Linear response relations in fluctuational electrodynamics

Near field radiative heat transfer and vacuum friction are just two instances of topics of technological and fundamental interest studied via the formalism of fluctuational electrodynamics. From the perspective of experiment and simulations, it is hard to precisely control and probe such nonequilibrium situations. Fluctuations in equilibrium are easier to measure, and typically can be related to nonequilibrium response functions by Green-Kubo relations. We consider a collection of arbitrary objects in vacuum, perturbed by changing the temperature or velocity of one object. Developing a method for computation of higher order correlation functions in fluctuational electrodynamics, we explicitly compare linear response and equilibrium fluctuations. We obtain a Green-Kubo relation for the radiative heat transfer, as well as a closed formula for the vacuum friction in arbitrary geometries in the framework of scattering theory. We comment on the signature of the radiative heat conductivity in equilibrium fluctuations.National Science Foundation (U.S.) (Grant DMR-12-06323)United States. Dept. of Energy (Grant DE-FG02-02ER45977