A cloak of invisibility is an item that prevents the wearer from being seen. 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 In classical mythology, the Cap of Invisibility (Ἅϊδος κυνέη ''(H)aïdos kyneē'' in Greek, lit. dog-skin of Hades) is a helmet or cap that can turn the wearer invisible, also known as the Cap of Hades or Helm of Hades. Wearers of the c ...

seen in ancient Greek myths. The motif falls under "D1361.12 magic cloak of invisibility" in the

Stith Thompson Stith Thompson (March 7, 1885 – January 10, 1976) was an American folklorist: he has been described as "America's most important folklorist". He is the "Thompson" of the Aarne–Thompson–Uther Index, which indexes folktales by type, and the ...

motif index scheme.

In folklore

Cloaks of invisibility are magical items found in folklore and

fairy tale A fairy tale (alternative names include fairytale, fairy story, magic tale, or wonder tale) is a short story that belongs to the folklore genre. Such stories typically feature magic, enchantments, and mythical or fanciful beings. In most cult ...

s. Such cloaks are common in

Welsh mythology Welsh mythology (Welsh: ''Mytholeg Cymru'') 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 Celti ...

; a "Mantle of Invisibility" is described in the tale ''

Culhwch and Olwen ''Culhwch and Olwen'' ( cy, Culhwch ac 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 Whi ...

'' (c. 1100) as one of

King Arthur King Arthur ( cy, Brenin Arthur, kw, Arthur Gernow, br, Roue Arzhur) is a legendary king of Britain, and a central figure in the medieval literary tradition known as the Matter of Britain. In the earliest traditions, Arthur appears as ...

's most prized possessions.Stephens (1998) p. 479 The mantle is described again, and in more detail, 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 Cassivellaunus was a historical British military leader who led the defence against Julius Caesar's second expedition to Britain in 54 BC. He led an alliance of tribes against Roman forces, but eventually surrendered after his location was revea ...

to assassinate the seven stewards left behind by Bran the Blessed and usurp the throne. In the English fairy tale ''

Jack the Giant Killer "Jack the Giant Killer" is a Cornish 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 folklor ...

'', the hero is rewarded with several magical gifts by a giant he has spared, among them a coat of invisibility.

Iona Iona (; gd, Ì Chaluim Chille (IPA: �iːˈxaɫ̪ɯimˈçiʎə, sometimes simply ''Ì''; sco, Iona) is a small island in the Inner Hebrides, off the Ross of Mull on the western coast of Scotland. It is mainly known for Iona Abbey, though ther ...

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''. The counterpart in Japan is the , 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.

Tarnkappe

Although occurrences in fairy tales are rare, 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.'' The cloak in German fairy tales may be traceable to the ''tarnkappe'' ("cloak of concealment"), such as the one that the hero Sîfrit (Siegfried) acquires from the dwarf Alberich in the

Middle High German Middle High German (MHG; german: Mittelhochdeutsch (Mhd.)) is the term for the form of German spoken in the High Middle Ages. It is conventionally dated between 1050 and 1350, developing from Old High German and into Early New High German. Hig ...

epic ''

Nibelungenlied The ( gmh, Der Nibelunge liet or ), translated as ''The Song of the Nibelungs'', is an epic poem written around 1200 in Middle High German. Its anonymous poet was likely from the region of Passau. The is based on an oral tradition of Germani ...

''. 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

. The ''tarnkappe'' (or ''tarnkeppelin'') is also owned by the dwarf king who is the title character in ''

Laurin Laurin is both a surname and a given name. Notable people with the name include: Surname * Anna-Lena Laurin (born 1962), Swedish composer * Camille Laurin (1922–1999), psychiatrist and politician in Quebec, Canada * Dan Laurin (born 1960), Swed ...

''. In different passages or variant manuscripts of these works, the ''tarnkappe'' is also referred to as the ''tarnhût'' (mod. Ger. '' Haut'' "skin") or ''hehlkappe'' (mod. Ger. '' hehlen'' "to hide").

Modern adaptations

In the original epic ''Nibelungenlied,'' the hero's cloak not only grants him

invisibility Invisibility is the state of an object that cannot be seen. An object in this state is said to be ''invisible'' (literally, "not visible"). The phenomenon is studied by physics and perceptual psychology. Since objects can be seen by light i ...

, but also increases his strength, to win over the Icelandic queen

Brünhild Brunhild, also known as Brunhilda or Brynhild ( non, Brynhildr , gmh, Brünhilt, german: Brünhild , label=Modern German or ), is a female character from Germanic heroic legend. She may have her origins in the Visigothic princess Brunhilda o ...

. In

Richard Wagner Wilhelm Richard Wagner ( ; ; 22 May 181313 February 1883) was a German composer, theatre director, polemicist, and conductor who is chiefly known for his operas (or, as some of his mature works were later known, "music dramas"). Unlike most op ...

's opera cycle ''

Der Ring des Nibelungen (''The Ring of the Nibelung''), WWV 86, is a cycle of four German-language epic music dramas composed by Richard Wagner. The works are based loosely on characters from Germanic heroic legend, namely Norse legendary sagas and the '' Nibe ...

'', 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.

In fiction

Raoul Walsh Raoul Walsh (born Albert Edward Walsh; March 11, 1887December 31, 1980) was an American film director, actor, founding member of the Academy of Motion Picture Arts and Sciences (AMPAS), and the brother of silent screen actor George Walsh. He wa ...

's film '' The Thief of Bagdad'', was released in the same year as the ''Die Nibelungen'' and also features a cloak of invisibility playing a pivotal role.

Edgar Rice Burroughs Edgar Rice Burroughs (September 1, 1875 – March 19, 1950) was an American author, best known for his prolific output in the adventure, science fiction, and fantasy genres. Best-known for creating the characters Tarzan and John Carter, ...

uses the idea of an invisibility cloak in his 1931 novel ''

A Fighting Man of Mars ''A Fighting Man of Mars'' is a science fantasy novel by American writer Edgar Rice Burroughs, the seventh of his Barsoom series. Burroughs began writing it on February 28, 1929, and the finished story was first published in '' The Blue Book Mag ...

''. 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 ''The Lord of the Rings'' is an epic high-fantasy novel by English author and scholar J. R. R. Tolkien. Set in Middle-earth, intended to be Earth at some time in the distant past, the story began as a sequel to Tolkien's 1937 children's bo ...

'', 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

Hobbit Hobbits are a fictional race of people in the novels of J. R. R. Tolkien. About half average human height, Tolkien presented hobbits as a variety of humanity, or close relatives thereof. Occasionally known as halflings in Tolkien's writings, ...

s were." Camouflaging cloaks form a central plot element in Samuel R. Delany's 1975 novel '' Dhalgren''. Cloaks of invisibility also exist in the ''

Harry Potter ''Harry Potter'' is a series of seven fantasy novels written by British author J. K. Rowling. The novels chronicle the lives of a young wizard, Harry Potter, and his friends Hermione Granger and Ron Weasley, all of whom are students at ...

'' 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 ''Harry Potter and the Deathly Hallows'' is a fantasy novel written by British author J. K. Rowling and the seventh and final novel of the main ''Harry Potter'' series. It was released on 21 July 2007 in the United Kingdom by Bloomsbury Publi ...

. In ''

The Secret History ''The Secret History'' is the first novel by the American author Donna Tartt, published by Alfred A. Knopf in September 1992. Set in New England, the campus novel tells the story of a closely knit group of six classics students at Hampden Colleg ...

'' by

Donna Tartt Donna Louise Tartt (born December 23, 1963) is an American novelist and essayist. Early life Tartt was born in Greenwood, Mississippi, in the Mississippi Delta, the elder of two daughters. She was raised in the nearby town of Grenada. Her fa ...

(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 Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...

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. However, cloaking a human-sized object at visible wavelengths appears to have low probability. Indeed, there appears to be a fundamental problem with these devices as "invisibility cloaks": On the other hand, a group of researchers connected with

Berkeley Lab Lawrence Berkeley National Laboratory (LBNL), commonly referred to as the Berkeley Lab, is a United States national laboratory that is owned by, and conducts scientific research on behalf of, the United States Department of Energy. Located in ...

and the

University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant un ...

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 (

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the ma ...

s) are used for a carpet cloak, which serves as an optical cloaking device. According to the lead investigator: 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 Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

. 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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefri ...

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: 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 Vladimir (Vlad) M. Shalaev (born February 18, 1957) is a Distinguished Professor of Electrical and Computer Engineering and Scientific Director for Nanophotonics at Birck Nanotechnology Center, Purdue University. Education and career Shalaev earn ...

, a professor of electrical and computer engineering at Purdue. "Ideally, if we make it real it would work exactly like

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 Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentar ...

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 Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...

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 In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, ...

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.

References

Bibliography

External links

Info on "stealth suit"
Invisibility Cloak Created in 3-D - BBC News

Invisibility Cloak One Step Closer - BBC News

* ttps://www.flickr.com/photos/jeisus2012/8267145697/in/set-72157631064216316/ The invisibility as a result of 4D SEIS field radiation effect
On The Quest To Invisibility - Metamaterials and Cloaking
(video), Prof. Andrea Alù at TEDxAustin, 2013. {{Authority control Electromagnetic radiation Fantasy tropes Recurrent elements in fairy tales Invisibility Magic items Fiction about invisibility

Emerging technologies