By Peter Weiss
The pitch of a blaring car horn rises as the vehicle approaches and falls as it moves away. That’s the Doppler effect, and it also occurs for electromagnetic radiation, enabling police to catch speeders with radar guns and astronomers to determine distances to stars.
Now, physicists in England have demonstrated a topsy-turvy Doppler shift in which a radio wave’s frequency rises as the source recedes. This inverse Doppler effect, first predicted in the 1940s, produces a frequency boost some 100,000 times greater than the drops of ordinary Doppler shifts, the researchers report in the Nov. 28 Science.
The large shift may make inverse Doppler useful for pushing radiation sources to yield frequencies that are now difficult to attain. One such hard-to-reach frequency range is terahertz radiation, which oscillates at trillions of cycles per second. It shows promise for medical imaging, security scanning, and many other applications (SN: 8/26/95, p. 136).
In the newly reported experiment, Nigel Seddon and Trevor Bearpark of BAE Systems, an aerospace and defense firm in Bristol, generated the Doppler reversal by following a strategy devised recently by Russian theorists.
At the heart of the scheme is a half-meter-long assembly of capacitors and electrical inductors into which the BAE team drives a brief, potent pulse of some 100 amperes of current. Whizzing along this transmission pathway at up to a tenth the speed of light, the powerful pulse creates all the needed inversion conditions, says Seddon.
To start with, the pulse alters the properties of each section of the pathway so that it exhibits what’s known as anomalous dispersion. Normally, the directions of waves and their energy match. However, in the altered sections of the new transmission pathway, the energy carried by electrical waves travels in the opposite direction of the waves themselves. Electromagnetic radiation can reflect from the sharp, moving boundary between anomalous regions and normal ones.
Besides creating anomalous dispersion, the pulse, like a boat churning up a wave with its bow, sheds an even-faster-moving radio wave that heads off in the direction opposite to that of the pulse. When that bow wave hits the start of the transmission line, it bounces back, catches up to the mobile boundary, and gets reflected again there.
That’s when the inverse Doppler effect kicks in. Reflecting off a receding boundary, an ordinary electromagnetic wave would Doppler-shift downward by a minute fraction of its frequency. However, with anomalous dispersion, the bow wave’s frequency jumps a whopping 20 percent of the original frequency.
That’s because of the boundary’s tremendous speed, Seddon says.
“Within the context of everyday experience, [this inverted effect] is certainly very unnatural,” comments theorist Evan J. Reed of the Massachusetts Institute of Technology. However, inverted Doppler may soon be seen beyond electrical systems. Last September, Reed and his colleagues unveiled a theoretical analysis showing that the reversal should be possible in light-manipulating materials, including photonic crystals (SN: 5/3/03, p. 276: Available to subscribers at Crystal Bash: Shocking changes to light’s properties).
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