Another Kuperberg exerpt
GK at Scott's blog, 28 October
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The alternative quantum gravity theorists seem much less concerned about the rigidity of general relativity in 3+1 dimensions. The approach is to assume a microscopic semblance of general relativity, then to conjecture that that implies macroscopic agreement. There are actually many microscopic semblances of general relativity — it is very easy to be creative here — but, according to Jacques and others, macroscopic convergence is a devastating restriction which is not ameliorated by microscopic resemblance. If we can believe people like Jacques on this point, then this is not a viable alternative.
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http://www.scottaaronson.com/blog/2005/10/insert-string-pun-here.html
this link might get the specific comment without having to scroll:
http://www.scottaaronson.com/blog/2005/10/insert-string-pun-here.html#113051729088456470
I was encouraged by his saying ALTERNATIVE QG theorists, as if he might be going to give his impressions of other approaches like Martin Reuter QEG and Renate Loll Causal Dynamical Triangulations. But he did not branch out and soon got back to talking about how he imagines LQG, what Witten says about LQG and so forth.
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The problem with the spin foam and and LQG calculations of entropy is that they are ad hoc calculations that are not part of a larger consistent framework.
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On the other hand, string theorists, and some non-string theorists too, have strong heuristic arguments that the main conjecture of LQG, that it converges to general relativity, is false.
It is true that the main conjecture of LQG is also much less rigorous, even as a conjecture. Nonetheless the heuristic argument against it is quite general and would require a very good new idea to circumvent. Without any kind of convergence to gravity, you're giving the name "quantum gravity" to something that you have no reason to believe is gravity.
So I don't see this as a case of complementary strengths and weaknesses. Rather, LQG looks more like a Naderite quest to compete with string theory. (I.e., Nader's struggle is to be an alternative to the Democrats, not to actually win elections.)
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In the above quote he mentions "spin foam and and LQG calculations of entropy". As far as i know there are no spinfoam calculations of BH entropy. It may sound as if he is considering something besides LQG, but I think he is continuing to repeat what he has heard about specifically LQG, and not including other approaches. He continued this morning as follows.
---quote Saturday 29 October---
One more thought this morning: It is completely reasonable to look for models of quantum gravity, or reality otherwise, that doesn't look like string theory. Two examples that come to mind are Matrix theory and 11-dimensional supergravity. Both of these arise as strong-coupling limits of string theory. But their definitions are completely different, and it is one reason that people believe that eventual definition of M-theory will also be completely different.
So the point is not to stay loyal forever to one narrow definition, although the old definition of superstring theory from 20 years ago has been monumentally useful. The point is that the string theory community is a big tent that will accept any viable fundamental model. Maybe not every string theorist is so accepting, but enough of them are. Witten, in particular.
The string theorists' intuition is that viability should imply a direct connection to string theory. That may seem unfair, but it could be well-founded.
But part of the Naderite mindset is that anything accepted into the big tent belongs to the other side, and therefore doesn't count as open-mindedness. It's an eternally divisive quest to compete with string theorists (or in the case of the real Nader, with Democrats). Merely writing papers in loop quantum gravity or whatever is just fine and it would be unfair to criticize Smolin or Rovelli or anyone else just for that. Perpetually offering it as an alternative to string theory seems closed-minded and self-defeating.
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