Why Relativity?

Limitations of Newtonian Theory

Our Universe, on the largest scales, appears to be homogeneous and isotropic. On those vast scales it is the force of gravity that governs its behaviour. Until the beginning of the 20th century, gravitational phenomena such as the motion of bodies in the Solar System were described in terms of Isaac Newton's theory of gravitation, written down by Newton some 250 years earlier. The successes of Newton's theory were truly remarkable. Not only did it provide an explanation of the orbits of bodies about the Sun, but it also made dramatic predictions that were subsequently verified: the return of Halley's comet and the existence of the planet Neptune. What more could one ask of a theory?

Despite those successes, two problems were evident even during the 19th century. There was the famous problem of the precession of the perihelion of Mercury's orbit about the Sun and there was the problem that Newton's theory could not address the cosmology of the time: a homogeneous and static Universe. Seeliger (1895) modified Newtonian gravity at large distances and his resulting cosmological model was perhaps the best one could do at the time.

However, at the end of 1915, Einstein published the first definitive version of his general theory of relativity, which provided a new theory of gravitation and with that came new cosmological models that could be tested against observations and applied to cosmology.

Throughout the rest of the 20th century numerous tests of Einstein's theory have established its overall validity for describing gravitational phenomena.