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String Theory predictions

  1. May 18, 2008 #1
    Hello everyone
    I create this thread as a copy of one thread with the same title in the subforum dedicated to string theory because nobody there answered my following question:

    I have a question regarding string theory (ST) predictions:
    I am not interested (in this thread) about the predictions related to what is specific about (ST) (what is not predicted by Quantum theory (QT) or General Theory of Relativity (GTR))

    I want to know only if (ST) by using its specific equation can predict the result of both (QT) experiments (such as the double slit experiment) and (GTR) experiments (such as observation in the sky)

    In other words, is (ST) currently able to reproduce the exact results of both (QT) and (GTR)?

    Can you give references if you have the answer.

    Thank you
  2. jcsd
  3. May 18, 2008 #2
    ST is based on quantum mechanics, and thus is in accordance with it. It also reproduces GTR in the low energy limit. These facts have been the main motivation for studying it and can be read in any text book. For LQG nothing comparable can be said, as far as I know.

    As for predictions, ST is exceedingly finely tuned and as such in principle makes an infinity of in principle testable predictions; however not in the ground state sector that we can access experimentally today. Perhaps we are lucky and the string scale is low as in some models (which is very unlikely though); in this case an experimantal verification would be possible by observing the stringy resonances.

    The possible existence of a landscape does not invalidate this statement; the landscape (whose existence is not even proven) refers to the set of possible ground states, but given any one such ground state, an infinite number of predictions can still be made in the massive sector. The point being that string theory is simply not an arbitrary, floppy and featureless theory: quite on the contrary, it is highly structured and finely tuned.

    All these issues have been said a countless number of times, but ignored and deliberatively confused by those self-declared critics who have their own reasons for doing so. Don't be misguided by those and the fools who mindlessly repeat this here all over without having any clue.
  4. May 19, 2008 #3
    Any references?
    Please note that I don't care about prediction not related to QT or GTR experiment.

    I am in no way against ST, especially if it is able to predict both QT and GTR because for me theories are not about reality but about prediction.

    I just want references proving that ST by using its own set of equation is able to predict QT experiment results (double slit experiment) and GTR experiment results (sky observation experiment), nothing more
  5. May 19, 2008 #4


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    QM is built in and put in by hand, so you don't really need to 'show' anything. It satisfies all the usual rules and axioms you would want (you know, hermitian operators, phi^2 is the probability, has a well defined classical limit, etc etc). It thus automatically would satisfy the two slit experiment for instance.

    What you do need to show is that it reduces to GR in the suitable low energy limit. That takes a bit more work, and you will need to consult a textbook (say Green, Schwarz, Witten) to see it done in detail. It really reduces to a type of supergravity + extra fields, which you can show reduces in a further low energy limit to GR at observable scales.

    The biggest problem is you need to show that it reduces to the correct matter content that we observe in our low energy regime (to wit, the standard model). Thats where the hiccups happen, b/c there are in principle, many ways to actually do this that are not necessarily unitarily equivalent. Its also never been done exactly, as phenomenologists can get close, but often have relic exotica that they don't necessarily like to be present. Anyway, ongoing active research area.
  6. May 19, 2008 #5
    Can it be used so to predict sky observations with the same accuracy than GTR?
  7. May 19, 2008 #6


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    The predictions would be identical since it *is* GR at those energies. Now if you are close to a blackhole or near the big bang, the predictions would differ by quantum stringy corrections.
  8. May 19, 2008 #7
    No, there are no predictions

    Unlike "moveon" (whoever he/she is) I'm not deliberately confusing or ignoring anything, rather making accurate scientific statements, unlike the misleading hype you see here. As for my motivations, the main one has always been to try and do something to combat hype of the kind you see here, which has done a huge amount of damage to theoretical physics.

    "moveon" seems to admit that string theorists have no idea what the ground state of the theory is and thus are unable to predict anything about physics at experimentally accessible energies. But, besides not knowing the ground state, string theorists don't know the energy scale of the theory, or even have a formulation of it valid outside of asymptotically small string coupling. As a result, they have no predictions at any scale. Characterizing this as "ST is exceedingly finely tuned and as such in principle makes an infinity of in principle testable predictions" is just absurd.

    If a string theorist starts going on about "predictions", ask them to produce some. What you will get are very generic answers ("it predicts gravity", yeah, 10d gravity, maybe 11d), followed by a list of excuses. As Feynman once said, "string theorists don't make predictions, they make excuses". He knew what he was talking about, and things have just gotten worse in the years since he has left us.
    Last edited: May 19, 2008
  9. May 19, 2008 #8
    No. String theory is indeed very finely structured and does not allow any deformation against its build-in structure. Do you really claim that scattering amplitudes are arbitrary and not determined once the ground state is given?
  10. May 19, 2008 #9


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    That main question, as I read it, in the OP is "does string theory agree with predictions made from quantum theory and general relativity?" These questions have both been answered: the first question is moot, and Haelfix has answered the second question quite correctly: since string theory reduces to GR in the appropriate limit, then it necessarily agrees with the predictions.

    I don't see the need for this thread to escalate into a "what does/doesn't string theory predict?" thread.
  11. May 19, 2008 #10


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    It seems to me that that claim is not written above. But I am intrigued: does the ground state fix the particle content completely, not allowing any further degeneracy of multiplets nor any kind of superselection sectors?
  12. May 19, 2008 #11
    Can I have references of good books that explain that nicely (not too complex)?
    Thank you
  13. May 20, 2008 #12


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    This point about 10d gravity keeps me intrigued also. We are told (by Witten) that the spin 3/2 fields appear as spin 1/2 fields in the 4dimensional spacetime after compactification. One could expect the same thing to happen for the spin 2 fields: to appear as spin 1 or spin 0 fields in the 4dimensional spacetime. If it were so, then it is not gravity. If it weren't, then some explanation about how susy becomes hidden after compactification should be exposed more explicitly.
  14. May 20, 2008 #13
    The only things that are well-defined are scattering amplitudes in flat 10-d space-time, for asymptotically small string coupling. For any kind of realistic background, where 6 dimensions have been compactified, or we live on a brane, etc., there is no well-defined theory. Since you don't have a non-perturbative string theory, you can't even begin to address certain obvious questions about string amplitudes: if you collide two strings together at Planck scale energies, what is the amplitude for producing a black hole?

    The bottom line is that there are no predictions. It is simply not true that you can use the 10d flat space-time perturbative scattering amplitudes to make a definitive, accurate predictions about Planck scale physics.
  15. May 23, 2008 #14
    Not too complex? Explain that nicely?

    The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next - by Lee Smolin

    Agree or disagree, it really is a perspective that is worth reading, comprehending, and considering.
  16. Jul 16, 2008 #15
    Of course there are predictions, in fact infinitely many. I just came out of a talk where detailed computations and predictions, down to the level of cross sections, were presented. This is work by Lust, Stieberger and Taylor to appear next week or so. What they do is to compute tree-level amplitudes for 4-gluon scattering and similar. Their results are amazingly model independent, the amplitudes they consider to not depend on CY moduli and alike; they take the form of a Veneziano like amplitude dressed up with the appropriate kinematical factors. The important part displays universal Regge behavior, and as said, does not depend on any of those dreaded moduli (nor on SUSY breaking etc). So this bypasses the story of 10^500 vacua alltogether.

    So these are universal robust predictions, and completely in line what I said before. The zero mode sector depends on the choice of vacuum state and predictions are hard to make there; but clearly once one accesses the sector of the stringy excitations, then clearly there will be stringy predictions! Of course in usual models the level spacing is in the order of the Planck scale and thus inaccessible. But the aim of this work is low string scale models, where the level spacing may be just a few TeV and thus well withing experimental reach. So there is a clear punchline: either one observes the extra predicted jets at the LHC, or not. If yes, case closed. If not, that class of models is excluded.

    This is not to say that these models are likely to be realized in nature nor that I believe in them, but I say this to make the point: string theory does make experimentally testable predictions, not only in principle but also, if we are very lucky, in practice. This is in striking contrast what you want to convey.
  17. Jul 16, 2008 #16
    You do realize who you're talking to, right?
  18. Jul 17, 2008 #17


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    Yes it can. However, it has been recently argued that in some cases the ST predictions on quantum interference may differ from those of the conventional quantum theory without strings:
  19. Jul 17, 2008 #18
    An expert authority with an impressive scientific record?
  20. Jul 18, 2008 #19
    I was referring to the fact that arguing the pros/cons of string theory with this person is tantamount to talking to a brick wall. Sorry, I assumed most people prefer not to waste their breath. My bad.

    There's no need to get defensive on anyone's behalf. My question had nothing to do with authority or scientific record. In fact, the author's book is one of the better lay reviews of particle physics I've ever read (second to Oerter's). No need to put words in anyone's mouth when "yes" would have sufficed.
    Last edited: Jul 18, 2008
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