LIGO stands for Laser Interferometer Gravitational-Wave Observatory. 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 (and still is as of 2007). The international LIGO Scientific Collaboration (LSC) is a growing group of researchers, some 600 individuals at roughly 40 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 is University of Florida Physicist David Reitze.
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 to Hulse and Taylor in 1993.
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. 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.
In August 2002, LIGO began its search for cosmic gravitational waves. 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), 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 is at the point where detection of gravitational waves—of significant astrophysical interest—is possible.
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), and its Fabry-Perot 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 use its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.
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.
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 collected data for several months. VIRGO (the French-Italian interferometric detector) joined in May 2007. The fifth science run was ended in the fall of 2007. It is hoped that after extensive analysis this may uncover perhaps two unambiguous detection events. This would be a milestone in the history of physics. In 2004, it was reported that theorists were estimating the chances of unambiguous direct detection by 2010 at one in six.
In February 2007 a short gamma ray burst, GRB070201 which came from the direction of the Andromeda Galaxy, failed to be observed by LIGO. This was significant as it ruled out the Andromeda Galaxy as the location of the event (provided LIGO is able to demonstrate detection of direct gravitational waves).
Before the sixth science run is started, a series of upgrades will be executed, resulting in an improved configuration called Enhanced LIGO with two or three times the sensitivity of Initial LIGO. Some of the planned improvements are:
Enhanced LIGO will culminate in the sixth science run (S6).
It is anticipated that this new instrument will 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 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, 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.
About LIGO and interferometric searches for gravitational waves:
About gravitational wave astronomy:
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