Black holes are interpreted to have a "break down" of general relativity at their point of singularity. The region near the singularity is expected to be described by some theory of quantum gravity.

Since quantum gravity is expected to describe the physics near the black hole center, black holes would make for great laboratory environments for observing the effects of quantum gravity.

The problem with using black holes to probe quantum gravity, other than them being very far away, is that black holes supposedly prevent anything from escaping beyond the event horizon.

I want to ask if there are any successful or serious theoretical attempts at overcoming the problem stated above.

(This question does not concern Hawking radiation.)

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    $\begingroup$ Are you asking whether any experiments are planned to probe the insides of black holes? Or are you asking about theoretical studies of the black hole interior? $\endgroup$ – John Rennie Aug 12 '15 at 7:47
  • $\begingroup$ @JohnRennie Thank you for the comment, I would like to ask about the theoretical studies of black hole interiors. I'll specify the question a bit further. $\endgroup$ – Otto Aug 12 '15 at 7:49
  • $\begingroup$ @Otto This stack exchange answer might clear your doubts: physics.stackexchange.com/a/156713/68645 $\endgroup$ – Praneet Srivastava Aug 12 '15 at 10:59
  • $\begingroup$ @HadrianEvan Thanks for the comment; I believe the question refers to something a bit different. It's true that a singularity arises from e.g. the schwarzschild solution, but the singularity is usually interpreted as non-physical. General relativity does not take quantum physics into account. According to the current leading theories, quantum physics is supposed to play a major role near the singularity. $\endgroup$ – Otto Aug 12 '15 at 12:47

Your premise is that quantum gravity has effects near the singularity and that the event horizons is a barrier to getting information our from within the horizon.

So a simple theoretical investigation is to look at a very very small black hole, one where the outside of the event horizon is still near the singularity and thus the quantum gravity effects are visible because they are happening outside the event horizon.

But if you don't have good experimental or observational data about small black holes (and if Hawking radiation exists such small black holes don't last long so are constantly changing) then you can't use this to test your theories.

And there are other approaches as well, approaches where spacetime is emergent and so the event horizon (like all of spacetime in such a theory) is just an approximation to something else.

If you are talking about theories that make predictions about situations we haven't been able to test and can't figure out how to test then the door is wide open. Its not wide enough for a layperson to just make up any theory whatsoever because it still has to agree with known results (which have very many particular details to get right) in regions where results have been tested. But there are many possibilities.

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  • $\begingroup$ Thanks for the answer Timaeus. Those are great points. Do you happen to know of any research exploiting a dynamic event horizon? Usually e.g. the schwarzschild solution is derived for static solutions (the black hole is a static, unchanging background metric). I notice there is investigation into perturbation theory of black holes, which leads to quasinormal mode oscillations. But those are simply small perturbations to the metric, so what about larger perturbations? $\endgroup$ – Otto Aug 13 '15 at 3:10

There are a number of attempts at constructing theories of quantum gravity, of which string theory and loop quantum gravity are the most developed. However none of these theories have been developed to a point where they can make uncontroversial predictions about what happens near a black hole singularity.

The only even passably convincing attempt is using a streamlined version of loop quantum gravity called loop quantum cosmology. When applied to the Big Bang singularity this predicts that gravity becomes repulsive at distances around the Planck length, and this prevents the singularity forming. The Big Bang becomes a bounce, and geodesics can be continued through the Big Bang. However there is limited acceptance of this as a valid description, and of course it describes a different type of singularity. I'm not sure what work has been done to describe the black hole singularity with LQG, though apparently some progress has been made.

Although you ask about the central singularity rather than the even horizon, I should mention the recent ideas about black hole firewalls. However these ideas remain highly controversial.

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  • $\begingroup$ I appreciate the answer, although I now realize I may have stated my question rather poorly. My question concerned theories which predict any information of the black hole center ("singularity") from escaping inside the black hole. The work done on LQG is still a very interesting read, thank you for that. $\endgroup$ – Otto Aug 12 '15 at 9:51

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