This is an ambitious book which suggests that a new picture of the nature of the universe is emerging from the study of thermodynamics, and that this picture will heal the breach between the scientific and the poetic view of man. Prigogine’s distinction as a scientist – the Nobel Prize for Chemistry in 1977 – requires that we take his views seriously, at least on the first of these claims. The major part of the book is devoted to explaining, in non-mathematical language, the new science that the authors see emerging, and to which Prigogine helped to give birth. The ideas are hard, but I think they succeed. There are places, particularly in their treatment of quantum theory, where readers without some previous knowledge may lose the thread; certainly I did. But anyone prepared to make a serious effort will get some insight into what is happening.
In this review, I aim to do three things. First, I will explain the new thermodynamics; here I will try to follow the authors’ account, not criticise it. Second, I will say something about the relevance of their ideas to biology. Finally, I will comment briefly on their significance for man’s view of himself.
The authors see the history of the physical sciences as dominated by two apparently incompatible approaches, dynamics and thermodynamics. Dynamics, stemming from Galileo and Newton, sees the world as deterministic and reversible. As Laplace argued, it follows from the dynamical world picture that, given the positions and velocities of all the particles in the universe at any instant, an all-powerful intelligence could calculate the future. It also follows that if, at some instant, the direction of motion of every particle was exactly reversed, then the universe would return to the states it had occupied at earlier times. To put the same point in another way, if a film of some sequence of events was run forwards and backwards, it would be impossible for a viewer to tell which direction was correct.
This deterministic and reversible picture is as true of Einstein’s dynamics as of Newton’s. It has no place for our subjective view of time. It is as if the whole past and future were eternally present, although we are forced to experience events in a particular sequence, just as, in the cinema, the whole reel of film is present all the time, but we are forced to view the frames in sequence. This dynamic picture seems to be contradicted by everyday experience, not only of subjective time, but also of the irreversible nature of most events. Shown a film, we have no difficulty in deciding whether it is being run in the correct order. Smoke does not flow towards a cigarette end as the cigarette lengthens.
Since the beginning of the 19th century, scientists have been increasingly interested in processes that do not run backwards. This interest originated from an effort to understand heat engines. A steam engine converts the chemical energy in coal into mechanical work: you cannot run it backwards, to turn mechanical work into coal. It will help to consider a simpler example of an irreversible process. A drop of water falling on the surface of a lake causes waves, which spread out as concentric rings, diminishing in amplitude as they go, until finally they disappear, leaving the lake smooth once more. One never sees a series of concentric rings converging on a point, and propelling a drop of water into the sky. If we ask what has happened to the energy of the water drop when the lake finally becomes smooth, the answer is that the velocity of the water molecules has increased infinitesimally: in other words, the lake has got a little hotter.
Thermodynamics arose as a way of describing and predicting the behaviour of systems in which irreversible changes occur, and in which energy is converted from one form to another – heat, mechanical work, electrical energy, chemical energy, and so on. The formulation of thermodynamics included the famous – I am tempted to say notorious – second law. This law differs from anything in classical dynamics, because it asserts that changes will occur only in one specific direction. It states that a certain quantity, entropy, can only increase (or, at equilibrium, remain constant) in a physically closed system. Increase in entropy really requires a mathematical definition, but can be understood qualitatively as saying that things will get more mixed up, random and unstructured. A vessel containing hot water at one end and cold at the other will come to contain lukewarm water everywhere, but a vessel containing lukewarm water will not separate out into hot and cold. When our drop of water falls into the lake, there is initially a set of molecules all moving in the same direction: finally, the directions are random.
At first sight, thermodynamics seems to have no more room for life than dynamics. Increase of entropy can explain death, but not life: why we rot, but not how we ripen. Indeed, the second law is regularly trotted out by the creationists as a reason why life could not have arisen without miraculous intervention. Before tackling this difficulty, however, I must say something about the attempts that have been made to reconcile reversible dynamics and irreversible thermodynamics. The crucial figures were Maxwell and Boltzmann. We can imagine a gas as consisting of elastic particles colliding with one another. The faster the particles move, the higher the temperature. Suppose that at one end of a container the particles are moving much faster than at the other. Boltzmann showed that the effect of a collision between a fast-moving and a slow-moving particle is, more often than not, to make their velocities more nearly equal. Consequently, the effect of collisions is an averaging out of the velocities: the gas becomes the same temperature everywhere. In fact, the velocities of the molecules do not become identical, but have a distribution around an average value. When an equilibrium is finally reached, this distribution no longer changes with time.
Thus a dynamic model of molecules obeying Newton’s laws can predict the increase of entropy demanded by the second law. But there is a snag, contained in the phrase ‘more often than not’ that I used above. Let us again return to the drop of water falling into a lake. Suppose that, when the waves have disappeared, the direction of motion of every molecule is exactly reversed. Does not the dynamic model predict that history will run backwards, and that converging rings would appear and propel a drop into the sky? Indeed it does. How then does Boltzmann’s argument help, since a dynamic model can predict either an increase or a decrease of entropy? The answer is that, for a flat surface to generate converging rings, the initial positions and velocities of every molecule in the lake would have to be precisely specified. If these values were varied, even slightly, no pattern would appear. In contrast, if we start with the falling drop, the initial values could be varied extensively, provided certain averages were maintained, and the visible behaviour would be unaltered.
Hence, from the point of view of dynamics, the law that entropy increases is merely probable. There are initial conditions for which entropy in a closed system will decrease, but they are very unlikely. Prigogine and Stengers argue that, in a technical sense, they are infinitely unlikely: it is this fact which, in their view, guarantees irreversibility, and the arrow of time.
Does this mean that there is some justice in the creationists’ claim that the origin of life required a miracle? After all, the entropy of a living organism is less than that of the nonliving matter from which it emerged. The short anwer to this is that the second law applies only to a closed system: that is, a system physically isolated from everything else. There is, however, a more interesting answer to the question, which emerges from a study of systems which are a long way from thermodynamic equilibrium; it is in this field that Prigogine’s work earned him his Nobel Prize. The point can best be explained by an example. If the plug is pulled out of a basin, the escaping water may form a vortex, with the water spinning round a central hole. If water is continuously added, so as to maintain the level in the basin, the vortex will persist. The movements of the water molecules in the vortex are far from random: in fact, they move coherently to form an ordered structure.
Such structures Prigogine has called ‘dissipative structures’. Their essential characteristic is that they require a continuing input of energy from without. In the case of the vortex, water must be continuously added to the basin. If that ceases, the basin will empty, and the water will flow to the lowest point it can reach, at which point all structure will disappear: the system has reached equilibrium. If a system is far from equilibrium, however, dissipative structures will form. Two other examples will illustrate the point. First, suppose that a thin layer of water is heated from below. If the temperature difference between top and bottom is great enough, an ordered flow of liquid is set up, in a hexagonal pattern known as Benard cells.
A second example, which has been of particular interest to biologists, is the chemical reaction known as the Zhabotinsky reaction. Chemical reactions proceed until an equilibrium is reached; the substances produced by the reaction become randomly mixed by a process of diffusion, so that, typically, all substances are uniformly distributed in space. In the Zhabotinsky reaction, however, a spatial pattern appears. Since some of the reactants are coloured, bands and rings of colour appear, and move slowly across the region in which the reaction is taking place. The first person to show mathematically that chemical reaction and diffusion can give rise to large-scale spatial patterns was Alan Turing, in 1952, shortly before his death.
In each of these examples – a vortex in a basin, Benard cells, Zhabotinsky’s reaction – there is an external supply of energy: the potential energy of the water, the heating from below, the chemical energy of the reactants. In each case, an ordered structure appears. The mathematical analysis of such structures, and of the conditions for their origin and maintenance, constitutes the new approach to science that Prigogine and Stengers describe. It is clear that living organisms can be viewed as dissipative structures. Life requires a continuous input of energy in the form of food or sunlight. So long as that input continues, complex structures can exist.
It is, therefore, somewhat surprising that few biologists have taken this view of life very seriously, although, as I shall explain, this neglect is not universal. A typical account of a living organism would start with a description of how the chemical reactions going on in cells are controlled by proteins, and go on to explain how the structure of those proteins is coded for in the genetic material, the DNA, and how, in evolutionary time, the DNA has been programmed by natural selection. As the terms ‘control’, ‘code’ and ‘programme’ indicate, the central concepts are derived from cybernetics and information theory. As Dawkins put it in The Selfish Gene, organisms can be viewed as ‘robot vehicles blindly programmed to preserve the selfish molecules known as genes’. There is not much room here for the organism as a vortex.
Some biologists, however, would argue that to think only of controls and programmes is to forget that there has to be an object to be controlled: ‘You use the snaffle and the curb all right, but where’s the bloody horse?’ To such people, Prigogine’s ideas will have an appeal. In the main, they are working on development. Developmental biologists tend to fall into two schools. For some, all that is needed is to understand how the information in the DNA is translated into adult structure; they seek to discover how groups of genes are switched on and off in different tissues. For others, the fundamental problem is the appearance, during development, of a spatially complex structure from a relatively homogeneous egg. Zhabotinsky’s reaction seems a better model than a computer programme for this process. Of course, no one imagines that Zhabotinsky’s reaction is more than an analogy, but it does seem to be the kind of process we should be looking for. A number of biologically more plausible processes have been proposed, and are being investigated. Ultimately, of course, there need be no contradiction between these two approaches. Even if we come to view embryos as dissipative structures, genes can still control those structures by determining which enzymes shall be present, and hence the rates at which chemical reactions shall proceed. But for the present, there is heated disagreement about what is the most fruitful way forward.
As it happens, I do not think that the biologists engaged in this enterprise have been directly influenced by non-equilibrium thermodynamics. Certainly my own interest was triggered in the Fifties by Turing’s paper. It is nice to know that the processes we are thinking about do not contradict the laws of thermodynamics, but in a sense we knew that already, or rather, we knew that if there was a contradiction, it would be so much the worse for thermodynamics. After all, eggs do turn into adults. What I cannot tell is whether the new thermodynamics is going to be of any more detailed use in analysing development. At present I confess I do not see how.
If I am uncertain about the relevance of non-equilibrium thermodynamics for biology, I am still more so about the claims the authors make for its poetic significance. Prigogine has remarked elsewhere: ‘I believe it is more helpful, more exhilarating, to be embedded in a living world than to be alone in a dead universe. And this is really what I try to express in my work.’ I can understand his discomfort with Einstein’s universe. Who would wish to live in an eternal and unchanging four-dimensional universe, and be forced, for no reason that could be understood, to move irrevocably along one of the dimensions, time? Better, surely, to be a changing and developing structure, of a kind that is both natural and predictable in a universe far from thermodynamic equilibrium. I agree, but only because the latter view is better science. A physics which does not permit the occurrence of birth, life and death (in that order) is bad physics.
I do not think we should embrace scientific theories because they are more hopeful, or more exhilarating. I would like to be able to say that we should embrace them because they are true, but that we can never know. The best we can do is to embrace them because they explain a lot of things, are not obviously false, and suggest some interesting questions. I feel sensitive on this matter because, as an evolutionary biologist, I know that people who adopt theories because they are hopeful finish up embracing Lamarckism, which is false, although perhaps not obviously so, or Creationism, which explains nothing, and suggests no questions at all. If non-equilibrium thermodynamics makes poets happier, so be it. But we must accept or reject it on other grounds.