Statistical Physics of Active Matter
By Active Matter we denote a collection of a large number of active entities. These entities are characterized by a continuous transformation of energy into directed motion. Examples of such active entities are animals (birds, fish, insects, etc.), humans, robots, drones, bacteria, active colloidal particles, micro tubuli driven by molecular motors, ... .
What we are not interested in
In the statistical physics of active matter we are not interested in details of the motion of individual entities. In particular we do not want to study how exactly a bird is flying, how it is changing its direction of motion and for what reasons it is doing this. Undebatable these are interesting and important questions regarding the biology of birds, however, those question are not the subject of statistical physics and not the most relevant questions regarding collective states of motion.
What we are interested in
In the statistical physics of passive particles (such as atoms, molecules), usually one does not study the individual trajectories of all the particles (at least this is not the purpose). Instead one is interested in quantities that are obtained as average values over a large number of particles such as temperature, momentum, density, pressure, heat capacity, etc.. The interplay of such quantities is governed by general physical laws and material specific parameters (that are determined by the details of the microscopic forces between the individual particles).
In active matter we consider a large collection of active entities as a material. That means for us, a flock of birds is considered as a material rather than a biological system. We are interested in discovering the general physical laws of such active materials that differ significantly from passive materials. The reason of this discrepancy lies in classical thermodynamic laws that rely on the concept of thermal equilibrium. The activity in active matter drives the system out of thermal equilibrium. The driving takes place in bulk, that means at every self-propelled particle. Hence, the driving can not be considered as a small perturbation of equilibrium systems. Thus, new theories have to be developed in order to reach an understanding of active matter that comes close to our current understanding of passive materials.
Active entities such as an animal are considerable more complex than passive particles such as molecules. In many cases we have only a limited understanding of the individual active entity. Improving the knowledge about individual active entities is an active field of research, that, however, is not in my expertise. In any way, it is nowadays not realistic to model a large collection of self-propelled particles microscopically realistic. Therefore it is reasonable to study simplified prototype models of active matter that carry some key features of activity. It is the general believing that the details of the exact microscopic dynamics are not so relevant for collective phenomena anyway.