Gravitomagnetic Induction Predicted From Einstein's Theory

February 11, 1997

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."
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University of Illinois at Urbana-Champaign

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