A wave-like motion in a gravitational field, produced when a mass is accelerated or otherwise disturbed. Gravitational waves travel through spacetime at the speed of light, and their amplitude is proportional to the rate of acceleration of the body producing them. The strongest sources are those with the strongest gravitational fields although the waves, like the force of gravity itself, are very weak. Gravitational waves were first observed directly in 2015 September by the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States (see gravitational radiation).