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?