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Science News, Jan 8, 2000 by Peter Weiss
Sensing ripples in the space-time sea from gravity's juggernauts
The starship suddenly shudders violently. "What was that?" the alarmed captain asks the ship's computer.
The computer deduces that a burst of gravitational waves has rolled through the spacecraft. These waves, it reports, have originated from a pair of gargantuan black holes, as near to the craft as the moon is to Earth.
The captain knew that the black holes--dense objects that exert such strong gravity that even light cannot escape--had been swirling around each other. But when they slammed together, merging into one, the collision sent a fast-moving ripple through the very fabric of space (or more accurately, of space-time) that was stronger than he expected. It alternately stretched and compressed everything and everyone in its path.
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"You are accustomed to gravitational waves so weak that only very delicate instruments can detect [their force]," the computer tells the captain. "Here, close to the coalescing holes, they were enormously strong."
Physicist Kip S. Thorne of the California Institute of Technology in Pasadena describes this far-future scenario at greater length in his book Black Holes and Time Warps (Norton, 1994). In it, he fulfills a dream of many contemporary scientists who specialize in the study of gravity and Albert Einstein's theory of general relativity--to somehow encounter gravitational waves.
Yet unlike the starship captain, today's scientists can't even claim familiarity with gravitational waves detected with sensitive instruments. For more than 30 years, using delicately balanced metal bars, researchers have tried unsuccessfully to discern the subtle stretching and shrinking that passing gravitational waves would cause (SN: 3/18/78, p. 169). The task is so challenging that detectors must measure changes in length that are less than a thousandth of the diameter of a proton.
Although detectors that rely on metal bars are still being improved, another technology has emerged that many researchers believe is more likely to capture the long-sought prize. In November, Caltech and the Massachusetts Institute of Technology completed the first phase of construction of a $300 million, two-site observatory called the Laser Interferometer Gravitational-Wave Observatory, or LIGO (SN: 2/29/92, p. 134), funded by the National Science Foundation.
Between now and 2002, LIGO scientists plan to install and fine-tune detector equipment. The observatory, flagship of a fleet of new laser-based gravitational-wave observatories being developed globally, is then expected to begin searching for waves.
If any of the instruments finally catch a gravitational wave, an important prediction of Einstein's 84-year-old theory of general relativity would be validated. Scientists could then begin subjecting the theory to rigorous tests at the tremendous gravity typical of extremely dense matter, such as black holes.
The feat would also mark the beginning of a fundamentally different way of probing the universe, particularly its dark and violent side. It might even provide a means of witnessing the universe at the merest instant after the Big Bang.
"LIGO is really the first step in what I see as the greatest challenge of the 21st century: Opening the gravitational-wave window to the universe," says cosmologist Michael S. Turner of the University of Chicago.
Soon after Einstein unveiled his theory of general relativity in 1916, scientists began to ponder its controversial predictions of gravitational waves.
General relativity merges space and time into a seamless four-dimensional entity. The presence of mass or energy curves space-time. That curvature manifests itself as an attractive force between objects--gravity.
Think of a ball resting on a rubber sheet. It dimples the sheet and any object on the curved surface rolls toward the ball, as if impelled by a force. If the ball rolls or jiggles, or if two balls spiral around each other, the sheet quivers with waves that rush outward.
Likewise, the motion of massive objects can generate waves of curvature--gravitational waves--that ripple through the fabric of space-time. Not all massive objects would be expected to trigger gravitational waves, according to theorists. Objects that move with perfect spherical symmetry, a spinning ball, for instance, provide such an exception.
No one has ever detected an actual gravitational wave. Nonetheless, the behavior of an unusual astrophysical object discovered in 1974 convinced most scientists that gravitational waves are real.
In that year, Russell A. Hulse and Joseph H. Taylor Jr., then both at the University of Massachusetts at Amherst, found a pair of extremely dense stellar cinders, known as neutron stars, rapidly orbiting each other. One of them, a pulsar, emits periodic bursts of radio waves as it orbits.
Gradual changes in the pulsation rate indicated that the stars were slowly spiraling toward one another. The slowdown perfectly matched the predictions of general rellativity if the pair was shedding energy in the form of gravitational waves (SN: 10/23/93, p. 262). For this insight, Hulse and Taylor won the Nobel Prize in Physics in 1993.
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