Hedrich, Reiner
(2011)
String Theory – Nomological Unification and the Epicycles of the Quantum Field Theory Paradigm.
[Preprint]
Abstract
String Theory is the result of the conjunction of three conceptually independent elements: (i) the metaphysical idea of a nomological unity of the forces, (ii) the modeltheoretical paradigm of Quantum Field Theory, and (iii) the conflict resulting from classical gravity in a quantum world  the motivational starting point of the search for a theory of Quantum Gravity. String Theory is sometimes assumed to solve this conflict: by means of an application of the (only slightly extended) modeltheoretical apparatus of (perturbative) Quantum Field Theory, interpreting gravity as the result of an exchange of gravitons, taken here to be dynamical states of the string. But, String Theory does not really solve the conflict. Rather it exemplifies the inadequacy of the apparatus of Quantum Field Theory in the context of Quantum Gravity: After several decades of development it still exists only in an essentially perturbative formulation (with minor nonperturbative extensions and vague ideas with regard to a possible nonperturbative formulation). And, due to its quantum field theoretical heritage, it is conceptually incompatible with central implications of General Relativity, especially those resulting from the general relativistic relation between gravity and spacetime. All known formulations of String Theory are backgrounddependent. And no physical motivation is given for this conceptual incompatibility.
On the other hand, although String Theory identifies all gauge bosons as string states, it was not even possible to reproduce the Standard Model. Instead, String Theory led to a multitude of internal problems  and to the plethora of lowenergy scenarios with different nomologies and symmetries, known as the String Landscape. All attempts to find a dynamically motivated selection principle remained without success, leaving String Theory without any predictive power. The nomological unification of the fundamental forces, including gravity, is only achieved in a purely formal way within the modeltheoretical paradigm of Quantum Field Theory  by means of physically unmotivated epicycles like higher dimensionality, CalabiYau spaces, branes, etc.
Finally, the possibility remains that some of the central (implicit) assumptions of String Theory are physically wrong. On the one hand, the idea of a nomological unity of the forces could be simply wrong. On the other hand, even if a nomological unity of all fundamental forces should be realized in nature, the possibility remains that gravity is not a fundamental force, but a residual, emergent and possibly intrinsically classical phenomenon, resulting from a quantum substrate without any gravitational degrees of freedom.
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