A perspective on active glassy dynamics in biological systems

Dynamics is central to living systems. In the last two decades, experiments have revealed that the dynamics in diverse biological systems - from intracellular cytoplasm to cellular and organismal aggregates - are remarkably similar to that in dense systems of inanimate particles in equilibrium. They show a glass transition from a solid-like jammed state to a fluid-like flowing state, where a moderate change in control parameter leads to an enormous variation in relaxation time. However, biological systems have crucial differences from the equilibrium systems: the former have activity that drives them out of equilibrium, novel control parameters, and enormous levels of complexity. These active systems showing glassy dynamics are known as active glasses. The field is at the interface of physics and biology, freely borrowing tools from both disciplines and promising novel, fascinating discoveries. We review the experiments that started this field, simulations that have been instrumental for insights, and theories that have helped unify diverse phenomena, reveal correlations, and make novel quantitative predictions. We discuss the primary characteristics that define a glassy system. For most concepts, we first discuss the known equilibrium scenario and then present the key aspects when activity is introduced. We end the article with a discussion of the challenges in the field and possible future directions.