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Is time absolute or dynamical in LQG and string theory?

  1. Jul 22, 2010 #1
    regarding the problem of time in QG,
    Is time absolute or dynamical in LQG?
    string theory?

    How does LQG and string theory deal with time in singularities in the center of black holes?
  2. jcsd
  3. Jul 22, 2010 #2


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    In LQC they typically include a matter field whose evolution can serve as clock. Time is not absolute. It is relational. The theory deals with correlations between observables---one of the observables is chosen to serve as clock.

    In other words, no time without some real clock, there must be some process.

    Black hole collapse has been studied in LQG by various people, look up Leonardo Modesto, Dah-wei Chiou, Kevin Vandersloot.
    As I recall those particular researchers find no singularity.
    Because of quantum corrections gravity becomes repellent at near planckian density and the collapsing spacetime region re-expands out the bottom--a socalled "bounce".
    However the issue is not settled. All the work so far has involved simplifying assumptions. Can't be sure yet what happens. I have to go. There is more to say and other researchers' work to mention. Maybe someone will, or i will get to it when I return.
  4. Jul 22, 2010 #3
    I thought the quantization procedure splits time and space and defines spatial coordinates with respect to time evolution.
  5. Jul 22, 2010 #4


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    You are talking about canonical quantization but what you are describing is the wrong way to conceptualize it. With canonical quantization of GR, what you get is essentially timeless*. The Hamiltonian is a constraint. This is why in a canonical quantization of GR something physical must be chosen to serve as clock, like i said. Then observables which depend on time are treated as correlations with the clock observable.

    Instead of saying "But I thought that..." and contradicting, maybe could you go back and try to understand what I said?

    Another thing to realize that LQG is not now presented as a quantization of GR. All that canonical quantization stuff from years back was basically heuristics. Read Rovelli's April paper to get an up-to-date perspective.

    It is really crucial that people who want to ask and talk about LQG should have examined that April paper:
    and understood as much of it as they can.
    It is essentially a complete reformulation of LQG, with a lot of insight and perspective expressed in ordinary words, as well as equations. A major effort to communicate a new perspective. I don't understand all of it by a long shot, but there are easy parts and I'm slowly struggling through the hard parts.

    *so-called "frozen time" formalism. Introducing real clocks and treating time relationally was the response adopted, some 10 years ago I guess.
    Last edited: Jul 23, 2010
  6. Jul 22, 2010 #5
    Has the LQG research community adopted Rovelli's reformulation? Seems a little early to say.
  7. Jul 23, 2010 #6


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    The slicing of spacetime M4 into time * space R * M3 assumes a certain topological structure of spacetime.

    This slicing does however not introduce any absolute time. This can be understood as follows: in canonical quantum gravity one expresses the local Lorentz symmetry plus 4-dim. diffeomorphism invariance in terms of certain operators forming an algebra. If this algebra stays intact after quantization (w/o anomaly) and can be implemented on the physical states the theory still has this symmetry even if it is not visible.

    The residue of the 4-dim. diff. inv.is just the Hamiltonian with H |phys> = 0.

    Look at angular momentum; you know the su(2) algebra as defined by the Pauli matrices. The angular momentum operators have the same commutation relations. No as you write down the angular momentum operators in cylinder coordinates it looks like as the z-coordinate has been singled out and is something special; this corresponds to the fact that m is always identified with Lz. Anyway - if you calculate the commutation relations you find that it's still su(2) and that the algebra stays intact; that is all what you need. Cylinder coordinates do not break the symmetry.
  8. Jul 23, 2010 #7
    I've just been reading a bit about this recently, and I was astonished to realise what was going on in canonical GR - it basically echoes what Tom said :

    The reference was http://arxiv.org/abs/gr-qc/0610057" [Broken] which summarises the situation nicely:

    Last edited by a moderator: May 4, 2017
  9. Jul 25, 2010 #8
    Since LQG does not yet proven to have semiclassical limit, can time dilation be calculated in LQG? Any reasons why it's 3+1?

    What about string theory? Can time dilation be computed in string theory?
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