Dynamical Casimir effect with Dirichlet and Neumann boundary conditions

We derive the radiation pressure force on a nonrelativistic moving plate in 1 + 1 dimensions. We assume that a massless scalar field satisfies either Dirichlet or Neumann boundary condition (BC) at the instantaneous position of the plate. We show that when the state of the field is invariant under time translations, the results derived for Dirichlet and Neumann BC are equal. We discuss the force for a thermal field state as an example for this case. On the other hand, a coherent state introduces a phase reference, and the two types of BC lead to different results.