Presence of negative entropies in Casimir interactions
  Negative entropy in connection with the Casimir effect at uniform temperature is a phenomenon rooted in the circumstance that one is describing a nonclosed system, or only part of a closed system. In this paper we show that the phenomenon is not necessarily restricted to electromagnetic theory, but can be derived from the quantum theory of interacting harmonic oscillators, most typically two oscillators interacting not directly but indirectly via a third one. There are two such models, actually analogous to the transverse magnetic (TM) and transverse electric (TE) modes in electrodynamics. These mechanical models in their simplest version were presented some years ago, by J. S. H{\o}ye et al., Physical Review E {\bf 67}, 056116 (2003). In the present paper we re-emphasize the physical significance of the mechanical picture, and extend the theory so as to include the case where there are several mediating oscillators, instead of only one. The TE oscillator exhibits negative entropy. Finally, we show explicitly how the interactions via the electromagnetic field contain the two oscillator models. 
Johan S. H{\o}ye, Iver Brevik, and Kimball A. Milton
quant-ph