 Complement Detectors——Continued
Getting into Space | Detecting Gravitational Waves | Complement Detectors
The LISA observatory will be complemented by both resonant-mass and laser-interferometry ground detectors——an international network of gravity wave observatories.
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Global network of ground observatories.
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J. Weber and resonant-mass detector. Courtesy of Louisiana State University/Department of Physics and Astronomy.
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Resonant-mass detectors, also called bar detectors, were pioneered by Joseph Weber, of the University of Maryland, in the early 1960s. These detectors use large (1-2 ton) aluminum cylinders. Passing gravitational waves stretch the bar and deposit mechanical energy. If the gravitational wave is near the resonant frequency of the aluminum cylinder, the cylinder "rings."
This type of detector operates at frequencies near 1000 hertz, which is where the largest gravitational waves——from supernovae explosions and the final in-spiral of neutron star binary systems——can be "heard." Resonant bar detectors are limited to a narrow frequency range by thermal and mechanical noise. More modern, cryogenic (cold) detectors are located in the United States (ALLEGRO in Louisiana), Switzerland (EXPLORER), and Italy (AURIGA near Padua and NAUTILUS at Frascati).
Ground-based laser interferometers are huge L-shaped instruments with each arm up to a few kilometers in length. Laser beams are bounced back and forth along the arms and reflected by mirrors at each end. These mirrors are like the test masses used in space-based instruments. To detect gravitational waves, small changes in the time needed for laser light to travel between suspended mirrors is measured. The advantage of this type of instrument is that they are not limited to a narrow frequency range. They are better able to detect waves arriving from different directions.
Because these observatories are located on Earth's surface, they will operate differently from LISA. The test masses are not free falling, but are suspended, and the interferometer arm lengths are much shorter than LISA's. They may be just a few kilometers, as compared to five million kilometers for LISA. Therefore, ground observatories will "hear" completely different sources, at completely different frequencies. For example, the Laser Interferometer Gravitational-Wave Observatory (LIGO), located in both Louisiana and Washington state, will "hear" the final few minutes of radiation from a neutron star falling into a stellar-mass black hole. LIGO will also "hear" radiation from neutron star-neutron star and stellar-mass black hole-black hole binaries. LISA will "hear" the final years of radiation from the coalescence of massive black holes, with masses 10 to a million times the mass of the Sun, as well as small objects like stars falling into supermassive black holes.
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