gravitational wave

In the language of Albert Einstein's general theory of relativity, gravitational radiation or gravitational waves (GWs) are ripples in the geometry of space and time.'' A less abstruse way to describe gravitational radiation is by drawing an analogy to the electromagnetic spectrum (light, infrared, radio, microwave, x-ray, etc.). Just as these represent forms of the free radiation or waves associated with electricity and magnetism, so GWs represent the radiation associated with the force of gravity. Einstein actually predicted their existence in 1916, the same year his paper on general relativity theory was published. He even calculated the radiation emitted from a binary star system (the strongest source known at the time) and concluded that the radiation was so weak that it had a negligible practical effect.'' For the next half century gravitational radiation remained a theoretical curiosity that was of no practical astrophysical significance. In the last two decades astrophysicists have discovered several new potential sources and have come to believe that it not only may be possible to detect gravitational waves directly but also that their emission may even be the dominant process in the evolution of some astrophysical objects. Electromagnetism and gravity are the only two fundamental, longrange forces in nature. Just as accelerated electric charges generate electromagnetic radiation, so do accelerated gravitational charges,'' that is, masses, generate gravitational radiation. Simply by analogy with electromagnetism, it is not surprising that gravitational waves are predicted by general relativity and every other viable theory of gravity. Furthermore, if gravity is to obey the laws of Einstein's special theory of relativity, then gravitational radiation must travel at the speed of light. where is the wavelength of the radiation. If the particles are electrons (which are responsible for most of the electromagnetic radiation we observe), the first factor alone is 10-43, which illustrates the incredible weakness of gravity. The second term in the preceding equation is proportional to the square of the ratio of the speed of the masses to the speed of light and is always less than 1. From this example it is clear that large, rapidly moving masses are the best sources of GWs. Figure 1. Oscillating masses (electric charges) are a source of gravitational (electromagnetic) waves. This same system can also absorb energy from an incident gravitational (electromagnetic) wave. Considering the inherent weakness of gravity, laboratory sources of gravitational radiation are nonexistent. For example, a 1-ton steel bar spun so rapidly that it is on the verge of being ripped apart by centrifugal force radiates less than 10-30 W. (This problem was considered by Einstein in 1918.) By contrast, existing detectors are only sensitive enough to detect such a source (at a distance of one wavelength) if it emits more than 106 W. Current hopes for directly detecting GWs are pinned on astrophysical sources in which massive bodies undergo tremendous accelerations. Short term binary star systems emit strongly (1025-1029 W) at frequencies from 10-4-10-3 mHz, but there are currently no detectors capable of detecting such sources even nearby. Even though not directly detectable, the energy loss from the short term binary pulsar PSR 1913+16 due to gravitational radiation has been measured by precise timing observations of the pulsar orbit. These measurements agree with the predictions of general relativity to within 1%, a result which is an important confirmation of the existence of GWs. A much stronger source is gravitational collapse to a black hole during which a large fraction of the mass of an entire star may be accelerated to velocities approaching the speed of light. It is expected that as much as 1049 W of GWs will be emitted from such a source in the form of a pulse of duration 0.001 s. It has also been conjectured that massive black holes (10