AFTER 60 years of trying, physicists have finally shown that the Doppler effect can be made to work in reverse. This may eventually allow electromagnetic radiation to be tuned to any desired region of the spectrum.
The normal Doppler effect is familiar enough: for example, sound waves from a police car’s siren rise in pitch as it approaches and fall as it recedes. That is because as the source moves nearer, each wave has slightly less far to travel than the one before it, so they arrive closer together in time than they left – at a higher frequency, in other words. The same effect occurs…
