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If String Theory explains/ leads to gravity, how does it not make predictions?

  1. Jun 8, 2012 #1
    If String Theory explains/"leads to gravity, how does it not make predictions?

    String Theory "predicts" gravity. It can explain gravity and it explains "small" structures.
    If String Theory includes these two topics shouldn't it predict what happens when Quantum Gravity becomes non-negligible.
    So how is it not testable/observable in black holes, singularities or similar situations?
     
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  3. Jun 8, 2012 #2

    mfb

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    Re: If String Theory explains/"leads to gravity, how does it not make predictions?

    String theory may be able to make predictions - however, it is not (yet?) at a point where you can "just apply string theory and predict reaction X". The first step would be a postdiction of particle physics and general relativity. At some point in the future, it might be possible to understand black holes with string theory - this could give testable predictions or at least explain the observations.
    However, I would not expect this within the next years...
     
  4. Jun 9, 2012 #3

    julian

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    Re: If String Theory explains/"leads to gravity, how does it not make predictions?

    The reason string theory doesnt make explanations is do with the lanscape isn't it??

    Also bit of a warning on the matter of string theory predicting gravity...there are a couple of reasons for believing this

    i) Adding up the effects of lots of gravitional interactions between grav waves leads to G.R

    2) for quantum strings to proagate consistently the Einstein equations should be fullfilled.


    Objection of i) first off some people dont like the idea that gravitation waves can come together and form a black hole say...some people claim the whole argument doesn't work (Padmanabhan).

    Secondly, although string theory might require the Einsteins equations hold, perturbative string theory only works when the spacetime is stationary: worldsheet supersymetry when combined with the GSO projection leads to spacetime-supersymmetry, but this requires a timelike killing vector field, i.e. the spacetimeis statiotnary.

    In actual fact pertpurbative string theory only works for a zero measure subset of solutions of Einstein's equations...so dont get too carried away about the connection between strings and GR.
     
  5. Jun 11, 2012 #4

    tom.stoer

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    Re: If String Theory explains/"leads to gravity, how does it not make predictions?

    There are several problems with "predictions" derived from string theory.

    1) some of them - like gravity - are post-dictions
    [it would be wrong to say that QCD predicts the existence of the strong interaction; it explains how it can be modelled, but it does not predict it]

    2) it suffers from the landscape problem, so it does not predict the standad model or some of its SUSY- or SUGRA-extensions
    [it is wrong to say that gauge theory predicts QCD; gauge theory allowes for QCD but this is only one possibility; gauge theory allowes for a world w/o unbroken SU(3) gauge invariance as well]

    These are two examples why string theory does not make predictions "which are experimentally testable in principle". In addition string theory seems to make no predictions which are testable in practice b/c in the low-energy sector (which is experimentally accessable) string theory solutions are tuned in the sense of 2) to fit to well-known low-energy theories whereas the true stringy behavior of elementary particles is by construction not visible in the low-energy regime.
     
    Last edited: Jun 11, 2012
  6. Jun 12, 2012 #5

    atyy

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    Re: If String Theory explains/"leads to gravity, how does it not make predictions?

    I don't know how mainstream this is, but here's one view by Gordon Kane:
    http://particle-theory.physics.lsa.umich.edu/kane/Colloq 2011.pdf

    "String theory is too important to be left to string theorists"
     
    Last edited: Jun 12, 2012
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