There is a large class of classical null-fronted metrics in which a free scalar field has an infinite number of conservation laws. In particular, if the scalar field is quantized, the number of particles is conserved. However, in more general null-fronted metrics, the null fronts are not suitable for specifying initial data and they cannot serve as a ‘‘time’’ parameter for canonical quantization.

• Received 31 January 1994

DOI:https://doi.org/10.1103/PhysRevD.50.7421