According to the general theory of relativity, space itself is affected by the movement of massive objects. Like a ripple in a lake caused by a fallen rock, gravitational waves ripple out from the source of the motion and radiate through space, possibly affecting other objects.
Gravitational waves distort an object's gravitational field. This distortion can cause the object to change shape: a spherical configuration could change into an ellipsoid.
Gravitational waves can affect all of space. Since nothing can travel faster than the speed of light, all of space can not be affected all at once. Instead, the waves spread out and flow across space.
The strongest gravitational waves come from very massive, dense objects that move at high velocities. The collapse of a black hole could put out a large amount of gravitational waves. But even the strongest gravitational waves should be extremely weak by the time they reach the Earth. By the time they get here, only a quadrillionth of their original strength remains.
Gravitational waves are hard to detect.
Until the 1960s, there were no gravitational wave detectors. During that decade, Joseph Weber built the first gravitational wave detectors. They were made of massive aluminum cylinders, cooled to low temperatures. These were expected to oscillate in reaction to gravitational waves.
Since then, detectors are generally the same, except they are sometimes made of niobium instead of aluminum. Even with these detectors, no conclusive evidence for gravitational waves has been found.
Scientists have suggested several different ways to look for gravitational waves. One idea is to use the Earth and spacecraft as free particles. Then, an observer would only have to look for oscillations in the time it takes for radio signals to travel between the two points.