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Does string theory predict additional long range forces gravity EM

  1. Sep 17, 2009 #1
    Does string theory predict additional long range forces gravity EM?

    One of the most touted features of String theory is that it correct predicts and even requires 2 infinite long range forces, gravity and E&M.

    I've heard that string theory though also predicts additional non-observed long range forces which are not observed, and allowing gravity and E&M to exist in string theory while suppressing the non-observed long range forces requires a lot of fine-tuning. Another way to hide these long-range forces is to associate them with dark energy.

    Is this correct? if gravity is a triumph of string theory then what about unseen forces that are equally an implication of string theory?
  2. jcsd
  3. Sep 17, 2009 #2
    The various string theories do generally have additional long range forces. Anytime you have extra U(1) gauge interactions e.g. (One thing to note with supersymmetry is that if there are gauge fields with scalar superpartners, these scalars also couple to charged fields like their vector-field partners, so you have to worry about those guys as well). As you mentioned, all these types of interactions may be suppressed, but not just through fine tuning; there are other situations like intersecting branes and domain walls in which these interactions can more naturally be suppressed.
    Some scalars, though, may indeed be useful phenomenologically for the strong-CP problem or Dark Energy if they are very weakly interacting, e.g., so that we would have missed them in experiments to date. I wouldn't say this is "hiding" them, but using them.
    Also, I wouldn't say that string theory makes specific predictions much more than quantum field theories do with regard to what kind of interactions to expect. String theory has a great flexibility in what kinds of low energy effective theories you can get from it. So to address your last question, yes string theory predicts gravitons and many other particles and interactions, but we see the first as a triumph and the latter as a challenge due to the fact that we haven't seen evidence for those new interactions in nature, so you have to push them out of the way according to experimental bounds. Another challenge is that there are expected to be numerous string scenarios that would be consistent with experiments to date and yet predict different beyond-the-standard-model interactions and particles. If you can't make tests to distinguish them, where do you go from there?
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