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Listening posts around the globe

Currently, scientists in listening posts all across the globe are busy trying to detect gravitational waves - using two distinct techniques.

Before we start describing the different projects, here is a map of their locations:

Map of world showing gravitational wave detectors

As can be seen from this map, gravitational wave detection is truly a world-wide endeavour.

Interferometric detectors

The most ambitious modern projects in search of gravitational waves are the interferometric detectors in development since the 1980s. After long periods of planning and constructions, there are currently the following projects:

GEO600 has an arm-length of 600m and is located in Ruthe, Germany (near Hannover). It is operated by the Hannover branch of the Albert-Einstein-Institute. The GEO collaboration includes the University of Glasgow, Cardiff University, and the AEI, as well as the University of Birmingham and the University of the Balearics.

LIGO is an abbreviation for "Laser Interferometer Gravitational Wave Observatory". The LIGO project consists of three interferometric detectors, two with arm lengths of 4 kilometers and one with arm length of 2 kilometers. They are located in Hanford, Washington, and Livingston, Louisiana.

Virgo is another European project, a collaboration between French and Italian physicists who have built a detector with an arm-length of 3 kilometres near Pisa in Italy. At the moment, Virgo is still being tested.

TAMA 300 is a detector with 300 metres arm-length, located in Tokyo. It is meant to serve as a prototype for the LCGT project, an advanced kilometre scale interferometric detector planned to be built in the Kamioka mines in Japan.

AIGO, the "Australian International Gravitational Observatory" is the only project in the Southern hemisphere to date. Located in Perth in Australia, it is a 80 metre prototype for developing advanced interferometric techniques. A larger interferometer is in the planning stage.

Listening posts around the globe? More than that, when one of the following projects comes to fruition:

The concept of LISA (Laser Interferometer Space Antenna), a space-based gravitational wave detector, has been studied jointly by the European and US space agencies ESA and NASA for 20 years. Due to shortage of money on the NASA side, currently ESA is studying a variant of the original LISA concept, called eLISA (or sometimes NGO), that ESA could afford on its own, should NASA not be able to come up with a financial contribution.

eLISA (evolved Laser Interferometer Space Antenna) is an ESA project for a space-based interferometric gravitational wave detector. eLISA consists of three spacecraft in orbit around the Sun moving in the formation of a near-equilateral triangle whose sides are 1 million kilometers long. eLISA is currently scheduled to fly in 2022.

Resonant detectors

The second kind of detectors are the so-called resonant detectors. The centre-piece of such a detector is a solid metal object. Passing gravitational waves make that central test-mass oscillate, and these oscillations can be measured and amplified to detect the gravitational waves. Currently, there are four groups looking for gravitational waves with the help of metal cylinders with masses of about one ton:

NAUTILUS is set up in Rome.

EXPLORER is another experiment of the Rome group; the detector itself, however, is located in the research centre CERN in Geneva.

ALLEGRO, short for "A Louisiana Low temperature Experiment and Gravitational wave Observatory", and the experiment itself, true to its name, is located at Louisiana State University in the US.

AURIGA, set up in Padova, Italy.

NIOBE in Perth, Australia.

The cylinder is the classical shape for resonant detector - first chosen by Joseph Weber, the pioneer of gravitational wave research, in the 1960s. However, other groups experiment with spherical tests masses, which have certain advantages, but, at the same time, present more of a technical challenge:

MiniGRAIL - GRAIL is short for "Gravitational Radiation Antenna In Leiden", and it will not surprise the reader that this spherical detector is located in Leiden in the Netherlands.

Mario Schenberg is a second spherical detector in the making, currently in the development phase in Saõ José dos Campos, Brazil.

Further Information

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.