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On String Theory's Predictiveness

  1. Sep 17, 2010 #1
    So I just finished reading Brian Greene's The Fabric of the Cosmos. All the parts before the string theory talk on inflationary cosmology were really illuminating (the stuff on Higgs fields was particularly enlightening). Anyway, I'm certainly not an expert on any of this stuff, (do the PF forums mind that? I'm sure the Ph.D's and grad students who actually work on cosmology related questions like in the Fine Structure Constant thread don't enjoy crackpots with no training making threads on their outlandish theories, but is it okay for someone with a strictly qualitative understanding to lurk in these forums and ask occasional questions?) but when I got to the string theory section, it was nice to see some updates on where Greene left off in The Elegant Universe (All the stuff about M-Theory). Anyway, my question is this.

    I notice a lot of people say that string theory makes no predictions, or that the ones it does make require astronomical (literally) amounts of energy to test. My question is
    A) Does this matter? We were all cool with accepting the stuff of atoms way before we could see them, so why is string theory necessarily different?
    B) Aren't there in fact tests that could be done that would give a lot of backing to string theory? In his book Greene mentions that the 3 non-gravitational forces are bound to our 3brane and gravity is free to drift off into the extra dimensions. Shouldn't this be verifiable (since then the gravitational force does not in fact diminish with respect to the distance squared between two bodies, it just appears to do that since we only look at stellar/planetary scales, not on teeny tiny scales where rolled up spatial dimensions matter). Isn't this a test that could be done (difficult, but plausible and not requiring particle accelerators the size of the solar system)? I realize that detecting gravitational waves has been difficult enough as it is, but this sort of a test seems like a "in our lifetime" test, as opposed to the particle accelerators the size of the solar system type of tests.

    Basically, I was hoping to know what other experts in cosmology thought about string theory (when the only information you have on string theory comes from the guy who wrote several books on it, it seems sensible to seek out other opinions... It's like asking the Toyota salesman if the Camry is the best car around). From a non-technical perspective, it does seem elegant in describing why there are the sub-atomic particles there are and why they have the specific masses they have, just like inflationary cosmology helped to explain why space is so flat and why entropy started so low. It seems like without string theory, there are a ton of "things are this way because that's the way they are" just like before the theory of inflationary cosmology low entropy start conditions were that way just because that's the way they are (sort of anthropic explanations, and who likes them?).

    Anyway, yea, just curious what people who don't make their livings off writing books on string theory think about string theory and its verifiability and potential implications on cosmology. Thanks!
  2. jcsd
  3. Sep 18, 2010 #2


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    Even before we could image individual atoms with scanning-tunneling electron microscopes, atoms themselves provided us with a model that provided a number of testable predictions. The inability of string theory to do this so far, then, is definitely a strike against accepting the theory.

    In the mean time, string theory does have many other promising aspects (the most significant of which is the unification of gravity with the other forces), and thus it seems to make sense to me that many theorists are rightly interested in working on it. We're likely to see a decrease in work in string theory over the next few years as the LHC ramps up, but I'd be willing to bet that we'll see it come back with a vengeance once we have plumbed most of what the LHC can tell us.

    In addition, while we may not be able to test string theory directly, string theory does provide a paradigm within which it is possible to produce specific models that can be tested, such as extra dimensions and branes.

    This is one of those specific models that I mention above. While we can't necessarily test any possible way in which our universe could be a 3+1 dimensional brane, there is a portion of the parameter space that we can very much investigate in the near future. Part of it has already been excluded. Sadly, the unknown parameters in such a theory can potentially take on values that we would never in our wildest dreams be able to detect, but if we're lucky, it is possible to verify the model.

    In the mean time, I think most people place string theory in the category of, "Interesting, needs work."
  4. Sep 18, 2010 #3


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    Agreed. I think string theory cannot possibly be wrong, but may be irrelevant. For now, I consider it a mathematical anomaly.
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