LISA - Hunting waves in space

The most spectacular project in the hunt for gravitational waves is LISA, the "Laser Interferometer Space Antenna". LISA is a joint project of ESA and NASA - an interferometric detector in space, with laser arms five million kilometres in length.

For LISA, three identical satellites will be brought into an orbit around the sun similar to that of the Earth. They will fly in an equilateral triangle formation, with a constant five million kilometres between each pair of satellites, following the Earth along its orbit at a distance of around 50 million kilometres. The configuration is sketched in the following image; the LISA triangle itself is pictured at three times its true size; if the satellites were drawn to scale (two metres in diameter), they would not be visible in the image:

The fact that the orientation of the triangle changes during each orbit makes it possible for researchers to determine the direction of gravitational waves reaching LISA.

Each satellite works similarly to a Michelson interferometer, with the two arms defined by two internal test masses in the satellite and one test mass in each of the others. Unlike a typical Michelson interferometer, LISA does not have a beam splitter. Instead, each satellite sends a laser beam to each of the other satellites. Because of the great distance, the laser beam that reaches a satellite is much too weak to reflect back. That's why the satellites do not contain mirrors - instead, they actively send laser light in response to all such light that reaches them. Comparison of the original laser beam and the response is used to monitor changes in the distance between the satellites. In this way, distance changes can be measured with a precision of 10 picometres (1 picometre = 10^-12 metres). For this, it is of crucial importance that the test masses remain undisturbed by external influences such as solar wind.

One of LISA's tasks is to listen to signals from the hot early phase of our universe, corresponding to a cosmic time of tiny fractions of a second. However, LISA's primary objective is the detection and examination of gravitational waves emitted by the supermassive black holes that reside in the centres of many galaxies. In addition, LISA will measure the signals of thousands of compact binary star systems in the Milky Way.

LISA is the result of decades of development in the area of laser interferomtry, propulsion and sensor technology. The new technologies are due to be tested in space by the LISA pathfinder mission in 2007, well before the mission commences in 2014. In both missions, the Max-Planck-Institute for gravitational physics plays a leading role.

[P. Aufmuth, Albert Einstein Institute and Leibniz University Hannover]

For background information on gravitational waves, check out Elementary Einstein, particularly the chapter Gravitational waves.

Related Spotlight topics on Einstein-Online can be found in the section Gravitational waves.

More information about LISA can be found on the project's websites:

LISA-website (ESA)
LISA-website (NASA)