Strong Emergence in Condensed Matter Physics

Abstract

This chapter argues that the physics of condensed matter, such as crystals, superconductors, magnets, liquids, cannot be fully reduced to the supposedly fundamental quantum mechanical theory for all the atoms of which the system consists. The author holds the view that there are in fact many reasons to reject the idea that the world of physics is causally closed with everything being determined bottom-up by fundamental microscopic laws. A considerable part of the chapter is devoted to showing how condensed-matter theory is done in practice. It is never done by starting with a microscopic theory for the interaction of all the atoms of the system. Instead, approximations, plausible assumptions, intuitive models, and phenomenological theories are used to mathematically describe and explain the properties of systems that consist of a macroscopic number of particles. The author argues that this is not merely a matter of convenience, but that there are fundamental and qualitative differences or even contradictions between the microscopic theory and the theory that is used in practice. These differences are in many cases due to the fact that the world is classical, with spatially localized objects, on the macroscopic scale, while quantum mechanics leads to superpositions of objects being in different locations. Concordantly, the theories used in condensed matter physics contain elements from classical physics as well as from quantum physics.