International Encyclopedia of Systems and Cybernetics

The transition by which time symmetry is broken.

The basic idea of irreversibility was developed in a rational way by the French philosopher E. QUINET (1870) and stated precisely by DOLLO (1893).

In 1924, A. LOTKA observed that absolute reversibility should be possible only if frictional resistances were totally absent, a completely illusory assumption in "real processes" which "are always irreversible" (1956, p.25-26).

A satisfactory theoretical basement for the concept of irreversibility has been finally established by I. PRIGOGINE and his Brussels school.

PRIGOGINE writes: "Rational mechanics knows only reversible time, whereas the direction of time plays a fundamental role in the biological and human sciences. It is true that in the Nineteeth Century the problem of the direction of time was incorporated in physics through the second law of thermodynamics. Still the contrast between the idea of evolution in physics and that in biology or sociology is striking. In physics, the increase of entropy expressed by the second law of thermodynamics shows a tendency toward a progressive "disorganization" of the system. On the other hand, biological or social evolution is accompanied by progressive structuration…" (1976, p.94). And: "Life corresponds… to an inscription of irreversibility into matter" (1985,p.9).

To explain irreversibility PRIGOGINE and his collaborators created the new thermodynamics of systems far away from equilibrium, which does not contradicts classical thermodynamics (quite the contrary), but generalizes it through new concepts related to complexity, such as giant fluctuations, stability thresholds and dissipative structures.

As a consequence of this work, the subjectivist interpretation of irreversibility (in physics and mechanics) as attributed to the observer incapacity to perceive the conditions of reversibility and to produce it, as could be done by MAXWELL's demon, is now abandoned, as well as CHANDRASEKAR's argument, quoted by PRIGOGINE, that "a process appears irreversible (or reversible) according as whether the initial state is characterized by a long (short) average time of recurrence compared to the time during which the system is under observation". PRIGOGINE writes: "According to this point of view, irreversibility would appear as an artefact due to the time scale of observation!" (1989, p.77).

Sensibility to initial conditions, as a basic characteristic of deterministic chaos, results in a new understanding of the meaning of irreversibility.