Imagine the things you would do if you had Harry Potter’s invisibility cloak. Everyone has had this fantasy, but now it seems that this staple of science fiction from Star Trek to Dr Who may be close to science fact, although it requires a little imagination, and a little faith in some extraordinary mathematics.
Within the last few months, a number of theories for developing cloaking devices have been unveiled. Two recent reports in the magazine Science have described how experimental "metamaterials" can change the way light bends around an object, to create an illusion that we might call a mirage.
Metamaterials are composite materials that are designed to have interesting properties, such as the ability to bend light. They contain microscopic rods or metallic rings that can be tinkered with to interact with light in controllable ways, such as to manipulate how quickly light travels when near particular parts of the material. However, despite our espionage fantasies, any invisibility cloaks made out of the material in the near future would be extremely heavy and thick, and you would not be able to see out of them.
Physicist Ulf Leonhardt, of the University of St Andrews and an author of one of the reports in Science, wrote "Imagine a situation where a medium guides light around a hole in it. The light rays end up behind the object as if they had travelled in a straight line. Any object placed in the hole would be hidden from sight. The medium would create the ultimate optical illusion: invisibility." This is like what happens to water when it runs around the outside of a smooth rock in a river, and occurs in our case here because of refraction - a characteristic of light where it takes the quickest, but not necessarily the shortest, path. We can see refraction by simply dropping a pen in a glass of water and observing that it looks like its bent, when we know its not.
Sir John Pendry of Imperial College London, author of the second report, also predicted that with sufficient funding, the first of these devices could be around within 5 years.
These devices could also be used to hide objects from other electromagnetic waves and even sound. This has obvious Defence applications. David Schurig of Duke University in North Carolina and Pendry’s co-author stated that this Defence goal "would be to conceal an object from discovery by agents using probing or environmental radiation." This is a different method of stealth than modern methods used to hide planes from radars. Current stealth technology revolves around reducing a plane’s "Radar Cross Section". Radars work by sending out electromagnetic radiation, and then detecting when it reflects back off its target. To reduce the amount of radiation that is reflected back by the plane, we can design the plane’s shape such that reflections do not go back in the direction they came, we can make it out of a material that is non-metallic and so less reflective, and we can paint it with paint that absorbs the radiation. But these methods never make a plane entirely invisible to radar. With this new technology, the hope is that the radiation does not even hit the plane in the first place.
Along with the problem of not being able to see out from the inside of such a material, is the fact that the more types of radiation against which that we make the material work – for instance, if the material cloaks against visible light and microwave radar – the more expensive and difficult the material is to produce.
Another recent study comes from Professor Graeme Milton, of the University of Utah, and Dr Nicolae-Alexandru Nicorovici, of the University of Technology, Sydney. They studied materials with bizarre optical properties first postulated in 1968 by Victor Veselago, a Russian physicist, to show that light could cancel itself out in some scenarios and make an object look invisible. This work remained a strange mathematical fantasy until six years ago with the creation of superlenses that can make objects, when placed near them, invisible. When an object is bathed in light of one colour, the light becomes trapped near the lens and "almost exactly cancels the light incident on each molecule in the object, so it has essentially no response to the incident light. Numerically we see that the molecule is effectively invisible."
This is a mathematical solution. The real test for any of these invisibility solutions will be when someone finally makes one and experiments with it. Until then, the best example of invisibility is that of Professor Susumu Tachi of Tokyo University, who made a suit with a video camera out the back, who's images were projected on the front of the suit, so it seems as though you were looking "through" the wearer. This didn’t quite work perfectly however, as you need to be looking from the right angle for it to be effective. So until our mathematical fantasies come true, we can only fantasise about a future where the Invisible Man is a possibility.
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