  After reading "The Casimir Effect" by Pluninen,.... I think that the Casimir effect can be shortly explained as: 'A measurable change in ground state energy because of change in the boundary conditions. This gives rise to a force, which often is named the retarded van der Waals force. I think that if the energy of the universe is constant, the loss / gain in the Casimir energy is nothing else than the corresponding gain / loss of potential energy between the boundaries (f. ex. parallell walls in a box, or extern electromagnetid field). This change in potential energy gives rise to a force ( F= - dV/d x ) which is the retarded var der Waals force. Comments? ! 
