ABSTRACT

This chapter discusses the density functional theory, which is a first principles method of describing real systems of condensed matter, whose practical implementation requires many concepts involved in the discussion of downwards causation. It describes an idealized model of the metal known as the electron gas. Downward causation is invoked less in physics than in the philosophy of physics, although, in the debate surrounding emergence, it is often raised as a possible explanation for the condensation of complex states of aggregative matter. Density functional theory is a form of mean field theory that has proven the most successful method of addressing the N-particle quantum problem in condensed matter systems. The sketched description of downward causation is very similar to the description of a much-used technique in theoretical physics: mean field theory. Mean field theory is most easily explained in terms of a simpleminded picture of a magnet.