The emergence of two new fields, the science of chaos and the science of networks, has changed the way in which we look at physical and social systems. From the first we learned that simple (in the sense of having few degrees of freedom) physical systems can undergo chaotic motions and display intricate trajectories. The double pendulum is one of the simplest systems of this kind. Initiated by Benoît Mandelbrot (1975) and Robert May (1976), the science of chaotic systems and fractals has already produced substantial achievements. This book relies only occasionally on the analysis of chaos; in contrast it relies heavily on the ideas of network science. Although it can be traced back to system theory which flourished in the 1960s and 1970s, network science really emerged in the late 1990s through the works of people such as Albert-László Barabási (2002), Sergei Maslov (2002), Steven Strogatz (2003) or Duncan Watts (2003). It has been instrumental in convincing us that what really matters in a system is its nodes, its links and their respective weights. Seen in this perspective, the real nature of the system, whether of physical, biological or social nature, is of little relevance.

But looking at physical and social systems in an abstract, purely structural way takes away much of their substance. The real challenge is to do real physics and real sociology in the framework of network theory. This is what we call “bridging the gap”.