Classical thermodynamics is the science of equilibrium: the concept of variation with time was viewed with suspicion; dissipative phenomena, such as friction, were considered a nuisance. Prigogine attacked the problem head-on, by introducing a quantitative concept of irreversibility. He produced a sound derivation of transport processes, relating the fluxes of energy and matter to the thermodynamic forces (such as gradients of temperature or density, or electric fields) that cause them. Irreversibility and entropy became the main themes of his subsequent work.
The next step was to consolidate these macroscopic notions by addressing their molecular basis. This subject had remained in the shadows since the last years of the nineteenth century with Ludwig Boltzmann's work on the kinetic theory of gases and a definition of entropy at the microscopic level. (It was even considered to be 'cursed' — Boltzmann committed suicide in 1906.) How could the irreversibility of real processes at the macroscopic scale be reconciled with the perfect time-reversibility of the (classical or quantum) law of motion for molecules? In the 1960s, surrounded by a small group of enthusiastic co-workers, Prigogine began to make crucial progress on this point, developing the first form of non-equilibrium statistical mechanics. (Another approach, developed independently by Nikolai Bogolyubov in the Soviet Union, turned out to be equivalent to Prigogine's.) As well as being quite a general formulation of the theory, there were other fruits of this endeavour, for it could be applied to a variety of systems, from gases and solids to plasmas.
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