Award-winning science writers Mary and John Gribbin reveal how Pullman's "His Dark Materials" trilogy ("Northern Lights", "The Subtle Knife" and "The Amber Spyglass") is rooted in scientific truth. Drawing on string theory and space-time, quantum physics and chaos theory, they answer questions such as: could parallel worlds like Will's and Lyra's really exist? How does Will's subtle knife cut through anything? Could there be a bomb like the one made with Lyra's hair? And, of course, what are the Dark Materials?