Sean Carroll in Preposterous Universe:
String theory was originally proposed as a relatively modest attempt to explain some features of strongly-interacting particles, but before too long developed into an ambitious attempt to unite all the forces of nature into a single theory. The great thing about physics is that your theories don’t always go where you want them to, and string theory has had some twists and turns along the way. One major challenge facing the theory is the fact that there are many different ways to connect the deep principles of the theory to the specifics of a four-dimensional world; all of these may actually exist out there in the world, in the form of a cosmological multiverse. Brian Greene is an accomplished string theorist as well as one of the world’s most successful popularizers and advocates for science. We talk about string theory, its cosmological puzzles and promises, and what the future might hold.