Andrew Zimmerman Jones, PhD, is the physics guide at About.com, where he writes lessons and explanations for common physics problems and questions. Andrew earned his degree in physics from Wabash College and his PhD in mathematics from Purdue University. He is the author of the previous edition of String Theory For Dummies. Alessandro Sfondrini is a theoretical physicist. He obtained his bachelor's and master's degrees in physics from the University of Padova. He is currently a Rita Levi-Montalcini Fellow and assistant professor in theoretical physics and mathematical methods and models at the University of Padova, as well as a member and IBM Einstein Fellow at the Institute for Advanced Study in Princeton, New Jersey.