Ilya Prigogine (1917–2003) Living Systems

news and views

Obituary governing the evolution of the system. These conditions arise, in particular, in Ilya Prigogine (1917–2003) living systems. Prigogine had thus created an important link between physics, Ilya Prigogine died on 28 May in Brussels, chemistry and biology (even extending after a long illness. Born in Moscow, he it to sociology and economics). His emigrated at an early age, as his family achievement was crowned with the award sought to escape the aftermath of the of the 1977 Nobel prize for chemistry — Bolshevik revolution. The family arrived the pinnacle of a long list of awards, prizes

first in Germany, then moved to Belgium and honorary degrees. M. MILLER /UNIV. TEXAS — the country that became Prigogine’s By the 1990s, Prigogine was again true homeland. He studied chemistry thinking about physics at the microscopic and physics at the Université Libre de level. Substantial progress had been made Bruxelles, and obtained his PhD in 1939 by mathematicians and physicists in the under the tutelage of Théophile De study of nonlinear dynamical systems, Donder, a remarkable professor, who and one of the most important features to pioneered the development of modern be uncovered was the intrinsic dynamical physics in Belgium. Extending and instability of most ‘non-integrable’ systems deepening De Donder’s work, Prigogine (even very simple ones). As a result, himself became a pioneer of the Prigogine introduced the crucial idea thermodynamics of irreversible processes. that the usual method of defining the state Classical thermodynamics is the science of a system by specifying exactly the of equilibrium: the concept of variation positions and momenta of all of its with time was viewed with suspicion; components (that is, a point in ‘phase dissipative phenomena, such as friction, space’) is not realistic, because a close, were considered a nuisance. Prigogine neighbouring state may evolve in a attacked the problem head-on, by completely different way. Rather, the state introducing a quantitative concept of should be described by an ensemble — a irreversibility. He produced a sound cluster of identical systems differing in derivation of transport processes, relating their initial conditions. The latter may the fluxes of energy and matter to the (but does not have to) be concentrated thermodynamic forces (such as gradients Pioneer of the around a single point in phase space. The of temperature or density, or electric fields) thermodynamics of description of the system’s evolution that cause them. Irreversibility and irreversible processes thus becomes statistical. In this way, the entropy became the main themes of his solutions of the equations are regularized subsequent work. that are far from equilibrium, Prigogine (divergences are suppressed), and The next step was to consolidate these and his increasingly numerous co-workers irreversibility appears as a welcome bonus macroscopic notions by addressing their uncovered an extraordinary phenomenon. in the theory. molecular basis. This subject had remained When the distance from equilibrium Like his mentor De Donder before him, in the shadows since the last years of (measured by some appropriate Prigogine was also a remarkable professor. the nineteenth century with Ludwig parameter) reaches a certain threshold, the In the United States, he was the founder Boltzmann’s work on the kinetic theory trajectory of a system meets a fork, or and director of the Center for Statistical of gases and a definition of entropy at bifurcation. The system may then leave the Mechanics at the University of Texas at the microscopic level. (It was even trajectory along which it has evolved from Austin (later renamed the Ilya Prigogine considered to be ‘cursed’ — Boltzmann equilibrium, and jump to a totally different Center for Studies in Statistical Mechanics committed suicide in 1906.) How could one. If the system is pushed even further, and Complex Systems); in 1959 he was the irreversibility of real processes at more bifurcations may appear, and when made director of the International Solvay the macroscopic scale be reconciled with the system is very far from equilibrium, it Institutes for Physics and Chemistry in the perfect time-reversibility of the may behave completely chaotically (as, for Brussels. His lectures were fascinating (classical or quantum) law of motion for example, the regular flow of a liquid may for students, as he preferred to leave molecules? In the 1960s, surrounded by a become turbulent). But, alternatively and out tedious details and instead include small group of enthusiastic co-workers, unexpectedly, the system might reach a parenthetical perspectives on art, music Prigogine began to make crucial progress new, ordered state — what Prigogine called and philosophy. His books for a general on this point, developing the first form a ‘dissipative structure’. audience, such as La Nouvelle Alliance of non-equilibrium statistical mechanics. Such states are particularly striking in (with I. Stengers), From Being to (Another approach, developed systems in which chemical reactions Becoming and his last work, La Fin des independently by Nikolai Bogolyubov proceed alongside the effects of diffusion Certitudes, were bestsellers around the in the Soviet Union, turned out to be or external forces: ordered structures of world. He was a true humanist, in the equivalent to Prigogine’s.) As well as different chemical composition appear, widest meaning of the word, and being quite a general formulation of the which may even propagate as ‘chemical attracted numerous disciples. His death theory, there were other fruits of this waves’.Two ingredients are vital if this is to closes an important chapter in the history endeavour, for it could be applied to a occur: the ‘open’ character of the system of science. Radu Balescu variety of systems, from gases and solids (meaning that it can exchange matter and Radu Balescu is at 86 Avenue A. Huysmans, to plasmas. energy with the external world), and the B-1050 Bruxelles, Belgium. Pushing the study further, to systems nonlinear character of the equations e-mail: [email protected]

30 NATURE | VOL 424 | 3 JULY 2003 | www.nature.com/nature