In the previous chapters we have occasionally included remarks about advanced topics. For example, we mentioned that the relativistic description of gravity requires an extension of special relativity, called general relativity. We also referred on occasion to the standard model of elementary particles and some of its properties. We described basic facts about supersymmetry, but we did not pursue how it might fit into a realistic theory. References were also made to string theory, which is the leading approach for constructing a fully unified quantum theory of particles and forces. In this chapter we will say a little more about each of these topics. The purpose is to convey a general impression and to whet the reader's appetite for exploring them further. The chapter concludes by discussing some important unsolved problems.

Relativity and gravity

When Einstein formulated special relativity he understood that a relativistic generalization of Newton's theory of gravity was not a simple matter. Rather, it requires new concepts and a new mathematical framework. Einstein dedicated himself to this project during the subsequent decade, which culminated in 1916 with a new theory of gravity, which he called general relativity. As will be discussed below, general relativity makes testable predictions for new phenomena that are not accounted for in Newton's theory. Some were verified right away and others in more recent times. At present, general relativity is tested to good precision (better than 1 percent in some cases) and no discrepancies have been observed.