Metamaterials

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.

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.

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. 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.