TECHNICOLOUR DREAM CLOAK

How to stage the ultimate disappearing act The most cunning paint job may be no use as camouflage. It won’t do much to hide you from the prying eye of a thermal camera, or silence an inadvertent sneeze. Even something as subtle as a shadow can give your presence away. To win in the ultimate game of hide-and-seek, what you need is an invisibility cloak that goes way beyond simply not being visible. That might seem a tall order, yet researchers are racing to perfect the technology. So could their devices make you disappear completely? This is where the future of concealment begins to look hazy. First, consider what it would take to make you vanish. If you could wear something that bends light rays from the wall behind you smoothly  around your body – like water flowing around a rock – then steer them towards someone looking in your direction, they would see nothing but the wall: no outline, no shadow. This trick is not too difficult to accomplish. Last year a team led by Hongsheng Chen from Zhejiang University in Hangzhou, China, used special optics to diffract light around a cat, making it vanish from view. Unfortunately, the effect only works at certain viewing angles. And the system isn’t portable – it uses a large and very heavy glass prism. Less cloak, more bunker. Rather more practical would  be clothes made of synthetic metamaterials – assemblies of  many small components that work together to warp light in unusual ways. The first metamaterial invisibility cloak was unveiled in 2006. Constructed from an array of concentric metal rings, it successfully hid a small metal object – but only from a beam of microwave radiation. Now, researchers are devising cloaks that work with visible light. Last year a team from Stanford University in California designed a metamaterial made from an array of tiny metal rings shaped like crescent moons. Their calculations suggest this high-tech “chain mail” could bend parts of the visible spectrum, hiding you completely in blue or green light, say. Don it in daylight and you would fade to a sort of ghostly, reddish hue. A material that bends light at all visible wavelengths is required if this monochrome spectre isn’t to betray you. But we have yet to find a An invisible man needs to be more than just unseen metamaterial that curves all wavelengths the same amount. In theory, there’s no reason why it couldn’t work across the visual spectrum, says Tie Jun Cui, a metamaterials expert at Southeast University in Nanjing, China, but most researchers believe it can’t be done. Certainly you will have more luck stifling the noise of a cough or sneeze. Sound waves zip faster through a light, stiff material than through air and, according to Steven Cummer, an engineer at Duke University in Durham, North Carolina, that makes an acoustic cloak entirely feasible. Constructed from stacks of thin materials with the right compressibility and density, this wrap will effectively soundproof your presence. A similar idea will help keep your thermal signature to yourself. Sebastien Guenneau and colleagues at the Fresnel Institute in Marseille, France, have used a two-dimensional metamaterial shield, made from a metal sheet patterned with rings, to divert heat around an object. If such a cloak can match the temperature of its outside edge to that of the surroundings, it should help keep even the hottest body out of sight. Lest you imagine that complete invisibility is within our grasp, think again. For now, there’s no way to combine technologies to simultaneously block visible light, heat and sound. The problem is that the materials to manipulate each of these waves work in different ways, and if you swathed yourself in a thick coat of cloaks stacked one on another, only the outer layer would perform as planned. For now, it might be better to stick with an old-fashioned cloak of paint.  ■