String theory developed over several decades through the combined efforts of multiple scientists; no one scientist invented it. However, in 1943, Werner Heisenberg started the original research program that led to the development of the theory.
String theory is popularly known as the "theory of everything," and its main purpose is to discover how gravity and quantum physics fit together. As of 2015, the theory is a work in progress. Other important scientists who contributed to the development of string theory in the second half of the 20th century include Stanley Mandelstam, Geoffrey Chew, Shoichi Sakata, Tullio Regge, John H. Schwarz and Joel Scherk.