This world is a drag--and scientists have proved it. By studying the dance of two Earth-orbiting satellites, physicists have detected the subtle twisting of spacetime around a massive, spinning object. The measurement is the most convincing demonstration yet of a hard-to-spot consequence of Albert Einstein's general theory of relativity.
General relativity predicts that a spinning mass drags the fabric of space and time around with it, much as a restless sleeper drags the sheets around while twisting and turning in bed. This effect, known as the Lense-Thirring, or "frame dragging," effect, is difficult to detect. To spot it, one has to observe how a spinning body changes the orientations of nearby gyroscopes.
Physicists Ignazio Ciufolini of the University of Lecce, Italy,
and Erricos Pavlis of Goddard Space Flight Center in Greenbelt,
Maryland, and colleagues tried to measure the effect in 1998 by
using two satellites, LAGEOS and LAGEOS II, as test gyroscopes. The
satellites--half-meter-wide mirrored spheres--were launched in 1976
and 1992 as targets for laser range finders, which can track their
position within a few centimeters. As the satellites spin around
Earth, the Lense-Thirring effect twists the planes of their orbits
slightly. The early measurements were "very rough," Ciufolini says,
because the uneven distribution of Earth's mass distorts the orbits
thousands of times more than does the Lense-Thirring effect.
Because the mass distribution was poorly known, Ciufolini and
colleagues had to make a few controversial estimates. As a result,
their value of the Lense-Thirring effect had a large
error—20%--and skeptics thought that there were subtle
problems with estimates that the researchers made about the
satellites' orbits. Now, thanks to better gravitational maps
produced by twin satellites known as GRACE, as well as improved
gravitational models and other refinements, the team now reports in
the 21 October issue of Nature a much firmer detection with
an error of about 10%. "This is the first reasonably accurate
measurement," says Neil Ashby, a physicist at the University of
Colorado, Boulder. Ciufolini predicts that a few more years of
observation will reduce the measurement's error to just a few
percent. By then, physicists hope, the gyroscope-laden satellite
Gravity Probe B, which was designed to detect the Lense-Thirring
effect, will have produced results with an error of about 1%--far
lower than the two LAGEOS satellites can achieve. Related
sites
More
about the LAGEOS satellites from NASA
Science
news article about Gravity Probe B
Probe B site at Stanford
University
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