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.The outline of the chapter is as follows: Introduction: The author tells how she struggled as a physics student trying to understand how the calculations presented in the lectures relate to the supposedly fundamental theory, until she came to see many years later that there are fundamental and interesting issues behind the questions she asked herself as a student.Example systems: Several examples from condensed-matter physics are given to illustrate the types of phenomena addressed in the chapter. A distinction is made between equilibrium systems, which can be separated from their environment without losing their properties, and nonequilibrium systems.Defining reduction and emergence: These concepts are defined with an emphasis on the difference between weak and strong emergence. It is explained that the reductionist method has been extremely successful in physics, but that nevertheless a discussion is needed concerning the quality of this reduction: is it complete, or is it incomplete. The relation to top-down causation is explained.Condensed matter research in practice: By quoting three Nobel laureates, it is shown how condensed-matter research is done in practise. These authors provide good arguments against a reductionist view. These three examples are supplemented by the author’s expertise in statistical physics, showing that the supposed reductionist explanations made in this field import ideas that are foreign to the microscopic theory.Arguments for strong emergence in physics: Based on the information provided so far, the author now gives six reasons for strong emergence, including dependence on the environment, stochasticity, and insensitivity to microscopic details.Answers to objections: The author responds to several objections that she often hears in discussions about reduction and emergence. The main objection is that future progress of physics might yield the missing full microscopic explanations. The author counters that her arguments give generic reasons why a full reduction is not possible in principle.