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In the late 1960s students had a rather revolutionary life in universities. In the midst of all this, Mark Granovetter, a PhD student at Harvard University, set himself to figure out how people and their jobs. He interviewed about a hundred people and sent out another 200 questionnaires in the Boston area.

Networks catch hold of you. They are enchanting and contagious. As a first ‘proof’ of these statements let me give you my own example. Just before starting to write this chapter, I sat on a train and watched a charming mother and her little daughter just opposite me. The baby fell asleep playing with her comforter. As I continued to watch, my mind went to work. What could be the periodicity of her suckling motions? Was it perhaps scale-free, showing sudden bursts of activity separated by longer and longer periods of stasis? Did it show self-organized criticality? Was this a punctuated equilibrium? My thoughts continued: What if I looked outside? Would I see fractals instead of trees and clouds?

Stability resembles intelligence: easy to imagine and hard to define. Here I will assess network stability at three different levels. First I will describe network perturbations and the concept of local dissipation and global connections. Later a number of different scenarios will be shown, where the perturbation is big or persistent enough to change the stability of the underlying bottom networks, and therefore, the structure of the current network is not preserved. Then stability relationships between the bottom and top networks will be discussed using the example of synchrony. Finally, I will describe the two basic design alternatives: evolution and engineering.

In the previous chapters we learned how weak links stabilize complex systems. The classical study of Granovetter (1973) demonstrated that weak links help the cohesion of society (see Chap. 1). Weak links are necessary for small-worldness, emerging in parallel with topological scale-freeness, and making a key contribution to the formation of nestedness (see Chap. 2). Weak links buffer noise, help relaxation, form barriers against cascading failures, and stabilize the coupled oscillators of bottom networks (see Chap. 3).

If you are ready, our journey will now begin. After entering Netland, we will first visit ‘simple networks’.^1 In the next section, I discuss proteins, essential building blocks of our cellular networks. They carry out almost all tasks in the cell and provide most of the cell’s shape, too. We shall begin with the enormous difficulties our proteins have to overcome to reach their final structure. In the next step, we examine their complex stability patterns using the idea of energy landscapes. Finally, we discover how weak links help them in all these processes.

This will be our second journey into Netland. We are going to visit cellular networks. I first introduce various cellular networks and continue with a wide variety of proteins which may all modulate the stability of our cells. In the last two sections, I also explain the role of stabilizing proteins in evolution, cancer and other diseases, and aging.

With this part of our trip, we discover more familiar parts of Netland. This chapter will be about ourselves. Our body is a network of cells and organs. Unfortunately, this complex world is largely unexplored from the standpoint of network behavior. This is partly because elements and modules are mixed up here. Sometimes an organ may be regarded as an element of the body net, whilst in other cases it has to be treated as a module of its cells. Moreover, the complexity of the interactions makes exploration and description even more difficult. This is why I will sometimes use examples from the animal world to bring out the network properties of this exciting network: our own body.

This is not only the time to get down to work, as I noted at the end of the last chapter, but also a time to thank you for your patience in coming along with me on this trip to Netland. We have reached an important point. We are just about to rise above ourselves. In the last chapter, we surveyed some of the networks in our body, and in this chapter the same body will be an element of a larger network, the social net. The current chapter will give me a good opportunity to understand my obsession with building social networks.

Now that you are back from the zoo, where you tried in vain to shake the six hands of all the ants for several hours,^1 it is time to start our fifth trip into Netland. Let us go and see what those macroscopic ants known as human beings can achieve. I will show you what type of networks we have figured out to support the last variety of social networks from the previous chapter.

On our sixth trip into Netland, we will go a level higher in complexity. In Chap. 8, we examined social networks where the elements were usually individual people. In Chap. 9, we saw some of the networks these communities produced. In this chapter, we investigate ‘supersocial’ nets, in which the elements are the social networks of the previous chapter, in the form of firms, investor groups or whole nations. Chapters 8 and 10 together show another nice example of nestedness.