Atomic and molecular systems interact with the electromagnetic field via their polarisabilities (induced electric or magnetic dipole moments). In the electronic ground state, this gives rise to the van der Waals interaction between atoms or to the Casimir-Polder interaction between an atom and a surface (macroscopic object). Both of them also contain a temperature-dependent component that becomes dominant at large distances. This happens typically beyond the Wien wavelength (a few microns at room temperature), but significant corrections appear sometimes at shorter distances (“thermal anomaly”). Mesoscopic objects like nano-particles can be described with similar tools. Next to atom-surface forces that lead to attraction and adsorption, the basic physical concepts also apply to momentum diffusion and spatial decoherence induced by the radiation field (“fluctuations of fluctuation-induced forces”).
We provide a tutorial review of the essential concepts and of the technical discussion in the Casimir community. It revolves around the challenge how to include material absorption into a quantum field theory description.