The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. Previously, gravitational waves had been inferred only indirectly, via their effect on the timing of pulsars in binary star systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of two black holes (of 36 M☉ and 29 M☉) and the subsequent ringdown of a single, 62 M☉ black hole remnant. The signal was named GW150914 (from gravitational wave and the date of observation 2015-09-14). It was also the first observation of a binary black hole merger, demonstrating both the existence of binary stellar-mass black hole systems and the fact that such mergers could occur within the current age of the universe.
This first direct observation was reported around the world as a remarkable accomplishment for many reasons. Efforts to directly prove the existence of such waves had been ongoing for over fifty years, and the waves are so minuscule that Albert Einstein himself doubted that they could ever be detected. The waves given off by the cataclysmic merger of GW150914 reached Earth as a ripple in spacetime that changed the length of a 1,120 km LIGO effective span by a thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width. The energy released by the binary as it spiralled together and merged was immense, with the energy of 3.0+0.5
−0.5 c M☉ (5.3+0.9
−0.8×10 joules or 5300+900
−800 foes) in total radiated as gravitational waves, reaching a peak emission rate in its final few milliseconds of about 3.6+0.5
−0.4×10 watts – a level greater than the combined power of all light radiated by all the stars in the observable universe.
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