Atomic Clocks and Fundamental Physics

Finally in this chapter we will describe contributions of atomic clocks that go beyond precision time-keeping and its applications, important though these are. Indeed it would be difficult to exaggerate the importance of the impact that such exquisitely stable time standards have already had on civilian and military technology and culture. However, the degree of precision, unimaginable only a few years ago, that has been achieved in the determination of quantum transition frequencies in a variety of atoms and ions, has made it possible to subject to ever greater scrutiny fundamental physical theory, theory that delves deep into the ultimate structure of the universe and the forces that shape its behavior, from the cosmic scale down to the subatomic scale. Tests for extremely subtle violations of symmetry properties that are at the foundation of unified field theories of our universe are now possible using earth-bound and spacecraft experiments based on atomic clocks.