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This question already has an answer here:

There is an apparent paradox in a Black hole. Keenan Pepper wrote:

Electromagnetic radiation cannot escape a black hole, because it travels at the speed of light. Similarly, gravitational radiation cannot escape a black hole either, because it too travels at the speed of light.

If this is true, then evidently a BH cannot exert a pull at the centre of a Galaxy, neither it can suck off the energy out of light trying to excape from it , etc?

EDIT: should I ask a new question about this? @John Rennie, saying that gravity is curvature, seems like shifting the problem (,like saying that electrostatic pull is the electric field). Relativity says that mass curves spacetime, but that supposes interaction. How can spacetime know that a BH is there, if it cannot communicate? a planet modifies surrounding spacetime in some way, and that , in its turn, affects bodies and light. The question now becomes: how does a BH acts on surrounding spacetime to make it curve, since nothing can excape it, except Hawkings radiation?

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marked as duplicate by John Rennie, Stan Liou, ACuriousMind, Kyle Kanos, Qmechanic Aug 5 '14 at 13:25

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  • $\begingroup$ This sounds like an interesting thought experiment disproving gravitons. But the existence of gravity waves has already been proven. $\endgroup$ – John Dvorak Aug 5 '14 at 9:58
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    $\begingroup$ Related: physics.stackexchange.com/q/937/2451 and links therein. $\endgroup$ – Qmechanic Aug 5 '14 at 9:59
  • $\begingroup$ I misunderstood this question at first due to strange wording. It is a very intriguing question, rewording it would be much appreciated. $\endgroup$ – Gödel Aug 5 '14 at 10:14
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There is a logical flaw in the assumption: gravitational radiation is not the cause of gravitational attraction. The latter is due to curvature of spacetime outside the black hole. For a black hole to attract/exert redshift on something, nothing has to propagate from inside the event horizon. Gravitational waves (radiation) merely represent moving disturbances in spacetime, they are not what causes the force of gravity, just like a ray of light does not pull you towards its source electromagnetically.

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    $\begingroup$ I think my own confusion on this topic comes from my (presumably flawed) understanding that mass induces curvature and the graviton is the particle mediating the "hey space, I'm massive, so bend now" type of message. So if the gravitons can't get past the horizon the space outside can't possibly curve in response. Does anything at all mediate between massive objects and spacetime curvature? What exactly is the mechanism curving space? $\endgroup$ – zibadawa timmy Aug 5 '14 at 10:55

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