Time and the interpretation of canonical quantum gravity

The unsatisfactory status of the interpretation of the wave function of the Universe in canonical quantum gravity is reviewed. The ‘‘naive interpretation’’ obtained by straightforwardly applying the standard interpretive rules to the canonical quantization of general relativity is manifestly unacceptable; the ‘‘WKB interpretation’’ has only a limited domain of applicability; and the ‘‘conditional probability interpretation’’ requires one to pick out a ‘‘preferred time variable’’ (or preferred class of such variables) from among the dynamical variables. Evidence against the possibility of using a dynamical variable to play the role of ‘‘time’’ in the conditional probability interpretation is provided by the fact (proven here) that in ordinary Schrödinger quantum mechanics for a system with a Hamiltonian bounded from below, no dynamical variable can correlate monotonically with the Schrödinger time parameter t, and thus the role of t in the interpretation of Schrödinger quantum mechanics cannot be replaced by that of a dynamical variable. We also argue that the interpretive problems of quantum gravity are not alleviated by the incorporation of observers into the theory. Faced with these difficulties, we seek a formulation of canonical quantum gravity in which an appropriate nondynamical time parameter is present. By analogy with a parametrized form of ordinary Schrödinger quantum mechanics, we make a proposal for such a formulation. A specific proposal considered in detail yields a theory which corresponds at the classical level to general relativity with an arbitrary, unspecified cosmological constant. In minisuperspace models, this proposal yields a quantum theory with satisfactory interpretive properties, although it is unlikely that this theory will admit sufficiently many observables for general spacetimes. Nevertheless, we feel that the approach suggested here is worthy of further investigation.