Implementing causality in the spin foam quantum geometry

We analyze the classical and quantum geometry of the Barrett-Crane spin foam model for four-dimensional quantum gravity, explaining why it has to be considering as a covariant realization of the projector operator onto physical quantum gravity states. We discuss how causality requirements can be consistently implemented in this framework, and construct causal transition amplitudes between quantum gravity states, i.e., realizing in the spin foam context the Feynman propagator between states. The resulting causal spin foam model can be seen as a path integral quantization of Lorentzian first order Regge calculus, and represents a link between several approaches to quantum gravity as canonical loop quantum gravity, sum-over-histories formulations, dynamical triangulations and causal sets. In particular, we show how the resulting model can be rephrased within the framework of quantum causal sets (or histories). (C) 2003 Elsevier B.V. All rights reserved.