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Non quantum gravity

  1. Sep 25, 2008 #1


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    arXiv:0809.4218 [ps, pdf, other]
    Title: Nonquantum Gravity
    Authors: Stephen Boughn
    Comments: submitted to Foundations of Physics
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    One of the great challenges for 21st century physics is to quantize gravity and generate a theory that will unify gravity with the other three fundamental forces of nature. This paper takes the (heretical) point of view that gravity may be an inherently classical, i.e., nonquantum, phenomenon and investigates the experimental consequences of such a model. At present there is no experimental evidence of the quantum nature of gravity and the liklihood of definitive tests in the future is not at all certain. If gravity is, indeed, a nonquantum phenomenon, then it is suggested that evidence will most likely appear at mesoscopic scales.
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  3. Sep 25, 2008 #2
    It is heretical, indeed. I seem to recall that the motivation behind the search for a theory of quantum gravity was that classical gravitation and quantum mechanics are too hard to unify (if it's even possible at all). I am curious as to what the experimental consequences of this classical model might be.
  4. Sep 25, 2008 #3


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    Well I mut say that I thought of this myself, and I guess as you keep not having evidence for quantum behaviour you get discouraged, but then we need to address the question how can gravity evolve?

    And this I think also brings us back to the question can we find something more foundational than the inertial mass<=>gravitatonal mass, basically I think that those in LHC hope finding Higgs Boson will shed some light on the origin of mass, although I'm not sure what is the mechanism, here, does this particle that is supposedly responsible for elementary particles possesing the property of mass, does it itself have mass, what other properties might it possess?
  5. Sep 25, 2008 #4


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    I think this is a good point.

    I think it's easy at first glance quickly convince oneself that since the quantum formalism, deals more with measurements and what we can measure, rather than what "really is going on". It is more "scientific" since it deals more with measurements, so this "measurement operator mechanics" seems to be the good stuff.

    I think this first impression is somewhat valid, however that doesn't mean that quantum mechanics has still ultimately attained this apparent ideal, it made a step in the right direction, but we're still left behind with the other foot. This essence of the measurement problem, and how to define the measurement in this spirit is still unsolved.

    This is partly why I personally find most attempts at a quantum gravity, that tries to somehow apply the plain QM, to "something" that is described in a classical spirit is very unattractive and naive, and even if some of them would make partial progress, it still has not answered to some of the deep issues.

    I think the problem is partly and the choice of abstractions used to described the world, and the apparent unavoidable nonlinearity that enters when you are trying to understand your own understanding.

    It seems that GR and QM has good points of their own. GR has understood partly some elements of nonlineary and relation between evolving references, and evolutions relative the reference. But this is a special case only. It only treats spacetime stuff, given a manifold.

    QM ignores this completely, but insteads takes the measurement issue seriously, the only problem is that it can't currently handle anything but a fixed reference.

    I think this problem of background dependence/independece must be understood to be deeper than just spacetime the metric. The same concept of background can be applied to generic abstractions when you describe anything relative to a reference, and the reference is responding (partially and ina cmoplex way) to the referenced. Einsteins equation, I think Einsteins Equations will one day be understood be a special case of a much more general abstraction.

    I agree with that paper that it's far from obvious (and perhaps not even likely) that this can be solved in a standard QM way. I think the nice parts of QM needs to be generalised. Rovelli seems to take the first step in his relational QM interpretation, but I think what he started can be improved differently than his own suggestions, and that the interpretation can be taken to suggest a reformulation.

    I think neither taking standard QM as an obvious unquestionable starting point nor taking Einsteins classical GR as the unquestionable starting point will solve the real problem.

  6. Sep 25, 2008 #5
    Absense of evidence is not evidence of absense. "No evidence" is not a valid premise for any argument or conclusion. It might be that GR can be derived as a generalization of flat spacetime QM, who knows?
  7. Sep 25, 2008 #6
    Fredrik, I think you're right, having either theory be "unquestionable" is a mistake.
    QM does seem the ideal starting point because we've already got 3 out of 4 forces.
  8. Sep 25, 2008 #7


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    Yes, but it depends on what we mean with "starting point". I think the missing points in QM goes way beyond just "squeezing in the remaining force" in the current framework. I think a makeover may be required. In this sense I am on board with those who think the problem of QG, requires revision of QM. But that does to me, not mean that QM predictions is wrong where currently tested, it rather means it may be a limiting case (emergent from a more fundamental abstraction).

    There is still a major difference between the "predictions of QM", and the current axioms and mathematical formulation making up the theory of QM.

    What a new theory need to reproduce, is the predictions QM makes in the tested domains. We do not need to reproduce the foundations and axioms of the current theory.

    To me the spirits to keep from QM, are the spirit of Bohr's, where we are careful to only make statements about things that is measurable. But my persoanl opinion is that this is largely simplified in QM. The measures/operators are given, but i don't think that's how the real world works. I think the construction of measures, are a process, and this is constrained by the degrees of freedom in question, and the degrees of freedom relates not only to the "projection operators" or communication channels, but also to the degrees of freedom where the communication ends. This is why QM alone (pretending we know nothing of GR) does not quite make sense from the point of view of a kind of a relational theory of measurement.

    I am personally gaining increasing confidence that constructing an evolving relational theory of measurement, is the most promising way to incorporate the relational nature of GR. This is a totally different way of thinking than the standard QFT approach.

    If measurements are relationally constructed, then it means the measurements are always fired relative to a "background" the only major point is that this "background" isn't fixed, it's dynamical and there is a nontrivial feedback between the the measurements beeing made and the evolution of this "background". There is no such thing in QM foundations.

    GR has it, but it's of limited scope and the concept of measurement is not as well abstracted in GR.

    So maybe we need to start somewhere in between, with some modified abstractions. Or maybe not! I guess noone knows. But thta's what I think.

    Last edited: Sep 25, 2008
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