LIGO

The LIGO Hanford Control Room

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 has been the largest and most ambitious project ever funded by NSF (and still is as of 2004). 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.

Mission

LIGO's mission is to observe directly 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 since observations were made of the binary pulsar PSR 1913+16, for which the Nobel Prize was awarded 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. 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. In August 2002, LIGO began its search for cosmic gravitational waves that are theoretically created in supernova collapses of stellar cores (which form neutron stars and black holes), collisions and coalescences of neutron stars or black holes, rotations of neutron stars with deformed crusts and the remnants of gravitational radiation created by the birth of the universe. 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.

Observatories

LIGO operates two gravitational wave observatories in unison: the Livingston Observatory in Livingston, Louisiana and the the Hanford Observatory, on the Hanford Nuclear Reservation, located near Richland, Washington. These sites are separated by about two thousand miles. This distance corresponds to a difference in gravitational wave arrival times of up to ten milliseconds, information which can help to determine the source of the wave in the sky.

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.

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. This arrangement provides one means to distinguish terrestrial seismic events from cosmic gravitational waves.

The 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.

The 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.

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.

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 out of resonance, and will cause the light currently in the cavity to become slightly out of phase with the incoming light.

After approximately 30 trips through the arms, the two separate beams leave the arms and recombine at the beam splitter. The 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), no light should arrive at the photodiode. When a gravitational wave passes through the interferometer, the beams become slightly out of phase, so some light arrives at the photodiode. 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.

Observations

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 (a hydrogen atom is about 5×10^-11 m). A baseline event is the inspiralling of two 10 Solar Mass black holes.

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 August 2005, sensitivity is rapidly approaching design specification. The baseline inspiral is typically expected to be observable if it occurs within about 8 million parsecs.

See also

• Tests of general relativity

External links

• LIGO Laboratory home site
• LIGO Hanford Observatory
• LIGO Livingston Observatory
• Amos, Jonathan. Gravity wave detector all set. February 18, 2003. BBC News.
• 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.
• Einstein@home "Einstein@home is a program that uses your computer's idle time to search for spinning neutron stars (also called pulsars) using data from the LIGO and GEO gravitational wave detectors."
• Livingston Observatory at Google Maps
• Hanford Observatory at Google Maps