Developed by Einstein in 1915, the general theory of relativity is the modern theory of gravity. This is a very complicated mathematical theory which in a nutshell says that space and time, collectively known as spacetime, is curved by the presence of matter and energy, and that the motion of matter and energy is influenced by this curvature.
When Einstein devised his theory, he had three experiments in mind to test its predictions. One was the gravitational bending of starlight around the sun. According to general relativity, the path of starlight which grazes the surface of the sun should bend by 1.75 arcseconds. This effect can be measured during a total eclipse. In 1919, Sir Arthur Eddington measured precisely this bending during an expedition in which he observed a total eclipse. This discovery made Einstein an immediate worldwide celebrity.
The second prediction is the precession of planetary orbits about the sun. According to general relativity, the perihelion of the orbit of mercury should precess about the sun by 43 arcseconds per century. This effect was in fact observed prior to Einstein’s theory and unexplained until predicted by the theory.
The third experimentally verified prediction is the slowing down of time due to gravity. This effect causes light to be redshifted in the presence of a strong gravitational field, i.e. near the surface of a massive star. This effect is observed in the spectra of massive stars. Taken to its extreme, this effect predicts the existence of black holes, i.e. objects which are so massive that nothing, not even light, can escape from their gravity. Black holes have been observed indirectly and now their existence is practically undisputed.