The Laser Interferometry Space Antenna (LISA)

The Laser Interferometry Space Antenna (LISA) is not due for launch until 2012 but tests of components are of course going forward now. LISA will search for gravity waves passing the sun's vicinity by watching how the distance between two test masses changes. A gravity wave can be thought of as a traveling disturbance in spacetime itself; such a wave would temporarily shorten and then lengthen the path between the test masses. In this case the masses would be 5 million km apart, an interval that would be monitored every instant by the interference of laser beams traveling back and forth between the masses. Actually three pairs of test masses would be mounted on three far-flung satellites, spread out in space in an equilateral triangle where each leg is 5 million km long, with all three craft in independent orbit around the sun (see LISA websites and http://lisa.jpl.nasa.gov/). While the spaceborne LISA would look for waves with very low frequencies (.001-.1 Hz), the earthbound detector LIGO would search for gravity waves in a higher frequency range (100-1000 Hz).

As an interim step toward deploying LISA, the European Space Agency (ESA) plans to launch in 2007 its Pathfinder mission, a craft serving as a miniature version of LISA, two free-floating test masses 35 cm apart (small thruster rockets will be used to reposition the spacecraft so its sides do not come in contact with the masses), will be tried out. The test, watching that the masses move along in parallel trajectories, is not unlike the famous (or apocryphal) experiment conducted by Galileo Galilei to affirm that two objects, one light and one heavy, would fall at the same rate from the Leaning Tower of Pisa. And to perform the test in 2007 some terrestrial tests have now been carried out in 2003. Basically, scientists at the Universita' di Trento (Italy) are attempting to understand all the possible forces, in addition to gravity, that could influence the motion of the test mass. In an ideal experiment, the test mass (2 kg or, in units of weight, about 20 newtons) would be hung from a thin wire and surrounded by all the apparatus that will accompany it into space, including the motion sensor needed to reorient the spacecraft, and all extraneous forces on the mass, down to a precision of a femto-newton (10^-15 newtons) would have to be accounted for if the desired levels of precision needed for LISA were to be achieved. Such precision is not possible with ground-based detectors, so the experimenters used not the full test mass, but a hollow facsimile. At this early stage in understanding, the Trento physicists found a satisfactorily "quiet" force environment, but there are still a fact of 10 away from the precision needed for Pathfinder and a factor of 100 away from the precision needed for LISA. (Carbone et al., Physical Review Letters, upcoming article; contact Stefano Vitale, 39-0461-881568 )