String Theory and the Quantization of Gravity

Abstract

Perhaps the main challenge in theoretical physics today is the unification of all interactions including. gravity. At present, string theories appear as the best candidates to achieve such an unification. However, several technical and conceptual problems remain and a quantum theory of gravity is still non-existent. Continuous effort over the last quarter of a century has demonstrated the many difficulties encountered in repeated attempts to construct such a theory and has also indicated some of the particular properties which an eventual complete theory will have to posses. The amount of work in that direction can be by now presented in two different sets which have most evolved (and remain) separated: (i) conceptual unification (introduction of the uncertainty principle in general relativity, the interpretation problem and the concept of ‘observables’, Q.F.T. in curved space time and by accelerated observers, Hawking radiation and its consequences, the Wheeler-De Witt equation and the “wave function of the universe”...) (ii) grand unification (the unification of all interactions including gravity from the particle physics point of view, in which, gravity is considered as a massless spin two particle ( the graviton), such as in supergravities, Kaluza-Klein theories and the more succefull: superstrings).