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A Dear Qubitzers, GR=QM

  1. Sep 20, 2017 #1
    In his recent letter to the physics community, Susskind says Gravity and Quantum Mechanics is inseparable. Read it here .
    Frankly, I do not fully understand his letter or the claim that he is making.
    What do you think?
     
  2. jcsd
  3. Sep 20, 2017 #2

    PeterDonis

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    If you don't already have a lot of background in this area, you should not expect to understand it. He is assuming that a lot of theoretical claims which, as far as I know, are not definitely established (many of them can't even be tested at all experimentally), are true, and working out a particular implication. But without a detailed understanding of all those theoretical claims, it's going to be hard to make sense of this letter. I don't follow all of it myself since I'm not familiar with all of the literature he is referring to.
     
  4. Sep 21, 2017 #3
    He's going one step beyond ER=EPR (the two ends of a wormhole can be considered as entangled in a QM sense, and two entangled particles can be considered as linked by a wormhole), and proposing that GR and QM imply each other. A weaker version of his claim is already implied, I think, in AdS/CFT duality: a quantum field theory without gravity (CFT) is equivalent to a string theory with gravity in a space with one more dimension (AdS).
     
  5. Sep 21, 2017 #4

    Demystifier

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    Is he saying that classical GR is equivalent to QM? If so, what is the GR equivalent of quantum uncertainty relations? Or of unitarity and superposition principle?
     
  6. Sep 21, 2017 #5
    Well he couldn't say that because all his arguments rest on AdS/CFT, which is not only quantum but stringily quantum. However, if classical GR is the low-energy effective field limit of string theory, he could say that classical GR follows from quantum strings.
     
  7. Sep 21, 2017 #6

    Demystifier

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    That's nothing new, we already know it from the 70's and 80's.
     
  8. Sep 21, 2017 #7

    Demystifier

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  9. Sep 21, 2017 #8

    vanhees71

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    Ehm, doesn't this fall under "personal speculation" and thus shouldn't be discussed here, although the speculation comes from a quite famous physicist? SCNR:oldconfused:
     
  10. Sep 21, 2017 #9

    Fra

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  11. Sep 21, 2017 #10
  12. Sep 21, 2017 #11
    susskind is a fanatic of string theory. string theory unifies quantum mechanics and general relativity but only mathematically.
    string theory is a belief that cannot be proven or disproven.
     
  13. Sep 21, 2017 #12

    PeterDonis

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    Good point, moved.
     
  14. Sep 21, 2017 #13

    PeterDonis

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    I don't think it's just personal speculation; the models Susskind is referring to are models that have been published in peer-reviewed papers and are used in a lot of ongoing research. But it was correctly pointed out that the topic does belong in the Beyond the Standard Model forum, so I have moved this thread there.
     
  15. Sep 21, 2017 #14
    Is there any physical theory that didn't start as a personal speculation? None that I can think of.

    However, yes, this does belong in "Beyond the SM."
     
  16. Sep 21, 2017 #15
    I think a better wording is "some already thought it from the 70's and 80's" (also related to your following post). Perhaps Susskind would say that recent theoretical developments (e.g. AdS/CFT, ER=EPR) provide more support for the idea that classical GR follows from quantum strings.
     
  17. Sep 21, 2017 #16
    In the sense of string theory being a set of nice mathematical ideas with no experimental evidence for, and in the sense of string theory seeming messy and incomplete (as compared to, for example, clean and consistent general relativity), I understand why one can think of string theory as not-even-wrong fashionable nonsense. But I believe some recent developments, e.g. the applications of AdS/CFT to condensed matter and nuclear plasma, indicate that the mathematics of string theory does relate to things that can be measured in the lab. Here, Susskind is proposing experiments, based on more powerful quantum computers that could be developed some day.
     
  18. Sep 22, 2017 #17
    Susskind says: "From the holographic AdS/CFT correspondence we may assume that observers, perhaps with human-like cognitive abilities, are possible in the bulk. Can a laboratory observer conrm the presence of such a bulk observer by doing experiments on the shell?..." This is a key point in the paper.

    But given AdS/CFT correspondence, if intelligent observers exist in the bulk, they also exist in the shell. So we have two mathematical descriptions of the same observer, but the two versions of this observer experience totally different things, for example one sees a world with spin-2 gravitons and the other sees a world with only spin-1 force carriers. What do you guys think?
     
  19. Sep 22, 2017 #18

    haushofer

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    I don't get this. To me, holography and its specific form the AdS/CFT correspondence, states the following: if you write down the Feynmandiagrams of a CFT like N=4 Super YM, somewhere deep down in those diagrams there is information about the structure of spacetime itself hidden. That's it. It's like a promise that a painting from a child somehow resembles a Van Gogh in a particular way. That's already amazing enough. Let's not make it mystical.
     
  20. Sep 22, 2017 #19
    Yes but not the same spacetime. The CFT lives on the boundary, and has info on the structure of spacetime in the bulk.
     
  21. Sep 22, 2017 #20

    Fra

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    I see the paper as a bunch of examples and arguments, which arent the "real thing" but serve to make a vision more plausible. For me the vision and thinking of howto make the arguments complete is the focus.

    As i see it the vision is that that dimensionality and connectivity, is just a way of coding information. The various correspondence principles is EXAMPLES of this within current theories. Also time evolution seems connection to some kind of computation (although i am not personally fond of the terminology, i like to think in terms of random walks guided by priors, rather than "computation", but once you think abbout it they two views are similar), and thus the examples of relating computation complexity with BH expansion.

    Regardless of the fate of string theory, I definitely dont think these is just conicidental notes. I think there is something profound here, that lies at the core of the problem of theoretical phyhsics: foundations of QM, QG, renormalisations etc that is yet to be revealed, understood and formalised. I think the core vision here
    has nothing a priori todo with string theory.

    Connectivity and metric spaces are also naturally related to information divergences and also intuitively complexity will spontaneously form as a way to encode as much information as possile, given finite capacity. And what ought to happen when two such systems interact? Well likely a negotiation resulting in an attractive force along the dimension where they are connected. Most probably a universal one, regardless of details (much like gravity). So there are many more examples that adds up and supports such a vision where the information perspective of QM likely can EXPLAIN gravity, and then i do not mean "explain" just as in string theory, but explain it in a way that is independent of string theory so that whenever you have "interacting" "computing" observers, trying to take control of their environment, a universal attraction is unavoidable. It will be implied.

    This is my reading out what what susskind says with "whenever there is QM there is Gravity". But of course, that makes no sense if we are just talking about regular QM. I think we need to improved its formalism, then and only then will these connetions be clear also so mathematicians will understand them.

    /Fredrik
     
    Last edited: Sep 22, 2017
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