Gravitational waves are ripples in the curvature of spacetime which propagate as waves, travelling outward from the source. They're caused when two massive objects bump together then send out shockwaves which squeeze and stretch the space and time around them.
Predicted in 1916 by Albert Einstein on the basis of his theory of general relativity, gravitational waves transport energy as gravitational radiation.
Before the direct detection of gravitational waves, there was indirect evidence for their existence. For example, measurements of the Hulse–Taylor binary system suggest that gravitational waves are more than a hypothetical concept. Potential sources of detectable gravitational waves include binary star systems composed of white dwarfs, neutron stars, and black holes. Various gravitational-wave are under construction or in operation, such as Advanced LIGO which began observations in September 2015.
On 11 February 2016, the LIGO and Virgo collaborations announced the first observation of gravitational waves.The waveform, detected on 14 September 2015 by Marco Drago, physicist at the Albert Einstein Institute in Hannover, Germany, matched the predictions of general relativity for the inward spiral and mergerof a pair of black holes and subsequent 'ringdown' of the resulting single black hole. The event happened at a distance of 1.3 billion light years where the two black holes circle one another. Both are about 100km across. One contains 36 times as much mass as the sun; the other, 29. They are locked in an orbital dance, a kilometre or so apart, that is accelerating rapidly to within a whisker of the speed of light. The signal was named GW150914.
This was the first observation of a binary black hole merger, demonstrating the existence of binary stellar-mass black hole systems, and that such mergers could occur within the current epoch of the Universe.
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