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Philosophy of Loop Quantum Gravity

  1. Nov 30, 2008 #1


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    I came across an interesting discussion about LQG's philosophy (through Googling "Cauchy surface" and "loop quantum gravity"):

    Approaching the Planck Scale From a Relativistic Point of View: A Philosophical Appraisal of Loop Quantum Gravity
    Christian Wüthrich
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  3. Nov 30, 2008 #2


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    Wüthrich is an interesting guy. He's young. He has a physics degree. There is hard content in his PhD thesis, equations. I think he understands LQG better than, say for example, string theorists normally do. I'd say he's on the ball about quantum gravity and asks interesting questions, and has insights.

    I didn't know about him until now. I'm glad you told us.

    I see he did his physics at Uni Bern, and then was at Pittsburgh, and Perimeter Institute, and is now tenure track at UC San Diego. He organized a summer school this year, somewhere in Switzerland, and got Carlo Rovelli to give talks.

    I actually wouldn't call what he does Philosophy. Even though he himself does! He is asking what does General Relativity tell us? How do we extend those lessons down to Planck scale? What does it mean to quantize spacetime geometry? Does GR actually need to be quantized? What should a quantum GR look like?

    In other words he is asking basic conceptual questions which should guide the construction of theory. There are times in physics when that is necessary. Einstein was at one of those junctions (1905-1915). He couldn't just write down equations and solve them and compare with data etc. He had to think at a fairly sophisticated level about basic concepts, time, distance, mass, measurement, different observers. It's not necessarily always trivial or useless to do that kind of thing.
  4. Nov 30, 2008 #3
    Without a viable theory of QG, do we really know that GR's lessons of DI and BI extend all the way down to the planckscale, or, perhaps, emerge after reaching a certain threshold.
  5. Nov 30, 2008 #4


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    That is exactly the kind of physics question that Christian Wütherich is investigating!
    Good for you for asking. Maybe you should try reading his PhD thesis. His thesis is the topic of this thread. It is called
    Approaching the Planck Scale from a Relativistic Point of View.
    Last edited: Nov 30, 2008
  6. Nov 30, 2008 #5
    I more or less raised the same issue in "String Field Theory and Background Independence" thread.

    I don't think it follows that b/c GR is BI + DI, that QG is BI + DI. Hopefully though some QG shows GR in the semiclassical regime.
    String theory does this in 10 D, LQG does not in any.
  7. Nov 30, 2008 #6

    The paper quoted states, page 12, "I will argue that the full spacetime diffeomorphism invariance cannot be recovered in the canonical formulation of the theory, at least not as it stands."
  8. Nov 30, 2008 #7


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    Markopoulou has an essay about "background independence" and what you're asking about, including some comments on Volovik's Fermi point idea:

    New directions in Background Independent Quantum Gravity
    Fotini Markopoulou
  9. Dec 1, 2008 #8


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    Thanks for digging up that paper Atyy! That sounds like potentially interesting - yet more papers to read. Unfotunately I noticed Wüthrich's paper is 238 pages :cry: I'll try to skim it during the week.

  10. Dec 1, 2008 #9


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    Wüthrich's discussion of background independence seems very close to Rovelli's book, which I don't understand (the temperature in Paris?). Here's a hilarious discussion about how difficult it is to define "diffeomorphism invariance" which I like:

    Some remarks on the notions of general covariance and background independence
    Domenico Giulini

    Incidentally that comes in a volume edited by Stamatescu that includes:

    The Canonical Approach to Quantum Gravity: General Ideas and Geometrodynamics
    Domenico Giulini, Claus Kiefer

    Loop and spin foam quantum gravity: a brief guide for beginners
    Hermann Nicolai, Kasper Peeters

    Hmmm, looks like Stamatescu's been dabbling in neuroscience! I shall have to read that.
  11. Dec 1, 2008 #10


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    BTW, I guess what attracted me to Wüthrich's work was not his discussion of background independence, but of the assumption that you can always slice spacetime up into space and time (global hyperbolicity, Cauchy surfaces etc). It looks like LQG takes that assumption. Does CDT?

    I understand in GR, global hyperbolicity is an assumption needed to formulate an initial data problem. Do we need to be able to formulate our problems as initial data problems to do science? Instinctively, I'd say yes. But in introductory GR, the way the Schwarzschild solution receives its interpretation doesn't seem to require an initial data formulation. So I'd guess no. Yet all numerical relativity seems to take that assumption, and apparently is required for cosmic censorship to work, without which GR is toast. So I'd guess yes! I'm totally confused :confused:
    Last edited: Dec 1, 2008
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