The Laser Interferometer Space Antenna (LISA)

This page is relevant to Drag-Free satellites because the LISA program to detect Gravity Waves from space requires that its satellites be Drag-Free to meet their performance specifications.

Gravity Waves compress and stretch space as they pass. These strains are extremely small, 10^-21 to 10^-23; so that they are very hard to detect. Nevertheless, it can be done.

Gravitational-Wave Detectors

Three types of Gravitation-Wave detectors either exist now, or will soon become operational, or are in the planning stages. These are:

1. Low-Temperature Resonant Bar-Detectors (not discussed here) which have been operational for several years but have not yet detected any confirmed Gravitational-Wave signals. See the Auriga or Louisiana State University websites. The present sensitivities of the bar detectors is about 10^-18 which isn't quite enough. See the paper by Michael Tobar (PDF), but new detector upgrades are planning to reach 10^-21 or 10^-22. (See the the paper by E. Cossia, July 22, Fourth LISA symposium.)

2. Long Ground-Based Laser Cavities. See LIGO, Geo600, TAMA300 (or TAMA300 Results), VIRGO, and ACIGA. These detectors use very long arms and resonant cavities to improve the sensitivity of the detector. For example, if the light bounces back and forth 100 times in the cavity, the effective path length of a 4-km arm becomes 400,000 meters, and 10^-21 times 400,000 meters = 4 x 10^-16 meters, a litte smaller than the size of an atomic nucleus. An example sensitivity is given by the GEO600 Sensitivity and Noise-Source graph or by the slide pages 43 and 44 of Jim Brau's Seminar (PDF) which show how the LIGO sensitivity will evolve with future improvements.

3. Very Long (5 x 10⁶ km) Space Laser Paths using Drag-Free Satellites to protect the reference masses from space disturbances such as solar radiation pressure or solar wind. This is the LISA project. See The NASA LISA Page, The LISA Hannover Homepage, European Space Agency (ESA) LISA Homepage, The JPL LISA Technology Plan, The LISA Pre-Phase A Report, Second Edition (PDF), and The LISA Status Report (10 MB PDF). LISA measures over a path length of 5 million km, and the distance change to be measured is about 5 x 10⁹ meters times 10^-23 = 5 x 10^-14 meters. Being in space also eliminates the seismic noise which limits the low-frequency response of the ground-based detectors.

There is a graph of the LIGO and LISA sensitivities on the GEO600 website. The text is in German but the graph is in English. It can be seen from the two curves that the space mission (LISA) is sensitive in a frequency range of 10^-4 to 0.1 Hz, and that it will be able to detect gravitational waves from compact binary pairs in our galaxy. Although it is not clear from this curve, the main mission of LISA is to study Massive Black Holes (MBH) throughout the entire Universe. By contrast, the ground-based Laser detectors (LIGO, GEO600, TAMA300, VIRGO, and ACIGA) are sensitive in the frequency range 10 to 1000 Hz. and are designed to detect catastrophic events such as a supernova explosion or the coalescence of neutron-star or Black-Hole binaries.

The sensitivities of the various detector types can be interpreted in terms of the distances out to which they can detect typical sources, and that in turn strongly effects how often an important event should be seen. The resonant-bar detectors with strain sensitivities of 10^-18 or 10^-19 can only detect sources in our Galaxy. The first generation of Laser Interferometers which are expected to have sensitivities of 10^-21 will be able to detect typical sources out to the Virgo cluster (about 20 Mpc or 65 million light years) or for some sources out to 100 or 200 Mpc. The second generation or LISA should achieve 10^-23 and will be able to detect sources in the entire observable Universe. See Kip Thorne's Paper (PDF) for a more detailed discussion of sources and distances.

Thus the bars so far have seen nothing since events such as supernova explosions or binary neutron-star/neutron-star mergers in our Galaxy are very rare. On the other hand simply improving the range to a radius of 20 Mpc (giving a much larger volume than that of the Galaxy) should give several events per year, and instruments at 10^-23 will see a large number of all kinds of events. Again see the slide pages 43 and 44 of Jim Brau's Seminar (PDF).

Third LISA Symposium at the Max Planck Institute in Golm

The third LISA Symposium was held in Golm (near Potsdam, near Berlin) Germany from July 11-14, 2000. (Also see the Conference Website.)

The nominal LISA Drag-Free design is a pair of single-axis cubical proof masses with their axes 60 degrees apart. An alternative design would be to use a single spherical proof mass. Solutions to some of the problems of spherical proof masses were discussed at the conference by Benjamin Lange. See Managing Spherical Proof Masses in Drag-Free Satellites with Application to the LISA Experiment.

The papers presented at LISA 3 have now been published in Classical and Quantum Gravity, Volume 18, Number 19, 7 October 2001.

The Amaldi 4 Gravitational-Wave Conference

The Fourth Edoardo Amaldi Conference on Gravitation Waves was held on July 8 - 13, 2001 at the University of Western Australia, Perth, Australia. The Department of Physics hosted the conference, and the chairman of the local organizing committee was Professor D. G. Blair.

The conference papers have now been reviewed for the May 2002 issue of Classical and Quantum Gravity. My paper on disturbances on a spherical proof mass is available on this website and has been accepted for the May issue of CQG.

Fourth LISA Symposium at Pennsylvania State University

The fourth LISA Symposium was held in State College, Pennsylvania July 20 - 24, 2002. See the LISA 4 program.

Additional Information

More information on Gravitational-Wave experiments can be obtained from a Bibliography of Gravitational Wave Experiments. Also see Gravitational-Wave Observatories.