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Whatever happens in there is not falsifiable nor provable to the outside. If for (amusing) example the interior consisted of 10^100 Beatles clones playing "Number Nine" backwards, do we know how to unscramble the Hawking radiation to divine this? The same question applies to this new firewall furor. So of what use is a description of the interior to our physics on the outside?

The only possibility of usefulness I can see is if our own universe can be described as an interior up to the cosmic horizon in de Sitter space. But that's only an "if".

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    $\begingroup$ I am almost tempted to downvote this since the question is formulated in a very confrontational tone, which could ignite unconstructive flame wars about legitimate black hole physics ... $\endgroup$ – Dilaton Apr 10 '13 at 19:32
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    $\begingroup$ How about the study can lead to new methods, techniques, ideas... that may be useful in solving other problems. $\endgroup$ – MBN Apr 10 '13 at 20:51
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    $\begingroup$ I have just watched a number of CERN conference videos convened to address the issue indico.cern.ch/… , and was impressed with the creativity displayed. But my personal issue is the blank fact that observation of an infalling event for an observer well outside produces a "frozen painting" at the event horizon of duration infinite time for that external observer. Thus my question above; the actual event horizon crossing event is never visible from outside. $\endgroup$ – Andrew Palfreyman Apr 11 '13 at 4:59
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    $\begingroup$ @Dilaton: No it's not. Lubos' comment contained a personal attack. The question itself contains none. It just questions a part of physics; which is legitimate. This is not a war. Nobody is attacking, you don't need to defend yourself. I don't see any part of the question which is confrontational. $\endgroup$ – Manishearth Apr 11 '13 at 10:09
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    $\begingroup$ @Dilaton: Look, the asker isn't saying that "This isn't justifiable, so physicists shouldn't do this". He's asking for a deeper understanding of the motivation. I would, too. The main purpose is not to cater to a lay audience, but that in no way implies that we shun lay questions. It doesn't matter if we assume that mainstream physics is legit. If someone wants to know why we pursue something (and is not asking in a philosphical manner), it will give them greater understanding if they understand the motivation. $\endgroup$ – Manishearth Apr 11 '13 at 11:10
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Simply because our final goal is a set of laws of physics that describes any part of the universe equally well. Let's say a physicist jumped into a black hole and saw that the interior of the black hole was composed entirely of John Lennon clones. His last thoughts before getting spaghettified would be "why?". From his perspective, physics is incomplete.

Sure, we probably can't use it to predict anything -- but modern physics is much less about predictions and much more about having a beautiful, mathematically rigorous model of the universe. Mathematical models with discontinuities usually aren't "beautiful", and John Lennon in black holes counts as a discontinuity if we take general relativity as our mathematical model of the universe. Which would mean that we will eventually have to replace our model (which is why knowing as much as we can about the inside is important). Besides, if our current theories partially fail inside a black hole, we need to patch that up.

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    $\begingroup$ Does the situation not have some analog in describing the physics of other verses in a multiverse, perhaps? In this case also the alternate regimes appear inaccessible. Perhaps one can talk about "gravity leaking between branes" in the verse case, or of "entanglement bridging the event horizon" in the BH case. But if we can agree that nothing that occurs in the "possibly different physics" of verses or BH interiors has any substantive effect on our physics right here, then isn't it justified to ignore the other stuff? I guess it depends where one draws the line between "curious" and "relevant" $\endgroup$ – Andrew Palfreyman Apr 11 '13 at 5:06
  • $\begingroup$ @AndrewPalfreyman: No, it doesn't. We can't jump to other universes. We can jump into a black hole. $\endgroup$ – Manishearth Apr 11 '13 at 8:52
  • $\begingroup$ Your point is well-taken. I would however have inserted the word "yet" in there re. hopping verses. And perhaps, in the interest of frankness, this as-yet-hypothetical process might putatively partially involve jumping into a black hole :) $\endgroup$ – Andrew Palfreyman Apr 11 '13 at 11:14
  • $\begingroup$ @Manishearth: "... because our final goal is a set of laws of physics that describes any part of the universe ...". If space and time end at the event horizon, the interior wouldn't actually be a part of our universe. $\endgroup$ – dcgeorge Jul 25 '14 at 15:24
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    $\begingroup$ Spacetime doesn't really end at the horizon. $\endgroup$ – Manishearth Jul 25 '14 at 15:54
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Whatever happens in there is not falsifiable nor provable to the outside.

General relativity predicts this, but a) no one has ever checked experimentally, and b) it seems to be incompatible with the rules of quantum physics. Every attempt at mixing quantum theory and GR has produced results like Hawking radiation that tell us that the black hole interior and exterior are not completely decoupled.

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    $\begingroup$ But isn't that the point? - that, although you can indeed dive in and do some experiments, there's no way you can tell the physics community back here anything about it. $\endgroup$ – Andrew Palfreyman Apr 11 '13 at 5:07
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    $\begingroup$ Who says that the physics community will remain "back here"? I can throw the Earth including all of the communities into a black hole – whether or not such an experiment is legal. Nature surely has prepared an answer to what the people will feel and a good theory of physics must be able to answer that, too. $\endgroup$ – Luboš Motl Apr 11 '13 at 6:41
  • $\begingroup$ As I mentioned in my original question, perhaps we already inhabit such an interior. $\endgroup$ – Andrew Palfreyman Apr 11 '13 at 11:06
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    $\begingroup$ @LubošMotl AFAIK we already proved that there exist undecidable statements in most branches of mathematics, why shouldn't there be undecidable physical questions? $\endgroup$ – Bakuriu Apr 11 '13 at 11:58
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    $\begingroup$ @AndrewPalfreyman: The existence of event horizons is an untested prediction of GR. Probably true, but we won't know until we've actually tried dropping a flashlight past where we think an event horizon is. $\endgroup$ – user1504 Apr 11 '13 at 12:22
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Why do we care about the physics inside black holes?

By Karl Popper's reckoning and the rules of general relativity, we probably shouldn't. From the outside, it's not being scientific to theorize about the inside. Whatever happens there is not falsifiable; not just difficult to falsify (like physics at the Planck scale or theories about the inside of the sun), but impossible in principle.

It may be mathematically useful for other reasons or philosophically interesting but it's not, according to Popper, scientific. If suicidal people jump in and survive, fine, it's valid science for them (while they last) but for us on the outside, it will never be scientific to theorize about the region.

That should be sufficient reason not to care, but there's something even more fundamental. Beyond the falsifiability issue, the greater reason not to care about the inside of a black hole is the more and more likely scenario that there isn't any inside to care about.

Raphael Bousso muses that space and time seem to "somehow" end at the horizon. And Joesph Polchinski sums it up this way: "... the inside of a black hole — it may not be there" ... "Probably that's the end of space itself; there's no inside at all."

Granted, these new ideas are speculative and controversial but what, really, is the evidence that the spacetime manifold does continue across the horizon? That looks to me like an unexamined assumption. The idea that an in-falling observer simply drifts on through the horizon assumes continuity of the manifold. The same assumption is there for the theory that entanglement bridges the horizon.

GR tells us that in-falling matter and energy never reach the horizon; firewall theory puts a vaporizing surface there; the membrane paradigm and stretched horizon theories completely ignore the inside and describe a one-sided surface with physical attributes; and Lynden-Bell and Katz's calculations put the mass of a black hole entirely in its external gravitational field.

These ideas strongly hint that there is no interior to worry about.

Unfortunately, all of this will upset a lot of very smart people who have invested so much time in theories about the interior, and rightfully so. But it also presents a great opportunity. If black holes are actual holes or cavities in the spacetime manifold, it ushers in an entirely new and untapped paradigm to explore. We live in interesting times, indeed.

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Can we be sure today that we'll never have some sort of faster-than-light travel, some unthinkable technology that transcends spacetime, or some other way that the black hole interior ceases to be unreachable and unknowable?

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    $\begingroup$ This should be a comment, not an answer. $\endgroup$ – sjasonw Apr 16 '13 at 23:43
  • $\begingroup$ @sjasonw Should it? The original question asked "why"? This answer addresses that question. Many of our greatest scientific advances were made as a byproduct of what could have appeared, at the time, like pure curiosity. Much of our current technology is derived from theoretical mathematical frameworks that had no intention of actually creating said technology. The truth is we simply cannot know what practical results pure research will have, or if it will have any, but it is required for any kind of real practical technological advancement. $\endgroup$ – Nicholas Oct 15 '14 at 14:30
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There used to be a branch of mathematics called "Surgery on Manifolds". Well it's still a branch of mathematics, but I do not hear much about it anymore. ( Maybe it's just me. )

This branch concerned itself with cutting out parts of a manifold and smoothly replacing them with other parts of a manifold.

A large part of your question relies on the question: "can I preform surgery on spacetime at an event horizon and replace it with some other physically viable interior?".

I think you would get different answers to the question depending on the person:

  • To a general physicist I think you will get the answer "No."
  • To a physicist expert in black hole physics I think you will get the answer "I hope not."
  • A mathematician would answer " You probably can."
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  • $\begingroup$ It's not clear to me how this actually answers the question of why we care about the interior of a black hole $\endgroup$ – Jim Mar 23 '15 at 18:39
  • $\begingroup$ @Jim, ... Grigori Perelman used the "surgery math" to prove the Poincaré conjecture. He found it was allowable to have certain kinds of three-dimensional singularities (linearly stretched spheres) in manifolds. A spherical cavity in a manifold sounds a lot like a black hole with nothing inside, a cavity in the spacetime manifold. Nothing inside, nothing to care about. $\endgroup$ – dcgeorge Jan 19 '18 at 4:26
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From my limited understanding of LHC physics, it has not been ruled out that particle accelerators can create subatomic-size objects which would behave very much like black holes, except for the sucking up of matter. Maybe this or other ways of creating black holes on demand could provide limited insight into their inner workings.

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Well, for one, we could be wrong: perhaps there is a way to "scope" underneath the black hole's horizon, but it seems highly unlikely. Nonetheless, if we find one, we have to revise our understanding and theories accordingly.

But of course, it is entirely logically possible (as demonstrated by the existence of consistent theories where such is true) that there is, in fact, no way, whatsoever, at all, to know empirically and thus it really could be a giant house filled with tiny gnomes.

However, none of those really answer why we'd want to know, only whether or not we can know. The reason why we want to know, I'd say, is because of curiosity. Many, if not most, if not even all, people are curious to at least some degree, though it varies considerably (quite so) from one person to another just how much. This is perennially exemplified by the child for whom being told that a certain closet and/or part of the house is "forbidden" without further explanation by its parents (and/or merely a "pacifying" explanation for which the child instinctively knows is not really the truth), only serves to titillate hir more and more into wanting to get in there and find out what is there, and who may do so or at least try to when parents or others who might deliver reprimand are not present.

And I choose this illustration somewhat carefully, for I feel it is apt: here, the Universe presents us with a similar "closed closet" in the form of a black hole. We want to know what is behind the dang door, darnnit! Open it up and spill the fweaking beans! And if we cannot get behind the door, then we will try and see if there are at least some "sneaky" "breadcrumbs" left behind - hence trying to understand/examine the Hawking radiation: it comes from the black hole after all, so maybe it also has something in it that tells us what is inside. Even moreso, if anything, this particular "closet" seems to be guarded rather closely - its barrier is actually one which in theory does let you in, but it does not let you out again, and once you go in, it condemns one to one's doom! Again, just as with the kid, the harsher the consequences that are threatened for transgressing the pact of secrecy, it is so often the case that hir curiosity cannot help but grow.

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Yes, the late Polchinski did muse that there's no interior at all, and that everything that ever "fell in" was in fact smeared across the event horizon. Let that be the working assumption here. Then may I propose a test which can be made in the region outside the event horizon to distinguish the "interior / no interior" models?

A careful examination of the gravitational field vectors in the region just outside the event horizon should resolve the issue. The field configuration should differ in the two cases, i.e. 1) A point singularity of mass M centrally located, vs. 2) The same mass M uniformly distributed within a thin shell at the event horizon.

In both cases the vector will point centrally (along a radius vector). However - and please correct me here if necessary -the magnitude of the gravitational attraction will differ in the two cases.

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  • $\begingroup$ The field configuration should differ in the two cases By Birkhoff's theorem, the fields don't differ. A careful examination of the gravitational field vectors in the region just outside the event horizon should resolve the issue. You would have to examine the Riemann curvature tensor. The equivalence principle says that the gravitational field vector is unobservable. $\endgroup$ – Ben Crowell Apr 20 at 20:03

protected by ACuriousMind Mar 23 '15 at 18:59

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