Gravity: Making Waves
LIGO’s Extended Family
If you fly into Baton Rouge, and you have a window seat, you’ll notice a strange sight. The mosaic of pine forest near Livingston becomes oddly parted. A perfect L cuts through 8 km of foliage, with the long, skinny concrete arms of the Laser Interferometer Gravitational-wave Observatory visible in the clearing.
But if you fly over Hanford, Washington; Pisa, Italy; Hannover, Germany; or Tokyo, Japan, you’ll realize that giant L’s aren’t peculiar to Louisiana. The world actually has not one, but five large-scale gravitational-wave detectors. Each is devoted to the collective cause of detecting, for the first time, gravitational radiation streaming from neutron stars, black holes, and other giant, moving masses in space.
Gravity from the Ground
As landmark as LIGO is, its reach is
limited. In fact, it can’t achieve its goals without a network of similar
interferometers, instruments that use light waves to detect a gravitational
wave’s minute warp on space. The network will be able to detect the same
sources that LIGO does, thereby verifying its observations.
The other interferometers that share LIGO’s earthly burden—LIGO’s Hanford, Washington, facility, Italy’s VIRGO, Germany’s GEO, and TAMA in Japan—have so far undergone various “dress rehearsals” together to compare and contrast initial data. Real collective runs using all detectors will hopefully begin no later than 2006.
If three of the detectors notice the same gravitational signal at once, they will be able to pinpoint where the source lives in space, says Neil Cornish, a gravitational-wave researcher and astrophysicist at Montana State University. “Because gravitational waves travel at the speed of light, they’ll be slight delays in the arrival times at the different detectors around the world,” he explains. Researchers can easily calculate these differences to triangulate the source location in space.
Locating such sources is a key step in plotting a brand-new map of space using gravitational information. Researchers can then scan the spot thoroughly with conventional optical and radio telescopes to add even greater detail about these sources.
Gravity from Space
Plans are underway for another interferometer
that will have a front-row seat to the cosmic objects that emit
gravitational waves: a seat from space itself. Slated for launch in 2013,
NASA’s Laser Interferometer Space Antenna (LISA) will boast the longest
interferometer arms in the Universe—each 5 million km long!
As space provides a near-perfect vacuum, LISA doesn’t need steel tubes or concrete to encase its arms. Instead, a virtual L will be created using a trio of satellites that will orbit the Sun together in a fixed arrangement. Each satellite will have its own laser directed towards the other two. The travel time of the beams leaving the satellite will be compared to that of the beams arriving from the distant satellites. This will determine whether or not a gravitational wave has affected the “arm” length.
LISA’s superior location and keen instrumentation won’t render ground-based interferometers useless, however. The detectors will complement one another by capturing waves at different points along the gravitational spectrum. Ground-based instruments are tuned to pick up the spastic, infrequent, high-frequency gravitational waves like those shed by binary objects ready to collide after billions of years of spiraling toward one another. LISA’s instrumentation, on the other hand, will capture the much slower, longer-lasting, lower-frequency waves launched during the early stages of these binaries’ spiraling-in process, thousands of years before they collide. Strong gravitational signals from these binary sources will pile up LISA’s detector with “an embarrassment of riches,” predicts Cornish.
