© Matt DePies, University of Washington.

String theory is the best candidate that we have for a theory of quantum gravity. What this means is that it is capable of describing both gravity and quantum effects, and is mathematically consistent. String theory is also very complicated, and performing the calculations and making sense of the answers is extremely difficult. As theorists work toward demonstrating that string theory is not only mathematically consistent, but also describes the world in which we live, they change the details and try out different ideas. So what we call string theory is actually a collection of different theories, with corresponding models of physics derived from them. The common element in all of these theories is that the fundamental objects are not point-like particles, but tiny strings that can form loops. The particles we normally think about correspond to vibrations of these loops of string. (Unit: 4)