In physics, gravitational waves are ripples in the curvature of spacetime that propagate as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1915 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as gravitational radiation. As objects with mass move around in spacetime, the curvature changes to reflect the changed locations of those objects. In certain circumstances, accelerating objects generate changes in this curvature, which propagate outwards at the speed of light in a wave-like manner. The illustration shows a pair of neutron stars orbiting each other very closely, sending gravitational waves pulsing along the 'skin' of spacetime.