Cloak of invisibility

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Cloak of invisibility
Illustration to Richard Wagner's "Das Rheingold".jpg
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
First appearanceAncient
GenreFolklore and fairy tales
In-story information
TypeMagical cape
FunctionRenders the wearer invisible

A cloak of invisibility is a fictional theme. In folklore, mythology and fairy tales, a cloak of invisibility appears either as a magical item used by duplicitous characters or an item worn by a hero to fulfill a quest. It is a common theme in Welsh and Germanic folklore, and may originate with the cap of invisibility seen in ancient Greek myths. The motif falls under "D1361.12 magic cloak of invisibility" in the Stith Thompson motif index scheme. There are recent attempts to create such a thing, and this device is subject to some scientific inquiry

Cap of invisibility helmet or cap that can turn the wearer invisible

In classical mythology, the Cap of Invisibility is a helmet or cap that can turn the host invisible. It is also known as the Cap of Hades, Helm of Hades. Wearers of the cap in Greek myths include Athena, the goddess of wisdom, the messenger god Hermes, and the hero Perseus. The Cap of Invisibility enables the user to become invisible to other supernatural entities, functioning much like the cloud of mist that the gods surround themselves in to become undetectable.

Stith Thompson was an American scholar of folklore. He is the "Thompson" of the Aarne–Thompson classification system, which indexes certain folktales by their structure and assigns them AT numbers. He also developed an alpha-decimal motif-index system for cataloging individual motifs.

Contents

In fiction

Cloaks of invisibility are magical items found in folklore and fairy tales. Such cloaks are common in Welsh mythology; a "Mantle of Invisibility" is described in the tale Culhwch and Olwen (c. 1100) as one of King Arthur's most prized possessions. [1] The mantle is described again, and in more detail, [1] in the Breuddwyd Rhonabwy, and is later listed as one of the Thirteen Treasures of the Island of Britain. A similar mantle appears in the Second Branch of the Mabinogi, in which it is used by Caswallawn to assassinate the seven stewards left behind by Bran the Blessed and usurp the throne. [1] [2]

Fairy tale Fictional story typically featuring folkloric fantasy characters and magic

A fairy tale, wonder tale, magic tale, or Märchen is an instance of a folklore genre that takes the form of a short story. Such stories typically feature entities such as dwarfs, dragons, elves, fairies, giants, gnomes, goblins, griffins, mermaids, talking animals, trolls, unicorns, or witches, and usually magic or enchantments. In most cultures, there is no clear line separating myth from folk or fairy tale; all these together form the literature of preliterate societies. Fairy tales may be distinguished from other folk narratives such as legends and explicit moral tales, including beast fables. The term is mainly used for stories with origins in European tradition and, at least in recent centuries, mostly relates to children's literature.

Welsh mythology

Welsh mythology consists of both folk traditions developed in Wales, and traditions developed by the Celtic Britons elsewhere before the end of the first millennium. As in most of the predominantly oral societies of prehistoric Britain, Welsh mythology and history was recorded orally by specialists such as druids. This oral record has been lost or altered as a result of outside contact and invasion over the years. Much of this altered mythology and history are preserved in medieval Welsh manuscripts, which include the Red Book of Hergest, the White Book of Rhydderch, the Book of Aneirin and the Book of Taliesin. Other works connected to Welsh mythology include the ninth-century Latin historical compilation Historia Brittonum and Geoffrey of Monmouth's twelfth-century Latin chronicle Historia Regum Britanniae, as well as later folklore, such as the materials collected in The Welsh Fairy Book by William Jenkyn Thomas (1908).

Culhwch and Olwen is a Welsh tale that survives in only two manuscripts about a hero connected with Arthur and his warriors: a complete version in the Red Book of Hergest, c. 1400, and a fragmented version in the White Book of Rhydderch, ca. 1325. It is the longest of the surviving Welsh prose tales. The prevailing view among scholars was that the present version of the text was composed by the 11th century, making it perhaps the earliest Arthurian tale and one of Wales' earliest extant prose texts, but a 2005 reassessment by linguist Simon Rodway dates it to the latter half of the 12th century. The title is a later invention and does not occur in early manuscripts.

In the English fairy tale Jack the Giant Killer , the hero is rewarded with several magical gifts by a giant he has spared, among them a coat of invisibility. Iona and Peter Opie observe in The Classic Fairy Tales (1974), that Jack's coat may have been borrowed from the Tale of Tom Thumb or from Norse mythology, but they also draw comparisons with the Celtic stories of the Mabinogion. [3]

Jack the Giant Killer

"Jack the Giant Killer" is an English fairy tale and legend about a young adult who slays a number of bad giants during King Arthur's reign. The tale is characterised by violence, gore and blood-letting. Giants are prominent in Cornish folklore, Breton mythology and Welsh Bardic lore. Some parallels to elements and incidents in Norse mythology have been detected in the tale, and the trappings of Jack's last adventure with the Giant Galigantus suggest parallels with French and Breton fairy tales such as Bluebeard. Jack's belt is similar to the belt in "The Valiant Little Tailor", and his magical sword, shoes, cap, and cloak are similar to those owned by Tom Thumb or those found in Welsh and Norse mythology.

Tom Thumb Character in literature

Tom Thumb is a character of English folklore. The History of Tom Thumb was published in 1621, and was the first fairy tale printed in English. Tom is no bigger than his father's thumb, and his adventures include being swallowed by a cow, tangling with giants, and becoming a favourite of King Arthur. The earliest allusions to Tom occur in various 16th-century works such as Reginald Scot's Discovery of Witchcraft (1584), where Tom is cited as one of the supernatural folk employed by servant maids to frighten children. Tattershall in Lincolnshire, England, reputedly has the home and grave of Tom Thumb.

The counterpart in Japan is the kakuremino(隠れ蓑), a magical "straw cape" or "raincoat" of invisibility. In the folktale of the "Peach Boy" Momotarō , one of the treasures the hero collects from the ogres is a cape of invisibility, paralleling the story of Jack the giant-slayer. [4]

Momotarō hero from Japanese folklore

Momotarō is a popular hero of Japanese folklore. His name translates as Peach Tarō, a common Japanese masculine name, and is often translated as Peach Boy. Momotarō is the title of various books, films and other works that portray the tale of this hero.

Tarnkappe

Although occurrences in fairy tales are rare, [5] the cloak of invisibility appears in the German tale The Twelve Dancing Princesses (KHM 133) and in The King of the Golden Mountain (KHM 92) in Grimm's Fairy Tales. [6] The cloak in German fairy tales may be traceable to the tarnkappe ("cloak of concealment"), [5] such as the one that the hero Sîfrit (Siegfried) acquires from the dwarf Alberich in the Middle High German epic Nibelungenlied . [7] The Grimms clarify that Sîfrit's kappe is a cape that covers not just the head but enshrouds the body, though in later times tarnkappe came to be regarded as a cap of invisibility. The tarnkappe (or tarnkeppelin [8] ) is also owned by the dwarf king who is the title character in Laurin . In different passages or variant manuscripts of these works, the tarnkappe is also referred to as the tarnhût (mod. Ger. Haut "skin") [7] [9] or hehlkappe (mod. Ger. hehlen "to hide"). [10] [11]

<i>The Twelve Dancing Princesses</i> German fairy tale

"The Twelve Dancing Princesses" is a German fairy tale originally published by the Brothers Grimm in 1812 in Kinder- und Hausmärchen as tale number 133. Its closest analogue is the Scottish Kate Crackernuts, where it is a prince who is obliged to dance every night.

The King of the Golden Mountain literary work

"The King of the Golden Mountain" is a German fairy tale collected by the Brothers Grimm in Grimm's Fairy Tales.

Sigurd fictional character in Germanic and Norse mythology

Sigurd or Siegfried is a legendary hero of Germanic mythology, who killed a dragon and was later murdered. It is possible he was inspired by one or more figures from the Frankish Merovingian dynasty, with Sigebert I being the most popular contender. Older scholarship sometimes connected him with Arminius, victor of the Battle of the Teutoburg Forest. He may also have a purely mythological origin. Sigurd's story is first attested on a series of carvings, including runestones from Sweden and stone crosses from the British Isles, dating from the eleventh century.

Modern adaptations

In the original epic Nibelungenlied, the hero's cloak not only grants him invisibility, but also increases his strength, to win over the Icelandic queen Brünhild. In Richard Wagner's opera cycle Der Ring des Nibelungen , the cloak becomes a magic helmet called the Tarnhelm, which also imparts the ability to transform upon its wearer. When Fritz Lang adapted Nibelungenlied for the movie screen in his 1924 film Die Nibelungen , Siegfried uses a veil or net of invisibility gained from the dwarf Alberich. Another film from the same year to use a cloak of invisibility was Raoul Walsh's The Thief of Bagdad , in which the cloak plays a pivotal role. 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, and the other members of the Fellowship of the Ring, were given cloaks by the Elves, and Samwise asked, "Are these magic cloaks?" The cloak given to Frodo camouflaged him so that the enemy could see "nothing more than a boulder where the Hobbits were."

Invisibility State of an object that cannot be seen

Invisibility is the state of an object that cannot be seen. An object in this state is said to be invisible. The term is often used in fantasy/science fiction, where objects cannot be seen by magical or technological means; however, its effects can also be demonstrated in the real world, particularly in physics and perceptual psychology classes.

Richard Wagner German composer

Wilhelm Richard Wagner was a German composer, theatre director, polemicist, and conductor who is chiefly known for his operas. Unlike most opera composers, Wagner wrote both the libretto and the music for each of his stage works. Initially establishing his reputation as a composer of works in the romantic vein of Carl Maria von Weber and Giacomo Meyerbeer, Wagner revolutionised opera through his concept of the Gesamtkunstwerk, by which he sought to synthesise the poetic, visual, musical and dramatic arts, with music subsidiary to drama. He described this vision in a series of essays published between 1849 and 1852. Wagner realised these ideas most fully in the first half of the four-opera cycle Der Ring des Nibelungen.

<i>Der Ring des Nibelungen</i> Cycle of four operas by Richard Wagner

Der Ring des Nibelungen, WWV 86, is a cycle of four German-language epic music dramas composed by Richard Wagner. The works are based loosely on characters from the Norse sagas and the Nibelungenlied. The composer termed the cycle a "Bühnenfestspiel", structured in three days preceded by a Vorabend. It is often referred to as the Ring Cycle, Wagner's Ring, or simply The Ring.

Camouflaging cloaks form a central plot element in Samuel R. Delany's 1975 novel Dhalgren .

Cloaks of invisibility also exist in the Harry Potter series of novels by J.K. Rowling. Harry Potter uses a Cloak of Invisibility, that was passed down to him by his father, to sneak into forbidden areas of his school and remain unseen. It is later revealed that this specific cloak was once owned by Death himself, making it one of the Deathly Hallows.

Also found in popular fiction, The Secret History , by Donna Tartt, 1992. The character, Richard, says, "I became expert at making myself invisible"...."Sunday afternoons, my cloak of invisibility around my shoulders, I would sit in the infirmary for sometimes six hours at a time....".

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. [12] [13] Other types of invisibility cloak are also possible, including ones that cloak events rather than objects.

However, cloaking a human-sized object at visible wavelengths appears to have low probability. [14] Indeed, there appears to be a fundamental problem with these devices as "invisibility cloaks": [15]

It's not yet clear that you're going to get the invisibility that everyone thinks about with Star Trek cloaking device or the Harry Potter's cloak. 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. [16] [17] 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: [18] [19]

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."

Furtherfurthermore, 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. [20]

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:

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." [22]

See also

Related Research Articles

Cloaking device theoretical device to render objects invisible

A cloaking device is a hypothetical or fictional stealth technology that can cause objects, such as spaceships or individuals, to be partially or wholly invisible to parts of the electromagnetic (EM) spectrum. However, over the entire spectrum, a cloaked object scatters more than an uncloaked object.

Metamaterial materials engineered to have properties that have not yet been found in nature

A metamaterial is a material engineered to have a property that is not found in naturally occurring materials. They are made from assemblies of multiple elements fashioned from composite materials such as metals and plastics. The materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable of manipulating electromagnetic waves: by blocking, absorbing, enhancing, or bending waves, to achieve benefits that go beyond what is possible with conventional materials.

A superlens, or super lens, is a lens which uses metamaterials to go beyond the diffraction limit. The diffraction limit is a feature of conventional lenses and microscopes that limits the fineness of their resolution. Many lens designs have been proposed that go beyond the diffraction limit in some way, but constraints and obstacles face each of them.

Negative-index metamaterial

Negative-index metamaterial or negative-index material (NIM) is a metamaterial whose refractive index for an electromagnetic wave has a negative value over some frequency range.

Terahertz metamaterial

A terahertz metamaterial is a class of composite metamaterials designed to interact at terahertz (THz) frequencies. The terahertz frequency range used in materials research is usually defined as 0.1 to 10 THz.

An acoustic metamaterial is a material designed to control, direct, and manipulate sound waves as these might occur in gases, liquids, and solids. The hereditary line into acoustic metamaterials follows from theory and research in negative index material. Furthermore, with acoustic metamaterials, controlling sonic waves can now be extended to the negative refraction domain.

David R. Smith is an American physicist and professor of electrical and computer engineering at Duke University in North Carolina. Smith's research focuses on electromagnetic metamaterials, or materials with a negative index of refraction.

Photonic metamaterial

A photonic metamaterial (PM), also known as an optical metamaterial, is a type of electromagnetic metamaterial, that interacts with light, covering terahertz (THz), infrared (IR) or visible wavelengths. The materials employ a periodic, cellular structure.

A seismic metamaterial, is a metamaterial that is designed to counteract the adverse effects of seismic waves on artificial structures, which exist on or near the surface of the earth. Current designs of seismic metamaterials utilize configurations of boreholes, trees or proposed underground resonators to act as a large scale material. Experiments have observed both reflections and bandgap attenuation from artificially induced seismic waves. These are the first experiments to verify that seismic metamaterials can be measured for frequencies below 100 Hz, where damage from Rayleigh waves is the most harmful to artificial structures.

A nonlinear metamaterial is an artificially constructed material that can exhibit properties not found in nature. Its response to electromagnetic radiation can be characterized by its permittivity and material permeability. The product of the permittivity and permeability results in the refractive index. Unlike natural materials, nonlinear metamaterials can produce a negative refractive index. These can also produce a more pronounced nonlinear response than naturally occurring materials.

Metamaterial cloaking

Metamaterial cloaking is the usage of metamaterials in an invisibility cloak. This is accomplished by manipulating the paths traversed by light through a novel optical material. Metamaterials direct and control the propagation and transmission of specified parts of the light spectrum and demonstrate the potential to render an object seemingly invisible. Metamaterial cloaking, based on transformation optics, describes the process of shielding something from view by controlling electromagnetic radiation. Objects in the defined location are still present, but incident waves are guided around them without being affected by the object itself.

A metamaterial absorber is a type of metamaterial intended to efficiently absorb electromagnetic radiation such as light. Furthermore, metamaterials are an advance in materials science. Hence, those metamaterials that are designed to be absorbers offer benefits over conventional absorbers such as further miniaturization, wider adaptability, and increased effectiveness. Intended applications for the metamaterial absorber include emitters, photodetectors, sensors, spatial light modulators, infrared camouflage, wireless communication, and use in solar photovoltaics and thermophotovoltaics.

History of metamaterials

The history of metamaterials begins with artificial dielectrics in microwave engineering as it developed just after World War II. Yet, there are seminal explorations of artificial materials for manipulating electromagnetic waves at the end of the 19th century. Hence, the history of metamaterials is essentially a history of developing certain types of manufactured materials, which interact at radio frequency, microwave, and later optical frequencies.

Theories of cloaking

Theories of cloaking discusses various theories based on science and research, for producing an electromagnetic cloaking device. Theories presented employ transformation optics, event cloaking, dipolar scattering cancellation, tunneling light transmittance, sensors and active sources, and acoustic cloaking.

Transformation optics

Transformation optics applies metamaterials to produce spatial variations, derived from coordinate transformations, which can direct chosen bandwidths of electromagnetic radiation. This can allow for the construction of new composite artificial devices, which probably could not exist without metamaterials and coordinate transformation. Computing power that became available in the late 1990s enables prescribed quantitative values for the permittivity and permeability, the constitutive parameters, which produce localized spatial variations. The aggregate value of all the constitutive parameters produces an effective value, which yields the intended or desired results.

A plasmonic metamaterial is a metamaterial that uses surface plasmons to achieve optical properties not seen in nature. Plasmons are produced from the interaction of light with metal-dielectric materials. Under specific conditions, the incident light couples with the surface plasmons to create self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons (SPPs). Once launched, the SPPs ripple along the metal-dielectric interface. Compared with the incident light, the SPPs can be much shorter in wavelength.

Artificial dielectrics are fabricated electromagnetic materials consisting of synthetic substances, usually constructed in an orderly arrangement, such as arrays. The arranged materials, and the distances between them are usually evenly spaced on, or within, a substrate. Hence, the arrays of inclusions are in a periodic or lattice structure. Also, the lattice spacing is smaller than the impinging electromagnetic wavelength. These were first conceptualized, constructed and deployed for interaction in the microwave frequency range in the 1940s and 1950s. The constructed medium, the artificial dielectric, has an effective permittivity and effective permeability, as intended.

Illusion optics is an electromagnetic theory that can change the optical appearance of an object to be exactly like that of another virtual object, i.e. an illusion, such as turning the look of an apple into that of a banana. Invisibility is a special case of illusion optics, which turns objects into illusions of free space. The concept and numerical proof of illusion optics was proposed in 2009 based on transformation optics in the field of metamaterials. It is a scientific disproof of the idiom 'Seeing is Believing'.

Illusion optics is an electromagnetic theory that can change the optical appearance of an object to be exactly like that of another virtual object, i.e. an illusion, such as turning the look of an apple into that of a banana. Invisibility is a special case of illusion optics, which turns objects into illusions of free space. The concept and numerical proof of illusion optics was proposed in 2009 based on transformation optics in the field of metamaterials. It is a scientific disproof of the idiom 'Seeing is Believing'.

References

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  11. Ettmüller (1829) ed.,
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  19. Zhang, Baile; Luo, Yuan; Liu, Xiaogang; Barbastathis, George (2011). "Macroscopic Invisibility Cloak for Visible Light". Physical Review Letters. 106 (3): 033901. arXiv: 1012.2238 . Bibcode:2011PhRvL.106c3901Z. doi:10.1103/PhysRevLett.106.033901. PMID   21405275.
  20. "Scientists Develop New Invisibility Cloak Technology". redOrbit. April 30, 2009.
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  22. "Student makes 'invisibility cloak'". Belfast Telegraph. 9 August 2011.

Bibliography

Further reading