Brian Greene is a physicist who specializes in string theory. His latest book is “The Fabric of the Cosmos: Space, Time, and the Texture of Reality.” Greene tells Steve Paulson that time appears to move in one direction only to complex organisms like people. At the atomic level, electrons don’t know one direction from another. So time travel ought to be theoretically possible in either direction. Einstein proved we can visit the future. Greene tells Steve how time travel to the past may still be a theoretical possibility.