Cloak of invisibility

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Alberich puts on the Tarnhelm and vanishes; illustration by Arthur Rackham to Richard Wagner's Das Rheingold

A cloak of invisibility is a theme that has occurred in fiction, and is a device which is under some scientific inquiry.

Cloaks of invisibility in fiction

Cloaks of invisibility are relatively rare in folklore; although they do occur in some fairy tales, such as The Twelve Dancing Princesses, a commoner trope is the cap of invisibility.^[1] The cap of invisibility has appeared in Greek myth: Hades was ascribed possession of a cap or helmet that made the wearer invisible.^[2] In some versions of the Perseus myth, Perseus borrows this cap from the goddess Athena and uses it to sneak up on the sleeping Medusa when he kills her. A similar helmet, the Tarnhelm, is found in Norse mythology. In the Second Branch of the Mabinogi, one of the important texts of Welsh mythology, Caswallawn (the historical Cassivellaunus) murders Caradog ap Bran and other chieftains left in charge of Britain while wearing a cloak of invisibility.^[3]

Edgar Rice Burroughs uses the idea of an invisibility cloak in his 1931 novel A Fighting Man of Mars. A scene in the movie, Erik the Viking rather humorously depicts the title character using a borrowed cloak of invisibility, which he does not realize works only on the foolish father of the princess to whom it belongs. His foes are so baffled by his bizarre behavior and false proclamations of his invisibility that they are too stunned to actually fight him, thus allowing him to easily defeat them.

Camouflaging cloaks form a central plot element in Samuel Delany's 1975 novel Dhalgren. A Cloak of Invisibility is presented as a key plot element in the Harry Potter series of novels by J.K. Rowling. Harry uses the cloak to sneak around. More recently, a cloaking device has been used in videogames, for example Battlefield Heroes and Team Fortress 2, where they aid stealth based characters. The cloaking devices appearing in Star Wars, Star Trek and Stargate, presents a similar notion in a science fiction form, but is generally used to hide larger scale objects, such as space ships. In science fiction cloaking, there is generally presented an assumed quasi-scientific, in-universe basis for the concept of achieving invisibility. Conversely, invisibility and cloaking is commonly presented in the science fantasy genre as a magical phenomenon, rather than in forms that rely on pure science.^[4]

Cloaks of invisibility in science

The expanded use of retroreflective material can be plainly seen in effect "invisibility". http://news.bbc.co.uk/2/hi/programmes/click_online/4097315.stm As reported by BBC World News as well as many others.http://news.bbc.co.uk/2/hi/7553061.stm The technology has emerged well beyond these reports dating back to 1994 and earlier. There are several other applications that are now in use that are available for public example and purchase.

Scientists in the US say they are a step closer to developing materials that could render people invisible.

Researchers at the University of California in Berkeley have developed a material that can bend light around 3D objects making them "disappear".

The materials do not occur naturally but have been created on a nano scale, measured in billionths of a metre.

The team says the principles could one day be scaled up to make invisibility cloaks large enough to hide people.

Stealth operations

The findings, by scientists led by Xiang Zhang, were published in the journals Nature and Science.

The light-bending effect relies on reversing refraction, the effect that makes a straw placed in water appear bent.

Previous efforts have shown this negative refraction effect using microwaves—a wavelength far longer than humans can see. In order to have the 'Harry Potter' effect, you just need to find the right materials for the visible wavelengths

Ortwin Hess

The new materials instead work at wavelengths around those used in the telecommunications industry—much nearer to the visible part of the spectrum.

Two different teams led by Zhang made objects made of so-called metamaterials—artificial structures with features smaller than the wavelength of light that give the materials their unusual properties.

One approach used nanometre-scale stacks of silver and magnesium fluoride in a "fishnet" structure, while another made use of nanowires made of silver.

Light is neither absorbed nor reflected by the objects, passing "like water flowing around a rock," according to the researchers. As a result, only the light from behind the objects can be seen.

Cloak and shadow

The fine structure of the material gives it light-bending abilities "This is a huge step forward, a tremendous achievement," says Professor Ortwin Hess of the Advanced Technology Institute at the University of Surrey.

"It's a careful choice of the right materials and the right structuring to get this effect for the first time at these wavelengths."

There could be more immediate applications for the devices in telecommunications, Prof Hess says.

What's more, they could be used to make better microscopes, allowing images of far smaller objects than conventional microscopes can see.

And a genuine cloaking effect isn't far around the corner.

"In order to have the 'Harry Potter' effect, you just need to find the right materials for the visible wavelengths," says Prof Hess, "and it's absolutely thrilling to see we're on the right track."

• On October 19, 2006, a joint effort between scientists from the United Kingdom and the United States produced a cloak that routed microwaves of a particular frequency around a copper cylinder in a way that made them emerge almost as if there were nothing there. The cloak was made from metamaterials. It cast a small shadow, which the designers hope to fix.

The device works only in two dimensions and only at a particular microwave frequency. Work on achieving similar results with visible light is in progress.^[5][6]

David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke University which demonstrated the first working "invisibility cloak" makes reference to a fundamental problem with these devices as "invisibility cloaks":^[7]

In 2006, David Miller at Stanford University proved ^[8] that there can be no invisible paint that will work against changing backgrounds. Equivalently, any attempt at optical cloaking using a material (including metamaterials) will fail if it is tested with a pulse. The reason is that the information required to reconstruct the pulse just on the far side of the object cannot get there fast enough if it has to go round the outside of the object at a speed no faster than the velocity of light.

Despite this, an American group of scientists say that a cloak could be very similar to the invisibility cloak in Harry Potter. The design calls for tiny metal needles to be fitted into a hairbrush-shaped cone at angles and lengths that would force light to pass around the cloak. This would make everything inside the cone appear to vanish because the light would no longer reflect off it. "It looks pretty much like fiction, I do realize, but it's completely in agreement with the laws of physics," said lead researcher Vladimir Shalaev, a professor of electrical and computer engineering at Purdue. "Ideally, if we make it real it would work exactly like Harry Potter's invisibility cloak," he said. "It's not going to be heavy because there's going to be very little metal in it."

• On April 30, 2009, two teams of scientists developed a cloak that rendered objects invisible to near-infrared light. Unlike its predecessors, this technology did not utilize metals, which improves cloaking since metals cause some light to be lost. Researchers mentioned that since the approach can be scaled down further in size, it was a major step towards a cloak that would work for visible light.^[9]

Problems of refraction and opacity

The headlined claims that laboratory results with metamaterials are demonstrations of prototype invisibility cloaks are highly misleading. This claim conflicts with two essential facts resulting from fundamental characteristics of the underlying metamaterial technology:

• These materials are, by nature, highly dispersive, hence light passing around a "cloaked" object would be strongly refracted (prisms are not invisible).
• Light passing through these materials would be partially (or almost completely) absorbed, making the shield partially (or almost completely) opaque.
• As Miller showed,^[8] perfect cloaking by materials is impossible anyway on causal grounds.

Perfect cloaking with active measurements and sources

Though perfect cloaking based on invisible paint is impossible, according to Miller,^[8] if detectors (such as microphones) and sources (such as loudspeakers) are placed round a volume and if a particular formula is used to calculate the signals to be fed to the sources, perfect cloaking is possible. Such perfect cloaking does require that the information can flow through the volume fast enough and the calculations can be performed fast enough so that the necessary information can get to the sources on the far side of the volume fast enough. As a result, perfect cloaking for light is still probably at least very difficult if not impossible. For sound waves, though, such perfect cloaking is possible in principle; an object could therefore be made invisible to sonar, for example.

See also

• Cap of invisibility
• Cloaking device

References

External links

• Cloaking and Invisibility: Fact and Fiction
• Three-dimensional optical metamaterial with a negative refractive index by Jason Valentine, Shuang Zhang, Thomas Zentgraf, Erick Ulin-Avila, Dentcho A. Genov, Guy Bartal, and Xiang Zhang. Nature advance online publication 11 August 2008.
• Optical Negative Refraction in Bulk Metamaterials of Nanowires by Jie Yao, Zhaowei Liu, Yongmin Liu, Yuan Wang, Cheng Sun, Guy Bartal, Angelica M. Stacy, and Xiang Zhang. Science 15 August 2008: Vol. 321. no. 5891, p. 930.