The larger the gravitational-wave detector, the more sensitive it will be. 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.
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 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. 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.