Detection made by LIGO and Virgo interferometers (2015)
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 "c"2 M☉ (×1047 joules or foes) in total radiated as gravitational waves, reaching a peak emission rate in its final few milliseconds of about ×1049 watts – a level greater than the combined power of all light radiated by all the stars in the observable universe.
The observation confirmed the last remaining directly undetected prediction of general relativity and corroborated its predictions of space-time distortion in the context of large scale cosmic events (known as strong field tests). It was heralded as inaugurating a new era of gravitational-wave astronomy, which enables observations of violent astrophysical events that were not previously possible and allows for the direct observation of the earliest history of the universe. On 15 June 2016, two more detections of gravitational waves, made in late 2015, were announced. Eight more observations were made in 2017, including GW170817, the first observed merger of binary neutron stars, which was also observed in electromagnetic radiation.
Gravitational waves.
Albert Einstein predicted the existence of gravitational waves in 1916, on the basis of his theory of general relativity. General relativity interprets gravity as a consequence of distortions in spacetime caused by the presence of mass, and further entails that certain movements or acceleration of these masses will cause distortions – or "ripples" – in spacetime which spread outward from the source at the speed of light. Einstein considered this mostly a curiosity, since he understood that these ripples would be far too minuscule to detect using any technology foreseen at that time. As a further consequence following from the conservation of energy, the energy radiated away by gravitational waves from a system of two objects in mutual orbit would cause them to slowly spiral inwards, although again, this effect would be extremely minute and thus challenging to observe.
One case where gravitational waves would be strongest is during the final moments of the merger of two compact objects such as neutron stars or black holes. Over a span of millions of years, binary neutron stars, and binary black holes lose energy, largely through gravitational waves, and as a result, they spiral in towards each other. At the very end of this process, the two objects will reach extreme velocities, and in the final fraction of a second of their merger a substantial amount of their mass would theoretically be converted into gravitational energy, and travel outward as gravitational waves, allowing a greater than usual chance for detection. However, since little was known about the number of compact binaries in the universe and reaching that final stage can be very slow, there was little certainty as to how often such events might happen.
Observation.
Gravitational waves can be detected indirectly – by observing celestial phenomena caused by gravitational waves – or more directly by means of instruments such as the Earth-based LIGO or the planned space-based LISA instrument.
Indirect observation.
Evidence of gravitational waves was first deduced in 1974 through the motion of the double neutron star system PSR B1913+16, in which one of the stars is a pulsar that emits electro-magnetic pulses at radio frequencies at precise, regular intervals as it rotates. Russell Hulse and Joseph Taylor, who discovered the stars, also showed that over time, the frequency of pulses shortened, and that the stars were gradually spiralling towards each other with an energy loss that agreed closely with the predicted energy that would be radiated by gravitational waves. For this work, Hulse and Taylor were awarded the Nobel Prize in Physics in 1993. Further observations of this pulsar and others in multiple systems (such as the double pulsar system PSR J0737-3039) also agree with General Relativity to high precision.
Direct observation.
Direct observation of gravitational waves was not possible for many decades following their prediction, due to the minuscule effect that would need to be detected and separated from the background of vibrations present everywhere on Earth. A technique called interferometry was suggested in the 1960s and eventually technology developed sufficiently for this technique to become feasible.
In the present approach used by LIGO, a laser beam is split and the two halves are recombined after traveling different paths. Changes to the length of the paths or the time taken for the two split beams, caused by the effect of passing gravitational waves, to reach the point where they recombine are revealed as "beats". Such a technique is extremely sensitive to tiny changes in the distance or time taken to traverse the two paths. In theory, an interferometer with arms about 4 km long would be capable of revealing the change of space-time – a tiny fraction of the size of a single proton – as a gravitational wave of sufficient strength passed through Earth from elsewhere. This effect would be perceptible only to other interferometers of a similar size, such as the Virgo, GEO 600 and planned KAGRA and INDIGO detectors. In practice at least two interferometers would be needed because any gravitational wave would be detected at both of these, but other kinds of disturbances would generally not be present at both. This technique allows the sought-after signal to be distinguished from noise. This project was eventually founded in 1992 as the Laser Interferometer Gravitational-Wave Observatory (LIGO). The original instruments were upgraded between 2010 and 2015 (to Advanced LIGO), giving an increase of around 10 times their original sensitivity.
LIGO operates two gravitational-wave observatories in unison, located apart: the LIGO Livingston Observatory () in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site () near Richland, Washington. The tiny shifts in the length of their arms are continually compared and significant patterns which appear to arise synchronously are followed up to determine whether a gravitational wave may have been detected or if some other cause was responsible.
Initial LIGO operations between 2002 and 2010 did not detect any statistically significant events that could be confirmed as gravitational waves. This was followed by a multi-year shut-down while the detectors were replaced by much improved "Advanced LIGO" versions.  In February 2015, the two advanced detectors were brought into engineering mode, in which the instruments are operating fully for the purpose of testing and confirming they are functioning correctly before being used for research, with formal science observations due to begin on 18 September 2015.
Throughout the development and initial observations by LIGO, several "blind injections" of fake gravitational wave signals were introduced to test the ability of the researchers to identify such signals. To protect the efficacy of blind injections, only four LIGO scientists knew when such injections occurred, and that information was revealed only after a signal had been thoroughly analyzed by researchers. On 14 September 2015, while LIGO was running in engineering mode but without any blind data injections, the instrument reported a possible gravitational wave detection. The detected event was given the name GW150914.
GW150914 event.
Event detection.
GW150914 was detected by the LIGO detectors in Hanford, Washington state, and Livingston, Louisiana, USA, at 9:50:45 UTC on 14 September 2015. The LIGO detectors were operating in "engineering mode", meaning that they were operating fully but had not yet begun a formal "research" phase (which was due to commence three days later on 18 September), so initially there was a question as to whether the signals had been real detections or simulated data for testing purposes before it was ascertained that they were not tests.
The chirp signal lasted over 0.2 seconds, and increased in frequency and amplitude in about 8 cycles from 35 Hz to 250 Hz. The signal is in the audible range and has been described as resembling the "chirp" of a bird; astrophysicists and other interested parties the world over excitedly responded by imitating the signal on social media upon the announcement of the discovery. (The frequency increases because each orbit is noticeably faster than the one before during the final moments before merging.)
The trigger that indicated a possible detection was reported within three minutes of acquisition of the signal, using rapid ('online') search methods that provide a quick, initial analysis of the data from the detectors. After the initial automatic alert at 9:54 UTC, a sequence of internal emails confirmed that no scheduled or unscheduled injections had been made, and that the data looked clean. After this, the rest of the collaborating team was quickly made aware of the tentative detection and its parameters.
More detailed statistical analysis of the signal, and of 16 days of surrounding data from 12 September to 20 October 2015, identified GW150914 as a real event, with an estimated significance of at least 5.1 sigma or a confidence level of 99.99994%. Corresponding wave peaks were seen at Livingston seven milliseconds before they arrived at Hanford. Gravitational waves propagate at the speed of light, and the disparity is consistent with the light travel time between the two sites. The waves had traveled at the speed of light for more than a billion years.
At the time of the event, the Virgo gravitational wave detector (near Pisa, Italy) was offline and undergoing an upgrade; had it been online it would likely have been sensitive enough to also detect the signal, which would have greatly improved the positioning of the event. GEO600 (near Hannover, Germany) was not sensitive enough to detect the signal. Consequently, neither of those detectors was able to confirm the signal measured by the LIGO detectors.
Astrophysical origin.
The event happened at a luminosity distance of megaparsecs (determined by the amplitude of the signal), or billion light years, corresponding to a cosmological redshift of (90% credible intervals). Analysis of the signal along with the inferred redshift suggested that it was produced by the merger of two black holes with masses of times and times the mass of the Sun (in the source frame), resulting in a post-merger black hole of  M☉. The mass–energy of the missing  M☉ was radiated away in the form of gravitational waves.
During the final 20 milliseconds of the merger, the power of the radiated gravitational waves peaked at about or 526 dBm – 50 times greater than the combined power of all light radiated by all the stars in the observable universe. The amount of this energy that was received by the entire planet Earth was about 36 billion joules, of which only a small amount was absorbed.
Across the 0.2-second duration of the detectable signal, the relative tangential (orbiting) velocity of the black holes increased from 30% to 60% of the speed of light. The orbital frequency of 75 Hz (half the gravitational wave frequency) means that the objects were orbiting each other at a distance of only 350 km by the time they merged. The phase changes to the signal's polarization allowed calculation of the objects' orbital frequency, and taken together with the amplitude and pattern of the signal, allowed calculation of their masses and therefore their extreme final velocities and orbital separation