On a summer day four years ago, a Stanford University computer-science professor named Andrew Ng held an unusual air show on a field near the campus. His fleet of small helicopter drones flew under computer control, piloted by artificial-intelligence software that could teach itself to fly after watching a human operator. By the end of the day, the copters were hot-dogging--flipping, rolling, even hovering upside down. It was a milestone for the field of "machine learning," the same area of artificial intelligence that lets Amazon recommend books based on a shopper's previous habits and helps Google tailor search results to a user's behavior. Mr. Ng and his team of graduate students showed that artificial-intelligence software could control one of the hardest-to-maneuver vehicles and keep it stable while flying at 45 miles an hour. That same year, Technology Review, published by the Massachusetts Institute of Technology, included Mr. Ng among the top 35 innovators in the world under the age of 35. Today Mr. Ng is an innovator in an entirely different setting: online education. He is a co-founder of the start-up Coursera, which works with 33 colleges to help them deliver free online courses. The co-founder of Coursera, an online start-up in which professors teach thousands of students at a time, says artificial intelligence could transform online education.