Universal relaxation and diffusion in many-body interacting systems:an outstanding unsolved problem

The purpose of this article is to bring the attention of research communities in China to the universal properties of relaxation and diffusion in a variety of complex systems with interactions.This remarkable universality has been brought out from experimental findings in inorganic,organic,polymeric,bio-molecular,colloidal,metallic,and ionically conducting systems with diverse chemical structures,and in various physical states including crystalline,glassy,liquid,molten,mixed,and hydrated,and with dimensions ranging from bulk to nano-meter size ［K.L.Ngai, Relaxation and Diffusion in Complex Systems.New York:Springer,2011］.The universal properties originate from the many-body nature of the irreversible processes, which are governed by some fundamental physics related to classical chaos originating from the anharmonic interaction potential. Recognition of the universality not only benefits research in many different fields, but also makes clear that many-body relaxation and diffusion in complex interacting systems is an important and unsolved problem in physics.Although it is neither a rigorous nor complete solution,the author's Coupling Model has given successful predictions about the universal properties of relaxation and diffusion,many of which are anomalous,in various different systems.It can be used as a stepping stone in the construction of a rigorous and fundamental theory to solve the problem,once and for all.