Invisibility Cloaks Possible, Study Says
by Sean Markey
for National Geographic News
May 25, 2006
 
Rarely, if ever, does physics news pique the interest of Pentagon brass, Harry Potter fans, and aspiring Romulans—those cloaking-device-wielding Star Trek baddies.

But a paper in tomorrow's issue of the journal Science might. In it researchers lay out design specs for materials that they say will be able to bend electromagnetic radiation around space of any size and shape.

The translation for Star Trek fans: Invisibility shields may not be science fiction for much longer.

(Also see "Invisibility Shields Planned by Engineers" [February 2005].)

The theoretical breakthrough is made possible by novel substances called metamaterials.

Invented six years ago, the man-made materials are embedded with networks of exceptionally tiny metal wires and loops.

The structures refract, or bend, different types of electromagnetic radiation—such as radar, microwaves, or visible light—in ways natural substances can't.

"[Metamaterials] have the power to control light in an unprecedented way," said Sir John Pendry, a theoretical physicist at England's Imperial College London.

"They can actually keep it out of a volume of space, but they can do so without you noticing that there's been a local disturbance in the light."

Theoretical Proof

The new study is by Pendry and physicists David R. Smith and David Schurig of Duke University in Durham, North Carolina.

The report explains not only how an invisibility cloak might work but also how to make one … in theory, at least.

While their study did not produce cloaking devices, the team offers mathematical proof that the materials work, as well as technical requirements for their creation.

The underlying idea, Pendry said, is that "you can take either rays of light or an electric field or a magnetic field, and you can move the field lines wherever you want."

"So in the specific instance of cloaking, you take the rays of light, and you just move them out of the area that you don't want them to go in. … Then you return them back to [their] original path."

Schurig likens the effect to a rock in a stream. The rock symbolizes a metamaterial cloaking shell. The water plays the role of electromagnetic radiation flowing around the cloaking shell.

"Downstream you can't necessarily tell that there was an object distorting the flow," he said, adding that, even from the side, the disturbance is hard to discern.

In theory, planes, tanks, cars, and even entire buildings could be concealed.

"There's no limit on what you put inside," Schurig said. "If you build a cloak with a certain hold volume, you can swap things in and out of there, and it doesn't matter what they are."

But there are some catches—money, for starters.

While the raw materials (copper wire, for example) are relatively cheap, metamaterials are, for now, labor intensive and therefore expensive to manufacture.

Currently, a lab's typical output in a single go might fill a coffee cup.

Knights and Wizards

So far researchers have only developed metamaterials that divert radar and microwaves—rather than light waves, which are the key to invisibility.

While that's good news for Air Force generals who want to conceal warplanes, it's bad news for wannabe wizards hoping for a magic cloak.

Metamaterials that control visible light are particularly elusive in large part because the required matrix of metal loops and wires must be "nanosize," or exceptionally small.

That's not to say the stuff can't be manufactured. But so far no one has figured out how, says Gennady Shvets, a physicist at the University of Texas at Austin, who studies metamaterials of optical frequencies.

Of the study, Shvets said, "It was not a result that could be achieved by brute force but required some ingenuity. … I think it's great."

Pendry, the lead study author, points out another limitation. "You can't design a cloak, even in theory, that's perfect at every frequency" of electromagnetic radiation.

But the physicist, who earned a knighthood for his earlier work with metamaterials, says the cloaks should be able to work over a range of frequencies.

"There is, in fact, a trade-off between how thick you let me make the cloak and how much bandwidth I can give you," he said.

An invisibility cloak, for example, would need to be quite thick in order to bend the rainbow of colors, or wavelengths, that make up the spectrum of visible light—a broadband cloak.

"If you let me make a very thick cloak with lots of design flexibility, I can give you a broadband cloak. If you say, 'Well, I want it to be really thin,' then the more narrowband it has to be."

Stealth capabilities may get all the attention, but the researchers say there are many other applications.

"What we have here is a completely new way of controlling light and electric fields," Pendry said.

"We've thought of a few simple things, like cloaking or excluding magnetic fields. But I'd be very surprised if those are the most important things you could do with it."

Smith, one of the Duke physicists, co-developed the first metamaterial while at the University of California, San Diego. He agrees with Pendry's optimistic forecast.

"This is just the start of what I think amounts to a lot of interesting things to come."

SOURCE: National Geographic