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Do we know for sure if gravity exists in quantum states?

  1. Sep 5, 2010 #1
    I was just wondering because it seems there is a contradiction in qm. If a quantum state can only be represented by an abstract statistic then would not gravity be equally subjective until decoherence occurs?

    And what about entanglement? It appears to act as a constant (an immediate one) over whatever distances never mind any gravitational influences.

    Maybe im crazy but that seems to suggest gravity aint happening in quantum states. Any evidence either way?
     
  2. jcsd
  3. Sep 5, 2010 #2

    tom.stoer

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    One can show that classical gravity couples to quantum states and induces quantum effects. One example is the gravitational red-shift of photons which can be measured via the Mössbauer effect.
     
  4. Sep 5, 2010 #3
    google
    neutron gravity quantum
    you get all sorts of articles of different credibility about quantum states created by gravity field.
     
  5. Sep 6, 2010 #4
    redshift doesnt prove gravity at quantum scale. You are talking light which is being measured hence its no longer in coherent state. I am talking about proof of gravity within an undisturbed quantum state.
     
  6. Sep 6, 2010 #5
    thanks will do.
     
  7. Sep 6, 2010 #6

    tom.stoer

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    What do you mean by "gravity in an undisturbed quantum state"; do you mean "a quantum state of the gravitational field"? If yes this is quantum gravity; if not then you always couple a quantum state to a gravitational field. It doesn't matter if you call this "disturbed", it's just an interaction. The particle doesn't care where the constant force (linear potential) comes from, whether it's a constant electric field or a gravitational field.

    The redshift is in some sense a proof of gravity at the quantum scale as it couples to a quantum object, a single photon. If you don't like the redshift + Mössbauer then you can use a kind of beam-splitter experiment where one beam is at constant gravitational potental whereas the other beam feels a different potential. Then instead of measuring an energy difference you can measure the phase shift.

    Anyway - it's a gravitational field coupled to a quantum system.
     
  8. Sep 6, 2010 #7
    My question was simple. Have we been able to measure/sense gravitaitonal effect within a un-decohered quantum state.

    But its okay becaus ei found another paper by some guys claming gravity is a macroscopic emergent property from quantum information, so i guess they are arguing gravity does not exist prior to decoherence.

    Anyways not to worry, thanks.
     
  9. Sep 6, 2010 #8

    tom.stoer

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    Hopefully you know that this is rather speculative!
     
  10. Sep 6, 2010 #9
    yes of course. However, considering no-one appears able to integrate qm and gravity..the whole idea of quantum gravity is in itself speculative.

    That didnt stop you bandying about the term as if its a fact :-)
     
  11. Sep 6, 2010 #10

    tom.stoer

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    To which term do you refer to? Quantum gravity?

    I do refer to a specific approach, but there is a rather general reason that something like quantum gravity MUST exist. Einstein's equation (formally) read

    G = T

    where G[g] is the spacetime and T are all other fields; T must be quantized whereas for G no such quantization is well established up to know. But we know that

    G = <T>

    is inconsistent, therefore some theory of quantum gravity MUST exist. Individual approaches may be speculative, but not the general idea.

    Btw.: how do you know that nobody is able to integrate gravity and the quantum? Have you studied all approaches and proven that they are all inconsistent or physically wrong? Congratulations!
     
  12. Sep 6, 2010 #11
    Its a well-known fact that various approaches to quantum gravity have failed thus far. Otherwise we would all have heard about it as a major breakthrough.

    Some theory of quantum gravity must exist? Well thats your opinion. I dont agree.
     
  13. Sep 6, 2010 #12

    tom.stoer

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    Which one has failed?
     
  14. Sep 6, 2010 #13
    As far as i know they've all failed. If you know of one which is now proven and accepted then please do share that breaking news with the rest of us :-)
     
  15. Sep 6, 2010 #14
    It's the total amount of energy that is required to test a theory of quantum gravity that is a the hurdle. But as Coldcall asserts, they may all be wrong(or maybe not).
     
  16. Sep 6, 2010 #15

    tom.stoer

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    Do you know what it means for a theory to have failed? It must have made at least one wrong prediction.

    What I see so far is rather different: present-day approaches towards quantum gravity can make some predictions which are subject to future experimental tests; some of them establish both an UV complete and finite theory with correct low-energy limit.
    CDT naturally has a phase which corresponds to macroscopic deSitter w/o any fine tuning or artificial input. CDT and LQG both indicate a "running spectral dimension" between 2 in the UV and 4 in the low-energy regime (LQG does not fix any dimension on the level of spin networks, so this can lead to a prediction of spacetime dimension). LQC (which is related but not derived from LQG) makes some testable predictions regarding the spectrum of primordial gravitational waves which can be seen as imprint in the CMB. LQC resolves big bang and black hole singularities; it points towards a natural explanation for inflation w/o artificial inflaton. Both string theory and LQG allow one to derive a microscopic picture for black holes micro state counting = entropy. From LQG (in its new formulation) one should be able to derive the long-range effective graviton propagator and therefore the correct semiclassical limit. The AS (asymptotic safety) approach provides another (rather different) setup which is UV complete and which to some extend explains the structure of Einsteins gravity as one "special point" in the "theory space" according to the renormalization group approach (via a non-Gaussian fixpoint). This approach backs up the results regarding spectral dimension 2 - 4.

    So all these approaches have not yet succeeded in defining a unique theory of quantum gravity. But saying that they have failed is absurd.
     
  17. Sep 6, 2010 #16

    tom.stoer

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    I agree, that's a correct statement
    :biggrin:
     
  18. Sep 6, 2010 #17
    Tom,

    "So all these approaches have not yet succeeded in defining a unique theory of quantum gravity. But saying that they have failed is absurd."

    You know that science is in big trouble when it hides behind semantic nonsense to deny something we all know to be a fact. That being, the failure (thus far) to combine qm and gravity into a unified theory or law.

    Anyways i think we all agree. My question was simple, i asked for proof, none was forthcoming, cheers.
     
  19. Sep 6, 2010 #18

    Pythagorean

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    I have some random questions about QGish ideas tom as long as this thread is bumping:

    1) can a quantum particle absorb gravitational energy? I mean, would all the quantum particles that make up a classical article individually absorb quantized gravitational potential energy as the classical object fell through a g-potential... or what?

    2) can (does?) gravity shape the potential landscape that a typical quantum particle exists in?

    3) of course, mass appears as a constant in introductory QM texts (I remember in solid state we had an effective mass, but I don't remember the context and I'm trying to keep it simple). But this is as an inertial term and doesn't necessarily imply gravity... unless one accepts Mach's principle. Is there any attempts at unification through Mach's principle?

    thanks!
     
  20. Sep 6, 2010 #19

    Pythagorean

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    well.. you DID see a theoretical proof stating that a quantum description of gravity must exist. I don't know how valid it is (it's the first time I've heard it) but I'm largely ignorant on the subject (even with a Bachelor's in physics).

    But you also saw lots of suggestive experimental evidence and falsifiable statements. So far, it sounds like good science to me. But I'm a meager grad student, so I desist.
     
  21. Sep 6, 2010 #20

    tom.stoer

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    The difference between "failed = demonstrably false" and "not yet succeeded" is not semantic nonsense but semantic clarity!

    What is missing is that one of these theories has been proven to be correct; but as you know one can't prove that a theory is correct, you can only prove that it's wrong (Popper). Of course I agree that there is still a long way to go, but there are promising approaches.
     
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