Final remarks on Part II

Abstract

This brief concluding chapter summarizes the general situation in semiclassical theory and quantum gravity. Even in the framework of the usual perturbative quantum field theory, there are several approaches leading to theoretically satisfactory models of quantum gravitational effects, starting from quantum field theory in curved spacetime. Here, the expression “satisfactory” does not mean perfectness, as there is no theoretically perfect model of quantum gravity. The chapter then goes on to review the main unsolved problems of quantum gravity, such as higher-derivative ghosts and instabilities and the cosmological constant problem. It concludes with the hope that the basic aspects of the models presented in this book will be useful for anyone who intends to start working in this fascinating area.