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Why do stringy people don't like LQG?

  1. Jun 19, 2009 #1


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    I know a couple of arguments why the loopy guys don't like string theory (perturbative quantization, background dependence, too many statements w/o proof, mysterious M theory, ...)

    But can anybody tell me why stringy people don't like loop quantum gravity?

    I mean serious arguments, not something like "Witten doesn't like it, either" or "string theory is the only game in trown". Why is the string community convinced that LQG is just wrong? What are the physical or mathematical reasons?

  2. jcsd
  3. Jun 19, 2009 #2
    I don't know whether I can answer to that, mainly because I am not certain whether the question is 100% true for "stringy people" to begin with (as well as the other way around-- I mean, it's quite a strong, generalized question).

    Meanwhile, you may want to take a look at these:

    http://arxiv.org/abs/0705.2222 (Ashtekar's FAQ on LQG; see second part of this paper)

    http://arxiv.org/abs/hep-th/0501114 (Loop quantum gravity: an outside view by H. Nicolai et al.)

    In any case, there is one point that I often think about: how far a "crash" of two schools is involved here. String theorists mostly coming from particle physics; LQG theorists mostly coming from general relativity.
  4. Jun 19, 2009 #3
    I hope new generations are more open minded and the "string wars" something of the past. I mean, that the younger ones could be wise enough to see possible benefits of certain techniques from both schools/approaches and amplify their vision towards a theory of quantum gravity.
  5. Jun 19, 2009 #4


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    Thanks for the hint - I already know both papers :-)

    The first one is rather helpful in order to understand the main open topics in LQG - nevertheless it's a LQG perspective. I mean all open issues are somehow technical and Ashtekar is very optimistic that they can be solved. I don't think that this is really the reason why you distrust the whole approach.

    The second one has been answered by Thiemann: http://arxiv.org/abs/hep-th/0608210 (Loop Quantum Gravity: An Inside View). Unfortunately I do not know if there is a response from Nicolai. Thiemann claims that most technical problems mentioned by Nicolai are solved or some progress is made at least.

    Nevertheless - thanks for the reminder; I will study the papers carefully a second time.
  6. Jun 19, 2009 #5


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    The string wars thing is old and boring and this post is just going to bait yet another rehash, with more or less laymen or nonspecialists (very few LQG or String theorist are active here) offering opinions about the merits. I suggest you look back a couple years ago for countless posts on the internet/usenets for the various arguments or pseudo arguments involved.

    Distler/Motl/CosmicVariance/Notevenwrong/Asymptotia/physics.research and so forth. The scientific content was actually minimal and the noise/confusion extreme outside of a few back and forths on Distlers blog where there was actual specialists from both sides present, so good luck.
  7. Jun 19, 2009 #6
    I think that Motl/Woit are both far too extreme in their positions to have any constructive dialog. And constructive dialog is what will need to happen to mend these fractures in the theoretical physics communities.

    I agree, and I think that in such a mathematical science we should be able to come to more of a consensus about how funding is allocated to the various branches of research; there may not be new experimental input for a long time.

    For better or worst most specialists are too busy working on their speciality to participate in these kinds of discussions. But I don't think that demerits the discussion, since the "string wars" is mostly fought on the battleground of public opinion, and as goes public opinion so goes to some extent scientific funding.

    For example, I don't know whether this is true, but since Lubos Motl left Harvard around the same time as Woit's and Smolin's books appeared, I have a theory that he was "let go" (failed to get tenure) in part because it was a particularly disfashionable time to be doing string theory, and that this is what fuels the anti-lqg comments on his blog.

    Another way to see the ongoing relevance of the string wars is by looking at the comments that are left by laymen on the articles on the various websites that aggregate technology and science news. Articles about string theory still cause heated debates, the laymen are carrying on the war. Even the TV show "Big Bang Theory", which is as close as most folks come to modern physics, portrays (circa 2009) the strings vs loops debate as sufficient to destroy a romantic relationship (haha, "what will we raise the children to believe?)

    The point is that public opinion matters, this is why the NSF funded the production of "The Elegant Universe" TV special in the first place. And right now, public opinion is mostly negative towards string theory, and I am worried that there is a lot of doubt and distrust of theoretical physics in general, and if people are looking down on theoretical physicists instead of looking up, then the world has its values system upside-down and that's dangerous.

    Having so far argued only for the existence of this thread, which I hope will be pleasant and specific, in my next post I will raise some answers to the bold question in the OP.
  8. Jun 19, 2009 #7
    I'm a fan of the LQ side due to the importance of GR in it's formulation.

    I love the mathematical aesthetics of string theory, it is intoxicating.

    I can fully understand why people wouldn't want to leave the conformting grips of string theory.

    It is so easy to get tangled up in there, so many questions, I get the impression that these people sleep on equations in dorm rooms. So much beautiful math, and it tantalizingly tries to connect to reality.

    I'm trying to write something which will be a mix from LQC and Stringy Models pushing a quantized GR with extended temporal interaction.

    We're all young, we should be asking new question, and working on these problems on our own if need be.

    Fighting just helps people who don't want things to change.

    We're all on the same side, the side that seeks to understand all things.
  9. Jun 19, 2009 #8
    Non Stringy Person here, offering an honest truce so all sides can understand this.
  10. Jun 19, 2009 #9


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    This is a constructive post and could be a useful thread especially if the question in bold is interpreted broadly as not limited to string folks but very generally "what do you think are the weak points in the LQG program?" and can you point out any obstacles to progress that you think are insurmountable?

    People in the LQG research community are sometimes self-critical and you get internal discussion of what are the most serious problems. We could try to gather some links to that "insider" crit as well.

    No need for hostile stuff or ad homs heaping scorn on fellow researchers or defensiveness or handwave dismissal obviously. that wasn't what Tom's opener post invited. Of course not that stuff!

    Personally I've found serious critique of LQG helpful, especially if really well informed. So I'd be happy if this thread could blossom into that. I might not be much of a contributor but I'd be an interested reader!
    Last edited: Jun 19, 2009
  11. Jun 19, 2009 #10


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    Sigh, fine. I'll put my 2 cents in for what its worth.

    The most obvious reason many HEP people aren't interested in LQG is b/c for a long time it was just about gravity. No standard model, no matter. This isn't just a little problem to be left for another day, in many ways its the whole shebang. The feeling is that even if LQG (or any other theory of pure quantum gravity) was successful, they will have found a mathematical model that is at best only a horribly simplified version of the real world, b/c in the real world there is also matter.

    Technically (and this is a theorem), at high energies there is no decoupling limit where you can decouple gravity from matter degrees of freedom. Even if you find a nonperturbative finite solution of quantized Einstein Hilbert gravity, there is absolutely nothing that guarentees that any future unknown interaction term(s) does not spoil the nice properties you found as you no longer live in the same universality class.

    This isn't strictly a statement about renormalizability vs nonrenormalizability, but the effects are the same. In order to make a prediction about actual experiments in the QG regime, you would need to know (in addition to your gravity solution) all the physics between the electroweak scale and the Planck scale with great accuracy.

    Conversely, in this sort of scenario the knowledge of how pure gravity works does not in principle shed any light whatsoever on whats allowed or not allowed in the matter sector and so can't be used to help model builders in their search for beyond the standard model physics.
    Last edited: Jun 19, 2009
  12. Jun 19, 2009 #11


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    @Marcus: thanks for your your support :-)

    @Haelfix: it was definitly not the intention to re-start something like the well-known Woit-Motl-... discussions. Waste of time!

    Regarding your last statement: the main reason seems to be that loop quantum gravity is only about quantum gravity :-) Yes, that's the limitation of the approach; it is not the intention to develop a ToE. But LQG is - as far as I know - consistent with almost all matter contents (electromagnetic fields and fermions have been investigated; I remember one statement regarding SUGRA).

    In general that's a strange argument! Ordinary quantum mechanics does not say a single word about gravity, nevertheless even physicists focussed on gravity take it seriously. So such a limitation does not kill an idea from the very beginning.

    Let me clarify my intention: as usual the inside view is mainly focussed on "technical details" (well-defined Hamiltonian etc.). If the loopy guys are able to figure this out, this will not change the way the string community thinks about the whole research program, I bet. So there may be an outside view which criticizes the whole approach - not because of technical problems but because of physical reasons.

    That's what I am interested in.

    Of course Nicolai's paper could be a good starting point.
  13. Jun 19, 2009 #12


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    Civilized, first of all, excuse me for my behavior that other day towards you.

    Secondly, the man went out not really because of the "string wars"... But because of certain "politcialy correct" stuff not related to physics.
  14. Jun 19, 2009 #13


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    I don't think you understand. LQG or any other pure QG theory may or may not be consistent with adding some arbitrary matter term, one can always do that, but thats not the point.

    The point is you cannot say anything about what the resulting theory actually *is* without knowing the full details of all the matter terms and then resolving.

    This is a bit subtle, and its hard to say in words. Its a bit like the QCD case, but upside down.

    Imagine you happen to live a regime where pure QCD was applicable in a world with only QCD and nothing else, and you had no idea about the *low* energy world and were interested in finding that out. One day, a prophet miraculously handed you a solution of gluon dynamics at some scale that he happened to have solved on his mountain. The problem is you don't know anything about quarks. How many of them there were, and what their nature was.

    Absent that knowledge, even though you think you understand the strong force, you still do not have any idea about real QCD. Your predictions would utterly fail an order or two of magnitude into the IR from wherever you started. Now lets say that someone handed you an observation of the top and bottom quarks. You would develop a theory that would work for awhile, but soon enough it would start going haywire again. Its important to note that its not just the new quark terms that are appearing that messes things up, but also the very physics of the gluons themselves are changing in some complicated function that depends on the other interactions that are present and the energy scale being probed. All of a sudden, you would start doubting the very foundation of the gluon dynamics that the prophet gave you in the first place (amusingly in this case: it might morph from a theory of particles to a theory of strings and fluxtubes)

    In other words, you cannot solve for these things seperatedly (say solve for the quarks, and then solve for the gluons and then add them together), its much more complicated than that b/c they feedback into each other in extremely complicated ways. Even today, with almost unlimited nuclear experimentation, we still have no idea what real QCD is in the IR.

    Now in the gravity case, this is almost infinitely worse, b/c its not just quarks but literally *everything* including physics we might not have even imagined yet. Everything *must* couple to gravity.
    Last edited: Jun 19, 2009
  15. Jun 19, 2009 #14
    You just took my job away.
  16. Jun 19, 2009 #15
    In as much as this thread is a brainstorming effort, I will offer my thoughts in "no particular order, one at a time" so that I don't get overwhelmed trying to order them. Also, even though I do condensed matter I also attend the weekly seminar on particles/strings at my university and so as much as possible I will try to share the opinions I've gleaned off of true string/particle theorists.

    (*) LQG does not connect well enough to the rest of modern physics.

    One of the things about putting in the extra work to get an article through peer review is that the referees will force you to reference other published articles. This has the side effect of making a researcher spend a good deal of their time connecting their ideas with what other people are doing or have done, even if said researcher does not like doing so and thinks it unnecessary, and has to stretch the applicability of the reference because their idea are so "out there." And so we get some genuine references along with a large number of "BS references" that are only put in to satisfy referees.

    Sounds like a bunch of BS, right (science in the sense of little "s") ? But guess what, the time-honored journal policies are wiser than any of us who find this exercise pointless, since in the long term this leads to a fully connected network of literature where the truly important connections are highlighted to future researchers and the BS references, if they don't ever pan out in future developments, are just forgotten.

    In comparison the LQG community exists almost on a totally seperate subnetwork, highly connected within itself, but barely connected to the larger network of physics as a whole. I'm not sure if this state of affairs is necessitated by the nature of LQG (is it really that different then everything else?) or if it is just a result from the near total reliance on ArXiV preprints to communicate results. Put another way, most scientists I know would find life much easier if they could plug away at their own ideas without being forced by reviewers to connect these ideas to other peoples work, but these same scientists know that they can't do this or else they will end up outside the mainstream ( a perhaps loaded sounding but I believe still honest reformulation of this thread's title would be "Why is LQG outside the mainstream?").

    I think this is very insightful. Of course string theorist do study GR more than almost any other type of physicists, and likewise I'm sure for loop theorists studying QFT, and so I think this comment really speaks to the approaches (rather than the training) of the two schools.

    In particular, the LQG core principles of diffeomorphism invariance and background independence bear a striking similarity to the principles that guided Einstein (and Hilbert) to develop GR. The fundamental starting point for LQG is not just that these symmetries exist in nature, but also that they are good symmetries to guide the development of the theory (just as Einstein used general covariance to guide the development of GR).

    String theory does not use such strong symmetries as input, instead it posits that relativistic strings exists and then writes down the most simple Lagrangian that respects Lorentz invariance. One of the immediate consequences is that the Lagrangian turns out to be conformally invariant, which is a symmetry that is comparable to diffeomorphism invariance.

    In other words, LQG posits an extreme degree of symmetry, while string theory posits an extremely different kind of matter/field content. The former approach is that taken by GR, and the latter is used in particle theory (for example by phenomenologists studying dark matter).

    Ultimately I think this is just a difference in approach, not a key reason why "stringy people don't like LQG."

    Right, even if LQG is not meant to describe the origin of non-gravitational interactions, it will still need to couple to these interactions in a well-defined way. In QFT we have already have a good understanding of how to couple fermions to gauge fields, so that's where the bar is set.

    This is an even stronger point that I had not heard before, I'd be interested to see how LQG diffuses this criticism.

    More later, I for one am really interested in what everyone has to say in this thread.
    Last edited: Jun 19, 2009
  17. Jun 19, 2009 #16
    Stringy people like to make things up. It's contrary to the very philosophy of LQG : making sure we need to.
  18. Jun 19, 2009 #17


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    Civilized, did you know that it was recently found that something similar to topological strings were naturally emerging from spin networks?

    BTW, you still has not forgave me.
  19. Jun 19, 2009 #18
    I completely forgive you MTd2, I don't think I organized my thoughts well enough in that thread, now that I see some thoughtful people on this board I am trying to think my posts through more.

    Do you mean string net condensates (Levin, Wen cond-mat/0407140) ? The work looks interesting, I think I'll read it soon (this subforum seems like a good place to discuss it as well). Of course, even though it involves the word "strings" I believe it is a low-dimensional theory, too low for them to be the quantized relativistic strings of string theory, afaik.
  20. Jun 19, 2009 #19


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  21. Jun 20, 2009 #20


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    My understanding is that LQG doesn't fit in this philosophy:
    1) Einstein gravity is not perturbatively renormalizable, therefore either
    2a) gravity is non-perturbatively renormalizable (Asymptotic Safety, CDT)
    2b) gravity is emergent (Strings, AdS/CFT, "condensed matter" approaches - the various emergent approaches differ in what else emerges)

    But LQG seems to divide the possibilities differently - background independent or not, with background independence being fundamental. Yet LQC, at least in its early days, was not fully background independent. So if LQC is acceptable as an interesting lead, then maybe background independence is not so important after all? Or is LQC not an interesting lead? What is the current situation with respect to LQC and background independence? What is the current situation with respect to LQC and LQG? If background independence is the true dividing line, then what is the flaw in the "either AS or emergence" philosophy? If there is no flaw in the "either AS or emergence" philosophy, then presumably LQG would be like AS - but if so, then why not just research AS?

    Of course, this philosophizing may be irrelevant - since Einstein used the wrong philosophy of "general covariance" to get the right theory of general relativity. So maybe background independence is not the true dividing line, but LQG is the correct theory.
    Last edited: Jun 20, 2009
  22. Jun 20, 2009 #21
    Ups I didn't mean to ignite another battle, I just answered a question by telling how one of the most prominent and influential particle physicists responded in a discussion.

    Point is that it is not so that string physicists "hate" LQG, as far as I can say; it rather seems the other way around, the whole war was started by certain circles to gain more visibility of their field. And indeed, LQG wasn't even on the radar screen for most; it was and I guess, still is, considered a niche field with limited scope. Some of the basic ingredients, like strange quantization procedures that don't even correctly reproduce the harmonic oscillator are considered dubious, and there is to my knowledge even today no convincing proof that LQG does reproduce GR in the long distance limit. LQG seems to be stuck in some kind of "topological phase" and from there is still a large step to be made in order to recover gravity (and the tricks that work in 3d don't apply to 4d).

    For these and other reasons this theory does not look compelling and promising to many as of today, certainly it is understood that LQG is work in progress --just like string theory--, and the moment some really convincing breakthroughs would be made, more people would study it. It is simply not so that people "hate" LQG but rather simply don't see a reason to study it; they prefer to do what appears more promising or interesting to them. Until now, as I said, LQG is considered as a side development with some potential (for example in the direction of background independence) but otherwise it is not taken very seriously.

    The negative tenor in this usually peaceful cohabitation of theoretical physicists was brought up by fabricating a "controversy" by certain authors of books and reports. If there are negative thoughts on the string physicists side, then because of this spin doctoring and instrumentalization of the general public, as an attempt to compensate for the lack of convincing science. It makes me sad to see the success of this strategy by reading through this forum, where most people do not have sufficient scientific insights in these matters but neverthelss have strong opinions!
  23. Jun 20, 2009 #22


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    I personally think that if one really wants to understand the mechanism behind some of this some of the commercial/financial and sociological arguments ARE "serious", in the sense that's it's possibly at least a part of the actual explanation. The competition for financial support of various research strategies unavoidably is a serious point.

    In an ideal world I guess everybody could get all the resources they want to do whatever research they want.

    But I'm the first to agree that this isn't really very interesting to discuss. This is apparently my impression of the the downside of physics as a proffession.

    But it's interesting, that not that many peoples seem seriously interested in discussing strategies without keep referring to the bulk opinion. I've got a feeling that those who are professional physicists to a certain extend have their hands tied, they really can not be too openminded at least officially because then there could be negative effects of the "downside of Science". This is the impression I also got from the personal contacts I've have with those doing string theory professionally. It becomes a compromise.

    Anyway given this competition for funding, all this is very expected.

    Maybe the "serious" physics arguments aren't really that many, or maybe they are simply subjective anyway. String theorist have their good reasons for their preference, and others have thier good reasons for their preferences. Who can judged which has the best arguments?

    I think so far (judged from my own non-professional and personal view) neither string theory or LQG (Rovelli style) has convinced me to be worth investing my life in. Maybe the answers is something in between, or something totally different.

    Given my relatively infinitesimal experience, I still add fwiw my strong opinions :)

    What I like about string theory is that it from a choice of fundamental microstructure and action of reality (string, branes etc) aims to infere all interactions in a unified manner.

    What I don't think like is that their is an informational ambigouity in the choice of this action and microstructure (apparently related to the landscape problem). Also it's somewhat ad hoc. If someone would find a more first principle explanation of the CHOICE of strings, and why this is the fundamental action, in a way that yields a navigationg principle in this landscape of their, I would have no problems at all with the extra dimensions and other stuff.

    So string theory has alot of background structure, not only background spaces, but more serious stuff like background logic - it's not that such things can be totally avoided, but the information content of this "background structure" is ignored. In a true inside view approach this doesn't make sense to me.

    What I don't like about rovellis LQG (judged from this book) is that his view of diff invarance pretty much goes hand in hand with the pure gravity idea, which doesn't make sense to me since to me I perceived it from my amateruish point of view to the extent he can convey his real ideas that he wants to construct a measurement theory without observers. Or alternatively with only god as an observer. This is what bugs me about the foudnations of rovelli. He tries to construct such a theory while explicitly ignoring some of the fundamental problems of QM, and how concepts like probability, statistics are to make and sense for events that "happen only once" like was mentioned earlier.

    So to me LQG also has alot of background structure, where the information context is left out of the physical interactions, by providing a birds view - a birds view that IMO is not physically justified.

    OTOH, if I am so unexperience and ignorant, then I must ask where my confidence comes from. Why hasn't the experts convinced me? I think so far neither string theory nor LQG can claim any superior status, as both are in my very humble and insignificant but still strong opinion still speculative and it's their problem to prove themselves, solve the open problems, suggest new applications that blow us off the chairs.

  24. Jun 22, 2009 #23


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    Thanks fort the replies. I’ll try to summarize some mathematical and/or physical issues leaving asside sociological or financial arguments. I do not say that they are not valid – I only want to stress what may focus is.
    [my comments in parenthesis]

    GR as it stands is not perturbatively renormalizable. Whereas LQG claims that using a different quantization procedure this problem does not show up, the HEP/string physicists insist on the possibility of perturbative calculations. So the claim is that this is not only a failure of the method but of the whole theory; it is not that a different quantization procedure is needed, but that a different approach (strings) of formulation of interacting "quantum field theories" is required.
    [I cannot understand this claim; for me it’s the quantization method that fails, not the whole theory.]

    A stronger argument is the fact the quantization methods used are not so straight forward and may be ill-defined. Several new ingredients not well understood in different contexts are used. Let me list a few:
    The algebra of constraints is solved step-wise with newly constructed or restricted Hilbert spaces.
    The algebra of constraints does not close off-shell. Instead the algebra closes only within a certain (physical) sub-space of the kinematical Hilbert space. Therefore one may lose the control on gauge anomalies.
    [I do not know if this has changed with Thiemann's master constraint approach.]
    The diffeomorphism constraint is rather obscure. Classically this constraint generates infinitesimal diffeomorphism transformations, but quantum mechanically the infinitesimal transformations and the corresponding operators cease to exist.
    The Hamiltonian constraint is not well-defined: it seems to be regulator dependent and has some severe ambiguities due to the very definition of quantum operators, not only due to their ordering.
    [I agree; the ambiguities of the Hamiltonian constraint are one major weakness of the theory. One would have to check rather rigorously if this spoils the consistency of the whole approach. One ambiguity, namely the Barbero-Immirzi parameter seems to be quite well understood in the meantime; it corresponds to the theta-angle in QCD.]

    There's no physical IR limit. or let’s phrase it differently: there is no proof that LQG has GR as its low-energy limit.
    [I agree; that is one major weakness up to now. I have to admit that I havent studied Rovelli's papers on the graviton propagator; maybe there are some additional insights on that topic.]

    Arguments from Haelfix: There is no decoupling limit for gravity and matter degrees of freedom in the UV. Therefore new (currently unknwon) interaction terms could spoil the UV properties of LQG. In order to make a prediction in the QG regime, one would need to know all the physics (quantum gravity plus ordinary matter) between the electroweak scale and the Planck scale.
    [It should be clear from the beginning that LQG focuses on gravity w/o any attempt to unify other forces with gravity, so this limitation is well-known. The argument seems to be valid only if we are forced to use a different class of theories (e.g. strings). If we assume that the structure of the matter theory remains valid (besides some additional technicalities like GUT, SUSY etc.), then LQG + matter theory (whatever it is) remains a mathematical well-defined framework.
    With ordinary gauge theories LQG should be consistent. For SUSY and SUGRA there are claims that the theory is consistent, too, but I do not know many papers on this subject, so perhaps further investigation is needed.
    I agree that if we are forced to consider a different class of theories (e.g. strings) in order to unify all interactions including gravity, then the LQG approach may cease to be a consistent framework for that new theory. It does neither provide any inside, if such a theory is required, nor how it could be constructed. As LQG is formulated today these ideas will come from the matter sector w/o any reference to gravity.
    I fully agree that LQG it is incomplete in that sense.]

    So there are basically three weaknesses in the LQG approach
    - quantization ambiguities, especially in the Hamiltonian constraint
    - missing IR limit; GR not recovered in the low energy limit
    - no approach towards a unification of gravity with other forces

    Last edited: Jun 22, 2009
  25. Jun 23, 2009 #24


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    I don't think that it is true that stringy people don't like loop quantum gravity, at least not for majority of string people. It would be more correct to say that they are not interested in it. For example, they are also not interested in solid-state physics, but it does not mean that they do not like solid-state physics.

    So, why they are not interested in it? Not because they think that it is wrong (most of them do not have a strong opinion regarding its correctness), but because they do not find it very promising in the program of finding the "theory of everything" (i.e., the theory with a power to unify all interactions). In other words, stringy people find loop quantum gravity not ambitious enough for their taste.
  26. Jun 23, 2009 #25
    Someone wrote (excuse my lack of proper quoting):

    "For better or worst most specialists are too busy working on their speciality to participate in these kinds of discussions."

    I wonder how many of them really know about this forum. I know of a very few coming eventually here, but I wonder how interesting it would be if they participated a little more. This would not take "so much" of their time. I can tell it from my own (professional) perspective (although I am not an expert in quantum gravity).

    I think there is a moral duty for the professional researcher to spend a small fraction of his/her time in order to share and discuss their research in a more accessible manner. PF is a good instrument for that matter.
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