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So, what'll it be?

  1. SMT, because...

    5 vote(s)
  2. LQG, because...

    4 vote(s)
  1. Oct 6, 2003 #1
    We already have a thread that discusses the differences between SFT and LQG. So, for this thread, all I ask is that you vote for your choice on the poll, and then give a reason.

    Reasons are basically just either what you don't like about the theory you didn't choose, or what you do like about the theory you chose.
  2. jcsd
  3. Oct 6, 2003 #2
    btw, the "because..." part is just a reminder that I'd very much appreciate your reason(s) for choosing as you do.
  4. Oct 6, 2003 #3
    I chose SMT (string/M- theory) for the following reasons:

    1) It's the first one I was exposed to, and so it's the one I'm more knowledgeable about.

    2) It's unification of QM and GR is really elegant, and has the added effect of explaining some things about the BB.

    I'll probably think of more reasons later, but this is what I was thinking about when I voted.
  5. Oct 6, 2003 #4


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    I don't choose either, or reject either. How's that for decisiveness? But seriously they both have advantages and disadvantages and neither is yet clear the Answer to It All. Both have a chance of developing into something very central. Both of them are fascinating to study indetail and have many beautiful things in them.
  6. Oct 6, 2003 #5


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    that is a darned intelligent attitude. kudos to you sA!
    and it is a scientists traditional attitude
    neither theory has been able to make numerical predictions sufficient to allow it to be tested

    it aint like Einstein comes out with the GR main equation in 1915 and in 1919 Eddington looks at the Pleiades during an eclipse and finds space is bent out of shape just the amount GR says to expect, that did not prove the whole theory but certainly was a bit of evidence

    and that (or so is my understanding) is just what we have not got for either of these two contemporary theories

    so special pleading about mathematical beauty or hype about how god wrote the universe in such and such a language or how the Titannic is built not to be able to sink because of its magic rabbits foot is not a scientific argument for accepting one or the other

    I happen to find LQG more interesting because I really like the curved space active-geometry description of gravity and I would like to see a simple direct quantization of GR's dynamic geometry. I am eager to see what the theoretical results are of directly quantizing GR. So you could have a poll about what you think is more interesting and I would vote.

    But when it comes to truth-tables I'm like selfAdjoint and I dont accept or reject either. And I dont ordinarily make book either, not being much of a gambler.
  7. Oct 6, 2003 #6


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    Mentat, must compliment you on sparking interesting discussion and will throw out an idea in case you want to think about it maybe for future theads:

    in several threads you have sort of equated Bojowald getting rid of the GR timezero singularity (his 2001 paper and followups by him and various) with a "bounce" predicted in M-theory, if I understand you. To me the things seem not analogous, you can see if this makes sense

    cosmologists use GR to model evolution of cosmos and the model failed at time zero (singularity, breakdown of the the model at that limit of applicability and failure to predict past the limit)
    and Bojowald quantized the Friedmann model in a way which happened to fix that glitch-----and it included matter and agreed with the classical limit which was merely the Friedmann model a very simple thing and it happened to predict prior contraction, i.e. a bounce

    the important thing is not to predict the "bounce" the important thing is to fix the broken place in the theory which cosmologists use and think works

    now cosmologists use GR to model the universe and they do not use Branes, as a rule.
    so fixing a singularity in Branes or predicting a "bounce" in branes is fixing the wrong theory and whatever you predict it does not apply to contemporary cosmology.

    Now it might in the future! one may speculate. And become relevant. Or it might not. But it does not seem to me to be analogous to quantum-patching over the actual BB singularity in the actual BB model that is being used.

    I am wondering, Mentat, if you see that as a valid distinction or not.

    Oh darn, Mentat's no longer online. Well how do you see it selfAdj?
    Does this make sense to you?
    Last edited: Oct 6, 2003
  8. Oct 6, 2003 #7
    I voted for superstring theory based solely on the theory as a model. If superstring/M-theory wins this poll we must agree to rename the theory to either Matrix Theory or Multidimensional Theory or anything else, anything but string theory would suite it's supremacy. I dunno why but something as supremely unified as this doesn't capture it's true meaning with the word "string".

    it's great to be back
  9. Oct 7, 2003 #8
    That's as good a choice as any (I only didn't include "neither" as a choice, since so many people subscribe to something other than these two theories, and so many others don't understand either of them, so the "neither" category would fill quickly, while the real purpose would be ignored). Thanks for responding.
  10. Oct 7, 2003 #9
    Well, I suppose it is a valid distinction, but I remember a post by jeff that seemed to indicate that SMT made use of much the same "fix", for the problem of the singularity, as the one you describe above (for LQG).

    I'll have to do some additional research, before really making up my mind on this particular issue.
  11. Oct 7, 2003 #10
    If you ask me, I won't choose any either. It's like deciding between Tony's pizza and Jack's pizza. Both have different qualities, but both have the same purpose, to satisfy my hunger.

    They both are there to do practically the same thing. A more fundimental understanding is needed. And in these theories, they both try to unify gravity into the equation. Promising? Yes. But it's no answer.
    Paden Roder
  12. Oct 7, 2003 #11
    Tony's pizza wins. They may both be around for the same reason, but one of them does it way better!
  13. Oct 7, 2003 #12


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    your reason for not including a response reserving judgement
    (skeptical of either until there is hard evidence) sounds
    plausible, Mentat, but not fully convincing

    in any case it denies at least two people the chance to vote in the poll

    so why not test your assumption?

    why not run the poll again, with a neither, or jury-is-out, third option?

    it might be that the third option would NOT fill up, it might get only two votes or so, and you would still see that our fellow posters at this particular forum are still overwhelmingly believers in the superiority and eventual vindication of string theory.

    I cant predict the outcome of such a poll but it seems to me an interesting possibility that in such a contest you would get one or two "neithers" and a large number of "string" votes
  14. Oct 7, 2003 #13


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    I dont know that there is any comparison, there are a ten-or-so papers that deal with fixing the cosmological (timezero) singularity by directly quantizing the Friedmann equation which cosmologists use. It would be tedious to list the titles and they are all online and easy to find at arxiv.

    It might be interesting if you would supply a link for some string/brane-type paper that purports to do something analogous and one could look and see what it actually did. I recall Paul Steinhardt did some stuff with "ekpyrotic" or "colliding branes" but it was rather imaginative and not very relevant to actual cosmology. I guess it could be presented as some explanation of BB by a bounce, but I would tend to be sceptical of that by Occam----too elaborate and 'creative' for the job. But if you think there is some such paper, then let us not rely on (possibly false) authority or hearsay, give a link and we'll look at it and see how it purports to fill the bill.
    Last edited: Oct 7, 2003
  15. Oct 7, 2003 #14


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    these discussions that refer to hearsay rather than actual papers dont seem to lead anywhere so I will try to do my part---even tho a mite tedious---by listing the papers I know about removing the timezero singularity by quantizing Friedmann (form of GR model used in cosmology) and the closely related prediction of inflation. All are easy to find online at arxiv just using title or author name.
    These are listed in random order as they came to hand.

    Bojowald---Absence of Singularity in Loop Quantum Cosmology
    Ashtekar et al---Mathematical Structure of Loop Quantum Cosmology
    Bojowald---Homogenous Loop Quantum Cosmology
    Bojowald/Vandersloot---Loop Quantum Cosmology, Boundary Proposals, and Inflation
    Golam Hossain---Hubble Operator in Isotropic Loop Quantum Cosmology
    Bojowald/Hinterleitner---Isotropic Loop Quantum Cosmology with Matter
    Hinterleitner/Major---Isotropic....with Matter II
    Alexander/Malecki/Smolin---Quantum Gravity and Inflation
    Bojowald---Quantum Gravity and the Big Bang
    Bojowald/Morales-Tecotl---Cosmological Applications of Loop Quantum Gravity
    Gambini/Pullin---Canonical Quantization of General Relativity in Discrete Spacetime
    Bojowald---Initial Conditions for a Universe
    Bojowald---Isotropic Loop Quantum Cosmology
    Bojowald---Inflation from Quantum Geometry

    Several survey articles also cover the removal of the BB singularity by quantizing the model and cite the relevant papers by Bojowald and others---but I've tried to limit the above list to research papers. A good survey article, though, is

    Ashtekar---Quantum Geometry in Action: Big Bang and Black Holes

    I have not mentioned some rather good exposition in recent PhD theses written, for the most part, by doctoral candidates in Germany, France, and the UK. some of these are online (but not at arxiv!). I do not recall seeing a PhD thesis written at a USA university that connects to this.

    You shouldnt feel disconcerted if you cant find string/brane papers that actually bear on removing the timezero singularity in GR by quantizing it, since that is not really string/brane's concern or turf, after all. And you may indeed find some links! In that case it would be delightful to have access to them so one could actually see what passes for predicting a timezero "bounce" in a brane context!
  16. Oct 8, 2003 #15
    I might try it some time, but I have seen enough votes gone bad in the past (some of which I started) to know that what you predict is rather unlikely. It's still possible, though, which is why I might try it out.
  17. Oct 8, 2003 #16
    I don't know of any paper on that now, and I don't think that I'll be able to look one up (I only get an hour on the internet per day (not counting Sundays, where I usually get no time at all) so my time is mostly taken up by the PFs and some E-mail), but I would be very interested in reading one, if someone else could find it.
  18. Nov 14, 2006 #17


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    I do not think that one has to choose between these two theories (strings and loops) because I do not view them as competitive. They try to solve different problems. In particular, the methods developed in LQG could be useful for quantization of strings as well.
  19. Nov 14, 2006 #18


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    the picture has changed considerably since 2003.

    Mentat's bi-polar choice "string versus LQG" looks oversimplified now.

    For an overview of approaches to QG, you might want to take a look at the TOC of Oriti's forthcoming book. I put a copy here
    (contained in a short essay of his where he talks about the book)
  20. Nov 14, 2006 #19
    Leonard Susskind writes in his book "The Cosmic Landscape : String Theory and the Illusion of Intelligent Design" that Lee Smolin's research is guided by his belief that LQG if true will probably turn out to be part of string theory.
  21. Nov 14, 2006 #20
    But the choice remains the 2 most active -- in the US, there are many political choices (libertarian, socialist, communist, green, Ross Perot's party) but it boils mostly down to Democons V Republicrats (not much of a difference really). In the US, besides PSU, it's really only string theory (i.e Princeton, Harvard, Rutgers, Stanford, MIT, etc.).
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