There were two major breakthroughs that revolutionized theoretical physics in the twentieth century: general relativity and quantum mechanics. General relativity is central to our current understanding of the large-scale expansion of the Universe. It gives small corrections to the predictions of Newtonian gravity for the motion of planets and the deflection of light rays, and it predicts the existence of gravitational radiation and black holes. Its description of the gravitational force in terms of the curvature of space-time has fundamentally changed our view of space and time: they are now viewed as dynamical. Quantum mechanics, on the other hand, is the essential tool for understanding microscopic physics. The evidence continues to build that it is an exact property of Nature. Certainly, its exact validity is a basic assumption in all string theory research.

The understanding of the fundamental laws of Nature is surely incomplete until general relativity and quantum mechanics are successfully reconciled and unified. That this is very challenging can be seen from many different viewpoints. The concepts, observables and types of calculations that characterize the two subjects are strikingly different. Moreover, until about 1980 the two fields developed almost independently of one another. Very few physicists were experts in both. With the goal of unifying both subjects, string theory has dramatically altered the sociology as well as the science.