of the article is in
Clifford M. Will
"The Confrontation between General Relativity and Experiment" (2001)
http://relativity.livingreviews.org/Articles/lrr-2001-4/
"Gravitational wave damping has been detected in an amount that agrees with general relativity to half a percent using the Hulse-Taylor binary pulsar,"
I also have something to say to the first question in comments there, to which the article author answered that he can't answer. The question was "Is the speed of gravity reduced by the medium through which it travels in a analogous manner to the slowing of the speed of light through various media?"
I was lucky enough to read Feynman Lectures recently, so as far as I understood him, the photons themselves don't really slow down in various media. That is, what we see as the result is the slowdown, but only because it's a new light (new photons) on the other side of the media. The photons on the other side are photons which were pushed out of the atoms of the media, that's why it appears that they come out slower. Inside, between the atoms of the media, photons still move at the speed of light.
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:
http://en.wikipedia.org/wiki/Ronald_Mallett
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.
Only virtual matter.
Which should still give off Cerenkov radiation in the old, boring way, even without tachyons.
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
As a slightly-related follow-on, see http://en.wikipedia.org/wiki/Cherenkov_radiation for a nice example of what happens in the case of a charged particle moving through a medium more quickly than the speed of light in that medium.