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Long ago, I realized this: (super)string theory can NOT give a well-defined/unique prediction of why the electron (muon, tau) or the neutrino (any flavor) masses have the masses we measure. String theory can NOT give a concrete prediction of any SM particle mass (even resonances like the Higgs and other particles). The question is: Is this lack of predictive ability of the mass of the SM "elementary particles" a hint that string theory in its current status is not complete or wrong? Or perhaps even better:

Even when string theory provides a framework in which you can accomodate every Standard Model field, and hence, a priori you can have the SM spectrum, we have not a string theory explanation of the values of the masses and charges. Should a explicit derivation of particle properties like (rest) masses and (bare) charges (like the electron charge value) be "predicted" and "derived" in a more fundamental version of the theory?

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    $\begingroup$ We already know that string theory is incomplete as it's currently formulated. We don't need "hints". $\endgroup$ – user1504 Jul 12 '13 at 17:21
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    $\begingroup$ Why do you even believe that mass ratios can be derived from first principles? Kepler thought for a while the ratios of the distances of the planets could be derived from first principles. That was wrong, too. $\endgroup$ – Urs Schreiber Jul 12 '13 at 19:02
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    $\begingroup$ This is too broad and much too opinion based, vote to close. $\endgroup$ – Dilaton Jul 12 '13 at 19:52
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    $\begingroup$ Ok, I could have voted to close as not a real question too, since I strongly doubt that you are really interested in hearing what experts have to say in the form of an answer or that you are ready to think about any answers you get seriously (apart from potentially incoming trolling answers that support your dismissive point of view). I rather suspect that by this post you just wanted to state your personal negative opinion about string theory on this site and that's it ;-) $\endgroup$ – Dilaton Jul 12 '13 at 20:08
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    $\begingroup$ @riemannium I think that why questions are not really answered by physics mathematical models, which is what string theory will be when/if finalized. The answers physics models give are of HOW. How from the axioms/postulates of the theoretical model the experimental data are described and new data predicted successfully. All the why questions are actually nested how answers that eventually hit the axioms/postulates. In a mathematical theory the answer would be because I chose the axioms. In a mathematical model of physics the answer is "because I fit the data by these axioms and predict new". $\endgroup$ – anna v Jul 13 '13 at 10:36
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None of our known theories of physics makes any predictions before specifying a model in the theory.

Think about it: There are many, many choices made to pick the standard model of particle physics out of the huge space of possible local QFTs. Think about how vast that space is, the "landscape of QFT": you can write down pretty much any local Lagrangian that is quantum anomaly free and get a QFT. Nothing in general QFT "predicts" that the world is described by Yang-Mills theory coupled to fermions with the gauge group and the coupling constants seen in the standard model of particle physics. These are a huge number of choices we humans make to fit QFT parameters. Only once all these choices are made, does the standard model start to predict anything.

That why it's called the standard model and not the standard theory! The standard theory is QFT, but that alone predicts nothing.

Same for the standard model of cosmology. Nothing in Einstein gravity predicts that the universe is modeled on large scales by an FRW model with positive cosmological constant. The back and forth with the assumptions made here is legendary (Einstein's "biggest blunder" in model building is now our Nobel worthy insight. Nothing predicted it.) Only after the FRW model is chosen, only after dark matter content is adjusted to fit observations, only then does Einstein gravity start to make predictions in cosmology. The "landscape of Einstein gravity" is huge and vast: think about all the possible solutions to Einstein's equations. There are not just 10^500 of them. It's certainly not a finite set, it's a hugely infinite-dimensional space.

One needs to remember this before getting worked up about the alleged lack of predictivity of string theory.

For in string theory it's the same: of course the theory itself doesn't predict anything. Just as QFT and Einstein gravity by themselves predict nothing. In each case one first needs to fix a model and then the theory make predictions about the remaining parameters.

The irony of this is: the constraints on model building in string theory are much stronger than in QFT. Not every model in QFT lifts to a model in string theory (the "Swampland" doesn't sit in the "Landscape"). On the other hand, string theory models generically contain Einstein-Yang-Mills theory. At least that. In QFT you can write down weird theories that have nothing to do with the real world. Like phi^4 theory. In string theory you cannot, since its much more constrained. String theory predicts that the world is not described by phi^4 theory. QFT cannot predict that. For instance.

So string theory is clearly more predictive than QFT! Think about it. In string theory we have a chance of arguing that there may be a finite number of models -- even if it is a large finite number such as 10^500. This is nothing against the hugely vastly infinite-dimensional space of choices of writing down local QFTs.

String theory, just as QFT and just as Einstein gravity, starts making predictions as soon as you specify a model. And that's precisely what string phenomenologist's do. They build models (or approximations to them, because in string theory its harder, since it is more constrained) and check against the known observational constraints. Then they refine their models. This process keeps going on check out the commented links at

http://ncatlab.org/nlab/show/string+phenomenology

Also check out the pertinent paragraphs at

http://ncatlab.org/nlab/show/string+theory+FAQ

And think about. Don't just repeat the "Oh my, oh my, string theory makes no predictions." that you hear in the blogosphere. Before you repeat this, think about what it means for physical theories to make predictions, how QFT makes predictions only once we choose the standard model.

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    $\begingroup$ Dear Urs, I have not said that String Theory does not make any predictions. In fact, the problem is quite the contrary. It does make too many (unseen) predictions. I have studied the theory during my master in Theoretical Physics. The fact that today I am not a big fan of string theory it does not mean I do not study it or that I can critize it. Constructive criticism is basic for the improvement of theories or models. Do you agree? I am only pointing out that string theory has a problem there, not that I can be more or less a stringer myself... $\endgroup$ – riemannium Jul 12 '13 at 19:28
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    $\begingroup$ This is exactly the nice reasonable answer such a way to confrontationally formulated question needs. How I'd like to upvote this a second or even third time ... ! $\endgroup$ – Dilaton Jul 12 '13 at 20:13
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    $\begingroup$ It's not about being a "fan" or a "stringer". It's about logical reasoning in science. String theory might be wrong, but not because it has more than one vacuum and not because it doesn't uniquely predict the observed elementary mass spectrum. That's not a logically sensible criticism. You might as well complain that it doesn't predict your telephone number. I would like to see people concentrate and have decent arguments about string theory. Feel free to try to trash it, that's what science is about, but know some good reasons. $\endgroup$ – Urs Schreiber Jul 12 '13 at 22:15
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    $\begingroup$ @MichaelBrown : I don't think that string phenomenology is not "practical". $\endgroup$ – Trimok Jul 13 '13 at 7:45
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    $\begingroup$ @Michael Brown, yes, I think back in the 20th century many hopes were just as irrational as many supposed debunkings are these days. It would be good if the grander theoretical physics community had a better sense of the logical foundations of its subject, such as to then be able to make objective, substantive judgements. Much of the criticism of string theory is based on confusion already about QFT (such as the common "string theory doesn't work because the perturbation series does not converge" or "because it doesn't have a unique vacuum"). $\endgroup$ – Urs Schreiber Jul 13 '13 at 14:19

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