What is LIGO?
What are gravitational waves?
What are LIGO's scientific goals?
What does a gravitational wave observatory look like?
How will the detectors sense gravitational waves?
Why are two installations necessary?
What are the prospects for international collaboration?




What is LIGO?

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a facility dedicated to the detection of cosmic gravitational waves and the measurement of these waves for scientific research. It consists of two widely separated installations within the United States, operated in unison as a single observatory. This observatory is available for use by the world scientific community, and is a vital member in a developing global network of gravitational wave observatories.

Funded by the National Science Foundation (NSF), LIGO was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, and by industrial contractors. Construction of the facilities was completed in 1999. Initial operation of the detectors began in 2001.


What are gravitational waves?

Artist's conception of gravitational waves
Artist's Conception of Gravitational Waves

Gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe, such as the collision of two black holes or shockwaves from the cores of supernova explosions. Gravitational waves are emitted by accelerating masses much as electromagnetic waves are produced by accelerating charges. These ripples in the space-time fabric travel toward Earth, bringing with them information about their cataclysmic origins, as well as invaluable clues as to the nature of gravity.

Albert Einstein predicted the existence of these gravitational waves in his 1916 general theory of relativity, but only now in the 21st Century has technology advanced to enable their detection and study by science. Although not yet detected directly, the influence of gravitational waves on a binary pulsar (two neutron stars orbiting each other) has been measured accurately, and was found to be in good agreement with original predictions. Scientists therefore have great confidence that gravitational waves do exist. Joseph Taylor and Russel Hulse were awarded the 1993 Nobel Prize in Physics for their studies in this field.


What are LIGO's scientific goals?

LIGO will be used to delve into the fundamental nature of gravity, and as such will throw open an entirely new window onto the universe. Its observations will cross many borders and it will serve as an investigational tool for both physics and astronomy.

Possible payoffs for physics:

General relativity describes gravity as a manifestation of the curvature of space-time. This description has been tested and proven correct in the solar system, where the static curvature due to the gravitational attraction of the planets and their satellites has been observed. LIGO will now permit scientists to test this description in the case of rapidly changing dynamical gravity (the space-time ripples of the gravitational waves), as well as for the extremely strong dynamical gravity of two black holes in collision.

More specifically, LIGO has the possibility to test several of general relativity's predictions:

Possible payoffs for astronomy:

Almost all of our present information about the distant universe is afforded us from electromagnetic waves. Until the 1930s, the only such waves accessible to astronomers were light waves, and optical telescopes used to study them revealed a largely serene universe of planets, stars, and galaxies. Then, through the 1940s, 50s, and 60s, the march of technology made possible entirely new types of observational tools - the radio telescope, the infrared telescope, the x-ray telescope - which looked at cosmic electromagnetic waves with wavelengths different from light. And because these radiations were different, they revealed wholly fresh stores of information. They presented another image of the universe, a vigorous and often violent side that included quasars, pulsars, and even the birth throes of stars. Gravitational waves, being radically different from all electromagnetic waves, have the potential to foster yet another revolution in our growing understanding of the universe.

Among things the study of gravitational waves might reveal are:


What does a gravitational wave observatory look like?

The larger the gravitational wave detector, the more sensitive it has the potential to be. LIGO employs a 4-foot diameter vacuum pipe arranged in the shape of an L with 4-kilometer (2.5-mile) arms. Since gravitational waves penetrate the earth unimpeded, these installations need not be exposed to the sky and are entirely shielded in a concrete cover. At the vertex of the L, and at the end of each of its arms, are test masses that hang from wires and which are outfitted with mirror surfaces. These mirrors are the sensors of gravitational waves. The main building situated at the vertex serves as the observatory's control center and houses vacuum equipment, lasers, computers, and personnel. Ultrastable laser beams traversing the vacuum pipes measure the effect of gravitational waves on the test masses. Confident detection of the very weak waves predicted requires two installations vastly separated.


How will the detectors sense gravitational waves?

Diagram of LIGO detector
Diagram of LIGO Detector
(click for larger view)


Gravitational waves are ripples in the fabric of space-time. When they pass through LIGO's L-shaped detector they will decrease the distance between the test masses in one arm of the L, while increasing it in the other. These changes are minute: just 10-16 centimeters, or one-hundred-millionth the diameter of a hydrogen atom over the 4 kilometer length of the arm. Such tiny changes can be detected only by isolating the test masses from all other disturbances, such as seismic vibrations of the earth and gas molecules in the air. The measurement is performed by bouncing high-power laser light beams back and forth between the test masses in each arm, and then interfering the two arms' beams with each other. The slight changes in test-mass distances throw the two arms' laser beams out of phase with each other, thereby disturbing their interference and revealing the form of the passing gravitational wave.


Why are two installations necessary?

At least two detectors located at widely separated sites are essential for the certain detection of gravitational waves. Regional phenomena such as micro-earthquakes, acoustic noise, and laser fluctuations can cause disturbances that simulate a gravitational wave event. This may happen locally at one site, but such disturbances are unlikely to happen simultaneously at two widely separated sites.

Settings for the LIGO observatories were selected by the National Science Foundation (NSF) near Livingston, Louisiana, and at Hanford, Washington. These venues, separated by nearly 2,000 miles, are both flat and large enough to accommodate the 4-kilometer interferometer arms. Both are also remote enough from urban development to ensure an environment of seismic and acoustic quiet, but still within convenient housing range for resident and visiting staff. The NSF selected these two sites after a nationwide open competition, which included a thorough evaluation of 19 proposed LIGO sites in 17 states, the endorsement of that evaluation by a national review panel, as well as an internal NSF review.


What are the prospects for international collaboration?

To determine (by triangulation) the exact celestial location of many gravitational-wave sources, and to extract all the other information the waves carry, more than the two sites will be required. For these studies LIGO is part of an international network of observatories, established in a collaborative arrangement with scientists in other countries. LIGO is a crucial component of this network. Other detectors nearing, or currently in, operation are Virgo (near Pisa, Italy), GEO 600 (near Hanover, Germany), and TAMA (in Tokyo, Japan); planning for an instrument is underway in Australia.