Adam Rupe and James P. Crutchfield
ABSTRACT: Coherent structures form spontaneously in nonlinear spatiotemporal systems and are found at all spatial scales in many natural phenomena from laboratory hydrodynamic flows and chemical reactions to ocean, atmosphere, and climate dynamics. Phenomenologically, they appear as key components that organize the behaviors in such systems. Despite a century of effort, they have eluded rigorous analysis and empirical prediction, with progress being made only recently. As a step in this, we present a formal theory of coherent structures in fully-discrete dynamical field theories. It builds on the notion of structure introduced by computational mechanics, generalizing it to a local spatiotemporal setting. The analysis' main tool employs the local causal states, which are used to uncover a system's spatiotemporal symmetries and which identify coherent structures as spatially- localized deviations from those symmetries. The approach is behavior-driven in the sense that it does not rely on directly analyzing spatiotemporal equations of motion, rather it considers only the spatiotemporal fields a system generates. As such it offers an unsupervised approach to discover and describe coherent structures. We illustrate the approach by analyzing coherent structures generated by elementary cellular automata, comparing the results with an earlier, dynamically-invariant-set approach that decomposes fields into domains, particles, and particle interactions.