Einstein expanded the special theory of relativity into a general theory (completed c.1916) that applies to systems in nonuniform (accelerated) motion as well as to systems in uniform motion. The general theory is principally concerned with the large-scale effects of gravitation and therefore is an essential ingredient in theories of the universe as a whole, or cosmology. The theory recognizes the equivalence of gravitational and inertial mass. It asserts that material bodies produce curvatures in space-time that form a gravitational field and that the path of a body in the field is determined by this curvature. The geometry of a given region of space and the motion in the field can be predicted from the equations of the general theory.
Details of the motions of the planet Mercury had long puzzled astronomers; Einstein's computations explained them. He stated that the path of a ray of light is deflected by a gravitational field; observations of starlight passing near the sun, first made by A. S. Eddington during an eclipse of the sun in 1919, confirmed this. He predicted that in a gravitational field spectral lines of substances would be shifted toward the red end of the spectrum. This has been confirmed by observation of light from white dwarf stars. Further confirmation has been obtained in more recent years from precision measurements using artificial satellites, the Viking lander on Mars, and Gravity Probe B (designed specifically to test the theory) as well as from detailed observations of pulsars.