Invisibility Cloaks For Muggles


Rarely, if ever, does physics news pique the interest of Pentagon brass, Harry Potter fans, and aspiring Romulans alike.

But a paper in tomorrow's issue of Science might. In it researchers lay out design specs for materials that can bend electromagnetic radiation around space of any size and shape.

The translation for Star Trek fans: Invisibility shields may no longer be science fiction.

The theoretical breakthrough is enabled 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, 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 by Pendry and Duke University physicists David R. Smith and David Schurig shows how—in theory, at least.

While their research did not produce cloaking materials, the team offers mathematical proof that they work and describe their technical requirements.

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, of Duke University, likens the effect to a rock in a stream. The rock represents a metamaterial cloaking shell and the water electromagnetic radiation flowing around it.

"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, even entire buildings could be concealed.

"There 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 their raw materials (copper wire, for example) are relatively cheap, metamaterials are, for now, labor-intensive and therefore expensive to manufacture.

Typical output might fill a coffee cup.

Knights and Wizards

More crucially, researchers have only developed metamaterials that divert radar and microwaves.

While that's good news for Air Force generals who want to conceal warplanes, it's bad news for wannabe wizards hoping for a coat that can turn them invisible.

Metamaterials that control visible light have been 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 Shvetz, a physicist at the University of Texas at Austin, who studies metamaterials of optical frequencies.

Of the study, Shvetz 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 nabbed a knighthood for his earlier work with metamaterials, says the cloaks can 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.

"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 narrow band it has to be."

Stealth capabilities may be useful to explain the technology, 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, the Duke physicist who co-developed the first metamaterial while at the University of California, San Diego, agrees. "This is just the start of what I think amounts to a lot of interesting things to come."

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