We investigate energy and momentum non-contact exchanges between two
arbitrary flat media separated by a gap. This problem is revisited as a
transmission problem of individual system eigenmodes weighted by a transmission
probability obtained either from fluctuational electrodynamics or quantum field
theory. An upper limit for energy and momentum flux is derived using a general
variational approach. The corresponding optimal reflectivity coefficients are
given both for identical and different media in interaction.Comment: accepted in Phys. Rev. B rapid communicatio