The Inverse Doppler Effect

[Tuckerfan]

Now all we need is a deflector dish we can reconfigure.

What if the speed of light is a constant only most of the time? What if gravity sometimes pushed instead of pulled? Scientists are increasingly asking what would seem like far-out questions regarding the laws and rules of physics after discovering conditions and materials where the rules don’t quite apply. Take the Doppler effect.

The Doppler effect is in use everywhere, everyday. Police use it to catch speeders. Satellites use it to track space debris and air-traffic controllers use it to monitor aircraft. The Doppler effect explains why the pitch changes from high to low when a police siren passes you on the street. As the siren moves toward you, it is catching up to and compressing the sound waves it produces, thus the higher pitch. When it passes, the sound expands to fill the increasing space between you and the noise. The sound waves are longer and the pitch is lower.

The inverse Doppler effect is not something you can hear, but understanding it could one day lead to important advances in optics and communications equipment.

Predicted in the 1940s, the inverse Doppler effect was first observed in 2003 by British researchers Nigel Seddon and Trevor Bearpark using an experimental magnetic, nonlinear transmission line sketched out by Avenir Belyantsev and Alexander Kozyrev in 2000. This nonlinear transmission line is a synthetic structure that allows electromagnetic waves to propagate along it in a new fashion. In the experiment, a pulse of current fed into the line acts as the moving “siren” or shockwave. It generates a radio frequency (RF) signal but as the pulse recedes, the spacing between the peaks and troughs in the waves tighten rather than loosen: the inverse of the Doppler effect. That’s just the opposite of what happens with sound waves when a siren passes you.

Reporting in the May 20 issue of the journal Physics Review Letters, University of Wisconsin-Madison Research Associate Alexander Kozyrev and Electrical and Computer Engineering Professor Dan van der Weide prove how an RF signal moving through this special transmission line can reverse itself and fall in sync with the shockwave in order to realize the inverse Doppler effect. They demonstrated that the shift arises from a complex and remarkable spatial structure of waves propagating along the line.