Consider the possibilities of an invisibility cloak. You are able to listen and interact with the world without being seen. You could hide your treasures in plain sight. You could hide your mistakes too.
Has Science made that possible? No. Not yet. But work in the blossoming field of metamaterials (man made material that can cover an object and make it invisible) means this is no longer crazy talk.
What does invisibility entail?
The world is visible to our eyes through light. Light comes in a range of flavours, more technically frequencies, which come in bands of energy. At the low end, radio waves and microwaves (with which we cook food and receive txt messages). Above that, infra red (like heat), visible light (like the rainbow) and UV (the black light in night clubs that makes your shirt glow). At the high end, X-ray radiation is used to see through your skin to your bones. This spectrum interacts with the matter around us: the way it interacts tells us about the matter. The color of a metal, for example, is due to how its atoms are arranged.
A good invisibility cloak would cover whole or large parts of the light spectrum. Imagine draping an invisibility cloak over a 100Watt speaker in the middle of a room. The odd person bumping into apparent 'thin air', the sheer confusion as to exactly where the loud noise was coming from, a wonder! Invisibility is most exciting when cloaking objects from the human eye but cloaking for other bands (such as the microwave and inferred; the radar and communication frequencies) is also important, and initially more practically feasible. This practical cloak should also work in 3D meaning that no matter from what direction the cloaked object was observed it would remain invisible.
In 1996 Sir John Pendry outlined the idea of metamaterials. These materials would be manufactured to channel light about an object causing it to be invisible at long ranges. At close range the object would be just detectable similar to a mirage or a clear glass. The meaning of close and short range will depend on the size of the object being cloaked.
How does this work? Think of a stream. Now place a rock in it. This rock provides little obstacle to the water's course: the water flows around it. Wade downstream some way and there is no disturbance from the rock. The water behaves as though it were not there.
Pendry's metamaterials cause the lightwaves to behave like the water described above. Instead of bouncing off the object, the light flows around it, interacting with what is going on behind it, where the shadow would normally be.
The realization of Pendry's ideas initially proved tricky. Near perfect invisibility depends on a lack of irregularities when the light is interacting with it (like a flawless diamond). Metamaterials require an ability to grow or etch structures/features on the nano scale (about 1, 000, 000th of a millimetre). Fortuitously, the advancement in microelectronics has meant that since 1996 huge technological leaps have been made in this area, recently allowing the first practical realizations of metamaterials to happen.
In 2006, D. Schurig and his team were the first to successfully cloak an object: a copper cylinder was successfully rendered invisible to microwave radiation. To most of humanity, the fact that the 'invisibility' was only for one frequency (and couldn't be adapted to others) and for one line-of-sight and for one sort of object, may not have appeared like much of an achievement. But in the scientific community, the first experimental proof of Pendry's ideas was HUGE.
More recently (in April 2009) J Valentine and the Berkley team, made a great stride forward by hiding a small imperfection on the surface of a mirror with an invisibility cloak. Though only applicable to very small (about 30 micrometres, 1000th of a milimetre) objects, the metamaterial used could be draped over an object of any shape. It worked for a range of frequencies (though still all infra red). More significantly, it has the possibility of being adapted to the light frequencies the human eye can see (the holy grail in invisibility research). The cloak is made of tiny holes in a material micrometres apart. These act to guide the light around the cloaked object. When light was reflected off the bump on the mirror without the cloak the outgoing light was changed from the path it would have taken if the mirror were perfectly flat. With the cloak applied the light behaved as though the bump was not there. The visible light region could be accessed by etching smaller holes in the material.
This sort of cloak could be used practically as an ('real life') eraser, correcting manufacturing mistakes on optics or electronics. As the manufacturing of metamaterials improve, further technological applications will become possible.
At the current rate of progress in the manipulation of solid materials, it is hard to predict when Pendry's metamaterials will appear in our day to day lives: it is a big step from bumps a tenth of the thickness of human hair to whole people. But the principle has been laid out.