This relaxation process is the very mechanism through which useful effects can be extracted from non-equilibrium systems. The efforts made to prod a system from one state to the next are externally imposed perturbations. The economy of engineering is to minimize the cost of these external perturbations with respect to the amount of useful outcome produced by the system. During a relaxation process a system traverses many non-equilibrium states, each characterized by the particular distribution of particles, energy and chemical composition. Unlike a system in equilibrium, the state of non-equilibrium cannot be described by a few macroscopic parameters such as mass density, temperature and pressure.
Radiation is often important in non-equilibrium systems.
Advanced computational methods are used to analyze the complex mathematical models of non-equilibrium systems and sophisticated experimental techniques are developed to get insight into details of the important processes and to verify theoretical models.