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”…..