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