Eps 1652: Super Easy Ways To Handle Your Extra Invisibility Cloak

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Tracy Bryant

Tracy Bryant

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Invisibility has been the objective for around 10 years, ever since John Pendry and colleagues at Imperial College in London proposed a method of channeling light above or around objects using substances with exotic optical properties called metamaterials. Invisibility cloaks conceal objects by steering light around them, such that an observer cannot see the object.
Invisibility means light that is being given off by you -- no matter its source -- should not be allowed to escape the cloak, where it could be seen by hunters. All the light falling upon an Invisibility Cloak is ultimately emanating from the same direction on the far side of the Cloak, and what the person standing on the far side of you sees is precisely what he or she would expect to see if you and the Cloak were both out. Furthermore, the light emerging from the cylindrical invisibility cloak experiences a shift of direction, which produces an additional shift of frequency, which causes additional distortion to an observer standing still watching the cloak as it is being zoomed by.
Even slight frequency shifts perceived from cloaks produce distortions, technically called -aberrations, which may hamper cloak invisibility. When the cylindrical cloak with its stealth properties is traveling at a fast rate relative to an observer, relativistic effects will shift the frequencies of light arriving at the cloak, such that light is no longer at the operating frequency of the cloak. While this work shows that higher speeds may have a detrimental effect on the invisibility cloak, scientists have also shown that, for the light of just the right frequency moving in just the right direction, the detrimental relativistic effects would be cancelled out, and the ideal cloaking could still be achieved.
Using this strategy, The scientists showed that there are infinitely many combinations of the frequencies and directions of the light that could cloak with any given relativistic velocity. Another type of cloak, which could bend light, would make objects invisible for only a specific amount of light. So, we could design a stealth cloak simply by drawing how we want waves to swirl around a cloaked object.
To enable incident plane waves propagating through an outer cloak to propagate longitudinally at the same effective speed as plane waves propagating in the absence of a cloak and the object , such that no distortions of the waveform are produced, and real invisibility is possible, we can design our cloak so that its outer layer is made from a fast-light scattering medium, that is, one that supports superluminal group speed of waves across the wideband spectrum -- that is, one that makes coefficient B 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 - 4 1 can be achieved, so that true invisibility is achieved. To allow the incident plane wave propagating through the external cloak travel longitudinally with the same effective velocity as that of a plane wave propagating in the absence of the cloak and the object , so that there is no waveform distortion and true invisibility can be attained, we may design our cloak such that its outer layer is made of a dispersive fast-light medium, i.e., a medium supporting a superluminal wave group velocity over a broadband -- equivalently, making the coefficient B 4,1 zero over broad spectral ranges . The induction of the double-layer, which is the same cloak, results in the total scattering at all angles is completely suppressed, leading to complete suppression, and thus invisibility, by decreasing the cross-sectional spread to become almost null. When light hits the cloak, a smaller volume of it cancels the scattered beams, effectively rendering three-dimensional objects transparent.
Light - or, in our experiments, microwaves - is deflected into a metamaterial cloak, appearing bent or flowing around cloaked objects. The metamaterial cloak is an actual device, forcing light to flow just like it would around a Romulan vessel being cloaked, meaning that this kind of stealthy device is plausible. The invisibility device developed by researchers at the University of Rochester also bends light, but not the same way as magic cloaks or metamaterials.
Most invisibility cloaks exploit the weird properties of metamaterials -- collections of structures put together in such a way as to interact with electromagnetic radiation in a very specific way. Metamaterials, for instance, have unique properties in that they interact with forces on a nanoscale in ways we can only interpret using quantum mechanics. In metamaterials, materials, structures, and properties are coupled in ways that yield unexpected results.
At this scale, we can describe the properties of a metamaterial using traditional structural engineering and materials science. Such materials have produced advances that range from covert devices that make objects almost invisible, to lenses that see details smaller than once thought to be the fundamental limits of optical resolution. Research on rendering objects invisible has made leaps and bounds just in the past few years.
One of these approaches has developed the carpeted coat, which can conceal tiny bits of matter beneath a layer of specialized metamaterial. GET is here not just because Harry Potter used his imaginary cloak so well, but because researchers are using high-tech metamaterials to build structures capable of twisting light around an object in order to make it hidden. Not only that, but trying to conceal that ultra-specific shade of blue can actually make the object you are cloaking more obvious.
Showing does not mean that you could create a stealthy cloak that perfectly redirects light to every hue of, say, blue. For the cloak to operate at the visible part of the electromagnetic spectrum, our eyes will detect only the light that appears to travel in a straight line from space behind the cloaked object, rather than appearing to move around it. The image projected from the background forward would have been obsolete, making it possible to say where a cloak of invisibility was, though admittedly much harder to see than just standing there.
A digital cloak essentially measures space in pixels, and in doing so, collects and radiates light in such a way that it makes anything that is being cloaked seem invisible to the human eye. When cloaking is used as an active disguise in video games, sometimes you will be able to see the person wearing it in this manner, because light is refracted around the overall form of the character or monster. Those who are familiar with Star Trek will recognize this form of invisibility device as a Romulan cape, and the drama it creates is played out like episode Balance of Terror of the original Star Trek TV series.