The phenom known as wizard Harry Potter may be coming to the end of its book and movie run, but one special effect from the series could become reality in a future theater of war. "Cloak of invisibility" technology is undergoing R&D by the U.S. Army Research Office (ARO). In his article "Scientists Seek to Hide Combat Forces" in this month's issue of SIGNAL Magazine, Technology Editor George I. Seffers discusses metamaterials-artificial, manmade materials with properties designed to avoid detection. Scientists can use nano-sized metal rods implanted into silicon and arranged to manipulate electromagnetic waves, including light waves in the visible spectrum, and microwave or infrared signals, which are not visible to the human eye. Natural materials like water, glass and air have a refractive index greater than one. Metamaterials have a negative refractive index. Dr. Richard Hammond, a theoretical physicist leading the ARO's invisibility effort, says these negative-index materials have opened up a whole new world of physics research:
We realized that if we can make negative-index materials, we can make light behave in almost any way we want. Now, we can do all kinds of things we couldn't do before.
Through two multidisciplinary university research initiatives, the ARO is working with two scientific teams from Duke University and Purdue University. Duke is studying cloaking materials that work with waveforms outside the visible spectrum, like microwaves or infrared. Purdue is creating an invisibility device for the visible spectrum. In 2006, Duke researchers made a major breakthrough with a small-scale cloaking device prototype that shielded an object within from microwaves. It deflects microwave beams so they flow around the hidden object, similar to water flowing around a rock, making it appear as if the object does not exist at all. Making an object invisible to light is the biggest challenge, as opposed to shielding something from radio waves or sound waves. Loss of light in the process has yielded skeptics, because the material works only with one color of light at a time. This is known as the bandwidth-or the dispersion-problem. A "cloak within a cloak" might work. This would include a different device for every color of light. Hammond notes the possibility of creating one metamaterial that will function as many, simultaneously shielding people or objects from all colors of light:
One of the big problems, at this point, is how to make the actual material. It's not like we can grab a blanket and become invisible, but maybe decades from now we'll have that technology. I think we have a lot of the theory worked out.
Even if scientists do create a cloaking device in decades to come, it likely will have some weakness, according to Hammond:
The thing about being invisible is that no one can see you, but you can't see anybody else, either. No light is reaching you, so you're blind as long as you're invisible.
He suggests having some type of window or cloaking technology that can be turned on and off at will. In addition, high-intensity laser beams will be a likely foil for the cloak, but researchers will tackle that problem after creating a working cloak. Cloaking technology that could enable troops to hide in plain sight is still in the R&D stage. Advances made by the U.S. Army Research Lab and participating universities, however, mean the ability to achieve invisibility is no longer just within the purview of the Stealth fighter of years past. How far into the future will "cloaks of invisibility" become standard-issue gear? Are other organizations studying this capability and working to collaborate? Share your thoughts here.