Nonlinear Physics deals with the understanding of phenomena on the human scale: chaos, instabilities, disorder, fracture, plasticity and turbulence on the one hand, but also pattern formation and the emergence of complex structures on the other hand. Nonlinear Physics starts where regular methods of theoretical physics fail: there is no small parameter, and therefore no perturbation theory. Often Nonlinear Physics deals with systems that are far from Equilibrium and that exhibit broken symmetries. Progress usually requires the introduction of specialized tools: scale invariance and scaling behaviour, renormalization, fractal geometry etc.
American Historical Association