'Sweeping it under the carpet' takes on new meaning.
An invisibility carpet would figure pretty low on Harry Potter's Christmas list, but it may prove to be the most realistic kind of cloaking device, according to new calculations.
Jensen Li and John Pendry of Imperial College London have come up with a theory for how to create a carpet that would cause anything swept under it to seem to disappear. The carpet could be made of regular silica and silicon, and would work across the spectrum of visible light — something no other cloaking scheme has yet been able to achieve. The authors' work appears on the ArXiv preprint server1.
Invisibility cloaks, shields and skins remain a dream for physicists and engineers, not to mention military organizations. Most modern schemes are based on 'metamaterials' - materials possessing carefully crafted internal structures that can alter the path of light.
The idea, says Pendry, seems complex but is actually fairly simple: the material can bend light by different amounts at different points, much as a heated column of air can cause the horizon to shimmer. "It's basically just an attempt to make a controlled mirage," he says.
To date, however, metamaterials have worked only at wavelengths longer than that of visible light, or at very specific colours or wavelengths. Li and Pendry's new scheme does better because it attempts to make an object look like a flat surface rather than making it disappear altogether.
That makes the requirements of the metamaterials involved less demanding, says Vladimir Shalaev, a researcher at Purdue Unversity in West Lafayette, Indiana. "This 'invisibility carpet' can be fabricated," he says.
So what would it look like? Well, Pendry says, it would actually look highly reflective, much like a mirror. That would make the carpet - not to mention anyone hiding under it - pretty conspicuous unless it was laid down on a mirrored surface. Even then, he says, "You wouldn't want to use it to hide people or anything really big".
Nevertheless, the carpet would be an "important step toward making the dream of invisibility true", says Shalaev. And, as impractical as it may seem, it would probably lead to some useful technologies, Pendry adds. Metamaterials could one day be used in a range of applications, from radar-invisible skins for ships and planes to optical computing. Building such devices will probably be based on knowledge gained from invisibility carpets and elsewhere, Pendry says.
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Brumfiel, G. How to weave an invisible rug. Nature (2008). https://doi.org/10.1038/news.2008.928