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Why the Black Hole Information Paradox Is Such a Problem 172

New submitter TheAlexKnapp writes: Here's a really nice explanation of the Black Hole Information Paradox for those who are unfamiliar with it. The article lays out the basic gist — that right now if you take two black holes, one made from the collapse of one type of star, and the second from the collapse of a different type, you can't tell which is which. Ethan Siegel points out that Hawking's big announcement was really just a small step heading towards a possible solution, and highlights that the paradox highlights the incompleteness of our understanding of some types of physics.
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Why the Black Hole Information Paradox Is Such a Problem

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  • by Anonymous Coward
    If I take two random iPhones from random strangers, I won't know which iPhone came from which stranger just by looking at the outside of them. But we don't call that a paradox.
    • Re:So? (Score:4, Insightful)

      by Fragnet ( 4224287 ) on Sunday September 06, 2015 @10:13AM (#50466999)
      You do know. There's information on and within both phones that's a record of which owner they had.
      • Re:So? (Score:4, Interesting)

        by Leuf ( 918654 ) on Sunday September 06, 2015 @10:18AM (#50467027)
        What if I make two different ice sculptures from the same source of water with the same mass but different shapes. After they melt it's impossible to tell which one was which. That's not a paradox.
        • Re:So? (Score:5, Insightful)

          by willworkforbeer ( 924558 ) on Sunday September 06, 2015 @10:27AM (#50467079)
          Obvious non-physicist here, but in absolute terms, information theory says that the original systems' (ice sculptures) information -- every particle's directions etc. was not destroyed, though it may be terribly difficult to ascertain by us at a later time.

          I don't pretend to understand why, but the fundamental premise is that information is a conserved property.
          • I'm not an astrophysicist either but I think that your mention of ice sculptures hits the nail on the head. When an ice sculpture melts the organization of the ice vanishes. But it also cools the air. The information content of the cold air increases. So the answers is the system preserves the information. Likewise when something is sucked into a blackhole it contributes it's mass but the organizational information it had (position, momentum, internal quantum states) is gone. On the otherhand as it's

            • So when the black hole evaporates by giving off Hawking radiation, who's to say the information (albeit all mixed together) isn't coming out in the particular spatio-temporal pattern of emanation of the radiation?

              Sure, the radiation seems (and is effectively to any observer) random, but it is well known that a random bitstring (k-random bitstring) can encode information, and in fact can be the most compact encoding of information.

              Random simply means you don't have the means or supplementary information to d

              • Random means exactly that. If there is a pattern how ever hidden it means it is no longer random.

                You can't take a melted ice sculpture and remake it .. It won't be exact. Only close.

                • The Kolmogorov-Chaitin definition of random says something to the effect that if there is no process simpler/shorter than the process which produced the sequence/arrangement of events which could predict what that sequence/arrangement of events would have been (and would evolve to next), then the sequence/arrangement is random. It is equivalent to saying that the process which produced the sequence/arrangement is maximally complex.

                  So if there is a deterministic pattern but it (or its originaing/generating p

              • Indeed when the information is reconfigured it is not erased, this is the mistake Hawking made thinking it was gone for good.

                Another interesting debate is Jack Semura and Todd Duncan who think information is erased in entropy (Laynes Law? definition of words problem?)
                https://www.quora.com/Are-Todd... [quora.com]
                Another article on the subject:
                http://olsonb.com/articles/jac... [olsonb.com]
                • by Bengie ( 1121981 )
                  Hawking never said what was happening, he only made an observation and could not think of a mechanism with which to allow information to be available.
          • So - when the pretty bubbles in my bath go down the plughole, is there some 'information' in my cesspit that tells me how they were configured? What's 'entropy' anyway?

          • by lgw ( 121541 )

            Obvious non-physicist here, but in absolute terms, information theory says that the original systems' (ice sculptures) information -- every particle's directions etc. was not destroyed, though it may be terribly difficult to ascertain by us at a later time.

            I don't pretend to understand why, but the fundamental premise is that information is a conserved property.

            This is just not true in quantum mechanics, at least if we're talking about what it's possible to observe (not merely practical). The QM definition of entropy is information loss (or that's my best attempt to translate the math to English - I've only studied QM a bit). QM entropy is a bit different from thermo and statistical entropy, though at large enough scales it all works out the same.

            There's a remarkable new idea linking black holes to quantum entanglement, going under the name ER=EPR [wikipedia.org] that, if true,

            • by Bengie ( 1121981 )

              The QM definition of entropy is information loss

              Yet quantum theory explicitly states information cannot be lost. Information not being lost is a corner stone of quantum theory. Your simplification is outright wrong.

              • by lgw ( 121541 )

                We're both technically correct (the best kind of correct). Entanglement means you can know everything (everything it's possible to know) about a system and yet know nothing about it's individual parts. As systems evolve they become entangled, and the information about the individual components is lost. Sure the total "count of bits of information" is conserved, but that's just a constraint on the allowed states of the system, and the actual details become increasingly vague.

    • Re:So? (Score:5, Interesting)

      by michelcolman ( 1208008 ) on Sunday September 06, 2015 @10:20AM (#50467033)

      Of course you are simplifying down to a ridiculous level, and I do understand that information is a pretty powerful concept in physics that carries a lot more weight than you would think, but still, I did not find anything useful in this article. The title of the Slashdot Summary was very promising: "why the black hole information paradox is such a problem", so I was hoping to finally see this question answered, but no, same old same old. Information gets conserved in all experiments we do outside black holes, so we kind of assume this must be some cosmic requirement (why?), and for some reason which is never properly explained we just can't accept that black holes would destroy information. Because... well, why exactly? Why is it such a problem that information would simply disappear in a black hole?

      Why would it be such a problem if information did simply disappear? Oh my god, entropy might go down in an isolated system, it's the end of physics as we know it! No it's not. Entropy is just a trick that works because of the statistically enormously small probability of it going down in a large macroscopic system without spending energy on it, but isn't that really all it is? Why does it get treated like one of the most important truths in the universe?

      • by Anonymous Coward

        Of course you are simplifying down to a ridiculous level, and I do understand that information is a pretty powerful concept in physics that carries a lot more weight than you would think

        In physics, sure. In biology, not so much. One would expect random events like mutations to decrease information and be a destructive process. That's what you need to turn a single-celled organism into a man - you need lots more information and you need it to be highly ordered. Where did it come from? How did it move from a less highly ordered state to a more highly ordered state when nothing else we observe does this on its own?

        These questions are taboo because they've become associated with theolo

        • by Anonymous Coward

          In physics, sure. In biology, not so much....These questions are taboo...

          No, in this particular case it is not so much taboo as just fundamentally misunderstanding things. Physical information in physics has a specific definition about the number of possible states within a system. Hand waving about the complexity of different stages of life doesn't show any impact in the amount of physical information in the system.

        • How did it move from a less highly ordered state to a more highly ordered state when nothing else we observe does this on its own?

          That's easy to answer: Energy was expended in moving from a less to a more ordered state. The general rule is that entropy almost always increases.

      • I also found the article's use of "paradox" and "problem" to be a bit grating. We are observing something here we don't understand, and maybe that's anathema to a physicist who thinks we already understand everything (hint: we don't), but it's not that shocking to me.

        However with the last two paragraphs of the article he clears things up a bit:

        Whenever there’s a conflict between what our best theories predict [...] that’s an omen of scientific advance. That paradox is such a problem because it tells us that something about our present understanding is, in some way, incomplete. Is there a new law of physics? Is there a new application of the currently existing laws that we’ve missed? Are these quantities not fundamentally conserved after all? Is the information really encoded in the final state somehow? Will quantum gravity eventually make this all clear?

        We hope to have the answer to this. But in the meantime, this paradox means we have a problem, and hence that we have more to learn. And for anyone curious about the scientific truths of the Universe, that’s an incredible thing: evidence that there’s still a whole lot more to be figured out.

        Well okay then. It's not so much a paradox or a problem as a (not totally unexpected to me) indication that our physics theories do not yet account for everything goi

      • Information gets conserved in all experiments we do outside black holes, so we kind of assume this must be some cosmic requirement (why?), and for some reason which is never properly explained we just can't accept that black holes would destroy information. Because... well, why exactly? Why is it such a problem that information would simply disappear in a black hole?

        Because the idea that physics is the same everywhere is one of the assumptions that our physics, especially relativity, is based upon. By stating that the physics at point A is the same as at point B allows us to make some assumptions and delete some variables via symmetry. This gives us the general theory of relativity not to mention science itself, which so far has been shown to be upheld. If the the physics in a black hole is different from the physics outside of it, then it most certainly is the end of

  • This is some high brow shit for Sunday morning
    • Well that article sure was not worth reading. But still it's an interesting question. One thing I don't understand is why the matter entering the black hole can't leave it's information behind in the conventional universe. I don't think that it is true that information is attached to the mass itself. As I understand it information content is a property of the universe. My understanding is that if you were to write down the position, momentum (or rather the density matrix, since things aw Quantum) ans an

      • Well that article sure was not worth reading.

        It's an Ask Ethan, just not on medium.com this time. Gotta wonder how the guy got slashdot to be his personal PR machine.

  • by Anonymous Coward

    Yea, I don't see why this is a problem, anymore than any other thing in physics today.

    Conservation of information is a quantum property. Black holes are a concept from general relativity (or a poor approximation in classical mechanics). Wake me when a verified theory of quantum gravity exists where this problem still exists... until then, this isn't news.

    • by Bengie ( 1121981 )
      Conservation of information is a property of causality.
      • But would destroying information violate causality? Would you be able to do something contradictory like the grandfather paradox or something like that by destroying information? I really don't see the problem.

        • Re:I RTFA, but... (Score:4, Interesting)

          by Bengie ( 1121981 ) on Sunday September 06, 2015 @12:13PM (#50467585)
          For every unique cause is a unique effect. You can know what the cause was based on the effect. If the effect disappears, you can't know what the cause was. Anyway, if information is destroyed, that means the energy it represented is destroyed which means it cannot contribute to the mass of a system. It's contradictory to say the mass of a blackhole increases if the information is destroyed. A blackhole is created by too much information and it is itself a blob of information. The question isn't "why does a blackhole destroy information", but "what mechanism does a blackhole use to allow information to be recovered".
          • by Anonymous Coward

            For every unique cause is a unique effect.

            This is not a requirement of causality, and nor is a requirement of most physics, only a consequence of current theories. Causality only requires that a cause precedes an effect, with a more complicated definition of "precede" in relativity. There is no uniqueness requirement, and macroscopic physics doesn't follow this, yet follows conservation of information.

            The question isn't "why does a blackhole destroy information", but "what mechanism does a blackhole use to allow information to be recovered".

            No, only if you make a bunch of assumptions. The whole point of the problem is that it is still open ended, even if there are some proposals to de

      • Re: I RTFA, but... (Score:4, Informative)

        by Anonymous Coward on Sunday September 06, 2015 @10:29AM (#50467095)

        No, it is more subtle than that. Conservation of information is closely tied to time reversal symmetry and energy conservation as a result of Noether's theorem. But in general it is not tied to casaulity.

    • Yea, I don't see why this is a problem, anymore than any other thing in physics today.

      I think what they're trying to say is: "Black Holes Matter".

      *ducks*

    • (Assuming I have this correctly.):

      A rule of physics had been that information is not destroyed. (It can be scrambled beyond recovery by any reasonable process, but it's still there.)

      Black holes make the information inside them inaccessible - no message gets out. Ok, it's still there but you can't get to it. All you can measure about a black hole is its mass, electric charge, and spin. All those other quantum numbers get hidden.

      But Hawking radiation - according to the first formulation - is vacuum virtua

  • by Anonymous Coward

    Leonard Susskind discusses this in a talk he gave a few years ago.
    https://www.youtube.com/watch?v=2DIl3Hfh9tY
    Pretty entertaining.

  • So, presumably we can't tell whether any particular black hole is artificial either?

    Shame, artificial black holes look like such a wonderful way for an advanced (for really advanced values of advanced) civilization to get both power and waste disposal taken care of....

    • "no hair" Theorem (Score:4, Informative)

      by l2718 ( 514756 ) on Sunday September 06, 2015 @10:23AM (#50467057)

      In general relativity (our theory of classical gravity, without quantum effects), there are several "no hair" theorems, saying that several types of black holes are completely determined by a few overall parameters (say mass, charge and angular momentum) and without regard to their history.

      We don't yet have a theory of quantum gravity, so we don't know if the quantum state of a black hole does retain information. It probably has to, but this is not understood. By the way, in any case classical GR would be an excellent approximation except in the case of very small black holes, so any information retained will not be actually accessible.

  • Are there any paradoxes concerning StartsWithABang and a sockpuppet?

  • Big Crunch (Score:5, Interesting)

    by CanEHdian ( 1098955 ) on Sunday September 06, 2015 @10:27AM (#50467075)

    Black holes are just a toy.

    Suppose gravity wins out after all, and the universe ends in a Big Crunch. One or a few Planck second(s) before the singularity, all the information in the universe should still be there. How?

    Suppose entropy wins and the universe ends in heat death. For bonus points assume all protons have decayed. All the information should still be preserved. How?

    • by Bengie ( 1121981 )
      Taken as a whole, the past present and future of a system can be described by its information at any point in time. Just think of forensics. It uses present information to figure out the state of the past. When talking about information at the Universe level, you can figure out the state of the past Universe perfectly assuming you have perfect access to all of the present information in the Universe.

      The other way to look at it is destroying information destroys the energy it represents. Since energy also
      • the past, present, and future of a system can be described by its information only if you store that information somewhere: example taking a video tape of a soccer game means you can relive the soccer game over and over again, but only because you stored that information on the video tape or SD card. So where is the universe storing all this meta information about information?

        Once you delete files off your hard drive, the information becomes scrambled - but you could only get the data back if you video
    • I think the term "heat death" should actually be renamed to "equilibrium death"... When the universe is in equilibrium due to the second law of thermodynamics, there aren't any cold spots or hot spots you just have equilibrium. Think of it: if it was all hot, it wouldn't be hot - it would be neutral.. all the cold spots would even out with the hot spots. Frozen planets would no longer be frozen, they would equal out at one temperature, hot planets would no longer be hot, they would all even out.

      An inte
      • Yes, equilibrium death is a better term. It's the universe in a uniform state. If it's uniform, there are only a few parameters needed to describe the universe.There is no way to "hide" more information than that within the universe itself.
  • Just because we cannot see the internal degrees of freedom does not mean that they do not exist. Someone who is inside the black hole can detect events that fall past him/her/it that we cannot see from outside the black hole. Does a tree falling in the woods make a sound if nobody was there to hear it? Yes.

    • by Rei ( 128717 )

      And we, outside the black hole, will "see" everything that the person inside the black hole sees as the black hole evaporates - albeit in a form mangled beyond recognition by the extreme forces at play.

      They fall in, and in their reference frame, they're chasing an event horizon (the apparent horizon) that always recedes away from them and which they never hit, until they're torn apart. We see them approach, see their time slow down, get dimmer and dimmer and more and more distorted, but never really "get th

      • I've considered this as well, though I don't think the person has to constantly chase the event horizon. In holes large enough, they can exist just fine inside of it, with the horizon constantly chasing them and experience less gravitational sheering than you experience right now on Earth. The problem I have with common black holes evaporating is that it would require the universe to entropy to the degree that background radiation doesn't get in the way of these holes evaporating.

        So it could be a while, i

    • by Intron ( 870560 )

      There is no "inside the black hole". It's a singularity - no volume, no structure. The event horizon is a boundary, not a container.

    • Just because we cannot see the internal degrees of freedom does not mean that they do not exist.

      If internal (hidden) degrees of freedom are impossible to detect then they are metaphysics and do not exist in the real world. If hidden degrees of freedom are merely impractical to detect, or beyond our science to detect, then they may exist. If hidden degrees of freedom have predictive value then they probably exist. However as of today, no theory relying on hidden degrees of freedom has shown any predictive value, in spite of huge numbers of research dollars having been tossed down that hole. Hidden degr

  • AFAIK from the point of view of an outside observer, it takes forever for a falling object to reach the event horizon. Given this, no object can ever be seen actually crossing said horizon; even the original star's collapsing matter is still just above it (or, alternately, it's just short of collapsing beyond it's Schwarzschild radius), merely very hard to see due to gravitational redshift. So... it seems to me that there's no two points in time for any outside observer where said observer could say informa

    • by Intron ( 870560 )

      Since the front part is traveling at the local speed of light, the back part of the infalling observer can't detect the front part reaching the event horizon.

      • by Bengie ( 1121981 )
        I used to think the event horizon was when acceleration was the speed of light, but it's not. It's when the escape velocity is the speed of light. An ever increasing blackhole would have an event horizon where the gravitational acceleration of the event horizon would asymptotically approach zero. All the event horizon means is you cannot have enough initial energy to gravitationally escape the blackhole.

        I don't know the math, but all they say is you can't gravitationally escape the blackhole, but they do
        • At some point an object make look as if it is moving away faster than light, meaning it is out of your light cone. You can still attempt to approach it and get closer to where it was but not where it is, but you'll never interact with it again.

          If an object is out of your light cone then you may not interact with it so it cannot appear to be moving away faster than light. Not only that (correct me if I am wrong please) the object always was out of your light cone, and you were never able to interact with it.

    • by Bengie ( 1121981 )

      Yet scientists claim passing the horizon should be unnoticeable.

      Because the event horizon is relative and different for all observers. One observer may see someone approaching the event horizon, but the one approaching the horizon may see that the horizon is still far away. "Passing" the horizon would be unnoticeable for the in-falling observer because from their frame of reference, they haven't passed it yet. Of course I don't think any observer would ever see any other object reach the event horizon, yet alone get past it.

  • The credit should go to Gerard T' Hooft for the holographic principle and this work for which he received a Nobel Prize. Hawking is just slurping up media credit because he's a household name. Let's not get into the whole Edison/Tesla thing and get it right folks.
  • I mean, with a sufficient supply of donuts you can assign a couple of cops to each star, and when they turn to black holes you can tell which is which as long as you don't lose sight of them...

    • by Pikoro ( 844299 )

      But as soon as they turn black, the cops will shoot them so I guess we'll never know.

  • If a black hole can be modelled as below from centre to event horizon a) A point gravity source b) A solid sphere with mass distributed evenly c) a distribution curve of mass from event horizon to centre d) a 2d shell and empty middle. Then won't behaviour of objects near eh be different and detectable ?

  • Why is it that the article can state things like "the known laws of physics break down" inside a black hole, yet insist that this particular law of physics, the conservation of information, shouldn't also break down and therefore results in a paradox.

    Who gets to decide which laws break down and which ones don't?

    Why isn't it all or nothing?
    • because the paradox is OUTSIDE the black hole. From our point of view, everything slows going into a black hole until the event horizon is reached. Information would be stuck there except the hole evaporates by hawking radiation (virtual pairs formed having one particle free to leave)

      • because the paradox is OUTSIDE the black hole. From our point of view, everything slows going into a black hole until the event horizon is reached. Information would be stuck there except the hole evaporates by hawking radiation (virtual pairs formed having one particle free to leave)

        Since when does a new particle going into a black hole mean it is losing mass? It seems to be gaining mass to me as it now has one more particle. Perhaps they don't actually evaporate, or they don't do it because of extra things falling into them.

  • The article says that all that is preserved, that is, information still obtainable, from a black hole is mass, electric charge, and spin (or angular momentum). I think also their plain old momentum is also preserved. What's interesting is that these are the first properties of matter to be discovered and understood in the study of physics. Newton describes mass and conservation of momentum, including angular momentum, and Benjamin Franklin discovered conservation of electric charge. I wonder if it's jus

  • "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka' but 'That’s funny...'" --- Isaac Asimov

    • When they discovered we were not designed by Jesus and evolved from ape like creatures, did they say "eureka", "that's funny", or "oh shit". I'm not sure.
  • How can you fit more information into an event horizon than it can hold? Isn't an event horizon a given size, a 2D spherical surface, existing in a universe that is limited by the resolution of the Planck distance? So how can I encode as much, about as many in-falling particles, as I like into it? If Hawking does not explain that adequately his theory is nonsense. Where he is going to find room to move to do this is a mystery, perhaps he will declare it to not be 2D after all and rather, a fractional dim

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