Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of massive objects, such as merging black holes or neutron stars. These waves travel at the speed of light and can stretch and compress space as they pass through it, creating tiny changes in distance that can be detected by sensitive instruments. The existence of gravitational waves was first predicted by Albert Einstein in 1916 as part of his General Theory of Relativity. They were directly observed for the first time in 2015 by the LIGO observatory, confirming Einstein's theory and opening a new way to observe the universe.