The study of complex systems has been a common arc in my career. It started with physics at the University of Chicago in the 1960s, where I was studying complex natural systems - atomic and molecular physics in particular. Later on, when I joined IBM Research and became a computer scientist, my main research interests were centered on large computer systems, including mainframes, supercomputers and distributed systems. In the last twelve years, my work has focused on the kinds of complex systems made possible by the advent of the Internet and the Web. Then, in the last five years, my interests have gravitated toward market-facing complex systems involving people and services.
What makes such complex systems complex? I found the most satisfying answer to this seemingly Socratic question in an excellent paper - Complexity and Robustness - by professors Jean Carlson and John Doyle from UC Santa Barbara and Cal Tech, respectively.
Complex systems, whether natural or engineered, are composed of many parts. But it is not the mere number of component parts that makes them complex. After all, a stone or a table is composed of huge numbers of molecules, yet we would not consider them complex. According to Carlson and Doyle, a truly complex system must consist of many different kinds of parts, intricate organizations and highly different structures at different levels of scale. Humans, bacteria, advanced microprocessors, modern airplanes, global enterprises, urban environments, national economies and healthcare delivery are all examples of complex systems exhibiting these massive, heterogeneous, intricate characteristics.