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Frame-dragging
Gravity Probe B used high-precision
gyroscopes (one at left) to measure
the twisting of spacetime by a spinning
object (right), a phenomenon predicted
by Einstein’s theory of general relativity.
change the orientation of the craft’s spinning spheres by only 39 milliarcseconds
per year, about the width of a human hair
seen from 400 meters.
After NASA pulled the plug in 2008,
private funding arranged by an executive
at Capital One Financial and the royal
family of Saudi Arabia bought some
extra time to clean up the data. By comparing the overall wobble of each sphere
with the tiny magnetic fluctuations on
its surface, the team worked out how the
patches were interacting. The researchers also discovered that the motion of
the revolving spacecraft could occasionally kick the gyros into ne w orientations.
“ What the Gravity Probe B team did to
understand this problem, sort it out and
get a credible answer was nothing short
of heroic,” says Clifford Will, a theoretical physicist at Washington University
in St. Louis who serves on the mission’s
science advisory board.
The results of this painstaking analysis, announced at a NASA press briefing
May 4 and scheduled for publication in
an upcoming Physical Review Letters,
reconfirm the geodetic effect with an
error of about 0.2 percent. Gravity Probe
B puts the frame-dragging effect at
37 milliarcseconds, with an error of
about 19 percent, far from the original
goal of 1 percent precision.
“This project has been a victim
of time,” says physicist Kenneth
Nordtvedt of Montana State University
in Bozeman, who points out that other
experiments have already measured
these effects.
Ignazio Ciufolini, a physicist at the
University of Salento in Lecce, Italy,
and Erricos Pavlis of the University of
Maryland, Baltimore County confirmed
frame-dragging by analyzing the orbits
of the two laser-ranged LAGEOS satellites (SN: 11/27/04, p. 348). Publishing
in Nature in 2004, the team reported an
error of 10 percent. Two other groups of
scientists in Germany and the United
States have since checked the analysis,
and a third satellite scheduled for launch
this year could help Ciufolini and Pavlis
improve their precision.
“We should be able to reach a test of
frame-dragging with an uncertainty
of almost 1 percent,” Ciufolini says.
Proponents of Gravity Probe B say
that general relativity, which is currently
incompatible with quantum mechanics,
should be tested in as many ways as possible. But the project’s ultimate legacy may
lie in its contributions to technology, not
science. GPS systems developed for the
spacecraft, for instance, now help farmers plant perfectly straight rows of corn.
“The technology needed to do this test
didn’t exist when the project started,”
says John Mester, a 19-year veteran of
the Gravity Probe B team at Stanford.
Mester hopes to help the team publish
a series of papers detailing the equipment they developed. But otherwise
their mission is complete.
“We’re basically done,” Mester says.
“None of us have a job anymore.” s
Back Story | GRAVIT Y PROBE A
of the many curious effects predicted by einstein’s general theory of relativity,
the slowing of time by gravity is (by comparison) relatively simple to grasp.
When gravity is stronger, time moves more slowly. this effect is often called
the “gravitational redshift” because it can be detected by its influence on the
color of light emitted from the surface of a massive body. (slowing of time
reduces the frequency of emitted light, shifting its color toward the red end of the
spectrum.) the gravitational redshift also affects
other sorts of radiation, such as radio waves.
in 1976, scientists from the smithsonian
astrophysical observatory and nasa’s marshall
space flight center tested this gravity-time
effect with an experiment called gravity probe a.
Launched to an altitude of 10,000 kilometers,
the probe (payload shown, left) contained a
hydrogen maser clock that emitted precisely
timed microwave signals until it crashed into
the atlantic ocean. by comparing signals from
the probe with an identical maser clock on the
ground, the scientists could determine whether
einstein’s expectations were fulfilled. and in fact,
the effect on the probe’s signals matched relativ-
ity’s predictions to within 7 parts in 100,000.
