Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Physical reality of string theory demonstrated

  1. Jul 6, 2009 #1

    Well, according to this article, at least.


    Just thought I'd share.

    Problem is, they didn't entirely describe high temp superconductivity.

    O well.
  2. jcsd
  3. Jul 6, 2009 #2
    I'm not quite sure what's new here
    Fermions and the AdS/CFT correspondence: quantum phase transitions and the emergent Fermi-liquid
    Maybe I misunderstand, but it seems to me that it would be quite disappointing if a fundamental theory would be confirmed by the prediction it does on the effective long range behavior of electrons in metal. Not that it is not interesting, but the hope is that it going to help with the opposite end of the spectrum, where gravity is relevant. Namely, the standard model should be able to produce the same results if we could perform the equivalent calculation. This last point, which has little to do with the relevance of string theory to describe Nature at a fundamental level (hence little to do with "demonstrating it") is quite interesting in its own.
  4. Jul 6, 2009 #3
    So, string theory (more accurately AdS/CFT) explains behavior in this one case where no other theory yet has, right?
    That's pretty cool, but is this really the first time such a thing has been accomplished?

    In any case, this does not at all "demonstrate physical reality" of the theory!
    To me, "demonstrating physical reality of a theory" means that your mathematical model is shown to be more useful or more accurate than previous models.
    This paper seems to promote optimism, which is good, but has nothing to do with the completeness of string theory.

    Misleading title on a science news article? That's old news. lol
  5. Jul 6, 2009 #4
    Not at all. In the realm of QCD, you will find several articles per month during the last 2 or 3 years where model calculations, often quite simple, allow new insights in the non-perturbative regime with astonishing agreement between data and those simple models, often increasing by one or two order of magnitudes, and also on much wider phase space, while respecting various analytical constraints (sum-rules, or other kinds of self-consistence, which were often believed to be quite hard and are now sometimes trivially respected). Yes, communication depends a lot on how easy it is to relate to the end result and/or the closeness of technological applications...

    But it's true that the paper is fascinatingly beautiful !
  6. Jul 6, 2009 #5
    Can the full text paper be viewed online for free? ;)
  7. Jul 6, 2009 #6
    Not right now sadly. Get to a University Library!
  8. Jul 7, 2009 #7
    This may be available on the arXive---try 0904.1993 [hep-th].

    Sorry, I don't know how to enter the address.

  9. Jul 7, 2009 #8


    User Avatar
    Science Advisor

    That's how:
    It can be downloaded for free.
  10. Jul 7, 2009 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Physical reality of string theory demonstrated [not]

    Demystifier, thanks for the arxiv link! I will pass it on.

    Coin had a cogent comment at Woit's blog:
    "... One thing I’ll note is that the grandiose and wildly inaccurate claim of the headline– “Physical reality of string theory demonstrated”– does not appear to be a claim anyone with Leiden University has made. It was added by ScienceDaily (where I think this article originated?). In fact, it wasn’t even a claim made in the ScienceDaily article itself– the claim appears only in the headline, and is in no way supported by anything in the article itself (which is much more conservative in its phrasing). A consistent problem in science journalism is that someone will write a decent article and then an editor will tack on an “exciting” but wildly inaccurate headline which changes the tenor or content of the piece itself. (Actually this is a frequent problem in many other kinds of journalism as well, especially in politics– most people writing in traditional journalism have no control over their own headlines.) If you ignore the headline and just look at what Leiden University team actually did you’ll find it doesn’t have anything to do with “demonstrating the physical reality of strings” (although what they did do sounds actually pretty awesome and like a legitimately positive development for AdS/CFT as a research program)."

    The usefulness of stringy mathematics applied to various other problems at altogether different scales has been shown already in several contexts. I think what Coin is saying is that the Leiden work does not show string validity as a fundamental description of nature, but rather shows the flexibility and agility of the mathematical resources that has grown up around string research.

    In this case they applied the AdS/CFT correspondence, as part of their mathematical toolkit. The Phys.org article makes this clearer, although it has the same dubious headline about demonstrating "physical reality".
    Last edited: Jul 7, 2009
  11. Jul 7, 2009 #10
    On his blog Woit says:

    I guess he doesn't know that the AdS/CFT duality is an exact correspondence, not an "approximation method." String theory consist of equations that describe relativistic quantized strings, and AdS/CFT allows us to test the validity of these equations. The only remaining question is whether quantized relativistic strings exist in nature on the scale of the Planck length, and AdS/CFT will continue to be silent on this subject, but the fact remains that the AdS/CFT duality can be and is actively being used to test the exact equations which make up string theory, it's not just a case of testing the general sort of mathematical techniques that are involved in string theory, but rather the quantized string dynamics, the predicted spectra, etc. Therefore these tests using AdS/CFT are verifying that we have found the correct description of quantized relativistic strings, and as I said the only remaining question is whether these exist in nature at the scale of the Planck length.
    Last edited: Jul 7, 2009
  12. Jul 7, 2009 #11


    User Avatar
    Science Advisor

    I thought the approximation was because the duality applied to supersymmetric versions of the theories in question, whereas the theories themselves were not supersymmetric?
  13. Jul 7, 2009 #12


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    You are referring to a post at N.E.W. that has not been linked-to yet in this thread or anywhere on Physicsforums.
    1. if you want to take something out of context from a blog and criticize it, how about giving a link to the blog post you are quoting? Many of us here will not have read the blog post you are quoting.
    2. If you think there is an error in the Woit blog you quoted, why not take it up directly there at the blog? Until some 5 minutes ago there were as yet no comments responding to that post.
    It would be more "Civilized" to challenge the person openly and give him a chance to reply to the aspersions cast, than to make what may be straw man innuendo over here out of sight.

    I don't believe Woit said anything about AdS/CFT being itself inexact, but you pretend he said the correspondence was an approximation (not that what it is being applied to is) and make a show of correcting the 'mistake'. I urge you to take that up directly with woit.

    To correct the omission I copied the comment at Woit blog, giving him the opportunity to respond if he cares to, and thinks it worth his while. It is now the first comment to that new post:
  14. Jul 8, 2009 #13
    In an ideal world, Woit would be right right about the authors responsibility against the title, but it is quite possible that the title was changed without him having any control or even being noticed, and Woit is responsible for permanently feeding the sterile pseudo-debates which spoil the scientific content of even this very sub-forum. Should the authors resign from their positions ? As far as I can tell, the free paper I linked to in the arXiv is perfectly respectable in this regards. Civilized is obviously wrong about Woit, and I doubt Woit would care.
  15. Jul 8, 2009 #14


    User Avatar
    Science Advisor

    I am sorry to ask this question over and over again, but I still was not able to find a reference were AdS/CFT is shown to be exact. All I know are
    - claims that this is the case
    - arguments coming from large-N and other limits were a correspondence between certain regimes can be established

    Are there articles were this missing link is discussed?
  16. Jul 8, 2009 #15
    OK, that would explain the use of the word "approximation" for the application of AdS/CFT to QCD, but in the same line Woit also mentions "strongly coupled condensed matter" systems, for which the duality is being applied directly to Hamiltonians we care about, as opposed to Lagrangians that are only similar to he ones we care about asin QCD/CFT.

    I found the statement through the link in your previous post, Marcus, and furthermore it is the most recent post on the top of Woit's blog, discussing the same press release and its derivatives which are the subject of this thread. Also, you might be interested to know that you can copy a string of text and put it into Google search surrounded by quotation marks to have the search engine locate and display pages which contain only that exact string of text, and that the statistical likelihood of any one of the uncommon phrases in the Woit quote I provided being found anywere else is so low that in practice one can use this technique locate the page I was quoting.

    According to Lubos Motl:

    Source: http://motls.blogspot.com/2009/02/are-adsqcd-and-adscmt-relevant-for.html

    Motl is closer to being an expert on AdS/CFT than either Woit or myself, his blog is widely read by the professional string community, and he has published well-cited articles with mainstream string theorists in the recent past. Regardless of what people think about Motl, look him up on SPIRES and compare him to Woit (or yourself), and I consider myself to be in good company if I am echoing his sentiments (incidentally, I had not read his blog post untill I made my comment).

    (1) Humanino is obviously wrong about Civilized, according to Lubos Motl.

    (2) I am disappointed that Marcus would copy my statement to Woit's blog without my permission. Looking at his professional record I am forced to conclude that Woit is an untenured ex-mathematician who has not contributed a peer reviewed scientific publication in over 15 years, and in my opinion he has recently done a lot of harm to physics with his anti-mainstream activism. As far as I am concerned, the less attention his blog gets, the better off physics will be.
    Last edited: Jul 8, 2009
  17. Jul 8, 2009 #16
    No such thing is available, it's a conjecture.
  18. Jul 8, 2009 #17


    User Avatar
    Science Advisor

    Yes, I think so. The claim is that the correspondence is exact, but the claim is not yet proved. http://arxiv.org/abs/gr-qc/0602037
  19. Jul 8, 2009 #18
    Of course this is true, but I would also add that mathematically speaking the existence of quantized gauge theories is itself only a conjecture, and that this would be a necessary prerequsite for a rigorous proof of the duality. In other words, the existence of QCD is "just" a conjecture. Rigorous proofs lag behind more than 30 years in high-energy physics, and they are not the right yardstick to apply.
    Last edited: Jul 8, 2009
  20. Jul 8, 2009 #19


    User Avatar
    Science Advisor

    True; but if string theory IS the ToE, than it must certainly proof its own existence and provide a way to derive QCD as an effective theory. If string theory does not do more than QCD but only in different regimes, then the two theories may be dual in the sense that they are both approximations to a more fundamental theory yet to be discovered.

    If this were the case then I would agree that the physical reality of string theory has been demonstrated (in the same sense as with QCD). But string theory as of now is doing much less than QCD in terms of physical predictions :-)
  21. Jul 8, 2009 #20
    Well... wait. Everyone keeps telling me that quantized gauge theories are, themselves, believed to be or treated as approximations rather than exact descriptions of reality. This usually comes in the form of saying something about renormalization and landau poles, and then explaining that all quantized field theories are actually "effective field theories".

    Though maybe I am conflating unrelated subjects here?
  22. Jul 8, 2009 #21
    Yes, the fact that QCD has not been rigorously proven to exist is a mathematical question which is distinct from QCDs role in describing reality. The problem of existence is that, as far as mathematicians are concerned, QCD is not defined --- we don't know what a quantum gauge theory really is in mathematical terms.

    The string theory side of AdS/CFT is highy restricted, as Ed Witten puts it in hep-th/9802150 :

    For example, the dynamics of a meson jet in a quark-gluon plasma is given by the dynamics of the endpoints of a string that is falling into a blackhole.
  23. Jul 8, 2009 #22


    User Avatar
    Science Advisor

    Is Lubos really saying the same thing? I think you said that AdS/CFT can be useful for understanding QCD/condensed matter, but that does not mean that string theory is the correct theory of gravity at the Planck scale. Lubos seems to be saying that if string theory is the correct theory of gravity at the Planck scale, that would explain why AdS/CFT is useful for QCD/condensed matter. "A critic could say that AdS/CMT and AdS/QCD don't tell us anything about the physical phenomena near the four-dimensional Planck scale - which is just too far from the typical scales of nuclear and condensed matter physics. That would be a true assertion but a somewhat vacuous one. ................... They're different groups of bats but what's important here is that they're connected with each other - they're two parts of the same AdS-like manifold. There is no way to separate them completely."
  24. Jul 9, 2009 #23
    Since this thread has to some extent intersected with discussion on Woit's blog I should probably mention that Woit has a new post up expressly about this paper. One interesting thing is that it turns out (contrary the conclusion I jumped to in the NEW post which Marcus kindly mirrored here...) the article found linked at EurekaAlert at the top of this thread as well as some other places was, in fact, a Leiden University press release copied verbatim... so this is not a case of a journalist going overboard but rather a case of a university PR department going overboard, which is maybe somewhat more embarrassing.

    --- --- ---

    One comment I'd have re: this thread is that it is I think really important to keep things like AdS/CFT separate in our minds from uppercase-letters "String Theory" in the sense of the String Theory TOE. The Leiden university research is about AdS/CFT, whereas statements like "physical reality demonstrated" would necessarily be statements about the String Theory TOE. The critical difference here is that in the String Theory TOE the strings actually have some sort of "physical reality" to them, whereas in AdS/CFT the strings are simply mathematical constructs. To say that a thing in physics has "physical reality" would mean, to me, that it's somehow got a leg up over Occam's razor-- that it has made some sort of unique prediction, that it has shown that there is a piece of information in this universe that has an explanation if you assume the physical reality of the thing but lacks an explanation otherwise.

    But AdS/CFT, by nature, can't provide that-- it can only ever have mathematical reality. The point of AdS/CFT is that it is a dual model-- the claim of AdS/CFT, the entire reason we're interested in it in the first place, is the (still only conjectured) idea that an AdS/CFT model is is exactly correspondent with a field theory. This means that not only can AdS/CFT not produce a unique prediction that the field theory did not, if it ever did that would imply the program had failed! AdS/CFT is a mathematical tool for solving field theory problems. Saying that strings have "physical reality" because the AdS/CFT mathematical tool correctly produced the answer to a field theory problem would be nothing more or less than saying that the number 4 has "physical reality" because you used the number 4 in solving a math problem about the moon orbiting the earth...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook