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.
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About Steven Weinberg
Steven Weinberg was a 20th-century American theoretical physicist. Steven Weinberg was an American theoretical physicist. He shared the 1979 Nobel Prize in Physics with Abdus Salam and Sheldon Glashow "for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current". Read more on Wikipedia →