I understand that light cannot escape from inside of an event horizon because the spacetime curvature is too warped for photons to escape. On the other hand, gravitational waves are ripples of spacetime curvature itself (as opposed to light being IN the spacetime), so if there is some internal dynamical structures inside of blackholes (forget about no hair theorem for the moment) that produce gravitational waves, can those gravitational waves escape from inside of event horizon?
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$\begingroup$ See also most any of these questions. $\endgroup$– Kyle KanosCommented Mar 5, 2015 at 3:11
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2$\begingroup$ @KyleKanos: I don't think that's a duplicate because it's asking about a static gravitational field not gravitational waves. But there is almost certainly a duplicate somewhere in the linked questions. $\endgroup$– John RennieCommented Mar 5, 2015 at 7:30
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1$\begingroup$ Not a duplicate. The question is different. $\endgroup$– KPMCommented Mar 5, 2015 at 7:36
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$\begingroup$ @JohnRennie: I agree that it's not an exact duplicate of the linked one. However, it would be a dupe of one or two from the other list I give which are closed as dupes of the one I chose and rather than close it as a dupe of a duplicate, I cut the middle man. $\endgroup$– Kyle KanosCommented Mar 5, 2015 at 13:01
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2 Answers
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First, if you accept that gravitational waves can't travel at fast than the speed of light in regular space, then you can move to the inside of a black hole and then imagine letting the light and the gravitational wave race each other as you fall freely.
As you fall freely then over a short time interval and a short distance everything looks normal to you, you don't notice the event horizon, but you'd say that the wave can't outrun the light beam.
If the light beam can't be overtaken by the wave and the light beam can't escape, the wave can't escape either.
That's it, gravitational waves can't escape the event horizon unless they can outrun light itself in normal circumstances.
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1$\begingroup$ what if, for some reason the black hole were pulsing, from horizon to center? $\endgroup$– anna vCommented Mar 5, 2015 at 5:23
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The answer to your question is that nothing can travel faster than light, and light can't escape through the event horizon. Therefore gravitational waves can't escape either. I give an algebraic proof that light can't escape in my answer to Why is a black hole black?, and a more visual proof in my answer to Would the inside of a black hole be like a giant mirror? (I'm linking my own answers because I remember them - there must be many similar answers on this site).
answered Mar 5, 2015 at 7:47
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$\begingroup$ "Therefore gravitational waves can't escape either" : it was before the eligo detection. Would you write the same after this information ? or is it more subtile ? $\endgroup$– user46925Commented Aug 3, 2016 at 22:42
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1$\begingroup$ @igael: this asks if GWs generated inside the horizon by some internal mnechanism can escape. The GWs detected by LIGO were generated outside the horizon. $\endgroup$ Commented Aug 4, 2016 at 4:18
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$\begingroup$ There is a big difference of masses before and after. It was outside the new BH and inside the 2 old. I suspect a false problem arising from semantics ... $\endgroup$– user46925Commented Aug 4, 2016 at 10:05