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 harnessing of these waves for scientific research. It consists of two widely separated installations within the United States, operated in unison as a single observatory. When it reaches maturity, this observatory will be open for use by the national community and will become part of a planned worldwide network of gravitational-wave observatories.

LIGO is being designed and constructed by a team of scientists from the California Institute of Technology and the Massachusetts Institute of Technology. LIGO is funded by the National Science Foundation (NSF). Construction of the facilities was completed in 1999. Initial operation of the detectors is scheduled for 2001 and the first data run is scheduled for 2003.


What are gravitational waves?

Artist's drawing of gravitational waves
Artist's Drawing of
Gravitational Waves

Gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe, for example by the collision of two black holes or by 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 to Earth, bringing with them information about their violent origins and about the nature of gravity.

Albert Einstein predicted the existence of these gravitational waves in 1916 in his general theory of relativity, but only since the 1990s has technology become powerful enough to permit detecting them and harnessing them for science. Although they have not yet been detected directly, the influence of gravitational waves on a binary pulsar (two neutron stars orbiting each other) has been measured accurately and is in good agreement with the predictions. Scientists therefore have great confidence that gravitational waves exist. Joseph Taylor and Russel Hulse were awarded the 1993 Nobel Prize in Physics for their discovery of this binary pulsar.


What are LIGO's scientific goals?

LIGO will be used for research into the nature of gravity, and it will open up an entirely new window onto the universe. It will thus be a scientific tool both for physics and for astronomy.

Possible payoffs for physics:

General relativity describes gravity as a manifestation of the curvature of space-time. This description has been tested and proved correct in the solar system, where gravity is weak and changes slowly due to the orbital motions of planets and their satellites. LIGO will permit scientists to test this description for rapidly changing, dynamical gravity (the space-time ripples of the gravitational waves), and also for the extremely strong, dynamical gravity of two black holes as they collide.

More specifically, LIGO has the possibility to:

  • Verify directly general relativity's prediction that gravitational waves exist.
  • Test general relativity's prediction that these waves propagate at the same speed as light, and that the graviton (the fundamental particle that accompanies these waves) has zero rest mass.
  • Test general relativity's prediction that the forces the waves exert on matter are perpendicular to the waves' direction of travel, and stretch matter along one perpendicular direction while squeezing it along the other; and also, thereby, test general relativity's prediction that the graviton has twice the rate of spin as the photon.
  • Firmly verify that black holes exist, and test general relativity's predictions for the violently pulsating space-time curvature accompanying the collision of two black holes. This will be the most stringent test ever of Einstein's general relativity theory.
Possible payoffs for astronomy:

Almost all our present information about the distant universe comes from electromagnetic waves. Until the 1940s, the only such waves accessible to astronomers were light waves; and optical telescopes, studying them, revealed a serene universe of planets, stars, and galaxies. In the 1940s, 1950s, and 1960s the march of technology made possible radio telescopes, infrared telescopes, and x-ray telescopes, which look at cosmic electromagnetic waves that have wavelengths different from light. Because these radiations differed from light, they brought us new kinds of information about the universe: they revealed the universe's violent side -- quasars, pulsars, and the birth throes of stars, for example. Gravitational waves, being radically different from all electromagnetic waves, have the potential to create yet another revolution in our understanding of the universe. Among the things they might reveal are these:

  • The spiralling together and coalescence of pairs of neutron stars (stars made of nearly pure nuclear matter); and in some cases the implosion of the coalesced star to form a black hole.
  • The swallowing of a neutron star by a black hole, and the collisions and coalescences of black holes.
  • The birth of a neutron star in a supernova explosion, and the pulsation and spin of this newborn neutron star.
  • Starquakes (analogs of earthquakes) in neutron stars, and details of how such starquakes change the star's shape and spin.
  • Gravitational waves produced at the moment when space and time came into being in the Big Bang creation of the universe.
  • Discoveries of which astronomers as yet have no inkling.


What does a gravitational wave observatory look like?

The larger the gravitational-wave detector, the more sensitive it will be. To detect the very weak waves that are predicted requires two installations, each with 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 covered by 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 are fitted with mirrors. The main building at the vertex serves as the control center and houses vacuum equipment, lasers, and computers. Ultrastable laser beams traversing the vacuum pipes measure the effect of gravitational waves on the test masses.

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 enter the LIGO 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. These tiny changes can be detected by isolating the test masses from all other disturbances, such as seismic vibrations of the earth and gas molecules in the air, and 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 tiny 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 unequivocal detection of gravitational waves. Local phenomena such as micro-earthquakes, acoustic noise, and laser fluctuations can cause a disturbance at one site, simulating a gravitational wave event, but such disturbances are unlikely to happen simultaneously at widely separated sites.

After a nationwide open competition, NSF selected sites near Livingston, Louisiana, and at Hanford, Washington, for the LIGO installations. The sites, which are separated by nearly 2,000 miles, are both flat and large enough to accommodate the 4-kilometer interferometer arms. Both are also far enough from urban development to ensure that they are seismically and acoustically quiet, but still within convenient distance of housing for resident and visiting staff. NSF selected the 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, and 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. Scientists in France and Italy are establishing a three-kilometer observatory near Pisa, Italy. Other efforts are underway in Britain, Germany, Japan and Australia.

Last modified October 2, 2001