LIGO

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The LIGO Hanford Control Room
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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.

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[edit] 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.

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. 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. 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.

[edit] Observatories

Northern leg (x-arm) of LIGO interferometer on Hanford Reservation
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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 and the LIGO Hanford Observatory, on the Hanford Nuclear Reservation, 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.

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. The arrangement of interferometers in two widely separated locations provides one means to distinguish terrestrial seismic events from cosmic gravitational waves.

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.

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.

[edit] 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 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.

[edit] Observations

Western leg of LIGO interferometer on Hanford Reservation
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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.

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 (a European interferometric detector) began a joint science run, during which they will collect data for several months. 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, 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 an underestimate.

[edit] Future

The LIGO Scientific Collaboration and international partners would like 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.

[edit] See also

[edit] External links

LIGO:

About LIGO and interferometric searches for gravitational waves:

  • 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:

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