CHAMPAIGN, Ill. -- A simple electromagnetic effect discovered in the
19th century may help explain one of the more exotic predictions of Einstein's
theory of relativity, a University of Illinois theoretician says.
"In relativistic physics, the gravitational field of a spinning object
[such as Earth] is different from the gravitational field of a non-spinning
object," said Stuart Shapiro, a U. of I. professor of physics and of
astronomy. "According to Einstein, a spinning object generates a magnetic-like
force in addition to the usual static force. This gravitomagnetic force
can induce matter currents in nearby moving objects."
Shapiro calls this effect "gravitomagnetic induction" and has
modeled it as the gravitational analog of Faraday induction in electromagnetism.
"The similarity is striking," Shapiro said. "Just as an electric
current can be induced in a wire by a changing magnetic field, a matter
current can be induced in an object by a changing gravitomagnetic field.
Although this matter current is just a stream of matter in motion, its effect
on an object -- say a neutron star orbiting a spinning black hole -- can
be very profound."
In a paper published Nov. 25 in Physical Review Letters, Shapiro presented
his model for gravitomagnetic induction and calculated its effect near rapidly
spinning black holes, where the gravitomagnetic force could rival the static
gravitational field in strength.
"The gravitomagnetic field of a rotating black hole would induce matter
currents in an inspiraling neutron star that could influence the star's
spin, internal structure and orbital motion," Shapiro said. "These
induced matter currents could affect the structure and stability of the
neutron star before the final plunge, accelerating the breakup of the star
and possibly leading to the formation of an orbiting disk around the black
hole."
The effects of gravitomagnetic induction would leave characteristic imprints
on the star's gravitational waves -- ripples of gravitation traveling at
the speed of light -- which might be discernible to future gravity wave
detectors. But researchers may not have to wait the estimated 10 years for
such detectors to become operational before receiving experimental verification
of the presence of gravitomagnetism.
According to relativity, the rotating mass of Earth itself should be the
source of a very weak gravitomagnetic field. One of the main tasks of the
Gravity Probe B satellite, scheduled for launch in 1999, is to detect and
measure Earth's gravitomagnetic field for the first time.
"Because Earth is spinning, its gravitomagnetic field will cause a
very sensitive gyroscope on the satellite to precess -- that is, the axis
will rotate -- in relation to the distant, 'fixed' stars," Shapiro
said. "The effect will be tiny, but noticeable."