Dear Juan, You asked:
I'm not sure if I can give you a satisfactory answer to this question! I think part of the difficulty is that there isn't yet a good theory of quantum gravity. That is, physicists can try to explain forces as produced by particles and fields in flat space (quantum field theory), and we can describe gravity as a warping of spacetime (general relativity), but we haven't figured out a way of combining those two schemes together. So thinking about gravitons and black holes together is very challenging!
With that disclaimer, I'll attempt to offer an explanation which at least sounds convincing to me. Let's start by looking at interactions between two charged particles (like two electrons). Quantum electrodynamics says that the force between the two electrons is the result of them exchanging photons with each other. But these aren't "real" photons -- we never detect them directly because they just travel from one electron to the other. Also, because we never see these virtual photons, they will take the opportunity to "bend the rules" without getting caught! The virtual photons don't have to conserve momentum or energy, and can travel back in time or faster than light. It sounds really strange, but when all the complicated math is worked out, everything turns out all right in the end. Nothing we can see or measure ever travels faster than light, or back in time.
Now take one of these electrons and put it inside a black hole's event horizon, leaving the other outside. Because virtual photons are such lawbreakers, they can still travel back and forth across the event horizon, between the two electrons. The result is that charged particles inside a black hole can still influence charged particles on the outside. In other words, the charge of a black hole can be measured.
The situation should be the same with massive particles and gravitons. In the language of particle physics, the force of gravity is mediated by the exchange of virtual gravitons. So mass inside the black hole ought to be able to make its presence known by swapping virtual gravitons across the event horizon with mass outside the black hole. This analogy isn't perfect. It ignores the brain-busting fact that these virtual gravitons are supposed to be what are actually producing the spacetime curvature in the first place, which is supposed to govern how the the particles that emitted the gravitons move through the universe. This is in some sense the chicken-and-egg conundrum of quantum gravity.
I hope this rather long response has helped to answer your question. My office-mates and I really enjoyed thinking about this issue! Please write back if you get confused by what I've written.
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