While the usual Doppler effect means that the frequency increases if the observer approaches the source - and decreases as they move away from each other - the theorists have speculated, since 1943, about the possibility that these rules may be interchanged. That would create an inverse Doppler effect.

In 2003, this situation was realized experimentally for the first time. Nigel Seddon and Trevor Bearpark in Bristol, United Kingdom, made a transmission line that contained magnetic induction coils and capacitors. The group and phase velocities of waves in the line pointed in opposite directions, a phenomenon known as anomalous dispersion. In most materials the two velocities are parallel to each other.

The British team then fired an electrical pulse through the transmission line. This had two effects: the pulse produced a moving barrier by creating a non-magnetic region as it moved along the line; it also generated a radio-frequency wave that travelled in the opposite direction to the pulse and at a greater speed.

This radio-frequency wave travelled back to the start of the transmission line, where it was reflected. The reflected pulse then caught up with the original pulse and was reflected from it in turn. However, it was not reflected with a lower frequency, as would be expected from the standard Doppler effect, but with a higher frequency.

Seddon and Bearpark say that these waves can travel at up to one-tenth the speed of light, and that the reflection of such waves from moving boundaries could be used to produce tuneable radiation sources that can be controlled over a large range of frequencies.