View source for LIGO

{{short description|Gravitational wave observatory site}}
{{about|LIGO observatories||LIGO Scientific Collaboration|and|Ligo (disambiguation)}}
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The '''Laser Interferometer Gravitational-Wave Observatory''' ('''LIGO''') is a large-scale physics experiment and observatory designed to detect cosmic [[gravitational wave]]s and to develop gravitational-wave observations as an astronomical tool.<ref name="Physics_Today_Oct_1999">{{cite journal |last1=Barish |first1=Barry C. |last2=Weiss |first2=Rainer |title=LIGO and the Detection of Gravitational Waves |journal=Physics Today |date=October 1999 |volume=52 |issue=10 |page=44 |doi=10.1063/1.882861 |bibcode=1999PhT....52j..44B }}</ref> Prior to LIGO, all data about the universe has come in the form of light and other forms of [[electromagnetic radiation]], from limited direct exploration on relatively nearby [[Solar System]] objects such as the [[Moon]], [[Mars]], [[Venus]], [[Jupiter]] and their moons, asteroids etc, and from [[Cosmic ray|high energy cosmic particles]]. Initially, two large observatories were built in the United States with the aim of detecting gravitational waves by [[laser]] [[interferometry]]. Two additional, smaller gravitational wave observatories are now operational in Japan ([[KAGRA]]) and Italy ([[Virgo interferometer|Virgo]]). The two LIGO observatories use mirrors spaced 4&nbsp;km apart to measure changes in length—over an effective span of 1120&nbsp;km—of less than one ten-thousandth the [[charge radius|charge diameter]] of a [[proton]].<ref name="LIGO-Facts">{{cite web |title=Facts |url=https://www.ligo.caltech.edu/page/facts |website=LIGO |quote=This is equivalent to measuring the distance from Earth to the nearest star to an accuracy smaller than the width of a human hair! |access-date=24 August 2017 |archive-url=https://web.archive.org/web/20170704030142/https://www.ligo.caltech.edu/page/facts |archive-date=4 July 2017 }} (that is, to [[Proxima Centauri]] at {{val|4.0208|e=13|u=km}}).</ref>

The initial LIGO observatories were funded by the United States [[National Science Foundation]] (NSF). They were conceived, built, and are operated by [[Caltech]] and [[MIT]].<ref>{{cite web |title=LIGO Lab Caltech MIT |url=https://www.ligo.caltech.edu |access-date=24 June 2016 }}</ref><ref>{{cite web |title=LIGO MIT |url=http://space.mit.edu/LIGO |access-date=24 June 2016 }}</ref> They collected data from 2002 to 2010, but no gravitational waves were detected during that period.

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]].<ref>{{cite web |title=Major research project to detect gravitational waves is underway |url=http://www.birmingham.ac.uk/news/latest/2015/05/gravitational-waves-28-05-15.aspx |website=University of Birmingham News |publisher=[[University of Birmingham]] |access-date=28 November 2015 }}</ref><ref>{{cite journal |last1=Shoemaker |first1=David |title=The evolution of Advanced LIGO |journal=LIGO Magazine |date=2012 |issue=1 |page=8 |url=http://www.ligo.org/magazine/LIGO-magazine-issue-1.pdf#page=8 |archive-date=16 November 2017 |access-date=28 November 2015 |archive-url=https://web.archive.org/web/20171116111338/http://ligo.org/magazine/LIGO-magazine-issue-1.pdf#page=8 }}</ref> 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,<ref>{{cite web |title=Revolutionary Grassroots Astrophysics Project "Einstein@Home" Goes Live |url=http://www.aps.org/newsroom/pressreleases/athome1.cfm |access-date=3 March 2016 |archive-date=9 April 2023 |archive-url=https://web.archive.org/web/20230409133728/https://www.aps.org/newsroom/pressreleases/athome1.cfm }}</ref><ref name="Census">{{cite web |title=LSC/Virgo Census |url=https://my.ligo.org/census.php |work=myLIGO |access-date=28 November 2015 }}</ref><ref name="Nature_2015_Sept_15" /> as well as 440,000 active [[Einstein@Home]] users {{as of|2016|December|lc=yes}}.<ref name="boinc">{{cite web |url=http://boincstats.com/en/stats/projectStatsInfo |title=BOINCstats project statistics |access-date=14 December 2016 |archive-date=1 October 2012 |archive-url=https://web.archive.org/web/20121001130529/http://boincstats.com/en/stats/projectStatsInfo |url-status=dead }}</ref>

LIGO is the largest and most ambitious project ever funded by the NSF.<ref>Larger physics projects in the United States, such as [[Fermilab]], have traditionally been funded by the [[United States Department of Energy|Department of Energy]].</ref><ref name="nsfligo">{{cite web |title=LIGO: The Search for Gravitational Waves |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=103042 |website=www.nsf.gov |publisher=National Science Foundation |access-date=3 September 2018 |language=en |archive-date=15 September 2016 |archive-url=https://web.archive.org/web/20160915004424/https://www.nsf.gov/news/news_summ.jsp?cntn_id=103042 }}</ref> In 2017, the [[Nobel Prize in Physics]] was awarded to [[Rainer Weiss]], [[Kip Thorne]] and [[Barry Barish]] "for decisive contributions to the LIGO detector and the observation of gravitational waves".<ref>{{cite web |title=The Nobel Prize in Physics 2017 |publisher=Nobel Foundation |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2017/press.html }}</ref>

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 [[List of gravitational wave observations|detections]] of gravitational waves.<ref>{{cite web |url=https://www.ligo.org/news/index.php#GWTC3-TGRwebinar |title=LSC News }}</ref><ref name="GWTC3">{{cite web |url=https://www.ligo.org/news/index.php#GWTC3 |title=LSC News }}</ref> 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]].<ref>{{cite journal |last1=The LIGO Scientific Collaboration |last2=the Virgo Collaboration |last3=Abbott |first3=B. P. |last4=Abbott |first4=R. |last5=Abbott |first5=T. D. |last6=Abraham |first6=S. |last7=Acernese |first7=F. |last8=Ackley |first8=K. |last9=Adams |first9=C. |last10=Adhikari |first10=R. X. |last11=Adya |first11=V. B. |date=4 September 2019 |title=GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs |journal=Physical Review X |volume=9 |issue=3 |article-number=031040 |doi=10.1103/PhysRevX.9.031040 |issn=2160-3308 |arxiv=1811.12907 |bibcode=2019PhRvX...9c1040A |s2cid=119366083 }}</ref> 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 2019<ref>{{cite web |url=https://twitter.com/ligo/status/1190283820228055041 |title=Welcome to O3b! |last=LIGO |date=1 November 2019 |website=@ligo |language=en |access-date=11 November 2019 }}</ref> until it was suspended on 27 March 2020 due to [[Coronavirus disease 2019|COVID-19]].<ref name="O3suspended">{{cite web |url=https://www.ligo.caltech.edu/news/ligo20200326 |title=LIGO Suspends Third Observing Run (O3) |date=26 March 2020 |access-date=15 July 2020 }}</ref> The O3 run included the first detection of the merger of a neutron star with a black hole.<ref name="GWTC3"/>

Subsequent gravitational wave observatories [[Virgo interferometer|Virgo]] in Italy, and [[KAGRA]] in Japan, which both use interferometer arms 3&nbsp;km long, coordinated with LIGO to continue observations after the COVID-caused stop, and LIGO's O4 observing run started on 24 May 2023.<ref name=":1">{{cite web |date=24 May 2023 |title=Gravitational-Wave Observatory Status |url=https://gwosc.org/detector_status/ |access-date=25 May 2023 |website=Gravitational Wave Open Science Center }}</ref><ref name=":0">{{cite journal |last=Castelvecchi |first=Davide |date=24 May 2023 |title=Gravitational-wave detector LIGO is back — and can now spot more colliding black holes than ever |url=https://www.nature.com/articles/d41586-023-01732-4 |journal=Nature |volume=618 |issue=7963 |pages=13–14 |language=en |doi=10.1038/d41586-023-01732-4 |pmid=37225822 |bibcode=2023Natur.618...13C |s2cid=258899900 |url-access=subscription }}</ref> LIGO projects a sensitivity goal of 160–190&nbsp;Mpc for binary neutron star mergers (sensitivities: Virgo 80–115&nbsp;Mpc, KAGRA greater than 1&nbsp;Mpc).<ref>{{cite web |title=LIGO, VIRGO AND KAGRA OBSERVING RUN PLANS |url=https://gwcenter.icrr.u-tokyo.ac.jp/en/archives/1581 |access-date=14 December 2021 }}</ref>

== History ==
=== Background ===
[[File:LIGO Hanford aerial 05.jpg|thumb|right|LIGO [[Hanford, Washington|Hanford]] Observatory]]
[[File:Ligo-livingston-aerial-03 599x400.jpg|thumb|right|LIGO [[Livingston, Louisiana| Louisiana]] 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]],<ref name="NSF">{{cite book |publisher=National Academies Press |pages=109–117 |isbn=978-0-309-09084-1 |url=http://www.nap.edu/catalog/10895/setting-priorities-for-large-research-facility-projects-supported-by-the-national-science-foundation |bibcode=2004splr.rept.....C |doi=10.17226/10895 |title=Setting Priorities for Large Research Facility Projects Supported by the National Science Foundation |year=2004 |journal=<!-- --> |author1=<!----> }}</ref><ref>{{cite journal |last=Gertsenshtein |first=M.E. |date=1962 |title=Wave Resonance of Light and Gravitational Waves |journal=Journal of Experimental and Theoretical Physics |volume=14 |page=84 }}</ref> 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.<ref>{{cite journal |date=1972 |journal=Quarterly Progress Report of the Research Laboratory of Electronics |author=Weiss, Rainer |title=Electromagnetically coupled broadband gravitational wave antenna |volume=105 |issue=54 |page=84 |url=https://dcc.ligo.org/P720002/public |access-date=21 February 2016 }}</ref> 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.<ref name="NSF"/>

Prototype interferometric gravitational wave detectors (interferometers) were built in the late 1960s by [[Robert L. Forward]] and colleagues at [[HRL Laboratories|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 [[Rainer Weiss|Weiss]] at MIT, and then by [[Heinz Billing]] and colleagues in [[Garching bei München|Garching]] Germany, and then by [[Ronald Drever]], [[James Hough]] and colleagues in Glasgow, Scotland.<ref>{{cite web |title=A brief history of LIGO |publisher=ligo.caltech.edu |url=https://www.ligo.caltech.edu/system/media_files/binaries/313/original/LIGOHistory.pdf |access-date=21 February 2016 |archive-url=https://web.archive.org/web/20170703113615/https://www.ligo.caltech.edu/system/media_files/binaries/313/original/LIGOHistory.pdf |archive-date=3 July 2017 }}</ref>

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&nbsp;km scale with adequate sensitivity.<ref name="NSF"/><ref name="buderi">{{cite journal |last=Buderi |first=Robert |date=19 September 1988 |title=Going after gravity: How a high-risk project got funded. |url=http://www.the-scientist.com/?articles.view/articleNo/9753/title/Going-After-Gravity--How-A-High-Risk-Project-Got-Funded/ |journal=The Scientist |volume=2 |issue=17 |page=1 |access-date=18 February 2016 }}</ref>

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 bei München|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 Eugen Vogt|Rochus E. Vogt]] (Caltech), was appointed. In 1988, a research and development proposal achieved funding.<ref name="NSF"/><ref name="buderi"/><ref name="jeffery">{{cite journal |last=Mervis |first=Jeffery |title=Funding of two science labs receives pork barrel vs beer peer review debate. |url=http://www.the-scientist.com/?articles.view/articleNo/12069/title/Funding-Of-Two-Science-Labs-Revives-Pork-Barrel-Vs--Peer-Review-Debate/ |journal=The Scientist |volume=5 |issue=23 |access-date=21 February 2016 }}</ref><ref name="waldrop">{{cite journal |last=Waldrop |first=M. Mitchell |date=7 September 1990 |title=Of politics, pulsars, death spirals – and LIGO |journal=Science |volume=249 |issue=4973 |pages=1106–1108 |doi=10.1126/science.249.4973.1106 |pmid=17831979 |bibcode=1990Sci...249.1106W }}</ref><ref>{{cite web |url=http://www.ligo.org/news/detection-press-release.pdf |title=Gravitational waves detected 100 years after Einstein's prediction |date=11 February 2016 |publisher=LIGO |access-date=11 February 2016 |archive-date=3 April 2016 |archive-url=https://web.archive.org/web/20160403025800/http://www.ligo.org/news/detection-press-release.pdf }}</ref><ref>{{cite journal |last=Irion |first=Robert |date=21 April 2000 |title=LIGO's mission of gravity. |journal=Science |volume=288 |issue=5465 |pages=420–423 |doi=10.1126/science.288.5465.420 |s2cid=119020354 }}</ref>

From 1989 through 1994, LIGO failed to progress technically and organizationally. Only political efforts continued to acquire funding.<ref name="NSF"/><ref name="cohen">{{cite web |url=http://oralhistories.library.caltech.edu/178/1/Barish_OHO.pdf |title=Interview with Barry Barish |work=Shirley Cohen |publisher=Caltech |year=1998 |access-date=21 February 2016 |archive-date=23 September 2015 |archive-url=https://web.archive.org/web/20150923084916/http://oralhistories.library.caltech.edu/178/1/Barish_OHO.pdf |url-status=dead }}</ref> Ongoing funding was routinely rejected until 1991, when the [[United States Congress|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.<ref name="jeffery"/><ref name="waldrop"/> By 1992, LIGO was restructured with Drever no longer a direct participant.<ref name="NSF"/><ref name="cohen"/><ref name="Cook">{{cite conference |last=Cook |first=Victor |date=21 September 2001 |title=NSF Management and Oversight of LIGO |conference=Large Facility Projects Best Practices Workshop |publisher=NSF }}</ref><ref>{{cite journal |last=Travis |first=John |date=18 February 2016 |title=LIGO: A$250 million gamble. |journal=Science |volume=260 |issue=5108 |pages=612–614 |doi=10.1126/science.260.5108.612 |pmid=17812204 |bibcode=1993Sci...260..612T }}</ref> 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.<ref name="NSF"/><ref name="cohen"/><ref>{{cite journal |last=Anderson |first=Christopher |date=11 March 1994 |title=LIGO director out in shakeup |journal=Science |volume=263 |issue=5152 |page=1366 |doi=10.1126/science.263.5152.1366 |pmid=17776497 |bibcode=1994Sci...263.1366A }}</ref><ref>{{cite news |last=Browne |first=Malcolm W. |date=30 April 1991 |title=Experts clash over project to detect gravity wave. |url=https://www.nytimes.com/1991/04/30/science/experts-clash-over-project-to-detect-gravity-wave.html?pagewanted=all |newspaper=New York Times |access-date=21 February 2016 }}</ref>

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,<ref name="NSF"/><ref name="Cook"/><ref>{{cite journal |last=Anderson |first=Christopher |date=11 March 1994 |title=LIGO director out in shakeup |journal=Science |volume=263 |issue=5152 |page=1366 |doi=10.1126/science.263.5152.1366 |pmid=17776497 |bibcode=1994Sci...263.1366A }}</ref> and the NSF made clear that LIGO had one last chance for support.<ref name="cohen"/> 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.<ref name="Nature_2014_July_14">{{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 }}</ref> 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.<ref name="NSF"/><ref name="jeffery"/>

=== 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.<ref>{{cite web |title=Gravitational wave detection a step closer with Advanced LIGO |url=http://spie.org/newsroom/technical-articles/videos/ligo-hanford-spie-video |publisher=SPIE Newsroom |access-date=4 January 2016 }}</ref><ref>{{cite web |title=Daniel Sigg: The Advanced LIGO Detectors in the era of First Discoveries |url=http://spie.org/x120637.xml |publisher=SPIE Newsroom |access-date=9 September 2016 }}</ref> Much of the research and development work for the LIGO/aLIGO machines was based on pioneering work for the [[GEO600]] detector at Hannover, Germany.<ref name="GEO600 a precursor">{{cite news |last=Ghosh |first=Pallab |url=https://www.bbc.com/news/science-environment-35524440 |title=Einstein's gravitational waves 'seen' from black holes |work=BBC News |date=11 February 2016 |access-date=18 February 2016 }}</ref><ref name="mpg2016">{{cite web |title=Gravitational waves detected 100 years after Einstein's prediction |url=https://www.mpg.de/9953360/gravitational-waves-detected |website=www.mpg.de |publisher=Max-Planck-Gelschaft |access-date=3 September 2018 |language=en }}</ref> By February 2015, the detectors were brought into engineering mode in both locations.<ref>{{cite web |title=LIGO Hanford's H1 Achieves Two-Hour Full Lock |url=https://www.advancedligo.mit.edu/feb_2015_news.html |date=February 2015 |archive-url=https://web.archive.org/web/20150922064706/https://www.advancedligo.mit.edu/feb_2015_news.html |archive-date=22 September 2015 }}</ref>

In mid-September 2015, "the world's largest gravitational-wave facility" completed a five-year US$200-million overhaul, bringing the total cost to $620 million.<ref name="Nature_2015_Sept_15">{{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 }}</ref><ref>{{cite magazine |first=Sarah |last=Zhang |title=The Long Search for Elusive Ripples in Spacetime |magazine=Wired |url=https://www.wired.com/2015/09/long-search-elusive-ripples-spacetime |date=15 September 2015 }}</ref> On 18 September 2015, Advanced LIGO began its first formal science observations at about four times the sensitivity of the initial LIGO interferometers.<ref>{{cite news |last=Amos |first=Jonathan |title=Advanced Ligo: Labs 'open their ears' to the cosmos |url=https://www.bbc.com/news/science-environment-34298363 |journal=BBC News |date=19 September 2015 |access-date=19 September 2015 }}</ref> Its sensitivity was to be further enhanced until it was planned to reach design sensitivity {{As of|alt=around 2021|2021|01|post=.}}<ref name="LIGO_dec_2015">{{cite web |title=Planning for a bright tomorrow: prospects for gravitational-wave astronomy with Advanced LIGO and Advanced Virgo |url=http://www.ligo.org/science/Publication-ObservingScenario/index.php |publisher=[[LIGO Scientific Collaboration]] |access-date=31 December 2015 |date=23 December 2015 }}</ref>{{Needs update|date=October 2025}}

==== Detections ====
On 11 February 2016, the LIGO Scientific Collaboration and [[Virgo interferometer|Virgo Collaboration]] published a paper about the [[First observation of gravitational waves|detection of gravitational waves]], from a signal detected at 09.51 [[Coordinated Universal Time|UTC]] on 14 September 2015 of two ~30 [[solar mass]] black holes merging about 1.3 billion [[light-years]] from Earth.<ref name="PhysRevLett.116.061102">{{cite journal |title=Observation of Gravitational Waves from a Binary Black Hole Merger |journal=Physical Review Letters |volume=116 |issue=6 |article-number=061102 |date=11 February 2016 |last=LIGO Scientific Collaboration and Virgo Collaboration |first=B. P. Abbott |doi=10.1103/PhysRevLett.116.061102 |pmid=26918975 |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |s2cid=124959784 }}</ref><ref name="Nature_11Feb16" />

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.<ref>{{cite book |url=https://cds.cern.ch/record/2131411 |title=New results on the Search for Gravitational Waves |year=2016 |series=CERN Colloquium }}</ref>

On 2 May 2016, members of the [[LIGO Scientific Collaboration]] and other contributors were awarded a [[Fundamental Physics Prize|Special Breakthrough Prize in Fundamental Physics]] for contributing to the direct detection of gravitational waves.<ref name=FPP-May2016>{{cite web |title=Fundamental Physics Prize – News |url=https://breakthroughprize.org/News/32 |publisher=Fundamental Physics Prize (2016) |access-date=4 May 2016 }}</ref>

On 16 June 2016 LIGO announced a [[GW151226|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.<ref name="chu"/>

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.<ref name = "Abbott2017">{{cite journal |doi=10.1103/PhysRevLett.118.221101 |pmid=28621973 |title=GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2 |journal=[[Physical Review Letters]] |date=1 June 2017 |author=B. P. Abbott |display-authors=etal |collaboration=[[LIGO Scientific Collaboration]] and [[Virgo interferometer|Virgo Collaboration]] |volume=118 |issue=22 |article-number=221101 |arxiv=1706.01812 |bibcode=2017PhRvL.118v1101A |s2cid=206291714 }}</ref><ref name = SciNews2017.06.01>{{cite journal |last=Conover |first=E. |title=LIGO snags another set of gravitational waves |journal=[[Science News]] |date=1 June 2017 |url=https://www.sciencenews.org/article/ligo-snags-another-set-gravitational-waves |access-date=3 June 2017 }}</ref> [[Laura Cadonati]] was appointed the first deputy spokesperson.<ref>{{cite web |url=https://news.gatech.edu/news/2017/04/20/college-sciences-professor-appointed-top-role-search-gravitational-waves |title=College of Sciences Professor Appointed to Top Role in Search for Gravitational Waves &#124; News Center |access-date=14 January 2023 |archive-date=28 February 2024 |archive-url=https://web.archive.org/web/20240228144106/https://news.gatech.edu/news/2017/04/20/college-sciences-professor-appointed-top-role-search-gravitational-waves }}</ref>

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.<ref>{{cite web |title=GW170814: A three-detector observation of gravitational waves from a binary black hole coalescence |url=https://dcc.ligo.org/LIGO-P170814/public/main |access-date=29 September 2017 }}</ref>

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.<ref>{{cite web |title=The Nobel Prize in Physics 2017 |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2017/press.html |website=Nobelprize.org |access-date=4 October 2017 }}</ref><ref name="BBC-20171003">{{cite news |last1=Rincon |first1=Paul |last2=Amos |first2=Jonathan |url=https://www.bbc.co.uk/news/science-environment-41476648 |title=Einstein's waves win Nobel Prize |work=[[BBC News]] |date=3 October 2017 |access-date=3 October 2017 }}</ref><ref name="NYT-20171003">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=2017 Nobel Prize in Physics Awarded to LIGO Black Hole Researchers |url=https://www.nytimes.com/2017/10/03/science/nobel-prize-physics.html |date=3 October 2017 |work=[[The New York Times]] |access-date=3 October 2017 }}</ref>

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.<ref>{{cite web |url=https://www.ligo.org/news/pr-O3resumes.pdf |title=LSC News |access-date=31 March 2019 |archive-date=27 March 2019 |archive-url=https://web.archive.org/web/20190327121626/https://www.ligo.org/news/pr-O3resumes.pdf }}</ref> Both ran until 27 March 2020, when the [[COVID-19 pandemic]] halted operations.<ref name="O3suspended" /> During the COVID shutdown, LIGO underwent a further upgrade in sensitivity, and observing run O4 with the new sensitivity began on 24 May 2023.<ref name=":1" />

== 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 [[Laser Interferometer Space Antenna|evolved Laser Interferometer Space Antenna]] (eLISA) and [[pulsar timing array]]s 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.<ref>{{cite web |title=Gravitational Wave Detectors and Sources |url=http://rhcole.com/apps/GWplotter/ |access-date=20 April 2014 |last1=Moore |first1=Christopher |last2=Cole |first2=Robert |last3=Berry |first3=Christopher |date=19 July 2013 }}</ref> These frequencies that aLIGO can detect are in the range of [[Hearing#Frequency range|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 in 1974, when observations of the binary pulsar [[PSR 1913+16]] showed an orbital decay which matched Einstein's predictions of energy loss by gravitational radiation. The Nobel Prize in Physics 1993 was awarded to [[Russell Alan Hulse|Hulse]] and [[Joseph Hooton Taylor, Jr.|Taylor]] for this discovery.<ref>{{cite web |title=The Nobel Prize in Physics 1993: Russell A. Hulse, Joseph H. Taylor Jr. |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1993/ |website=nobelprize.org }}</ref>

Direct detection of gravitational waves had long been sought. Their discovery has launched a new branch of astronomy to complement [[Electromagnetic radiation|electromagnetic]] telescopes and [[neutrino]] observatories. [[Joseph Weber]] pioneered the effort to detect gravitational waves in the 1960s through his work on [[Weber bar|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.<ref name="obituary">{{cite web |title=Obituary: Dr. Robert L. Forward |url=http://www.spaceref.com/news/viewpr.html?pid=9328 |archive-url=https://archive.today/20130902222420/http://www.spaceref.com/news/viewpr.html?pid=9328 |archive-date=2 September 2013 |website=www.spaceref.com |date=21 September 2002 |access-date=3 September 2018 |language=en }}</ref>

In fact as early as the 1960s, and perhaps before that, there were papers published on wave resonance of light and gravitational waves.<ref>{{cite journal |author=M.E. Gertsenshtein |title=Wave Resonance of Light and Gravitational Waves |journal=JETP |volume=41 |issue=1 |pages=113–114 |year=1961 |url=http://www.jetp.ac.ru/cgi-bin/e/index/e/14/1/p84?a=list |access-date=19 January 2016 |archive-date=6 February 2016 |archive-url=https://web.archive.org/web/20160206055044/http://www.jetp.ac.ru/cgi-bin/e/index/e/14/1/p84?a=list }}</ref> 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.<ref>{{cite journal |title=On the detection of low frequency gravitational waves |first1=M. E. |last1=Gertsenshtein |first2=V. I. |last2=Pustovoit |journal=[[Journal of Experimental and Theoretical Physics|JETP]] |volume=43 |pages=605–607 |date=August 1962 }}</ref> The authors argued that by using interferometers the sensitivity can be 10<sup>7</sup> to 10<sup>10</sup> times better than by using electromechanical experiments. Later, in 1965, [[Vladimir Braginsky|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 wave]]s—of significant astrophysical interest—is now possible.<ref>{{cite journal |title=Astrophysical Sources of Gravitational Radiation |journal=[[Annual Review of Nuclear and Particle Science]] |volume=44 |issue=44 |pages=655–717 |doi=10.1146/annurev.ns.44.120194.003255 |doi-access=free |year=1994 |last1=Bonazzola |first1=S |last2=Marck |first2=J A |bibcode=1994ARNPS..44..655B }}</ref>

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 star]]s 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 [[Big Bang|birth of the universe]]. The observatory may, in theory, also observe more exotic hypothetical phenomena, such as gravitational waves caused by oscillating [[cosmic string]]s or colliding [[Domain wall (string theory)|domain walls]].

== Observatories ==
<!-- This section is linked from [[LIGO Livingston Observatory]] -->
LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory ({{coord|30|33|47|N|90|46|28|W}}) in [[Livingston, Louisiana]], and the LIGO Hanford Observatory, on the [[Hanford Site|DOE Hanford Site]] ({{coord|46|27|19|N|119|24|28|W}}), located near [[Richland, Washington]]. These sites are separated by 3002&nbsp;km straight line distance through the earth, but 3030&nbsp;km 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. With the difference in arrival times at three different facilities one can [[trilaterate]] the source of the wave.
This is done by adding a third instrument like [[Virgo interferometer|Virgo]], located at an even greater distance, in Europe.<ref>{{cite web |title=Location of the Source |url=http://www.sr.bham.ac.uk/gwastro/what-are-we-looking-for/4 |website=Gravitational Wave Astrophysics |publisher=University of Birmingham |access-date=28 November 2015 |archive-url=https://web.archive.org/web/20151208133633/http://www.sr.bham.ac.uk/gwastro/what-are-we-looking-for/4 |archive-date=8 December 2015 }}</ref>

Each observatory supports an L-shaped [[ultra high vacuum]] system, measuring 4&nbsp;km on each side. Up to five [[interferometer]]s 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&nbsp;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&nbsp;km interferometer, the [[Fabry–Pérot]] arm cavities had the same optical finesse, and, thus, half the storage time as the 4&nbsp;km interferometers. With half the storage time, the theoretical strain sensitivity was as good as the full length interferometers above 200&nbsp;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}}
The parameters in this section refer to the [[#Advanced LIGO|Advanced LIGO]] experiment. The primary interferometer consists of two beam lines of 4&nbsp;km length which form a power-recycled [[Michelson interferometer]] with [[Gires–Tournois etalon]] arms. A pre-stabilized 1064&nbsp;nm [[Nd:YAG laser]] emits a beam with a power of 20&nbsp;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&nbsp;W. From the beam splitter the light travels along two orthogonal arms. By the use of partially reflecting mirrors, [[Fabry–Pérot interferometer|Fabry–Pérot cavities]] are created in both arms that increase the effective path length of laser light in the arm from 4&nbsp;km to approximately {{val|1200|u=km}}.<ref>{{cite web |title=LIGO's Interferometer |url=https://www.ligo.caltech.edu/page/ligos-ifo }}</ref> The power of the light field in the cavity is 100&nbsp;kW.<ref name="PRL-20160211" />

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 (waves)|phase]] (antiphase) with the incoming light. The cavity will therefore periodically get very slightly out of [[Coherence (physics)|coherence]] and the beams, which are tuned to [[Interference (wave propagation)|destructively interfere]] at the detector, will have a very slight periodically varying detuning. This results in a measurable signal.<ref>
{{cite web |author=Thorne, Kip |author1-link=Kip Thorne |title=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) |url=http://www.pmaweb.caltech.edu/Courses/ph136/yr2012/1227.1.K.pdf |year=2012 |access-date=11 February 2016 }}</ref>

After an equivalent of approximately 280 trips down the 4&nbsp;km length to the far mirrors and back again,<ref>{{cite web |title=LIGO's Interferometer |url=https://www.ligo.caltech.edu/page/ligos-ifo }}</ref> 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.

=== Noise ===
Since the signal being measured is so small, the LIGO project has carefully measured, documented, and eliminated many forms of noise. Mechanical/seismic noise sources can move objects in the optical system, such as the mirrors, the photon injectors, etc. Electric and magnetic noise can vibrate the permanent magnets and electronics, and radio waves can couple to electronics in the interferometer controls. These noises may drown out the real signal, or produce spurious signals. Background noise and unknown errors (which happen daily) are in the order of 10<sup>−20</sup>, while gravitational wave signals are around 10<sup>−22</sup>. 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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, 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Much complexity in the instrument is in reducing these spurious couplings, and in monitoring the environment to detect previously unknown sources of noise in order to mitigate them. Other than careful insulation of the equipment, the methods of mitigation also include:
* If the noise source is produced by nearby human activity, then the human activity can be adjusted or relocated. For example, the main road near the LHO site was repaved to reduce vibrations caused by trucks.<ref name=":2">{{cite journal |last1=Effler |first1=A |last2=Schofield |first2=R M S |last3=Frolov |first3=V V |last4=González |first4=G |last5=Kawabe |first5=K |last6=Smith |first6=J R |last7=Birch |first7=J |last8=McCarthy |first8=R |date=5 February 2015 |title=Environmental influences on the LIGO gravitational wave detectors during the 6th science run |url=https://iopscience.iop.org/article/10.1088/0264-9381/32/3/035017 |journal=Classical and Quantum Gravity |volume=32 |issue=3 |article-number=035017 |doi=10.1088/0264-9381/32/3/035017 |arxiv=1409.5160 |bibcode=2015CQGra..32c5017E |issn=0264-9381 }}</ref>
* If the noise source is predictable, then the equipment can adjust directly to cancel out the noise source. This is true for Earth tides.
* If the noise source is not predictable, but measurable, and its effect on the equipment is known, then the noise can be recorded and subtracted from the signal afterwards. This is true for seismic activities.

Seismic vibration is a major source of noise, mitigated by seismic insulation, as well as by sensitive recording of seismic activity in the vicinity of the site, which allows the effect of seismic activity to be subtracted away from the signal. [[Earth tide]]s produce significant but highly predictable stretching of the equipment, which is cancelled out by a system which adjusts the position of the chambers or the laser frequency, based on the predicted stretching. There are also many "microseismic" sources that produce ground vibrations, including ocean storms, dam operations, forest logging, trucks driving on a road 2&nbsp;km away, walking heavily in a control room, etc. When under a strong wind, the walls of the building can tilt, which deforms the floor within 10&nbsp;m of the walls, creating "tilt noise". Even the [[Thermal expansion|thermal expansion and contraction]] of the building itself creates creaks and thumps. Similarly, acoustic vibrations can shake the equipment. This includes wind, nearby vehicles, [[propeller aircraft]], [[Heating, ventilation, and air conditioning|HVAC]] of a building 300&nbsp;m away, [[Fan (machine)|cooling fans]], etc.<ref name=":2" /><ref>{{cite web |last=Schofield |first=Robert |title=LIGO environmental influences |url=https://people.ligo-wa.caltech.edu/~robert.schofield/iLIGOenvironmentalInflueinces.htm }}</ref><ref>{{cite journal |last1=Nguyen |first1=P |last2=Schofield |first2=R M S |last3=Effler |first3=A |last4=Austin |first4=C |last5=Adya |first5=V |last6=Ball |first6=M |last7=Banagiri |first7=S |last8=Banowetz |first8=K |last9=Billman |first9=C |last10=Blair |first10=C D |last11=Buikema |first11=A |last12=Cahillane |first12=C |last13=Clara |first13=F |last14=Covas |first14=P B |last15=Dalya |first15=G |date=22 July 2021 |title=Environmental noise in advanced LIGO detectors |url=https://iopscience.iop.org/article/10.1088/1361-6382/ac011a |journal=Classical and Quantum Gravity |volume=38 |issue=14 |page=145001 |doi=10.1088/1361-6382/ac011a |arxiv=2101.09935 |bibcode=2021CQGra..38n5001N |issn=0264-9381 }}</ref><ref name=":3">{{citation |last1=Fiori |first1=Irene |title=Environmental Noise in Gravitational-Wave Interferometers |date=2021 |work=Handbook of Gravitational Wave Astronomy |pages=1–72 |editor-last=Bambi |editor-first=Cosimo |url=https://link.springer.com/10.1007/978-981-15-4702-7_10-1 |access-date=2 July 2025 |place=Singapore |publisher=Springer Singapore |language=en |doi=10.1007/978-981-15-4702-7_10-1 |isbn=978-981-15-4702-7 |last2=Effler |first2=Anamaria |last3=Nguyen |first3=Philippe |last4=Paoletti |first4=Federico |last5=Schofield |first5=Robert M. S. |last6=Tringali |first6=Maria C. |bibcode=2021hgwa.bookE..10F |editor2-last=Katsanevas |editor2-first=Stavros |editor3-last=Kokkotas |editor3-first=Konstantinos D. |url-access=subscription }}</ref> Other sources of noise include a case of ravens pecking on frost-covered pipes connected to a [[Liquid nitrogen|LN2]] cryopump on hot days.<ref>{{cite web |date=19 July 2017 |title=aLIGO LHO Logbook: Why the GW channel detects thirsty black ravens along with colliding black holes |url=https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=37630 |access-date=1 July 2025 |website=alog.ligo-wa.caltech.edu }}</ref> Some noises are transient with no identifiable cause.<ref>{{cite journal |last1=Coughlin |first1=S. |last2=Bahaadini |first2=S. |last3=Rohani |first3=N. |last4=Zevin |first4=M. |last5=Patane |first5=O. |last6=Harandi |first6=M. |last7=Jackson |first7=C. |last8=Noroozi |first8=V. |last9=Allen |first9=S. |last10=Areeda |first10=J. |last11=Coughlin |first11=M. |last12=Ruiz |first12=P. |last13=Berry |first13=C. P. L. |last14=Crowston |first14=K. |last15=Katsaggelos |first15=A. K. |date=16 April 2019 |title=Classifying the unknown: Discovering novel gravitational-wave detector glitches using similarity learning |url=https://link.aps.org/doi/10.1103/PhysRevD.99.082002 |journal=Physical Review D |volume=99 |issue=8 |article-number=082002 |doi=10.1103/PhysRevD.99.082002 |arxiv=1903.04058 |bibcode=2019PhRvD..99h2002C }}</ref>

Electromagnetic noise can couple with electronic and magnetic equipment. Such examples include building heaters, large motors, lights or relatively near-by high-voltage power lines up to 4&nbsp;km away from the site.<ref name=":2" /> Since [[Barkhausen effect|Barkhausen noise]] is suspected to be relevant, magnets at noise-critical locations of aLIGO are made of [[Samarium–cobalt magnet|samarium cobalt]], which has lower Barkhausen noise.<ref>{{cite journal |last1=Aston |first1=S M |last2=Barton |first2=M A |last3=Bell |first3=A S |last4=Beveridge |first4=N |last5=Bland |first5=B |last6=Brummitt |first6=A J |last7=Cagnoli |first7=G |last8=Cantley |first8=C A |last9=Carbone |first9=L |last10=Cumming |first10=A V |last11=Cunningham |first11=L |last12=Cutler |first12=R M |last13=Greenhalgh |first13=R J S |last14=Hammond |first14=G D |last15=Haughian |first15=K |date=7 December 2012 |title=Update on quadruple suspension design for Advanced LIGO |url=https://iopscience.iop.org/article/10.1088/0264-9381/29/23/235004 |journal=Classical and Quantum Gravity |volume=29 |issue=23 |article-number=235004 |doi=10.1088/0264-9381/29/23/235004 |bibcode=2012CQGra..29w5004A |issn=0264-9381 |url-access=subscription }}</ref>

Because gravity cannot be shielded, the motion of massive objects can create varying gravitational attraction on the test masses in LIGO, called ''gravity gradient noise'' or ''Newtonian noise''. This can be caused by air and soil density variation. Even the gravitational effect of a single human walking within 5&nbsp;m of the mirror already approaches the noise floor, and the LIGO corner building was designed to keep people at least 10 m from all test masses during normal operations.<ref>{{cite journal |last1=Thorne |first1=Kip S. |last2=Winstein |first2=Carolee J. |date=24 September 1999 |title=Human gravity-gradient noise in interferometric gravitational-wave detectors |url=https://link.aps.org/doi/10.1103/PhysRevD.60.082001 |journal=Physical Review D |language=en |volume=60 |issue=8 |article-number=082001 |doi=10.1103/PhysRevD.60.082001 |arxiv=gr-qc/9810016 |bibcode=1999PhRvD..60h2001T |issn=0556-2821 }}</ref> Cosmic ray events may also create noise in the detector.<ref name=":3" />

== Observations ==
[[File:LIGO on Hanford Reservation.jpg|thumb|300px|Western leg of LIGO [[interferometer]] on [[Hanford 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 4&nbsp;km mirror spacing by about {{val|e=−18|u=m}}, less than one-thousandth the [[charge radius|charge diameter]] of a [[proton]]. Equivalently, this is a relative change in distance of approximately {{val|e=-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 {{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 interferometer|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&nbsp;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&nbsp;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.<ref>{{cite press release |title=LIGO Sheds Light on Cosmic Event |first=Kathy |last=Svitil |url=http://www.caltech.edu/news/ligo-sheds-light-cosmic-event-1367 |date=2 January 2008 |access-date=14 February 2016 |publisher=[[California Institute of Technology]] }}</ref>

=== Enhanced LIGO ===
[[File:Northern leg of LIGO interferometer on Hanford Reservation.JPG|thumb|300px|Northern leg (x-arm) of LIGO [[interferometer]] on [[Hanford Reservation]]]]
After 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.<ref>{{cite tech report |first1=Rana |last1=Adhikari |first2=Peter |last2=Fritschel |first3=Sam |last3=Waldman |title=Enhanced LIGO |url=https://dcc.ligo.org/public/0007/T060156/001/T060156-01.pdf |id=[https://dcc.ligo.org/LIGO-T060156-x0/public LIGO-T060156-01-I] |date=17 July 2006 }}</ref> 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&nbsp;km detectors.<ref>{{cite web |url=https://ligonews.blogspot.com/2009/06/firm-date-set-for-start-of-s6.html |title=Firm Date Set for Start of S6 |first=Dave |last=Beckett |date=15 June 2009 |website=LIGO Laboratory News }}</ref> 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 Hz<ref>{{cite journal |last1=Danilishin |first1=Stefan L. |last2=Khalili |first2=Farid Ya. |last3=Miao |first3=Haixing |date=29 April 2019 |title=Advanced quantum techniques for future gravitational-wave detectors |url=http://link.springer.com/10.1007/s41114-019-0018-y |journal=Living Reviews in Relativity |language=en |volume=22 |issue=1 |page=2 |doi=10.1007/s41114-019-0018-y |arxiv=1903.05223 |bibcode=2019LRR....22....2D |s2cid=119238143 |issn=2367-3613 }}</ref>]]
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.<ref name="Nature_2015_Sept_15" />

The first observing run operated at a sensitivity roughly three times greater than Initial LIGO,<ref name="LIGO_sep_2015">{{cite web |title=The Newest Search for Gravitational Waves has Begun |url=https://www.ligo.caltech.edu/news/ligo20150918 |publisher=LIGO Scientific Collaboration |access-date=9 September 2017 |last=Burtnyk |first=Kimberly |date=18 September 2015 |archive-url=https://web.archive.org/web/20170704121637/https://www.ligo.caltech.edu/news/ligo20150918 |archive-date=4 July 2017 |quote=LIGO's advanced detectors are already three times more sensitive than Initial LIGO was by the end of its observational lifetime }}</ref> and a much greater sensitivity for larger systems with their peak radiation at lower audio frequencies.<ref>{{cite journal |last1=Aasi |first1=J |title=Advanced LIGO |journal=Classical and Quantum Gravity |date=9 April 2015 |volume=32 |issue=7 |article-number=074001 |doi=10.1088/0264-9381/32/7/074001 |arxiv=1411.4547 |bibcode=2015CQGra..32g4001L |s2cid=118570458 }}</ref>

On 11 February 2016, the LIGO and [[Virgo interferometer|Virgo]] collaborations announced the [[first observation of gravitational waves]].{{r|Nature_11Feb16|PRL-20160211}} The signal, named [[GW150914]],<ref name="PRL-20160211" /><ref name="Naeye">{{cite news |last=Naeye |first=Robert |url=http://www.skyandtelescope.com/astronomy-news/gravitational-wave-detection-heralds-new-era-of-science-0211201644/ |title=Gravitational Wave Detection Heralds New Era of Science |work=Sky and Telescope |date=11 February 2016 |access-date=11 February 2016 }}</ref> was recorded on 14 September 2015, just two days after Advanced LIGO started collecting data following the upgrade.<ref name="Nature_11Feb16" /><ref name="cho2016">{{cite journal |last1=Cho |first1=Adrian |title=Here's the first person to spot those gravitational waves |url=https://www.science.org/content/article/here-s-first-person-spot-those-gravitational-waves |journal=Science |date=11 February 2016 |doi=10.1126/science.aaf4039 }}</ref><ref name="BBC_11Feb16">{{cite news |title=Gravitational waves from black holes detected |url=https://www.bbc.co.uk/news/science-environment-35524440 |work=BBC News |date=11 February 2016 }}</ref> It matched the [[Tests of general relativity|predictions of general relativity]]<ref name="Pretorius2005">{{cite journal |last1=Pretorius |first1=Frans |title=Evolution of Binary Black-Hole Spacetimes |journal=Physical Review Letters |volume=95 |issue=12 |article-number=121101 |year=2005 |issn=0031-9007 |doi=10.1103/PhysRevLett.95.121101 |pmid=16197061 |arxiv=gr-qc/0507014 |bibcode=2005PhRvL..95l1101P |s2cid=24225193 }}</ref><ref name="CampanelliLousto2006">{{cite journal |last1=Campanelli |first1=M. |author-link=Manuela Campanelli (scientist) |last2=Lousto |first2=C.O. |author-link2=Carlos Lousto |last3=Marronetti |first3=P. |last4=Zlochower |first4=Y. |year=2006 |title=Accurate Evolutions of Orbiting Black-Hole Binaries without Excision |journal=Physical Review Letters |volume=96 |issue=11 |article-number=111101 |arxiv=gr-qc/0511048 |bibcode=2006PhRvL..96k1101C |doi=10.1103/PhysRevLett.96.111101 |issn=0031-9007 |pmid=16605808 |s2cid=5954627 }}</ref><ref name="BakerCentrella2006">{{cite journal |last1=Baker |first1=John G. |last2=Centrella |first2=Joan |author2-link=Joan Centrella |last3=Choi |first3=Dae-Il |last4=Koppitz |first4=Michael |last5=van Meter |first5=James |title=Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes |journal=Physical Review Letters |volume=96 |issue=11 |article-number=111102 |year=2006 |issn=0031-9007 |doi=10.1103/PhysRevLett.96.111102 |pmid=16605809 |arxiv=gr-qc/0511103 |bibcode=2006PhRvL..96k1102B |s2cid=23409406 }}</ref> for the inward spiral and [[Stellar collision|merger]] of a [[Binary black hole|pair]] of [[black hole]]s 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.<ref name="chu">{{cite news |last1=Chu |first1=Jennifer |title=For second time, LIGO detects gravitational waves |url=https://news.mit.edu/2016/second-time-ligo-detects-gravitational-waves-0615 |access-date=15 June 2016 |work=MIT News |publisher=MIT |date=15 June 2016 }}</ref> The signal was named [[GW151226]].<ref>{{cite journal |url=http://www.ligo.org/science/Publication-GW151226/index.php |title=GW151226: Observation of Gravitational Waves from a 22 Solar-mass Binary Black Hole Coalescence |journal=Physical Review Letters |volume=116 |issue=24 |article-number=241103 |date=15 June 2016 |bibcode=2016PhRvL.116x1103A |last1=Abbott |first1=B.P. |last2=Abbott |first2=R. |last3=Abbott |first3=T.D. |last4=Abernathy |first4=M.R. |last5=Acernese |first5=F. |last6=Ackley |first6=K. |last7=Adams |first7=C. |last8=Adams |first8=T. |last9=Addesso |first9=P. |last10=Adhikari |first10=R.X. |last11=Adya |first11=V.B. |last12=Affeldt |first12=C. |last13=Agathos |first13=M. |last14=Agatsuma |first14=K. |last15=Aggarwal |first15=N. |last16=Aguiar |first16=O.D. |last17=Aiello |first17=L. |last18=Ain |first18=A. |last19=Ajith |first19=P. |last20=Allen |first20=B. |last21=Allocca |first21=A. |last22=Altin |first22=P.A. |last23=Anderson |first23=S.B. |last24=Anderson |first24=W.G. |last25=Arai |first25=K. |last26=Araya |first26=M.C. |last27=Arceneaux |first27=C.C. |last28=Areeda |first28=J.S. |last29=Arnaud |first29=N. |last30=Arun |first30=K.G. |display-authors=3 |arxiv=1606.04855 |doi=10.1103/PhysRevLett.116.241103 |pmid=27367379 |s2cid=118651851 }}</ref>

The second observing run (O2) ran from 30 November 2016<ref>{{cite web |title=VIRGO joins LIGO for the "Observation Run 2" (O2) data-taking period |date=1 August 2017 |url=http://www.virgo-gw.eu/docs/AdV_joins_O2_en.pdf |publisher=LIGO Scientific Collaboration & VIRGO collaboration |access-date=20 October 2017 |archive-date=10 October 2017 |archive-url=https://web.archive.org/web/20171010204215/http://www.virgo-gw.eu/docs/AdV_joins_O2_en.pdf }}</ref> to 25 August 2017,<ref name=O3_update>{{cite web |title=Update on the start of LIGO's 3rd observing run |date=24 April 2018 |url=https://www.ligo.org/news/index.php#O3updateApr18 |access-date=31 August 2018 |quote=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. }}</ref> with Livingston achieving 15–25% sensitivity improvement over O1, and with Hanford's sensitivity similar to O1.<ref>{{cite news |title=Advanced LIGO ramps up, with slight improvements |last1=Grant |first1=Andrew |date=12 December 2016 |journal=[[Physics Today]] |issue=11 |article-number=12128 |quote=The bottom line is that [the sensitivity] is better than it was at the beginning of O1; we expect to get more detections. |doi=10.1063/PT.5.9074 }}</ref> In this period, LIGO saw several further gravitational wave events: [[GW170104]] in January; [[GW170608]] in June; and [[List of gravitational wave observations|five others]] between July and August 2017. Several of these were also detected by the Virgo Collaboration.<ref>[https://dcc.ligo.org/LIGO-P1800307/public GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs]</ref>{{r|Chu17}}<ref>{{cite web |url=https://www.ligo.caltech.edu/news/ligo20170927 |title=Gravitational waves from a binary black hole merger observed by LIGO and Virgo }}</ref> Unlike the black hole mergers which are only detectable gravitationally, [[GW170817]] came from the [[neutron star collision|collision of two neutron stars]] and was also detected electromagnetically by gamma ray satellites and optical telescopes.<ref name=Chu17>{{cite press release |title=LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars |url=https://www.ligo.caltech.edu/page/press-release-gw170817 |date=16 October 2017 |first=Jennifer |last=Chu |publisher=LIGO }}</ref>

The third run (O3) began on 1 April 2019<ref>{{cite web |url=https://www.ligo.caltech.edu/news/ligo20190502 |title=LIGO and Virgo Detect Neutron Star Smash-Ups }}</ref> and was planned to last until 30 April 2020; in fact it was suspended in March 2020 due to [[Coronavirus disease 2019|COVID-19]].<ref name="O3suspended"/><ref>{{cite web |title=Observatory Status |url=https://www.ligo.caltech.edu/page/observatory-status |archive-url=https://web.archive.org/web/20200409072730/https://www.ligo.caltech.edu/page/observatory-status |archive-date=9 April 2020 |url-status=live |work=LIGO |date=23 March 2020 |access-date=23 June 2020 }}</ref><ref>Diego Bersanetti: [https://indico.cern.ch/event/577856/contributions/3422625/ Status of the Virgo gravitational-wave detector and the O3 Observing Run], EPS-HEP2019</ref> 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.<ref>{{cite web |url=https://www.ligo.caltech.edu/news/ligo20200106 |title=LIGO-Virgo network catches another neutron star collision }}</ref>

The fourth observing run (O4) was planned to start in December 2022,<ref>{{cite web |title=LIGO Laboratory statement on long term future observing plans |url=https://www.ligo.caltech.edu/page/ligo-lab-statement-long-term-future-observing-plans |access-date=22 March 2022 |website=LIGO Lab }}</ref> but was postponed until 24 May 2023. O4 is projected to continue until February 2025.<ref name=":1" /> As of O4, the interferometers are operating at a sensitivity of {{val|155|-|175|u=Mpc}},<ref name=":1" /> within the design sensitivity range of {{val|160|-|190|u=Mpc}} for binary neutron star events.<ref>{{cite journal |title=Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO, Advanced Virgo and KAGRA |journal=Living Reviews in Relativity |date=24 November 2020 |doi=10.1007/s41114-020-00026-9 |arxiv=1304.0670 |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Abraham |first4=S. |last5=Acernese |first5=F. |last6=Ackley |first6=K. |last7=Adams |first7=C. |last8=Adya |first8=V. B. |last9=Affeldt |first9=C. |last10=Agathos |first10=M. |last11=Agatsuma |first11=K. |last12=Aggarwal |first12=N. |last13=Aguiar |first13=O. D. |last14=Aiello |first14=L. |last15=Ain |first15=A. |last16=Ajith |first16=P. |last17=Akutsu |first17=T. |last18=Allen |first18=G. |last19=Allocca |first19=A. |last20=Aloy |first20=M. A. |last21=Altin |first21=P. A. |last22=Amato |first22=A. |last23=Ananyeva |first23=A. |last24=Anderson |first24=S. B. |last25=Anderson |first25=W. G. |last26=Ando |first26=M. |last27=Angelova |first27=S. V. |last28=Antier |first28=S. |last29=Appert |first29=S. |last30=Arai |first30=K. |volume=23 |issue=1 |page=3 |pmid=33015351 |pmc=7520625 |bibcode=2020LRR....23....3A |display-authors=1 }}</ref>

The fifth observing run (O5) is projected to begin in late 2025 or in 2026.<ref name=":1" />

== Future ==

=== LIGO-India ===
{{main|INDIGO}}
[[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 [[National Science Foundation|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 roadmap<ref name="gwic2010">{{cite web |title=The future of gravitational wave astronomy |url=https://gwic.ligo.org/roadmap/Roadmap_100814.pdf |publisher=Gravitational Waves International Committee |access-date=3 September 2018 |archive-date=30 July 2017 |archive-url=https://web.archive.org/web/20170730062043/https://gwic.ligo.org/roadmap/Roadmap_100814.pdf }}</ref> issued by the [[Gravitational Wave International Committee|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.<ref>{{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 |article-number=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 }}</ref><ref>{{citation |arxiv=1102.5421 |title=Networks of Gravitational Wave Detectors and Three Figures of Merit |journal=Classical and Quantum Gravity |volume=28 |issue=12 |article-number=125023 |date=25 April 2011 |first=Bernard F. |last=Schutz |bibcode=2011CQGra..28l5023S |doi=10.1088/0264-9381/28/12/125023 |s2cid=119247573 }}</ref> Improvements in localization averages are predicted to be approximately an order of magnitude, with substantially larger improvements in certain regions of the sky.

The [[National Science Foundation|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.<ref>{{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 |archive-url=https://web.archive.org/web/20130411020012/http://www.gravitycentre.com.au/wp-content/uploads/2009/10/Science-article-about-LIGO-Australia.pdf |archive-date=11 April 2013 }}</ref> The first potential distant location was at [[AIGO]] in [[Western Australia]],<ref>{{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 }}</ref> 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.<ref>[https://2009-2017.state.gov/r/pa/prs/ps/2012/06/192271.htm U.S.-India Bilateral Cooperation on Science and Technology] meeting fact sheet&nbsp;– dated 13 June 2012.</ref> 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.<ref>[https://www.nsf.gov/nsb/meetings/2012/0823/major_actions.pdf Memorandum to Members and Consultants of the National Science Board] {{Webarchive|url=https://web.archive.org/web/20210403041125/https://www.nsf.gov/nsb/meetings/2012/0823/major_actions.pdf |date=3 April 2021 }}&nbsp;– dated 24 August 2012</ref> In India, the project was presented to the [[Department of Atomic Energy]] and the [[Department of Science and Technology (India)|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.<ref>{{cite tweet |user=PMOIndia |author=Office of the Prime Minister of India |number=699931256008511492 |title=Cabinet has granted 'in-principle' approval to the LIGO-India mega science proposal for research on gravitational waves. |date=17 February 2016 }}</ref>

A site near pilgrimage site of [[Aundha Nagnath]] in the [[Hingoli district]] of state [[Maharashtra]] in [[western India]] has been selected.<ref>{{cite news |title=First LIGO Lab Outside US To Come Up In Maharashtra's Hingoli |url=http://www.ndtv.com/india-news/first-ligo-lab-outside-us-to-come-up-in-maharashtras-hingoli-1456355 |date=8 September 2016 |journal=[[NDTV]] }}</ref><ref>{{cite web |url=https://www.iucaa.in/~li_events/limma_2019/Day3_LIProj_Souradeep_LIMMA2019.pdf |title=LIGO-India: Origins & site search |last=Souradeep |first=Tarun |date=18 January 2019 |page=27 |url-status=live |archive-url=https://web.archive.org/web/20190915091406/https://www.iucaa.in/~li_events/limma_2019/Day3_LIProj_Souradeep_LIMMA2019.pdf |archive-date=15 September 2019 |access-date=15 September 2019 }}</ref>

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 [[crore]]s.<ref name="approval">{{cite news |url=https://timesofindia.indiatimes.com/india/cabinet-clears-rs-2600-crore-ligo-india-observatory-to-come-up-in-maharashtra-will-be-part-of-global-network/articleshow/99309532.cms |title=Cabinet clears Rs 2,600-crore LIGO-India; Observatory to come up in Maharashtra, will be part of global network |newspaper=The Times of India |date=7 April 2023 }}</ref>

=== 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.<ref>{{cite web |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=297414 |title=Upgraded LIGO to search for universe's most extreme events |website=www.nsf.gov |date=14 February 2019 |language=en |access-date=9 April 2020 }}{{Dead link|date=October 2025 |bot=InternetArchiveBot }}</ref> The changes would almost double Advanced LIGO's sensitivity,<ref>{{cite journal |title=Prospects for doubling the range of Advanced LIGO |journal=Physical Review D |volume=91 |issue=62005 |article-number=062005 |date=16 March 2015 |first1=John |last1=Miller |first2=Lisa |last2=Barsotti |author-link2=Lisa Barsotti |first3=Salvatore |last3=Vitale |first4=Peter |last4=Fritschel |first5=Matthew |last5=Evans |first6=Daniel |last6=Sigg |doi=10.1103/PhysRevD.91.062005 |arxiv=1410.5882 |url=http://dspace.mit.edu/bitstream/handle/1721.1/96149/PhysRevD.91.062005.pdf |bibcode=2015PhRvD..91f2005M |s2cid=18460400 }}</ref><ref>{{cite conference |title=Getting an A+: Enhancing Advanced LIGO |first=Michael E. |last=Zucker |url=https://dcc.ligo.org/LIGO-G1601435/public |id=LIGO-G1601435-v3 |date=7 July 2016 |conference=LIGO–DAWN Workshop II |conference-url=https://wiki.ligo.org/LSC/LIGOworkshop2016/WebHome }}</ref> and increase the volume of space searched by a factor of seven.<ref>{{cite web |url=https://www.popularmechanics.com/space/telescopes/a26362135/ligo-gravitational-wave-observatory-update/ |title=LIGO Gravitational Wave Observatory Getting $30 Million Upgrade |last=Thompson |first=Avery |website=www.popularmechanics.com |language=en-US |access-date=17 February 2019 |date=15 February 2019 }}</ref> The upgrades include:
* Improvements to the mirror suspension system.<ref>{{cite news |url=https://www.bbc.com/news/science-environment-47213202 |title=Black hole detectors to get big upgrade |last=Ghosh |first=Pallab |date=15 February 2019 |access-date=17 February 2019 |language=en-GB }}</ref>
* 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 [[Optical coating|mirror coatings]] with lower mechanical loss.<ref>{{cite web |url=https://dcc.ligo.org/LIGO-T1800042/public |title=LIGO-T1800042-v5: The A+ design curve |website=dcc.ligo.org |access-date=9 April 2020 }}</ref>

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 [[Uncertainty principle|quantum mechanical limit]] on their product.<ref>{{cite web |title=The Quantum Enhanced LIGO Detector Sets New Sensitivity Record |url=http://ligo.org/science/Publication-SqueezedVacuum/index.php }}</ref> 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.<ref>{{cite journal |last1=Tse |first1=M. |last2=Yu |first2=Haocun |last3=Kijbunchoo |first3=N. |last4=Fernandez-Galiana |first4=A. |last5=Dupej |first5=P. |last6=Barsotti |first6=L. |author-link6=Lisa Barsotti |last7=Blair |first7=C. D. |last8=Brown |first8=D. D. |last9=Dwyer |first9=S. E. |last10=Effler |first10=A. |last11=Evans |first11=M. |date=5 December 2019 |title=Quantum-Enhanced Advanced LIGO Detectors in the Era of Gravitational-Wave Astronomy |journal=Physical Review Letters |volume=123 |issue=23 |article-number=231107 |doi=10.1103/PhysRevLett.123.231107 |pmid=31868462 |bibcode=2019PhRvL.123w1107T |doi-access=free |hdl=1721.1/136579.2 |hdl-access=free }}</ref> 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&nbsp;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&nbsp;Hz.<ref name="ISWP15">{{cite report |url=https://dcc.ligo.org/LIGO-T1500290/public |title=Instrument Science White Paper |last1=McClelland |first1=David |last2=Evans |first2=Matthew |date=8 October 2015 |publisher=LIGO Scientific Collaboration |id=LIGO Document T1500290-v2 |last3=Lantz |first3=Brian |last4=Martin |first4=Ian |last5=Quetschke |first5=Volker |last6=Schnabel |first6=Roman }}</ref> Plans call for the glass mirrors and [[Nd:YAG laser|1064&nbsp;nm lasers]] to be replaced by even larger 160&nbsp;kg silicon test masses, cooled to 123&nbsp;K (a temperature achievable with [[liquid nitrogen]]), and a change to a longer laser wavelength in the range 1500–2200&nbsp;nm at which silicon is transparent. (Many documents assume a wavelength of 1550&nbsp;nm, but this is not final.)

Voyager would be an upgrade to A+, to be operational around 2027–2028.<ref>{{cite tech report |author=LIGO Scientific Collaboration |title=Instrument Science White Paper |url=https://dcc.ligo.org/public/0113/T1400316/004/T1400316-v5.pdf |id=LIGO-T1400316-v4 |publisher=LIGO |date=10 February 2015 |access-date=23 June 2020 }}</ref>

=== Cosmic Explorer ===
A design for two larger observatories with longer arms is called "[[Cosmic Explorer (gravitational wave observatory)|Cosmic Explorer]]". This is based on the LIGO Voyager technology, including a similar LIGO-type L-shape geometry, but with 20 and 40&nbsp;km arms. It is currently planned to be on the surface. It has a higher sensitivity than [[Einstein Telescope]] for frequencies beyond 10&nbsp;Hz, but lower sensitivity under 10&nbsp;Hz.<ref name=ISWP15/>

== See also ==
* [[BlackGEM]]
* [[Cosmic Explorer (gravitational wave observatory)|Cosmic Explorer]], a proposed American third-generation gravitational wave detector
* [[Einstein Telescope]], a proposed 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
* [[Fermilab Holometer]]
* [[GEO600]], a gravitational wave detector located in Hannover, Germany
* [[Laser Interferometer Space Antenna|Laser Interferometer Space Antenna (LISA)]]
* [[LISA Pathfinder]]
* [[North American Nanohertz Observatory for Gravitational Waves]]
* [[PyCBC]], an open source software package to help analyze LIGO data
* [[Richard A. Isaacson]]
* [[Taiji Program in Space]], a future Chinese space-based gravitational wave detector
* [[Tests of general relativity]]
* [[TianQin|TianQuin]], a future Chinese space-based gravitational wave detector
* [[Virgo interferometer]], an interferometer located close to Pisa, Italy

== Notes ==
{{reflist|30em|refs=
<ref name="PRL-20160211">{{cite journal |author=Abbott, B.P. |title=Observation of Gravitational Waves from a Binary Black Hole Merger |journal=[[Phys. Rev. Lett.]] |volume=116 |issue=6 |article-number=061102 |year=2016 |doi=10.1103/PhysRevLett.116.061102 |display-authors=etal |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |pmid=26918975 |s2cid=124959784 }}</ref>
<ref name="Nature_11Feb16">{{cite journal |title=Einstein's gravitational waves found at last |journal=Nature News |url=http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 |date=11 February 2016 |last1=Castelvecchi |first1=Davide |last2=Witze |first2=Witze |doi=10.1038/nature.2016.19361 |s2cid=182916902 |access-date=11 February 2016 |doi-access=free }}</ref>
}}

== References ==
{{refbegin|}}
* [[Kip Thorne]], ITP & Caltech. ''[http://online.kitp.ucsb.edu/online/plecture/thorne/ Spacetime Warps and the Quantum: A Glimpse of the Future.]'' Lecture slides and audio
* [[Barry C. Barish]], Caltech. ''[https://cds.cern.ch/record/304956?ln=en The Detection of Gravitational Waves.]'' Video from CERN Academic Training Lectures, 1996
* [[Barry C. Barish]], Caltech. ''[https://www.youtube.com/watch?v=qSPki_1DI38 Einstein's Unfinished Symphony: Sounds from the Distant Universe]'' Video from IHMC Florida Institute for Human Machine Cognition 2004 Evening Lecture Series.
* [[Rainer Weiss]], ''[https://web.archive.org/web/20110721132922/http://www.ligo.caltech.edu/docs/P/P720002-00.pdf 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&nbsp;– JETP Vol. 43 pp.&nbsp;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&nbsp;– M.E. Gertsenshtein&nbsp;– JETP Vol. 41 pp.&nbsp;113–114 (July 1961)
* Gravitational electromagnetic resonance, V.B. Braginskii, M.B. Mensky&nbsp;– GR.G. Vol. 3 No. 4 pp.&nbsp;401–402 (1972)
* Gravitational radiation and the prospect of its experimental discovery, V.B. Braginsky&nbsp;– Usp. Fiz. Nauk Vol. 86 pp.&nbsp;433–446 (July 1965). English translation: Sov. Phys. Uspekhi Vol. 8 No. 4 pp.&nbsp;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&nbsp;– GR.G. Vol. 11 No. 6 pp.&nbsp;407–408 (1979)
* On the propagation of electromagnetic radiation in the field of a plane gravitational wave, E. Montanari&nbsp;– gr-qc/9806054 (11 June 1998)
{{refend}}

== Further reading ==
* {{cite web |last1=Barish |first1=Barry C. |title=The Science and Detection of Gravitational Waves |url=https://labcit.ligo.caltech.edu/~BCBAct/talks00/Alberta/LakeLouisepaper.PDF |year=2000}}
* {{cite book |last=Bartusiak |first=Marcia |title=Einstein's unfinished symphony: listening to the sounds of space-time |url=https://archive.org/details/einsteinsunfinis00bart_0 |url-access=registration |publisher=Joseph Henry Press |location=Washington, DC |year=2000 |isbn=978-0-425-18620-6}}
* {{cite book |last=Saulson |first=Peter |title=Fundamentals of interferometric gravitational wave detectors |url=https://archive.org/details/fundamentalsofin0000saul |url-access=registration |publisher=World Scientific |location=Singapore River Edge, NJ |year=1994 |isbn=978-981-02-1820-1}}
* {{cite book |last=Collins |first=Harry M. |title=Gravity's shadow the search for gravitational waves |url=https://archive.org/details/gravitysshadowse0000coll |url-access=registration |publisher=University of Chicago Press |location=Chicago |year=2004 |isbn=978-0-226-11378-4}}
* {{cite book |last=Kennefick |first=Daniel |title=Traveling at the speed of thought: Einstein and the quest for gravitational waves |url=https://archive.org/details/travelingatspeed0000kenn |url-access=registration |publisher=Princeton University Press |location=Princeton, NJ |year=2007 |isbn=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}}
* {{cite book |last=Collins |first=Harry, M. |title=Gravity's kiss: the detection of gravitational waves |publisher=MIT Press |location=Cambridge, MA & London |year=2017 |isbn=978-0-262-03618-4}}

== External links ==
{{wiktionary|ligo}}
{{commons category}}
{{refbegin}}
* [https://www.ligo.org/magazine/ LIGO Newsletters] Excellent wide-audience newsletters published twice-yearly in March and September. From Issue 1 (September 2012) through to present day.
* [http://ligo.org LIGO Scientific Collaboration] web page
* [http://www.ligo.org/science/outreach.php LIGO outreach] webpage, with links to summaries of the Collaboration's scientific articles, written for a general public audience
* [http://www.ligo.caltech.edu LIGO Laboratory]
* [http://ligonews.blogspot.com LIGO News] blog
* [http://www.livingligo.org Living LIGO] blog: answering questions about LIGO science and being a scientist by LIGO member Amber Stuver
* [https://web.archive.org/web/20101107064702/http://www.advancedligo.mit.edu/ Advanced LIGO homepage]
* [http://arquivo.pt/wayback/20160517170735/http://geco.phys.columbia.edu/ Columbia Experimental Gravity]
* [https://archive.today/20041206082807/http://sciencebulletins.amnh.org/astro/f/gravity.20041101/ American Museum of Natural History film and other materials on LIGO]
* [https://web.archive.org/web/20060907131305/http://www.ligo.caltech.edu/~ajw/40m_upgrade.html 40 m Prototype]
* [http://www.ligo-wa.caltech.edu/ligo_science/earth_motion.html 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.
* [https://web.archive.org/web/20060721201701/http://elmer.tapir.caltech.edu/ph237/CourseMaterials.html 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)
* [http://www.black-holes.org/the-science/gravitational-wave-astronomy Caltech Tutorial on Relativity]&nbsp;– An extensive description of gravitational waves and their sources.
* [https://news.mit.edu/2016/rainer-weiss-ligo-origins-0211 Q&A: Rainer Weiss on LIGO's origins] at news.mit.edu
* [http://cerncourier.com/cws/article/cern/64361 LIGO: a strong belief] {{Webarchive|url=https://web.archive.org/web/20160329053913/http://cerncourier.com/cws/article/cern/64361 |date=29 March 2016 }}, 2/11/16 CERN Courier Interview with Barry Barish (18 March 2016 publication date).
* {{YouTube|gmmD72cFOU4|Video (3:10): LIGO Orrey (1 December 2018)}}
{{refend}}

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