Non-additive entropies and statistical mechanics at the edge of chaos: a bridge between natural and social sciences

Abstract

The Boltzmann–Gibbs (BG) statistical mechanics constitutes one of the pillars of contemporary theoretical physics. It is constructed upon the other pillars—classical, quantum, relativistic mechanics and Maxwell equations for electromagnetism—and its foundations are grounded on the optimization of the BG (additive) entropic functionalSBG=−k∑ipiln⁡pi. Its use in the realm of classical mechanics is legitimate for vast classes of nonlinear dynamical systems under the assumption that the maximal Lyapunov exponent ispositive(currently referred to asstrong chaos), and its validity has been experimentally verified in countless situations. It fails however when the maximal Lyapunov exponentvanishes(referred to asweak chaos), which is virtually always the case with complex natural, artificial and social systems. To overcome this type of weakness of the BG theory, a generalization was proposed in 1988 grounded on the non-additive entropic functionalSq=k((1−∑ipiq)/(q−1)) (q∈R; S1=SBG). The indexqand related ones are to be calculated, whenever mathematically tractable, from first principles and reflect the specific class of weak chaos. We review here the basics of this generalization and illustrate its validity with selected examples aiming to bridge natural and social sciences.This article is part of the theme issue ‘Thermodynamics 2.0: Bridging the natural and social sciences (Part 2)’.