Gravitational waves are waves of spacetime curvature produced by the relative motion of gravitating masses and which propagate away at the speed of light. They were first predicted by Albert Einstein as a consequence of his general theory of relativity, appearing as "ripples in spacetime curvature". Hundreds of these gravitational waves have since then been observed, first indirectly using binary-pulsar observations and, since 2015, directly through dedicated observatories.