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Loop Quantum Gravity and String Theory

  1. Jun 29, 2007 #1
    I'm having a little bit of trouble distinguishing between the two.

    Basically LQG states that the quantum structure of spacetime is composed of spin-networks which are composed of strings just with their ends tied, I guess you could say. LQG is also background independent. It says that these spin-networks are space and time. Time is the constantly shifting spin-networks and space (volume) is the "nodes" in the middle of the looped strings. Is all that basically right?

    Is string theory suggesting that space is made out of strings that extend infintley? But string theory is background dependent. Why is it even being researched if we know that a theory of quantum gravity has to be background independent? And what are "string bits"?

    I've just starting reading about this stuff so bear with me.
  2. jcsd
  3. Jun 29, 2007 #2


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    there are three very recent short papers you should look at immediately if you are interested in non-string QG and Quantum Cosmology (QC)

    The Dark Side of a Patchwork Universe

    this gives a 7 page description of "the universe according to loop quantum gravity"
    pages 3 - 10, of which the first 3 pages give a GENERAL outline of the LQG approach he needs for cosmology
    and the other four pages a specializing it down to the quantum model of the universe.

    the other two you should look at are a couple of recent ones by Ashtekar, one is introductory for beginning researchers, the other is a survey talk invited by a major international conference and intended as an up-to-date review for specialists in other fields.

    An Introduction to LQG through Cosmology
    "This introductory review is addressed to beginning researchers. Some of the distinguishing features of loop quantum gravity are illustrated through loop quantum cosmology of FRW models. In particular, these examples illustrate: i) how `emergent time' can arise; ii) how the technical issue of solving the Hamiltonian constraint and constructing the physical sector of the theory can be handled; iii) how questions central to the Planck scale physics can be answered using such a framework; and, iv) how quantum geometry effects can dramatically change physics near singularities and yet naturally turn themselves off and reproduce classical general relativity when space-time curvature is significantly weaker than the Planck scale.
    20 pages, 4 figures, Introductory Review. "

    Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions
    "As per organizers' request, my talk at the 11th Marcel Grossmann Conference consisted of two parts. In the first, I illustrated recent advances in loop quantum gravity through examples. In the second, I presented an overall assessment of the status of the program by addressing some frequently asked questions. This account is addressed primarily to researchers outside the loop quantum gravity community.
    21 pages, to appear in the Proceedings of the 11th Marcel Grossmann Conference"

    STS I would advise you to download these, print them out, and look them over, whether or not you think you can understand them. It will give you a very useful up-to-date impression of what the field is about.
    There is TECHNICAL progress (possibly quite important) happening in other areas, like spinfoam models including matter and Feynman diagrams of particle field theory.
    But the most visible aspects of what is going on in have to do with cosmology. there are rapid developments going on in QC (quantum cosmology) and to some extent they may begin to influence the rest of quantum gravity research.

    It is highly significant that Ashtekar chose to INTRODUCE LQG by way of its applications to cosmology. This is pedagogically a smart choice because you see interesting applications right away.

    In its classical form, cosmology uses a SIMPLIFIED version of Einstein classic Gen Rel---simplified by assumptions of symmetry and uniformity, so there are only a handful of variables and parameters to worry about. Analogously quantum cosmology uses a symmetry reduced model which is much simpler than the full LQG theory. Fortunately the universe seems to be fairly uniform at large scale and so one can do this, and it makes the whole business more intuitive and comprehensible.

    So again I would strongly advise having a look at these three, short, recent papers----to get an idea of what Loop gravity and cosmology folks are really up to.

    String is a totally different story. I wouldn't try to study them simultaneously because in practice (what the leading researchers are actually working on) they are very different. After a while you might begin to make comparisons contrasts analogies at various levels and in various corners and subdepartments.
    But to begin by trying to understand in tandem would be, I think, a systematic error.
    Last edited: Jun 29, 2007
  4. Jun 29, 2007 #3


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    You could go a long way just on the first 5 or 6 pages of the Bojowald paper alone.

    He has the best short characterization of the essential Loop (or more generally nonstring) approach that I can remember seeing, on page 5 in section 2.2 on Relational Evolution
    where he says
    ... This presents an implicit description of space-time not as a manifold but as the relational dynamics of the fields defined on it. We do not refer to A(t) and V(t) as functions of a coordinate time variable t which, in a general relativistic situation, could be chosen arbitrarily. We rather eliminate t in regions where V(t) is invertible by inserting t(V ) in A(t). The variable A then evolves with respect to the “internal time” V ...

    One way to think of this is that the usual idea of spacetime continuum (realized in the conventional mathematical form of a differentiable manifold as invented by Riemann around 1850) is gauge.
    At a fundamental level there is NO MANIFOLD. There is only the relational dynamics between observables.

    there is no absolute time (as if a god with a wrist watch was watching) there are only quantum observables corresponding to some better or worse real clocks.

    so time-evolution of a system corresponds to how one observable varies with another (which is designated to be the "clock").

    The remarkable thing is that this way (which sounds a mite awkward) can work and has indeed succeeded in doing things that AFAIK no other approach can claim, including:

    1. get rid of bigbang singularity without any extra assumption
    2. show that inflation happens naturally and generically right at first, without anything put in by hand or 'fine-tuned'
    3. offer a plausible explanation for accelerated expansion (the 'dark energy' effect) with nothing put in by hand, no ad hoc cosmological constant.
    4. give a handle on what could be prior to the beginning of expansion---a prior contraction phase
    5. present some tantalizing possibilities to TEST hypotheses observationally that phenomenologists who specialize in testing theories are interested in and working on.

    I don't know of any other approach that has achieved all of these things or a comparable list of results. And the progress here has been all since 2001.
    It is just in the past 6 or 7 years that this has happened.

    Historically Quantum Gravity was studied in the 1970s, 1980s, 1990s, and Loop Quantum Gravity goes back to 1990. What I am talking about is a radical speedup in the rate of progress since 2000-2001

    A lot of those points 1 thru 5 that I mentioned are discussed in Bojowald's paper where he give CONCRETE EXAMPLES and REFERENCES to earlier papers that you can download from arxiv. That is why the Bojowald paper is so helpful. It is all there, in brevity, with concreteness, and references to backup.

    So you get right to the front edge without wasting a lot of time. I know it is hard to understand, but I suggest you try it anyway and see what you can get out.
    Last edited: Jun 30, 2007
  5. Jun 30, 2007 #4


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    STS I hope you are savy enough to realize that just because a certain model gets rid of the BB singularity and replaces it with a bounce that this proves a bounce actually happened:smile:
    the model still has to be tested.
    (and indeed people are working on the problems of how to do it.)

    in a sense the most important thing here is the MINIMALITY and the Occamness. do the simplest quantization of the equations that everybody likes to use (the Einstein Gen Rel and the Friedman symmetry-reduced version used in cosmology)----add nothing or as little as you possibly can----then try to answer the questions everybody wants to know, like what happened before the big bang and down inside a black hole and why does expansion accelerate and all that. minimality of the means and relevancy of the results. plus you then insist on near-term testability.
  6. Jun 30, 2007 #5
    Marcus, thanks for the links to those papers. I've been looking for a good source of info on LQG at about that level.
  7. Jul 1, 2007 #6
    Wow, marcus. I apprieciate your help. I'll be sure to read over those.
  8. Jul 2, 2007 #7


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    First, strings are not extended infinitely. Instead, they length is typically of the order of the Planck scale. Also, space is not made out of strings, but strings move through the space.

    Second, we know that classical gravity is background independent, but we do not know if it is true for quantum gravity as well.

    Third, quantum gravity is not why string theory has been researched in the first place. Instead, it turned out that quantum gravity is only one of string theory's (originally unexpected) predictions.
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