LIGO stands for Lesser Inner Greater Outer. 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 and Caltech. It is sponsored by the National Science Foundation (NSF). At the cost of $365 million (in 2002 USD), it was the largest and most ambitious project ever funded by NSF^[1] (and still is as of 2007). The international LIGO Scientific Collaboration (LSC) is a growing group of researchers, some 400 individuals at roughly 40 institutions, working to analyze the data from LIGO and other detectors, and working toward more sensitive future detectors. Kip S. Thorne Professor Kip Stephen Thorne, Ph. ... Ron Drever is a Scottish physicist who co-founded the LIGO project, and was a co-inventer of the Pound-Drever-Hall technique for LASER stabilization, among other accomplishments. ... The California Institute of Technology (commonly referred to as Caltech)[1] is a private, coeducational research university located in Pasadena, California, in the United States. ... Rainer (Rai) Weiss is professor of physics emeritus at MIT. // Weiss was born in Berlin, Germany. ... “MIT” redirects here. ... The logo of the National Science Foundation The National Science Foundation (NSF) is an independent United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. ...

Mission

LIGO's mission is to directly observe gravitational waves of cosmic origin. These waves were first predicted by Einstein's Theory of General Relativity in 1916, when the technology necessary for their detection did not yet exist. Gravitational waves were indirectly confirmed to exist when observations were made of the binary pulsar PSR 1913+16, for which the Nobel Prize was awarded in 1993. In physics, a gravitational wave is a fluctuation in the curvature of spacetime which propagates as a wave, traveling outward from a moving object or system of objects. ... General relativity (GR) or general relativity theory (GRT) is the theory of gravitation published by Albert Einstein in 1915. ... PSR B1913+16 is a pulsar in a binary star system, in orbit with another star around a common center of mass. ... The Nobel Prizes (Swedish: ), as designated in Alfred Nobels will in 1895, are awarded for physics, chemistry, physiology or medicine, literature, and peace. ...

Direct detection of gravitational waves has long been sought, for it would open up a new branch of astronomy to complement electromagnetic telescopes and neutrino observatories. Some progress in detection occurred with the work of Joseph Weber in the 1960s on resonant mass bar detectors, which continue to be used at six significant sites worldwide. By the 1970s, scientists including Rainer Weiss realized the applicability of interferometry to gravitational wave measurements. Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... For other uses, see Neutrino (disambiguation). ... Joseph Weber (May 17, 1919 – September 30, 2000) was an American physicist. ... The 1960s decade refers to the years from 1960 to 1969, inclusive. ... A Weber bar is a device used in the detection of gravitational waves first devised and constructed by physicist Joseph Weber at the University of Maryland. ... The 1970s decade refers to the years from 1970 to 1979, also called The Seventies. ... Rainer (Rai) Weiss is professor of physics emeritus at MIT. // Weiss was born in Berlin, Germany. ... It has been suggested that Optical interferometry be merged into this article or section. ...

In August 2002, LIGO began its search for cosmic gravitational waves. Predicted significant emissions of gravitational waves are expected from binary inspiral systems (collisions and coalescences of neutron stars or black holes), supernova collapses of stellar cores (which form neutron stars and black holes), 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, interferometer physicists have believed that technology is at the point where detection of gravitational waves—of significant astrophysical interest—is possible. For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... For other uses, see Supernova (disambiguation). ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... For other uses, see Black hole (disambiguation). ... A cosmic string is a hypothetical 1-dimensional topological defect in the fabric of spacetime. ... A domain wall is a theoretical 2-dimensional singularity. ... For the band, see 1990s (band). ...

Observatories

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

LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory in Livingston, Louisiana( 30°29′55″N, 90°44′54″W) and the LIGO Hanford Observatory, on the Hanford Nuclear Reservation (coordinates of central complex: 46°27′28″N, 119°24′35″W^[2]) , located near Richland, Washington. These sites are separated by 3,002 kilometers (1,876 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. Image File history File linksMetadata Download high resolution version (1984x1488, 706 KB) Summary Northern leg of LIGO interferometer on Hanford Reservation looking north. ... Image File history File linksMetadata Download high resolution version (1984x1488, 706 KB) Summary Northern leg of LIGO interferometer on Hanford Reservation looking north. ... LIGO stands for Lesser Inner Greater Outer. ... Livingston is a town located in Livingston Parish, Louisiana. ... Pacific Northwest National Laboratory Fast Flux Test Facility (partially decommissioned) B-Reactor (decommissioned) Hanford Site Categories: | ... Hanford Site plutonium production reactors along the Columbia River during the Manhattan Project. ... Richland Police Station in foreground. ... Triangulation can be used to find the distance from the shore to the ship. ...

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. Ultra high vacuum (UHV) is the regime of characterised by pressures lower than about 10-7 Pascal or 100 nanopascals (~10-9 torr). ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ...

A half-length interferometer can be operated in parallel with a primary interferometer. This second detector is half the length at 2 kilometers (1.25 miles), but its Fabry-Perot arm cavities have twice the optical finesse and thus the same storage time and shot noise sensitivity. To gravitational waves, the half-length interferometer has the same sensitivity as the full-length interferometers. To seismic displacement noise, however, the half-length interferometer is twice as sensitive. The arrangement of interferometers in two widely separated locations provides one means to distinguish terrestrial seismic events from cosmic gravitational waves. In optics, a Fabry-Perot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly-reflecting mirrors. ... Photon noise simulation. ...

The LIGO Livingston Observatory houses one laser interferometer in the primary configuration. This observatory was successfully upgraded in 2004 with hydraulics to use active seismic isolation to insulate the optics from terrestrial disturbances such as nearby logging. LIGO stands for Lesser Inner Greater Outer. ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ...

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 use its original passive seismic isolation system due to limited geologic activity in Southeastern Washington. Pacific Northwest National Laboratory Fast Flux Test Facility (partially decommissioned) B-Reactor (decommissioned) Hanford Site Categories: | ...

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 Fabry-Perot arms. A pre-stabilized laser emits a 10 watt beam 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-Perot cavities that store the beams and increase the effective path length. A Michelson interferometer for use on an optical table. ... In optics, a Fabry-Perot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly-reflecting mirrors. ... A transverse mode of a beam of electromagnetic radiation is a particular intensity pattern of radiation measured in a plane perpendicular (i. ...

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 the length of one or both of the cavities. This length change will bring the cavity very slightly out of resonance, and will cause the light currently in the cavity to become very slightly out of phase with the incoming light.

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. Photodiode closeup A photodiode A photodiode is a semiconductor diode that functions as a photodetector. ...

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 "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. Image File history File linksMetadata Download high resolution version (1984x995, 300 KB) Summary Western leg of LIGO on Hanford Reservation as seen from east. ... Image File history File linksMetadata Download high resolution version (1984x995, 300 KB) Summary Western leg of LIGO on Hanford Reservation as seen from east. ... General relativity (GR) or general relativity theory (GRT) is the theory of gravitation published by Albert Einstein in 1915. ... For other uses, see Proton (disambiguation). ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ...

By fourth Science Run at the end of 2004, the LIGO detectors had demonstrated sensitivities in measuring these displacements to within a factor of 2 of their design.

As of November 2005, sensitivity had 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, averaged over all directions and polarizations. In November 2005, LIGO and GEO 600 (the German-UK interferometric detector) began a joint science run, during which they will collect data for several months. VIRGO (the French-Italian interferometric detector) joined in May 2007. The fifth science run is expected to be complete in late 2007. It is hoped that after extensive analysis this may uncover perhaps two unambiguous detection events. Ongoing events • Abramoff-Reed gambling scandal • Al Jazeera bombing memo • Avian influenza (H5N1) outbreak • Black sites scandal • Conservative leadership race (UK) • Fuel prices • Irans nuclear program • Jilin chemical plant explosions • Kashmir earthquake • Malawi food crisis • Malaysian prisoner abuse scandal • New Delhi bombings investigation • Niger food crisis • North Indian cyclone... This article is about the unit of length. ... GEO or Geo may refer to any of the following: Geo or gio, is a creek (inlet) or gulley in the Orkney and Shetland Islands GEO (newspaper), a popular scientific magazine Geo (microformat), a microformat for marking up WSG84 geographical coordinates in (X)HTML Geo (automobile), a brand of entry... Virgo (Latin for virgin, symbol , Unicode ♍) is a constellation of the zodiac. ...

This would be a milestone in the history of physics, but how likely is it to happen soon? In 2004, it was reported that theorists were estimating the chances of unambiguous direct detection by 2010 at one in six, but many physicists think this is a pessimistic estimate.^[3]

Future

Enhanced LIGO

When the fifth science observing run (S5) ends in the autumn of 2007, a series of upgrades will be executed, resulting in an improved configuration called Enhanced LIGO^[4] with two or three times the sensitivity of Initial LIGO. Some of the planned improvements are:

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

Enhanced LIGO will culminate in the sixth science run (S6).

Advanced LIGO

The LIGO Scientific Collaboration and international partners plan to build Advanced LIGO (formerly referred to as "LIGO 2") to improve the sensitivity of Initial LIGO (LIGO 1) by more than a factor of 10. This new detector would be installed at the LIGO Observatories to replace the present detector once it has reached its goal of a year of observation, and is hoped would transform gravitational wave science into a real observational tool.

It is anticipated that this new instrument would see gravitational wave sources possibly as often as daily, with excellent signal strengths, allowing 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.

But if and when even one verified gravitational wave event is observed by any of the worldwide detectors, it will be a truly exciting moment for all astronomers and astrophysicists worldwide who have waited so long for such an event to be seen.

LISA

LISA, the Laser Interferometer Space Antenna, is a proposed joint project of NASA and the European Space Agency to build a laser interferometer gravitational wave detector consisting of three spacecraft in solar orbit. LISA will be sensitive to gravitational waves in a different frequency band than LIGO, so the two experiments will complement each other. The LISA is the Laser Interferometer Space Antenna experiment. ... This article is about the American space agency. ... “ESA” redirects here. ...

References

• Dr. Kip Thorne, ITP & Caltech. Spacetime Warps and the Quantum: A Glimpse of the Future. Lecture slides and audio

Kip S. Thorne Professor Kip Stephen Thorne, Ph. ...

Books about LIGO

• Fundamentals of Interferometric Gravitational Wave Detectors by Peter R. Saulson, ISBN 9810218206.
• Einstein's Unfinished Symphony by Marcia Bartusiak, ISBN 0425186202.
• Gravity's Shadow: The Search for Gravitational Waves by Harry Collins, ISBN 0226113787.

Notes

1. ^ Larger physics projects in the United States, such as Fermilab, have traditionally been funded by the Department of Energy.
2. ^ Both legs of the interferometer are easily visible from space in satellite photography.
3. ^ Adhikari, Rana (2007). Private communication.
4. ^ Adhikari, Fritschel, and Waldman. LIGO technical document LIGO-T060156-01-I. July 17th, 2006.

Aerial view of the Fermilab site. ... The United States Department of Energy (DOE) is a Cabinet-level department of the United States government responsible for energy policy and nuclear safety. ...

See also

• Tests of general relativity
• VIRGO, for a European gravitational wave detector.
• GEO 600, for a gravitational wave detector located in Hannover, Germany.
• Einstein@Home, for a distributed gravitational wave detection program.
• Gravitational Wave

Tests of Einsteins general theory of relativity did not provide an experimental foundation for the theory until well after it was introduced in 1915. ... Virgo (Latin for virgin, symbol , Unicode ♍) is a constellation of the zodiac. ... Geo 600 is a gravitational wave detector located in Hannover, Germany. ... Map of Germany showing Hanover Hanover (in German: Hannover [haˈnoːfɐ]), on the river Leine, is the capital of the state of Lower Saxony (Niedersachsen), Germany. ... Einstein@Home is a distributed computing project running on the Berkeley Open Infrastructure for Network Computing (BOINC) software platform. ... In physics, a gravitational wave is a fluctuation in the curvature of spacetime which propagates as a wave, traveling outward from a moving object or system of objects. ...

External links

LIGO:

• LIGO Laboratory home site
• LIGO Hanford Observatory
• LIGO Livingston Observatory
• Livingston Observatory at Google Maps
• Hanford Observatory at Google Maps
• Advanced LIGO homepage
• 40m Prototype
• Columbia Experimental Gravity
• Einstein's Messengers - the award-winning 20-minute LIGO documentary produced by the National Science Foundation
• American Museum of Natural History film and other materials on LIGO
• Interferometric Detection of Gravitational Waves: 5 Needed Breakthroughs (2Physics.com)

About LIGO and interferometric searches for gravitational waves: The logo of the National Science Foundation The National Science Foundation (NSF) is an independent United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. ...

• 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.

About gravitational wave astronomy:

• 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.