Quantum Theory of Gravity. I. The Canonical Theory
Abstract
Following an historical introduction, the conventional canonical formulation of general relativity theory is presented. The canonical Lagrangian is expressed in terms of the extrinsic and intrinsic curvatures of the hypersurface x^{0}=constant, and its relation to the asymptotic field energy in an infinite world is noted. The distinction between finite and infinite worlds is emphasized. In the quantum theory the primary and secondary constraints become conditions on the state vector, and in the case of finite worlds these conditions alone govern the dynamics. A resolution of the factorordering problem is proposed, and the consistency of the constraints is demonstrated. A 6dimensional hyperbolic Riemannian manifold is introduced which takes for its metric the coefficient of the momenta in the Hamiltonian constraint. The geodesic incompletability of this manifold, owing to the existence of a frontier of infinite curvature, is demonstrated. The possibility is explored of relating this manifold to an infinitedimensional manifold of 3geometries, and of relating the structure of the latter manifold in turn to the dynamical behavior of spacetime. The problem is approached through the WKB approximation and HamiltonJacobi theory. Einstein's equations are revealed as geodesic equations in the manifold of 3geometries, modified by the presence of a "force term." The classical phenomenon of gravitational collapse shows that the force term is not powerful enough to prevent the trajectory of spacetime from running into the frontier. The asyet unresolved problem of determining when the collapse phenomenon represents a real barrier to the quantumstate functional is briefly discussed, and a boundary condition at the barrier is proposed. The state functional of a finite world can depend only on the 3geometry of the hypersurface x^{0}=constant. The label x^{0} itself is irrelevant, and "time" must be determined intrinsically. A natural definition for the inner product of two such state functionals is introduced which, however, encounters difficulties with negative probabilities owing to the barrier boundary condition. In order to resolve these difficulties, a simplified model, the quantized Friedmann universe, is studied in detail. In order to obtain nonstatic wave functions which resemble a universe evolving, it is necessary to introduce a clock. In order that the combined wave functions of universecumclock be normalizable, it turns out that the periods of universe and clock must be commensurable. Wave packets exhibiting quasiclassical behavior are constructed, and attention is called to the phenomenological character of "time." The innerproduct definition is rescued from its negativeprobability difficulties by making use of the fact that probability flows in a closed finite circuit in configuration space. The article ends with some speculations on the uniqueness of the state functional of the actual universe. It is suggested that a viewpoint due to Everett should be adopted in its interpretation.
 Publication:

Physical Review
 Pub Date:
 August 1967
 DOI:
 10.1103/PhysRev.160.1113
 Bibcode:
 1967PhRv..160.1113D