Ripples in the fabric of spacetime itself, traveling at the speed of light and caused by accelerating massive objects, particularly merging black holes or neutron stars. These waves stretch and compress space as they pass through, and were first directly detected in 2015.
The concept was predicted by Albert Einstein in 1916 as part of his general theory of relativity, combining 'gravitational' from Latin 'gravitas' (weight) and 'waves' describing the ripple-like propagation. Einstein himself doubted they could ever be detected due to their incredibly small effects.
Gravitational waves are the universe's most subtle messengers, carrying news of the most violent events across billions of light-years! When LIGO first detected them in 2015, it was like gaining a completely new sense - we went from being cosmically deaf to hearing the actual sounds of black holes colliding. These ripples in spacetime are so tiny that they change the length of LIGO's 4-kilometer arms by less than 1/10,000th the width of a proton, yet they've opened an entirely new window for exploring the universe.
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