In general relativity, gravity is a distortion of spacetime due to mass. Its effects travel (if that's the right word) at the speed of light. In the SM all 3 other known interactions are mediated by exchange of particles.

A major unsolved problem is the belief that there should be a way to unify these, perhaps by some kind of quantum gravity, and perhaps by means of a gravitational exchange particle, hypothetically dubbed the "graviton".

In a region of extreme mass, a distortion in spacetime has an intuitive meaning, so it's easy to understand how a black hole can exist (gravity doesn't "leave" anywhere so it never has to pass through the EH). But in a theory where gravity is a result of exchange particles, how can that exchange occur, once an event horizon exists? Would a discrete gravitational boson have to have no self-interaction, conveying a kind of gravitational immunity, or would event horizons have an exclusion carved out ("nothing can leave except gravitons")?

Theories of quantum and exchange-particle gravity have been developed for decades, so this issue must have arisen and been considered in them.

How do current proposals for quantum gravity and "gravity based on an exchange particle" get around this apparent problem, or prevent it arising?

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    $\begingroup$ Related: physics.stackexchange.com/q/937/2451 $\endgroup$ – Qmechanic May 7 '19 at 10:53
  • $\begingroup$ I've thought about this, but I feel like it takes the concept of 'force-mediating particles' (that too virtual) too far. $\endgroup$ – Avantgarde May 7 '19 at 11:20
  • $\begingroup$ From Stan Liou's answer to the question Qmechanic has linked: "rather than gravity having a special property that enables it to cross the horizon, in a certain sense gravity can't cross the horizon, and it is that very property that forces gravity outside of it to remain the same". $\endgroup$ – PM 2Ring May 7 '19 at 14:27
  • $\begingroup$ The issue exists for EM. I think everyone agrees that a charged black hole exerts electromagnetic force on charged particles outside it. How do the virtual photons mediating this force cross the horizon? I assume the answer boils down to the fact that virtual particles are not real particles, are off the mass shell, and can “go” faster than light. Is this the wrong way to think about it? $\endgroup$ – G. Smith May 7 '19 at 16:05
  • $\begingroup$ @G.Smith From math.ucr.edu/home/baez/physics/Relativity/BlackHoles/… In this sense the black hole is a kind of "frozen star": the gravitational field is a fossil field. The same is true of the electromagnetic field that a black hole may possess. $\endgroup$ – PM 2Ring May 15 '19 at 14:10

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