Albert Einstein predicted the existence of gravitational waves in 1916 as part of the theory of general relativity. LIGO will detect ripples in space-time by using a device called a laser interferometer, in which the time it takes light to travel between suspended mirrors is measured with high precision using controlled laser light. Two mirrors hang far apart, forming one “arm” of the interferometer, and two more mirrors make a second arm perpendicular to the first. Viewed from above, the two arms form an “L” shape. Laser light enters the arms through a beam splitter located at the corner of the L, dividing the light between the arms. The light is allowed to bounce between the mirrors repeatedly before it returns to the beam splitter. If the two arms have identical lengths, then interference between the light beams returning to the beam splitter will direct all of the light back toward the laser. But if there is any difference between the lengths of the two arms, some light will travel to where it can be recorded by a photo-detector.
The space-time ripples cause the distance measured by a light beam to change as the gravitational wave passes by, and the amount of light falling on the photo-detector to vary. The photo-detector then produces a signal defining how the light falling on it changes over time. The laser interferometer is like a microphone that converts gravitational waves into electrical signals. Three interferometers of this kind were built for LIGO – two near Richland, Washington, and the other near Baton Rouge, Louisiana. LIGO requires at least two widely separated detectors, operated in unison, to rule out false signals and confirm that a gravitational wave has passed through the Earth.
URL:
http://events.internet2.edu/2008/fall-mm/sessionDetails.cfm?session=10000236&event=911
www.ligo.org
www.ligo.caltech.edu
www.ligo.mit.edu
Scientific Collaboration (LSC) Council – Members and Observers:
USA:
Caltech; Eastern Michigan University; LIGO Hanford Observatory; LIGO Livingston Observatory; Louisiana State University; MIT; National Science Foundation; Northwestern University; Penn State University; Stanford University; Syracuse University; University of Colorado at Boulder; University of Florida; University of Michigan; University of Oregon Eugene; University of Wisconsin
France:
Observatoire de Paris-Meudon; IN2P3
Germany:
Max-Planck-Instituts für Quantenoptik – Garching
Japan:
TAMA Project National Astronomical Observatory
Russia:
Moscow State University
UK:
University of Glasgow
