A theory announced by A. Einstein in 1915 that describes how space and time are affected by the gravitational fields of matter, and how space and time change as seen by an observer on an accelerating object. The theory predicts that gravitational fields change the geometry of space and time, causing it to become curved. This curvature is apparent in a number of ways. First, light is bent in a gravitational field, a prediction that was confirmed by photographic measurements of the positions of stars near the limb of the Sun made during a total solar eclipse in 1919. The same effect manifests itself in a delay in radio signals from distant space probes as the signals pass the limb of the Sun. The curvature of space near the Sun also causes the perihelion point of Mercury’s orbit to move forward, by 43″ per century more than predicted by I. Newton’s theory of gravity (see advance of perihelion). In the orbits of pulsars in binary systems, the advance of periastron can amount to several degrees per year.

Another effect predicted by general relativity is the redshift of light caused by gravity. This has been demonstrated in the redshift of lines in the spectra of the Sun and, more noticeably, white dwarfs. Other predictions of the general theory include the expanding Universe; the gravitational lens effect; gravitational waves; singularities; and the invariance of the universal gravitational constant, G. General relativity was developed from the principle of equivalence between gravitational and inertial forces.