The Relativity Mission
Relativity is either of two theories of physics developed by the German-born American physicist Albert Einstein. Those theories are (1) the special theory of relativity, which was published in 1905; and (2) the general theory of relativity, announced in 1915. Einstein's theories explain the behavior of matter, energy, and even time and space. They are two of the "foundation blocks" upon which modern physics is built.
This site contains informaiton about NASA's Gravity Probe B that confirmed Two Einstein Space-Time Theories and has links to other resources.