Colloquium: What do we mean by ‘active matter’?
Rev. Mod. Phys. 98, 031001 – Published 13 July, 2026
DOI: https://doi.org/10.1103/wd4f-q7kv
Abstract
As the study of active matter has developed into one of the most rapidly growing subfields of condensed matter physics, more and more kinds of physical systems have been included in this framework. While the word ‘active’ is often thought of as referring to self-propelled particles, it is also applied to a large variety of other systems. Developing novel forms of active matter, as attempted, for example, in the framework of quantum active matter, requires a clear idea of what active matter is. Active matter is defined here based on commonly employed criteria as a system that is driven out of equilibrium by a local energy influx and that generates motion and/or forces whose direction is not determined by external fields. The limits of such a definition and to what extent a unified definition of ‘active matter’ is still possible are also critically discussed. Moreover, we discuss the definition of an ‘active field theory,’ where ‘active’ is used as an attribute of a theoretical model rather than of a physical system. We show that the usage of the term ‘active’ requires agreement on a description that neglects certain degrees of freedom. The meaning of this word is discussed both in general terms and via the specific examples of chemically driven particles, particles with nonreciprocal interactions, intracellular phase separation, and quantum active matter.
