It currently offers 11 courses, for free, in subjects such as computer programming, statistics and mathematics, plus a robocar programmer’s workshop with Thrun himself. It rustles up some instructors from the likes of Rutgers and the state universities of Virginia and Utah. Other teachers are experts from industry. Faculty pay runs between $5,000 and $10,000 per course. Many of Udacity’s students are midcareer professionals who want to sharpen specific skills. Udacity later this year is expanding into the humanities. Thrun says the service will always have “a free path,” but the idea is eventually to charge for certificates or enhanced features such as chat.

It was only last year that Thrun seemed like a fast-track scholar thriving within academia. In eight years he rose from a Ph.D. student at the University of Bonn to a tenured post in Stanford’s computer science department (with a stint in between at Carnegie Mellon). “I was a popular professor,” Thrun says. “My teaching ratings were usually good. I could take complicated subjects and explain them in an entertaining way.”

Even so, professor Thrun privately knew something was wrong. In many of his classes students fared much worse on the midterm exams than he expected. He says he had fallen into the “lecturing trap,” in which the instructor looks brilliant and a handful of top-performing students create the appearance of a lively class—but most students aren’t keeping pace. Thrun needed a way to engage all students.