Newly theorized active cloaking could achieve broadband invisibility
Mathematicians at the University of Utah have recently announced they have elaborated an innovative way to shield two-dimensional objects from all types of waves, from electromagnetic to those caused by natural events like earthquakes and tsunamis, leading the way to a completely new approach to achieving invisibility.
Unlike former techniques, most notably those that rely on metamaterials to effectively bend electromagnetic waves in such a way to make objects invisible to a narrow-band signal, the new method developed by the scientists uses destructive interference to actively neutralize (and later rebuild) the waves that are directed at an object.
The idea behind this technique — destructive interference — is incredibly simple. If you throw two rocks in a puddle close enough to each other, you can see the interference between the waves generated by the two objects. Because wave amplitude is additive, interference can be exploited to completely destroy an incoming wave by detecting its characteristics (wavelength, phase, amplitude) and generating a second wave with the same wavelength and amplitude but different phase, so that every peak in the incoming wave will correspond to a trough in the second generated wave, and vice versa, so that the sum of the two amplitudes will be zero at any given point.
In a recent edition of the journal Optics Express, the mathematicians presented their results explaining how, in a two-dimensional environment, three cloaking devices can effectively generate a 'quiet zone' so that objects placed within the region are virtually invisible to incoming waves for a wide range of frequencies, as illustrated in the video at the end of this article.
The main advantage of this approach over metamaterials is that it can act on a much wider bandwidth, shielding objects up to ten times the wavelength involved, which raises hope for cloaking larger objects. Metamaterials are by comparison very narrow-banded, as their behavior depends heavily on the frequency being cloaked. "Maybe you'd be invisible to red light, but people would see you in blue light," Graeme Milton, senior author of the research and a distinguished professor of mathematics at the University of Utah, explained.
The technique, however, also comes with its drawbacks: in order to work correctly, it needs as much information on the incoming wave as it can gather, which might constitute a problem when the signal characteristics change unpredictably, and requires placement of numerous sensors. Moreover, the mathematicians only demonstrated this working principle on a two-dimensional surface, even though they believe it shouldn't be hard to extend active cloaking to the third dimension, making it fit for much more practical applications.
Because visible light has tiny wavelengths — around 380 to 750 nanometers — only microscopic objects could be made invisible by the new method. But even though cloaking from light is still very far from being achieved, this field of research could have more immediate applications in building new types of antennas and for military stealth technology.
The video below illustrates the working principle of active cloaking.