So what happens to the effective field theories of electroweak, strong, and gravitational interactions at energies of order 10¹⁵–10<sup>18… - Steven Weinberg

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So what happens to the effective field theories of electroweak, strong, and gravitational interactions at energies of order 10¹⁵–10¹⁸ GeV? I know of only two plausible alternatives. One possibility is that the theory remains a quantum field theory, but one in which the finite or infinite number of renormalized couplings do not run off to infinity with increasing energy, but hit a fixed point of the renormalizable group equations. ... The other possibility, which I have to admit is a priori more likely, is that at very high energy we will run into really new physics, not describable in terms of a quantum field theory. I think that by far the most likely possibility is that this will be something like a string theory.