The twin LIGO detectors are sensitive to a wide range of frequencies, bracketing those detectable by the highly tuned ALLEGRO and other acoustic bar detectors: “from about 50 Hertz—an octave above the lowest note on a piano—to 10,000 Hertz, about that of the squeak of a mouse,” says Weiss. And the LIGO detectors are not alone. Somewhat smaller versions are operating in Germany, Italy, and Japan. In addition to searching for signals from supernovae, astronomers hope they can capture the entire glissando accelerating up to the death chirp of binary neutron stars coalescing into black holes. LIGO is so sensitive, in fact, that eventually it should detect supernova explosions, in-spiraling neutron stars, and black holes swallowing gases (and burping) all the way out to the Virgo Cluster, some 45 million light-years away. “We’re already within tasting distance of this!” Weiss exclaims.
Gravitational astronomers’ dearest hopes, however, lie on drawing boards. NASA and the European Space Agency are planning the Laser Interferometer Space Antenna, a constellation of three spacecraft that will orbit the sun in formation, 20 degrees behind Earth. When completed and launched in 2014, LISA will be the largest spaceborne instrument ever built.
Like LIGO, LISA would operate as an interferometer, but instead of being L-shaped with split beams recombined at the apex, LISA’s three spacecraft will form an equilateral triangle, all three spacecraft sending beams that travel in both directions along each side and are reflected back by small free-floating test masses. Instead of being two and a half miles long, each of LISA’s arms will be a little more than three million miles long—so long that the laser beam will need more than 16 seconds to travel its length.
Why such long arms? Signal. LISA is being designed to be sensitive to frequencies from below 0.1 Hertz down to 0.0001 Hertz—frequencies with wavelengths so long that the detector must be extremely large in order to sense them. “In that frequency range, the universe is doing a lot of big, exciting, violent stuff,” says Robin “Tuck” Stebbins, the U.S. LISA project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The number of sources giving off gravitational radiation at such long wavelengths is expected to be so huge that investigators worry about a “confusion limit,” where only the loudest sources can be separated from the combined din. LISA’s size will make it so much more sensitive than ALLEGRO and LIGO that Stebbins says, “If LISA doesn’t see thousands of signals at turn-on, it’s broken.”
Astronomers have seen indirect evidence of gravitational waves, most recently in May, when NASA’s Chandra X-ray Observatory measured the orbital period of two white dwarf stars circling each other. Einstein’s theory predicts that massive in-spiraling stars will shed energy as gravitational waves and that, as the system loses energy, the two stars will move closer together. Although the Chandra observations confirm the prediction—the orbital period of the stars is decreasing, so they are drawing closer to each other—the cause of that behavior remains unproved. And it will remain so until LISA, operating in space, senses a movement in its tiny test masses of a half-billionth of an inch, the subtle shiver of a gravitational wave passing by.