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. LIGO is available for use by the world scientific community, and is a vital member in a developing global network of gravitational wave observatories. Albert Einstein first predicted gravitational waves in his 1916 general theory of relativity, but for years their effects were regarded as too small to measure. LIGO, supported by the National Science Foundation, now participates in an international quest to detect gravitational waves.
An interferometer is an instrument that utilizes the interference of light waves for precise measurement. Interference occurs whenever a pair of coherent waves come together at the same time and place. If the crests of both waves coincide, or they are in phase, the waves sum together and the resultant wave amplitude is increased (constructive interference). If a crest of a wave meets a trough of another wave, the two waves cancel and the overall amplitude is decreased (destructive interference). The terms "interferometry" and "interferometer" are both derived from the word interference. There are many designs for interferometers, but they all operate on a similar principle. LIGO's design is based on a Michelson-Morley interferometer: A beam of light coming from a single source is split into two different perpendicular light beams traveling into the arms of the interferometer. These beams are then recombined so they interfere with each other.
Gravitational waves cyclically compress space-time in one direction while stretching it in the perpendicular direction. When a gravitational wave reaches Earth, the wave changes the relative lengths of the interferometer arms. This produces changing interference patterns in the interferometer's recombined beam. LIGO measures the relative lengths of the arms by looking at these interference patterns through a photodetector. By measuring this interference, the gravitational wave can be detected. To reduce the rate of false signals, the LIGO project consists of two indentical interferometers. They are located in Hanford, Washington, and Livingston, Louisiana. The arms of the LIGO interferometers are 4 km in length (a little less than 2.5 miles). LIGO's instruments are the largest gravitational wave detectors ever built.
LIGO has the possibility to test several of Einstein's General Relativity predictions, as well as provide insight into fields of astronomical study. LIGO will allow scientists to:
LIGO is funded by the National Science Foundation and operated by the California Institute of Technology and the Massachusetts Institute of Technology. This material is based upon work supported, in part, by the National Science Foundation under Grant PHY-0852870. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Image Credits: LIGO Lab, Tobin Fricke, NASA, The Hubble Heritage Team, B. Rankins