What Are Gravitational Waves?
Gravitational waves can be understood by analogy to electromagnetic waves. Just as the rapid motion of charges in the antenna of a radio transmitter generates electromagnetic waves, so the rapid motion of masses generates gravitational waves. However, because of the relative weakness of gravity, an effective gravitational wave transmitter needs very large masses moving very rapidly. Gravitational radiation is different from electromagnetic radiation in some important respects. For example, there is no negative mass and gravitational waves of the simplest wave geometry, known as dipole radiation, cannot be made.
Gravitational waves are often described as ripples in spacetime, and they manifest themselves as a time-varying strain, that is, a fractional variation in the light travel time between points in space. Consequently, the size of the distance change is proportional to the distance over which the measurement is made. Since the motions will be generally extremely small, detection will be made easier by very large antennas, and therein lies one of the principal motivations for a space antenna.
How Are Gravitational Waves Made?
Because of gravity's weakness, it takes masses comparable to, and preferably much larger than, the Sun to make a gravitational wave transmitter from which we can hope to detect a signal. An effective transmitter would be made of two compact objects, like white dwarf stars, neutron stars or black holes, in tight orbits around each other. The denser the objects, the more mass can fit in smaller orbits, and the shorter the period of their orbit.
Are there such transmitters giving observable signals?
Our current knowledge of compact stars suggests that there should be very many, spread throughout our galaxy. In addition, it seems likely that there are other, much more powerful transmitters containing massive black holes elsewhere in the Universe. Some of the most interesting sources are difficult or impossible to observe by conventional electromagnetic means, making their numbers and characteristics uncertain, but all the more intriguing. For example, two massive black holes, both the size of a million Suns, spiraling in toward each other in a galactic nucleus anywhere in the Universe would produce easily detected signals. Their coalescence would be one of the most energetic events in the Universe, but it would likely go unobserved with present instruments.
How Are Gravitational Waves Detected?
Several detection techniques have been developed, and are being tried. "Receivers" for detecting gravitational waves generally rely on measuring the changing separations between test masses, analogous to the motions of charges in a radio antenna. However, because of gravity's inherent weakness, the motions are usually extraordinarily small. One method measures the vibration of resonant bars that are excited like tuning forks by the ripples in spacetime. Another times the arrival of radio waves from distant spacecraft looking for a rhythmic variation in the propagation time. A variant on this method uses the radio signals from distant pulsars.
There is an important related observation that demonstrates the existence of gravitational waves. In the pioneering observations of pulsars in binary star systems by Hulse and Taylor, for which they received the Nobel Prize, the orbital period of the two stars increased as they spiraled inward at the rate expected for energy loss by gravitational radiation.
The first technique used to search for gravitational wave used a solid metal cylinder. The fundamental vibrational mode of the bar changes in amplitude and phase as a gravitational wave passed through the bar. Modern versions of these detectors are cooled to near absolute zero and use resonant transducers increased sensitivity and bandwidth, Several groups are also constructing resonant detectors using spheres rather than cylinders, which have the added advantage of being omnidirectional in their sensitivity and capable of determining the direction of an incoming gravitational wave.
A promising detection scheme now being brought into operation uses the great displacement sensitivity of laser interferometry to measure changing separations between widely separated test masses. There are two main experimental challenges: to measure distance changes around 10-21 of the distance between the masses, and to isolate the test masses from all disturbances larger than gravitational waves.
There are several long ground-based detectors under construction. The US project, called LIGO is building 4 km long detectors in Hanford, Washington and Livingston, Louisiana. The French/Italian Virgo Project is building a 3 km detector in Cascina, Italy. The GEO Project, sponsored by Germany and the UK, is building a 600 m detector near Hannover, and the Japanese are working on a 300 m detector near Tokyo known as TAMA.
The Laser Interferometer Space Antenna (LISA) is a gravitational-wave detector based on an interferometer in space with 5 million kilometers separating its test masses. As with the ground-based interferometric detectors, the experimental challenges are monitoring the separations of the masses with sufficient precision and isolating the test masses from disturbances comparable to those caused by the gravitational waves.
Ground-based and space-based detectors operate in complementary frequency bands and the sources in these bands are quite different. In the lower frequency band accessible to space detectors, the sources tend to be detectable for long times, a year or more; in the higher frequency band of ground detectors, the sources tend to be catastrophic bursts, detectable for at most a few minutes. The LIGO signal band is as different from the LISA signal band as gamma rays are different from visible light.
Stephen M. Merkowitz
NASA / Goddard Space Flight Center
Greenbelt, MD 20771
Tel: (301) 286-9412
Fax: (301) 286-1684
Last Update: December 19, 2001
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