|Laser Interferometer Gravitational-wave Observatory|
The LIGO Hanford Control Room
|Organization||LIGO Scientific Collaboration|
|Location||Hanford Nuclear Reservation, Washington
|First light||August 23, 2002|
|Telescope style||Gravitational-wave Observatory|
LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. Cofounded in 1992 by Kip Thorne and Ronald Drever of Caltech and Rainer Weiss of MIT, LIGO is a joint project between scientists at MIT, Caltech, and many other colleges and universities. It is sponsored by the National Science Foundation (NSF). At the cost of $365 million (in 2002 USD), it is the largest and most ambitious project ever funded by the NSF.
Observations at LIGO began in 2002 and ended in 2010; no unambiguous detections of gravitational waves have been reported. The original detectors were disassembled and are currently being replaced by improved versions known as "advanced LIGO", scheduled to be operational by 2014.
The international LIGO Scientific Collaboration (LSC) is a growing group of researchers, over 800 individuals at roughly 50 institutions, working to analyze the data from LIGO and other detectors, and working toward more sensitive future detectors. The current spokesperson for the LIGO Scientific Collaboration, re-elected in March 2013, is Louisiana State University Professor of Physics and Astronomy, Gabriela González.
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. Gravitational waves were indirectly suggested to exist when observations were made of the binary pulsar PSR 1913+16, for which the Nobel Prize was awarded to Hulse and Taylor in 1993.
Direct detection of gravitational waves has long been sought. Their discovery would launch 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.
In fact as early as the 1960s, and perhaps before that, there were papers published on wave resonance of light and gravitational waves (V.B.Braginsky, L.P.Grishchuck, A.G.Doroshkevieh, M.B.Mensky, I.D.Novikov, M.V.Sazhin and Y.B.Zeldovisch). Based on this phenomenon 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, "On the detection of low frequency gravitational waves," M.E.Gertsenshtein and V.I.Pustovoit, JETP Vol.43, p. 605-607 (August 1962). The authors argued that by using interferometers the sensitivity can be 107-1010 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.
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 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 currently hypothetical phenomena, such as gravitational waves caused by oscillating cosmic strings or colliding domain walls. Since the early 1990s, physicists have believed that technology has evolved to the point where detection of gravitational waves—of significant astrophysical interest—is now possible.
LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory ( ) in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site (coordinates of central complex: ), located near Richland, Washington. These sites are separated by 3,002 kilometers (1,865 miles). 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 triangulation, the difference in arrival times can determine the source of the wave in the sky.
At the Hanford Observatory, a second interferometer operates in parallel with the primary interferometer. This second detector is half the length at 2 kilometers (1.25 miles), and its Fabry–Pérot arm cavities have the same optical finesse and thus half the storage time. With half the storage time, the theoretical strain sensitivity is as good as the full length interferometers above 200 Hz but only half as good at low frequencies.
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, as well as one half-length interferometer. Hanford has been able to retain its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.
Members of the public can tour both observatories, either by special arrangement or on regular "open house" days. The LSC webpages also feature a wide range of information and resources for students, teachers and the general public, including summaries of the Collaboration's scientific articles written for a general audience.
The primary interferometer at each site consists of mirrors suspended at each of the corners of the L; it is known as a power-recycled Michelson interferometer with Fabry–Pérot arms. A pre-stabilized laser emits a beam of up to 200 Watts that passes through an optical mode cleaner before reaching a beam splitter at the vertex of the L. There the beam splits into two paths, one for each arm of the L; each arm contains Fabry–Pérot cavities that store the beams and increase the effective path length.
When a gravitational wave passes through the interferometer, the space-time 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 with the incoming light. The cavity will therefore periodically get very slightly out of resonance 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. Note that the effective length change and the resulting phase change are a subtle tidal effect that must be carefully computed because the light waves are affected by the gravitational wave just as much as the beams themselves.
After an equivalent of approximately 75 trips down the 4 km length to the far mirrors and back again, 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 resonance (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, so some light arrives at the photodiode, indicating 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 which produces 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. Observers compare signals from both sites to reduce the effects of noise.
Based on current models of astronomical events, and the predictions of the general theory of relativity, gravitational waves that originate tens of millions of light years from Earth are expected to distort the 4 kilometer mirror spacing by about 10−18 m, less than one-thousandth the charge diameter of a proton. Equivalently, this is a relative change in distance of approximately one part in 1021. 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 2 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 1021 over a 100 Hz bandwidth. The baseline inspiral of two roughly solar-mass neutron stars is typically expected to be observable if it occurs within about 8,000,000 parsecs (26,000,000 ly), or the vicinity of our Local Group of galaxies, 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. It is hoped that after extensive analysis, data from this run may uncover two unambiguous detection events. This would be a milestone in the history of physics. In 2004, it was reported that theorists estimated the chances of an unambiguous direct detection by 2010 at one in six.
In February 2007, GRB 070201, a short gamma-ray burst, arrived at Earth from the direction of the Andromeda Galaxy, a nearby galaxy. The prevailing explanation of most short gamma-ray bursts is the merger of a neutron star with either a neutron star or 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 is predicated on LIGO eventually demonstrating a direct detection of gravitational waves.
In August 2010 a radio pulsar was discovered through analysis of data from radio telescopes by the LIGO collaboration. The detection was made using the "Einstein@home" personal computer collaboration through LIGO and BOINC. Although significant, discovery of this radio pulsar was not a detection of gravitational waves nor did it involve the LIGO interferometers.
Enhanced LIGO 
After the completion of Science Run 5, initial LIGO was upgraded with certain Advanced LIGO technologies that resulted in an improved-performance configuration dubbed Enhanced LIGO. Its aim was a best-effort goal of achieving twice the sensitivity of initial LIGO by the end of the run, unlike the NSF-contracted sensitivity goals of the Initial and Advanced instruments. 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. It concluded in October 2010, and the disassembling of the original detectors began. An estimated four-year long effort to install and commission the Advanced LIGO detectors is currently underway.
Advanced LIGO 
The LIGO Laboratory, funded by the National Science Foundation with contributions from the GEO 600 Collaboration and ANU and Adelaide Universities in Australia, and with participation by the LIGO Scientific Collaboration, is building Advanced LIGO. This new detector is designed to improve the sensitivity of initial LIGO by more than a factor of 10, and is currently being installed at both LIGO Observatories, replacing the original detectors. Scheduled for interferometer acceptance in 2014, the Advanced LIGO system is anticipated to transform gravitational wave science into a powerful observational tool. As of end-2012, the Project phase is more than 80% complete and is on track to complete on time and on budget.
LIGO scientists expect this new instrument to see gravitational wave sources possibly as often as daily, with excellent signal strengths. It should allow details of the waveforms to be read off and compared with theories of neutron stars, black holes, and other highly relativistic objects. The improvement of sensitivity will allow the one-year planned observation time of initial LIGO to be equaled in just several hours.
LIGO-India is a collaborative project proposed by the LIGO Laboratory and the Indian Initiative in Gravitational Observations (IndIGO) to create a world-class gravitational-wave detector in India. The LIGO Laboratory, with permission from the U.S. 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 two planned Hanford Advanced LIGO detectors to be installed, commissioned, and operated by an Indian team of scientists in a facility to be built in and by India.
The motivations behind this project are clear and compelling. 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 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 comparable sensitivity detectors and forms the core of this international network. Yet further detectors are extremely important. A fourth site not in the plane formed by the present three and distant from them all greatly improves source localization ability. Studies indicate that the localization of sources by a network that includes a detector in India would provide significant improvements. Improvements in localization averages are predicted to be approximately an order of magnitude, with substantially larger improvements in certain regions of the sky.
The project requires the support and agreement of both governments in addition to the LIGO Laboratory and IndiGO. The project was discussed at a Joint Commission meeting between India and the US in 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. In India, the project has been presented to the Department of Atomic Energy and the Department of Science and Technology for approval and funding. Final approval is pending.
LISA, the Laser Interferometer Space Antenna, is a proposed gravitational-wave detection project to build a laser interferometer consisting of three spacecraft in solar orbit. As initially conceived, LISA would be sensitive to gravitational waves in a different frequency band from that of the LIGO detectors, making the two experiments complementary of each other. LISA began as a joint project of NASA and the European Space Agency (ESA), but in April 2011 NASA announced that it would be unlikely to continue its LISA partnership with the ESA, due to funding limitations. The ESA is planning to begin a full revision of the mission's concept commencing in February 2012.
eLISA/NGO (evolved Laser Interferometer Space Antenna / New Gravitational Wave Observatory) 
The eLISA constellation would comprise three spacecraft operating in a V formation, with the central craft having two free-falling “test masses” that define the endpoint of the two interferometer arms while the other two craft have the end-points of the interferometer arms. The three craft would orbit the Sun and initially have a near-equilateral triangular formation with both baselines 106 km long (total, 2 x 1 Gm). The goal is detection in a frequency range from about 0.1 mHz to 1 Hz.
In May 2012, eLISA/NGO was one of three candidate projects under review for ESA funding, these were: JUICE (JUpiter ICy moons Explorer); ATHENA, the (Advanced Telescope for High-ENergy Astrophysics); and, eLISA/NGO. Due to the pull-out of NASA funding in early 2011 following cost over-runs on NASA's James Webb Space Telescope mission at the same time as the legacy of the 2008 financial crisis, ESA was only able to commit to one of the three Cosmic Vision L-class (large mission) projects under consideration, and selected JUICE to go forwards as the L1 mission. However, ESA is open to review both eLISA/NGO and ATHENA again in 2013 for funding consideration as the L2 mission.
A paper published in January 2012 discussed the European New Gravitational Wave Observatory (NGO) mission (derived from the previous LISA proposal) and referred to this derivative proposal by an informal name “eLISA”, and proposed that it will survey in the low-frequency gravitational wave band (about 0:1 mHz to 1 Hz) with sufficient sensitivity to detect interesting individual astrophysical sources out to cosmological distances represented by a redshift z = 15.
See also 
- Einstein Telescope, for a European third-generation gravitational wave detector.
- Einstein@Home, for a volunteer distributed computing program one can download in order to help the LIGO/GEO teams analyze their data.
- GEO 600, for a gravitational wave detector located in Hannover, Germany.
- Fermilab Holometer
- Tests of general relativity
- Virgo interferometer, an interferometer similar to LIGO, located close to Pisa, Italy.
- List of laser articles
- Larger physics projects in the United States, such as Fermilab, have traditionally been funded by the Department of Energy.
- LIGO Fact Sheet at NSF
- Homepage of the LSC can be found at http://www.ligo.org/ and features summaries of the Collaboration's scientific articles, written for a general public audience. These science summaries can be found at http://www.ligo.org/science/outreach.php.
- http://www.lsu.edu/ur/ocur/lsunews/MediaCenter/News/2011/04/item28979.html, LSU official announcement of González as new LSC Spokesperson, April 2011.
- "Astrophysical Sources of Gravitational Radiation".
- Both legs of the interferometer are easily visible in aerial photography.
- Thorne, Kip (2004). "Chapter 26.5: The Detection of Gravitational Waves (in "Applications of Classical Physics chapter 26: Gravitational Waves and Experimental Tests of General Relativity", Caltech lecture notes)". Retrieved 2010-08-02.
- Adhikari, Rana (2007). Private communication.
- "LIGO Sheds Light on Cosmic Event". 2007-12-20. Retrieved 2007-12-21.
- "Pulsar Discovery by Global Volunteer Computing". 2010-08-12. Retrieved 2010-12-08.
- "Home computer finds rare pulsar". 2010-08-12. Retrieved 2010-12-08.
- Adhikari, Fritschel, and Waldman. LIGO technical document LIGO-T060156-01-I. July 17th, 2006.
- Firm Date Set for Start of S6, by Dave Beckett, 6/15/2009, LIGO Laboratory News
- Jonathan Amos (April 16, 2008). "Gravity 'ripples' hunt upgraded". BBC News.
- http://gwic.ligo.org/roadmap/Roadmap_100814.pdf GWIC Developmental Roadmap.
- https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=90988 "Improved Source Localization with LIGO India," in the LIGO Document Control Center.
- http://arxiv.org/abs/1102.5421 "Networks of Gravitational Wave Detectors and Three Figures of Merit," at arXiv.org.
- http://www.state.gov/r/pa/prs/ps/2012/06/192271.htm U.S.-India Bilateral Cooperation on Science and Technology meeting fact sheet – dated June 13, 2012.
- http://www.nsf.gov/nsb/meetings/2012/0823/major_actions.pdf Memorandum to Members and Consultants of the National Science Board – dated August 24, 2012
- http://news.discovery.com/space/a-farewell-to-lisa-110408.html, Discovery News announcement of LISA Project status, April 2011.
- "eLISA/NGO (evolved Laser Interferometer Space Antenna / New Gravitational Wave Observatory)". Max Planck Society. Retrieved 2012-09-01.
- "eLISA/NGO – mission". Max Planck Society. Retrieved 2012-09-01.
- "eLISA/NGO – mission sensitivity". Max Planck Society. Retrieved 2012-09-01.
- "eLISA science case is published". Max Planck Institute for Gravitational Physics. 2012-05-30. Retrieved 2012-09-01.
- "JUICE is Europe’s next large science mission". ESA. 2012-05-02. Retrieved 2012-09-04.
- "ESA makes a big choice". ScienceBlogs. 2012-04-18. Retrieved 2012-09-04. "see page 18 of cited Science Program Committee report"
- Doing science with eLISA:Astrophysics and cosmology in the millihertz regime, PDF, Pau Amaro-Seoane et al, arXiv:1201.3621v1 [astro-ph.CO] 17 Jan 2012, accessed 20 January 2012
- Kip Thorne, ITP & Caltech. Spacetime Warps and the Quantum: A Glimpse of the Future. Lecture slides and audio
- Rainer Weiss, 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 – JETP Vol.43 p. 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 – M.E.Gertsenshtein – JETP Vol.41 p. 113-114 (July 1961)
- Gravitational electromagnetic resonance, V.B.Braginskii, M.B.Mensky – GR.G. Vol.3 No.4 p. 401-402 (1972)
- Gravitational radiation and the prospect of its experimental discovery, V.B.Braginsky – Soviet Physics Vol.86 p. 433-446 (July 1965)
- 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 – GR.G. Vol.11 No.6 p. 407-408 (1979)
- On the propagation of electromagnetic radiation in the field of a plane gravitational wave, E.Montanari – gr-qc/9806054 (June 11, 1998)
Further reading 
- Einstein's Unfinished Symphony by Marcia Bartusiak, ISBN 0-425-18620-2.
- Fundamentals of Interferometric Gravitational Wave Detectors by Peter R. Saulson, ISBN 981-02-1820-6.
- Gravity's Shadow: The Search for Gravitational Waves by Harry Collins, ISBN 0-226-11378-7.
- Traveling at the Speed of Thought by Daniel Kennefick, ISBN 978-0-691-11727-0
- 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
- Advanced LIGO homepage
- Columbia Experimental Gravity
- American Museum of Natural History film and other materials on LIGO
- 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.
- 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 – An extensive description of gravitational waves and their sources.