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Information loss paradox

  1. Mar 16, 2008 #1
    can someone explain information loss paradox in layman terms without using any quantum physics terms...i read in some sites but i couldnt understand as they are explaining using some jargon terms...
     
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  3. Mar 17, 2008 #2

    Fredrik

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    General relativity says that black holes are kind of like enormous elementary particles. A black hole has a mass, a total electric charge, an angular momentum, and no other properties whatsoever. If that's true, then all the information about anything that falls into a black hole is gone forever.

    Quantum mechanics on the other hand says that if you know the initial state, you can calculate the final state, and vice versa. So if everything is described by quantum mechanics, this must hold even if the matter described by the initial state collapses to a black hole.

    That's the paradox. GR says the information is gone. QM says it isn't.

    A black hole will eventually evaporate through a process called Hawking radiation. The black hole will lose mass by emitting radiation until it's gone. The radiation is the final state of the matter that collapsed to the black hole or later fell into it. So if QM is correct, then the radiation must somehow contain all the information that's needed to reconstruct the initial state. (E.g. if you throw your hard drive into a black hole, all the information about the pornography you downloaded is present in the radiation). That would be pretty surprising, because we're talking about a kind of thermal radiation, a lot like the infrared radiation you can feel from a hot stove. That kind of radiation is normally completely random, but if QM is correct, the quantum states of particles in the radiation from the black hole must somehow be correlated so that the information is present in the correlations.

    (Correlations in the Hawking radiation is not the only resolution of the paradox that has been proposed. It's just the one that's the easiest to understand).

    Scientific American article by Leonard Susskind: http://staff.science.uva.nl/~jdeboer/gr/susskind.pdf
     
    Last edited: Mar 17, 2008
  4. Mar 17, 2008 #3
    Thank you Fredrik for your clear explanation...I have one doubt...the only difference between blackhole and other other heavenly bodies like earth,moon is the heavy gravitational field...if we drop some thing like my hard drive on earth it will be on the earth's surface and if we drop it on moon it will be definetely on the moon's surface...likewise the same hard drive should be on the blackhole's surface,isn't it? then how GR says information is lost ...information is on the blackhole's surface,am i right?only difference is that hard drive cant come back to our world since the escape velocity is greater than c....and after some time because of hawking radiation my hard drive will come back to our world i think else law of conservation of energy would be violated...clear my doubts..
     
  5. Mar 18, 2008 #4

    Demystifier

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    I do not think that it is correct. GR is a causal theory. In principle, from the exact state of a classical black hole at a FINITE time after the start of collapse, you can reproduce the initial conditions of matter that were present before the collapse. (This is analogous to the fact that if you take a porno picture and cut it in a million of pieces and mix them all up, your mother/girl friend/wife can still reproduce the original picture. :biggrin: )

    Now let me try to explain the information paradox in simple terms by myself.
    Quantum black hole creates pairs of particles. One of the members of the pair escapes from the black hole, while the other falls into the black hole and remains there. During this process the outside energy increases (due to the particles that escape), so the total energy conservation implies that the inside energy must decrease. Eventually, the energy of the black hole drops to zero. GR says that the black hole with zero mass contains nothing, i.e., that its information content is zero. On the other hand, this zero mass black hole should still contain the information carried by particles that fell into the black hole and remained there, i.e., the information content should not be zero.

    Now, what is the solution of the paradox? To be clear, there are many proposals and there is no consensus among experts. In my opinion, the solution is that the particles that fall into the black hole are virtual (not real), so the information carried by them is not physical. For the details see
    http://xxx.lanl.gov/abs/0708.0729 [Eur. Phys. J. C 54 (2008) 319]
     
    Last edited: Mar 18, 2008
  6. Mar 18, 2008 #5
    From the Leonard Susskind article:
    Since when is this true?
    QM and classical are not the same thing! (edit; in regard to microreversibility)
    If a deck of cards are randomly shuffled to the point of taking advantage of the HUP within QM then how can there be any “microreversibility” to reconstruct the data within the context of QM?
    If we are to take the HUP as serious and correct part of QM that denies that the classical as ever being able to produce a complete explanation of reality; how are we to expect QM to apply microreversibility without accounting for the principle contained in HUP.

    Is Susskind trying to tell us for situations outside a black hole that Hawking and ’t Hooft have abandoned the HUP in favor a Super-deterministic version of BM.
    That is to say toss an unopened deck of cards into the Sun and by looking at the end products of the incineration we could determine the information printed on the cards to tell if it was a Poker or UNO deck of cards?
    In principle that may be true for a Classical interpretation.
    But QM has an uncertainty principle that would make discovering that past information just as impossible to determine as predicting both the future location and momentum of an individual particle.

    AFAIK the HUP still applies to QM, and this “paradox” being unique to black holes vs. information detail lost into the sun makes no sense.
     
    Last edited: Mar 19, 2008
  7. Mar 18, 2008 #6
    Perhaps the information is stored on the surface of the black hole as waves generated by what and how objects fall onto it. Afterall, two black holes merging are not expected to instantly form a perfectly symmetrical sphere, so there would be peaks and valleys associated with how they merge, right? Then the radiation would be correlated to these waves on the surface which would reflect information about what fell in. Does this sound plausible? just guessing.
     
  8. Mar 18, 2008 #7

    Ken G

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    I think the problem that underlies all these primarily philosophical issues is that people continually treat any branch of physics, be it quantum mechanics or classical mechanics, as if they were intended to be, or succeeded as, axiomatically complete descriptions of reality. But they weren't, and they aren't. In quantum mechanics, there is no evidence at all that we gain insight into a macro system by describing it with a "wave function" (except insofar as one recovers the classical principles that underlie quantum mechanics), and in classical mechanics, we already know that the concept of an "exact trajectory" is inconsistent with precise observation. But above all, both of those theories are reversible in time, so we had to invent an entirely new approach, thermodynamics, to treat very complex systems. Now, who among us can claim the concept of "irreversibility" is not vastly effective at treating much of reality (and allows the patent office to duck the endless stream of perpetual motion inventions)? Yet this concept appears nowhere in either classical or quantum mechanics. If someone says, "there's no such thing as irreversibility in principle", I say, "you have a different interpretation of what a principle is than I do-- because I think irreversibility is as valuable a principle as any!"

    So I agree with RandallB that I don't see "what's the big deal" with information loss in black hole physics, but I don't distinguish quantum or classical mechanics in that regard-- they are all snippets of our descriptions of reality that simply don't describe everything we need to understand about our universe. I just don't see the value of arguments that start out "assuming quantum mechanics is a complete description...", any more than a physicist in 1900 was making any sense when they said "assuming Newton's laws give a complete description...". This isn't what physics is for, it's not a "philosophy engine", it's a way to predict systems-- and the fingerprints of the physicist are all over the result in the axiomatic structure chosen.
     
    Last edited: Mar 18, 2008
  9. Mar 18, 2008 #8

    Fredrik

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    Actually, almost the opposite is true. The gravitational field is about the only thing a black hole has in common with a star.

    A black hole doesn't have a surface.

    The hard drive can't come back in one piece from the region inside the event horizon, and its mass must eventually come back in the form of Hawking radiation. You got those details right, but it's still far from obvious that the information stored on the hard drive will be present in the radiation.

    Also, GR doesn't say anything about Hawking radiation. According to GR a black hole will stick around forever, and can only get bigger, not smaller. So GR definitely says that the information is gone. Hawking radiation is a prediction of another theory, quantum field theory in curved space-time, which is a partial union of quantum mechanics and GR. That prediction doesn't solve the problem, it merely suggests a possible way out: correlations in the Hawking radiation. If the information is contained in the Hawking radiation, we still have to explain how it got there. One mechanism that has been suggested is quantum teleportation.
     
    Last edited: Mar 19, 2008
  10. Mar 18, 2008 #9

    Fredrik

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    The geometry of space-time around the black hole will contain information about the matter that collapsed, but it won't contain all of it.

    I don't think the idea of a zero mass black hole makes sense.
     
  11. Mar 19, 2008 #10

    Fredrik

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    He didn't say that they are. I haven't read his article recently, but it seems to me that all he says is that time evolution is reversible in both theories.

    I don't think the claim is that we can really reconstruct the data. (If it is, I agree that it must be wrong). I think the claim is just that the quantum state of the matter after the black hole has evaporated contains all the information we would need to calculate the initial state. That state is of course unknowable. We can't even measure the direction of an electron's spin, so how could we measure the quantum state of the Hawking radiation emitted by a black hole?

    I think he's just saying that time evolution is given by exp(-iHt), which is a unitary (and therefore invertible) operator. I assume he's aware that it's not possible to measure a quantum state.

    Since classical and quantum mechanics both say that time-evolution is reversible, they both say that no information is lost when you throw something into the Sun. But GR says that nothing can ever leave a black hole. There isn't even any Hawking radiation in GR.
     
  12. Mar 19, 2008 #11

    Fredrik

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    The point if of course that if information really is lost, then QM is wrong!
     
  13. Mar 19, 2008 #12

    Demystifier

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    It does, at least formally, as a mathematical limit of the black hole with the mass M->0.
    See, e.g., the discussion around Eq. (3) in
    http://xxx.lanl.gov/abs/hep-th/0402145 [Int.J.Mod.Phys. D14 (2005) 2257]
     
  14. Mar 19, 2008 #13
    If we say information is lost in blackhole since it is not in our world, which would also mean blackhole is also lost from our world... I personally believe that information is not lost..we say a thing is lost only if
    1. we cant get it back
    2. we dont know the location
    In this case, we know the location which is the blackhole then why we say information is lost and have to worry about it...I think there is no problem.. we are creating an imaginary problem and trying to solve it...a paradox arises if a theory gives unbelievable results and also contradictory to reality...either theory should be wrong or paradox should be wrong...
     
  15. Mar 19, 2008 #14

    Demystifier

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    You missed the point. The black hole evaporates, so at the end there is no black hole in which information could be located.
     
  16. Mar 19, 2008 #15

    Demystifier

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    Any Cauchy surface (which includes both the exterior and the interior of the black hole) will contain ALL information about the matter that collapsed.
     
  17. Mar 19, 2008 #16

    Ken G

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    But I'm saying that we don't have "right" and "wrong" in physics, and never did-- all we ever had or have is "is successfully chosen to treat this situation". That's why we still use Newton's laws far more often than Einstein's treatment in published papers on dynamics. The question is not "is quantum mechanics right", it is, "why on Earth would we ever want to put that question as if it was a meaningful absolute?" Physicists throughout history made that mistake: "assuming our current understanding is complete, we may infer..."

    Using theory to try and decide if information is lost in black holes is mistaking physics for a "philosophy engine". Make a testable prediction.
     
    Last edited: Mar 19, 2008
  18. Mar 19, 2008 #17

    Fredrik

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    That doesn't sound right to me, but I haven't been able to find an argument that proves you wrong, so maybe you're right.

    Anyway, I've been reading the Wikipedia article and parts of the references in it, and it seems that even if I was right when I said that "GR says the information is gone" (and I may not have been), that isn't what the experts have in mind when they talk about the information paradox. The paradox is that when a black hole has evaporated completely, the final state doesn't contain all the information we would need to calculate the initial state.
     
  19. Mar 19, 2008 #18

    Fredrik

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    Yes, theories aren't really "right" or "wrong", they are only consistent with experiments to various degrees. Anyone who knows anything about physics knows that. But the thing is, quantum mechanics has so far been 100% consistent with all experiments. It would be a major discovery to find something that can't be described by quantum mechanics.

    It isn't a mistake when these guys are doing it. It's more like assuming that the square root of 2 is rational in order to prove that it isn't. These guys are just trying to do verify that there's a contradiction between theories, and then learn something from it.

    This isn't any different in principle than to study the contradiction between Maxwell's equations and Newtonian mechanics. Special relativity could have been discovered even before the Michelson-Morley experiment just by noting that there is a contradiction between the best theories of that time, and by trying to answer the question "what do we have to change to avoid the contradiction if we don't change Maxwell's equations?".
     
  20. Mar 19, 2008 #19

    Ken G

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    You mean all experiments for which it can be used to make a testable prediction. We are not talking about that here, this is my point. What is the test on the table here?
    Thermodynamic reversibility can't be described by quantum mechanics alone. I think you mean it would be a major discovery to find something for which quantum mechanics makes a testably wrong prediction. If so, then I agree. But that in no way suggests we should use quantum mechanics to build philosophies around things we cannot even think of a practical test.
    My contention is that they are only learning something if they take their theories to be something they are not-- something other than a means for making testable predictions or ways of organizing existing data.
    But to examine that, we have to ask, what would have happened if the speed of light was so great that there was never an experimental way to decide between Newton and Maxwell? Of course the conclusion would have been what it actually was-- that Maxwell's theory only applies in the frame of the ether, we just don't have the experimental precision to tell what that frame is. But my way of thinking would have led instead to the conclusion that the contradiction is as irrelevant as the number of angels that can fit on a pin-- until there is a difference in a testable prediction. I'n not saying there's no point in looking for inconsistencies, I'm saying there's no point in looking for untestable inconsistencies- because consistency is only a requirement if one thinks that our theories are something more than what they are. I would say that inconsistency is only a factor to take into account when choosing the theory to apply, and I cite as evidence all the times we use Newton's laws instead of Maxwell's equations to treat electrodynamical systems of slow-moving charges.
     
    Last edited: Mar 19, 2008
  21. Mar 19, 2008 #20
    I agree that the concept of “irreversibility” appears to be so evidently true that it should be considered an important principle to find in any theory expecting to be “complete”.

    But I disagree that the idea appears nowhere in quantum mechanics.
    IMO "irreversibility" is an inevitable consequence, and can be directly derived from the HUP in QM.
    Just as it defines our inability to specifically predict both the future location and momentum of an individual particle, the HUP tells us we cannot predict with certainty future events or information.
    In the same manner, QM in principle would have no expectation of being able to define a single specific past configuration necessary to establish our current reality. It seems to me that in principle QM uncertainty cuts both ways; no predetermined future from our current state, and our current state cannot be considered preordained from a deterministic past. Therefore, the reversibility of information is just as uncertain as the predictability of the future.

    As theories go, both Classical and General Relativity might allow for such a deterministic view in principle within their theories, but neither have resolve the simultaneous prediction of both location and momentum for an individual particle and only some views of GR (such as Hawking QFT in curved space-time) allow Blackholes to “evaporate”. The argument that Susskind is promoting in his article may apply to defending such a principle in classical or GR theories, but I find the argument that the principle applies to QM as simply misguided very likely wrong and certainly not demonstrated to be more complete than CI-QM. At least to the extent that CI-QM has not been convincing Wrong in the

    As to those that begin by "assuming quantum mechanics is a complete description..."
    That misrepresents what Niels Bohr said and meant by CI-QM. His point was that QM was as complete as is possible, and that no other description (such as Einstein expected) could ever be more complete. IMO no one has established a more complete interpretation of QM, which would eliminate the uncertainty principle to allow the reversible reconstruction of destroyed information.

    Although I personally do not think QM is complete, uncertainty and "irreversibility" of information loss are principles I believe will survive a more complete theory that might replace it someday. But that’s a personal opinion, probably based on my belief in free will.
     
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