David Lorimer talks to Nobel prize-winner Ilya Prigogine about introducing the arrow of time into the basic laws of physics.

Ilya Prigogine has had a lifelong preoccupation with time. Years ago he wrote the gnomic phrase "time precedes existence". Only now does he claim to begin to understand it. What, he asks, do we and a rock have in common? Time, which is both the unifying element and the source of diversity.

The thrust of his current work is the establishment of an evolutionary perspective in physics, which would make it consistent with our understanding of cosmology and biological evolution. At present we have a physics in which time is reversible and essentially an illusion and a biology in which time is crucial: without the reality and irreversibility of time, evolution could not have occurred.

Prigogine was born in Moscow in 1917, and his family emigrated when he was four, first to Lithuania and then on to Paris and Brussels, where he now lives. Since 1959 he has been director of the Solvay Institutes of Physics and Chemistry and since 1984 has held a chair in the University of Texas at Austin. His father was a chemical engineer and his mother a trained musician who taught him to read piano scores before he could even read books. This marriage of art and science was a decisive influence and encouraged him to develop and maintain a broad outlook which included chemistry, physics, art, history, literature and philosophy.

ÁñÁ«ÊÓƵ

Two of his early philosophical influences were Alfred North Whitehead and Henri Bergson. He sees his current physics work as underpinning Whitehead's organic process view of life. Bergson was convinced that if time existed in us, it must also exist in the universe. And if time is real, so are novelty and creativity. The reality of novelty and creativity implies in turn an emergent, evolutionary universe and not a deterministically programmed one. Prigogine's work starts where Bergson and Whitehead left off, moving from philosophy to physics: if the flow of time and duration are fundamental on most levels of description, then they should also be found in basic physics.

With this type of background it was natural that Prigogine turned first to thermodynamics. The legacy of the 19th century left us with two conflicting views of nature: the deterministic and time-reversible view based on the law of dynamics and the evolutionary views associated with the second law of thermodynamics - the idea that the universe is a machine gradually running down into increasing disorder. Both views have been immensely successful. How could they be related and delineate their respective domains of application? Prigogine considers that there have been two basic steps towards clear classification. The first refers to the macroscopic, thermodynamic level. Traditionally, thermodynamics was mainly applied to equilibrium situations, but Prigogine focused his attention on non-equilibrium situations. His research led to many surprising findings.

ÁñÁ«ÊÓƵ

An example is Prigogine's theory of dissipative structures, for which he was awarded the Nobel Prize for chemistry in 1977, which shows how ever higher levels of complexity can emerge in nature in far from equilibrium situations. The existence of dissipative structures therefore proves the constructive role of the arrow of time associated with irreversibility. In such systems fluctuations and instabilities lead to bifurcations, points at which the system spontaneously self-organises into a new pattern. And, as the title of one of Prigogine's earlier books states, order can emerge out of chaos. Bifurcations introduce a basic element of unpredictability. At these points several possibilities are open to the system, one of which will actually be realised. Therefore the future involves probabilities, in contrast with stable dynamical systems (such as a pendulum) whose future can be predicted by deterministic laws.

This naturally has essential consequences for the description of evolution. A deterministic view of evolution implies that the film has already been made and is simply unfurling in an entirely predictable sequence. We are automata, but we deceive ourselves into thinking that we are free; our freedom is only apparent. This debate over free will and determinism has a long ancestry. The theological determinism implied in divine omniscience and omnipotence was subsequently translated into various forms of scientific and social determinism ranging from the genetic, neural or biochemical to the psychological and cultural. The key question is whether the future can be predicted with certainty or is it always essentially a question of probabilities?

Which brings us back to the dilemma we began with: the irreversible nature of time is asserted in cosmology and biology, but physics is based on a formulation of the laws of nature in which there is no distinction between past and future; in other words time is reversible and therefore illusory, as Einstein implied. The paradox is that the universe itself is full of irreversible transformations which imply an arrow of time (such as ourselves) and yet the basic laws of physics are said to be reversible. The results mentioned in connection with non-equilibrium thermodynamics show that irreversibility cannot be the result of our approximations. Non-equilibrium structures are as real as equilibrium ones. How then can the arrow of time and limited predictability be introduced into the basic laws of physics? This is where the recent work of Prigogine and his group in Brussels and Austin comes in.

Traditional formulations of the laws of physics for individual cases or experiments in classical mechanics are associated with the idea of trajectories, and in quantum mechanics with the idea of the wave function and its collapse; then for ensembles (a large number of individual cases) there is the corresponding statistical description. It was always assumed that these two descriptions - individual and statistical - were equivalent, but Prigogine has shown that this equivalence is broken for important classes of unstable systems, both in classical and quantum mechanics.

The proof of this statement requires appropriate mathematical tools which have been developed only recently. In short the statistical description leads to new solutions that are irreducible to classical trajectories or quantum wave functions. As a result the meaning of the fundamental laws of physics is changed. They no longer express certainties but possibilities and time symmetry is broken, making the process irreversible rather than reversible. We therefore obtain an extension of Newton's and Schroedinger's dynamics to account for unstable dynamic systems. In the classic Popperian sense of a new formulation containing but transcending the existing one, Prigogine's theory suggests that stable systems are actually a subset of unstable systems and not vice-versa as was previously assumed.

ÁñÁ«ÊÓƵ

Prigogine quotes his friend Leon Rosenfeld that "no physical concept is sufficiently defined without the knowledge of its domain of validity". It is precisely this domain of validity of the basic concepts of physics which he is delineating in relation to instability and chaos; it is also important to stress that his co-workers have recently validated some of the main predictions of his approach by an extensive computer programme.

Irreversibility, which is an emergent property like phase transitions, is therefore a fundamental property of nature, since, according to Prigogine, it is intrinsic to unstable chaotic systems . In other words, the statistical description is not simply an expression of our ignorance. Many physicists like Murray Gell-Mann still claim that irreversibility can be explained by "coarse graining", that is by saying that it results from our limited human approximations, and that for a really well-informed observer the world would appear perfectly time reversible. Prigogine strongly contests this view, maintaining that "irreversibility subsists, whatever the precision of our experiments".

An important illustration of this controversy can be found in Prigogine's understanding of the quantum measurement problem and the well-known conundrum of the role of the observer in the measurement. He refers to a recent article in the magazine Scientific American in which Steven Weinberg highlights "a stubborn duality in the role of intelligent life in the universe": on the one hand when a system is not observed it behaves in a perfectly deterministic (and reversible) way in agreement with Schroedinger's equation of quantum mechanics; on the other hand, when the system is actually measured by an external observer, it violates this determinism. In this second case, possible outcomes of measurements can no longer be predicted with certainty in advance, but can only be expressed in terms of probabilities.

ÁñÁ«ÊÓƵ

In this view irreversibility is therefore introduced by the observer or experimenter rather than being inherent in the basic physics. Prigogine's conceptual scheme involves instability or chaos leading to probability and in turn to irreversibility. It is these notions of instability, probability and irreversibility which now become fundamental, so that the ideas of trajectory or wave function become particular cases which are only valid for stable systems. The duality inherent in the traditional formulae of quantum mechanics - on the one hand Schrodinger's equation; on the other the collapse of the wave function - is therefore avoided.

The wider implications of this new point of view in physics are far-reaching. Classical science emphasised stability and equilibrium. Now we discover fluctations, instabilities and evolutionary patterns at all levels. This is not only true in science as the second half of the century has been characterised by social instability and a crisis of control and planning, as well as by recent self-organising phenomena like the Internet. We have to find a "narrow passage" between the Scylla of determinism and Charybdis of randomness.

We are not at the end of physics, but rather at the end of predictability and certainty, which means that physics needs to include novelty and creativity. And a science in which creativity and participation in the construction of the world are intrinsic is a science which overcomes the widespread alienation associated with the traditional scientific outlook. In Prigogine's "new rationality", probability will no longer be seen as ignorance nor science as equivalent to certainty. Time is real and the future is open: we live not simply in an "open society" but also in an open universe.

David Lorimer is director of the Scientific and Medical Network.

ÁñÁ«ÊÓƵ