Invisible Man Almost Visible

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Two prescriptions for an invisibility cloak have been unveiled by physicists in the United Kingdom and the United States. The researchers say that in principle the technologies needed for building these devices already exist. "Invisibility is visibly close," says Ulf Leonhardt of the University of St Andrews in Scotland, one of the researchers behind the proposals.

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He and John Pendry of Imperial College London, UK, and their co-workers have independently described similar ways to create an invisible 'hole' in space, inside which objects can be hidden. They say it is possible to guide light around the hole, rather like water flowing around a rock in a river, so that the object inside it cannot be seen.

Light rays are bent when they pass between materials with different refractive indices, such as air and water. But bending light so that it passes round a region of space and emerges travelling along the same line as it was initially is a difficult trick, requiring an invisibility cloak made from materials with a 'tunable' refractive index.

Such substances have been made, in the form of so-called metamaterials. These are built from rings or coils of metal wire, etched into printed circuit boards and glued together, which act as antennae that interact with the electromagnetic field of incoming light and modify the paths the light takes. Such metamaterials can have bizarre optical properties: for example, having negative refractive index, so that they bend light the 'wrong' way.

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