Gravity travels at the speed of light, meaning that a mass feels the
gravitational attraction of another mass as if the mass was where it was
r/c ago, where r is the distance between them. We feel the gravitational
attraction of say, alpha centauri, as if it was at its position 4.3 years ago,
so if it nova'd, we'd feel the change in gravity at the same time we saw the
nova.
Einstein did explain gravity as the curvature of spacetime. The geometric
rules caused by gravity, called the "metric", can be time dependant, thus there
can be propagating waves. Gravity waves are, according to theory, produced by
accelerating masses, just as electromagnetic waves (light) are produced by
accelerating charges. Any massive object in an orbit would thus emit gravity
waves.
Ted Baltz
[email removed]