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LHC, supersymmetry and strings

  1. Oct 16, 2007 #1

    Demystifier

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    One of the things that LHC will experimentally search for are supersymmetric partners of the Standard-Model particles. Maybe it will found them, maybe it will not. But what if it will? The existence of supersymmetry does not imply the existence of superstrings. However, string theory is consistent only if it is also supersymmetric. In other words, string theory implies supersymmetry, that is, the existence of supersymmetric partners.
    Thus, if LHC finds supersymmetry, one of the predictions of string theory will be experimentally confirmed. If that happens, will you give more credit to string theory? Will you start to think that string theory is more likely to be correct than you thought so far?
     
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  3. Oct 16, 2007 #2
    Yes, I'd find it (a little) more appealing. But I do not agree with you that ST predicts supersymmetry. As far as I understand it, supersymmetry is a precondition for constructing ST. I'd really find it much more appealing if ST could offer clear and testable predictions, that is, if it could be falsifiable.
     
  4. Oct 16, 2007 #3
    Can you clarify the difference between prediction and precondition?

    So if astronomers discovered a gigantic cosmic string (this is edward witten's favourite way of varifying string theory) you would be only a bit less dismissive of string theory unless someone could provide a way to falsify string theory?
     
  5. Oct 16, 2007 #4

    arivero

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    So now Witten is astronomer? I'd thought his favorite way should be to find some proof of 11-dim gravity in a way such that the extra seven dimensions are acted by SU(3)xSU(2)xU(1).
     
  6. Oct 16, 2007 #5
    IF the LHC finds superpartners, it would be a great triumph for string theory. However, that is a very big IF. So far, all experiments looking for SUSY effects have seen exactly nothing. In fact, only in the past two days there have been four eprints reporting on null searches for SUSY, namely

    http://www.arxiv.org/abs/0710.2372
    http://www.arxiv.org/abs/0710.2378
    http://www.arxiv.org/abs/0710.2502
    http://www.arxiv.org/abs/0710.2897
     
  7. Oct 16, 2007 #6
    precondition: a condition that must be fulfilled before other things can happen or be done
    prediction: an indication of the outcome of an experiment in a laboratory setting (or based on the observation of a phenomenon in nature)

    I'm not waiting for the day that ST will be falsified. I'm only expecting that ST can be formulated as a falsifiable theory, like any theory is supposed to be. There is a big difference between the two. Think about it.
     
  8. Oct 16, 2007 #7

    Fra

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    I think I objected to this in another thread. If you take the view that ST really isn't a theory (that makes straight predictions), but rather a framework with contains many possible (but somewhat specific) theories or types of theories, then the falsification should be applied to the framework, because there is no theory. It's a framework of theories.

    So what does falsification of a framework possibly mean?

    Perhaps that no successful theory exist in the framework? seems reasonable?

    But then the "string strategy" really is seriously incomplete unless this framework is equipped with a method or principle that allows theories to evolve or be selected in the framework.

    Strongly constrained but pre-tuned (narrowed down) frameworks can do with a random selection and still be very fit, but as the framework inflates in complexity the entire strategy stalls unless there is a guiding principle. That's my opinion at least.

    Given we have such strategy, and the framework as such is good, it seems the falsification should be replaced by a quantiative measure of support, where a good strategy is a "quick learning" strategy or a "quick selection".

    I figure that in this way, the efficient strategy (good framework and good adaption) is the winning one that will dominate, all others will be less frequent.

    I think this applies not only to human endavours but also to how evolution in nature works. The poppian thinking of falsification feels sort of out of date.

    /Fredrik
     
  9. Oct 16, 2007 #8

    Demystifier

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    But string theory is already formulated as a falsifiable theory. For example, it clearly predicts that there are particles with masses of the order of the Planck mass. The fact that experimentalists are not able to verify this because their accelerators are not strong enough does not make string theory unfalsifiable. If such energies were experimentally available but such particles were still unseen, that would be a clear experimental proof that string theory was wrong.

    The problem, of course, is to find a LOW energy prediction of string theory. But LQG, for example, suffers from the same problem. I do not understand why it is considered a drawback of string theory but not of LQG. Double standard?
     
    Last edited: Oct 16, 2007
  10. Oct 16, 2007 #9
    If supersymmetry is as you say a “precondition for constructing string theory”, string theory requires supersymmetry to actually exist in nature to make sense. If supersymmetry doesn`t exist in nature, string theory can never make any sense as a physical theory since it is as you say a “precondition for constructing string theory”. Therefore should one not say that string theory does indeed predict supersymmetry, since supersymmetry is as you say a “precondition for constructing string theory”?

    IMHO, there is no substantial difference between the two in how you’re using them here.

    Suppose a theory was falsifiable in principle, but not in practice. Would this satisfy you, because it’s possible that this may turn out to be the case. It`s even logically possible that there are correct theories of our universe that are not falsifiable even in principle. It may turn out that the only basis on which our belief in a theory can ever rest is it’s explanatory power. I hope this does not turn out to be the case, but how can we be sure it won`t?

    Is this the main source of your unease with string theory as far as you understand it?
     
    Last edited by a moderator: Oct 16, 2007
  11. Oct 16, 2007 #10
    I think it is a problem for both: ST and LQG. And I never said otherwise.

    Although I find LQG approaches more interesting (because my background mostly comes from GR, not particle physics), it is not the case that I find LQG less problematic than ST.

    Actually, if I would go to work on quantum gravity, I'd study both approaches (their main fundamentals), find useful tools and ideas from them, but I'd also think about other ideas. In special, I'd go far back into their ontological status. I'm starting to feel like I am a 'structural realist' when it comes to how our scientific knowledge evolves over difficult matters.
     
  12. Oct 16, 2007 #11
    Well, I suppose there are many currently accepted definitions of what science is.

    I am a professional scientist (an astrophysicist turned into software engineer turned into condensed matter physicist) and use the traditional scientific method in my daily work.

    But I respect other interpretations. So it's fine for me if there are people working under the frame of 'a theory falsifiable in principle'. I myself crossed a bit that territory some times.
     
  13. Oct 16, 2007 #12
  14. Oct 16, 2007 #13
    Were these first or second order phase transitions? :smile:
     
  15. Oct 16, 2007 #14

    marcus

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    Demy, isn't it true that string is a framework within which various theories with various theories might be constructed? Perhaps it would help to restate the problem. It might be too much to ask that there be a collective prediction that would refute the whole philosophy. But I leave that up to you since you know more about string thinking.

    In any case I would say that LQG stands for a bunch of (different but related) approaches.
    Different theories within the LQG cluster might make predictions and be falsifiable. The problem (as I would restate it in the LQG case) is to take one particular approach and see if you can refute it, experimentally or otherwise.

    For example the approach that Lee Smolin has been working on since 2005 (and a halfdozen other people) depends critically on 4D. If evidence of extra dimension were seen this would shoot it down.

    It also has only the ordinary particles---no room for partners. So if evidence of SUSY were found, that would shoot it down.
     
    Last edited: Oct 16, 2007
  16. Oct 16, 2007 #15

    Demystifier

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    I would say there is no much qualitative difference between strings and loops in that regard. The difference is only quantitative, in the sense that the string approach contains a much larger number of (different but related) possibilities. This is the price payed for the fact that the string program is much more ambitious than the loop program.

    If the ONLY goal of the string approach was to have a consistent quantum gravity, then I would certainly prefer loops over strings. But the string approach is much more than a quantum theory of gravity.
     
    Last edited: Oct 16, 2007
  17. Oct 16, 2007 #16

    Demystifier

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    Anyway, marcus, would YOU find strings more appealing if supersymmetry would be found by LHC?
     
  18. Oct 16, 2007 #17

    marcus

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    I didn't realize that you think the "loop program" is only to make a consistent quantum gravity. If you think that, then naturally you would suppose that the string program is much more ambitious.
     
  19. Oct 16, 2007 #18
    As has been repeated many times in this forum, string theory appears to be based on a unique but as of yet undiscovered set of principles with different solutions describing different phenomenologies.

    I too use the term LQG this way. But I’m unaware of any serious LQG analog of the developments in string theory that suggest that these approaches are parts of the same theory. This shouldn’t be surprising though since IMHO research in LQG has always been characterized much more by free-wheeling invention than natural discovery.
     
    Last edited by a moderator: Oct 16, 2007
  20. Oct 16, 2007 #19

    Demystifier

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    Are you talking about dualities and the hypothetic M-theory? I think it is more honest to say that "it is believed" or that "there is some evidence", rather than "appears to be".
     
  21. Oct 16, 2007 #20
    Why? Have you ever heard of anyone arguing for the uniqueness of string theory on some other basis?

    I disagree.
     
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