We sometimes say that string theory is the only consistent theory of quantum gravity. It’s the only game in town. This is an observation mostly based on various types of circumstantial evidence. Whenever you try something that deviates from string/M-theory, you run into inconsistencies. Sometimes you don’t run into inconsistencies but something else happens. Many good ideas that were thought to be “competitors” to string theory were shown to be just aspects of some (usually special) solutions to string theory (noncommutative geometry, CFT, matrix models, and even the Hořava-Lifshitz class of theories have been found to be parts of string theory), and so on. And decades of attempts to find a truly inequivalent competing theory have utterly failed. That’s not a complete proof of their absence, either, but it is evidence that shouldn’t be completely ignored.
But that doesn’t mean that the statement that every consistent theory of quantum gravity has to be nothing else than another approach to string/M-theory is just an expression of vague feelings, a guesswork, or a partial wishful thinking. We don’t have the “most complete proof” of this assertion yet – this fact may be partly blamed on the absence of the completely universal, most rigorous definition of both “quantum gravity” and “string theory”. But there exist partial proofs and this paper is an example.
— Every theory of quantum gravity is a part of string theory: a partial proof
— A successful test in \(AdS_3\)
— Lubos Motl
2015.05.06 Wednesday ACHK