Gravity´s rainbow [gravitational waves]

Electromagnetic waves are ubiquitous in science and technology, but gravity waves are proving difficult even to find. The challenge of detecting gravity waves is met by engineering a suitable detector - a classic case of detecting a wanted signal above the noise. In this case, noise is almost any movement or vibration, and for this reason, the largest terrestrial gravity wave observatories have been established in carefully chosen locations. They all work at audio frequencies, say 10 Hz to 10 kHz, which is thought to be a good range for the detection of signals from stellar sized objects. Unfortunately, frequencies below this range are swamped by seismic noise and cannot be detected on Earth. Signals in this frequency range are expected from massive black holes, which radiate in the unfamiliar millihertz range and have periods measured in minutes or hours. The proposed engineering solution for the detection of these low frequency gravity waves is the laser interferometer space antenna (LISA). This joint NASA/ESA mission is planned for launch in 2012 and will be the first gravitational wave detector in space. The basic principle of LISA involves measuring gravity-wave induced changes in distance between freely floating ´test masses´ on board three separate spacecraft. The article outlines the design of LISA and its mission.