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

  1. Oct 16, 2003 #1

    I went to a lecture last january where it was mentioned that physicists don't work much on LQG and why it's such a dead end but m- theory isn't. Could someone explain this?

    Also, can someone give me references to explain why the problems of LQG are so serious and so why the above is true but m-theorys serious problems don't make it a dead end? I think it is safe to assume the above would not be thought for no good reason, I just couldn't understand the lecture.

    Also, is it better to start for m-theory with green schwartz witten or polchinski and what mininmum knowledge do I need? Also, is there an introduction or paper on line? Thanks.
  2. jcsd
  3. Oct 16, 2003 #2
    I have a suspicion. It is subject to scoff, but here it is anyway.

    M-theory and its supersymmetric neighbors use tons of advanced mathematics, which means employment and career advancement opportunities galore for the mathematical adept. In other words, it has developed a flourishing life of its own. Maybe Loop Quantum Gravity uses only a normal range of mathematical techniques.

    Pat Schwartz's math preparation for string theory --->
  4. Oct 16, 2003 #3
    the importance of d-branes and m-theory were discovered after greene schwartz and witten was written, so you won t learn any m-theory from there. polchinski is newer.

    but i think it is easier to learn basic string theory from GSW.

    also, there is a new book by zwiebach that is supposed to hit the shelves in january, string theory for undergrads, which will teach you quite a bit of string theory with a minimum of prerequisites. but this book is not available to the public yet.
  5. Oct 16, 2003 #4


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    It's a groundless claim, so it would be difficult to find a recent (2003) reputable paper explaining why one is a dead end and the other isnt. However there is a polarized perspective! So you can find papers by Relativists (GR theory people) who have one viewpoint and papers by Particle Theorists who have a different one. Neither kind---if he is responsible and speaking to peers---will say that the other guy's line is a dead end. But they will see things from radically different perspectives.

    There is a survey "Loops versus Strings" written by a string theorist Enrique Alvarez for an audience of particle theorists as invited survey talk to a recent conference "What comes after the Standard Model?". If you want a reputable String-side view try him: search arxiv with Alvarez or the title Loops versus Strings.

    For an opposing viewpoint, read Smolin's recent survey "How far are we from the quantum theory of gravity". It is respectful of string theory and makes a careful (objective it seems to me) comparison, one from the loop-side however.

    Interestingly (to me at least) the most serious recent critiques of stringy research have been coming not from loop-people (who mostly mind their own business) but from people such as Tom Banks and Peter Woit whose research interests have nothing to do with LQG.

    What members of the string community say on the outside, to members of the general public, is a separate chapter and requires a separate discussion
    Last edited: Oct 16, 2003
  6. Oct 16, 2003 #5
    Re: Re: Why is LQG a dead end but not m-theory?

    You sound confident but the idea that relatively speaking LQG generates little interest among physics people is not a "claim" it is a fact and if you don't know this I'm pretty sure that you won't be able to tell me about the problems with LQG. But if you can explain to me the problems I would appreciate it because I don't want to spend eons trying to figure it out when the experts don't think it is a worthwhile theory. Also there must be some serious problems since physicists aren't interested in it very much. Also, what do you mean by polarized perspective? do you mean your perspective?

    But thankyou very much for the references I appreciate it.
  7. Oct 16, 2003 #6


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    The impression I've gotten at this point is:

    People who study LQG think that ST is fundamentally misguided because it ignores the most important question about merging GR with quantum mechanics, "What does quantized GR look like?"

    The people who study ST think that it is literally impossible to quantize 4-D General Relativity, and thus LQG is doomed from the start. Instead, they look to expand the standard model in novel ways hoping to find an alternate theory of gravity.
  8. Oct 16, 2003 #7


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    I've been looking into some LQG papers about trying to get to current physics from LQG. And I've also seen papers from the stringy side trying to do that too. And just today I read an intro paper on doing that with Connes' noncommutative geometry. That's three independent ways of "going deeper" than the standard model, and trying to get the standard model as a limiting case. They all fail. None of them is any closer than the others.

    String partisans like to say that LQG fails to have a low energy limit of GR. But they don't mention that for twenty years now they have claimed to have a low energy limit of the Standard model and have never truly achieved it. Their models look a bit like the SM if you squint and cross your eyes, but not up close.

    Similarly noncommutative geometry can produce things that remind you of the standard model, but aren't.

    The LQG case is interesting because they are trying to fit quantum physics from the ground up into their background free webs. And they get so far and run into very subtle math problems. They have had breakthroughs in the past that weren't as widely broadcast as the stringy duality breakthrough, but they need another one now.

    For Strings, there is reason to think that producing things that look like the standard model isn't a big achievement. The Coleman-Mandula theorem says that in Minkowski spacetime, with point particles, any theory that goes beyond the standard model would look like it it. So the low energy limits of stringy physics, where the strings, etc. look like points would fall under that theorem, and anything consistent they might produce in the lower energy range would have to look a lot like the standard model. Even with that help, they can't close the gap.
  9. Oct 16, 2003 #8
    Very interesting! But the point of my question is still being missed. Given the evenhanded way you guys talk about the LQG versus string theory/m-theory you would think that the interest in them would be equal but really the opposite is the case. So either string theory people are just irrational and prejudiced against LQG or there are good reasons why the problems with LQG are more serious than in string theory. 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. At least that's how I feel. So what I want to know is why LQG is inferior to string theory, which virtually must be the case given that hardly anybody works on LQG. What other reason could there be since string theorists probably understand LQG too, or at least as well as anyone here. Do you guys not think the lack of interest in LQG says something about LQG worth considering?

    For example why do " people who study ST think that it is literally impossible to quantize 4-D General Relativity, and thus LQG is doomed from the start."?
  10. Oct 16, 2003 #9


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

    You are very welcome! The claim that "LQG a dead end but not m-theory" does not have a logical basis. It belongs to the realm of propaganda. So the question you asked, namely "why is it true" does not make sense.

    However the string research establishment is well entrenched and the sheer volume of stringy research is impressive. If that is what you want to talk about, numbers of pre-prints in the archive, you are very welcome to discuss it! Here are some numbers. I did a search at arxiv.org, by year from 1998 to present, using keywords "string", or "brane", or "M-theory" appearing in the abstract:

    1998-----1260 papers
    1999-----1373 papers
    2000-----1459 papers
    2001-----1400 papers
    2002-----1211 papers
    "past year"-----715 papers

    The "past year" list is for November 2002 thru October 2003, a substitute to make up for not having the full year for 2003. Personally I do not consider numbers of papers in theoretical physics signifies much----the research fields go thru changes of fashion---especially when the counts are not weighted by a citations gauge of how influential or uninfluential the papers are.
    I wouldnt expect anybody else to attribute much importance to the numbers either. But you may be curious to compare these with the much smaller LQG numbers for the same time period.

    I did a search on keywords "loop quantum gravity" or "loop quantum cosmology" or "spin foams" appearing in the abstract and found:

    "past year"-----68

    Since these are just the los alamos preprint archive robot's raw response, I would not take them too seriously. Sorry about the meaninglessness but its the only source of numbers readily at hand. Ignore any apparent trends. At most they show a sort of order of magnitude imbalance in the research effort----at least 10 times more stringy research is being supported. Maybe 50 times! There are one would guess at least 10 times more research positions---just as a guesstimate, it may be much more. Probably a good many more than 10 times as many people. I dont want to suggest that this is necessarily an advantage to the progress of the theory! Or indicative of some intrinsic merit---it's just the condition things are in.

    Instead of looking at the raw numbers of papers---which indicate rather more about fashion and the control of funding institutions---you might learn more that's real about the merits and progress of the research itself by reading Smolin's 90 page side-by-side progress comparison: "How far are we from the quantum theory of gravity?"

    http://arxiv.org/hep-th/0303185 [Broken]

    Or the counterpart survey "Loops versus Strings" by the string theorist Enrique Alvarez
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  11. Oct 16, 2003 #10


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    Do you want just to argue with someone or do you want to read articles and learn something. People in particle physics had some mistaken ideas early on about the impossibility of quantizing GR.
    (who knows eventually it may prove impossible, but the dire predictions have not panned out)

    Rovelli who is a science historian as well as a GR expert, analyses the reasons in his new book "Quantum Gravity". It is interesting to see how so many HEP people got on the present track. You are asking why so many had the (now questionable) notion that it was "doomed from the start" and Rovelli devotes part of an interesting history chapter to this! So if you want to know you can read up on it.

    But in the present this is not so interesting, I think, because I see people switching interest to Loop Gravity and a growing amount of energy in the work done in this field. When I see new authors of papers and look at their PAST papers (which the LANL archive makes easy) then I sometimes see that the new Loop author has earlier been writing String papers.

    Rather than dwell on past mistakes I would suggest trying to understand recent trends. Like this months Strings meets Loops symposium at Berlin---a first of its kind---I think it presages a more balanced attendance at future conferences and eventually a more balanced distribution of funding and research positions. This presupposes continued rapid progress in LQG (which has made significant gains this year)
    Last edited: Oct 16, 2003
  12. Oct 17, 2003 #11


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    Just to say, Marcus, your arxiv search probably missed most of the Ashtekar, et al. papers, as well as those of Rovelli and associates since they both tend to avoid the term Loop Quantum Gravity and its abbreviation LQG.

    I've posted before that it's a mistake to judge a field by the number of people working in it. Until the late 1930s the annual congresses of people working in quantum theory only had a dozen or so people attending. As late as the 1950s it was a small enough community world wide that everybody knew each other (except for the USSR, of course).

    Over on s.p.r. it's been pointed out that "all particle physicists" is a tiny subset of "all physicists".
  13. Oct 17, 2003 #12
    I'm not talking about past mistakes. I'm talking about the unresolved problems of LQG. Since part of "recent trends" is going to include new ways to address old problems we should understnd what those problems are. It's getting hard not to assume that your nonrespones to this question are either just irrational prejudice or lack of understanding or unwillingness to even consider in an honest way what the challenges of LQG are. Of courese I can try to understand the papers but I don't think I know enough to really understand them. That's why I'm asking you to just outline them for me to give me persective before I read papers. I 'm not being argumentative.

    For example what is the hamiltonian constraint and why can't it be solved? Is this related to why they can't prove that LQG is realistic since it can't get GR? Why can't they get GR? I don't see how you can be confident without being familiar with all of this.
  14. Oct 17, 2003 #13


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    november issue focus mag,

    Witten showed that all five superstring theories are just rough descriptions of a single,overarching
    idea, which he dubbed M theory, some theorists argued that the M represented mother,
    mysterious, or even magic,but its connection with superstrings is clearest if it stands for membrane,
    the five superstring theories then emerge as merely the multi-dimensional edges of 11 dimensional
    membranes, all but four of whose dimensions are curled up to small for us to see.
    even M theory may ultimatly prove to lack the power requierd to answer all these questions.
    edward witten advanced study Princeton.
    this is what joe public is being told
    Last edited: Oct 17, 2003
  15. Oct 17, 2003 #14
    Yes, but why are physicists so much more confident in M theory than LQG?
  16. Oct 17, 2003 #15


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    I heartily agree. For a theoretical field it seems like half a dozen or a dozen good ones has been enough in the past, and fashion does not have so much to do with validity as it does with other circumstances. I dont think those numbers mean anything---but it seemed to me that one of the other posters was asking about numbers and wanted to discuss them, so for the sake of concreteness there they are.
  17. Oct 17, 2003 #16


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    posted by eigenguy.

    Yes, but why are physicists so much more confident in M theory than LQG?
    plebs view.

    1 reward
    2 endless possibilities
    3 years of future work
    4 they are not
  18. Oct 17, 2003 #17
    No, it's a mistake to judge a field ONLY by the number of people working in it. For people like us who are not experts it makes sense to ask why most experts feel this or that way.

    Why stand on personal opinion when you can find out for sure? That's all I'm trying to do.

    Again I ask what is the hamiltonian constraint and why can't it be solved? Is this related to why they can't prove that LQG is realistic since it can't get GR? Why can't they get GR? I don't see how you can be confident without being familiar with all of this.
  19. Oct 17, 2003 #18


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    Just to re-iterate, we are both skeptical that the number of people who happen to be working in a field directly reflects the merit of that line of research. But frivolously or not, this morning I set out to verify that my search did NOT underrepresent LQG and found, to my amusement, that the first thing I looked for "Quantum Gravity and the Big Bang" a survey of loop quantum cosmology by Bojowald given this summer at a cosmology and fundamental physics conference was in fact not on the robot's search results list! I wanted to be able to reassure you that my search was effective and instead immediately found anecdotal evidence that it was not. (but I still think it gives a roughly accurate idea of the disproportion---something like a factor of 10 to 50)

    You are right that several senior loop people dont use the term LQG---they simply say quantum gravity, or quantum geometry, or quantum GR, and omit the word loop. But I was searching in the abstract text and one would imagine that even if the title does not say "loop" that it will occur in the abstract somewhere. So I still think that all or nearly all of ashtekar and rovelli papers were caught.

    indeed some stuff that shouldnt have been caught got included by mistake but not enough to distort the proportion---this occurred in the search of abstract text on["string" OR "brane" OR "M-theory"]as well but didnt seem significant there either.
  20. Oct 17, 2003 #19
    Even if there are 1000 times more string theory papers than LQG ones, it still doesn't explain to me why this is the case. What is it about LQG that makes it so less interesting to researchers in this field? I just don't get it. Is there a "fatal" flaw and if so what is it and if there isn't what is it about LQG that is less attractive than string theory? Is it really just that the math is less interesting or it's a smaller theory ? Isn't there some physics thing about LQG that people don't like compared to string theory. I feel discouraged to giving up looking for an answer here.
  21. Oct 17, 2003 #20
    I'm getting worried that I'm offending people here. I really don't mean to do that and I am very sorry if it comes across that way. I am just having a hard time understanding the papers or finding where it says what's wrong with LQG which explains why this is more bothersome to people than whats wrong with string theory.
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