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Doppler Done Backward


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You’re daydreaming at a railroad crossing while a train whistles past. As the engine passes, you expect to hear the pitch drop as the Doppler effect dictates. Instead, it rises. Your eyes widen. What gives? Ask Nigel Seddon and Trevor Bearpark, physicists with the R&D arm of BAE Systems, a defense contracting firm in Bristol, U.K. In the 28 November issue of Science, they describe an electronic setup that generated an inverse Doppler effect, something theorists have pondered since 1943.

The standard Doppler effect occurs when sound or other waves come from or bounce off a moving object. If that object is approaching, the waves are essentially pushed closer together, so their frequency rises. If the object is receding, the waves spread out, and the frequency drops.

To make the opposite happen, Seddon and Bearpark took advantage of a property called anomalous dispersion. In familiar materials, such as copper wire, the overall energy in the wave and the phase (the relative position of a single point) move in the same direction. But in materials with anomalous dispersion, the energy moves forward but the phase moves in reverse (see figure). To create a transmission line with anomalous dispersion, Seddon and Bearpark linked a series of electronic devices called capacitors and inductors. When they then sent a large pulse of current into their line, the moving pulse altered the magnetic structure of the line, creating a nonmagnetic region in its wake. That produced a moving barrier between the magnetic and nonmagnetic regions.

At the same time the pulse acted like a shock wave that generated a radio frequency (RF) wave that traveled backward toward the start of the transmission line, reflected off the front of the apparatus, and then moved back toward the original pulse. Traveling faster than the original electromagnetic pulse, the RF wave caught up with the receding barrier and bounced off it. Because it was moving through a material with anomalous dispersion, the phase of the RF wave traveled in the opposite direction from its energy. As a result, when the wave hit the receding barrier, it reflected not with a lower frequency as usual but with a higher frequency. “We were absolutely flabbergasted” that the effect occurred as was predicted, Seddon says of the day the readings came across the oscilloscope. “For a good hour we just played with the experiment.”

“It’s great work,” says Nader Engheta, a professor of electrical and systems engineering at the University of Pennsylvania in Philadelphia. Seddon and Bearpark suggest that the effect could spawn the development of cheap, compact devices that turn out a broad range of gigahertz-frequency electromagnetic pulses and could be used for nondestructive testing of materials, among other uses.