On heat flow and non-reciprocity

Heat transfer properties in non-reciprocal systems are discussed. An ideal experiment employing microwave or optical isolators is considered in order to investigate the possibility for a spontaneous transfer of energy between two black-bodies at common temperatures. Under the validity of the adopted assumptions on the examined physical system, the effect appears to be weak but feasible. We consider that the relevance of detailed balance for thermal equilibrium and its close connections with reciprocity confer interest to the close inspection of these problems started here.