Michio Kaku on loop quantum gravity

[marcus]

I think the division between particle physicists and relativists is more sociological than anything. On the other hand background independence/dependence is much more philosophical. Its then interesting that the AS approach to QG is very much and background independent. In Reuter's talk at the AS conference last month Horava asked Reuter why did he insisted on BI and Reuter simply answered that why do you(stringy people) insist on exactly the opposite.



I think the LQG program has always assumed AS even though they do not say it explicitly. I think the interesting question is how is AS achieved in any given approach to QG.

That's a really interesting comment. I've been wondering about the relation of LQG to Asymptotic Safety. What you say hadn't occurred to me. How do you think it is somehow assumed?

Is Loop already operating at the bare coupling constant scale? If so, how can it give the right answers in the large-scale limit? This relationship between LQG and AS is one of the most obscure puzzles for me. What I expect is that when it is finally resolved it will reveal something non-trivial. I want very much to hear any comment about this.

 

[atyy]

I think the division between particle physicists and relativists is more sociological than anything. On the other hand background independence/dependence is much more philosophical. Its then interesting that the AS approach to QG is very much and background independent. In Reuter's talk at the AS conference last month Horava asked Reuter why did he insisted on BI and Reuter simply answered that why do you(stringy people) insist on exactly the opposite.

I think BI is very much sociological too. In these comments about LQG, I only mean LQG of Rovelli's introduction in his QG book and reviews and Smolin's TTWP. LQC is background dependent (so far), yet the LQG people don't reject it. AdS/CFT is the most BI thing in quantum gravity so far, and it is stringy. And I think the more general difficulty in formulating string theory non-perturbatively is producing some very interesting heuristics http://arxiv.org/abs/0907.2939.

I think the LQG program has always assumed AS even though they do not say it explicitly. I think the interesting question is how is AS achieved in any given approach to QG.

I agree the words of LQG do imply AS. Two ways:
1) Gravity is not effective + renormalization group ---> AS
2) BI ~ diffeomorphism invariance ~ general covariance of the Lagrangian ---> AS
So to me this is a criticism of LQG that they do not follow the logical implications of their words. However, I think the mathematics of LQG (the whole field, not just Rovelli and Smolin) is pointing in a different way. I would watch Oriti's, Freidel's, Livine's, Rivasseau's, Gurau's etc explorations of GFT, and Thiemann and friends' exploration of the Bergmann-Komar group. The LQG people doing AS-related stuff are a bit removed from LQG: Bahr and Dittrich (I'm assuming their "perfect action" comes from a fixed point, as it does it QCD), and Krasnov (not AS per se, but he has a long discussion about how it is/isn't related to AS in his 78 page paper).