Gravity-Wave Detector Comes Online

Richard Gaughan

Despite the best attempts of researchers, conclusive proof of the existence of gravitational waves has remained elusive. But several next-generation projects around the world with the sensitivity to detect these waves are coming into service, including the Tama300 interferometer, on the Mitaka campus of the National Astronomical Observatory. These new systems promise to usher in an era of gravity-wave astronomy.

The Tama300 interferometer has achieved continuous stable operation. The device has the ability to detect the gravity waves created by the coalescence of two 1.4-solar-mass neutron stars within 10,000 parsecs.

The existence of gravitational waves is one of the few predictions of Einstein's general theory of relativity that remains unverified experimentally. Analogous to the way that oscillating electric charges can generate electromagnetic waves, oscillating masses are thought to produce gravitational waves. But there is a key difference: Gravitational radiation displays a quadrupole rather than a dipole pattern. Gravity waves consequently are very weak.

Hopeful astronomers with the Tama collaboration recently demonstrated the sensitivity required to detect the gravity waves created by the coalescence of two 1.4-solar-mass neutron stars within 10,000 parsecs. The detector is an actively stabilized Fabry-Perot Michelson interferometer with two equal 300-m arms. A specially designed 10-W Nd:YAG laser from Sony Corp. provides the interferometer beams, and a 10-m mode cleaner eliminates higher-order transverse modes and stabilizes the laser frequency.

Masaki Ando, a research associate in the department of science at the University of Tokyo and one of the Tama collaborators, said that gravity waves are responsible for any changes in the optical path lengths of the interferometer. Detectors at the cavity mirrors and beamsplitter provide input to a control system that keeps the interferometer stable.

Any corrections are recorded, and the wave signals are extracted from the feedback signal of the stabilization loop. In other words, Ando said, the signal from the control system -- the errors that it suppresses -- indicates how much the mirrors tried to move in a gravity-wave event.

New astronomy

The Tama300 interferometer has achieved continuous stable operation for 24.8 hours, and it took measurements for more than 160 hours in a two-week period last year, but it is not easy. "For long-term operation, the rejection of seismic motion is most important," Ando explained. "Usually, seismic motion is huge in the daytime [from] human and industrial activities." The researchers have installed an active and passive seismic-isolation system to compensate for this source of error.

But with the potential for false detection, how will Tama300 conclusively identify gravitational waves? In the collision and merger of neutron stars, the waveform is well-predicted by theory, and any disturbance can be checked against the prediction.

For other sources, it's not as easy. "In the case of supernova explosions," Ando said, "we will investigate the coincidence with the other observations (electromagnetic wave and neutrino). If gravitational-wave signals are observed with multiple detectors at the same time, we can declare the detection with confidence."

He believes the potential rewards are worth the collaboration's painstaking efforts. "We are helping create the new field of gravitational-wave astronomy."