 As remarked in the introduction, there is no single, generally agreed-upon body of theory in quantum gravity. The majority of the physicists working in the field focus their attention on string theory, an ambitious program which aims at providing a unified theory of all four interactions. A non-negligible minority work on what is now called loop quantum gravity, the goal of which is simply to provide a quantum theory of the gravitational interaction simpliciter. There is also significant work in other areas, including approaches that don’t really involve the quantization of a theory at all. [Good recent reviews of the theoretical landscape include Carlip 2001, Smolin 2001 (Other Internet Resources section below), 2003, Penrose 2004, and Oriti, ed, 2009.] But there is no real consensus, for at least two reasons.