See Inside November 2010

Going with the Flow: Can the Similarity between Gravitation and Electromagnetism Be Exploited?

Could electrical transformers have a gravitational analogue?

If the sign of a successful scientific theory is that you get more out of it than you put in, then the most successful of all must be Einstein’s general theory of relativity. Starting from a few simple principles and earthy thought experiments, such as what would happen if you got stuck in a falling elevator, general relativity predicts everything we know about gravity and much we never suspected. In the latest example, John Swain of Northeastern University suggests that it might be possible to build a gravitational transformer that transfers kinetic energy just as an electrical transformer transfers electrical energy.

The idea is based on the uncanny resemblance between the equations of general relativity and those of electricity and magnetism. The gravitational attraction that makes apples fall is analogous to an electric field, with mass playing the role of electric charge. And just as the motion of electric charges gives rise to a magnetic field, so the motion of mass gives rise to a “gravitomagnetic” field. Earth’s spin, for instance, tugs on satellites in an effect known as frame dragging.

A steady flow of mass is analogous to DC electric power; an unsteady flow, to AC power. If you juxtapose two wires carrying AC power, the current in one creates an oscillating magnetic field that in turn generates, or “induces,” current in the other wire, thereby transferring power—voilà, a transformer. So why shouldn’t the same work for mass flows? Gravitomagnetism could then convey power from one flow to a nearby one.

Physicist and futurist Robert L. Forward made an offhand remark about this possibility in a 1961 paper, and Swain proposes that the process occurs naturally during, for example, the formation of black holes. Scientists might even pull it off in the lab. “There are lots of situations where one would expect these transformerlike effects,” Swain says.

Any time someone talks about manipulating gravity in the lab, though, it blows some physicists’ fuses. Claims even by the most reputable of researchers have fizzled [see “A Philosopher’s Stone,” by George Musser; News Scan, Scientific American, June 2002]. Swain’s idea strikes relativity experts as vague and probably unobservable. “I notice that Swain never puts in actual numbers to calculate the size of anything,” says Clifford M. Will of Washington University in St. Louis.

The analogy between gravitation and electromagnetism is only approximate, adds Giovanni Modanese of the University of Bolzano in Italy. Theorists have yet to prove that gravitomagnetism can induce currents; if anything, he suspects it can’t. But it is remarkable that a century-old theory still remains a topic of such lively debate.

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