Natural philosophers have been seeing signs of 'life' in non-living matter ever since Aristotle. But that motley enterprise has always been handicapped by a lack of clarity about what qualifies as life-like. When in 1828 Robert Brown first saw pollen grains dancing in suspension, he imagined that this jiggling activity revealed the 'vital force' animating all matter. But the association of random motion with heat defeated any easy equivalence of motion and life. When 60 years later another botanist, Friedrich Reinitzer, discovered spontaneous molecular alignment in liquid crystals, Ernst Haeckel leapt to the conclusion that organization, not motion, is life's most fundamental feature. Erwin Schrödinger refined that idea by suggesting it is non-equilibrium organization — the ability, as he put it, to feed on 'negative entropy' — that characterizes the living state.

One of the attractions of the notion of 'active matter' — the constituents of which move of their own accord in a non-thermal fashion, using free energy in the environment to sustain a non-equilibrium state — is that it unites all of these ideas about life's defining characteristics, while showing that none of them are to be equated with life itself. All the same, this field unifies concepts from the biological and abiotic sciences, so that, for example, schooling fish can be regarded as a kind of self-propelled, macroscopic liquid crystal, and bacterial swarms — the prototypical form of active matter — can be analysed using the physical scientists' conceptual tools of statistical mechanics and hydrodynamics.

Credit: PHILIP BALL

Thus active matter shows that research at the interface of the living and non-living can be productively pursued without having to bother with the arbitrary old question of where to draw the boundary. Equally valuable is the focusing of attention on the mesoscale, the traditional regime of colloid science, as a locus of new phenomena made possible by relative liberation from both gravity's enervating tug and heat's crazy battering. Active matter injects fresh energy into the materials engineer's old (by now) discipline of complex fluids.

A recent review1 outlines the physics of colloid-sized objects that 'swim', ranging from flagella-driven bacteria to Janus bimetallic microparticles propelled by an electrocatalytic reaction on one face. (Even in that seemingly simple case the propulsion mechanism is subtle, probably a kind of electrophoresis created by proton flow in the surrounding fluid.) The range of non-equilibrium behaviour in such systems, both natural and artificial, is vast2. Giomi et al. have shown, for example, how filaments propelled by motor proteins, such as the actin–myosin system, can produce nematically ordered arrangements with pulsatile activity that pumps solvent3. A new theoretical study4 of this sort of system reveals ever-evolving patterns, including rings, spirals and aster-like entities reminiscent of those formed by kinesin-driven microtubules in mitosis and seen in a pioneering early study of active matter5.

When that latter work was published, it was so unusual that no one knew quite what to do with it. One might say the same today of all active matter, but this will change, and concurrently so will our view of matter itself. It's not so much that life is the means by which matter achieves coordination and coherence; rather, we'll find life, here as elsewhere, to be a particularization of something more general.