Michio Kaku on loop quantum gravity

In summary, Kaku is known for promoting string/M-theory through various books and media platforms. When asked about loop quantum gravity, he believes that string theory is the only true theory of everything as it does not "blow up" and contains matter. He also mentions that loop theories are not competitors as they only contain pure gravity and become divergent when matter is added. However, there is ongoing research on incorporating matter fields in loop quantum gravity. Despite Kaku's popularity, some view his exaggerated and misleading statements as unhelpful in promoting scientific understanding.
  • #1
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Kaku is famous for promoting string/M-theory in a variety of books from hyperspace to physics of impossible, to name a few. He also popularizes string theory in radio and sci-fi documentaries.

He was asked what he thought of loop quantum gravity:

Antimatter
How much closer are we at this point to a Theory of Everything? Are the string and loop theories the best we have right now?

Michio Kaku
Personally, I think we already have the theory of everything, in the form of string theory. Loop theories are not really a competitor, since loop theories do not contain any matter, plus they are probably divergent. Loop gravity theories only contain pure gravity. Once matter is added, they blow up horribly. It's possible that when these loops touch, they also blow up. So far, string theory is the only theory which does not blow up, and contains matter. The problem with string theory is that we cannot test it directly, since we are a puny Type 0 civilization. The true realm of string theory is the Planck energy, which only a Type III civilization can manipulate at will.


http://rogerobregon.com/michiokaku.htm [Broken]

Questions?
Is it true loop theories do not contain matter, only contain pure gravity, and "blow up" when matter is added?

What does Kaku mean when he says when loops in LQG touch, they blow up?
 
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  • #2
Perhaps he is referring to renormalizability in GFT?
 
  • #3
atyy said:
Perhaps he is referring to renormalizability in GFT?

Is there a renormalizability problem in GFT?
 
  • #4
ensabah6 said:
Is there a renormalizability problem in GFT?

Oriti's review (http://arxiv.org/abs/0912.2441) says they are perturbatively divergent, but at least some graphs are Borel summable (Rivasseau's approach, I think Freidel et al use a different idea, not sure how they are related). I think the work is at a very early stage. Also, I hope GFT goes beyond LQG (which I dislike, because it seems to ignore Asymptotic Safety). It's interesting that Oriti seems to think Asymptotic Safety unlikely, and Rivasseau at the Asymptotic Safety conference at Perimeter also distinguished between AS and GFT. GFT can incorporate matter, but whether it is the matter of our universe is a different matter (http://arxiv.org/abs/hep-th/0512113, http://arxiv.org/abs/0903.3475).
 
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  • #5
atyy said:
Oriti's review (http://arxiv.org/abs/0912.2441) says they are perturbatively divergent, but at least some graphs are Borel summable (Rivasseau's approach, I think Freidel et al use a different idea, not sure how they are related). I think the work is at a very early stage. Also, I hope GFT goes beyond LQG (which I dislike, because it seems to ignore Asymptotic Safety). It's interesting that Oriti seems to think Asymptotic Safety unlikely, and Rivasseau at the Asymptotic Safety conference at Perimeter also distinguished between AS and GFT. GFT can incorporate matter, but whether it is the matter of our universe is a different matter (http://arxiv.org/abs/hep-th/0512113, http://arxiv.org/abs/0903.3475).

Then perhaps Kaku's remarks on LQG are not wrong, given he was speaking to a lay audience. Telling a lay audience that LQG blows up may be ordinary English for non-renormalizable. i.e adding matter to LQG makes it non-renormalizable.

Do we know that string theory computation of graviton scattering amplitude is pertubatively finite and borel summable?

Since both SUSY and GUT remain experimentally unverified, I'm not entirely sure that string theory can be called a TOE since TOE is a larger symmetry that includes gravity and GUT that then gets broken to give us gravity + SM.
 
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  • #6
ensabah6 said:
Then perhaps Kaku's remarks on LQG are not wrong, given he was speaking to a lay audience. Telling a lay audience that LQG blows up may be ordinary English for non-renormalizable. i.e adding matter to LQG makes it non-renormalizable.

Do we know that string theory computation of graviton scattering amplitude is pertubatively finite and borel summable?

Since both SUSY and GUT remain experimentally unverified, I'm not entirely sure that string theory can be called a TOE since TOE is a larger symmetry that includes gravity and GUT that then gets broken to give us gravity + SM.

Kaku's like a (bad) crackpot in his popular persona! But try searching for "Borel summable" here, apparently there are many series in string theory that are not Borel summable
http://books.google.com.sg/books?id=9d-JpKu-2u0C&dq=kaku+conformal&source=gbs_navlinks_s
 
  • #7
I really don't like Kaku he always make science sound like something out of star trek to try to wow the public. He talks about making light sabers and traveling to parallel universes. I don't really give his opinions any credit. He just writes books that mislead the public.
 
  • #8
Finbar said:
I really don't like Kaku he always make science sound like something out of star trek to try to wow the public. He talks about making light sabers and traveling to parallel universes. I don't really give his opinions any credit. He just writes books that mislead the public.

I agree, still, can LQG accommodate matter fields? Can spinors, weak interaction, and chiral fermions be added to LQG? If two loops touch "It's possible that when these loops touch, they blow up"?
 
  • #9
ensabah6 said:
I agree, still, can LQG accommodate matter fields? Can spinors, weak interaction, and chiral fermions be added to LQG? If two loops touch "It's possible that when these loops touch, they blow up"?
:rofl:

Come on 'Sabah give us a break. Loops are not physical objects that can touch.
And LQG is not based on loops. The name is a misnomer which arose c 1990 for historical reasons.
The public Kaku would have to be a total ******* to say something like that. Canonical LQG is based on spin-networks, and these cannot touch. Each one describes a quantum state of spatial geometry. Two networks=two states.
No amount of talking down to a lay audience could justify that degree of made-up misrepresentation and distortion.

The term Atty used was "(bad) crackpot". I.e. dysfunctional as a mouthpiece for science. I don't see how you expect to promote serious discussion starting with a mind-**** sound-bite.
 
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  • #10
marcus said:
:rofl:

Come on 'Sabah give us a break. Loops are not physical objects that can touch.
And LQG is not based on loops. The name is a misnomer which arose c 1990 for historical reasons.
The public Kaku would have to be a total ******* to say something like that. Canonical LQG is based on spin-networks, and these cannot touch. Each one describes a quantum state of spatial geometry. Two networks=two states.
No amount of talking down to a lay audience can justify that degree of made-up misrepresentation and distortion.

The term Atty used was "(bad) crackpot". I.e. dysfunctional as a mouthpiece for science. I don't see how you expect to promote serious discussion starting with a mind-**** sound-bite.

Hi, In addition to Kaku, I've seen Lubos also claim that LQG cannot accommodate SM matter fields in a physically realistic model. Nicolai also levels, in LQG Outside view, something along the lines that "loop theories do not contain any matter, plus they are probably divergent. Loop gravity theories only contain pure gravity. Once matter is added, they blow up horribly"

Kaku might be a crackpot, but these views on LQG seem to be echoed at least by Lubos and Nicholai. I know Jacques Distler also wrote of LQG on his blog, critical.
 
  • #11
Well then, start a thread based on what Hermann Nicolai said. He's credible. So get an exact quote from him. Just don't start a thread based on a Kaku-ery. It's too ridiculous. :biggrin:
 
  • #12
Finbar said:
I really don't like Kaku he always make science sound like something out of star trek to try to wow the public. He talks about making light sabers and traveling to parallel universes. I don't really give his opinions any credit. He just writes books that mislead the public.

I agree to the extent that I know him. All I know, though, is quotes by other people. Whenever I see a statement attributed to Kaku, it's silly. Sometimes extravagantly so.
He may have written sensible things in his books which (having no reason to look at them) I wouldn't know about.

I think it's a mistake to put Kaku centerstage in a discussion and approve of 'Sabah referring to a 2005 paper by Hermann Nicolai. And his paraphrasing what he thinks Nicolai said.

As long as 'Sabah, the thread-starter, is happy with that, why don't we look at what Hermann Nicolai said about Loop?
 
  • #13
ensabah6 said:
Hi, In addition to Kaku, I've seen Lubos also claim that LQG cannot accommodate SM matter fields in a physically realistic model. Nicolai also levels, in LQG Outside view, something along the lines that "loop theories do not contain any matter, plus they are probably divergent. Loop gravity theories only contain pure gravity. Once matter is added, they blow up horribly"

Kaku might be a crackpot, but these views on LQG seem to be echoed at least by Lubos and Nicholai. I know Jacques Distler also wrote of LQG on his blog, critical.

My thread is to assess the accuracy of the claim, not who made it. Still since you have high regard for Nicholai here's a link

Loop quantum gravity: an outside view
Authors: Hermann Nicolai, Kasper Peeters, Marija Zamaklar
(Submitted on 14 Jan 2005 (v1), last revised 18 Sep 2005 (this version, v4))

Abstract: We review aspects of loop quantum gravity in a pedagogical manner, with the aim of enabling a precise but critical assessment of its achievements so far. We emphasise that the off-shell (`strong') closure of the constraint algebra is a crucial test of quantum space-time covariance, and thereby of the consistency, of the theory. Special attention is paid to the appearance of a large number of ambiguities, in particular in the formulation of the Hamiltonian constraint. Developing suitable approximation methods to establish a connection with classical gravity on the one hand, and with the physics of elementary particles on the other, remains a major challenge.

http://arxiv.org/abs/hep-th/0501114

"Developing suitable approximation methods to establish a connection with classical gravity on the one hand, and with the physics of elementary particles on the other, remains a major challeng"

Since it's pdf I can't cut and paste but page 5 "matter couplings anything goes" concludes "is it possible to obtain the correct answer for pion decay when fermions are coupled to electromagnetism in LQG or would LQG predict pi mesons are neutral stable particles" top of page 6.
 
  • #14
ensabah6 said:
Loop quantum gravity: an outside view
Authors: Hermann Nicolai, Kasper Peeters, Marija Zamaklar
(Submitted on 14 Jan 2005 (v1), last revised 18 Sep 2005 (this version, v4))

... Developing suitable approximation methods to establish a connection with classical gravity on the one hand, and with the physics of elementary particles on the other, remains a major challenge.

http://arxiv.org/abs/hep-th/0501114
...

Good! You have an actual quote from a responsible critic! BTW I assume you know that Loop and Spinfoam formalisms merged starting 2007. The picture has been completely revised and Loop exists in two forms: canonical and covariant.

Both versions of Loop are being actively worked on, both are evolving, and since 2008 the signs are that they are thoroughly compatible--two aspects of the same theory.

I don't know how relevant what Nicolai et al wrote in 2005 is now, since the picture has changed so much. Presumably major challenges remain! That's hardly a bad thing. New researchers are joining in, progress is being made. Presumably one major challenge is what Barrau and Grain are writing about---finding the observational footprint of Loop---its early universe effect in the cosmic microwave background. Or not finding--falsifying.

Another major challenge should be connecting with Standard Model matter. Rovelli described where that stands--how matter is to be included--in his talk at the Strings 2008 conference. I don't think any impossibility or incompatibility has been discovered, it is just a good area for future research. But perhaps you can find some up-to-date expert source that says what Rovelli described in 2008 to the string theorists is impossible or would "blow up" whatever that is supposed to mean :biggrin:

See if you can find some up-to-date (2008-2009) critique by Nicolai! That would be interesting. I have to go but I'll check in later.
 
  • #15
marcus said:
Good! You have an actual quote from a responsible critic! BTW I assume you know that Loop and Spinfoam formalisms merged starting 2007. The picture has been completely revised and Loop exists in two forms: canonical and covariant.

Both versions of Loop are being actively worked on, both are evolving, and since 2008 the signs are that they are thoroughly compatible--two aspects of the same theory.

I don't know how relevant what Nicolai et al wrote in 2005 is now, since the picture has changed so much. Presumably major challenges remain! That's hardly a bad thing. New researchers are joining in, progress is being made. Presumably one major challenge is what Barrau and Grain are writing about---finding the observational footprint of Loop---its early universe effect in the cosmic microwave background. Or not finding--falsifying.

Another major challenge should be connecting with Standard Model matter. Rovelli described where that stands--how matter is to be included--in his talk at the Strings 2008 conference. I don't think any impossibility or incompatibility has been discovered, it is just a good area for future research. But perhaps you can find some up-to-date expert source that says what Rovelli described in 2008 to the string theorists is impossible or would "blow up" whatever that is supposed to mean :biggrin:

See if you can find some up-to-date (2008-2009) critique by Nicolai! That would be interesting. I have to go but I'll check in later.

I've not had much success in Nicolai's more recent statements, but Lubos does make this claim,

http://motls.blogspot.com/2006/06/top-ten-results-in-loop-quantum.html

7) Coupling to all the standard forms of matter fields are understood, including gauge fields, spinors, scalars and higher p-form gauge fields. Inclusion of matter fields does not affect the finiteness and discreteness of the area and volume observables.


It is known that loop quantum gravity does not predict any extra matter and even the ad hoc inclusion of the realistic matter leads to profound inconsistencies. One cannot include spinors with chiral interactions (required for weak interactions) and probably not even scalars that could lead to the Higgs mechanism. The fact that loop quantum gravity does not imply the existence of other forces and matter is not a mild problem because it is more or less known that in consistent quantum theories of gravity,
 
  • #16
ensabah6 said:
I've not had much success in Nicolai's more recent statements,
...

Nicolai is an intelligent string theorist---one who belies the childish idea that string and loop researchers are "at war" with each other. He supports and encourages loop research. I believe he was instrumental, as longtime member of the annual Strings conference scientific committee, for getting Rovelli to give an invited talk at Strings 2008. He demonstrates that the "two camps" mentality an unenlightened waste of time. Nicolai is a string theorist who hires loop researchers at his institute. The "two camps" picture is just something to appeal to immature imaginations.


You have offered something said in 2006 by someone whose statements I have never found reliable.

We've discussed the fact that loop underwent a revolution in 2007. How about showing us 2008-2009 critique by a respected academic? Someone of Nicolai's stature, if you can't find something by Nicolai himself.
 
  • #17
marcus said:
Nicolai is an intelligent string theorist---one who belies the childish idea that string and loop researchers are "at war" with each other. He supports and encourages loop research. I believe he was instrumental, as longtime member of the annual Strings conference scientific committee, for getting Rovelli to give an invited talk at Strings 2008. He demonstrates that the "two camps" mentality an unenlightened waste of time. Nicolai is a string theorist who hires loop researchers at his institute. The "two camps" picture is just something to appeal to immature imaginations.



You have offered something said in 2006 by someone whose statements I have never found reliable.

We've discussed the fact that loop underwent a revolution in 2007. How about showing us 2008-2009 critique by a respected academic? Someone of Nicolai's stature, if you can't find something by Nicolai himself.

http://arxiv.org/abs/hep-th/0601129

Loop and spin foam quantum gravity: a brief guide for beginners
Authors: Hermann Nicolai, Kasper Peeters
(Submitted on 18 Jan 2006 (v1), last revised 16 Feb 2006 (this version, v2))

Abstract: We review aspects of loop quantum gravity and spin foam models at an introductory level, with special attention to questions frequently asked by non-specialists.


page 1 "An important issure remains the consistent incorpations of matter couplings"
 
  • #18
Again you have not offered anything current. The approach was transformed in 2007, you offer a Nicolai critique from 2006.

I read both Nicolai papers when they came out. I thought they were helpful, and also that Nicolai learned something between his 2005 and his 2006 papers.

His 2006 paper looked to me like constructive criticism, the kind of thing that points out issues that need to be clarified and which everybody benefits from.

A lot of progress has been made on a number of fronts since the changes in 2007. I really would like it if you could dig up some current (2008-2009) criticism by a respected academic string theorist. Someone, as I said, of Nicolai's stature if you can't find anything up-to-date by Nicolai himself.
 
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  • #19
I think even though LQG is not QFT its not unreasonable to state that it relies on the assumption that gravity is asymptotically safe in the sense that it doesn't give infinities as answers to physical observables.

I've attended lectures by Rovelli on the covariant formalism and my understanding is that the question of renormalizability hasn't yet been addressed. But this is because the frame work for the theory is still being built. I think the hope is that GFT will somehow be merged with LQG such that the renormalizability question can be addressed. So i think its not fair to say LQG ignores asymptotic safety rather it just hasn't got that far yet. The same was true of QFT when it was first being formulated, renormalisability only really came as an after thought.




Marcus, I think your comments on LQG are misleading. There is a limit that must be taken in LQG/spin foams that must be taken to make sure the theory is UV complete. What Rovelli has been working recently on is the low energy dynamics of the covariant theory such that he does not yet take the limit.


The ERG (Reuter, Percacci etc.) approach is coming at the problem from the other side. It starts with renormalisation but has not yet told us what degrees of freedom are being quantized e.g. what the bare action is.



Matter is again another hurdle but the LQG guys do what they can. If they can first prove it works with pure gravity that will be an achievement. Next they need to address matter.
 
  • #20
Finbar said:
I think even though LQG is not QFT its not unreasonable to state that it relies on the assumption that gravity is asymptotically safe in the sense that it doesn't give infinities as answers to physical observables.

I've attended lectures by Rovelli on the covariant formalism and my understanding is that the question of renormalizability hasn't yet been addressed. But this is because the frame work for the theory is still being built. I think the hope is that GFT will somehow be merged with LQG such that the renormalizability question can be addressed. So i think its not fair to say LQG ignores asymptotic safety rather it just hasn't got that far yet. The same was true of QFT when it was first being formulated, renormalisability only really came as an after thought.

Marcus, I think your comments on LQG are misleading. There is a limit that must be taken in LQG/spin foams that must be taken to make sure the theory is UV complete. What Rovelli has been working recently on is the low energy dynamics of the covariant theory such that he does not yet take the limit.

The ERG (Reuter, Percacci etc.) approach is coming at the problem from the other side. It starts with renormalisation but has not yet told us what degrees of freedom are being quantized e.g. what the bare action is.

Matter is again another hurdle but the LQG guys do what they can. If they can first prove it works with pure gravity that will be an achievement. Next they need to address matter.

This is a great post! Thanks! It is a real contribution. If my remarks have been misleading I will try to figure out how, and correct them.
I think the stance you take is constructive. I also think that it is interesting to see what the LQG researchers have NOT yet addressed. Like Asymptotic Safety. Atyy said earlier that he "didn't like" that LQG seems to ignore AS. To me that looks like a potential for explaining why constants run and how the UV fixed point happens (if in fact it does.)

There is a great value in having approaches which "come from the other side" (as you said in your post.) To paraphrase what you said---Reuter Percacci start by putting renormalization in, they start with running constants, but they don't yet tell us the underlying microscopic machinery. Coming from the other side, the loopsters offers some very interesting microscopic machinery but (although they are already talking about addressing renormalization and are certainly aware--eg Ashtekar's comments) they don't yet explain the renormalization group flow using their machinery.

My comments are not intended to suggest that the loop program has solved all its problems or is certain of success! What I find exciting, in fact, is the combination of risk and the remarkable progress made since around 2006 in both loop cosmo and the main theory. See that other thread for example, papers by Ashtekar et al on LQC inflation and by Barrett et al on covariant LQG asymptotics.

I'd like it if 'Bah or someone could dig up some actual competent critiques of loop by respectable people (as I said, someone on par with Nicolai). Knowledgeable criticism can be enormously helpful. Yours included. Please spell it out in as much detail as you have time for.
 
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  • #21
Don't you think it's more fun if LQG heads away from AS? I'm sure Rovelli is not going to enjoy being rescued by particle physicists! :tongue2:

I hope we can get unification of gravity and matter in GFT, as Livine and friends seem to be getting matter to emerge from some GFTs.

Another interesting route away is maybe Han and Thiemann's speculation of the Bergmann-Komar group as being more fundamental than the diffeomorphism symmetry.
 
  • #22
atyy said:
Don't you think it's more fun if LQG heads away from AS? I'm sure Rovelli is not going to enjoy being rescued by particle physicists! :tongue2:
...
Drollery aside, I don't understand what you are talking about, Atyy. The loop program is hardly in trouble, or in need of rescue :biggrin:
 
  • #23
marcus said:
I'd like it if 'Bah or someone could dig up some actual competent critiques of loop by respectable people (as I said, someone on par with Nicolai). Knowledgeable criticism can be enormously helpful. Yours included. Please spell it out in as much detail as you have time for.

I don't but how does recent research change the 2006 string criticism on how well LQG handle matter couplings?

Thanks
 
  • #24
ensabah6 said:
I don't but how does recent research change the 2006 string criticism on how well LQG handle matter couplings?

Thanks

Although this review motivates LQG in Rovelli's usual bizarre way (particle physicists versus relativists), the later sections are quite even-handed. (So Marcus, I would say if AS works, it's Rovelli's and Smolin TTWP's LQG that needs rescuing :biggrin: - fortunately LQG is more than those two guys, there's Reichenbach, Thiemann, Freidel ... :smile:)

http://relativity.livingreviews.org/Articles/lrr-2008-5/ [Broken]
"The coupling of fermions to the theory [206, 207, 54, 287] works easily. All the important results of the pure GR case survive in the GR + fermions theory. ...

The extension of the theory to the Maxwell field [169, 126] and Yang–Mills [290] also works smoothly ...

The coupling of matter in the spin-foam and group field theory formalism is not yet clear, in spite of considerable work in this direction. ..."
 
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  • #25
Merry Christmas and Happy New Year by the way! Too much was going on and I didn't get around to replying to your holiday greeting.
Reisenberger?
 
  • #26
marcus said:
Reisenberger?

Yeah! :redface: :rofl:

marcus said:
Merry Christmas and Happy New Year by the way! Too much was going on and I didn't get around to replying to your holiday greeting.


 
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  • #27
atyy said:
Don't you think it's more fun if LQG heads away from AS? I'm sure Rovelli is not going to enjoy being rescued by particle physicists! :tongue2:

I hope we can get unification of gravity and matter in GFT, as Livine and friends seem to be getting matter to emerge from some GFTs.

Another interesting route away is maybe Han and Thiemann's speculation of the Bergmann-Komar group as being more fundamental than the diffeomorphism symmetry.

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.
 
  • #28
Finbar said:
I really don't like Kaku he always make science sound like something out of star trek to try to wow the public. He talks about making light sabers and traveling to parallel universes. I don't really give his opinions any credit. He just writes books that mislead the public.
Strings with its extra dimensions and landscape allows for all kinds of speculation including worm holes, braneworlds, cosmic strings, parallel universes, multiverses, time travel, type 2 civilization, etc.

I suspect one reason Kaku disses LQG is that LQG, unlike string theory, doesn't offer sci-fi star trek wow scenarios. If nature is simply quantized 4d GR + SM, with quantized GR disallowing for wormholes and time travel, I doubt there's much Kaku or anyone to sci-fi wow with.
 
  • #29
ensabah6 said:
I still fail to understand the importance of BI should the LHC -- or other experiments, discovers SUSY and/or higher dimensions. If no evidence of either is forthcoming from LHC or other observations, like proton decay or dark matter detection, string theories lack of BI still doesn't seem to matter.
IMHO, the only thing that matters is making contact with experiments and real world predictions. If LQG is BI and makes wrong predictions, or if LQC makes predictions of gravity waves effect in Planck era Big Bang on polarization of CMB which is or is not verified. I would put credibility on a theory that lacks BI but is experimentally or observationally confirmed, than a theory that is BI and is inconsistent with experiments.

Well you could say the same of anything, unitarity, Lorentz invariance, locality. In the end the experiments are what matters.
 
  • #30
Finbar said:
Well you could say the same of anything, unitarity, Lorentz invariance, locality. In the end the experiments are what matters.

One of criticism of LQG is that LQG has a fixed topology whereas string theory proves topology can change.
Maybe topology does change -- but not in a manner predicted by string theory. I've wondered how this can be experimentally tested.
 
  • #31
Finbar said:
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.
 
  • #32
Finbar said:
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.

Finbar said:
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).
 
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