Net worker: where are your friends in cyberspace? Closer than you might think, says Internet researcher Jon Kleinberg.

JON KLEINBERG helps us see the invisible networks that pervade our lives. A professor of computer science at Cornell, he teaches a class with the economist David Easley that covers, Kleinberg says, "how opinions, fads and political movements spread through society; the robustness and fragility of food webs and financial markets; and the technology, economics and politics of Web information and online communities." If it sounds like "Intro to How the World Works," that's the general idea.

Some of Kleinberg's research builds on social psychologist Stanley Milgram's famous 1960s experiments into the "small-world phenomenon." Milgram enlisted a random group of people in …