Tuesday, February 27, 2007

how to make an invisibility cloak

Researchers say they are rapidly closing in on new types of materials that can throw a cloak of invisibility around objects, fulfilling a fantasy that is as old as ancient myths and as young as "Star Trek" and the Harry Potter novels.

Unlike those tales of fictional invisibility, the real-life technologies usually have a catch. Nevertheless, limited forms of invisibility might be available to the military sooner than you think.

"We're very confident that at radar frequencies, these materials can be implemented on a time scale of 18 months or so," John Pendry of Imperial College London told MSNBC.com.

Pendry's research team is one of two groups whose results were posted Thursday on the journal Science's Web site in advance of print publication. The two papers lay out different theoretical methods for creating invisibility, not only for radar but potentially for optical wavelengths as well.

Still more teams are out there with ideas to make things invisible — using methods ranging from superlenses that cancel out the light from nearby objects to actual cloaks onto which video can be projected as a moving camouflage. The most exotic technologies involve "metamaterials," blends of polymers and tiny coils or wires that twist the paths of electromagnetic radiation.

"There are recipes for controlling metamaterials," explained University of Pennsylvania electrical engineer Nader Engheta, who published his own invisibility recipe last year. "Metamaterials are very interesting products."

The latest research papers describe how metamaterial could be fabricated to bend light in carefully curved paths around the object to be hidden, so that an observer would see right through it — or more accurately, right around it — to the other side.

This diagram shows how light rays could theoretically be bent around a concealed object, making it seem as if an observer were looking straight through the object.

"The cloak would act like you've opened up a hole in space," Duke University's David Smith, one of Pendry's co-authors, explained in a news release. "All light or other electromagnetic waves are swept around the area, guided by the metamaterial to emerge on the other side as if they had passed through an empty volume of space."

Pendry told MSNBC.com that the cloak wouldn't reflect any light, and wouldn't cast a shadow either. "It would be like Peter Pan had lost his shadow," he said, referring to the fictional character who had to have his shadow stitched back on.

Dreams come true, with a few catches
Theoretically at least, the metamaterial could work like the helmet of invisibility celebrated in Greek myth, or the cloaking device that hid Romulan and Klingon vessels in the "Star Trek" series, or the invisibility cloak that came in so handy for Harry Potter in J.K. Rowlings' novels.

"Fiction has predicted the course of science for some time. ... Maybe these Harry Potter novels were ahead of their time," Pendry said, half-jokingly.

Of course, there are some scientific catches that the tale-tellers never had to worry about:

For a total invisibility effect, the waves passing closest to the cloaked object would have to be bent in such a way that they would appear to exceed relativity's light speed limit. Fortunately, there's a loophole in Albert Einstein's rules of the road that allows smooth pulses of light to undergo just such a phase shift.
The invisibility effect would work only for a specific range of wavelengths. "There is a price to be paid if you want a thin cloak, in that it operates only over a narrow range of frequencies," Pendry said.
The cloak could be made to cover a volume of any shape, but "you can't flap your cloak," Pendry said. Moving the material around would spoil the effect.
The tiny structures embedded in the metamaterial would have to be smaller than the wavelength of the electromagnetic rays you wanted to bend. That's a tall order for optical invisibility, because the structures would have to be on the scale of nanometers, or billionths of a meter. It's far easier to create radar invisibility, Pendry said: "You're talking millimeters" — that is, thousandths of a meter.
The radar application is of great interest to military outfits such as the Defense Advanced Research Projects Agency, which funded Pendry's team. "Radar is a defense technology, and if you wish to hide from it, this sort of cloak would be a good way of doing it," he said. Such a technology would be "far superior to stealth," he said.

If optical cloaks could be designed, that would be of interest to the military as well. "One obvious thing would be that you could construct a hutch in which you could hide a tank, and the hutch would make it appear as though the tank wasn't there. ... You could also think of weightier things, like submarines or battleships, where you might want to put some of this stuff," Pendry said.

Civilian applications, too
There'd be plenty of applications in the civilian world as well, even for rudimentary cloaking devices. For example, you could create receptacles to shield sensitive medical devices from disruption by MRI scanners, or build cloaks to route cellphone signals around obstacles. "You may wish to put a cloak over the refinery that is blocking your view of the bay," Duke University's David Schurig, another of Pendry's co-authors, was quoted as saying.

While Pendry's team proposed constructing all-over cloaking devices, the other research paper published Thursday describes a simpler method that would involve shaping the metamaterials into cylindrical cloaking devices. The method could also work to block sound waves — like the cone of silence on the "Get Smart" TV show, but not as silly.

The catch here is that the invisibility effect would work only if you were on the same plane as the hidden object. "You could look on top of it, and look inside the cloak," said the paper's author, Ulf Leonhardt of the University of St. Andrews in Scotland.

Leonhardt told MSNBC.com that "potentially a mixture of the two schemes will lead to a practical design." He said the paper from Pendry's team gave him some additional ideas to work with.

"I read it for the first time just last Friday, and I've come up already with something new," he said.