Number 295 (Story #2), November 13, 1996 by Phillip F. Schewe and Ben Stein A GRAVITOMAGNETIC FIELD , according to the theory of general relativity, arises from moving matter (matter currents) just as an ordinary magnetic field arises from moving charges (electrical currents). The analogy is so apt that the equations describing this "magnetic" component of gravity can essentially be adapted from Maxwell's equations for electromagnetism by replacing the charge density with the mass density and the charge current with the mass current. The rotating Earth, containing a lot of matter in motion, is the source of such a very weak gravitomagnetic force. Indeed, measuring this field is one of the tasks of the upcoming Gravity Probe B a satellite. Near a rapidly rotating black hole the gravitomagnetic force is much more potent and rivals the static gravitational field in strength. Stuart Shapiro of the University of Illinois (shapiro@astro.physics.uiuc.edu) asserts that in analogy with the Faraday effect, whereby a changing magnetic force can induce electrical currents, a changing gravitomagnetic force emanating from a rapidly rotating black hole can induce matter currents inside a nearby object---say, a neutron star spiraling in toward the hole. This induced vorticity will influence the spin of the star, its internal structure, and its orbital motion. These effects in turn might be discernible in gravitational waves reaching the future LIGO and VIRGO detectors on Earth. (Phys.Rev. Lett., 18 November; figure available on 11/14 at www.aip.org/png.)