Washington DC
AFTER a search lasting 80 years, scientists announced this week that they
have found evidence for a curious prediction of Einstein鈥檚 general theory of
relativity. Two teams of astronomers have observed the telltale signs that dense
bodies such as black holes and neutron stars pull space and time around with
them as they spin, a phenomenon known as 鈥渇rame dragging鈥�.
Ever since Einstein formulated the general theory of relativity, which links
gravity, space and time, physicists have been trying to test its predictions.
For instance in 1919, the British astronomer Arthur Eddington showed during a
solar eclipse that the Sun warps the path of starlight. And in 1974, astronomers
confirmed the existence of gravitational waves鈥攔ipples in space-time that
can be emitted by massive stars.
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This week, at a meeting of the American Astronomical Society in Estes Park,
Colorado, astronomers reported evidence for another of the theory鈥檚 predictions.
They have seen signs that spinning objects twist time and space as they rotate.
Kip Thorne, an astrophysicist at the California Institute of Technology in
Pasadena, says that this is a vital test of Einstein鈥檚 ideas. 鈥淎side from the
direct detection of gravitational waves, frame dragging is the most important
补谤辞耻苍诲.鈥�
Einstein said that any spinning object should drag space and time as it
turns. The effect is so small, however, that it should only be measurable near
bodies with intense gravitational fields, such as neutron stars or black holes.
When one of these superdense objects has a star orbiting nearby, its gravity can
drag material away from the star to form an accretion disc. As matter in the
disc spirals down towards the object at its centre, it heats up and emits
X-rays.
General relativity apart, you would expect an accretion disc to remain facing
in the same direction. But frame dragging should change the picture, causing the
orbit of the matter in the accretion disc to wobble around the spin axis of the
massive object at its centre.
A team led by Wei Cui of the Massachusetts Institute of Technology looked for
this effect by observing accretion discs around several possible black holes.
Luigi Stella of the Astronomical Observatory of Rome led another group who did
the same for 15 neutron stars. Both teams measured the X-ray brightness of the
discs using NASA鈥檚 Rossi X-Ray Timing Explorer satellite.
In both cases, the brightness of the emitted X-rays varied in a way that
suggests that each disc is indeed wobbling. 鈥淭he area of the X-ray emitting
region you see changes,鈥� says Cui. 鈥淭he oscillation agrees very well with the
辫谤别诲颈肠迟颈辞苍蝉.鈥�
鈥淚t鈥檚 extremely exciting that there is some effect,鈥� says Francis Everitt, a
Stanford University physicist who runs the Gravity Probe B project, a satellite
that will measure frame dragging by putting gyroscopes in orbit
(鈥淢usic of the spheres鈥�, 快猫短视频, 31 August 1996, p 28).
鈥淏ut it鈥檚 not a
quantitative result,鈥� he says. Everitt hopes Gravity Probe B will reveal the
effect鈥檚 strength.
Mitch Begelman of the University of Colorado at Boulder agrees that further
observations are needed. 鈥淚t will be hard to convince people beyond reasonable
doubt; there could be spots on the disc or flares rotating around,鈥� he says.
鈥淏ut if they鈥檙e able to nail this down, it鈥檚 a momentous discovery.鈥�