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Strings, Loops and LHC.

  1. Sep 16, 2008 #1
    So, what are the theories' predictions which are going to be tested on LHC?

    And if they won't be found, Does it mean they will be refuted or not?
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  3. Sep 16, 2008 #2


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    The nonstring quantum gravity concepts that I'm familiar with favor 3D space. I believe at least one approach---the Triangulation QG developed by Ambjorn and Loll---would be outright refuted if evidence were found of extra spatial dimensionality.

    Their approach (Causal Dynamical Triangulations, CDT) has a popular account in a recent issue of SciAm---if you're not familiar with it, click on the Loll QG link in my sig. In this approach, the dimensionality around a point is a quantum observable and depends on scale. At the scale probed by LHC the spatial dimensionality should always be 3. At much higher energies and smaller scales the dimensionality is predicted to decline. Under no circumstances is it predicted to be more.

    On the other hand, a basic introductory treatment of Loop QG, such as given by Rovelli, is formulated with spatial dimension n (variable). As a general approach, I expect Loop could accomodate extra dimensions. But some of the results are for the special case of 3D space ( 4D spacetime) and would be invalidated. So Loop would be at least partially refuted also, if evidence of extra dimensions were found. At least in my personal view.

    So I think that nonstring QG approaches, particularly Ambjorn and Loll's CDT but possibly others, actually risk falsification by making a prediction which the LHC can test. At least one approach says flatly and categorically that extra dimensions will NOT be detected by LHC.
    Last edited: Sep 16, 2008
  4. Sep 16, 2008 #3
    That's all nice and good, but from your post I guess string theory cannot be refuted from LHC, no matter what they find.
  5. Sep 16, 2008 #4
    The LHC certainly cannot be blamed : the problem with string theory is whether it can be refuted at all.
  6. Sep 16, 2008 #5


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    If they don't find supersymmetry, will string theory be less likely? Especially given the landscape problem?
  7. Sep 16, 2008 #6
    In string theory, you can have supersymmetry on the worldhseet or in the target space. I need to check, but I think there are even other scenario, in particular, I'm not sure what's going on with Witten's twistor string in 4D.
  8. Sep 16, 2008 #7


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    If they don't find the Higgs, does that mean anything for strings? Or is it just trouble for the standard model?
  9. Sep 16, 2008 #8
    If you disprove supersymmetry, you disprove string theory, basically... the problem is that you can't disprove supersymmetry. If the LHC does not find supersymmetry you've only disproven "TeV-scale supersymmetry". It may be supersymmetry is still real, it just happens at a higher scale than the LHC can see.

    However! If the LHC does not find supersymmetry-- if TeV-scale supersymmetry is wrong-- then supersymmetry is much, much less useful. For example this means supersymmetry is not the solution to something called the "hierarchy problem", which is one of the reasons people were proposing supersymmetry in the first place. So if supersymmetry is not visible at the LHC scale, then very many people will stop believing supersymmetry is true-- because although it has not been disproven, not by a long shot, the reasons people hoped supersymmetry was true will be taken away, instead almost the only reason to believe in supersymmetry anymore will be so that string theory can survive. But I think this will not make it less "likely", just harder to believe in :P
  10. Sep 16, 2008 #9
    I agree with the general idea that "if LHC does not find TeV-scale supersymmetry, string theory will loose appeal". However, worldsheet supersymmetry does not imply spacetime supersymmetry. If that was established, that would be a profound result in string theory.
  11. Sep 16, 2008 #10
    Hi humanino,

    I'm afraid I don't understand this sentence. Do you think you could elaborate? What is the difference between these two things?
  12. Sep 16, 2008 #11
    IIRC, string theory can be refuted in favor of discrete QG approaches, one of the cornerstones of string theory is perfect Lorentz invariance, and most alternative QG theories predict Lorentz breaking at sufficiently high energies. It's not something that can be tested by LHC, and it's not very testable overall, but it is there.

    Now, if the LHC does not find Higgs, THAT would be interesting. So far all evidence collected by Tevatron does point (tentatively) towards there being no Higgs at all. (Either that, or we're extremely unlucky and Nature chose to put Higgs almost exactly in the point where we're least likely to find it)
  13. Sep 16, 2008 #12
    I'm quite certain some people here would be more qualified to comment in depth.

    BUSSTEPP Lectures on String Theory describes them well. See for instance Fig.7 for the link between worldsheet and target space. In the RNS formalism, you start with worldsheet supersymmetry to include fermions in the spectrum, and then later GSO projection removes unphysical degrees of freedom and realizes spacetime supersymmetry. In the light-cone Green-Schwarz formalism, target space supersymmetry is explicit from the onset.

    In any case, the attempt to have physical string theories without are numerous, and not quite dead I think :smile:
    How Strings Make Do without Supersymmetry: An Introduction to Misaligned Supersymmetry
    Lorentz invariance can be spontaneously broken in string theory
    Lorentz Invariance Violation from String Theory
    Last edited: Sep 16, 2008
  14. Sep 17, 2008 #13
    In essence, string theory is a perfect theory that can explain anything and everything we may or may not see in the universe.
  15. Sep 17, 2008 #14
    So it's philosophy and not science then.
  16. Sep 17, 2008 #15


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    My view of string theory is to see it as a framework which rather constrains the "possible theories", in that sense it could "at best" possibly be a theory of theories, which would not be bad, but there are things missing for it to really appeal to me as such.

    I see two possible modes of failure, either the actual theories that best fits nature, or the basic idea of "what is a theory or physical law" doesn't fit the string framework. But to find out I figure we have to exhaust the landscape of possible theories in order to truly falsify the string framework.

    But then the obvious next issue is that of effiency. If the string framework does allow for a large number of possibilities, and are unable to come up with at least some means of navigation through this hypothesis space, then the framework risk beeing starved to death before proving it's viability.

    Doesn't sound too bad does it? :) Consider a lifeform who is so flexible that it's beeing able to pretty much evolve and adapt to any possible new environment. Now that would not be bad!! The only slight problem is that of speed. In a quickly changing environment, the lifeform will die or get destabilized unless it evolves in a timeley manner. And if the landscape is as big problem as some think, then it seems doing a random walk in that landspcae is going to result in anything by timely responsetimes.

    That's my main objection on the string framework. Other than that, I think the general idea of a framework is rather an evolved and sophisticated. Beeing easily shot down as such, is not a trait in my opinion, so I differ with Poppers view here.

    From what I know of Poppers analysis of this, he does not put enough weight on the problem and effiency of hypothesis generation in the event of falsification. He thinks this is not a question for physics. I disagree with that. Paradoxally the point I raise in defense of the concept of frameworks (if we accept to thing of string theory assuch) is also, in the above sense, also what seems to be a major problem with it.

    I don't think the idea of a framework is totally wrong, I just thinkg that perhaps "strings" aren't the one we're looking for.

  17. Sep 17, 2008 #16


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    I couldn't help this reflection, intended as musings with pun (not meant to upset anyone, in particular not Popper;)

    Suppose this is as hard as it seems, I'm not sure what's better. Beeing starved to death, or beeing shot down?

    I would guess Popper would choose the latter, and consider the problem that the "theory operating the hypothesis generator" will also starve unless it produces an theories that generates a return to the investments someone elses problem.

    I see both options as equally bad. It seems somewhere there is a balance between staying sufficiently openminded to not mistake the unlikely for impossible, and OTOH not mistake lack of certainty of lack of guidance.

    I think Popper must have had alot of nightmare about uncertain induction that he rather die of starvation, than using an unreliable guide called induction. At least he would go down with pride :approve:

  18. Sep 26, 2008 #17
    Marcus,what with positive predictions of those nonstring thoeries?

    I mean besides they are against more dimensions, do they have some testable predictions?
  19. Sep 26, 2008 #18


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    Thanks for asking, I think you could hardly imagine a stronger test. If extra dimensionality is found, that shoots down the whole nonstring QG range of approaches. That is falsifiability in spades, deluxe version.

    So nonstring QG is being rigorously tested by the Large Hadron Collider, in a properly Poppery sense, whereas superstring/M ideas are not.

    I can't pretend to speak for any of the research communities, since I am just an interested observer. I am still hoping that before April 2009 (when they expect to start running collisions) some absolutely brilliant theorist will come up with a really interesting prediction. It could happen. I mean theorist of any kind, nonstring QG or other. So far what people are talking about strikes me as mainly related to refining and elaborating the Standard Model.
    Last edited: Sep 26, 2008
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