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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".

Qmechanic
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Andrew Palfreyman
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    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 ... – Dilaton Apr 10 '13 at 19:32
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    @Dilaton: meh, looks like a legit question asked out of good faith to me. The Beatles thing was a joking example. :) – Manishearth Apr 10 '13 at 19:44
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    @Manishearth maybe, we will see what happens ... ;-). I think it can nicely be answered in principle. One just has to be wary about nonconstrictive answers and comments, but maybe it will be ok – Dilaton Apr 10 '13 at 19:51
  • @Dilaton, by horizon you mean "AdS boundary" I hope? There is no reason to believe a BH event horizon has a CFT living on it. – Vibert Apr 10 '13 at 19:54
  • @Vibert yes that s what I meant. Do you have a nice answer to the question ? I deleted the comment since it was not correctly formulated. – Dilaton Apr 10 '13 at 19:57
  • I don't have a nice answer since I've never been fully involved in the question. The gist is that the BH is in contact with the rest of the universe, for example there is radiation far away from the horizon which is entangled with the states inside the BH. So you cannot just believe that general states factorize as "inside of BH" times "outside of BH" but you need a full description to get the entropy correct etc. – Vibert Apr 10 '13 at 20:21
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    Usefulness is a much stricter criterion than falsifiability. –  Apr 10 '13 at 20:37
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    How about the study can lead to new methods, techniques, ideas... that may be useful in solving other problems. – MBN Apr 10 '13 at 20:51
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    One of my former bosses would agree with @Dilaton, 'cause of course the universe is made of Monkees! – Izkata Apr 10 '13 at 20:54
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    I am sorry but reading the question again, I really dont like its tone. It reminds me too much about the negative attitudes found in news articles, the media, certain blogs, etc concerning certain parts of fundamental physics that can only theoretically or indirectly experimentally be investigated with the current technoligies. The firewall discussion produced some friutfull insights too, as Lumo for example regularely reported as something new appeared; it is not just a crazy furor as the OP alludes. So I cant holt back but I'll remove my downvote it the OP formulates the question – Dilaton Apr 10 '13 at 22:00
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    less confrontational. I am sick and tired of observing to many reports in the media the goal of which is nothing but incite "controversies" by "SUSY is in the Hospital" and similar titles etc and I dont like it if questions formulated in a similarly "controversial" style appear here on physics SE too. – Dilaton Apr 10 '13 at 22:03
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    @Dilaton. Regardless question tone reflecting hostility, it suggests OP has had little or no real exposure to the reasoning which falsifies furor over firewalls. If OP reads Lubos multiple posts from 7/12 forward at TRF why AMPS fails, OP will also appreciate the importance and reason that the number of Beatles, Monkees and even Trolls will and must stay in black holes due QM and relativity. – user12811 Apr 11 '13 at 01:14
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    I have just watched a number of CERN conference videos convened to address the issue http://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=222307 , 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. – Andrew Palfreyman Apr 11 '13 at 04:59
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    The hypotheses about the inside-the-black-hole phenomena are surely falsifiable as long as the scientist, the potential falsifier, is ready to sacrifice her life. She may just jump in and see whether there's life afterwords, if there's life afterwords, and what it is. Someone may also throw the whole planet inside the black hole so the whole mankind, including Beatles fans, will be forced to observe what's going on. Whether something is useful is subjective. – Luboš Motl Apr 11 '13 at 06:12
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    @LubošMotl: Please be nice :/ – Manishearth Apr 11 '13 at 08:52
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    @Manishearth Lumo is right from a physics point of view and the question IS written in a too confrontational manner. So nobody has to wonder if physicists working on such topics feel provoked or even slightly insulted and they have the right to express this. I understand and somehow share Lumo's feelings. – Dilaton Apr 11 '13 at 09:51
  • @LubošMotl Hi Lumo, could you write a nice answer to gauge the negative impression the style of this question leaves? A very nice answer is urgently needed here ... – Dilaton Apr 11 '13 at 09:56
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    @Dilaton: That does not justify rudeness (I edited it out) – Manishearth Apr 11 '13 at 10:01
  • @Manishearth then the question should be edited too, the question itself is rude against certain topics and physicists working on it! It should not be the case that on a serious physics site it is allowed to attack physics and the corresponding physicists that work on certain topics, whereas the physicists are not allowed to defend themself and their work against such attacks. The question the should have been closed as not constructive until it is reformulated in a less confrontational style ... – Dilaton Apr 11 '13 at 10:05
  • @Manishearth people have the right to say that such confrontational questions are not appreciated here. The question can be formulated in a nice friendly manner without changing the physics content, and somebody (obtimally the OP) should do it. You know I like questions about black holes, but not if they are formulated in such an insulting tone. – Dilaton Apr 11 '13 at 10:09
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    @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. – Manishearth Apr 11 '13 at 10:09
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    @Dilaton: Did I delete the whole comment? No. I just removed the part that was a personal attack. Please stop getting yourself worked up over non-issues. Step back for a moment and survey the situation. – Manishearth Apr 11 '13 at 10:10
  • @Manishearth the question insinuates that certain legitimate parts of physics are not physics in fact and should therefore not be allowed to be persued, this is an insulting attack against certain legitimate physics topics and the people working on them. Such things should clearly not be tolerated here. We have enough of this in the popular media etc ..., it should not be tolerated or even encouraged by denying people the right to defend themself here ! – Dilaton Apr 11 '13 at 10:12
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    @Dilaton: He's basically saying "I personally don't see why we need to know about the interiors of BHs. Could you please explain to me why we are looking for it?" It's not meant to be an insult. Any question where someone is confused about the motivation behind something will seem like an insult if you approach it that way. If I am confused about the motivation behind something, I will first say why I don't see any motivation. That's not an insinuation, it's a statement of my current thought process. – Manishearth Apr 11 '13 at 10:19
  • @Manishearth I think on a serious physics site with the targetted audience of students and grown up physicists it should not be needed that people have to justify WHY they are doing what they are doing, why it is legitimate and interesting etc.That what main stream physicists are working on IS legitimate and justified itself does not need to be questioned here in my opinion since as David Zaslavsky recently said the main purpose of this site is not to cater a lay audience ... In my opinion, a basic assumption everybody should agree upon, is that ALL of mainstream physics persued is legitimate. – Dilaton Apr 11 '13 at 11:01
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    @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. – Manishearth Apr 11 '13 at 11:10
  • That leaves open the definition of "mainstream physics". I note that questions concerning different types of multiverse get very jumped upon here, for example, on just such unfalsifiability grounds. –  Mar 23 '15 at 20:22

9 Answers9

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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.

Manishearth
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    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" – Andrew Palfreyman Apr 11 '13 at 05:06
  • @AndrewPalfreyman: No, it doesn't. We can't jump to other universes. We can jump into a black hole. – Manishearth Apr 11 '13 at 08:52
  • 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 :) – Andrew Palfreyman Apr 11 '13 at 11:14
  • @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. – dcgeorge Jul 25 '14 at 15:24
  • @Manishearth: "... modern physics is much less about predictions and much more about having a beautiful, mathematically rigorous model of the universe." Au contraire, science is all about prediction. If a model of reality doesn't allow for predictions, it's philosophy, not science. – dcgeorge Jul 25 '14 at 15:25
  • @dcgeorge Predictions are the final goal; but a mathematically rigorous model is how you get there -- most of the theoretical research is driven by model-making rather than immediate predictive ability. – Manishearth Jul 25 '14 at 15:53
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    Spacetime doesn't really end at the horizon. – Manishearth Jul 25 '14 at 15:54
  • @Manishearth: "Spacetime doesn't really end at the horizon." Perhaps not but Polchinski and Bousso have both made comments to that effect; granted, they did add "somehow" into their comments. Whether it does or not is, IMHO, the most interesting question going these days. – dcgeorge Jul 25 '14 at 18:05
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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.

user1504
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    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. – Andrew Palfreyman Apr 11 '13 at 05:07
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    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. – Luboš Motl Apr 11 '13 at 06:41
  • As I mentioned in my original question, perhaps we already inhabit such an interior. – Andrew Palfreyman Apr 11 '13 at 11:06
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    @LubošMotl AFAIK we already proved that there exist undecidable statements in most branches of mathematics, why shouldn't there be undecidable physical questions? – Bakuriu Apr 11 '13 at 11:58
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    @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. – user1504 Apr 11 '13 at 12:22
  • Hi 1504 ;-), if you could elaborate a bit about the relationship between the interior and the exterior I would like your answer even more than I already do ... I think this is at the heart of the constructive part of the question and would nicely clarify it. – Dilaton Apr 11 '13 at 19:32
  • @Dilaton Personally, I think that's a different question (and a level of technical detail that seems inappropriate for the question asked above). In any case, you're more likely to get an answer if you ask everyone in a new question, than if you only ask me here. – user1504 Apr 11 '13 at 19:59
  • @user1504 yeah, you are probably right. Maybe I'll try to formulate a follow up question but I have to think about it to not produce too much confused fluff, or Lumo (if he sees it) and probably others will give me (rightly so) a nice hairdryer treatmet because I should know better ... :-D – Dilaton Apr 11 '13 at 21:01
2

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."
Mouse.The.Lucky.Dog
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  • It's not clear to me how this actually answers the question of why we care about the interior of a black hole – Jim Mar 23 '15 at 18:39
  • @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. – dcgeorge Jan 19 '18 at 04:26
2

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.

The_Sympathizer
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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.

dcgeorge
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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?

DarenW
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    This should be a comment, not an answer. – sjasonw Apr 16 '13 at 23:43
  • @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. – Nicholas Oct 15 '14 at 14:30
0

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.

l0b0
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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.

Andrew Palfreyman
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  • 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. –  Apr 20 '19 at 20:03
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If a Paul McCartney was going to sing you a lullaby that described how the mass of a black hole is constituted, would you not want to hear it?

One argument that some people don't like is the "space and time ending at the horizon" one and that's because quasars existed for most of this eon's history, once they were common and their energy conversion processes outstrip nuclear fusion (plus the fatter the accretion disk, the more localized and useful the energy output is). Without mass concentrations of this type (black hole: in a form still unknown) such efficiency in energy conversion would be unavailable to us.

Since not everyone on this planet has enough energy it might naively seem beneficial were we able to distribute more.

And, for the long-term, should we wish to store energy, there is no better way.

Wookie
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