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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| First observation of gravitational waves | 1/6 | https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves | reference | science, encyclopedia | 2026-05-05T10:01:16.701964+00:00 | kb-cron |
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 c2 M☉ (5.3+0.9−0.8×1047 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+0.5−0.4×1049 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 ====