Quantum Gravity Commentary

During the last couple days, some interesting commentary on quantum gravity has appeared at a couple places on the web. One is at John Baez’s latest edition of his proto-blog This Week’s Finds in Mathematical Physics. John is mainly writing about operads, but he begins by saying a bit about why he’s working on pure math rather than quantum gravity these days:

Work on quantum gravity has seemed stagnant and stuck for the last couple of years, which is why I’ve been turning more towards pure math.

He mentions the “landscape” and the problems it is causing for string theory, suggesting a reason Susskind’s “anthropic” nonsense is getting attention:

perhaps it’s because nobody really knows how to get string theory to predict experimental results! Even after you chose a vacuum, you’d need to see how supersymmetry gets broken, and this remain quite obscure.

But instead of spending time bashing string theory, John admirably also has a critical take on his own side of the LQG/string theory controversy, noting that

it has major problems of its own: nobody knows how it can successfully mimic general relativity at large length scales, as it must to be realistic! Old-fashioned perturbative quantum gravity failed on this score because it wasn’t renormalizable. Loop quantum gravity may get around this somehow… but it’s about time to see exactly how.

Jacques Distler also has an interesting posting about quantum gravity, based on his introductory lecture to the string theory class he is teaching this semester. He explains what some of the generic problems with quantum gravity are, from an effective field theory/renormalization group point of view, and how string theory gets around them. There are also some interesting comments about observables in quantum gravity and the signficance in this context of non-trivial gauge transformations at infinity. Unfortunately, unlike John, Jacques doesn’t believe in being very explicit about the problems his side is having (to be fair, maybe that’s the topic of another lecture). He does mention background independence and refers to discussion elsewhere, where students could learn about the lack of a non-perturbative formulation of the theory. But his claim that string theory “provides a unique, or nearly unique UV completion” seems to me seriously misleading, and deserving of elaboration lest the uninitiated get the wrong idea.

Jacques does deal in a somewhat peculiar way with a commenter named Jason who is happy with the idea of a quantum gravity theory that can’t predict anything at all at the Planck scale. Instead of making the obvious point that believing in a theory that can’t predict anything is not what scientists do, Jacques writes

Careful, Jason. A certain self-anointed String Theory gadfly might hear you.

Perhaps Jacques meant to write “self-appointed”, since I’d never thought of myself as a “gadfly” until Sean Carroll recently referred to me as such. If I were the sort to self-anoint, I suppose I’d prefer something more serious sounding than “String Theory gadfly”, maybe “String Theorist’s worst nightmare”…..

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56 Responses to Quantum Gravity Commentary

  1. woit says:

    Please, no more comments about your favorite ideas about physics, unrelated to the original posting. Stop doing this here, I’ll delete anything more of this kind.

  2. Shantanu says:

    Probably you know this already but a video of Joe Lykken’s talk
    at this year’s SSI is online.

  3. John Baez says:

    dan writes:

    >Since Loll’s causal dynamic triangulation appears to have a
    >well-behaved semi-classical limit, with non-trivial predictions
    >on the planck scale, shouldn’t that excite you to doing research
    >in QG?

    It does excite me; I think it’s one of the most exciting things to come along in quantum gravity during the last few years! Everyone should read this for a less technical description of what Ambjorn, Jurkiewicz and Loll have done – or these for more detail. I talked about this stuff in the issue of This Week’s Finds covering the 2004 Marseille conference on loops and spin foams, so you can also read that.

    Unfortunately the most important work being done by these authors isn’t the sort of thing I’m good at. It involves lots of computer calculations. I’ve tried to get some computer whizzes interested, but so far nothing has come of it. So, I expect I’ll watch from the sidelines for a while.

    There’s one place I can *imagine* helping out. Their theory makes crucial use of a time coordinate. This should wash out when they take the continuum limit, but it might not – in which case they would be studying not quantum gravity, but some other theory in a different “universality class”.

    One can investigate this issue numerically. But it would be nice to find a variant of their model which did not make use of a chosen time coordinate, to simply sidestep this issue.

    That’s the sort of thing I can *imagine* being able to do… but I haven’t actually been able yet. Since I’m making so much more progress on various kinds of math, I’ve been doing more of that.

  4. dan says:

    “There’s one place I can *imagine* helping out. Their theory makes crucial use of a time coordinate.”

    Perhaps you should help them out. To what extent can CDT be applied to LQG or spinfoam, or to what extent can LQG-spinfoam, including calculations of BH entropy, be applied to CDT?

  5. Chris W. says:

    Follow-up on JB’s comment: See this lovely set of slides by Renate Loll, from a talk apparently given in 2002.

  6. Chris W. says:

    Slightly off-topic, but very interesting:

    The Information Geometry of Space and Time
    gr-qc/0508108 (Ariel Caticha, SUNY-Albany)

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