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Inspired by the gravtiomagnetic analogy, I would expect that just as a charged tachyon would emit normal (electromagetic) Cerenkov radiation, any mass-carrying warp drive would emit gravitational Cerenkov radiation. The gravitomagnetic approximation may well break down near the mass, but "sufficiently far" from it, this would still be valid. Is that correct?

Specifically, let's suppose there is a moving closed surface S, such that on and outside S the gravitomagnetic equations are approximately valid (no assumptions about interior), such that it moves with a velocity greater than $c$, and such that it "carries mass", in the sense that the closed surface intergral of the gravitational field strength around S is negative (net inward gravitational field).

In general relativity, is this situation even possible? If so, would it emit gravitational radiation? If so, how fast would it lose energy (mass)?

I am motivated by the recent media hype around the Alcubierre metric. Nevertheless, it is a general question applying to any proposed "moving warp bubble" solution of general relativity. (As opposed to, say, a pair of "stargates", or a "warp corridor", or whatever -- if a mass $M$ travels through a stargate, it might be that the gate through which it enters could get heavier by $M$, and the gate through which it leaves could get lighter by $M$. Then this particular question wouldn't arise.)

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Such an object would not emit gravitational radiation/waves. Only certain objects/systems of objects can do that (e.g. binary neutron stars). – HDE 226868 Aug 30 '14 at 22:56
@HDE226868: any stress-energy souce with a time-varying quadrupole moment will emit gravitational radiation. Certainly, the setting up and tearing down of the warp bubble would involve a time varying gravitational field involving (exotic) matter, so, genericically, I'd expect gravitational radiation. – Jerry Schirmer Aug 18 '15 at 15:21
I do not understand why you resort to hypothetical tachyons to explain your question. Normal charged matter already does that when it moves faster through a medium than the speed of light in that medium (which is lower than the speed of light in a vacuum). – Sentry Jul 6 at 16:55

The basic metric for the Alcubierre drive is asymptotically flat (it reduces to flat space as $r \rightarrow \infty$). Hence the far field for the metric will be flat so no emission of gravitational waves. Nothing actually goes faster than light in there (indeed, in the ideal case, the spaceship doesn't even move at all)

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Asymptotic flatness doesn't force zero gravitational radiation. The metric for a binary black hole system is asymptotically flat, too, after all. – Jerry Schirmer Aug 18 '15 at 15:22

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