Heinz Georg Schuster: Complex Adaptive Systems

Chapter 1

Introduction

1.1 Motivation

If we look at the activities of modern physics we get the feeling that its main frontiers of current research have moved to the extremes of dimensions.

Figure 1.1: A coordinate system whose axes measure length scales, time scales and different degrees of disorder. Complex adaptive systems (CAS) are located right in the middle on human length and time scales and between order and disorder.

Physicists explore quarks and quasars, they resolve femtosecond flashes of laserlight and analyze cosmic background radiation for signs of the beginning of the universe billions of years ago. But at the same time age-old questions, like where we come from or who we are seem to have disappeared out of focus. Fortunately this picture has changed during recent years. We can draw into Fig. 1.1 another axis which reaches from an ordered crystal to randomly moving gas atoms. Whereas we have understood the extremes, i.e. the crystal by quantum mechanics and the disordered gas by the means of statistical mechanics, the middle region, which is much less understood, has recently become another exciting frontier of physical research. In this middle region at the border between order and disorder and on human length and time scales reside the complex adaptive systems (CAS) which relate to the questions just mentioned and which we will be studying in this book. The word complex comes from the Latin ''complexus'' and means a whole made up of many interacting, interwoven parts. This describes for example our intuitive picture of the human brain. The parts are the neurons, which are connected in an intricate, interwoven way by synapses. So the brain, which is certainly the most complex system on human scales, is our most prominent example of a complex adaptive system. By adaptive we mean that the system can change its behavior under external influences. We can learn with our brain. Some other examples of complex adaptive systems are the interwoven chemical reactions involved in prebiotic evolution, the many different reaction schemes of the immune system, ecosystems with their mutual interdependencies and evolutionary games in which a group of competing players (species) discovers new strategies by adaptation (see Table 1). But these are the systems which are most closely related to the problems raised above. The question of our origin is related to prebiotic evolution, which describes the step from chemistry to biology, and the question of our identity is related to the working of our brain. So, how could we come to grips with complex adaptive systems and what has this all to do with physics?
Table 1 : Examples of Complex Adaptive Systems

Prebiotic evolution	Eigen and Schuster (1979)
Darwinian evolution	Maynard Smith and Szamary (1995)
Chemical networks	Kauffman (1993)
Ecological networks	Sigmund (1993)
Insect colonies	Bonnabeau, Dorigo and Theraulaz (1999)
Immune system	Segel and Cohen (2000)
Nervous system	Kandel and Squire (2000)
Economic networks	Arthur, Durlauf and Lane (1997)
Social networks	Frank (1998)
Communication networks	Albert, Jeong and Barabasi (2000)
Transportation networks	Narguney (2000)
Evolutionary games	Hofbauer and Sigmund (1998)

Heinz Georg Schuster: Complex Adaptive Systems