Quantum Field Theory with Null-Fronted Metrics
  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, with more general null-fronted metrics, field quantization cannot be interpreted in terms of particle creation and annihilation operators, and the physical meaning of the theory becomes obscure. 
Orit Levin and Asher Peres (Department of Physics, Technion - Israel   Institute of Technology, Haifa, Israel)