Shtetl-optimized, an attractive blog

[marcus]

http://www.scottaaronson.com/blog/

see what you think,
I think he writes well and has original thoughts.

sometimes the topics are related to quantum gravity

scott aaronson is in computing, at Waterloo, incidentally where Perimeter is but no connection AFAIK

 

[selfAdjoint]

I noticed Distler's comment. Distler seems to have taken on the role of seriously critiquing every seriously proposed alternative to string theory as a source of gravity. As such I certainly am not able to counter-critique, but I note that his "devastating" criticisms of the deep technical problems of LQG don't seem to faze Smolin or Baez.

This is where I personally would like to see the issues thrashed out, rather than the Woit-Motl son-et-fumiere; I only regret that I am unable to contribute to the debate.

 

[Kea]

nice blog

Distler seems to have taken on the role of seriously critiquing every seriously proposed alternative to string theory as a source of gravity.

I feel that much of Distler's criticism is probably correct, although I haven't bothered to follow the details of all his arguments. What continues to floor me is the supreme confidence that standard LQG is the only considered alternative to Strings.

 

[selfAdjoint]

I feel that much of Distler's criticism is probably correct, although I haven't bothered to follow the details of all his arguments. What continues to floor me is the supreme confidence that standard LQG is the only considered alternative to Strings.

Kea, I don't get that feeling at all. Note that at Loops 05 there were a number of non-LQG theories on show, especially of course the CDT ones, but also for example the completely other asymptotic safety approach (which Distler also found sufficiently prominent to bother with a critique).

If what you mean is that your category-logical approach is not as famous, well, neither are some that interest me, like the Jordan algebra ones. You are such a valuable contributor here, I hope you aren't going to succumb to mere envy.

 

[Kea]

...for example the completely other asymptotic safety approach (which Distler also found sufficiently prominent to bother with a critique)

Dear selfAdjoint

Oh, yes. I had forgotten about the Asymptotic Safety critique. But then I did qualify the comment with the word much. And of course I am disappointed that our own pet theories don't even rate a mention, but I wasn't harping on that. It's just that there are a lot of good ideas out there and I don't see how some String theorists can be so sure that they have fairly evaluated them all!

All the best
Kea

 

[marcus]

Kuperberg comments on LQG at Scott Aaronson blog

selfAdjoint mentioned the interesting comments following Scott's entry

Insert "string" pun here
http://www.scottaaronson.com/blog/2005/10/insert-string-pun-here.html

The most extensive, and the ones most concerned with non-string QG, are by Greg Kuperberg. He is a mathematician at UC Davis. Publishes in a lot of fields (combinatorics, probability, computing, differential geometry, metric geometry, algebraic topology--homotopy theory,...). Young and prolific, and both his parents are also active mathematicians publishing their research----they are at Auburn, Georgia. Quite a family!

I think it could help to exerpt some samples of Kuperberg's comment and copy it here, giving sense of how a young mathematician may see the current situation quantum gravity, in the present information climate.

-----quote from Kuperberg-----
Scott: You're right, there isn't a serious alternative to quantum probability. There are alternatives, but they aren't serious. If I were clueless, I could accuse you of ignoring alternatives like biological computation. But the truth is that biological computation isn't a real alternative to quantum computation, moreover not all quantum computation experts ignore it.

Loop quantum gravity is to string theory as biological computation is to quantum computation. (At least, that is my very strong outside impression.) This is not a perfect analogy, because the LQG leaders have spent much more time playing up their effort as an alternative. Moreover Witten and some others have not ignored it. They have read the main LQG papers, and they have talked to the LQG leaders. They say that LQG isn't credible as quantum gravity (although maybe some of it is credible as something else), and it sounds to me like an informed judgment.

It isn't triumphalism. Modulo some very likely conjectures, string theory is a consistent perturbative model of quantum gravity. (Perturbative means that all calculations are formal power series in h-bar.) Consistency of LQG is an unlikely conjecture.[/color] ... When Witten says "the only real idea", he means the only consistent mathematics known to be relevant to the question.

It's a mistake to study inconsistent mathematics. Also to study consistent but irrelevant mathematics.[/color] ...

It's also important to remember that the Internet decomposes into intellectual echo chambers, one of them being the echo chamber of skeptics of string theory.
----endquote---

I have bolded some parts to show the main message. Apparently it is that that the ONLY alternative QG he will consider is some (probably the canonical) version of LQG which he says has been examined by Witten. And this ONLY alternative is NOT A REAL ALTERNATIVE.

Others when they have made similar arguments have referred to a version of LQG which was examined last year by Hermann Nicolai et al (in the Outsider's View paper) and which Smolin pointed out has not been much worked on for several years. When authorities are cited in this fashion one rarely has any way of telling what version of what QG approach the authority was actually examining, and what the actual objection was.

I blued two uses of the word consistent. Sometimes it means consistent with Gen Rel in the sense of having been shown to have the right largescale limit. In other contexts it can mean INTERNALLY or "mathematically" consistent. At times it may sound almost as if it is being used persuasively, that is as a buzz-word or part of a reassuring slogan.

 

[marcus]

Another Kuperberg exerpt

GK at Scott's blog, 28 October
---quote---
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.
---endquote---
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.

---quote---
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.
...
...

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.)
---endquote---

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.
---endquote---

 

[Chronos]

I agree with Kea. If you look hard at CDT, category theory leaps off the page and slaps you in the face.

 

[Kea]

I agree with Kea. If you look hard at CDT, category theory leaps off the page and slaps you in the face.

Dear Chronos

So sweet! It sounds as if you have slaved away at the CDT papers. Perhaps you could start a new thread about this?