Over the past generation, the U.S. military has spent billions trying to enable its airplanes to elude enemy radars using so-called “stealth” technologies. Scientists are increasingly pushing the envelope with even more sophisticated techniques that could render such weapons—and perhaps even troops themselves—ever closer to invisible.
Harry Potter fans need to chill—these technologies won’t make someone standing in front of you disappear. At least not yet. Rather, they’re designed to foil an expanding roster of electro-magnetic detectors designed to betray one’s presence via radio waves. An expanding family of radars, in other words, and its relatives.
All this leads to mumbo-jumbo and complex mathematical equations that only a relatively few physicists can understand, but the bottom line is easy to see: there may be a way in the future to cloak military forces, as well as other objects, for purposes ranging from secret surveillance, to sneak attacks, to improved wireless connectivity. A trio of new articles in journals of the American Physical Society details the evolving research.
“We believe that the development of the concepts and ideas proposed in this paper may open new and exciting research lines in the field of cloaking and invisibility,” Pai-Yen Chen, Christos Argyropoulos and Andrea Alù write in a paper partly funded by the Pentagon’s Defense Threat Reduction Agency, the Air Force Office of Scientific Research, and the National Science Foundation. They involve an “active” cloak, thinner than older “passive” cloaks, that uses amplifiers to paint the surface of the object to be hidden with electricity. The technology is designed to shield the cloaked object from detection across a far broader slice of the radio spectrum.
“Our work shows a practical venue to make objects transparent to radio waves, which may be of interest to camouflage from radars for defense applications,” Alù says of his paper, Broadening the Bandwidth of Metamaterial Cloaks with Non-Foster Metasurfaces. “Also, civil applications may benefit from our findings. We could make structural components or antennas disappear to neighboring antennas in a complex wireless communication system, improving the links and efficiency.” A companion paper, Do Cloaked Objects Really Scatter Less? by Alù and Francesco Monticone, highlights the downside of passive cloaks, which trade invisibility in one part of the electromagnetic spectrum for increased detectability in others.
Michael Selvanayagam and George V. Eleftheriades of the University of Toronto write, in a third paper partly funded by the Natural Sciences and Engineering Research Council of Canada, that their research shows how an actively-cloaked item can generate “a camouflaging-type behavior” as well as making it “disguised as another object.”
Instead of using the traditional cloaking shell to guide radio waves around the object to be hidden, Selvanayagam and Eleftheriades envelope a cloaked object “with tiny antennas that radiate a field that cancels out all the reflections—more precisely, scattering—by the object,” Eleftheriades says of their paper, Experimental Demonstration of Active Electromagnetic Cloaking. He adds that the process he and Selvanayagam are promoting can do more than merely hide: “If instead of radiating a field to cancel the scattered field, we radiate something else, we can make the object to look as if it has a different shape, size, material composition and even location, to some extent.”
Eleftheriades says true invisibility—the way normal people understand it—may be within reach. “What is needed to make our approach work for visible light, is to surround the object with optical antennas and control what they radiate—color, amplitude and delay—precisely,” he says. Such antennas “are an active area of research globally,” Eleftheriades adds, but “how to control them though is not there yet.”
Well, what did you expect him to say? A search of Pentagon contracting documents apparently dates the most recent invisibility work back to 2010, with a 2012 modification. It’s an Air Force program seeking “Tactical Capabilities for Battlefield Airmen and Security Forces.” One of those capabilities is called “Clandestine Mobility”:
The focus should be on detection avoidance and solutions that reduce signatures to cloak Special Operations Forces’ presence in the area of operations. These technologies must be rugged, lightweight, compact, waterproof, and operable in all light and weather conditions and must be comparable or better in performance (speed, capacity, armor, weapon integration, etc.) than current capabilities.
Funny that there doesn’t appear to be any follow-on work. Perhaps such efforts have been moved into the Defense Department’s so-called “black” budget, where billions of dollars are spent—invisibly—every year. After all, the Pentagon spent years cloaking its stealthy F-117 and B-2 warplanes behind multiple veils of secrecy. It’s a safe bet that as certain elements inside the U.S. military reach the vanishing point, you’ll never see them coming.