Cloak of invisibility
|Cloak of invisibility|
Alberich puts on the Tarnkappe and vanishes; illustration by Arthur Rackham to Richard Wagner's Das Rheingold
|Plot element from Folk lore and fairy tales|
|Genre||Folklore and fairy tales|
|Function||Renders the wearer invisible|
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 more common trope is the cap of invisibility. The cape of invisibility has appeared in Greek myth: Hades was ascribed possession of a cap or helmet that made the wearer invisible. 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.
Edgar Rice Burroughs uses the idea of an invisibility cloak in his 1931 novel A Fighting Man of Mars. The movie, Erik the Viking humorously depicts the title character using a cloak of invisibility, which he does not realize apparently works only on elderly men. In the Lord of the Rings, Frodo's elven cloak camouflaged him so that the enemy could see "nothing more than a boulder where the Hobbits were"; this magical item was used in the Dungeons & Dragons game. In the Walt Disney movie Now You See Him, Now You Don't Dexter Riley explains to principal Higgins of Medfield College the principles of invisibility cloaking through a liquid that can bend light.
Camouflaging cloaks form a central plot element in Samuel R. Delany's 1975 novel Dhalgren and the Harry Potter series of novels by J.K. Rowling. Harry uses the cloak to sneak into forbidden areas of the school.
In "A Very Potter Musical" it is stated that invisibility cloaks can be used to kick wiener dogs, pretend to be a ghost and scare people, avoid facing your own reflection in the mirror, and fake your own death.
More recently, cloaking devices have been used in videogames, for example Battlefield Heroes, Team Fortress 2 and the Halo series, where they aid stealth-based characters. Also, CrossFire has a game mode (appropriately called Ghost Mode) with the Black List Terrorists cloaked, and using stealth to detonate specific targets.
The cloaking devices appearing in Star Wars, Star Trek and Stargate, present a similar notion in a science fiction form, but are 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.
In science 
On October 19, 2006, a cloak was produced 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 obscures a defined two dimensional region and only at a particular microwave frequency. Work on achieving similar results with visible light is in progress. Other types of invisibility cloak are also possible, including ones that cloak events rather than objects.
|“||It's not yet clear that you're going to get the invisibility that everyone thinks about with Harry Potter's cloak or the Star Trek cloaking device. To make an object literally vanish before a person's eyes, a cloak would have to simultaneously interact with all of the wavelengths, or colors, that make up light.||”|
On the other hand, a group of researchers connected with Berkeley Lab and the University of California, Berkeley believe that cloaking at optical frequencies is indeed possible. Furthermore, it appears within reach. Their solution to the hurdles presented by cloaking issues are dielectrics. These nonconducting materials (dielectrics) are used for a carpet cloak, which serves as an optical cloaking device. According to the lead investigator:
|“||We have come up with a new solution to the problem of invisibility based on the use of dielectric (nonconducting) materials. Our optical cloak not only suggests that true invisibility materials are within reach, it also represents a major step towards transformation optics, opening the door to manipulating light at will for the creation of powerful new microscopes and faster computers.||”|
Furthermore, a new cloaking system was announced in the beginning of 2011 that is effective in visible light and hides macroscopic objects, i.e. objects that can be seen with the human eye. The cloak is constructed from ordinary, and easily obtainable calcite. The crystal consists of two pieces configured according to specific parameters. The calcite is able to refract the light around a solid object positioned between the crystals. The system employs the natural birefringence of the calcite. From outside the system the object is not visible "for at least 3 orders of magnitude larger than the wavelength of light in all three dimensions." The calcite solves for the limitations of attempting to cloak with metallic inclusions - this method does not require a nanofabrication process as has become necessary with the other methods of cloaking. The nanofabrication process is time consuming and limits the size of the cloaked region to a microscopic area. The system works best under green light. In addition the researchers appear to be optimistic about a practical cloaking device in the future:
|“||In summary, we have demonstrated the first macroscopic cloak operating at visible frequencies, which transforms a deformed mirror into a flat one from all viewing angles. The cloak is capable of hiding three-dimensional objects three to four orders of magnitudes larger than optical wavelengths, and therefore, it satisfies a layman's definition of an invisibility cloak: namely, the cloaking effect can be directly observed without the help of microscopes. Because our work solves several major issues typically associated with cloaking: size, bandwidth, loss, and image distortion, it paves the way for future practical cloaking devices.||”|
Another 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."
Furthermore, 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.
Problems of refraction and opacity 
The headlined claims that laboratory results with metamaterials are demonstrations of prototype invisibility cloaks conflicts with two 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).
- Currently light passing through these materials is partially absorbed, making the shield partially opaque.
- Perfect cloaking by materials may be problematic, when taking causality into account.
Acoustic cloaking 
Though perfect cloaking based on invisible paint is impossible 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.
According to Fermat’s Principle, light follows the trajectory of the shortest optical path, that is, the path over which the integral of the refractive index function is minimal. Therefore, the refractive index of an optical medium determines how light propagates within it. Consequently, by a suitable choice of refractive index profile for an optical medium, light rays can be bent around and made to propagate in closed loops…
Janos Perczel, 22, an undergraduate student at St Andrews University in Fife, has developed an optical sphere which could be used to create an "invisibility cloak". He said that by slowing down light by way of an optical illusion, the light can then be bent around an object to "conceal" it. Attempts have already been made to create invisibility cloaks but research shows that efforts are limited because any cloak would only work within certain backgrounds. But by slowing down the rays of light, Mr Perczel says the cloak wearer can move around ever-changing backgrounds.
Mr Perczel, from Hungary, came up with the idea under the guidance of "invisibility expert" Professor Ulf Leonhardt, who teaches at the university's school of physics and astronomy. The student recognised the potential of the invisible sphere and spent eight months fine tuning his project. The key development lies in the ability of the sphere, an optical device, to not only remain invisible itself but to slow light.
According to Prof Leonhardt, all optical illusions can slow down rays of light and the sphere can be used to bend this illusion around an object, reflecting off it and making it appear to be invisible. Mr Perczel added: "When the light is bent it engulfs the object, much like water covering a rock sitting in a river bed, and carries on its path, making it seem as if nothing is there. Light however can only be sped up to a speed faster than it would travel in space, under certain conditions, and this restricts invisibility cloaks to work in a limited part of the spectrum, essentially just one colour. This would be ideal if somebody was planning to stand still in camouflage. However, the moment they start to move, the scenery would begin to distort, revealing the person under the cloak. By slowing all of the light down with an invisible sphere, it does not need to be accelerated to such high speeds and can therefore work in all parts of the spectrum."
See also 
- Maria Tatar, The Annotated Brothers Grimm, p 332 W. W. Norton & company, London, New York, 2004 ISBN 0-393-05848-4
- Edith Hamilton, Mythology, p 29, ISBN 0-451-62702-4
- Gantz, Jeffrey (translator) (1987). The Mabinogion, p. 80. New York: Penguin. ISBN 0-14-044322-3.
- John Clute and Peter Nicholls, The Encyclopedia of Science Fiction, "Invisibility", p 625 ISBN 0-312-13486-X
- Peter N. Spotts (2006-10-20). "Disappear into thin air? Scientists take step toward invisibility.". The Christian Science Monitor. Retrieved 2007-05-05.
- Sean Markey (2006-10-19). "First Invisibility Cloak Tested Successfully, Scientists Say". National Geographic News. Retrieved 2007-05-05.
- Robert F. Service and Adrian Cho (17 December 2010). "Strange New Tricks With Light". Science 330 (6011): 1622. Bibcode:2010Sci...330.1622S. doi:10.1126/science.330.6011.1622. PMID 21163994.
- "Invisibility Cloak Demonstrated!". Computing News. 2006. Retrieved 2007-05-05.
- Berkely Lab News Center Yarris, Lynn (May 1, 2009). "...Berkeley researchers create an invisibility cloak" (Available online). Invisibility cloaking research. Lawrence Berkeley National Laboratory (LBNL). Retrieved 2011-03-23.
- Valentine, Jason; Li, Jensen; Zentgraf, Thomas; Bartal, Guy; Zhang, Xiang (2009). "An optical cloak made of dielectrics". Nature Materials 8 (7): 568–71. Bibcode:2009NatMa...8..568V. doi:10.1038/nmat2461. PMID 19404237.
- Chen, Xianzhong; Luo, Yu; Zhang, Jingjing; Jiang, Kyle; Pendry, John B.; Zhang, Shuang (2011). "Macroscopic invisibility cloaking of visible light" (free download of this open access article). Nature Communications 2 (2): 176. arXiv:1012.2783. Bibcode:2011NatCo...2E.176C. doi:10.1038/ncomms1176. PMC 3105339. PMID 21285954.
- Zhang, Baile; Luo, Yuan; Liu, Xiaogang; Barbastathis, George (2011). "Macroscopic Invisibility Cloak for Visible Light". Physical Review Letters 106. arXiv:1012.2238. Bibcode:2011PhRvL.106c3901Z. doi:10.1103/PhysRevLett.106.033901.
- "Scientists Develop New Invisibility Cloak Technology". redOrbit. April 30, 2009.
- Miller, David A. B (2006). "On perfect cloaking". Optics Express 14 (25): 12457–12466. Bibcode:2006OExpr..1412457M. doi:10.1364/OE.14.012457.
- "Student makes 'invisibility cloak'". Belfast Telegraph. 9 August 2011.
Further reading 
- Cloaking and Invisibility: Fact and Fiction by Professor David R. Smith - Electrical and Computer Engineering - Duke University (May 28, 2006)
- 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.
- Leonhardt, Ulf; Smith, David R (2008). "Focus on Cloaking and Transformation Optics". New Journal of Physics 10 (11): 115019. Bibcode:2008NJPh...10k5019L. doi:10.1088/1367-2630/10/11/115019.
- Hiding a realistic object using a broadband terahertz invisibility cloak by Fan Zhou, Yongjun Bao, Wei Cao, Colin Stuart, Jianqiang Gu, Weili Zhang, and Cheng Sun. Scientific Reports 1 2011.
- Vanishing point - The Guadian Info on "stealth suit"
- Invisibility Cloak Created in 3-D - BBC News
- Invisibility Cloak One Step Closer - BBC News
- How Invisibility Cloaks Work - HowStuffWorks
- Plasmonic invisibility effect - Live Science
- Scientists Aim to Duplicate Harry Potter's Invisibility Cloak - Live Science
- The invisibility as a result of 4D SEIS field radiation effect