The Invisible Man made real
26 May 2006
Cloaks that make objects invisible will be made within 18 months, say UK and US scientists. Changes to sub-wavelength structural details, rather than the chemical composition of these materials, will make objects disappear before our eyes, claims the team.
Sir John Pendry, from Imperial College London, pioneered the field of metamaterials, where structural nuances, not chemistry, control properties. To interact with light and create properties that Pendry says are 'difficult or impossible to find in nature', the only condition placed on the material is that its features must be smaller than the wavelength of the incoming light.
These materials can be made from wires, which can carry a current, and coils, which set up magnetic fields. 'It doesn't matter if the wires are made from copper or silver,' said Pendry, 'you don't have to work with atoms. All you have to do is have a structure in there that is a lot smaller than the light.'
Pendry has shown that electromagnetic fields can be redirected around an object at will by altering the structure of a material. This led his team to design a cloaking material that light can flow round, much like water in a stream flowing round an obstacle: further downstream, the flowing water shows no signs that it was previously diverted. When light waves join up, the object they have gone round can't be seen.
Pendry likened the cloaking device to a controlled version of a mirage on a hot road, where changing temperature alters the refractive index of the air above the road's surface making a strange image of the sky appear. By having control over this process, optical effects can be created that cause invisibility.
But there is disagreement brewing in the metamaterial world. Ulf Leonhardt at the University of St Andrews, UK, claims it is impossible to make cloaking devices that achieve perfect invisibility, 'one can never completely hide from waves,' he said.
Pendry disagrees. With his wavelength-specific devices, light from a single wavelength can make an object entirely invisible. A cloak could make an object invisible to red light, but as the light shifts to green and blue wavelengths, parts of the object would become visible, he said, adding that, with the help of colleagues at Duke University, US, these materials will be made within 18 months.
Pendry says that to go beyond radar cloaking, which uses long wavelengths of around 3cm, a new generation of materials is needed. 'It's now becoming urgent. We have all these things we know we can do when the chemistry is right,' he told Chemistry World. He hopes that new metals or alloys will be put to use in cloaking objects not only for military stealth uses, but also in such things as cloaking an object that gets in the way of an air traffic controller's radar device.
J B Pendry, D Schurig and DR Smith, Sciencexpress, 2006, (DOI: 10.1126/science.1125907)