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If Superstring theory is not a TOE what is it?

  1. Apr 30, 2015 #1
    If superstring theory works, has no infinities, etc. Then what is wrong with it? It must describe some Universe unless there is a contradiction in it somewhere? If it predicts all particles and gravity what is missing from it? Is it not compatible with cosmology for instance?

    Is Superstring theory just an approximation? (Like classical mechanics is an approximation to quantum mechanics setting h=0, and c-->infinity ).

    So is superstring like M-theory as classical mechanics is to quantum mechanics, i.e. merely an approximation?

    i.e. classical mechanics is perfectly consistent on it's own (if we think of the world as consisting only of billiard balls), but there are some things it can't explain like radioactivity.

    In that case is there something that superstrings can't explain that we need M-Theory for?

    By M-Theory I mean some kind of theory with some parameters that can smoothly be tuned to get the various string theories.
     
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  3. Apr 30, 2015 #2

    ShayanJ

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    There are two main things which are wrong with superstring theory.
    1) It still is not able to propose a unique model for the universe and string theorists don't know which model should they consider to describe our universe.
    2) One property that any physical theory must have, is that we should be able to design experiments such that among their possible results, some of them, if realized as the actual results, will disprove the theory. String theorists are not able to design such experiments yet.
     
  4. Apr 30, 2015 #3

    DaveC426913

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    Concur with Shyan.
    String theory
    - has a huge number of parameters that can be set to a huge number of values. Why the particular values that make our universe we don't know. Makes it kind of useless.
    - is untestable. By strict definition, a theory that is untestable is not science.
     
  5. Apr 30, 2015 #4

    ChrisVer

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    The reason you need the M-theory is because the String Theories appear in many (5) versions. The M-Theory is proposed in order to unite those 5 versions under 1 theory. So in fact there will not be 5 distinct types of superstrings, but they will be just the results of limits of this 1 theory, and so can be related to one another.
    So I am not sure about the explanation- what does superstring explain? nothing. If it explained something, it would be a physical theory... The reason of the need of M-Theory (that in fact we know nothing about) is to play the role of the relationship between the different scenarios.
    Now people has also discovered the "F-theory"..

    Superstring theory is just the string theory together with supersymmetry, so that it could also contain the fermions (something the string theory / bosonic strings couldn't)..
     
  6. Apr 30, 2015 #5
    But supposing we set all the parameters of string theory, that will presumably model some sort of Universe.
    But like classical physics doesn't explain radioactivity, is there something that superstring theory fails to predict?
    I'm looking for an answer like "Superstring theory can't contain protons". or "Superstring theory can't explain an expanding universe". or something like that.
    For example we had to abandon classical physics because it is incompatible with experiments on radioactivity and the double slit experiment.
    I wonder if there is a physical experiment that would force us to rule out superstrings and say it is a good approximation but...?

    I wonder if in M-Theory those limits which are the 5 string theories, are physical limits? Setting free parameters? Or just approximations in the same way as setting h=0 or c-->infinity? i.e. would M-theory have any free paramters or could we set them all to 1 in the same way as setting h=c=G=1?
     
  7. Apr 30, 2015 #6

    ShayanJ

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    Yes, but what universe? We don't want something that models some universe, we want something that models our universe!
    We didn't abandon classical physics. In fact right now, classical physics is much more credible than string theory. At least we still call it physics!(And always use it when we don't need more advanced things!)
    In fact I should say that the main thing which is wrong with string theory is not that it can't contain something, but that it can contain everything you want! I don't know how that sounds to you but its a bad thing for a physical theory!
    Those experiments didn't rule out classical physics, but just helped us to know its limits.
    That's exactly one of the problems of string theory. We don't know such an experiment!
     
    Last edited: Apr 30, 2015
  8. Apr 30, 2015 #7
    That's kind of my point. If we can't rule out superstring theory, why do we need M-Theory?
     
  9. Apr 30, 2015 #8

    ChrisVer

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    Again, because there are many superstring theories and you have found that they are somehow related. To put it simply, they seem to be different faces of the same dice.

    But still, the ability of a theory to describe more things that you naturally observes, makes it unfalsifiable...and this has not to do with its ability to see the limits of h or c or whatever (M-Theory is not a mathematically developed theory, so you can't speak on its grounds), but on how you choose to do the compactifications and what mechanism chooses the "right"/realistic one (by the way we don't know a realistic one).
     
  10. Apr 30, 2015 #9

    atyy

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    What is the difference between string theory and the standard model of particle physics? The different metastable states are just like the many different materials we see in our everyday life.
     
  11. Apr 30, 2015 #10

    ChrisVer

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    Could you make this question a little easier to comprehend? What do you mean we meet many different matterials in our everyday life? I have never seen a different universe... leave aside the fact that no compactification so far has been successful to reproduce our universe.
    To me, metastability means to lie in a vacuum that is not so favorable compared to others, but some mechanism keeps you "shielded" in that. I don't know if the vacua we get from the compactification are different and not degenerate. I guess they are degenerate and that's why some people try to answer the landscape problem by introducing the anthropic principle.

    The fast difference I can find is that the SM has been successful in making testable predictions and so far it has worked in excellent efficiency. Also you can't compare the SM (an effective theory, that we know it is not supposed to be "The One" ) with the string theory.
     
  12. Apr 30, 2015 #11

    DaveC426913

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    The difference is that string theory proposes a mechanism by which general relativity and quantum mechanics are reconcilable; it does so by postulating a workable model of quantum gravity, something our current standard model has repeatedly and exasperatingly failed to do.
     
  13. Apr 30, 2015 #12

    atyy

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    What I mean is that I don't think it is reasonable to criticize string theory for having many emergent low energy universes, just like the standard model. Of course, this was not the unification some theorists like Weinberg dreamt of. But from the condensed matter viewpoint, Weinberg's dream was not reasonable, because if even the standard model has many emergent low energy universes, then any theory of everything will have them.
     
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