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B Collapse and unitary evolution

  1. Jul 28, 2018 #1
    In his book "the black hole war" Susskind writes that in quantum mechanics information cannot be lost because it is unitary. As collapse is not unitary does he say that collapse never occurs?
     
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  3. Jul 28, 2018 #2

    Demystifier

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    Not necessarily. In the usual collapse interpretation of QM, the non-unitary evolution happens only at the instant of measurement. So even if one accepts the collapse interpretation associated with measurement, one still expects that evolution should be unitary before measurement. On the other hand, black hole evaporation seems to imply a non-unitary evolution before the measurement.
     
  4. Jul 28, 2018 #3
    my question is about Susskind's point of view.
    According to him
    "Hawking's conclusions violated one of the most basic scientific laws of the universe, the conservation of information"
    it seems that he is talking about unitarity. is unitarity a basic law that exists before the birth or a black hole during its life and during its evaporarion?
     
  5. Jul 28, 2018 #4

    PeterDonis

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    I believe Susskind favors the MWI, which is a no collapse interpretation, so he would probably say that yes, collapse never occurs.

    However, as @Demystifier says, the issue with black holes does not depend on whether collapse occurs; it is present even in "no collapse" interpretations like the MWI.
     
  6. Jul 29, 2018 #5
    susskind does not use the word collapse.
    he recalls that black holes evolution can be equvalently described by two
    theories GR in 3+1 dimension and an ADS quantum theory in a different space.
    he concludes that Hawking lost the war because information is never lost in quantum mechanics
    is there a bias in his conclusion?
     
  7. Jul 29, 2018 #6
    i think that wigner's friend provides an argument against objective collapse.
    when wigner chooses something to be measured there is a quantum object in a spacetime region outside him.
    wigner himself is not a quantum object.
    suppose that there is a collapse in this spacetime region.
    wigner's friend can do another choice and include wigner as a quantum object in a larger spacetime region
    for him wigner is in a superposition and collapse could occur later when he will decide to do delayed measurement
    may be collapse is observer dependent whereas unitarity is more fundamental
     
  8. Jul 29, 2018 #7

    atyy

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    There is only a bit of bias. The AdS/CFT correspondence is one of the most stunning developments in theoretical physics. It may provide a complete theory of quantum gravity in some universe. The caveats are that although there is much evidence for the correspondence, it remains unproved; also it may not be able to describe qusntum gravity in our universe.
     
  9. Jul 29, 2018 #8
    i have of course no answer to the measurement problem but i think that collapse is a wrong concept.
    mw interpretation is not the only possibible way to avoid it. collapse only concerns individual measurements.
    Statisrical ensemble interpretation does not need collapse and unitarity is safe.
     
  10. Jul 29, 2018 #9

    atyy

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    The statistical ensemble interpretation has collapse. If you wish to preserve unitarity, then you should look at either Bohmian mechanics or the Many-worlds interpretation as apptoaches to the measurement problem.
     
  11. Jul 29, 2018 #10

    PeterDonis

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    Why not?
     
  12. Jul 29, 2018 #11
    What do you think about Susskind's sentence: in quantum mechanics informationd can never be lost?
    this is the core of his conclusion

    i read this
    "The first merit of Maldacena's work has been to solve the paradox of information, at least in the particular case of a black hole in AdS5. The latter is indeed equivalent to a hot plasma on the boundary, characterized by the temperature of Hawking TH and described by a gauge theory; both have the same entropy. In addition, plasma obeys the usual rules of quantum mechanics, and in particular it evolves unitarily, which automatically leads the black hole to evolve also unitarily and to respect the principles of quantum mechanics [6]. This result led Hawking to revise its position and to announce in 2005 that the paradox was indeed solved by the AdS / CFT correspondence for the conservation of information."

    this is written by Jean Pierre Luminet
    any collapse would break the unitarity. so physicists insist on the fact that unitarity is a law of quantum field theory.
    one often read that states evolve unitarily except when they do not....
     
  13. Jul 29, 2018 #12

    PeterDonis

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    Please give a specific reference. We can't comment on out of context quotes.

    The models you are talking about do not contain any measurements, so the question of whether collapse takes place or not is irrelevant. These models are just the same as, for example, the "internals" of a double slit experiment, where even collapse interpretations agree that the evolution of the wave function is unitary; the only "collapse" is at the end of the experiment when the pattern is observed on the detector screen. The equivalent of that in the models you refer to is the universe in the infinite future, when all of the black holes have evaporated and all that is left is an infinite expanse of radiation at extremely low temperature. What "unitary evolution" means in this context is that, for a hypothetical observer in that infinite future universe, they can't tell from any of their measurements whether the infinite expanse of radiation came from the evaporation of black holes or from some other process (like matter-antimatter annihilation leaving only radiation behind) that didn't involve black holes at all.

    Note also that all of these models are set in anti-de Sitter space, which is not the universe we live in. A major unresolved issue in this field is whether similar models can be constructed in de Sitter space, which is at least approximately like the universe we live in.
     
  14. Jul 29, 2018 #13
    it is in "the black hole war" by Susskind at the end of paragraph 22
     
  15. Jul 29, 2018 #14

    nrqed

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    I have
    I have always wondered why the absorption of matter by a black hole could not be considered a type of measurement, which would then take care of the loss o information. I am sure this is stupid for some reason but I have never seen a clear explanation why so.
     
  16. Jul 29, 2018 #15

    PeterDonis

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    Which is a pop science book, so it's not a valid source for discussion here. You need to look at actual textbooks or peer-reviewed papers.
     
  17. Jul 30, 2018 #16

    bhobba

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    Yes he is - we all are. There is a coarse graining argument that shows how our classical world emerges from the scale below us - but that scale is quantum and so is our scale - its just - as I mention below different physics emerges at different scales - but it's all quantum stuff. You cant choose to view and not view him as a quantum object - he is one all the time.

    Just reviewing re-normalization theory at the moment and one of the big insights of Ken Wilson was this idea of different but self similar physics in different scales of nature. Its a very interesting phenomena:
    https://websites.pmc.ucsc.edu/~wrs/Project/2014-summer seminar/Renorm/Wilson-many scales-Sci Am-79.pdf

    You have to be very wary of things said by the early pioneers like Wigner - great mathematical physicist though he was. Things have moved on a lot since then and from today's vantage things like Wigner's friend are not the issues they were in the early days.

    Thanks
    Bill
     
    Last edited: Jul 30, 2018
  18. Jul 30, 2018 #17

    bhobba

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    I just want to mention Susskind does write some excellent actual textbooks for a 'lay' audience under the Theoretical Minimum series. You just need a bit of calculus and its quite accessible if you think a bit - you are not not spoon fed - its real physics. They of course can be freely discussed here. Popular writings, while often interesting to read, I read them myself, are problematical to the aims of a forum like this.

    Thanks
    Bill
     
  19. Jul 30, 2018 #18

    PeroK

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    Suppose we say that "Wigner is a physicist". Can that be translated into quantum mechanics? I guess it's possible that you could define some properties of a bunch of atoms that represented physicist from non-physicist. But, it seems to me, that it may be impossible to disentangle that property from the atomic configuration.

    Or, "Wigner is a US Citizen".

    How could you, by studying the atomic configuration of a bunch of atoms tell whether they represented a US Citizen?

    In that case, if citizenship is a defining property of a human being, then a human being is not a quantum object.

    In general, the question is whether a complex system can assume properties not inherent in the underlying atomic configuration. The reductionist position would be that it cannot.

    But, does that mean that what someone has written or achieved in life is either not a defining part of them or is inherent in their current atomic structure?

    In any case, Wigner cannot be a quantum object but a continuously evolving set of quantum systems. He's effectively never the same quantum system from one moment to the next.
     
    Last edited: Jul 30, 2018
  20. Jul 30, 2018 #19

    martinbn

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    Why is that?
     
  21. Jul 30, 2018 #20

    Fra

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    This seems a wise statement that is as close to the truth we can come, and not as but not as silly as it may first seem.

    As I see it quantum mechanics, deduces the future expectations of the state given expectations of the current state, from requring a consistent self-evoluation of the state. Its is from the self-consistent evolution (given that no perturbations to the deductive system takes place), that unitarity follows deductively.

    But lets not forget that the deductive system here is effectively part of the "initial conditions". Ie. one can not realy separate initial conditions from laws, except during special conditions. Conditions that happens to be met in regular particle physics (where we have a massive detector controlloing and preparing subatomic events in a small detector), but not in cosmology.

    So the idea that unitarity in quantum mechanics "must hold" universally when we talk about general QG or cosmological models, is IMO a fallacy. The reaons for unitarity in quantum mechanics makes sense, but it should be equally clear why one can not wildly extrapolate this beyond the scope of the observer control.

    "future evolves as per my expectations, except when they do not" actually constitutes the basis for action, in the context of evolution. Ie. the best we can do, is to act according to our best prognosis, and revise or die when wrong. I expect physical law to be no differently constructed.

    I this view one will understand consistent unitarity as a kind of attractor or steady state in theory space and observer population ecology. The biggest difference between that and thinking that its some eternal mathematical constraint is when it comes to try to understand unification of forces. There need not be a conflict between them.

    At the heart of the black hole info paradox is indeed the observer role in theory formulation - which is an open issue. So there is not really a "paradox".

    /Fredrik
     
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