Japan plans underground gravitational-wave detector

An underground site and cryogenic mirrors are the hallmarks of Japan’s planned gravitational-wave detector. In June the Japanese government approved about two-thirds of the estimated ¥15.5 billion ($190 million) tab for building the Large-scale Cryogenic Gravitational-wave Telescope (LCGT).

The funding granted so far covers construction, but not the equipment to cool the interferometer’s four mirrors. Money for that and for digging perpendicular tunnels some 200 m below ground to accommodate the 3-km-long arms of the interferometer will be sought in the next few years, says project principal investigator Takaaki Kajita of the University of Tokyo’s Institute for Cosmic Ray Research.

Tests using a prototype detector with 100-m-long arms found that seismic noise from wind, human activities, and other sources was attenuated about two orders of magnitude underground compared with on the surface, Kajita says. The LCGT will share the Kamioka Mountain location some 250 km northwest of Tokyo with the Super-Kamiokande neutrino detector and other experiments, but it will have its own entrance to avoid disturbances.

The interferometer’s 30-kg mirrors will be cooled to 20 K to reduce thermal noise. When decisions were being made for the LCGT 10 years ago, the choice was between cold mirrors or heavy, ton-scale ones, says Kajita. “People thought that cooling the mirrors must be easier.” The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) is taking a different tack—using 40-kg monolithic silica mirrors and an all-glass suspension system (see story on page 31). But, says Advanced LIGO project leader David Shoemaker of MIT, “We appreciate them doing the pathfinding on cryogenic mirrors.”

The LCGT, like Advanced LIGO, should be able to detect the merger of two neutron stars at a distance of around 700 million light-years.

For now, the LCGT is funded entirely by the Japanese government. But Kajita says groups of scientists in the US, China, and Taiwan are interested in collaborating. He adds, “since the information obtained by any single detector is limited, it is really essential to form a worldwide network of gravitational detectors.” If all goes smoothly, the LCGT could be fully online by 2017.