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ScienceWeek
GENERAL RELATIVITY: CONFIRMATION OF FRAME-DRAGGING
The following points are made by Neil Ashby (Nature 2004 431:918):
1) General relativity predicts that a spinning mass distorts space-time -- one of a variety of "gravitomagnetic" phenomena that are absent in Newtonian gravity. Unfortunately, gravitational forces are so weak that it is useless to try to detect this warping of space-time unless the mass is very large and spinning rapidly -- an astronomical body, say, such as the Earth or the Sun, or a neutron star. After many years' work, and through the analysis of millions of "laser-ranging" measurements made from more than 50 Earth-based stations, Ciufolini and Pavlis(1) confirmed the twisting effect of the spinning Earth on the orbits of two artificial satellites. The remarkable precision they achieved was due in large part to recent improvements in the modelling of Earth's gravitational field.
2) According to general relativity, a spinning flywheel imparts a twist to space and time in its proximity that can affect a nearby gyroscope. If a frictionless gyroscope is placed near the flywheel's axis of rotation, the gyroscope's spin axis will be dragged along in the direction of the flywheel's rotation. However, if the gyroscope is placed near the flywheel's perimeter, its spin axis will be dragged in a direction opposite to the flywheel's rotation. First analyzed by Joseph Lense and Hans Thirring, soon after Einstein published his general theory of relativity, the phenomenon is known as the "Lense-Thirring effect" -- or "frame-dragging", because the spin axes of ideal gyroscopes could be used to track the directions of coordinate axes in an inertial reference frame.
3) Frame-dragging is one aspect of the class of relativistic phenomena loosely known as "gravitomagnetism", through their analogy with the effects of ordinary magnetic forces on moving electric charges. General relativity describes gravity in terms of a set of ten independent quantities (the components of a second-rank tensor). The gravitomagnetic terms, however, vanish unless the mass producing the gravitational field is moving. Among the effects caused by gravitomagnetic forces are precessions (similar to the wobble of the axis of a spinning top) of the orbital plane of a satellite around a spinning body (such as Earth), or precessions of the orbit of an Earth satellite as the Earth-satellite system orbits the Sun. This latter effect is usually called "de Sitter" or "geodetic precession".
4) Precise measurement of these effects predicted by relativistic gravity theories is crucial, as they have important implications for our view of the Cosmos. Gravitomagnetic effects can be significant in many astrophysical systems. For example, in the binary pulsar B1913+16, geodetic precession of the orbits may cause the pulsed beam from this star to precess out of our line of sight in a few dozen years, and to reappear some centuries later(2). However, the uncertainties in such distant systems are usually too large for astronomical objects to be used for precision tests of gravitomagnetic effects.(2-5)
References:
1. Ciufolini, I. & Pavlis, E. C. Nature 431, 958-960 (2004)
2. Kramer, M. Astrophys. J. 509, 856-860 (1998)
3. Williams, J. G., Newhall, X. X. & Dickey, J. O. Phys. Rev. D 53, 6730-6739 (1996)
4. http://earthobservatory.nasa.gov/Library/GRACE/grace2.html
5. Reigber, C. et al. J. Geodyn. (in the press)
Nature http://www.nature.com/nature
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Related Material:
ON GENERAL RELATIVITY
The following points are made by Clifford Will (citation below):
1) During the two decades 1960-80, the subject of general relativity experienced a rebirth. Despite its enormous influence on scientific thought in its early years, by the late 1950s general relativity had become a sterile, formalistic subject, cut off from the mainstream of physics. It was thought to have very little observational contact, outside of cosmology and a few tests. It was believed to be an extremely difficult subject to learn and comprehend. It was also viewed as a field that was full of ambiguities and unanswerable questions.
2) One of the outgrowths of the renaissance of general relativity that occurred between 1960 and 1980 has been a change in attitude about the importance and use of the theory. Its importance as a fundamental theory of the nature of spacetime and gravitation has not been diminished in the least: if anything it has been enhanced by the flowering of research in the subject that has taken place. Its importance as a foundation for other theories of physics has been strengthened by current searches for unified and grand unified quantum theories of nature that incorporate gravity along with other interactions.
3) But the real change in attitude about general relativity has been its use as a tool in the real world. In astrophysics, for example, the general relativistic bending of light in gravitational lenses can help astrophysicists probe the structure of galaxies. General relativistic effects in the binary pulsar gave a high-precision determination of the mass of the pulsar. Had the result been very different from 1.4 solar masses it could have affected our understanding of supernovae in close binary systems. Neutron-star mass limits from general relativity are important in the observational search for black holes.
4) Finally, gravitational radiation may one day provide a completely new tool for exploring and examining the Universe. Relativity even plays a role in everyday life. For example, the gravitational redshift effect on clocks /must/ be taken into account in satellite-based navigation systems, such as the US Global Positioning System, in order to achieve the required positional accuracy of a few meters or time transfer accuracy of a few nanoseconds.
Clifford Will: The Renaissance of General Relativity. in: Paul Davies (ed.): The New Physics. Cambridge University Press, Cambridge UK 1989, p.7,33.
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Related Material:
ON GRAVITATIONAL WAVES
The following points are made by B.C. Barish and R. Weiss (Physics Today October 1999):
1) The idea of gravitational waves was already implicit in the 1905 special theory of relativity, with its finite limiting speed for information transfer. The explicit formulation for gravitational waves in general relativity was put forward by Einstein in 1916 and 1918. He showed that the acceleration of masses generates time-dependent gravitational fields that propagate away from their source at the speed of light as warpages of spacetime. Such a propagating warpage is called a "gravitational wave".
2) The best empirical evidence we have of the existence of gravitational radiation is indirect. It comes from the 1974 discovery and beautiful observations, by Russell Hulse and Joseph Taylor, of the first binary pulsar ever found. Exploiting the clockwork pulsar signal from the neutron star, they were able to monitor the orbital period of the binary star system with exquisite precision and confirm that it was indeed gradually speeding up at just the rate predicted for the general-relativistic emission of gravitational waves.
3) The direct detection of gravitational waves will mark the opening of a new window on the near and far reaches of the Cosmos. For physics, its most important promise is the direct observation of gravitation in highly relativistic settings, so that one can test general relativity in the strong-field limit, where it is not merely a small correction to Newtonian gravity. In that limit, the strong curvature of the spacetime geometry should show us fundamentally new physics.
Physics Today http://www.physicstoday.org
ScienceWeek http://scienceweek.com
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