The traditional particle interpretation of states of quantum field theory in Minkowski spacetime relies on the Minkowski spacetime symmetry, i.e., the Poincare group uplifted to the Hilbert space. We argue that Weinberg’s approach to particles and interactions in Minkowski spacetime utilizes the representations of the inhomogeneous Lorentz transformations in the Hilbert space dissociated from the spacetime symmetry. Thus it provides a particle interpretation of states in general nonstationary curved spacetimes straightaway. We give Lagrangians for scalars, spinors and vector fields supporting the argument and show that these classical fields contribute to the cosmological constant.