THREE ROADS TO QUANTUM GRAVITY

The most obvious questions—e.g., "What are time and space?"—are nearly impossible to answer. Smolin (The Life of the Cosmos), professor of physics at Pennsylvania State University, offers "a report from the front" of the current "revolution" in theories of space and time. Aimed at beginners, this careful treatment of the search for a single theory that brings together relativity and quantum theory will draw in potential explorers. Illus. (July 15)