Waving Back: Research

There are now three gravitational wave detectors in the US, two in Hanford, Washington and one in Livingston, Louisiana, which together make up the Laser Interferometer Gravitational Observatory (LIGO), a Caltech-MIT collaboration. Each detector employs an interferometer with a pair of perpendicular arms of equal length, with a state-of-the art laser as the light source.

The gravitational wave detector at Hanford , Washington . The long perpendicular sheds house the two arms of the interferometer. (credit: LIGO Laboratory)

The structure that houses the interferometer beams at the Livingston , Louisiana LIGO laboratory. (credit: LIGO Laboratory)

The distance that Advanced LIGO can search will be ten times that of LIGO, leading to a thousandfold increase in the volume of space (the volume of a sphere scales as the radius cubed) that can be searched (credit: LIGO Laboratory)

The three LISA satellites will function as a gravitational wave detector in space, with a baseline of five million kilometers. (credit: NASA/JPL)

Since the gravitational wave signal is so small, noise reduction is a major challenge. LIGO physicists isolate the interferometer mirrors from Earth movements and even compensate for them in real time by applying delicate magnetic forces to the mirrors. Moreover, the experiment is performed in a high vacuum, only about a trillionth of atmospheric pressure. Since the interferometer's arms are kilometers long, this is one of the largest evacuated spaces ever made. In all, noise is reduced by a factor of ten billion.

Seismic noise is particularly difficult to suppress because gravitational waves are predicted to share much of the frequency range of earthquakes. To help discriminate signal from noise, the two detectors in Hanford were designed with very different lengths—their perpendicular arms are 2 km and 4 km long. Since a passing gravitational wave produces a change in length proportional to the length itself, the signal from the longer detector should be twice as large as the signal from the shorter one. But a signal from an earthquake, or a local source of noise, won't scale up with the length in the same way, so seismic noise, even masquerading as a gravitational wave, can be rejected.

Likewise, the physical separation of the two laboratories helps eliminate spurious signals. The LIGO interferometers are extraordinarily sensitive—they respond to trees falling in a nearby forest and to small local earthquakes. To rule out signals from these kinds of events, the detectors in Hanford and Livingston operate simultaneously, and only signals that appear in both places at the same time are considered candidates for gravitational waves.

So far, LIGO has not detected any gravitational waves, but LIGO has met its design requirements and is a functioning observatory. The current LIGO is a prototype for Advanced LIGO, which will increase sensitivity by a factor of ten and the volume of space that can be searched for gravitational wave sources by a factor of a thousand (the searchable volume scales as the cube of the linear distance to the observed object-see drawing). Advanced LIGO should begin observations in 2013 and is expected to find gravitational waves.

Even Advanced LIGO will still be limited by the lengths of its arms and the challenges of seismic noise. Planning is underway for a gravitational wave laboratory in space, the Laser Interferometer Space Antenna (LISA), a space-based laser interferometer jointly sponsored by the European Space Agency (ESA) and NASA, originally to be launched in 2015. LISA will array three satellites in an equilateral triangle three million kilometers on a side and look for gravitational waves with frequencies less than one milli-Hertz (one thousandth of a cycle per second)—a range of frequencies inaccessible to LIGO because of seismic wave noise.

You can participate in the computer analysis of LIGO data through the Einstein@home project. If you have a high-speed internet connection, LIGO can apply your computer's unused processing power to search for signals from gravitational waves. Learn more at http://www.einsteinathome.org .