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Bee on Consistency and some interesting comments

  1. Aug 3, 2007 #1

    marcus

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    http://backreaction.blogspot.com/2007/07/consistency.html

    Bee took the time to write a thoughtful essay about general relativity and why you might or might not want to quantize it---and some broader thoughts about theories (what their limitations are, how you verify them.)

    Perhaps I am not saying accurately what the essay is about, and you should go there and read it and decide for yourself.

    There are a lot of extra bits----extra thoughts, bits of news, a cameo appearance by El Lumo, witty sayings, a charcoal drawing of a naked physicist with glasses, and so on.
     
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  3. Aug 3, 2007 #2

    marcus

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    as I understand the essay you don't see what it is about until you get to the brief final section "Bottom Line", and also read Moshe Rozali's comment.

    Here is the comment:
    ===quote Moshe===

    As for Einstein gravity, the fact that it is non-renormalizable strongly suggests that it is an effective description of something completely different. To paraphrase Ted Jacobson, you don't quantize the metric for the same reason you don't go about quantizing ocean waves. There are people out there who strongly object to this viewpoint, blaming the problem on the lack of background independence. Who knows, maybe they are right.
    ==endquote==

    I think einstein gravity is, in fact, renormalizable, and that Martin Reuter et al have accumulated a lot of credible evidence

    so I would say that the classic unquantized theory of the metric is an effective description of something very much like itself.

    But Moshe still puts his point in a clear, persuasive, and memorable way---and he acknowledges other people with different opinions might be right. So it's good.

    The first 90 percent of Bee's essay is generalities about theories, setting up for a good discussion by the commenters----in a few days she got over 70 comments by mostly intelligent articulates--- the art of the saloniste. how to set out beforehand the thematic appetizers and punchbowl. how to get the right people to show up.

    But here's her conclusion
    ==quote Bee==

    Bottomline

    Much like classical electrodynamics, Einstein's field equations too have a source term whose dynamics one needs to know. The system can be closed with an equation of state for each component. This theory is self-consistent [6], and it is consistent with all available observations. It reaches its limits if one asks for the microscopic description of the constituents. The transition from the macro- to the microscopic regime can be made for the sources of the gravitational field, but not also for the coupled gravitational field (oh, and then there's the CC, but this is a known problem).

    Two theories that yield the same predictions for all observables I'd call equivalent (if you don't like that, accept it as my definition of equivalence.) But our observations are limited, and unlike the case of classical electrodynamics not being consistent with the stability of the atom, there is presently no observational evidence in disagreement with classical gravity.

    For me this then raises the question:

    Is there more than one theory that is self-consistent, self-contained and consistent with all present observations?


    In a recent comment, Moshe remarked:"To paraphrase Ted Jacobson, you don't quantize the metric for the same reason you don't go about quantizing ocean waves." That sounds certainly reasonable, but if I look at water close enough I will find the spectral lines of the hydrogen atom and evidence for its constituents. And their quantization. To me, this just doesn't satisfactory solve the question what the microscopic structure of the 'medium', here space-time, is.
    ....
    ==endquote==

    My answer to the question in blue would be that there might very well be more than one.

    But if they are not equivalent (really only one) by Bee's definition, then they will make different predictions about FUTURE experimental outcomes. One of the candidate theories will predict some surprising new stuff that we didnt think to look for yet----like a shape to the expansion of the universe that differs in some subtle way from what LambdaCDM tells us it should look like. Theories should predict some unexpected curious new phenomena---as well as do the other things Bee mentions.
     
    Last edited: Aug 3, 2007
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