What makes a system a system, and not simply a collection of elements, are the connections and interactions between its components, as well as the effect that these linkages have on its behavior. For example, it is the interrelationship between capital and labor that makes an economy; each component taken separately would not suffice. The two must interact for economic activity to take place, and complexity and surprise often reside in these connections. The following is an illustration of this point.

Certainly the most famous question of classical celestial mechanics is the n-body problem, which comes in many forms. One version involves n point masses (a simplifying mathematical idealization that concentrates each body’s mass into a point) moving in accordance with Newton’s laws of gravitational attraction and asks if, from some set of initial positions and velocities of the particles, there is a finite time in the future at which either two (or more) bodies will collide or one (or more) bodies will acquire an arbitrarily high energy and thus escape the system. In the special case when n = 10, this is a mathematical formulation of the question, “Is our solar system stable?”

The behavior of two planetary bodies orbiting each other can be written down completely in terms of the elementary functions of mathematics, such as powers, roots, sines, cosines, and exponentials. Nevertheless, for the extension to just three bodies it turns out to be impossible to combine the solutions of the three two-body problems to determine whether the three-body system is stable. Thus, the essence of the three-body problem resides somehow in the way in which all three bodies interact. Any approach to the problem that severs even one of the linkages between the bodies destroys the very nature of the problem. Here is a case in which complicated behavior arises as a result of the interactions between relatively simple subsystems.

Emergence

A surprise-generating mechanism dependent on connectivity for its very existence is the phenomenon known as emergence. Emergence refers to unexpected global system properties, not present in any of the individual subsystems, that emerge from component interactions. A good example is water, whose distinguishing characteristics are its natural form as a liquid and its noninflammability—both of which are totally different than the properties of its component gases, hydrogen and oxygen.

The difference between complexity arising from emergence and that coming only from connection patterns lies in the nature of the interactions between the various components of the system. For emergence, attention is not placed simply on whether there is some kind of interaction between the components but also on the specific nature of those interactions. For instance, connectivity alone would not enable one to distinguish between ordinary tap water, which involves an interaction between hydrogen and oxygen molecules, and heavy water (deuterium), which involves an interaction between the same components but with an extra neutron thrown into the mix. Emergence would make this distinction. In practice it is often difficult (and unnecessary) to differentiate between connectivity and emergence, and they are frequently treated as synonymous surprise-generating mechanisms.

Emergence in an ant colony

Like human societies, ant colonies achieve things that no individual member can accomplish. Nests are erected and maintained; chambers and tunnels are excavated; and territories are defended. Individual ants acting in accord with simple, local information carry on all of these activities; there is no master ant overseeing the entire colony and broadcasting instructions to the individual workers. Each individual ant processes the partial information available to it in order to decide which of the many possible functional roles it should play in the colony.

Recent work on harvester ants has shed considerable light on the processes by which members of an ant colony assume various roles. These studies identify four distinct tasks that an adult harvester-ant worker can perform outside the nest: foraging, patrolling, nest maintenance, and midden work (building and sorting the colony’s refuse pile). It is primarily the interactions between ants performing these tasks that give rise to emergent phenomena in the ant colony.

When debris is piled near their nest opening, nest-maintenance workers abound. Apparently, the ants engage in task switching, by which the local decision of each individual ant determines much of the coordinated behavior of the entire colony. Task allocation depends on two kinds of decisions made by individual ants. First, there is the decision about which task to perform, followed by the decision of whether to be active in this task. As already noted, these decisions are based solely on local information; there is no centralized control keeping track of the big picture.

Once an ant becomes a forager it never switches to other tasks outside the nest. When a large cleaning chore arises on the surface of the nest, new nest-maintenance workers are recruited from ants working inside the nest, not from workers performing tasks on the outside. When there is a disturbance, such as an intrusion by foreign ants, nest-maintenance workers switch tasks to become patrollers. Finally, once an ant is allocated a task outside the nest, it never returns to chores on the inside.

The foregoing ant colony example shows how interactions between various types of ants can give rise to patterns of global work allocation in the colony, emergent patterns that cannot be predicted or that cannot even arise for isolated ants. The next section presents an example of emergence in an artificial financial market.