Quantum Gravity
As stated in Sect. 3.2, the motivations for QG include the desire for unification, as well as the desire for a single framework for both matter and spacetime (rather than a number of different theories describing different domains).
Yet, QG it is not necessarily a unified theory, nor a theory of everything. In the first case, it may be a semiclassical theory, which is a non-unified combination of GR and QFT, and in the second case, it may just be a quantum theory of gravity, and not a theory that combines gravity with the standard model forces. In other words, although it is expected to be more-fundamental than GR and QFT, QG is not necessarily a fundamental theory [7].Currently, there is no QG. Instead, there are a number of approaches to finding a theory (i.e., different research programs), of various stages of development, with none, as yet, yielding novel, experimentally testable predictions. One of these approaches, string theory, alleges to be a unified theory of everything. An additional claim for string theory’s being a candidate for a fundamental (i.e., final) theory is its UV completeness—using a symmetry in the theory known as “T-duality”, an argument can be made that string theory describes a minimal length [20]. Thus, string theory apparently says there can be no new physics “beyond” that described by the approach. According to string theory, then, we should stop digging.
But, as described above (Sect. 1), even if string theory is correct, its UV completeness does not alleviate the epistemic worry: It is possible that new physics exists beyond, and that a more-fundamental theory be necessary (one not featuring the minimal length described by string theory). In other words, it is possible that string theory, even if correct, turns out to be an effective theory after all.
String theorists, however, believe that this is not the case, and that the approach will produce a final theory.[36] The problem, though, is that the approach does not satisfy many of the other conditions on a fundamental theory—in particular, it is only known perturbatively, it is background dependent, and its parameters are not uniquely determined (string theory is not a single theory, but a huge “landscape” of possible theories). Additionally, the approach is not level comprehensive, because of overlaps—different theories (known as dual theories) can potentially be interpreted as describing the “same physics” [21]. All of these factors drive string theorists to search for a more fundamental type of string theory underlying the currently-known “versions” of it. (Researchers in other approaches to QG, however, are duly sceptical of a positive outcome).
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