International Encyclopedia of Systems and Cybernetics

The expression was created by A.S. EDDINGTON (1929).

However, in the laws of classical physics and quantum mechanics, it is impossible to distinguish between past and future. (See "Reversibility and entropy").

Nevertheless, daily experience shows us that new entities appear "at any moment", only to decay "later on" and that we cannot do without implicit or explicit references to that enigmatic perception that we call "time".

In P. COVENEY's words: "… as the physicist Ludwig BOLTZMANN discovered, it is not possible to base the arrow of time directly on equations that ignore it. His failed attempt to recon cile microscopic mechanics with the second law of thermodynamics gave rise to the "irreversibility paradox" (1990, p.51).

The "legalization" of time's arrow in physics required more than 100 years, in three steps:

1) The introduction by R. CLAUSIUS in 1850 of the concept of entropy as a quantity that relentlessly increases in relation to progressive dissipation of heat and levelling of internal energy states in an isolated system.

The formulation of the 2nd Law as an inequality implies in fact giving a privileged direction to time.

2) However, as no real systems are absolutely isolated and as they can maintain successfully their internal organization through absorption of energy from their environment, a more realistic approach was necessary. This was done by I. PRIGOGINE with his theorem of minimum entropy production in systems near their equilibrium (1945) (1947) However, this theorem still explains stability, more than irreversible transformations and/or decay

3) One more (and decisive) step was taken by P. GLANSDORFF and I. PRIGOGINE (1971). They showed that systems far-from-equilibrium could jump from one stable state to one of some alternate others, through a bifurcation brought about by dissipation of energy through giant fluctuations that could not anymore be damped.

From another viewpoint, as noted by M. HAAS "Time is perhaps the most ineluctable foe of symmetry" (1967, p.71).

It could also be said that time's arrow is the basic vector of causality, which could never become manifest in "no time".

In a somewhat more precise perspective, P. MANZELLI states: "The irreversibility of natural phenomena, i.e. the reality of the arrow of time, can be described… as a fundamental consequence of the evolutive asymmetrical character of the dynamics of matter/information/ energy transformations in nature" (1993, p.234).

As experimental proofs became numerous, it can be admitted that the "arrow of time" is now firmly established in a scientific perspective. What remains now to be found out is how and why we perceive time physiologically and psychologically.