Activity and the arrow of time in biological systems
Living systems function out of thermodynamic equilibrium. We present a non-invasive method to identify and quantify such non-equilibrium dynamics based on broken detailed balance. Using this method, we study active dynamics in beating flagella and non-periodic fluctuations of primary cilia. In addition, we show with a model how tracer filaments can be employed in this framework to infer non-equilibrium dynamics in active cytoskeletal networks on different length scales: the filament’s normal modes exhibit current cycles in phase space, revealing information about activity. A distinguishing feature of active biological networks is that local forces produced by multiple active units, such as molecular motors, are accumulated and propagated by fiber networks to control dynamics at larger scales. Although these motors are traditionally regarded as the defining elements of biological force generation, here we show that the surrounding network also plays a central role in this process. Indeed, rather than merely propagating forces like a simple elastic medium, fiber networks produce emergent, dramatically amplified stresses.