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Invisibility and Reverse Light
2006.05.26 (Fri) 11:11
We freely admit it: we totally geek out when science fiction concepts crawl closer and closer to becoming reality. As reported by both Reuters and MSNBC, scientists think they may actually be able to engineer that old standby of traditional fantasy: the invisibility cloak!
Two separate teams of researchers have come up with theories on ways to use experimental "metamaterials" to cloak an object and hide it from visible light, infrared light, microwaves and perhaps even sonar probes.
The concept begins with refraction — a quality of light in which the electromagnetic waves take the quickest, but not necessarily the shortest, route. This accounts for the illusion that a pencil immersed in a glass of water appears broken, for instance.
"Imagine a situation where a medium guides light around a hole in it," Physicist Ulf Leonhardt of Britain's University of St. Andrews, wrote in one of the reports, published in Friday's issue of the journal Science.
The light rays end up behind the object as if they had traveled in a straight line.
"Any object placed in the hole would be hidden from sight. The medium would create the ultimate optical illusion: invisibility,["] Leonhardt wrote.
Currently, "cloaking devices" like those used on stealth craft operate by misdirecting reflected electromagnetic waves — like radar — which renders them "invisible" to certain sensors.
This technology is a far different story. Rather than odd reflections, it relies on refraction, and sends any photons along on their way as if there was nothing stopping them in the first place.
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."
But MSNBC also points out some salient factors regarding the manufacture of invisibility gear:
- 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.
And Reuters mentions that:
Anyone making such a cloak would have to choose what form of radiation one wanted invisibility from, Shurig [sic] said. The invisibility would work both ways — a person hidden from the visible light spectrum would have to use infrared or sonar or microwaves to see out, he said.
Even with these drawbacks, though, the potential inventions and uses are truly, truly keen. Even in mundane spheres, some great ideas have been put forth — for instance, cloaking large structures in radiowave-specific invisibility cloaks to enable cell phone signals to get through to frequently "dropped" areas, or carefully shielding sensitive medical equipment from just the right "bad" electromagnetic waves while still allowing them to function in the wavelengths they require. The MSNBC article has numerous links, including one to the Pendry, et al paper published on the subject.
And, speaking of doing neat tricks with light, this news comes on the tail of an absolutely fantastic breakthrough earlier this month, when Robert Boyd of the University of Rochester managed to make light travel so fast it went backwards:
As if to defy common sense, the backward-moving pulse of light travels faster than light.
[says Robert Boyd,] "Theory predicted that we could send light backwards, but nobody knew if the theory would hold up or even if it could be observed in laboratory conditions."
Boyd recently showed how he can slow down a pulse of light to slower than an airplane, or speed it up faster than its breakneck pace, using exotic techniques and materials. But he's now taken what was once just a mathematical oddity — negative speed — and shown it working in the real world.
"It's weird stuff," says Boyd. "We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end. Through experiments we were able to see that the pulse inside the fiber was actually moving backward, linking the input and output pulses."
Holy...shit. This stuff is so mind-numbingly amazing we're clapping our hands at the sheer genius and absurdity of it. Check out the University of Rochester article for some good analogies, great explanations, and super graphics illustrating the process. Our minds are racing with interesting concepts — both real and fictional — that could develop from this fascinating observation.
Thanks go out to BJS for the Rochester link (via Jake, who had a smart entry in the latest Skeptics' Circle).
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[ Filed under: % Science & Technology ]
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BigHeathenMike, 2006.05.26 (Fri) 21:48 [Link] »
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