Humans have long been fascinated by the concept of invisibility. From H.G. Wells' Invisible Man to Harry Potter's cloak of invisibility, purveyors of fiction have pondered what one would do if one could move about unseen. Invisibility is often portrayed as a perfect transparency– ala the Invisible Man– however this method is in conflict with the laws of nature as we understand them. Moreover, a transparent person would be plagued with a host of difficulties that seem quite insurmountable. Any consumed food or drink would be embarrassingly visible as it meanders through the digestive system, and these visible nutrients would immediately begin to integrate into the body. That's to say nothing of wardrobe problems and social difficulties.
The competing approach to invisibility involves some sort of cloaking device to route photons around an object. This method is somewhat more feasible, but of course it comes with its own unique set of complications. For instance, if all the outside light is diverted around something, no light is able to reach an observer inside, leaving them unable to see out.
These difficulties and others have long left all serious speculation about invisibility lodged safely in the distant future. But this is no longer so. In October of 2006, Professor Sir John Pendry of the Imperial College London announced the successful creation of a rudimentary cloaking device which nudges the idea a bit closer to reality. Perhaps most surprising of all, the whole concept rests on a fairly simple physical principle of light– one that requires no electricity to operate, and that every high-schooler learns in basic physics.
In essence, Sir John's invisibility cloak relies on refraction, the same property of light seen when a prism casts a rainbow. Refraction can also be seen by poking a pencil into a glass of water. The underwater portion will appear to be offset from the rest because of the bending of light as it moves from one medium to another– from water to air. A few years ago Sir John and his physicist friends reflected upon the idea of using refraction to bend light completely around an object. If this were possible, the light would emerge on the other side, unchanged, as if the object were not there at all.
Of course this simple idea isn't quite so simple in application. The researchers' first obstacle was the precision light-bending this method requires. There simply weren't any materials with quite the right properties to bend light in the necessary semi-circle, nor were any naturally occurring materials good candidates for the position. So the scientists looked to metamaterials– substances whose electromagnetic properties are dependent upon tightly designed internal structures rather than on their chemical composition.
Guided by a theoretical design published in an earlier paper, and working in concert with researchers at Duke University, Sir John and his team created a five-inch round cloak using a metamaterial structured in two-dimensional concentric rings, specifically designed for this purpose. This unique configuration is thought to be one of the most complex metamaterial structures ever made. Their first goal was to make a material that was "invisible" to microwave radiation since microwaves are a longer wavelength than visible light– millimeters rather than nanometers– and therefore easier to manipulate.
In the laboratory the researchers placed their cloak inside a test chamber, turned on the microwave emitter, and monitored the detector on the other side. Their spiffy new metamaterials worked flawlessly. The inside of the small cloak was completely unaffected by the microwaves aimed at it, while the outside registered readings as if the cloak weren't there at all.
Of course, the need for the right materials isn't the only difficulty in making a true invisibility cloak. If they wish to develop their prototype into something less limited, Sir John's team must address a number of issues. First is the wavelength problem. The cloak as it currently exists can only work for a very narrow range of wavelengths. In the context of visible light, it's as if the cloak was only invisible to red light, while still perfectly visible in blue. Broadening the range means making the cloak significantly thicker, which could severely limit applications. Additionally, creating metamaterials which can do the same for visible wavelengths is considerably more tricky, since the metamaterial's precision structures must be as small as the wavelength they are meant to affect, and visible light waves are on the scale of millionths of a millimeter.
The other weakness in the current design is that the invisibility only works in one plane. The object enclosed by it cannot be seen from the side, but it can be seen clearly from above or below. To remedy this, Sir John and some of his co-authors at Duke are working to push the cloak into the third dimension. Should they succeed with both of these challenges a new difficulty emerges– if the object within the cloak is invisible to the outside world, then no light is able penetrate the cloak, and therefore any occupants would be blind. This is not necessarily a problem if one wishes to hide a stationary object, but it creates difficulty when one wants to conceal people or cameras. Allowing a cloak to work effectively while on the move presents yet another challenge.
Naturally the military has expressed interest in the budding technology, as well as providing funding for its development. Of particular interest is the fact that radar wavelengths are very close to that of microwaves, meaning that a radar-invisible cloak will be possible much sooner than one which is invisible to the eye. This would provide the armed forces with a radar-defeating technology far beyond even that of current stealth aircraft.
Even though all of the wrinkles have yet to be ironed out, the basic breakthrough proves that the concept has merit. An invisibility cloak exists. The Defense Advanced Research Projects Agency (DARPA) hopes that Sir John's cloaking device can help make some of their problems disappear within the next few years, and civilian applications could soon follow. In any case, it is now certain that an invisibility cloak will someday be possible. Whether or not it is practical remains to be seen.