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{{short description|Gravitational wave detector}}{{For|the Latvian holiday|LÄ«go}}{{Distinguish|LigoLab}}{{Use dmy dates|date=September 2019}}

factoids

LIGO Livingston Observatory: {{Coord33N46Wdisplay=inline|name=LIGO Livingston Observatory}}| locmapin = noHanford Site, Washington (state)>Washington and Livingston, Louisiana, US USA|relief=1| border=infobox| width = 300| float = right| caption = LIGO observatories in the Contiguous United States| label = LIGO Livingston Observatory }}}}The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool.JOURNAL, Barish, Barry C., Weiss, Rainer, LIGO and the Detection of Gravitational Waves, Physics Today, October 1999, 52, 10, 44, 10.1063/1.882861, 1999PhT....52j..44B, Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These observatories use mirrors spaced four kilometers apart which are capable of detecting a change{{clarify|date=February 2024}} of less than one ten-thousandth the charge diameter of a proton.WEB, Facts,weblink LIGO, This is equivalent to measuring the distance from Earth to the nearest star to an accuracy smaller than the width of a human hair!, 24 August 2017,weblink 4 July 2017, dead, (that is, to Proxima Centauri at {{val|4.0208|e=13|u= km}}).The initial LIGO observatories were funded by the United States National Science Foundation (NSF) and were conceived, built and are operated by Caltech and MIT.WEB, LIGO Lab Caltech MIT,weblink 24 June 2016, WEB, LIGO MIT,weblink 24 June 2016, They collected data from 2002 to 2010 but no gravitational waves were detected.The Advanced LIGO Project to enhance the original LIGO detectors began in 2008 and continues to be supported by the NSF, with important contributions from the United Kingdom's Science and Technology Facilities Council, the Max Planck Society of Germany, and the Australian Research Council.WEB, Major research project to detect gravitational waves is underway,weblink University of Birmingham News, University of Birmingham, 28 November 2015, JOURNAL, Shoemaker, David, The evolution of Advanced LIGO, LIGO Magazine, 2012, 1, 8,weblink The improved detectors began operation in 2015. The detection of gravitational waves was reported in 2016 by the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration with the international participation of scientists from several universities and research institutions. Scientists involved in the project and the analysis of the data for gravitational-wave astronomy are organized by the LSC, which includes more than 1000 scientists worldwide,WEB, Revolutionary Grassroots Astrophysics Project "Einstein@Home" Goes Live,weblink 3 March 2016, WEB, LSC/Virgo Census,weblink myLIGO, 28 November 2015, as well as 440,000 active Einstein@Home users {{as of|2016|December|lc=yes}}.WEB,weblink BOINCstats project statistics, 14 December 2016, LIGO is the largest and most ambitious project ever funded by the NSF.Larger physics projects in the United States, such as Fermilab, have traditionally been funded by the Department of Energy.WEB, LIGO: The Search for Gravitational Waves,weblink www.nsf.gov, National Science Foundation, 3 September 2018, en, 15 September 2016,weblink dead, In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry C. Barish "for decisive contributions to the LIGO detector and the observation of gravitational waves".WEB, The Nobel Prize in Physics 2017, Nobel Foundation,weblink Observations are made in "runs". {{As of|2022|January|df=US}}, LIGO has made three runs (with one of the runs divided into two "subruns"), and made 90 detections of gravitational waves.WEB,weblink LSC News, WEB,weblink LSC News, Maintenance and upgrades of the detectors are made between runs. The first run, O1, which ran from 12 September 2015 to 19 January 2016, made the first three detections, all black hole mergers. The second run, O2, which ran from 30 November 2016 to 25 August 2017, made eight detections: seven black hole mergers and the first neutron star merger.JOURNAL, The LIGO Scientific Collaboration, the Virgo Collaboration, Abbott, B. P., Abbott, R., Abbott, T. D., Abraham, S., Acernese, F., Ackley, K., Adams, C., Adhikari, R. X., Adya, V. B., 2019-09-04, GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs, Physical Review X, 9, 3, 031040, 10.1103/PhysRevX.9.031040, 2160-3308, 1811.12907, 2019PhRvX...9c1040A, 119366083, The third run, O3, began on 1 April 2019; it was divided into O3a, from 1 April to 30 September 2019, and O3b, from 1 November 2019WEB,weblink Welcome to O3b!, LIGO, 2019-11-01, @ligo, en, 2019-11-11, until it was suspended on 27 March 2020 due to COVID-19.WEB,weblink LIGO Suspends Third Observing Run (O3), 26 March 2020, 15 July 2020, The O3 run made the first detection of a merger of a neutron star with a black hole.The gravitational wave observatories LIGO, Virgo in Italy, and KAGRA in Japan are coordinating to continue observations after the COVID-caused stop, and LIGO's O4 observing run started on 24 May 2023.WEB, 24 May 2023, Gravitational-Wave Observatory Status,weblink 2023-05-25, Gravitational Wave Open Science Center, JOURNAL, Castelvecchi, Davide, 2023-05-24, Gravitational-wave detector LIGO is back — and can now spot more colliding black holes than ever,weblink Nature, 618, 7963, 13–14, en, 10.1038/d41586-023-01732-4, 37225822, 2023Natur.618...13C, 258899900, LIGO projects a sensitivity goal of 160–190 Mpc for binary neutron star mergers (sensitivities: Virgo 80–115 Mpc, KAGRA greater than 1 Mpc).WEB, LIGO, VIRGO AND KAGRA OBSERVING RUN PLANS,weblink 14 December 2021,

History

Background

(File:LIGO Hanford aerial 05.jpg|thumb|right|LIGO Hanford Observatory)(File:Ligo-livingston-aerial-03 599x400.jpg|thumb|right|LIGO Livingston Observatory)The LIGO concept built upon early work by many scientists to test a component of Albert Einstein's theory of general relativity, the existence of gravitational waves. Starting in the 1960s, American scientists including Joseph Weber, as well as Soviet scientists Mikhail Gertsenshtein and Vladislav Pustovoit, conceived of basic ideas and prototypes of laser interferometry,BOOK, National Academies Press, 109–117, 978-0-309-09084-1,weblink 2004splr.rept.....C, 10.17226/10895, Setting Priorities for Large Research Facility Projects Supported by the National Science Foundation, 2004, JOURNAL, Gertsenshtein, M.E., 1962, Wave Resonance of Light and Gravitational Waves, Journal of Experimental and Theoretical Physics, 14, 84, and in 1967 Rainer Weiss of MIT published an analysis of interferometer use and initiated the construction of a prototype with military funding, but it was terminated before it could become operational.JOURNAL, 1972, Quarterly Progress Report of the Research Laboratory of Electronics, Weiss, Rainer, Electromagnetically coupled broadband gravitational wave antenna, 105, 54, 84,weblink 21 February 2016, Starting in 1968, Kip Thorne initiated theoretical efforts on gravitational waves and their sources at Caltech, and was convinced that gravitational wave detection would eventually succeed.Prototype interferometric gravitational wave detectors (interferometers) were built in the late 1960s by Robert L. Forward and colleagues at Hughes Research Laboratories (with mirrors mounted on a vibration isolated plate rather than free swinging), and in the 1970s (with free swinging mirrors between which light bounced many times) by Weiss at MIT, and then by Heinz Billing and colleagues in Garching Germany, and then by Ronald Drever, James Hough and colleagues in Glasgow, Scotland.WEB, A brief history of LIGO, ligo.caltech.edu,weblink 21 February 2016,weblink 3 July 2017, dead, In 1980, the NSF funded the study of a large interferometer led by MIT (Paul Linsay, Peter Saulson, Rainer Weiss), and the following year, Caltech constructed a 40-meter prototype (Ronald Drever and Stan Whitcomb). The MIT study established the feasibility of interferometers at a 1-kilometer scale with adequate sensitivity.JOURNAL, Buderi, Robert, 19 September 1988, Going after gravity: How a high-risk project got funded.,weblink The Scientist, 2, 17, 1, 18 February 2016, Under pressure from the NSF, MIT and Caltech were asked to join forces to lead a LIGO project based on the MIT study and on experimental work at Caltech, MIT, Glasgow, and Garching. Drever, Thorne, and Weiss formed a LIGO steering committee, though they were turned down for funding in 1984 and 1985. By 1986, they were asked to disband the steering committee and a single director, Rochus E. Vogt (Caltech), was appointed. In 1988, a research and development proposal achieved funding.JOURNAL, Mervis, Jeffery, Funding of two science labs receives pork barrel vs beer peer review debate.,weblink The Scientist, 5, 23, 21 February 2016, JOURNAL, Waldrop, M. Mitchell, 7 September 1990, Of politics, pulsars, death spirals – and LIGO, Science, 249, 4973, 1106–1108, 10.1126/science.249.4973.1106, 17831979, 1990Sci...249.1106W, WEB,weblink Gravitational waves detected 100 years after Einstein's prediction, 11 February 2016, LIGO, 11 February 2016, JOURNAL, Irion, Robert, 21 April 2000, LIGO's mission of gravity., Science, 288, 5465, 420–423, 10.1126/science.288.5465.420, 119020354, From 1989 through 1994, LIGO failed to progress technically and organizationally. Only political efforts continued to acquire funding.WEB,weblink Interview with Barry Barish, Shirley Cohen, Caltech, 1998, 21 February 2016, Ongoing funding was routinely rejected until 1991, when the U.S. Congress agreed to fund LIGO for the first year for $23 million. However, requirements for receiving the funding were not met or approved, and the NSF questioned the technological and organizational basis of the project. By 1992, LIGO was restructured with Drever no longer a direct participant.CONFERENCE, Cook, Victor, 21 September 2001, NSF Management and Oversight of LIGO, Large Facility Projects Best Practices Workshop, NSF, JOURNAL, Travis, John, 18 February 2016, LIGO: A$250 million gamble., Science, 260, 5108, 612–614, 10.1126/science.260.5108.612, 17812204, 1993Sci...260..612T, Ongoing project management issues and technical concerns were revealed in NSF reviews of the project, resulting in the withholding of funds until they formally froze spending in 1993.JOURNAL, Anderson, Christopher, 11 March 1994, LIGO director out in shakeup, Science, 263, 5152, 1366, 10.1126/science.263.5152.1366, 17776497, 1994Sci...263.1366A, NEWS, Browne, Malcolm W., 30 April 1991, Experts clash over project to detect gravity wave.,weblink New York Times, 21 February 2016, In 1994, after consultation between relevant NSF personnel, LIGO's scientific leaders, and the presidents of MIT and Caltech, Vogt stepped down and Barry Barish (Caltech) was appointed laboratory director,JOURNAL, Anderson, Christopher, 11 March 1994, LIGO director out in shakeup, Science, 263, 5152, 1366, 10.1126/science.263.5152.1366, 17776497, 1994Sci...263.1366A, and the NSF made clear that LIGO had one last chance for support. Barish's team created a new study, budget, and project plan with a budget exceeding the previous proposals by 40%. Barish proposed to the NSF and National Science Board to build LIGO as an evolutionary detector, where detection of gravitational waves with initial LIGO would be possible, and with advanced LIGO would be probable.{{citation |title=Physics: Wave of the future|journal=Nature |volume=511 |issue=7509 |pages=278–81 |first=Alexandra |last=Witze|date=16 July 2014 |bibcode=2014Natur.511..278W |doi=10.1038/511278a |pmid=25030149 |doi-access=free }} This new proposal received NSF funding, Barish was appointed Principal Investigator, and the increase was approved. In 1994, with a budget of US$395 million, LIGO stood as the largest overall funded NSF project in history. The project broke ground in Hanford, Washington in late 1994 and in Livingston, Louisiana in 1995. As construction neared completion in 1997, under Barish's leadership two organizational institutions were formed, LIGO Laboratory and LIGO Scientific Collaboration (LSC). The LIGO laboratory consists of the facilities supported by the NSF under LIGO Operation and Advanced R&D; this includes administration of the LIGO detector and test facilities. The LIGO Scientific Collaboration is a forum for organizing technical and scientific research in LIGO. It is a separate organization from LIGO Laboratory with its own oversight. Barish appointed Weiss as the first spokesperson for this scientific collaboration.

Observations begin

Initial LIGO operations between 2002 and 2010 did not detect any gravitational waves. In 2004, under Barish, the funding and groundwork were laid for the next phase of LIGO development (called "Enhanced LIGO"). This was followed by a multi-year shut-down while the detectors were replaced by much improved "Advanced LIGO" versions.WEB, Gravitational wave detection a step closer with Advanced LIGO,weblink SPIE Newsroom, 4 January 2016, WEB, Daniel Sigg: The Advanced LIGO Detectors in the era of First Discoveries,weblink SPIE Newsroom, 9 September 2016, Much of the research and development work for the LIGO/aLIGO machines was based on pioneering work for the GEO600 detector at Hannover, Germany.NEWS, Ghosh, Pallab,weblink Einstein's gravitational waves 'seen' from black holes, BBC News, 11 February 2016, 18 February 2016, WEB, Gravitational waves detected 100 years after Einstein's prediction,weblink www.mpg.de, Max-Planck-Gelschaft, 3 September 2018, en, By February 2015, the detectors were brought into engineering mode in both locations.WEB, LIGO Hanford's H1 Achieves Two-Hour Full Lock,weblink February 2015, dead,weblink 22 September 2015, By mid-September 2015, "the world's largest gravitational-wave facility" completed a five-year US$200-million overhaul at a total cost of $620 million.{{citation |title=Hunt for gravitational waves to resume after massive upgrade: LIGO experiment now has better chance of detecting ripples in space-time|journal=Nature |volume=525 |issue=7569 |pages=301–302 |first=Davide |last=Castelvecchi|date=15 September 2015 |doi=10.1038/525301a |pmid=26381963 |bibcode=2015Natur.525..301C |doi-access=free }}MAGAZINE, Sarah, Zhang, The Long Search for Elusive Ripples in Spacetime, Wired,weblink 15 September 2015, On 18 September 2015, Advanced LIGO began its first formal science observations at about four times the sensitivity of the initial LIGO interferometers.NEWS, Amos, Jonathan, Advanced Ligo: Labs 'open their ears' to the cosmos,weblink BBC News, 19 September 2015, 19 September 2015, Its sensitivity was to be further enhanced until it was planned to reach design sensitivity {{As of|alt=around 2021|2021|01|post=.}}WEB, Planning for a bright tomorrow: prospects for gravitational-wave astronomy with Advanced LIGO and Advanced Virgo,weblink LIGO Scientific Collaboration, 31 December 2015, 23 December 2015,

Detections

On 11 February 2016, the LIGO Scientific Collaboration and Virgo Collaboration published a paper about the detection of gravitational waves, from a signal detected at 09.51 UTC on 14 September 2015 of two ~30 solar mass black holes merging about 1.3 billion light-years from Earth.JOURNAL, Observation of Gravitational Waves from a Binary Black Hole Merger, Physical Review Letters, 116, 6, 061102, 11 February 2016, LIGO Scientific Collaboration and Virgo Collaboration, B. P. Abbott, 10.1103/PhysRevLett.116.061102, 26918975, 1602.03837, 2016PhRvL.116f1102A, 124959784, Current executive director David Reitze announced the findings at a media event in Washington D.C., while executive director emeritus Barry Barish presented the first scientific paper of the findings at CERN to the physics community.BOOK,weblink New results on the Search for Gravitational Waves, 2016, CERN Colloquium, On 2 May 2016, members of the LIGO Scientific Collaboration and other contributors were awarded a Special Breakthrough Prize in Fundamental Physics for contributing to the direct detection of gravitational waves.WEB, Fundamental Physics Prize – News,weblink Fundamental Physics Prize (2016), 4 May 2016, On 16 June 2016 LIGO announced a second signal was detected from the merging of two black holes with 14.2 and 7.5 times the mass of the Sun. The signal was picked up on 26 December 2015, at 3:38 UTC.The detection of a third black hole merger, between objects of 31.2 and 19.4 solar masses, occurred on 4 January 2017 and was announced on 1 June 2017.JOURNAL, 10.1103/PhysRevLett.118.221101, 28621973, GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2, Physical Review Letters, 1 June 2017, B. P. Abbott, etal, LIGO Scientific Collaboration and Virgo interferometer, Virgo Collaboration, 118, 22, 221101, 1706.01812, 2017PhRvL.118v1101A, 206291714, JOURNAL, Conover, E., LIGO snags another set of gravitational waves, Science News, 1 June 2017,weblink 3 June 2017, Laura Cadonati was appointed the first deputy spokesperson.WEB,weblink College of Sciences Professor Appointed to Top Role in Search for Gravitational Waves | News Center, A fourth detection of a black hole merger, between objects of 30.5 and 25.3 solar masses, was observed on 14 August 2017 and was announced on 27 September 2017.WEB, GW170814 : A three-detector observation of gravitational waves from a binary black hole coalescence,weblink 29 September 2017, In 2017, Weiss, Barish, and Thorne received the Nobel Prize in Physics "for decisive contributions to the LIGO detector and the observation of gravitational waves." Weiss was awarded one-half of the total prize money, and Barish and Thorne each received a one-quarter prize.WEB, The Nobel Prize in Physics 2017,weblink Nobelprize.org, 4 October 2017, NEWS, Rincon, Paul, Amos, Jonathan,weblink Einstein's waves win Nobel Prize, BBC News, 3 October 2017, 3 October 2017, NEWS, Overbye, Dennis, Dennis Overbye, 2017 Nobel Prize in Physics Awarded to LIGO Black Hole Researchers,weblink 3 October 2017, The New York Times, 3 October 2017, After shutting down for improvements, LIGO resumed operation on 26 March 2019, with Virgo joining the network of gravitational-wave detectors on 1 April 2019.WEB,weblink LSC News, Both ran until 27 March 2020, when the COVID-19 pandemic halted operations. During the COVID shutdown, LIGO underwent a further upgrade in sensitivity, and observing run O4 with the new sensitivity began on 24 May 2023.

Mission

File:LIGO detector sensitivity curve.png|thumb|300px|upright=2| Detector noise curves for Initial and Advanced LIGO as a function of frequency. They lie above the bands for space-borne detectors like the evolved Laser Interferometer Space Antenna (eLISA) and pulsar timing arrays such as the European Pulsar Timing Array (EPTA). The characteristic strains of potential astrophysical sources are also shown. To be detectable the characteristic strain of a signal must be above the noise curve.WEB, Gravitational Wave Detectors and Sources,weblink 20 April 2014, Moore, Christopher, Cole, Robert, Berry, Christopher, 19 July 2013, These frequencies that aLIGO can detect are in the range of human hearing.]]LIGO's mission is to directly observe gravitational waves of cosmic origin. These waves were first predicted by Einstein's general theory of relativity in 1916, when the technology necessary for their detection did not yet exist. Their existence was indirectly confirmed when observations of the binary pulsar PSR 1913+16 in 1974 showed an orbital decay which matched Einstein's predictions of energy loss by gravitational radiation. The Nobel Prize in Physics 1993 was awarded to Hulse and Taylor for this discovery.WEB, The Nobel Prize in Physics 1993: Russell A. Hulse, Joseph H. Taylor Jr.,weblink nobelprize.org, Direct detection of gravitational waves had long been sought. Their discovery has launched a new branch of astronomy to complement electromagnetic telescopes and neutrino observatories. Joseph Weber pioneered the effort to detect gravitational waves in the 1960s through his work on resonant mass bar detectors. Bar detectors continue to be used at six sites worldwide. By the 1970s, scientists including Rainer Weiss realized the applicability of laser interferometry to gravitational wave measurements. Robert Forward operated an interferometric detector at Hughes in the early 1970s.WEB, Obituary: Dr. Robert L. Forward,weblinkweblink" title="archive.today/20130902222420weblink">weblink dead, 2 September 2013, www.spaceref.com, 21 September 2002, 3 September 2018, en, In fact as early as the 1960s, and perhaps before that, there were papers published on wave resonance of light and gravitational waves.JOURNAL, M.E. Gertsenshtein, Wave Resonance of Light and Gravitational Waves, JETP, 41, 1, 113–114, 1961,weblink 19 January 2016, 6 February 2016,weblink" title="web.archive.org/web/20160206055044weblink">weblink dead, Work was published in 1971 on methods to exploit this resonance for the detection of high-frequency gravitational waves. In 1962, M. E. Gertsenshtein and V. I. Pustovoit published the very first paper describing the principles for using interferometers for the detection of very long wavelength gravitational waves.JOURNAL, On the detection of low frequency gravitational waves, M. E., Gertsenshtein, V. I., Pustovoit, Journal of Experimental and Theoretical Physics, JETP, 43, 605–607, August 1962, The authors argued that by using interferometers the sensitivity can be 107 to 1010 times better than by using electromechanical experiments. Later, in 1965, Braginsky extensively discussed gravitational-wave sources and their possible detection. He pointed out the 1962 paper and mentioned the possibility of detecting gravitational waves if the interferometric technology and measuring techniques improved.Since the early 1990s, physicists have thought that technology has evolved to the point where detection of gravitational waves—of significant astrophysical interest—is now possible.JOURNAL, Astrophysical Sources of Gravitational Radiation, Annual Review of Nuclear and Particle Science, 44, 44, 655–717, 10.1146/annurev.ns.44.120194.003255, free, 1994, Bonazzola, S, Marck, J A, 1994ARNPS..44..655B, In August 2002, LIGO began its search for cosmic gravitational waves. Measurable emissions of gravitational waves are expected from binary systems (collisions and coalescences of neutron stars or black holes), supernova explosions of massive stars (which form neutron stars and black holes), accreting neutron stars, rotations of neutron stars with deformed crusts, and the remnants of gravitational radiation created by the birth of the universe. The observatory may, in theory, also observe more exotic hypothetical phenomena, such as gravitational waves caused by oscillating cosmic strings or colliding domain walls.

Observatories

LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory ({{Coord|30|33|46.42|N|90|46|27.27|W}}) in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site ({{Coord|46|27|18.52|N|119|24|27.56|W|}}), located near Richland, Washington. These sites are separated by 3,002 kilometers (1,865 miles) straight line distance through the earth, but 3,030 kilometers (1,883 miles) over the surface. Since gravitational waves are expected to travel at the speed of light, this distance corresponds to a difference in gravitational wave arrival times of up to ten milliseconds. Through the use of trilateration, the difference in arrival times helps to determine the source of the wave, especially when a third similar instrument like Virgo, located at an even greater distance in Europe, is added.WEB, Location of the Source,weblink Gravitational Wave Astrophysics, University of Birmingham, 28 November 2015, dead,weblink" title="web.archive.org/web/20151208133633weblink">weblink 8 December 2015, Each observatory supports an L-shaped ultra high vacuum system, measuring four kilometers (2.5 miles) on each side. Up to five interferometers can be set up in each vacuum system.The LIGO Livingston Observatory houses one laser interferometer in the primary configuration. This interferometer was successfully upgraded in 2004 with an active vibration isolation system based on hydraulic actuators providing a factor of 10 isolation in the 0.1–5 Hz band. Seismic vibration in this band is chiefly due to microseismic waves and anthropogenic sources (traffic, logging, etc.).The LIGO Hanford Observatory houses one interferometer, almost identical to the one at the Livingston Observatory. During the Initial and Enhanced LIGO phases, a half-length interferometer operated in parallel with the main interferometer. For this 2 km interferometer, the Fabry–Pérot arm cavities had the same optical finesse, and, thus, half the storage time as the 4 km interferometers. With half the storage time, the theoretical strain sensitivity was as good as the full length interferometers above 200 Hz but only half as good at low frequencies. During the same era, Hanford retained its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.

Operation

{{gravitational_wave_observatory_principle.svg}}The parameters in this section refer to the Advanced LIGO experiment. The primary interferometer consists of two beam lines of 4 km length which form a power-recycled Michelson interferometer with Gires–Tournois etalon arms. A pre-stabilized 1064 nm emits a beam with a power of 20 W that passes through a power recycling mirror. The mirror fully transmits light incident from the laser and reflects light from the other side increasing the power of the light field between the mirror and the subsequent beam splitter to 700 W. From the beam splitter the light travels along two orthogonal arms. By the use of partially reflecting mirrors, Fabry–Pérot cavities are created in both arms that increase the effective path length of laser light in the arm from 4 km to approximately 1,200 km.WEB, LIGO's Interferometer,weblink The power of the light field in the cavity is 100 kW.When a gravitational wave passes through the interferometer, the spacetime in the local area is altered. Depending on the source of the wave and its polarization, this results in an effective change in length of one or both of the cavities. The effective length change between the beams will cause the light currently in the cavity to become very slightly out of phase (antiphase) with the incoming light. The cavity will therefore periodically get very slightly out of coherence and the beams, which are tuned to destructively interfere at the detector, will have a very slight periodically varying detuning. This results in a measurable signal.WEB, Thorne, Kip, Kip Thorne, Chapter 27.6: The Detection of Gravitational Waves (in "Applications of Classical Physics chapter 27: Gravitational Waves and Experimental Tests of General Relativity", Caltech lecture notes),weblink 2012, 11 February 2016, After an equivalent of approximately 280 trips down the 4 km length to the far mirrors and back again,WEB, LIGO's Interferometer,weblink the two separate beams leave the arms and recombine at the beam splitter. The beams returning from two arms are kept out of phase so that when the arms are both in coherence and interference (as when there is no gravitational wave passing through), their light waves subtract, and no light should arrive at the photodiode. When a gravitational wave passes through the interferometer, the distances along the arms of the interferometer are shortened and lengthened, causing the beams to become slightly less out of phase. This results in the beams coming in phase, creating a resonance, hence some light arrives at the photodiode and indicates a signal. Light that does not contain a signal is returned to the interferometer using a power recycling mirror, thus increasing the power of the light in the arms. In actual operation, noise sources can cause movement in the optics, producing similar effects to real gravitational wave signals; a great deal of the art and complexity in the instrument is in finding ways to reduce these spurious motions of the mirrors.NEWS, Doughton, Sandi, Suddenly there came a tapping: Ravens cause blips in massive physics instrument at Hanford,weblink 14 May 2018, The Seattle Times, 14 May 2018, Background noise and unknown errors (which happen daily) are in the order of 10−20, while gravitational wave signals are around 10−22. After noise reduction, a signal-to-noise ratio around 20 can be achieved, or higher when combined with other gravitational wave detectors around the world.JOURNAL, Abbott BP, Abbott R, Abbott TD, Acernese F, Ackley K, Adams C, Adams T, Addesso P, Adhikari RX, Adya VB, Affeldt C, Afrough M, Agarwal B, Agathos M, Agatsuma K, Aggarwal N, Aguiar OD, Aiello L, Ain A, Ajith P, Allen B, Allen G, Allocca A, Altin PA, Amato A, Ananyeva A, Anderson SB, Anderson WG, Angelova SV, Antier S, Appert S, Arai K, Araya MC, Areeda JS, Arnaud N, Arun KG, Ascenzi S, Ashton G, Ast M, Aston SM, Astone P, Atallah DV, Aufmuth P, Aulbert C, AultONeal K, Austin C, Avila-Alvarez A, Babak S, Bacon P, Bader MK, Bae S, Bailes M, Baker PT, Baldaccini F, Ballardin G, Ballmer SW, Banagiri S, Barayoga JC, Barclay SE, Barish BC, Barker D, Barkett K, Barone F, Barr B, Barsotti L, Barsuglia M, Barta D, Barthelmy SD, Bartlett J, Bartos I, Bassiri R, Basti A, Batch JC, Bawaj M, Bayley JC, Bazzan M, Bécsy B, Beer C, Bejger M, Belahcene I, Bell AS, Berger BK, Bergmann G, Bernuzzi S, Bero JJ, Berry CP, Bersanetti D, Bertolini A, Betzwieser J, Bhagwat S, Bhandare R, Bilenko IA, Billingsley G, Billman CR, Birch J, Birney R, Birnholtz O, Biscans S, Biscoveanu S, Bisht A, Bitossi M, Biwer C, Bizouard MA, Blackburn JK, Blackman J, Blair CD, Blair DG, Blair RM, Bloemen S, Bock O, Bode N, Boer M, Bogaert G, Bohe A, Bondu F, Bonilla E, Bonnand R, Boom BA, Bork R, Boschi V, Bose S, Bossie K, Bouffanais Y, Bozzi A, Bradaschia C, Brady PR, Branchesi M, Brau JE, Briant T, Brillet A, Brinkmann M, Brisson V, Brockill P, Broida JE, Brooks AF, Brown DA, Brown DD, Brunett S, Buchanan CC, Buikema A, Bulik T, Bulten HJ, Buonanno A, Buskulic D, Buy C, Byer RL, Cabero M, Cadonati L, Cagnoli G, Cahillane C, Calderón Bustillo J, Callister TA, Calloni E, Camp JB, Canepa M, Canizares P, Cannon KC, Cao H, Cao J, Capano CD, Capocasa E, Carbognani F, Caride S, Carney MF, Carullo G, Casanueva Diaz J, Casentini C, Caudill S, Cavaglià M, Cavalier F, Cavalieri R, Cella G, Cepeda CB, Cerdá-Durán P, Cerretani G, Cesarini E, Chamberlin SJ, Chan M, Chao S, Charlton P, Chase E, Chassande-Mottin E, Chatterjee D, Chatziioannou K, Cheeseboro BD, Chen HY, Chen X, Chen Y, Cheng HP, Chia H, Chincarini A, Chiummo A, Chmiel T, Cho HS, Cho M, Chow JH, Christensen N, Chu Q, Chua AJ, Chua S, Chung AK, Chung S, Ciani G, Ciolfi R, Cirelli CE, Cirone A, Clara F, Clark JA, Clearwater P, Cleva F, Cocchieri C, Coccia E, Cohadon PF, Cohen D, Colla A, Collette CG, Cominsky LR, Constancio M, Conti L, Cooper SJ, Corban P, Corbitt TR, Cordero-Carrión I, Corley KR, Cornish N, Corsi A, Cortese S, Costa CA, Coughlin MW, Coughlin SB, Coulon JP, Countryman ST, Couvares P, Covas PB, Cowan EE, Coward DM, Cowart MJ, Coyne DC, Coyne R, Creighton JD, Creighton TD, Cripe J, Crowder SG, Cullen TJ, Cumming A, Cunningham L, Cuoco E, Dal Canton T, Dálya G, Danilishin SL, D'Antonio S, Danzmann K, Dasgupta A, Da Silva Costa CF, Dattilo V, Dave I, Davier M, Davis D, Daw EJ, Day B, De S, DeBra D, Degallaix J, De Laurentis M, Deléglise S, Del Pozzo W, Demos N, Denker T, Dent T, De Pietri R, Dergachev V, De Rosa R, DeRosa RT, De Rossi C, DeSalvo R, de Varona O, Devenson J, Dhurandhar S, Díaz MC, Dietrich T, Di Fiore L, Di Giovanni M, Di Girolamo T, Di Lieto A, Di Pace S, Di Palma I, Di Renzo F, Doctor Z, Dolique V, Donovan F, Dooley KL, Doravari S, Dorrington I, Douglas R, Dovale Álvarez M, Downes TP, Drago M, Dreissigacker C, Driggers JC, Du Z, Ducrot M, Dudi R, Dupej P, Dwyer SE, Edo TB, Edwards MC, Effler A, Eggenstein HB, Ehrens P, Eichholz J, Eikenberry SS, Eisenstein RA, Essick RC, Estevez D, Etienne ZB, Etzel T, Evans M, Evans TM, Factourovich M, Fafone V, Fair H, Fairhurst S, Fan X, Farinon S, Farr B, Farr WM, Fauchon-Jones EJ, Favata M, Fays M, Fee C, Fehrmann H, Feicht J, Fejer MM, Fernandez-Galiana A, Ferrante I, Ferreira EC, Ferrini F, Fidecaro F, Finstad D, Fiori I, Fiorucci D, Fishbach M, Fisher RP, Fitz-Axen M, Flaminio R, Fletcher M, Fong H, Font JA, Forsyth PW, Forsyth SS, Fournier JD, Frasca S, Frasconi F, Frei Z, Freise A, Frey R, Frey V, Fries EM, Fritschel P, Frolov VV, Fulda P, Fyffe M, Gabbard H, Gadre BU, Gaebel SM, Gair JR, Gammaitoni L, Ganija MR, Gaonkar SG, Garcia-Quiros C, Garufi F, Gateley B, Gaudio S, Gaur G, Gayathri V, Gehrels N, Gemme G, Genin E, Gennai A, George D, George J, Gergely L, Germain V, Ghonge S, Ghosh A, Ghosh A, Ghosh S, Giaime JA, Giardina KD, Giazotto A, Gill K, Glover L, Goetz E, Goetz R, Gomes S, Goncharov B, González G, Gonzalez Castro JM, Gopakumar A, Gorodetsky ML, Gossan SE, Gosselin M, Gouaty R, Grado A, Graef C, Granata M, Grant A, Gras S, Gray C, Greco G, Green AC, Gretarsson EM, Groot P, Grote H, Grunewald S, Gruning P, Guidi GM, Guo X, Gupta A, Gupta MK, Gushwa KE, Gustafson EK, Gustafson R, Halim O, Hall BR, Hall ED, Hamilton EZ, Hammond G, Haney M, Hanke MM, Hanks J, Hanna C, Hannam MD, Hannuksela OA, Hanson J, Hardwick T, Harms J, Harry GM, Harry IW, Hart MJ, Haster CJ, Haughian K, Healy J, Heidmann A, Heintze MC, Heitmann H, Hello P, Hemming G, Hendry M, Heng IS, Hennig J, Heptonstall AW, Heurs M, Hild S, Hinderer T, Ho WC, Hoak D, Hofman D, Holt K, Holz DE, Hopkins P, Horst C, Hough J, Houston EA, Howell EJ, Hreibi A, Hu YM, Huerta EA, Huet D, Hughey B, Husa S, Huttner SH, Huynh-Dinh T, Indik N, Inta R, Intini G, Isa HN, Isac JM, Isi M, Iyer BR, Izumi K, Jacqmin T, Jani K, Jaranowski P, Jawahar S, Jiménez-Forteza F, Johnson WW, Johnson-McDaniel NK, Jones DI, Jones R, Jonker RJ, Ju L, Junker J, Kalaghatgi CV, Kalogera V, Kamai B, Kandhasamy S, Kang G, Kanner JB, Kapadia SJ, Karki S, Karvinen KS, Kasprzack M, Kastaun W, Katolik M, Katsavounidis E, Katzman W, Kaufer S, Kawabe K, Kéfélian F, Keitel D, Kemball AJ, Kennedy R, Kent C, Key JS, Khalili FY, Khan I, Khan S, Khan Z, Khazanov EA, Kijbunchoo N, Kim C, Kim JC, Kim K, Kim W, Kim WS, Kim YM, Kimbrell SJ, King EJ, King PJ, Kinley-Hanlon M, Kirchhoff R, Kissel JS, Kleybolte L, Klimenko S, Knowles TD, Koch P, Koehlenbeck SM, Koley S, Kondrashov V, Kontos A, Korobko M, Korth WZ, Kowalska I, Kozak DB, Krämer C, Kringel V, Krishnan B, Królak A, Kuehn G, Kumar P, Kumar R, Kumar S, Kuo L, Kutynia A, Kwang S, Lackey BD, Lai KH, Landry M, Lang RN, Lange J, Lantz B, Lanza RK, Larson SL, Lartaux-Vollard A, Lasky PD, Laxen M, Lazzarini A, Lazzaro C, Leaci P, Leavey S, Lee CH, Lee HK, Lee HM, Lee HW, Lee K, Lehmann J, Lenon A, Leon E, Leonardi M, Leroy N, Letendre N, Levin Y, Li TG, Linker SD, Littenberg TB, Liu J, Liu X, Lo RK, Lockerbie NA, London LT, Lord JE, Lorenzini M, Loriette V, Lormand M, Losurdo G, Lough JD, Lousto CO, Lovelace G, Lück H, Lumaca D, Lundgren AP, Lynch R, Ma Y, Macas R, Macfoy S, Machenschalk B, MacInnis M, Macleod DM, Magaña Hernandez I, Magaña-Sandoval F, Magaña Zertuche L, Magee RM, Majorana E, Maksimovic I, Man N, Mandic V, Mangano V, Mansell GL, Manske M, Mantovani M, Marchesoni F, Marion F, Márka S, Márka Z, Markakis C, Markosyan AS, Markowitz A, Maros E, Marquina A, Marsh P, Martelli F, Martellini L, Martin IW, Martin RM, Martynov DV, Marx JN, Mason K, Massera E, Masserot A, Massinger TJ, Masso-Reid M, Mastrogiovanni S, Matas A, Matichard F, Matone L, Mavalvala N, Mazumder N, McCarthy R, McClelland DE, McCormick S, McCuller L, McGuire SC, McIntyre G, McIver J, McManus DJ, McNeill L, McRae T, McWilliams ST, Meacher D, Meadors GD, Mehmet M, Meidam J, Mejuto-Villa E, Melatos A, Mendell G, Mercer RA, Merilh EL, Merzougui M, Meshkov S, Messenger C, Messick C, Metzdorff R, Meyers PM, Miao H, Michel C, Middleton H, Mikhailov EE, Milano L, Miller AL, Miller BB, Miller J, Millhouse M, Milovich-Goff MC, Minazzoli O, Minenkov Y, Ming J, Mishra C, Mitra S, Mitrofanov VP, Mitselmakher G, Mittleman R, Moffa D, Moggi A, Mogushi K, Mohan M, Mohapatra SR, Molina I, Montani M, Moore CJ, Moraru D, Moreno G, Morisaki S, Morriss SR, Mours B, Mow-Lowry CM, Mueller G, Muir AW, Mukherjee A, Mukherjee D, Mukherjee S, Mukund N, Mullavey A, Munch J, Muñiz EA, Muratore M, Murray PG, Nagar A, Napier K, Nardecchia I, Naticchioni L, Nayak RK, Neilson J, Nelemans G, Nelson TJ, Nery M, Neunzert A, Nevin L, Newport JM, Newton G, Ng KK, Nguyen P, Nguyen TT, Nichols D, Nielsen AB, Nissanke S, Nitz A, Noack A, Nocera F, Nolting D, North C, Nuttall LK, Oberling J, O'Dea GD, Ogin GH, Oh JJ, Oh SH, Ohme F, Okada MA, Oliver M, Oppermann P, Oram RJ, O'Reilly B, Ormiston R, Ortega LF, O'Shaughnessy R, Ossokine S, Ottaway DJ, Overmier H, Owen BJ, Pace AE, Page J, Page MA, Pai A, Pai SA, Palamos JR, Palashov O, Palomba C, Pal-Singh A, Pan H, Pan HW, Pang B, Pang PT, Pankow C, Pannarale F, Pant BC, Paoletti F, Paoli A, Papa MA, Parida A, Parker W, Pascucci D, Pasqualetti A, Passaquieti R, Passuello D, Patil M, Patricelli B, Pearlstone BL, Pedraza M, Pedurand R, Pekowsky L, Pele A, Penn S, Perez CJ, Perreca A, Perri LM, Pfeiffer HP, Phelps M, Piccinni OJ, Pichot M, Piergiovanni F, Pierro V, Pillant G, Pinard L, Pinto IM, Pirello M, Pitkin M, Poe M, Poggiani R, Popolizio P, Porter EK, Post A, Powell J, Prasad J, Pratt JW, Pratten G, Predoi V, Prestegard T, Prijatelj M, Principe M, Privitera S, Prix R, Prodi GA, Prokhorov LG, Puncken O, Punturo M, Puppo P, Pürrer M, Qi H, Quetschke V, Quintero EA, Quitzow-James R, Raab FJ, Rabeling DS, Radkins H, Raffai P, Raja S, Rajan C, Rajbhandari B, Rakhmanov M, Ramirez KE, Ramos-Buades A, Rapagnani P, Raymond V, Razzano M, Read J, Regimbau T, Rei L, Reid S, Reitze DH, Ren W, Reyes SD, Ricci F, Ricker PM, Rieger S, Riles K, Rizzo M, Robertson NA, Robie R, Robinet F, Rocchi A, Rolland L, Rollins JG, Roma VJ, Romano JD, Romano R, Romel CL, Romie JH, RosiÅ„ska D, Ross MP, Rowan S, Rüdiger A, Ruggi P, Rutins G, Ryan K, Sachdev S, Sadecki T, Sadeghian L, Sakellariadou M, Salconi L, Saleem M, Salemi F, Samajdar A, Sammut L, Sampson LM, Sanchez EJ, Sanchez LE, Sanchis-Gual N, Sandberg V, Sanders JR, Sassolas B, Sathyaprakash BS, Saulson PR, Sauter O, Savage RL, Sawadsky A, Schale P, Scheel M, Scheuer J, Schmidt J, Schmidt P, Schnabel R, Schofield RM, Schönbeck A, Schreiber E, Schuette D, Schulte BW, Schutz BF, Schwalbe SG, Scott J, Scott SM, Seidel E, Sellers D, Sengupta AS, Sentenac D, Sequino V, Sergeev A, Shaddock DA, Shaffer TJ, Shah AA, Shahriar MS, Shaner MB, Shao L, Shapiro B, Shawhan P, Sheperd A, Shoemaker DH, Shoemaker DM, Siellez K, Siemens X, Sieniawska M, Sigg D, Silva AD, Singer LP, Singh A, Singhal A, Sintes AM, Slagmolen BJ, Smith B, Smith JR, Smith RJ, Somala S, Son EJ, Sonnenberg JA, Sorazu B, Sorrentino F, Souradeep T, Spencer AP, Srivastava AK, Staats K, Staley A, Steinke M, Steinlechner J, Steinlechner S, Steinmeyer D, Stevenson SP, Stone R, Stops DJ, Strain KA, Stratta G, Strigin SE, Strunk A, Sturani R, Stuver AL, Summerscales TZ, Sun L, Sunil S, Suresh J, Sutton PJ, Swinkels BL, SzczepaÅ„czyk MJ, Tacca M, Tait SC, Talbot C, Talukder D, Tanner DB, Tápai M, Taracchini A, Tasson JD, Taylor JA, Taylor R, Tewari SV, Theeg T, Thies F, Thomas EG, Thomas M, Thomas P, Thorne KA, Thorne KS, Thrane E, Tiwari S, Tiwari V, Tokmakov KV, Toland K, Tonelli M, Tornasi Z, Torres-Forné A, Torrie CI, Töyrä D, Travasso F, Traylor G, Trinastic J, Tringali MC, Trozzo L, Tsang KW, Tse M, Tso R, Tsukada L, Tsuna D, Tuyenbayev D, Ueno K, Ugolini D, Unnikrishnan CS, Urban AL, Usman SA, Vahlbruch H, Vajente G, Valdes G, Vallisneri M, van Bakel N, van Beuzekom M, van den Brand JF, Van Den Broeck C, Vander-Hyde DC, van der Schaaf L, van Heijningen JV, van Veggel AA, Vardaro M, Varma V, Vass S, Vasúth M, Vecchio A, Vedovato G, Veitch J, Veitch PJ, Venkateswara K, Venugopalan G, Verkindt D, Vetrano F, Viceré A, Viets AD, Vinciguerra S, Vine DJ, Vinet JY, Vitale S, Vo T, Vocca H, Vorvick C, Vyatchanin SP, Wade AR, Wade LE, Wade M, Walet R, Walker M, Wallace L, Walsh S, Wang G, Wang H, Wang JZ, Wang WH, Wang YF, Ward RL, Warner J, Was M, Watchi J, Weaver B, Wei LW, Weinert M, Weinstein AJ, Weiss R, Wen L, Wessel EK, Weßels P, Westerweck J, Westphal T, Wette K, Whelan JT, Whitcomb SE, Whiting BF, Whittle C, Wilken D, Williams D, Williams RD, Williamson AR, Willis JL, Willke B, Wimmer MH, Winkler W, Wipf CC, Wittel H, Woan G, Woehler J, Wofford J, Wong KW, Worden J, Wright JL, Wu DS, Wysocki DM, Xiao S, Yamamoto H, Yancey CC, Yang L, Yap MJ, Yazback M, Yu H, Yu H, Yvert M, Zadrożny A, Zanolin M, Zelenova T, Zendri JP, Zevin M, Zhang L, Zhang M, Zhang T, Zhang YH, Zhao C, Zhou M, Zhou Z, Zhu SJ, Zhu XJ, Zimmerman AB, Zucker ME, Zweizig J, 6, GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral, Physical Review Letters, 119, 16, 161101, October 2017, 29099225, 10.1103/PhysRevLett.119.161101, LIGO Scientific Collaboration & Virgo interferometer, Virgo Collaboration, 1710.05832, 2017PhRvL.119p1101A, free,

Observations

File:LIGO on Hanford Reservation.jpg|thumb|300px|Western leg of LIGO interferometer on Hanford ReservationHanford Reservation{{see also|First observation of gravitational waves|List of gravitational wave observations}}Based on current models of astronomical events, and the predictions of the general theory of relativity,{{r|Pretorius2005|CampanelliLousto2006|BakerCentrella2006}} gravitational waves that originate tens of millions of light years from Earth are expected to distort the {{convert|4|km|adj=on}} mirror spacing by about {{val|e=−18|u=m}}, less than one-thousandth the charge diameter of a proton. Equivalently, this is a relative change in distance of approximately one part in {{10^|21}}. A typical event which might cause a detection event would be the late stage inspiral and merger of two 10-solar-mass black holes, not necessarily located in the Milky Way galaxy, which is expected to result in a very specific sequence of signals often summarized by the slogan chirp, burst, quasi-normal mode ringing, exponential decay.In their fourth Science Run at the end of 2004, the LIGO detectors demonstrated sensitivities in measuring these displacements to within a factor of two of their design.During LIGO's fifth Science Run in November 2005, sensitivity reached the primary design specification of a detectable strain of one part in {{10^|21}} over a {{val|100|u=Hz}} bandwidth. The baseline inspiral of two roughly solar-mass neutron stars is typically expected to be observable if it occurs within about {{convert|8|e6pc|e6ly|lk=on}}, or the vicinity of the Local Group, averaged over all directions and polarizations. Also at this time, LIGO and GEO 600 (the German-UK interferometric detector) began a joint science run, during which they collected data for several months. Virgo (the French-Italian interferometric detector) joined in May 2007. The fifth science run ended in 2007, after extensive analysis of data from this run did not uncover any unambiguous detection events.In February 2007, GRB 070201, a short gamma-ray burst arrived at Earth from the direction of the Andromeda Galaxy. The prevailing explanation of most short gamma-ray bursts is the merger of a neutron star with either a neutron star or a black hole. LIGO reported a non-detection for GRB 070201, ruling out a merger at the distance of Andromeda with high confidence. Such a constraint was predicated on LIGO eventually demonstrating a direct detection of gravitational waves.PRESS RELEASE, LIGO Sheds Light on Cosmic Event, Kathy, Svitil,weblink 2 January 2008, 14 February 2016, California Institute of Technology,

Enhanced LIGO

File:Northern leg of LIGO interferometer on Hanford Reservation.JPG|thumb|300px|Northern leg (x-arm) of LIGO interferometer on Hanford ReservationHanford ReservationAfter the completion of Science Run 5, initial LIGO was upgraded with certain technologies, planned for Advanced LIGO but available and able to be retrofitted to initial LIGO, which resulted in an improved-performance configuration dubbed Enhanced LIGO.TECH REPORT, Rana, Adhikari, Peter, Fritschel, Sam, Waldman, Enhanced LIGO,weblink LIGO-T060156-01-I, 17 July 2006, Some of the improvements in Enhanced LIGO included:

• Increased laser power
• Homodyne detection
• Output mode cleaner
• In-vacuum readout hardware

Science Run 6 (S6) began in July 2009 with the enhanced configurations on the 4 km detectors.WEB,weblink Firm Date Set for Start of S6, Dave, Beckett, 15 June 2009, LIGO Laboratory News, It concluded in October 2010, and the disassembly of the original detectors began.

Advanced LIGO

(File:Simplified diagram of an Advanced LIGO detector.png|thumb|300px|Simplified diagram of an Advanced LIGO detector (not to scale).)(File:AdvLIGO noise curve.webp|thumb|300px|Design sensitivity of Advanced LIGO interferometer with major noise sources, maximum sensitivity is around 500 HzJOURNAL, Danilishin, Stefan L., Khalili, Farid Ya., Miao, Haixing, 29 April 2019, Advanced quantum techniques for future gravitational-wave detectors,weblink Living Reviews in Relativity, en, 22, 1, 2, 10.1007/s41114-019-0018-y, 1903.05223, 2019LRR....22....2D, 119238143, 2367-3613, )After 2010, LIGO went offline for several years for a major upgrade, installing the new Advanced LIGO detectors in the LIGO Observatory infrastructures.The project continued to attract new members, with the Australian National University and University of Adelaide contributing to Advanced LIGO, and by the time the LIGO Laboratory started the first observing run 'O1' with the Advanced LIGO detectors in September 2015, the LIGO Scientific Collaboration included more than 900 scientists worldwide.The first observing run operated at a sensitivity roughly three times greater than Initial LIGO,WEB, The Newest Search for Gravitational Waves has Begun,weblink LIGO Scientific Collaboration, 9 September 2017, Burtnyk, Kimberly, 18 September 2015,weblink 4 July 2017, LIGO’s advanced detectors are already three times more sensitive than Initial LIGO was by the end of its observational lifetime, and a much greater sensitivity for larger systems with their peak radiation at lower audio frequencies.JOURNAL, Aasi, J, Advanced LIGO, Classical and Quantum Gravity, 9 April 2015, 32, 7, 074001, 10.1088/0264-9381/32/7/074001, 1411.4547, 2015CQGra..32g4001L, 118570458, On 11 February 2016, the LIGO and Virgo collaborations announced the first observation of gravitational waves.{{r|Nature_11Feb16|PRL-20160211}} The signal was named GW150914.NEWS, Naeye, Robert,weblink Gravitational Wave Detection Heralds New Era of Science, Sky and Telescope, 11 February 2016, 11 February 2016, The waveform showed up on 14 September 2015, within just two days of when the Advanced LIGO detectors started collecting data after their upgrade.JOURNAL, Cho, Adrian, Here's the first person to spot those gravitational waves,weblink Science, 2016-02-11, 10.1126/science.aaf4039, NEWS, Gravitational waves from black holes detected,weblink BBC News, 11 February 2016, It matched the predictions of general relativityJOURNAL, Pretorius, Frans, Evolution of Binary Black-Hole Spacetimes, Physical Review Letters, 95, 12, 121101, 2005, 0031-9007, 10.1103/PhysRevLett.95.121101, 16197061, gr-qc/0507014, 2005PhRvL..95l1101P, 24225193, JOURNAL, Campanelli, M., Lousto, C.O., Marronetti, P., Zlochower, Y., Accurate Evolutions of Orbiting Black-Hole Binaries without Excision, Physical Review Letters, 96, 11, 111101, 2006, 0031-9007, 10.1103/PhysRevLett.96.111101, 16605808, gr-qc/0511048, 2006PhRvL..96k1101C, 5954627, JOURNAL, Baker, John G., Centrella, Joan, Joan Centrella, Choi, Dae-Il, Koppitz, Michael, van Meter, James, Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes, Physical Review Letters, 96, 11, 111102, 2006, 0031-9007, 10.1103/PhysRevLett.96.111102, 16605809, gr-qc/0511103, 2006PhRvL..96k1102B, 23409406, for the inward spiral and merger of a pair of black holes and subsequent ringdown of the resulting single black hole. The observations demonstrated the existence of binary stellar-mass black hole systems and the first observation of a binary black hole merger.On 15 June 2016, LIGO announced the detection of a second gravitational wave event, recorded on 26 December 2015, at 3:38 UTC. Analysis of the observed signal indicated that the event was caused by the merger of two black holes with masses of 14.2 and 7.5 solar masses, at a distance of 1.4 billion light years.NEWS, Chu, Jennifer, For second time, LIGO detects gravitational waves,weblink 15 June 2016, MIT News, MIT, 15 June 2016, The signal was named GW151226.JOURNAL,weblink GW151226: Observation of Gravitational Waves from a 22 Solar-mass Binary Black Hole Coalescence, Physical Review Letters, 116, 24, 241103, 15 June 2016, 2016PhRvL.116x1103A, Abbott, B.P., Abbott, R., Abbott, T.D., Abernathy, M.R., Acernese, F., Ackley, K., Adams, C., Adams, T., Addesso, P., Adhikari, R.X., Adya, V.B., Affeldt, C., Agathos, M., Agatsuma, K., Aggarwal, N., Aguiar, O.D., Aiello, L., Ain, A., Ajith, P., Allen, B., Allocca, A., Altin, P.A., Anderson, S.B., Anderson, W.G., Arai, K., Araya, M.C., Arceneaux, C.C., Areeda, J.S., Arnaud, N., Arun, K.G., 3, 1606.04855, 10.1103/PhysRevLett.116.241103, 27367379, 118651851, The second observing run (O2) ran from 30 November 2016WEB, VIRGO joins LIGO for the "Observation Run 2" (O2) data-taking period, 1 August 2017,weblink LIGO Scientific Collaboration & VIRGO collaboration, 20 October 2017, 10 October 2017,weblink" title="web.archive.org/web/20171010204215weblink">weblink dead, to 25 August 2017,WEB, Update on the start of LIGO's 3rd observing run, 24 April 2018,weblink 31 August 2018, the start of O3 is currently projected to begin in early 2019. Updates will be provided once the installation phase is complete and the commissioning phase has begun. An update on the engineering run prior to O3 will be provided by late summer 2018., with Livingston achieving 15–25% sensitivity improvement over O1, and with Hanford's sensitivity similar to O1.NEWS, Advanced LIGO ramps up, with slight improvements, Grant, Andrew, 12 December 2016, Physics Today, The bottom line is that [the sensitivity] is better than it was at the beginning of O1; we expect to get more detections., 10.1063/PT.5.9074, In this period, LIGO saw several further gravitational wave events: GW170104 in January; GW170608 in June; and five others between July and August 2017. Several of these were also detected by the Virgo Collaboration.GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs{{r|Chu17}}WEB,weblink Gravitational waves from a binary black hole merger observed by LIGO and Virgo, Unlike the black hole mergers which are only detectable gravitationally, GW170817 came from the collision of two neutron stars and was also detected electromagnetically by gamma ray satellites and optical telescopes.PRESS RELEASE, LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars,weblink 16 October 2017, Jennifer, Chu, LIGO, The third run (O3) began on 1 April 2019WEB,weblink LIGO and Virgo Detect Neutron Star Smash-Ups, and was planned to last until 30 April 2020; in fact it was suspended in March 2020 due to COVID-19.WEB, Observatory Status,weblinkweblink 2020-04-09, live, LIGO, 2020-03-23, 2020-06-23, Diego Bersanetti: Status of the Virgo gravitational-wave detector and the O3 Observing Run, EPS-HEP2019 On 6 January 2020, LIGO announced the detection of what appeared to be gravitational ripples from a collision of two neutron stars, recorded on 25 April 2019, by the LIGO Livingston detector. Unlike GW170817, this event did not result in any light being detected. Furthermore, this is the first published event for a single-observatory detection, given that the LIGO Hanford detector was temporarily offline at the time and the event was too faint to be visible in Virgo's data.WEB,weblink LIGO-Virgo network catches another neutron star collision, The fourth observing run (O4) was planned to start in December 2022,WEB, LIGO Laboratory statement on long term future observing plans,weblink 2022-03-22, LIGO Lab, but was postponed until 24 May 2023. O4 is projected to continue until February 2025. As of O4, the interferometers are operating at a sensitivity of 155-175 Mpc, within the design sensitivity range of 160-190 Mpc for binary neutron star events.JOURNAL, Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO, Advanced Virgo and KAGRA, Living Reviews in Relativity, 24 Nov 2020,weblink 13 May 2024, The fifth observing run (O5) is projected to begin in late 2025 or in 2026.

Future

LIGO-India

LIGO-India, or INDIGO, is a planned collaborative project between the LIGO Laboratory and the Indian Initiative in Gravitational-wave Observations (IndIGO) to create a gravitational-wave detector in India. The LIGO Laboratory, in collaboration with the US National Science Foundation and Advanced LIGO partners from the U.K., Germany and Australia, has offered to provide all of the designs and hardware for one of the three planned Advanced LIGO detectors to be installed, commissioned, and operated by an Indian team of scientists in a facility to be built in India.The LIGO-India project is a collaboration between LIGO Laboratory and the LIGO-India consortium: Institute of Plasma Research, Gandhinagar; IUCAA (Inter-University Centre for Astronomy and Astrophysics), Pune and Raja Ramanna Centre for Advanced Technology, Indore.The expansion of worldwide activities in gravitational-wave detection to produce an effective global network has been a goal of LIGO for many years. In 2010, a developmental roadmapWEB, The future of gravitational wave astronomy,weblink Gravitational Waves International Committee, 3 September 2018, 30 July 2017,weblink dead, issued by the Gravitational Wave International Committee (GWIC) recommended that an expansion of the global array of interferometric detectors be pursued as a highest priority. Such a network would afford astrophysicists with more robust search capabilities and higher scientific yields. The current agreement between the LIGO Scientific Collaboration and the Virgo collaboration links three detectors of comparable sensitivity and forms the core of this international network. Studies indicate that the localization of sources by a network that includes a detector in India would provide significant improvements.{{Citation |url=https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=90988 |title=Improved Source Localization with LIGO India |journal=Journal of Physics: Conference Series |volume=484 |issue=1 |pages=012007 |date=28 September 2012 |first=Stephen |last=Fairhurst |id=LIGO document P1200054-v6|bibcode=2014JPhCS.484a2007F |arxiv=1205.6611 |doi=10.1088/1742-6596/484/1/012007 |s2cid=118583506 }}{{Citation |arxiv=1102.5421 |title=Networks of Gravitational Wave Detectors and Three Figures of Merit |journal=Classical and Quantum Gravity |volume=28 |issue=12 |pages=125023 |date=25 April 2011 |first=Bernard F. |last=Schutz|bibcode = 2011CQGra..28l5023S |doi = 10.1088/0264-9381/28/12/125023 |s2cid=119247573 }} Improvements in localization averages are predicted to be approximately an order of magnitude, with substantially larger improvements in certain regions of the sky.The NSF was willing to permit this relocation, and its consequent schedule delays, as long as it did not increase the LIGO budget. Thus, all costs required to build a laboratory equivalent to the LIGO sites to house the detector would have to be borne by the host country.{{Citation |url=http://www.gravitycentre.com.au/wp-content/uploads/2009/10/Science-article-about-LIGO-Australia.pdf |title=U.S. Physicists Eye Australia for New Site of Gravitational-Wave Detector |journal=Science |volume=329 |issue=5995 |page=1003 |first=Adrian |last=Cho |date=27 August 2010 |doi=10.1126/science.329.5995.1003 |bibcode=2010Sci...329.1003C |pmid=20798288 |url-status=dead |archive-url=https://web.archive.org/web/20130411020012weblink |archive-date=11 April 2013 }} The first potential distant location was at AIGO in Western Australia,{{Citation |url=https://dcc.ligo.org/LIGO-T1000251/public |title=Report of the Committee to Compare the Scientific Cases for AHLV and HHLV |date=13 May 2010 |id=LIGO document T1000251-v1 |first1=Sam |last1=Finn |first2=Peter |last2=Fritschel |first3=Sergey |last3=Klimenko |first4=Fred |last4=Raab |first5=B. |last5=Sathyaprakash |first6=Peter |last6=Saulson |first7=Rainer |last7=Weiss}} however the Australian government was unwilling to commit funding by 1 October 2011 deadline.A location in India was discussed at a Joint Commission meeting between India and the US in June 2012.U.S.-India Bilateral Cooperation on Science and Technology meeting fact sheet â€“ dated 13 June 2012. In parallel, the proposal was evaluated by LIGO's funding agency, the NSF. As the basis of the LIGO-India project entails the transfer of one of LIGO's detectors to India, the plan would affect work and scheduling on the Advanced LIGO upgrades already underway. In August 2012, the U.S. National Science Board approved the LIGO Laboratory's request to modify the scope of Advanced LIGO by not installing the Hanford "H2" interferometer, and to prepare it instead for storage in anticipation of sending it to LIGO-India.Memorandum to Members and Consultants of the National Science Board â€“ dated 24 August 2012 In India, the project was presented to the Department of Atomic Energy and the Department of Science and Technology for approval and funding. On 17 February 2016, less than a week after LIGO's landmark announcement about the detection of gravitational waves, Indian Prime Minister Narendra Modi announced that the Cabinet has granted 'in-principle' approval to the LIGO-India mega science proposal.TWEET, PMOIndia, Office of the Prime Minister of India, 699931256008511492, Cabinet has granted 'in-principle' approval to the LIGO-India mega science proposal for research on gravitational waves., 17 February 2016, A site near pilgrimage site of Aundha Nagnath in the Hingoli district of state Maharashtra in western India has been selected.NEWS, First LIGO Lab Outside US To Come Up In Maharashtra's Hingoli,weblink 8 September 2016, NDTV, WEB,weblink LIGO-India: Origins & site search, Souradeep, Tarun, 18 January 2019, 27, live,weblink 15 September 2019, 15 September 2019, On 7 April 2023, the LIGO-India project was approved by the Cabinet of Government of India. Construction is to begin in Maharashtra's Hingoli district at a cost of INR 2600 crores.NEWS,weblink Cabinet clears Rs 2,600-crore LIGO-India; Observatory to come up in Maharashtra, will be part of global network, The Times of India, 7 April 2023,

A+

Like Enhanced LIGO, certain improvements will be retrofitted to the existing Advanced LIGO instrument. These are referred to as {{em|A+}} proposals, and are planned for installation starting from 2019 until the upgraded detector is operational in 2024.WEB,weblink Upgraded LIGO to search for universe's most extreme events, www.nsf.gov, en, 2020-04-09, The changes would almost double Advanced LIGO's sensitivity,JOURNAL, Prospects for doubling the range of Advanced LIGO, Physical Review D, 91, 62005, 062005, 16 March 2015, John, Miller, Lisa, Barsotti, Lisa Barsotti, Salvatore, Vitale, Peter, Fritschel, Matthew, Evans, Daniel, Sigg, 10.1103/PhysRevD.91.062005, 1410.5882,weblink 2015PhRvD..91f2005M, 18460400, CONFERENCE, Getting an A+: Enhancing Advanced LIGO, Michael E., Zucker,weblink LIGO-G1601435-v3, 7 July 2016, LIGO–DAWN Workshop II,weblink and increase the volume of space searched by a factor of seven.WEB,weblink LIGO Gravitational Wave Observatory Getting $30 Million Upgrade, Thompson, Avery, www.popularmechanics.com, en-US, 17 February 2019, 15 February 2019, The upgrades include:

• Improvements to the mirror suspension system.NEWS,weblink Black hole detectors to get big upgrade, Ghosh, Pallab, 15 February 2019, 17 February 2019, en-GB,
• Increased reflectivity of the mirrors.
• Using frequency-dependent squeezed light, which would simultaneously decrease radiation pressure at low frequencies and shot noise at high frequencies, and
• Improved mirror coatings with lower mechanical loss.WEB,weblink LIGO-T1800042-v5: The A+ design curve, dcc.ligo.org, 2020-04-09,

Because the final LIGO output photodetector is sensitive to phase, and not amplitude, it is possible to squeeze the signal so there is less phase noise and more amplitude noise, without violating the quantum mechanical limit on their product.WEB, The Quantum Enhanced LIGO Detector Sets New Sensitivity Record,weblink This is done by injecting a "squeezed vacuum state" into the dark port (interferometer output) which is quieter, in the relevant parameter, than simple darkness. Such a squeezing upgrade was installed at both LIGO sites prior to the third observing run.JOURNAL, Tse, M., Yu, Haocun, Kijbunchoo, N., Fernandez-Galiana, A., Dupej, P., Barsotti, L., Lisa Barsotti, Blair, C. D., Brown, D. D., Dwyer, S. E., Effler, A., Evans, M., 2019-12-05, Quantum-Enhanced Advanced LIGO Detectors in the Era of Gravitational-Wave Astronomy, Physical Review Letters, 123, 23, 231107, 10.1103/PhysRevLett.123.231107, 31868462, 2019PhRvL.123w1107T, free, 1721.1/136579.2, free, The A+ improvement will see the installation of an additional optical cavity that acts to rotate the squeezing quadrature from phase-squeezed at high frequencies (above 50 Hz) to amplitude-squeezed at low frequencies, thereby also mitigating low-frequency radiation pressure noise.

LIGO Voyager

A third-generation detector at the existing LIGO sites is being planned under the name "LIGO Voyager" to improve the sensitivity by an additional factor of two, and halve the low-frequency cutoff to 10 Hz.REPORT,weblink Instrument Science White Paper, McClelland, David, Evans, Matthew, 8 October 2015, LIGO Scientific Collaboration, LIGO Document T1500290-v2, Lantz, Brian, Martin, Ian, Quetschke, Volker, Schnabel, Roman, Plans call for the glass mirrors and to be replaced by even larger 160 kg silicon test masses, cooled to 123 K (a temperature achievable with liquid nitrogen), and a change to a longer laser wavelength in the 1500–2200 nm range at which silicon is transparent. (Many documents assume a wavelength of 1550 nm, but this is not final.)Voyager would be an upgrade to A+, to be operational around 2027–2028.TECH REPORT, LIGO Scientific Collaboration, Instrument Science White Paper,weblink LIGO-T1400316-v4, LIGO, 2015-02-10, 2020-06-23,

Cosmic Explorer

A design for a larger facility with longer arms is called "Cosmic Explorer". This is based on the LIGO Voyager technology, has a similar LIGO-type L-shape geometry but with 40 km arms. The facility is currently planned to be on the surface. It has a higher sensitivity than Einstein Telescope for frequencies beyond 10 Hz, but lower sensitivity under 10 Hz.

See also

• BlackGEM
• Einstein Telescope, a European third-generation gravitational wave detector
• Einstein@Home, a volunteer distributed computing program one can download in order to help the LIGO/GEO teams analyze their data
• GEO600, a gravitational wave detector located in Hannover, Germany
• Holometer
• North American Nanohertz Observatory for Gravitational Waves
• Richard A. Isaacson
• PyCBC, an open source software package to help analyze LIGO data
• Tests of general relativity
• Virgo interferometer, an interferometer located close to Pisa, Italy
• Laser Interferometer Space Antenna (LISA)
• LISA Pathfinder
• Taiji Program in Space, a space-based Chinese gravitational wave detector

Notes

References

• Kip Thorne, ITP & Caltech. Spacetime Warps and the Quantum: A Glimpse of the Future. Lecture slides and audio
• Barry C. Barish, Caltech. The Detection of Gravitational Waves. Video from CERN Academic Training Lectures, 1996
• Barry C. Barish, Caltech. Einstein's Unfinished Symphony: Sounds from the Distant Universe Video from IHMC Florida Institute for Human Machine Cognition 2004 Evening Lecture Series.
• Rainer Weiss, weblink" title="web.archive.org/web/20110721132922weblink">Electromagnetically coupled broad-band gravitational wave antenna, MIT RLE QPR 1972
• On the detection of low frequency gravitational waves, M.E. Gertsenshtein and V.I. Pustovoit â€“ JETP Vol. 43 pp. 605–607 (August 1962) Note: This is the first paper proposing the use of interferometers for the detection of gravitational waves.
• Wave resonance of light and gravitational waves â€“ M.E. Gertsenshtein â€“ JETP Vol. 41 pp. 113–114 (July 1961)
• Gravitational electromagnetic resonance, V.B. Braginskii, M.B. Mensky â€“ GR.G. Vol. 3 No. 4 pp. 401–402 (1972)
• Gravitational radiation and the prospect of its experimental discovery, V.B. Braginsky â€“ Usp. Fiz. Nauk Vol. 86 pp. 433–446 (July 1965). English translation: Sov. Phys. Uspekhi Vol. 8 No. 4 pp. 513–521 (1966)
• On the electromagnetic detection of gravitational waves, V.B. Braginsky, L.P. Grishchuck, A.G. Dooshkevieh, M.B. Mensky, I.D. Novikov, M.V. Sazhin and Y.B. Zeldovisch â€“ GR.G. Vol. 11 No. 6 pp. 407–408 (1979)
• On the propagation of electromagnetic radiation in the field of a plane gravitational wave, E. Montanari â€“ gr-qc/9806054 (11 June 1998)

Further reading

• WEB, Barish, Barry C., The Science and Detection of Gravitational Waves,weblink 2000,
• BOOK, Bartusiak, Marcia, Einstein's unfinished symphony : listening to the sounds of space-time,weblink registration, Joseph Henry Press, Washington, DC, 2000, 978-0-425-18620-6,
• BOOK, Saulson, Peter, Fundamentals of interferometric gravitational wave detectors,weblink registration, World Scientific, Singapore River Edge, NJ, 1994, 978-981-02-1820-1,
• BOOK, Collins, Harry M., Gravity's shadow the search for gravitational waves,weblink registration, University of Chicago Press, Chicago, 2004, 978-0-226-11378-4,
• BOOK, Kennefick, Daniel, Traveling at the speed of thought : Einstein and the quest for gravitational waves,weblink registration, Princeton University Press, Princeton, NJ, 2007, 978-0-691-11727-0,
• Janna Levin (2016). Black hole blues : and other songs from outer space. New York: Alfred A. Knopf. {{ISBN|978-0307958198}}
• BOOK, Collins, Harry, M., Gravity's kiss: the detection of gravitational waves, MIT Press, Cambridge, MA & London, 2017, 978-0-262-03618-4,

External links

{{Wiktionary|ligo}}{{Commons category}}

• LIGO Newsletters Excellent wide-audience newsletters published twice-yearly in March and September. From Issue 1 (September 2012) through to present day.
• LIGO Scientific Collaboration web page
• LIGO outreach webpage, with links to summaries of the Collaboration's scientific articles, written for a general public audience
• LIGO Laboratory
• LIGO News blog
• Living LIGO blog: answering questions about LIGO science and being a scientist by LIGO member Amber Stuver
• weblink" title="web.archive.org/web/20101107064702weblink">Advanced LIGO homepage
• weblink" title="arquivo.pt/wayback/20160517170735weblink">Columbia Experimental Gravity
• weblink" title="archive.today/20041206082807weblink">American Museum of Natural History film and other materials on LIGO
• weblink" title="web.archive.org/web/20060907131305weblink">40 m Prototype
• Earth-Motion studies A brief discussion of efforts to correct for seismic and human-related activity that contributes to the background signal of the LIGO detectors.
• weblink" title="web.archive.org/web/20060721201701weblink">Caltech's Physics 237-2002 Gravitational Waves by Kip Thorne Video plus notes: Graduate level but does not assume knowledge of General Relativity, Tensor Analysis, or Differential Geometry; Part 1: Theory (10 lectures), Part 2: Detection (9 lectures)
• Caltech Tutorial on Relativity â€“ An extensive description of gravitational waves and their sources.
• Q&A: Rainer Weiss on LIGO's origins at news.mit.edu
• LIGO: a strong belief, 2/11/16 CERN Courier Interview with Barry Barish (18 March 2016 publication date).
• {{YouTube|gmmD72cFOU4|Video (3:10): LIGO Orrey (1 December 2018)}}

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