A broken egg cannot spontaneously unbreak, and a drop of ink once mixed in water cannot spontaneously unmix. Nature is full of such irreversible phenomena, actions that cannot undo themselves. This irreversibility is quantified by the so-called entropy production rate, which, according to the second law of thermodynamics, is always positive [1]. Thus, one can think of the entropy production rate as a measure of the flow or “arrow” of time for a system. However, measuring this parameter is tricky for complex systems, such as the brain, that have nontrivial, complex interactions between their constituent elements. Now Christopher Lynn of the City University of New York and Princeton University and colleagues present a method to quantify entropy production in such a system [2, 3] (Fig. 1). The team applies its method to the activity of neurons in the retina of a salamander as the system responds to a series of complex visual images. Their work opens the door to the quantitative analysis of the arrow of time in complex biological systems, such as the neuronal networks in the brain, where the model could potentially enable a quantitative understanding of the neural basis of our perception of the passage of time.
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