School of Physics Astrophysics

LIGO

Aerial view of one LIGO interferometer

The Laser Interferometer Gravitational Wave Observatory (LIGO) is a collaboration seeking to detect gravitational waves (as predicted by Einstein in 1916 as a part of the theory of general relativity). The LIGO scientific collaboration seeks to detect gravitational waves, to use gravitational waves to explore the fundamental physics of gravity, and to develop gravitational wave observations as a tool of astronomical discovery. Work towards this goal includes research on, and development of techniques for, gravitational wave detection; and the development, commissioning and exploitation of gravitational wave detectors. The LIGO scientific collaboration is currently made up over 600 members from over 50 institutions and 11 countries. A/prof Andrew Melatos is the local lead at the University of Melbourne. His group's work with LIGO focuses on building theoretical models for gravitational wave emission and searching for gravitational waves in LIGO data.

How LIGO works

LIGO will detect the 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 photodetector.

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 photodetector to vary. The photodetector 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 LlGO -- two near Richland, Washington, and the other near Baton Rouge. Louisiana. LlGO 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.

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