Gravitational waves are tiny ripples in the fabric of space-time. They are caused by violent and energetic events across the Universe, and they travel at the speed of light. Albert Einstein predicted the existence of gravitational waves in 1916, in his general theory of relativity.
Einstein's work in mathematics showed that massive accelerating objects, such as neutron stars or black holes, would disturb space-time. In a similar way, stones thrown into a pond disturb the surface of the water.
Gravitational waves are observed by measuring tiny changes to the distance between two detectors, as the wave passes through. A laser beam shines from one detector to the other. As space-time is squeezed, the distance between the detectors decreases very slightly. As space-time is stretched, the distance increases very slightly.
On 14th September 2015, the LIGO and Virgo Collaborations announced the first detection of gravitational waves. They originated from a pair of merging black holes. Since this initial announcement, LIGO has confirmed two more gravitational wave detections. There are currently several new gravitational wave detectors under construction. LISA will be the first space-based gravitational wave detector.