.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) 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. Scientists involved in the project and the analysis of the data for gravitational-wave astronomy are organised by the LIGO Scientific Collaboration which includes more than 900 scientists worldwide.[1][2] LIGO is funded by the National Science Foundation (NSF), with important contributions from the UK Science and Technology Facilities Council, the Max Planck Society of Germany, and the Australian Research Council.[3][4] By mid-September 2015 "the world's largest gravitational-wave facility" completed a 5-year US$200-million overhaul at a total cost of $620 million.[2][5] LIGO is the largest and most ambitious project ever funded by the NSF.[6][7]

Initial LIGO operations between 2002 and 2010 did not detect any gravitational waves. This was followed by a multi-year shut-down while the detectors were replaced by much improved "Advanced LIGO" versions.[8] As of February 2015, two such advanced detectors (one in Livingston, Louisiana and the other in Hanford, Washington) have been brought into engineering mode.[9] On September 18, 2015, Advanced LIGO began its first formal science observations at about four times the sensitivity of the initial LIGO interferometers.[10] Its sensitivity will be further enhanced until it reaches design sensitivity around 2021.[11]

On 11 February 2016, the LIGO collaboration announced the detection of gravitational waves, from a signal detected at 09.51 UTC on 14 September 2015[12] of two 30 solar mass black holes merging together around 1.3 billion years ago.[13][14]

Mission
Detector noise curves for Initial and Advanced LIGO as a function of frequency. They lie above the bands for space-borne detectors like the evolved Laser Interferometer Space Antenna (eLISA) and pulsar timing arrays such as the European Pulsar Timing Array (EPTA). The characteristic strain of potential astrophysical sources are also shown. To be detectable the characteristic strain of a signal must be above the noise curve.[15]

LIGO's mission is to directly observe gravitational waves of cosmic origin. These waves were first predicted by Einstein's general theory of relativity in 1916, when the technology necessary for their detection did not yet exist. Their existence was indirectly confirmed when observations of the binary pulsar PSR 1913+16 in 1974 showed an orbital decay which matched Einstein's predictions of energy loss by gravitational radiation. The Nobel Prize in Physics 1993 was awarded to Hulse and Taylor for this discovery.[16]

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.[17]

In fact as early as the 1960s, and perhaps before that, there were papers published on wave resonance of light and gravitational waves.[18] 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.[19] 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 explosions of massive stars (which form neutron stars and black holes), accreting neutron stars, rotations of neutron stars with deformed crusts, and the remnants of gravitational radiation created by the birth of the universe. The observatory may, in theory, also observe more exotic hypothetical phenomena, such as gravitational waves caused by oscillating cosmic strings or colliding domain walls. Since the early 1990s, physicists have thought that technology has evolved to the point where detection of gravitational waves—of significant astrophysical interest—is now possible.[20]

Observatories
Northern leg (x-arm) of LIGO interferometer on Hanford Reservation

LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory (30°33′46.42″N 90°46′27.27″W) in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site (46°27′18.52″N 119°24′27.56″W), located near Richland, Washington. These sites are separated by 3,002 kilometers (1,865 miles). 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.[21]

Each observatory supports an L-shaped ultra high vacuum system, measuring 4 kilometers (2.5 miles) on each side. Up to five interferometers can be set up in each vacuum system.

The LIGO Livingston Observatory houses one laser interferometer in the primary configuration. This interferometer was successfully upgraded in 2004 with an active vibration isolation system based on hydraulic actuators providing a factor of 10 isolation in the 0.1–5 Hz band. Seismic vibration in this band is chiefly due to microseismic waves and anthropogenic sources (traffic, logging, etc.).

The LIGO Hanford Observatory houses one interferometer, almost identical to the one at the Livingston Observatory. During the Initial and Enhanced LIGO phases, a half-length interferometer operated in parallel with the main interferometer. For this 2 km interferometer, the Fabry–Pérot arm cavities had the same optical finesse, and thus half the storage time, as the 4 km interferometers. With half the storage time, the theoretical strain sensitivity was as good as the full length interferometers above 200 Hz but only half as good at low frequencies. During the same era, Hanford retained its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.

Operation

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 Gires–Tournois etalon 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.[needs update]

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.[22]

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.

Observations
Western leg of LIGO interferometer on Hanford Reservation

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 million parsecs (26×10⁶ ly), or the vicinity of the Local Group, averaged over all directions and polarizations. Also at this time, LIGO and GEO 600 (the German-UK interferometric detector) began a joint science run, during which they collected data for several months. Virgo (the French-Italian interferometric detector) joined in May 2007. The fifth science run ended in 2007, after extensive analysis of data from this run did not uncover any unambiguous detection events.

In February 2007, GRB 070201, a short gamma-ray burst, arrived at Earth from the direction of the Andromeda Galaxy. The prevailing explanation of most short gamma-ray bursts is the merger of a neutron star with either a neutron star or 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.[23]

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.[24] 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.[25] It concluded in October 2010, and the disassembling of the original detectors began. By mid-September 2015, LIGO Scientific Collaboration included more than 900 scientists worldwide.[2]
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, has installed the new Advanced LIGO detectors in the LIGO Observatory infrastructures. This new detector is designed to improve the sensitivity of initial LIGO by more than a factor of 10 once fully commissioned.

The LIGO Laboratory started the first Observing Run 'O1' with the Advanced LIGO detectors in September 2015 at a sensitivity roughly 4 times greater than Initial LIGO for some classes of sources (e.g., neutron-star binaries), and a much greater sensitivity for larger systems with their peak radiation at lower audio frequencies.[26]

Further Observing runs will be interleaved with commissioning efforts to further improve the sensitivity. It is aimed to achieve design sensitivity in 2019.[27]
Future

LIGO-India

LIGO-India is a proposed collaborative project between 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 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 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[28] 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. 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.[29][30] Improvements in localization averages are predicted to be approximately an order of magnitude, with substantially larger improvements in certain regions of the sky.

The NSF was willing to permit this relocation, and its consequent schedule delays, as long as it did not increase the LIGO budget. Thus, all costs required to build a laboratory equivalent to the LIGO sites to house the detector would have to be borne by the host country.[31] The first potential distant location was at AIGO in Western Australia,[32] however the Australian government was unwilling to commit funding by the 1 October 2011 deadline.

A location in India was discussed at a Joint Commission meeting between India and the US in June 2012.[33] 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.[34] 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 has not been granted yet.
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
Fermilab Holometer
GEO600 – for a gravitational wave detector located in Hannover, Germany
List of laser articles
Tests of general relativity
Virgo interferometer – an interferometer similar to LIGO, located close to Pisa, Italy

Notes

"LSC/Virgo Census". myLIGO. Retrieved 28 November 2015.
Castelvecchi, Davide (15 September 2015), Hunt for gravitational waves to resume after massive upgrade: LIGO experiment now has better chance of detecting ripples in space-time, Nature News, retrieved 12 January 2016
"Major research project to detect gravitational waves is underway". University of Birmingham News. University of Birmingham. Retrieved 28 November 2015.
Shoemaker, David (2012). "The evolution of Advanced LIGO" (PDF). LIGO Magazine (1): 8.
Zhang, Sarah (15 September 2015). "The Long Search for Elusive Ripples in Spacetime".
Larger physics projects in the United States, such as Fermilab, have traditionally been funded by the Department of Energy.
LIGO Fact Sheet at NSF
"Gravitational wave detection a step closer with Advanced LIGO". SPIE Newsroom. Retrieved 4 January 2016.
"LIGO Hanford's H1 Achieves Two-Hour Full Lock". February 2015.
Amos, Jonathan (19 September 2015). "Advanced Ligo: Labs 'open their ears' to the cosmos". BBC News. Retrieved 19 September 2015.
"Planning for a bright tomorrow: prospects for gravitational-wave astronomy with Advanced LIGO and Advanced Virgo". LIGO Scientific Collaboration. 23 December 2015. Retrieved 31 December 2015.
"Gravitational waves from black holes detected". BBC News. 11 February 2016.
Castelvecchi, Davide; Witze, Witze (February 11, 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. Retrieved 2016-02-11.
"Gravitational waves detected 100 years after Einstein's prediction" (PDF). LIGO. February 11, 2016. Retrieved 11 February 2016.
Moore, Christopher; Cole, Robert; Berry, Christopher (19 July 2013). "Gravitational Wave Detectors and Sources". Retrieved 20 April 2014.
"The Nobel Prize in Physics 1993: Russell A. Hulse, Joseph H. Taylor Jr.". nobelprize.org.
California Institute of Technology announces death of Robert L Forward September 22, 2002
M.E. Gertsenshtein (1961). "Wave Resonance of Light and Gravitational Waves". JETP (USSR) 41 (1): 113-114.
Gertsenshtein, M. E.; Pustovoit, V. I. (August 1962). "On the detection of low frequency gravitational waves". JETP 43: 605–607.
"Astrophysical Sources of Gravitational Radiation".
"Location of the Source". Gravitational Wave Astrophysics. University of Birmingham. Retrieved 28 November 2015.
Thorne, Kip (2012). "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)" (PDF). Retrieved 2016-02-11.
"LIGO Sheds Light on Cosmic Event". 2007-12-20. Retrieved 2007-12-21.
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
Aasi, J (9 April 2015). "Advanced LIGO". Classical and Quantum Gravity 32 (7): 074001. arXiv:1411.4547. doi:10.1088/0264-9381/32/7/074001.
Abbott, B P (2 April 2013). "Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO and Advanced Virgo". arXiv:1304.0670. Bibcode:2013arXiv1304.0670L.
GWIC Developmental Roadmap p. 97
Fairhurst, Stephen (28 Sep 2012), Improved Source Localization with LIGO India, LIGO document P1200054-v6
Schutz, Bernard F. (25 Apr 2011), Networks of Gravitational Wave Detectors and Three Figures of Merit, arXiv:1102.5421, Bibcode:2011CQGra..28l5023S, doi:10.1088/0264-9381/28/12/125023
Cho, Adrian (27 August 2010), "U.S. Physicists Eye Australia for New Site of Gravitational-Wave Detector" (PDF), Science 329 (5995): 1003, Bibcode:2010Sci...329.1003C, doi:10.1126/science.329.5995.1003
Finn, Sam; Fritschel, Peter; Klimenko, Sergey; Raab, Fred; Sathyaprakash, B.; Saulson, Peter; Weiss, Rainer (13 May 2010), Report of the Committee to Compare the Scientific Cases for AHLV and HHLV, LIGO document T1000251-v1
U.S.-India Bilateral Cooperation on Science and Technology meeting fact sheet – dated June 13, 2012.

Memorandum to Members and Consultants of the National Science Board – dated August 24, 2012

References

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)