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(a) The conventional gravitational field around a massive object. (b) The "gravitomagnetic" field around a rotating massive object. (c) Matter currents induced in a neutron star spiralling in toward a rapidly rotating black hole.

A gravitomagnetic field, according to Einstein's theory of general relativity, arises from moving matter (matter current) just as an ordinary magnetic field arises from moving charges (electrical currents). This analogy is so apt that the equations describing this "magnetic" component of gravity can essentially be adapted from Maxwell's equations for electromagnetism.

Near a rapidly rotating black hole the gravitomagnetic force is very potent and rivals the strength of the huge "static" component of the field that exists even if the black hole were not rotating. The rotating Earth, containing a lot of matter in motion, is the source of a very weak gravitomagnetic force. The upcoming Gravity Probe B satellite will set out to measure the Earth's gravitomagnetic field.

Stuart Shapiro of the University of Illinois asserts that a changing gravitomagnetic force emanating from a rapidly rotating black hole can induce matter currents inside a nearby object, such as a neutron star spiralling in toward the hole. This induced vorticity could have dramatic effects on the properties of the star, such as its spin, internal structure, and orbital motion. These effects might be discernible in gravitational waves reaching the future LIGO and VIRGO detectors. (Illustration by Malcolm Tarlton, AIP.)

This research is reported by Stuart Shapiro in Physical Review Letters, 77, 4487 (18 November 1996).

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