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Critical Behavior in Physics and Probabilistic Formal Languages

We show that in many data sequences — from texts in different languages to melodies and genomes — the mutual information between two symbols decays roughly like a power law with the number of symbols in between the two. In contrast, we prove that Markov/hidden Markov processes generically exhibit exponential decay in their mutual information, which explains why natural languages are poorly approximated by Markov processes. We present a broad class of models that naturally reproduce this critical behavior. They all involve deep dynamics of a recursive nature, as can be approximately implemented by tree-like or recurrent deep neural networks. This model class captures the essence of probabilistic context-free grammars as well as recursive selfreproduction in physical phenomena such as turbulence and cosmological inflation. We derive an analytic formula for the asymptotic power law and elucidate our results in a statistical physics context: 1-dimensional “shallow” models (such as Markov models or regular grammars) will fail to model natural language, because they cannot exhibit criticality, whereas “deep” models with one or more “hidden” dimensions representing levels of abstraction or scale can potentially succeed.