# Synopsis: Tuning in to gravity

#### Upper Limit on Gravitational Wave Backgrounds at 0.2 Hz with a Torsion-Bar Antenna

Koji Ishidoshiro, Masaki Ando, Akiteru Takamori, Hirotaka Takahashi, Kenshi Okada, Nobuyuki Matsumoto, Wataru Kokuyama, Nobuyuki Kanda, Yoichi Aso, and Kimio Tsubono

Published April 20, 2011

Depending on their source, gravity waves—never-detected ripples in spacetime that result from massive accelerating bodies—could appear at almost any frequency. Each gravity-wave detector is designed to be sensitive over a different stretch of the spectrum. Now, writing in Physical Review Letters, a team of scientists in Japan describes early tests of a detector that is sensitive over a frequency range not currently completely captured by others.

Last year, Masaki Ando, at Kyoto University, and colleagues proposed building a detector that senses gravity waves by tracking the relative angle between two suspended bar-masses. This torsion-bar antenna, or “TOBA,” would be sensitive to gravity waves between $1\phantom{\rule{0.333em}{0ex}}\text{mHz}$ and $1\phantom{\rule{0.333em}{0ex}}\text{Hz}$, a frequency range below that of the ground-based detector LIGO, but above that of the proposed space-based interferometer LISA.

In a first test of their detector’s sensitivity, the same group has designed a mini-version consisting of an upside-down T-shaped bar (about $22\phantom{\rule{0.333em}{0ex}}\text{cm}$ across). A magnet on one end of the bar allows the team to suspend the mass, free of contact, from a superconducting pivot, while a laser interferometry setup tracks the deflections in the bar.

The miniature TOBA provides an early estimate of the detector’s sensitivity, but the team expects three more stages of design before they scale up to a final version with $10$-$\text{m}$-wide bars. – Jessica Thomas