## The discovery of gravitational waves

On 14 September 2015 at 09:50:45 UTC the two gravitational wave detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO)—one at Hanford, Washington and the other at Livingston, Louisiana—simultaneously observed a transient gravitational-wave signal. The signal exhibited the classic waveform predicted by Einstein’s general relativity theory for a binary black hole merger, sweeping up in frequency from 35 to 250 Hz, and exhibited a peak gravitational-wave strain of 1.0 × 10^{−}^{21} at the detectors.^{1}

The two detectors recorded the same signal, which matched the predicted waveform for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a statistical significance greater than 5.1σ (where 1σ represents 1 standard deviation).^{2} In other words, the detection is highly likely to be real.

The source lies at a luminosity distance of about 1.3 billion light-years corresponding to a redshift z ≈ 0.09.^{3} The two initial black hole masses were 36 M_{⊙} and 29 M_{⊙},^{4,5} and the final black hole mass is 62 M_{⊙}, with the equivalent of 3 M_{⊙} radiated as gravitational waves. The observations demonstrate for the first time the existence of a binary stellar-mass black hole system but, more importantly, **the first direct detection of gravitational waves and the first observation of a binary black hole merger.**