Why is LQG a dead end but not m-theory?

[wolram]

http://arxiv.org/list/gr-qc/0311

a list incuding tests for gravity theories 2003

 

[nonunitary]

Most high energy physicists today are convinced that loop quantum gravity is *probably* a dead end, and therefore they don't work on it. Of course, they can be wrong. However, I don't think that they are wrong.

However, I think that they can be wrong.


Loop quantum gravity is a different type of theory. Its goal is not to explain the origin of quarks; leptons; electromagnetic force; weak force; strong force. Its only goal is to claim that Einstein's gravity can be relatively easily reconciled with quantum mechanics. Most leaders of theoretical physics believe that this opinion has been proved wrong beyond any doubts. General relativity treated as quantum theory has a lot of technical as well as conceptual problems - and the divergences in Feynman's diagrams are the most obvious problem.



The loop quantum gravity people argue that these calculations that clearly show the inconsistency of quantized general relativity should not be taken seriously. We should not trust our perturbative calculations, they say, and they believe that if we just reparameterize the variables in a relatively obscure way, the problem will be solved.

This is NOT what the LQG people say. The problem when you want to trash a theory as you do with LQG, is that sometimes you do not quote in a precise way. What a LQG people may say is that the perturbative methods are doomed to fail because they assume a continuum picture at all scales. This assumption might simply be wrong. Besides, LQG is NOT a reparametrization of variables (obscure or not, that'a another problem). This would be dumb to claim. For anyone that really understands what´s going on, it is clear that the asumptions made on the construction of the non-perturbative methods are way beyond a simple reparametrization.

There is a lot of evidence that it is not true, and there is no evidence that it is true. Loop quantum gravity essentially plays with a sort of randomly chosen simple discrete system that is, in some formal sense, equivalent to Einsteinian gravity at very short distances. Nevertheless we know that if we discretize a system, we are not guaranteed that the physics will follow the original laws at the long distance scale. Just the opposite: there is a principle that everything, every term, every correction, every possible interaction whose existence is not forbidden by a symmetry, WILL occur.


Again this depiction of LQG is essentially, a random choice of words that simply give the impresion of LQG being a random theory. It is simply NOT true that LQG is build by a discretization of anything. For anybody who understands it, it is clear that the theory is defined in the continuum. The discrete picture of the geometry is an outcome not an input.


If we apply this to loop quantum gravity, it turns out that more or less smooth space will NOT exist at all. There is no evidence whatsoever that smooth space can emerge from loop quantum gravity. Most likely, the spin networks want to be crumpled to a very chaotic web whose radius is comparable to the Planck length.


This is your very particular prediction of what will happen in LQG. While it is true that there is no evidence that a smooth space will ever emerge, there is no evidence that it will NOT emerge. It is a bit like the fantasy that whenever we know what M theory is, then it will be an ultra-cool theory solving all our expectations. It may or may not happen. We don't know.

Imagine. It is a theory based on all possible - and very unlikely - prejudices that people had about the real world decades ago, and it is very likely that it does not allow the smooth space to exist. This simply means that most likely, it contradicts *all* observations that have ever been made. Is there something in the real physics Universe that can exist without the space?


This, most likely, is not what LQG will have as a description of what you want to call space. This claim is based in your prejudices that a theory that does not look like string theory is wrong.


Loop quantum gravity contains no successful self-consistency checks - unlike thousands of mathematical checks that just work out in string theory. Every attempt to calculate something in loop quantum gravity always leads to a result that seems wrong.

NOT true

The black hole entropy was calculated with a wrong coefficient by a factor of ln(2)/sqrt(3).pi. Ridiculous numbers. Attempts have been made to remove this discrepancy, and they have led to other predictions - for example about the quasinormal modes of the black holes - and these predictions have been shown incorrect again.

Not correct. The BH calculation for type I IH is used to fix the parameter once and for all. It has been checked in ALL cases (i.e. the self-consistency check you mention), that this number is consistent with: distorted non-rotating horizons, rotating horizons, and with the following couplings: Maxwell field, dilaton, and for a non-nomimally coupled scalar field. These are highly non-trivial consistency checks.
About the quasi-normal modes, it is not clear what they mean and whether LQG should even predict anything about them. Whay is clear is that they do NOT represent a failure of LQG as you claim.


Nothing seems to work, and loop quantum gravity seems to be a perfect example of the disasters and inconsistencies that one encounters if she wants to describe quantum gravity without string theory.

Also not true.


String theory really seems to be the only game in town.


This claim is not even wrong. Even Brian Greene has conceded that it is not the only game in town (but the must fun he adds). Personal tastes on what is more fun are ok, but denying a theory as being "not in the game", based on missinformation and some prejudices is not fair game.


When Strominger and Vafa calculated the correct black hole entropy from string theory in 1995, loop quantum gravity people wanted to do the same. It did not quite work. Because today we are expecting the experiments (not only accelerators) to confirm many possible cosmic scenarios that emerged from string theory and that are very exciting and reasonable.


The logic of this phase is scary. Is really your argument that LQG can NOT calculate the entropy because we are expecting very interesting experiments in the future?

No comments.



I know it is difficult to understand the different situation of string theory and loop quantum gravity without actually going into the technical details,...

I couldn´t agree more


.. but I also know that loop quantum gravity simply can't compare with strings once you *do* study the details.


... but when will YOU study the details?

 

[Stingray]

Originally posted by eigenguy
I think the former possibility is implausible since physics is an exact science and not politics and the people who work on this stuff are very very bright and just want to find the truth any way they can before they die.

Physics is not an exact science. No such thing exists unless you consider mathematics to be science. There is a LOT of politics in physics. Most of that is just because there is so much to know. A standard graduate education gives you a practically insignificant fraction of the "known" body of knowledge in physics.

String theory and LQG are both extremely difficult sets of ideas, and I would be surprised if more than a small handful of people truly understood everything significant that is known about both. Many people I've met in string theory have no clue what LQG is about, and vice versa.

My understanding of the split between string theory and LQG is just an historical accident. Fundamental physics was dominated by high energy phenomena for a very long time. New things were discovered every time a new accelerator experiment was done. All of these unexplained phenomena obviously generated a lot of interest, and the field became very large very quickly. By contrast, relativity has always had a small theoretical following, and essentially no experimental work.

String theory tends to do things the particle physics way, and LQG tends to take GR as more important. Since there are a lot more advisors around who've had a history in particle physics rather than GR, more people tend to go that way. All of that in turn generates the popular books, etc which make string theory even more well known. And all of that generates the funding :)

Another thing that makes string theory popular is that it attempts to explain everything - LQG only tries to explain gravity.

 

[lumidek]

Failures of LQG

Dear nonunitary,

I sort of misunderstood who you are, but it is probably polite to answer anyone who is interested in physics. Based on your text, I guess that you've read Three Roads to Quantum Gravity by Smolin or something like that, right, and you want to learn how real physics looks like, don't you? You might find my answers helpful, especially because it seems obvious that there are many things that I can teach you about LQG - I've worked on it and studied it, one could say "in detail".

You have probably a very, very long way to go if you want to learn something about quantum gravity, but you should not be scared because of this long way in front of you.

First: continuity of space. Well, yes, various LQG people are saying various things, but the more important thing is the truth. The fact is that the variables to describe gravity are chosen in such a way that the areas are quantized. One needs about 5 minutes to understand why the area quantization is a direct consequence of the choice of the variables. For example, see the 10-page paper

http://arxiv.org/abs/gr-qc/9806079

The presentation could be written on 3 pages, too, if one used the mathematical language that most up-to-date theoretical physicists know, and these 3 pages would essentially contain everything about LQG that is at least a little bit interesting. If you chose a different gauge theory parameterization of the metric, you could not obtain the usual area quantization known in LQG. In this sense, the area quantization is put in, because it determines the way how you should translate the metric into the language of gauge fields.

Different parameterizations lead to different results. There are thousands of other ways to discretize non-renormalizable theories such as gravity, and all of them lead to a similar type of problems. The special feature of LQG is that it assumes the metric to be a good variable at the Planck scale, which we just know to be naive. In my opinion, this characteristic feature of LQG is more of a disadvantage than an advantage.

One of the most far-reaching insights in particle physics of the 1970s is the concept of Renormalization Group (RG) that implies that field theories and their degrees of freedom are associated with a scale, and the necessary description varies with the scale. In fact, we can derive how the equations of gravity are affected by the change of the scale, and in the case of gravity, we know how this force looks at astronomical scales, and therefore we know that it must look different at short distance scales. The UV divergent structure of GR proves that Einstein's equations are missing new physics and new degrees of freedom at short distances - new physics that is necessary to reconcile gravity with quantum mechanics.

LQG assumes that Einstein's equations are essentially correct exactly in the regime where we know they must be wrong, and because physics at different scales is guaranteed to be different, physics of LQG at long distances can't agree with Einstein's equations i.e. with the observations.

Concerning non-uniqueness of LQG: there are hundreds of different unrelated versions of it once we want to include matter and not just gravity - and gravity without matter is really useless. There are many ways how people try to add gauge fields and fermions; none of them can really reproduce quantum field theory, and a really unsolvable problem seems to be adding the scalar fields (i.e. the Higgs scalar that is more or less necessary for electroweak symmetry breaking) to the LQG framework.

LQG has not even proved that the "canonical formalism" is equivalent to the path-integral-like formalism - i.e. the spinfoams - and there are good reasons to be afraid that they can't be really equivalent. All of these things are random and mostly unrelated ideas - whose intersection is made of the wrong prejudices about the Planck scale geometry - and there does not seem to be a working theory behind them.

There is an extremely strong evidence (almost a proof) - based on the Renormalization Group - that smooth space can't emerge from LQG. There is comparably strong evidence that LQG can't be compatible with special relativity - even most LQG practitioners admit that, and some of them even want to transmute this obvious flaw into a virtue. I don't know what you meant by your comparison with M-theory: M-theory certainly has no problems to get smooth space, gravity and essentially all other required particles and forces at long distances. What is the point of this confusing comparison?

Concerning your sentence "This claim is based in your prejudices that a theory that does not look like string theory is wrong", let me tell you something, and let us hope that it won't make you too frustrated. That claim of mine - that the tens of the *assumptions* of LQG have been proved incorrect - was not really based on prejudices but rather thousands of insights that people have accumulated in thousands of papers about field theory, particle physics, and its gravitational extension (string theory) during the last 30 years, and it is not too likely that all these insights are incorrect.

Concerning the Immirzi parameter. The Immirzi parameter is a multiplicative constant that measures how wrong prediction of the black hole entropy - based on a very unconvincing and heuristic treatment of LQG - predicts unless one tries to cheat. If one tries to cheat, he can argue that this discrepancy does not really matter and things can be "renormalized". Unfortunately all attempts to calculate the "renormalization" of Newton's constant - the Immirzi parameter - have failed so far, and the research direction based on quasinormal modes is the newest example of this fact.

There is nothing surprising or non-trivial about the LQG calculation of the entropy of the horizons. The fact that the result is proportional to the area was calculated by Hawking, not LQG, and it was inserted as input to the heuristic argument based on LQG. When one assumes that the entropy comes from objects spread over the horizon (and he or she simply cuts the interior of the black hole by hand, without explaining why is it allowed), it is not surprising that the entropy will turn out to be proportional to the area. However, the proportionality factor - the only non-trivial number that might have been calculated - is wrong.

Well, you say that it is "not clear whether LQG should predict anything about them". Well, a theory of gravity must predict everything about the gravitational equation. It might be questionable whether a theory XY is able to calculate AB, but one thing is clear: LQG is not able to predict the QN modes, it is also not able to predict the existence of gravitons, Newton's force, the quantitative black hole thermodynamical properties - simply nothing that we usually call "gravitational phenomena". The reason is simple: it is a discrete system that is formally related to geometry, but its physics has nothing to do with geometry.

Concerning LQG, experiments and black hole entropy - the answer to your question is no. My point was only to demonstrate that the claimed "successes" of LQG are just parodies about real physics and that LQG has no reasonable prediction whatsoever that the future experiments might test. You may be sad about it, you might cry, but it is the last thing that you can do about this fact.

I hope that you will find this text helpful, and that you will kindly forgive me that I won't reply to your personal attacks or comments "you are wrong" that are not supported by any argument. At any rate, it is sort of amazing how the "popular science" can often differ from real science. There are so many people who are interested in telepathy; ESP; LQG and so forth - even though these fields are considered to be "most likely incorrect" by most of the scientists (especially in the case of the ESP phenomena). There are no new jobs for the LQG people in the USA. I sort of wonder where does the idea that LQG is something that can be compared to string theory come from?

Sincerely Yours
Luboš

 

[nonunitary]

Dear Lumidek,

Thank you very much for your message trying to explain your views of LQG. I don't argue that you have studied it "in detail", but I still am under the impression that you do not make justice of it. I agree that you may not like it, that you think is "most likely" not correct, and that you feel compelled to attack it in any possible way. I also agree that one should just look at the facts. I will not compare LQG to any other theory of quantum gravity (including strings), so let's keep the discussion about LQG. Also, from your response, it seems obvious that there are many things that I can teach you about LQG, and I am offering my advise in case you want to take it.

The ultimate test for a theory is whether it is compatible with existing experiments, and not whether we like many dimensions, variables, symmetries, of RG arguments.
Let me borrow your style for claiming things, and assert that: There is no single experiment up to date that contradicts LQG.

Does LQG have many challanges in front of her? yes.
After all, it is not a finished theory, but one can safely say that it IS a theory that has some predictions and when all the steps that need to be completed are completed, will have more predictions. Only then, by comparing with experiments can one tell whether it is dead or alive. Same is true with any other theory.

One can make bets, and use "common sense", academic demographics, "we are more than you" kind of arguments, or the like to give psicological support to them. Unfortunately those are only of anecdotic value, and one has to resort to the results.

In all of your postings here and in S.P.R. you have not supported you
claims about the wrong-ness of LQG, and I would like it very much if you could make them precise and organized. We know you are a very talented physicist and certainly able to write it to the physics community, in, say, a posting to the arXives. (Also in my previous message, there was no personal attack. Telling you that you don't understand something is not a personal attack.) I would like to see in particular: The proof, - based on the Renormalization Group - that smooth space can't emerge from LQG, and that issue of the Immirzi parameter. Why is is a wrong number? If one takes LQG as it is and asks questions one has to give a number for this parameter. The is no test that tells us that LQG is inconsistent, which would be the case if one needed different values of it for different processes and tests.

I am sure that if you make you case based on strong evidence, even LQG practitioners will appreciate it. Would you do that for us "physics boys"?



Nonunitary

 

[jeff]

Originally posted by lumidek
I sort of wonder where does the idea that LQG is something that can be compared to string theory come from?

I've noticed this tendency mainly in older members. I think they're intimidated by the complexity of string theory and prefer to believe that the comparatively simple LQG program is equally viable. I've posted this opinion a number of times.

 

[ranyart]

Originally posted by jeff
I've noticed this tendency mainly in older members. I think they're intimidated by the complexity of string theory and prefer to believe that the comparatively simple LQG program is equally viable. I've posted this opinion a number of times.

oops!