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Conventional wisdom says "no", lack of new predictions being the main criticism. Johansson and Matsubara review string theory from various empiricist perspectives, and the best they can say (for string theory) is that it is not the right time to drop it. Yet. They add that it still dominates physics only because "those involved have strong realist convictions".

But many conventional criticisms give me an impression of missing the point. Does one really need to believe in reality made of math, like Tegmark, to embrace unification, with or without new predictions? At the time of Galileo experimental results were at a premium, and theoretical speculations in the absence of empirical constraints were plenty. Today the situation is nearly reversed, experimental methodologies are plenty and routine, we have far more data than we can process, let alone organize and put to use. Experiments in modern chemistry, for example, are little more than analog substitutes for quantum mechanical computations that we are unable to perform efficiently, no one seeks to falsify QM there. The vast empirical input condensed in the Standard Model (SM) and General Relativity (GR) is far more constraining on any unifying theory than any new test can possibly be.

So constraining that so far not a single mathematically coherent (not just heuristic) string theory that reproduces SM and GR in appropriate limits has been proposed. And even the Standard Model itself is riddled with inconsistencies and ad hoc tricks, new tests aren't going to fix that either. Predicting new phenomena is nice, but that was never string theory's main motivation, the incompatibility of SM and GR was. Smolin, one of string theory's chief critics, admits that it unifies SM, and unlike it at least has resources to incorporate gravity too. Moreover, common expectation is that phenomena unexplainable by SM and GR do not manifest below the so-called "Planck energy", which is so huge that it will remain inaccessible for decades anyway, for practical reasons.

The same logic of scientific method that emphasizes empirical testing at early stages of a science discounts it at more advanced stages, when theoretical unification of already available data becomes more important. Since unifying theories maintain contact with reality through already tested lower level theories their independent testing may no longer be a priority, shouldn't they be judged on fulfilling their theoretical goals instead.

Question: Realists may like string theory for revealing truths about hidden reality they believe in, but why shouldn't empiricists love it as a vehicle of unification even if it makes no new testable predictions (which is unlikely)?

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    Can you limit yourself to 1 question per question? also, what do you mean by "modern empiricists" in your question title? – virmaior Mar 13 '16 at 11:15
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    @virmaior Modern empiricists are positivists and post-positivists, J&M specifically use Popper, Kuhn and Lakatos as examples, and self-identify as naturalized epistemologists. But for all their differences the main complaint is the same, no new predictions. The 1 question is in the title, the rest just clarify its context and motivation. At least that was the idea, I am open to editing suggestions. – Conifold Mar 14 '16 at 20:37
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    @draks Physicists seem to think that it is needed to eliminate divergencies in SM in a coherent manner, and fix GR's time travel problems. But more pragmatically, the clearer and the more streamlined formulation of a theory, the more people get involved in developing and applying it, and the more efficient uses we are likely to make of it. I mentioned Tegmark because of his "mathematical universe", J&M mention Susskind, who mocks empiricists as "Popperazzi". – Conifold Mar 14 '16 at 22:25
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    If the universe is mathematical, based upon some axioms, maybe Gödel's incompleteness could be used to reveal that it is not possible to prove i.e. test/measure a theory that connects SM and GR..."Popperazzi" wuahahaha... – draks ... Mar 14 '16 at 22:35
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    Are you asking us to evaluate string theory? Tall order! If string theory is falsifiable but hasn't been falsified then empiricists should embrace it if there are no other more convincing theories. How can a theory be falsifiable and not make predictions though? Is it that it does not make predictions beyond what current theories do? Or is it that the predictions cannot be tested. If they cannot be tested then that would halt an empiricist in their tracks. – igravious May 27 '16 at 10:12
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An empiricist should treat string theory as they do any other theory. They should study the theoretical framework as well as the experimental evidence and if either

(1) There is no ability to ever test the predictions

(2) An irreconcilable contradiction is found in the data

are proven to be true then they should abandon the theory. As of now, neither of those have happened so the theory should not be abandoned. At the same time, an empiricist wouldn't be doing their empirical duty if they accepted string theory as being true without evidence. Supersymmetry (Witten) is integral to string theory and any evidence of it, while it does not out right prove string theory, is more evidence that corroborates the story. The paper you linked briefly mentioned experiments concerning the quark-gluon plasma and it states:

One example is the use of string formalism for describing quark-gluon plasma; see for instance the review by Gubser (2009) and references therein. If such attempts would be empirically successful, it would show the usefulness of the mathematics of string theory; but it would not say anything about the string theory interpreted as a theory about the fundamental interactions or its ability to solve the three fundamental problems stated in section 2.

(Karch provides an account of progress in gauge/gravity (introduction provided by Polchinski) descriptions of quark-gluon plasmas.) Now, it is true that a confirmation of the mathematics behind string theory should not cause an empiricist to believe in string theory; however it also is not, of course, irreconcilable evidence that disproves string theory. At this point the position of empiricists should be agnostic and they should be seeking more evidence. They should not have an a definitive opinion on its truth as of yet.

String theory, as well as all other forms of quantum gravity, are in desperate need of experiments. Empiricists need experimental data to validate their hypotheses. As such, empiricists should be working with theoretical physicists to come up with ways to test string theory. Whether they should then believe string theory to be true or believe it to be false depends on the results of the experiments. If it turns out that string theory cannot ever be experimentally verified then empiricists would have to abandon it.

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