In the "Star Trek" universe, cloaking devices on Romulan and Klingon spaceships create all sorts of tactical nightmares for their human foes. Hiding in plain sight is certainly a handy trick for a person, too, as fans of "The Invisible Man" and the "Harry Potter" series know well.
Science has given us glimpses, as it were, of how these anti-detection technologies might be possible. But full-fledged invisibility cloaks like those of science fiction and fantasy remain quite a ways off.
"I won't call it impossible, but it's implausible what you see in 'Harry Potter,'" David Smith, professor of electrical and computer engineering at Duke University, said. "That's perfect movie invisibility — too perfect."
Nevertheless, research into rendering objects invisible has made leaps and bounds just in the last several years. Partial cloaks that work like sophisticated camouflage — much like the shimmering distortion of the Predator alien in the 1987 movie of the same name — might be more realistically achievable, Smith said.
The rise of cloaking tech
Smith leads the group at Duke that demonstrated the deflection of microwaves around a two-dimensional cylinder back in 2006, making it appear almost as though the cylinder were not actually there. The effect is similar to water flowing around a rock and resuming its course, as if the former obstacle was not there at all.
This breakthrough result and many of the others since made use of so-called metamaterials. These custom-crafted metals, plastics or other materials have structures — and therefore properties — unlike those found in nature. The metamaterials' design allows them to manipulate electromagnetic waves in prescribed ways.
"Artificial materials, where you have so much more flexibility, have allowed this theoretical idea [of cloaking] to really move forward," Smith said.
Traditional attempts at cloaking have focused on reducing electromagnetic emissions and light reflection from an object. The B-2 stealth bomber, for instance, has engines in its wings to cut down on heat exhaust, plus the aircraft is coated with radio wave-absorbing paint to thwart radar. But objects still cast shadows this way, which can spoil an attempt at truly turning ghost.
Bending light to our will
Recent work with metamaterials has extended their deceptive capabilities to wavelengths in the optical range. One such approach devised a "carpet cloak" that could hide a tiny lump of material under a special metamaterial layer. Another approach relied on natural, chunky calcite crystals to manipulate visible light with a certain vibration direction, or polarization.
In short, progress continues to be made. The metamaterial and calcite techniques could see real-world applications soon, with the former preventing antennas from interfering with each other, for example.
Yet what's been accomplished thus far has been limited in terms of wavelength range and types of light that have been duped into doing our bidding.
In microwave and visible bands, state-of-the-art cloaking device-like technologies have been able to address only "a tiny portion, at best, of any one of those bands, let alone the whole thing," Smith said.
To cloak a dynamic object moving through free space, like the dreadlocked Predator creature romping through the jungle, metamaterials have a long way to go. "Give us one hundred years with other technologies that come in, and we might have something a lot closer than you'd think," Smith said.
In the meantime, would-be invisible men might pin their hopes on an entirely different technological method to cloaking, one that fools the eye by emitting light, rather than detouring it around a hidden object.
The concept involves covering the object or person in micro-cameras and tiny screens, all hooked up to fancy software and an energy source.
"The micro-cameras take images of what is behind the person. Then the screens project that image in front so it seems like you're looking through the person," explained Sidney Perkowitz, a physicist at Emory University in Atlanta, Ga. "You would need perhaps hundreds of thousands of devices, but if they were linked wirelessly and had fast enough computing, you could have the illusion be fast enough for a real-time effect."
The idea is similar to how cuttlefish change the complexion of their skin with astonishing speed and accuracy to mimic the hues and patterns of coral and rock they nestle against. Crude versions of this sort of "active" or "adaptive" camouflage are in the works by the U.S. military and other armed forces.
What of cloaking devices for future spaceships? "If you cloak something the size of a human, I don’t think it will be too much [more difficult] to cloak a battleship," Smith said.
A problem that will limit all cloaking technologies, however, is the speed of light. With a flawless cloak, diverted light waves arrive at a potential observer without any delay. But re-routing light waves from and then back to a normal, straight-line path means the light has to cover more distance, which of course takes more time, Smith said.
The delay means some diverted light arrives late; the delay is slight, but probably still enough to mar the cloaking effect." To some extent, these things are always going to be detectable," Smith said.
"Star Trek" writers got around this quandary pseudo-scientifically by proposing that a cloaking device warps space itself, thus delivering diverted light to an observer right on time. Not an easy trick to pull off, though, Smith said. "It would take a ridiculous amount of energy to make something like that feasible."
Still, even in the relative near-term, cloaking tech should make the move from the lab into the field. "I think invisibility cloaks have largely moved from a physics problem that has been solved," said Perkowitz, "to an engineering problem that may be solved."
Plausibility score: A total disappearance-style cloaking device as depicted in sci-fi and fantasy is out of bounds. But extremely sophisticated camouflage that lets you melt right into the background looks like a go. On balance, then, we give the cloaking concept three out of four Rocketboys.
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