"York time" of general relativity, has been developed by Charles Wang. This work has later been further developed by him and his collaborators to an approach of identifying and quantizing time amenable to a large class of scale-invariant dilaton gravity-matter theories. The problem of quantum cosmology The problem of quantum cosmology is that the physical states that solve the constraints of canonical quantum gravity represent quantum states of the entire universe and as such exclude an outside observer, however an outside observer is a crucial element in most interpretations of quantum mechanics. See also ADM formalism Ashtekar variables Canonical quantization Canonical coordinates Diffeomorphism Hole argument Regge Calculus Loop quantum gravity is one of this family of theories. Loop quantum cosmology (LQC) is a finite, symmetry reduced model of loop quantum gravity. Problem of time Notes References Sources Mathematical methods in general relativity Quantum gravity Physics beyond the Standard Model