DARPA's ambitious project is to create armor for soldiers that will render them -- and their shadows -- invisible.

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Metamaterials

Researchers are currently exploring the possibilities associated with an artificial type of matter called metamaterials.

Naturally occurring matter exhibits behavior based on the molecules that make it up -- the atomic material that composes the finished product determines what properties the product will have. For instance, take the relationship between wood and light. Wood, like all natural matter, reflects and refracts light. But just how much light it reflects and refracts depends on how the electromagnetic waves of the light interact with the particles -- like electrons -- that make up the wood.

With metamaterials, the sum of the parts, not the parts themselves, determines how the material behaves. Researchers have found that by using certain materials -- like gold and copper arranged in certain patterns and shapes -- they can combine the properties of those materials. In other words, unlike natural matter, metamaterials' behavior depends on the properties of the materials that make it up and the way the materials are put together.

So how do metamaterials make something appear invisible? To simplify it, Duke University's David R. Smith suggests this: Imagine a fabric woven of thread. In this fabric, light is only allowed to flow over the threads (meaning it can't travel into the nooks and crannies between the threads). If you punch a hole in the fabric with a pin, light will go around the hole and resume its original course of travel, since light can only travel over the thread. So to the light waves, the hole doesn't exist. If you put an object in the hole, the light waves would go around the object too, effectively rendering the object invisible [source: Technology Review].

This is what metamaterials theoretically can do: They guide light around an object, rather than reflect or refract the light. So to the light waves -- and the human eye that perceives them -- the object might as well not even be there. If the light waves can be guided by the metamaterials around the object and back to its original course, the object wouldn't cast a shadow, either. This is another goal of using metamaterials to create cloaking devices.

Smith is one of several researchers using metamaterials to manipulate microwaves -- the electromagnetic waves used in radar. To effectively manipulate an electromagnetic wavelength of any kind, the metamaterial used has to be smaller than the wavelength. Since microwaves' wavelengths are measured in centimeters, scientists have the technology to create metamaterials small enough to manipulate them, moving waves around an object. A stealth bomber sheathed in the right layer of metamaterials, for example, could be invisible to radar. The shield would be visible, but radar wouldn't be able to detect the plane.

Making the entire aircraft invisible to the naked eye is a bigger challenge. For one thing, we don't currently have the technology to manufacture materials on the small scale required to manipulate light waves. Light wavelengths are measured in nanometers (billionths of a meter), and the metamaterials needed to block light must be even smaller than that. Another challenge is that a metamaterial cloaking device would have to be arranged to manipulate light on the entire visible spectrum, because different colors exist on different wavelengths. And lastly, a cloaking device would plunge a person on the inside into darkness, as the light that would normally reach him or her would be diverted around the cloaking device.

If research and funding for metamaterials continues at its current pace, these challenges could soon be overcome. But there are other challenges that must be solved before the technology is practical. One demand of the DARPA project is that it be asymmetrical. This means that the wearer on the inside should be able to see out, but he or she would be invisible to anyone outside the suit. Once these problems are worked out, the army of the future may be very hard to spot.

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