When does matter become alive? Can the laws of physics describe living systems? To answer those vexing questions, early natural philosophers invoked notions of vitalism. But now we know better. Describing the living state is as much a physical problem as a biological one, and the modern tools of condensed-matter and nonequilibrium physics are two of the microscopes of choice for attacking it.
Starlings flock in the sky above the Otmoor wetlands in Oxfordshire, England.
Starlings flock in the sky above the Otmoor wetlands in Oxfordshire, England.
At the end of the 20th century, the field of active matter emerged as scientists increasingly and successfully treated living systems as materials. Broadly put, active matter refers to a large class of self-driven systems, each composed of many interacting entities that can independently and locally convert ambient free energy into sustained work or motion. In his latest book, Active Matter Within and Around Us: From Self-Propelled Particles to Flocks and Living Forms, the well-known author and complex-systems scientist Len Pismen provides a bird’s-eye view of the young but rapidly growing field.
As the book’s title suggests, active matter is pervasive throughout the natural world. It ranges in scale from the thermally buffeted motions of motor proteins in a cell to the mesmerizing swirls formed by large flocks of birds. In the past decade and a half, researchers have developed synthetic and artificial active systems that use such inanimate components as colloids, grains, or robots to ingeniously mimic lifelike behavior.
The science underlying active matter spans physics, biology, and materials science, and covering it all in a 200-page book is a difficult task. But it’s one that Pismen takes up admirably. Active Matter Within and Around Us presents a curated display of recent developments in the field of active matter from the vantage point of an experienced surveyor.
The book’s eight chapters can be roughly grouped into three parts. The first is devoted to large-scale and continuum models of active fluids that display polar and nematic orientational order. Well-known models of flocking physics and active liquid crystals are described alongside their basic phenomenology and some experimental realizations. Phase separation caused by crowding makes a brief appearance, and the dynamic role played by topological defects that are often present in active fluids is appropriately emphasized.
Although some open theoretical issues are highlighted, they are often quite technical and at times inaccurate or misleading. For instance, the book claims that the Toner–Tu equations—which are used to describe flocks as a continuum fluid—rely on momentum conservation, Galilean invariance, and an equation of state for pressure. But those assumptions were explicitly broken in the model’s formulation.
The focus shifts in the second part to microscopic active agents, both synthetic and biological. Mechanisms for propelling colloids and drops through viscous fluids are discussed, as are the particles’ interactions and collective effects in suspensions. Chapter 4 dives straight into the movements of microorganisms and their mechanisms, including swarming, biofilm formation, and flagellar and ciliary motility. It also covers how the synchronization of flagella and cilia affects bacterial swimming.
Eukaryotes are covered next in a lightning-fast review of the cytoskeletal components and architecture, the dynamics and patterns present within a cell, and the role those factors play in cellular locomotion. That background motivates coarse-grained active-gel models that permit theoretical descriptions of mechanics and dynamics at both the single-cell and tissue levels. A brief excursion into how shape change emerges from growth and plasticity seems a bit tangential, but it perhaps portends the final two chapters.
The third part delves deeper into biology by covering such topics as tumor invasion; the dynamics of collective cell migration; and the spreading, jamming, folding, and shaping of tissues. Fundamental ideas of morphogenesis are highlighted—including positional information, symmetry breaking, morphogen gradients, and mechanical feedback—but the discussion remains brief. I appreciate the inclusion of sections on plant tissues. The book concludes, perhaps a bit abruptly, with a short digression into biomimetic systems and soft robotics.
Overall, Active Matter Within and Around Us manages to fulfill its promise of emphasizing the physical, biological, and technological aspects of active matter—at least to the extent that can be expected in 200 pages. Nonetheless, it is not a casual read, nor is it a pedagogical introduction to active matter. For the latter, the many extensive review articles that have been written on the subject remain the best option.
Because of the book’s brisk pace, its heavy bias toward theory, and its often-impressionistic descriptions of phenomena without equations, it is unlikely to be useful to beginners except perhaps to whet their appetites for further study. But for attentive and motivated readers who are willing to dive into the literature, Active Matter Within and Around Us hints at many open questions and problems that are waiting to be solved.
Suraj Shankar is a junior fellow in the Harvard Society of Fellows. His research focuses on problems in soft matter, statistical physics, and physical biology.
