Quantum Gravity from General Relativity

This chapter is devoted to canonical quantum gravity and particularly to its main representative, loop quantum gravity. General relativity (GR) is our best theory of gravity and one of the best confirmed theories in the history of science. The most straightforward way to a theory of quantum gravity is by means of the semi-classical approach to quantum gravity. Causal det theory takes the central insight of GR to be that, for causally sufficiently non-pathological relativistic spacetimes, the causal structure of a spacetime determines its full geometry up to a conformal factor. The central question for CST is whether the postulated causal sets generically give rise to relativistic spacetimes, or an empirically indiscernibly close surrogate thereof. In CST, there exists a sketch of how one can recover aspects of spacetimes from the fundamental causal sets. As the canonical quantization procedure has thus encountered a formidable roadblock, physicists are seeking ways to circumvent the problem.