CoursesCourse 16
Network science
Complex systems are held together by their connections as much as by their parts. This course follows the science of networks, from the six steps between two strangers to the hubs that hold the Web and the cell together, and asks what the maps tell us about failure, spread, and control. It also says plainly which famous claims have since been disputed.
Start lesson 1Lessons
- 1 Networks everywhereA tree in Ohio, and fifty million people in the dark. How dots and lines describe a grid, a cell, and a society, why any two people are only a few steps apart, and why that is not yet the whole story.
- 2 Hubs and the rich-get-richerIn a world drawn by chance everyone would have about the same number of friends. The Web, the cell, and the airline map do not look like that. What a map of the Web showed, how hubs grow, and how a latecomer can still win.
- 3 What the hubs doHubs make a network hard to break by accident and easy to break on purpose. How failure and disease travel through hubs, and how far the claim that real networks are scale-free survives a closer look.
- 4 Groups, bridges, and controlA karate club that split in two, the weak ties that bring us news, and the question of whether a network can be steered by a few well-chosen nodes. What the maps can and cannot tell a leader.
You’ll be able to
- Describe a system as nodes and links, explain a cascading failure, and say why short distances and strong clustering can exist together in large networks.
- Explain what a power law and a scale-free network are, how growth with preferential attachment produces hubs, and how fitness lets latecomers overtake early leaders.
- Explain why hub-dominated networks are robust against random failure but fragile against targeted attack, why diseases spread differently on them, and how the claim that real networks are scale-free has been challenged.
- Describe communities, the strength of weak ties, and the idea of controlling a network through a few driver nodes, with the limits of each.
About this course
This course covers 网络科学, the study of systems as webs of connections. The question that runs through it is a simple one: when a system does something no single part does, such as a blackout, a market crash, or an epidemic, how much of the explanation lies in who is connected to whom? It touches several courses in this series. Course 10, Thinking, fast and slow, asked how a single mind decides; here we ask how many minds, linked together, behave. Course 9, Sex, love, and romantic relationships, ended on how people find one another, and the social networks of Lessons 1 and 4 are the wider setting for that.
The structure of the argument follows the source course: the small world, hubs and the rich-get-richer, what hubs do to failure and spread, and then groups, weak ties, and control. Around it we add what is ours: every number is checked against the original papers (several popular figures, such as the date of the 2003 blackout and the success rate of the famous letter experiment, turned out to be wrong), hands-on activities, background for readers in China and the United States, and an honest “Another view” in each lesson. Network science has its own controversy, about whether real networks are scale-free at all, and we give it a full section rather than pass over it.