Doesn’t string theory make predictions at very high energy?

Not really. The standard claim that if you go to high enough energies string theory predicts that you will see string excitations with linearly rising energy and characteristic string theory scattering amplitude behavior are based on perturbative string theory. Non-perturbatively, it is very unclear if this behavior will survive: what if you start producing black-hole states, for instance? The standard conjecture about perturbative string theory is that it is just one special corner of a theory called “M-theory”, which generically does not necessarily contain string-like states at all. Yes, if you were to see characteristic string-like behavior at the Planck scale, you would have good evidence for string theory, but string theory unification does not require this at all, with many models not having this behavior.

If you don’t believe me, maybe you’ll believe Arkani-Hamed, see this posting, which includes:

In the question session, he made the same point I often end up arguing with string theory proponents about, saying (1:14) that if “you can do experiments at the string scale, wouldn’t help you at all”. The idea that you would see string excitations on a compactified space he characterizes as a misguided old idea from the 1990s. If there’s a landscape, the possibilities are so complex for Planck scale behavior that you can’t predict what experiments at that scale would see.

For more, see here

http://mathoverflow.net/questions/133203/explanations-for-mathematicians-about-the-falsifiability-or-not-of-string-the

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