New Material Creates Inverse Doppler Effect

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The Doppler Effect is the reason why car horns drop in tone as they go by, and radar speedometers work. But a new material may actually create an inverse Doppler Effect.

The Doppler Effect is trotted out early in physics courses because it is a concept that can be demonstrated every day. When an object makes waves, whether they are sound or light, they have a characteristic wavelength. The crests are a certain distance apart. When the object moves toward an observer, it keeps emitting waves at the same frequency, but those waves are no longer uniform. The crests of the waves that are emitted in the direction that the object is moving get crushed a little closer together as the object ‘catches up' to the wave it just emitted. Meanwhile, the waves on the far side of the object spread apart when the object leaves these just-emitted waves behind.

In sound, the shorter wavelengths mean higher-pitched sounds. This is why car horns are slightly high pitched as they move toward a person, and drop to lower pitches as they pass the person by. The same thing happens when any object emits waves. The Doppler Effect can be used to show the speed of something moving through water, as the waves of water crowd close or are left behind. It even works with light; a shift between wavelengths shows an object either advancing on or retreating from the observer. As light comes toward a person, it shifts to shorter, bluer wavelengths. As it retreats, it shifts to look more red.


But not if it's traveling through a material of close-set silicon rods. This material has negative permittivity and negative permeability. Permittivity is a measure of how a material refracts electromagnetic waves. For example, a glass of water will refract waves so that a spoon place half-in, half-out of the glass will appear ‘broken'. The waves of light from underneath the water are bent when they hit the air, making them appear to come from a different place than the waves of light that reach the eye from above the surface of the water. Permeability is the measure of how a material allows the formation of a magnetic field. A good conductor will have a high permeability.

In the late sixties, it was thought that a material could be formed that would have both a negative permeability and a negative permittivity. This material would cause negative refraction – in other words, it would bend light in precisely the opposite direction that it moves in conventional materials. Since light emitted by a moving object would move through its surrounding material in precisely the opposite way it usually did – it would cause an inverse Doppler Effect. Testing has shown that this is the case. An infrared laser red shifted when moving towards an observer and blueshifted when moving away. The Doppler Effect has finally toppled. Looks like it's impossible to trust anything anymore.

Via Physics World and Nature.