Passivity, Causality and Locality

I report on recent work which uses passivity, an expression of the Second Law of Thermodynamics, as a selection criterion for reference (vacuum) states of quantum field theories over (certain) curved space-times. The representations associated with such states manifest properties which are physically desirable. Among others, causality and (weak) locality properties are predicted. Though the quantum fields in such representations are local in the selected state, they need not be local on the entire representation space. The curious properties of such nonlocal but weakly local models will be discussed from the point of view of a new approach to constructing quantum field models.