Einstein found that the simplest way to include all effects is to replace gravity by space-time curvature. Thus gravity is reduced to geometry. Objects follow a geodesic (shortest possible) path in curved space-time.

In 1893, Oliver Heaviside postulated the existence of gravitational radiation, based on an analogy with electromagnetic radiation.  Einstein, using his new theory of gravitation, predicted the existence of gravitational radiation in 1916, a century ago. In 1957, Feynman and Bondi pointed out that such radiation is in principle detectable, and five years later it was suggested that the best bet for observing it would be by using an interferometer. In 1994, construction began on two LIGO observatories, which became operational in 2002. After several upgrades of sensitivity, LIGO detected an enormous pulse of gravitational radiation from two merging black holes, on Sept. 14, 2015.  A number of later mergers have been observed.  One binary neutron star collision has also been observed.

The first event observed so far, in a tiny fraction of a second, radiated 3 solar masses worth of energy! This is a power output about 50 times greater than the total summed optical output of all stars in the observable universe. The second event radiated one solar mass.  As more advanced detectors are being constructed, worldwide, a totally new area of astronomy has begun! LISA is a system of three satellites that will form a space-based gravitational wave observatory.  Table of LIGO observations to date.