The force between two macroscopic conducting surfaces in a volume that only contains an electromagnetic field. The zero-point energy of the electromagnetic field depends on the mode frequencies of the field, which in turn depend on the boundary conditions on the field. Thus, changing the positions of the surfaces changes the mode frequencies and zero-point energy of the field. This energy change can be expressed as a potential energy per unit area of the plates (as a function of distance between the plates), which leads to a force between the plates. The existence of these forces was predicted by the Dutch physicist Hendrik B. G. Casimir (1909– ) in the 1940s and detected experimentally by M. J. Sparnaay in 1958. Both the sign and magnitude of the Casimir effect depend critically on the geometry of the surface.