The Laser Interferometer Gravitational Observatory (LIGO) was an ambitious experimental program designed to directly detect gravitational radiation. The LIGO project, which began construction in 1998, began operating in September 2002. LIGO consisted of two observatories: the LIGO Hanford Observatory near Richland, Washington, and the LIGO Livingston Observatory near Baton Rouge, Louisiana. Other gravitational radiation detectors include: the GEO 600 project located in Hanover, Germany, (a collaboration between UK and Germany); the VIRGO detector located at the European Gravitational Observatory outside Pisa, Italy, (a collaboration between France and Italy); TAMA 300 in Japan; and the Australian International Gravitational Research Center outside Crawley, Western Australia. Together with LIGO, these instruments are the first components of what will become a worldwide network of gravitational-wave detectors. As well as attempting to directly verify the existence of gravitational waves, the LIGO observatories will open new ways to observe and measure the composition and evolution of the universe.
Einstein's theory of general relativity relates the curvature of space-time at any point to the mass and energy that exists nearby. General relativity predicts a number of phenomena--such as the bending of light by stars and the precession of the planet Mercury--which have been well verified by direct observations. General relativity also predicts the existence of gravitational waves, ripples in space-time curvature that travel at the speed of light. The existence of these ripples was indirectly verified by Princeton University astronomers Joseph Taylor and Russell Hulse in 1974 by observing a pair of neutron stars in the Milky Way galaxy. One of the stars is a pulsar, a regular source of radio wave emissions that can be detected at Earth. After measuring the frequency of these pulses over a long time, it was found that the pulse frequency became shifted due to a loss of energy from the binary star system. This was in agreement with general relativity, which predicted loss of energy from the system associated with the emission of gravitational radiation.
However, no matter how well an indirect measurement supports a theory, it is possible to come up with an alternative explanation that will also reproduce that measurement. Accordingly, because direct measurements are always the most desirable, LIGO is designed to directly detect the gravitational radiation.
The LIGO apparatus consists of an L-shaped tunnel with two arms. At the end of each arm is a mirror, and a laser beam splitter at the juntion of the two arms. Each arm has the same length. When a laser beam is shot into the tunnel, it goes through the beam splitter, so both beams have the same phase initially. The beams are reflected from the mirrors and recombine at the original entry point, where the intensity of the beam is measured by a photodetector. As long as both arms have the same length, the beams interfere constructively and the maximum intensity will be measured. If a gravitational wave passes through the apparatus, the length of the tunnel arms will change; one arm will become slightly shorter, and the other will become slightly longer. The laser beams will then be out of phase when they recombine (because they traveled different distances) and there will be some measurable destructive interference.
The challenge in making these measurements is to minimize vibrations caused by seismic activity or human activity that could disrupt the experiment and lead to false signals. To minimize these effects, sophisticated automatic mirror control systems have been developed to keep the mirrors from moving due to vibrations. In addition, new vacuum technology has been developed to minimize scattering from gas atoms, and high precision lasers with minimal frequency variation are incorporated to avoid false interference patterns. However, even with these precautions, false signals can appear. This is the reason for having multiple LIGO sites; a seismic vibration in Washington will not appear at the same time in Louisiana, so signals at one site can be compared to signals from other sites to weed out false signals. Comparing the signals detected by LIGO with other observatories will enable researchers to triangulate the signals and find their point of origin in the sky. In this way, LIGO and the other observatories will become a huge gravity-wave telescope capable of yielding new data about the composition and behavior of the universe.
LIGO observations began in 2002 and were completed in 2010, during which time no gravitational waves were detected. The LIGO equipment was then shut down in preparation for construction of a more advanced form of the experiment known as Advanced LIGO, originally scheduled to go into operation in 2014. Installation of the required equipment for Advanced LIGO was completed in May 2014, and initial testing and operation began at that time. Full operation of the system was expected to begin in March 2015, with final implementation of all systems scheduled for some time in 2017.