Since MIT and Harvard started edX, their joint experiment with free online courses, the venture has attracted enormous attention for opening the ivory tower to the world. But in the process, the world will become part of an expensive and ambitious experiment testing some of the most interesting--and difficult--questions in digital education. Can community-college students benefit from a new form of hybrid learning, based on a mix of local instruction and edX content? Can colleges tap alumni as teaching volunteers? Can labs be reinvented in the style of online video games? EdX and its collaborators are developing tools and teaching models to answer these questions. And they view the project as a means to study even deeper problems, like understanding how people forget--and creating strategies to prevent it. The author reports five ways that edX could change education.