How Do Quasi-Particles Exist?

Abstract

Quasi-particles emerge in solids. In the context of the debate on scientific realism, their concept is puzzling. While quasi-particles seem to be fake entities rather than full fledged physical particles, they can nevertheless be used as markers, etc., in crystals. This has led some authors (e.g., Gelfert) to argue that, even though one can use them as technological tools, they cannot be said to ‘exist’ in the way ordinary particles do. Hence they seem to counter Hacking’s reality criterion, “If you can spray them, they exist.” However, this line of reasoning misses the crucial point that quasi-particles are real collective effects of the constituents of a solid. In order to spell out the way in which they do or don’t exist, i.e., their ontological status, their particle properties are discussed in detail. It is instructive to compare them with the field quanta of a quantum field, on the one hand, and subatomic matter constituents, on the other. All these particle concepts differ substantially from the classical particle concept. Not only does this discussion shed light on the specific way in which quasi-particles exist, but it may also clarify the ontological status of quantum phenomena in general.