Invisibility is the state of an object which cannot be seen. An object in this state is said to be invisible (literally, "not visible"). The term is usually used as a fantasy/science fiction term, where objects are literally made unseeable by magical or technological means; however, its effects can also be seen in the real world, particularly in physics and perceptional psychology.
Since objects can be seen by light in the visible spectrum from a source reflecting off their surfaces and hitting the viewer's eye, the most natural form of invisibility (whether real or fictional) is an object which neither reflects nor absorbs light (that is, it allows light to pass through it). In nature, this is known as
transparency, and is seen in many naturally occurring materials (although no naturally occurring material is 100% transparent).
Visibility also depends on the eyes of the observer and/or the instruments used. Thus an object can be classified as "invisible to" a person, animal, instrument, etc. In the research of sensorial perception invisibility has been shown to happen in cycles.
An object may be classified as "invisible" if it cannot be noticed by use of sight due to environmental
factors other than the fact that it doesn't reflect light. An object that might normally be seeable may be classified as invisible if it is:
- Behind an object.
- The same colour or pattern as the background (camouflage)
- In an environment which is too dark or too bright.
- In a particular observer's blind spot.
- utilizing video/image capture (background capture), dynamic modification of background image data transmitted to object attached display causes invisibility to human sight within the human sight light/photonic frequency range.
Theoretical and practical physics
offer several causes of invisibility. An object may be invisible if it is:
- So massive that its escape velocity exceeds the speed of light (such objects are called black holes)
- Transparent (such as air and many other gases)
- Emitting or reflecting light outside the wavelength range of visible light. (Radiation is generally invisible by this means.) Unfortunately, this would result in any obscured human being becoming not invisible and transparent, but completely opaque and resembling a human-shaped black hole.
- A recent breakthrough (2006) at Imperial College London has shown that invisibility is possible by using specifically patterned crystals made up of nanoscale boxes that hold electrons. When light hits these crystals, it becomes entangled within the boxes, causing the object to become transparent.
- Theoretically, it is possible to make an object invisible, if the object has the same refractive index as the surrounding medium (e.g. air.) (This is the mechanic used in HG Well's The Invisible Man.)
Technology can be used theoretically or practically to render real-world objects invisible:
- Making use of real-time image displayed on a wearable display, scientists are able to create a see-through effect, if not invisibility. This is known as active camouflage.
- Though stealth technology is cited as invisibility to radar, all officially disclosed applications of the technology can only reduce the size and/or clarity of the signature detected by radar.
- In some science fiction stories, a hypothetical "cloaking device" is used to make objects invisible. On Thursday October 19 2006 a team effort of researchers from Britain and the U.S announced the development of a real cloak of invisibility, though it is only in its first stages.
- In filmmaking, people, objects, or backgrounds can be made to look invisible on camera through a process known as chroma keying.
Engineers and scientists have performed various kinds of research to investigate the possibility of finding ways to create real optical invisibility for objects.
- Although it has been shown that making opaque objects perfectly invisible ("non-scattering scatterers") is impossible, 2006 theoretical work predicts that the imperfections need not be serious, and metamaterials may make real-life "cloaking devices" practical. The technique is suspected to be applied to radio waves within five years, and eventually visible light is a possibility. The theory that light waves can be acted upon the same way as radio waves is now a popular idea among scientists and can be compared to a stone in a river, in where the water passes around it, but leaving no trace of a stone being in the water slightly down-stream. Comparing light waves to the water and whatever object that is being "cloaked" to the stone, the desire is to have light waves pass around that object, leaving no visible aspects of it, possibly not even a shadow. This is the technique depicted in the 2000 television portrayal of The Invisible Man.
- Two teams of scientists worked separately to create two "Invisibility Cloaks" from 'metamaterials' engineered at the nanoscale level. They demonstrated for the first time the possibility of cloaking 3-dimensional (3-D) objects with artificially engineered materials that redirect radar, light or other waves around an object, like water flowing around a smooth rock in a stream. While one uses a type of fishnet of metal layers to reverse the direction of light, the other uses tiny silver wires. Xiang Zhang, of the University of California, Berkeley said: "In the case of invisibility cloaks or shields, the material would need to curve light waves completely around the object like a river flowing around a rock. An observer looking at the cloaked object would then see light from behind it, making it seem to disappear."
- Researcher Jason Valentine's team made a material that affects light near the visible spectrum, in a region used in fibre optics: 'Instead of the fish appearing to be slightly ahead of where it is in the water, it would actually appear to be above the water's surface. It's kind of weird. For a metamaterial to produce negative refraction, it must have a structural array smaller than the wavelength of the electromagnetic radiation being used." Valentine's team created their 'fishnet' material by stacking silver and metal dielectric layers on top of each other and then punched through with holes. The other team used an oxide template and grew silver nanowires inside porous aluminum oxide at tiny distances apart, smaller than the wavelength of visible light. This material refracts visible light.
- The Imperial College London research achieved results with microwaves. An invisibility cloak layout of a copper cylinder was produced in May, 2008, by physicist Professor Sir John Pendry. Scientists working with him in the US put the idea into practice.
Invisibility, usually by magic, is a popular theme in fantasy, mythology and Ceremonial magic. It is often used as a gameplay device in role-playing and strategy games. Typically, people or objects can be rendered completely invisible by several means:
- Magical objects such as rings, cloaks and amulets can be worn to grant the wearer permanent invisibility.
- Magical potions can be consumed to grant temporary invisibility.
- Magic spells can be cast on people or objects, usually giving temporary invisibility.
- Some mythical creatures can make themselves invisible at will, such as some versions of Leprechaun, and Chinese dragons in some tales, which can shrink so small that humans cannot see them.
- An invisibility ritual is described in the manuscripts of the Hermetic Order of the Golden Dawn and appears on the Book The Golden Dawn by Israel Regardie.
- In the Artemis Fowl series, fairies can become invisible, or shield, by vibrating faster than the eye can see.
In some works, magical invisibility is "psychic" invisibility; while the character could be normally seen by anyone who notices him, the magic distracts anyone who actually could notice him. Such invisibility can be betrayed by mirrors or other reflective surfaces.
Where magical invisibility is concerned, the issue may arise of whether the clothing and items carried by the invisible wearer/carrier are also rendered invisible. In general, they are, but in some instances, clothing remains visible and must be removed for the full invisibility effect -- as for instance, the movie The Incredibles, where Violet Parr can only turn her clothing invisible when it has been specially made to allow it.
Invisible colour paradox
Since something that is invisible has no colour
associated with it, it is somewhat paradoxical
to imagine an object that is both invisible and coloured. This idea is most famous in the parody goddess
, the Invisible Pink Unicorn
Sight while invisible
According to the laws of physics, a perfectly invisible person would necessarily be blind, no matter how their invisibility were achieved. In order to see light, it must be absorbed by the retina
, but in order for a person to be invisible, the body must not absorb light. So to retain sight at least pupil sized holes in the cloak would be necessary in front of the pupils and directly behind them on the back of the person as light isn't being transmitted through. In fact, according to the no cloning theorem
of quantum mechanics, they could not even make a copy of the photons so they could see one copy and allow the other copy to pass through or around them. This idea was first discussed by Mat Ryer a computer software engineer based in London.
This physical barrier appears to offset the advantage of any perfect invisibility method, unless one's intent was simply to hide and be still, letting the danger pass. On the other hand, a practical invisibility method need not allow light of all frequencies to pass all the time, so there may be ways around this limitation. For example, if the wearer of a perfect invisibility device had goggles that allowed him or her to perceive infrared light while the invisibility device only diverted visible light, the wearer would be effectively invisible to the human eye while still being able to see heat sources.
Alternatively, many works of fiction portray invisibility as a magic achievement, and since paranormal magic may be interpreted as breaking the laws of physics, it could theoretically allow sight. Invisibility is often utilized in science fiction and fantasy works which people go into with a healthy suspension of disbelief, anyway.
One of the few fictional examples of a double-blind cloak comes from the Thrawn Trilogy of Star Wars novels. Grand Admiral Thrawn's cloaking devices make the ships wielding them invisible, but also prevent those inside the ship from seeing out. Thus, most of the time, ships using this type of cloak remain stationary, dropping the cloak just before battle. (See the beginning of Specter of the Past for an example of this tactic.)
In the Halo video game series, the "active camouflage" power-up renders the wearer only partially invisible — the visible silhouette of the wearer is likely necessary so that the wearer's retinas can absorb what little light they need to see (though it also exists for game balance issues). In the video game Quake, picking up a magic ring turns the player invisible to monsters for thirty seconds. In multiplayer deathmatch mode, only the player's eyes are visible, giving his opponents only a small clue to his location. Of course, with eyes being visible, light can be absorbed and the player can see.
People have attributed invisibility to things that are mythical
, things that do not exist or things are of a religious or supernatural
nature in order to explain why they are not apparent. In the Middle Ages
, fern seeds
were thought to be invisible since ferns don't have seeds. They were also said to grant invisibility.
In medieval astronomy
, the crystal spheres
that held up the sun, moon, stars, and planets were invisible. Historically, creatures such as goblins
have also been described as invisible or able to become invisible. Currently, many entities or phenomena whose existence is disputed, such as ghosts
, and auras
, are also ascribed invisibility. In religion
are commonly thought to be invisible, at least part of the time. Indeed, the omnipresence
attributed to the monotheistic God
of Abrahamic religions
would seem to require invisibility, since otherwise God would be constantly visible to all people because of inhabiting all places.
The idea of being unseen and hence undetectable has fascinated mankind for generations. This concept of invisibility has been explored in many myths
and video games
, some of them comedies.
- Mythology and folklore
- In many myths and legends, gods, spirits, fairies, angels, and demons are often invisible or can choose to become invisible at will.
- The ring of Gyges is described in a story in Plato's The Republic. A peasant finds a ring in the tomb of a dead king which allows him to become invisible at will. Plato has him enter the palace, seduce the queen, and plot to kill the king, arguing that power, such as this, corrupts absolutely.
- The hero Perseus went equipped with a cap of invisibility to kill Medusa.
- A magic cloak, made by Alberich the dwarf, granted invisibility to Sigurd.
- In German fairy tales, magical caps called tarnkappes are worn by dwarfs. The caps can make an entire village of dwarfs invisible.
- In The Twelve Dancing Princesses, the old soldier is able to follow the princesses by use of an invisibility cloak
- In The King of the Gold Mountain, the hero can sneak into the home of his treacherous wife by means of a cloak of invisibility
- Modern fiction
- What Was It? A Mystery (1859) by Fitz James O'Brien
- The Crystal Man (1881) by Edward Page Mitchell
- The Damned Thing (1894) by Ambrose Bierce
- The Horla (1897) by Guy de Maupassant
- The Invisible Man (1897) by H. G. Wells is a well-known novel about invisibility, later made into a film and several TV series.
- The Dunwich Horror (1929) by H. P. Lovecraft
- One function of the One Ring in J. R. R. Tolkien's The Lord of the Rings series was to render the user invisible. Unfortunately, it had an evil influence with negative effects on the wearer's actions.
- The Philadelphia Experiment (1984) is a movie about an attempt to make a ship invisible.
- The film Predator and animated television series like Batman Beyond and Max Steel depict a kind of stealth mode that is a partial invisibility where the subject is largely transparent and/or translucent. While a concentrated look in optimal lighting conditions can spot it, it still makes the wearer extremely hard to see which eases stealth movement.
- In comic books, there are superheroes such as the Invisible Woman (who can bend light around herself without distortion) that have the power to become invisible at will as well as magicians like Doctor Strange who have invisibility spells in their possession.
- In the Invisible Man TV series, a former thief is given the ability to become invisible thanks to a gland in his brain that secrets a light-bending chemical called quicksilver through his body, the only catch being that prolonged use of the chemical turns him insane unless he receives regular injections.
- In the film Hollow Man (2000), an injection is tested on animals (and eventually, humans) which made the organism invisible, organ by organ. Unfortunately, the serum can only be counteracted by electricity, and the long term effects of invisibility proved serious.
- In The Hitchhiker's Guide to the Galaxy "trilogy" of novels by Douglas Adams, the Somebody Else's Problem field is a humorous concept of a field which makes people believe the object in question is "somebody else's problem" and therefore do not see it. The concept of the "somebody else's problem field", as it is explained in the book, bases off of a statement to the effect that actual, invariable, invisibility is basically impossible and that the field is merely a way to make something close to being invisible by actually making it hard to notice deliberately.
- In the video game The Legend of Zelda: Majora's Mask, the "stone mask" item works similarly. It causes the wearer to be "as plain and uninteresting as a rock" and thus functionally invisible, causing enemies to ignore their presence. The soldier that gives Link the mask is invisible to the player without the use of the Lens of Truth.
- In the Star Trek universe, some ships of the Klingon Empire, Romulan Star Empire, as well as the Federation ship USS Defiant, are equipped with hardware known as cloaking devices that conceal them from most varieties of scans, including visual.
- Wonder Woman flys around in an invisible airplane, which also makes her invisible while she is in the vehicle.
- The popular Japanese sci-fi animation film Ghost in the Shell (1995) as well as its TV series Ghost in the Shell:Stand Alone Complex (2002) deals with invisibility of cyborg bodies using thermoptics. This is one of many cases of invisibility found in anime.
- In the popular 1998 video game StarCraft, units known as Dark Templar are able to bend light around themselves using psionic energies, effectively rendering themselves invisible as extremely effective stealth units. Observers, small reconnaisance drones, also have the same ability. Terran Ghosts and Wraiths carry cloaking devices, which drain energy. Also, Protoss Arbiters produce an invisibility field around them that makes every allied unit within the field invisible, but not the Arbiter itself nor other Arbiters. Invisible units can´t be attacked, but attentive players may be able to spot them by their transparent silhouette.
- In the Harry Potter series, an Invisiblity cloak allows the wearer to be literally invisible. However, they must cover themselves in order for the cloak to take effect.
- In Mystery Men, one of the characters boasts the ability to turn invisible so long as he is naked and no one is looking at him. Under this idea, he defeats Cassanova Frankenstein's defenses, ultimately leading to the group's final victory.
- In the new TV series Heroes, one of the secondary characters, Claude, possesses the power of invisibility, keeping it permanently on to escape detection by the Company.
- The Cheshire Cat from Alice's Adventures in Wonderland can turn invisible and leave certain parts of himself visible, most notably his smile.
- In the book Things Not Seen, a boy named Bobby is rendered invisible when radiation causes his electric blanket to malfunction.
- In the popular video game series, Metal Gear Solid, there exists a device which the user can wear that allows light to become distorted around the user and, hence, rendering him invisible. Such a device is called the Stealth Camoflauge and plays a mediocre role in the story but can be obtained via completing the game after fulfilling certain parameters. Visually, it makes a distorted image of the background so that the player can see his character while making the character, himself, completely invisible to the Non Playable Characters.
- In the book Masters of Solitude by Marvin Kaye and Parke Godwin there is a Girdle of Solitude which, when worn, renders the wearer invisible. There is an associated helmet with goggles without which the wearer is blind while invisible.
- In the video game Xenosaga, the character Ziggurat 8 (aka. "Ziggy") has the ability to become invisible, but he rarely uses it due to its tendency to malfunction at inopportune moments.
- In the video game Oddworld: Abe's Oddyssey the main character can become totally invisible even to the player by a bug in the PlayStation version in the first level by triangle jumping first on a mine, then through an electric forcefield. The only way as to "see" him is to use his psychic powers. However, the slightest fall will kill him.
- In the manga Translucent, a strange disease renders it's victims partially invisible (translucent), with no other side effects. Emotions affect the partial invisibility, with the ever present danger of slipping into permanent complete invisibility as a complication.
- In the computer game Crysis the player's character wears a nanosuit which has the ability to render the player invisible to enemies for a short time.
- In the television show Danny Phantom the main character and other ghosts in the series can make themselves, objects, and other people invisible.
- In the television show Spongebob Spongebob and Patrick purchase an invisibility can that when sprayed turns everything it touches invisible.
- The Japanese made for TV drama Invisible Girl Ea's titular character, Ea, is turned invisible before the show begins, and shifts uncontrollably between visible and invisible (with a emphasis on invisibility) throughout. Various things cause her invisibility to malfunction, both when it is on and off, including electricity, increased heartbeat, being touched or touching things, and bright flashes of light. Noteworthy due to the excessive nudity in the show -- Ea's invisibility powers malfunction whenever she touches clothing, preventing her from getting dressed. A major part of the show's plot is trying to undo Ea's invisibility powers so she can live a normal life.
- The Mipedian tribe in Perim in Chaotic consists of reptiloids with the ability to turn invisible.
- In the Team Fortress 2, a multiplayer video game, the Spy class can use his cloaking device to activate invisibility for short period of time.
- (Command & Conquer series) Nod stealth tanks, commandos, upgraded Avatar Mechs and some Black Hand soldiers use a cloaking device, the Lazarus Shield. This shield had a slight shimmer in the shape of a man or steath tank in Renegade, due to "imperfections." It was probably so one could not simply go invisible and destroy a base unhindered. It is a passive ability, and will only turn off when firing. Scrin Assimilator engineer analogs are invisible when stationary. The Disruption Tower, stealth generator, and mobile stealth generator cloak wide areas of terrain. Turrets, Pitbulls, Attack bikes, and standard infantrymen or buzzers can detect invisible units/structures. Col. Burton, Jarmen Kell, and Black Lotus (Generals hero characters) are invisible only when not attacking, bombing, or uploading viruses/hacking. American stealth fighters are invisible only when not attacking.
- In Act of War: Direct Action and High Treason, all sides use invisibility to some degree. TFT Comanche helicopters and GlobalHawk spy planes come standard with stealth capabilities. Transport rotorcraft can be upgraded to "stealth capability", and are invisible until attack or loading. United States Marine Corps. use stealth bombers. The Consortium has a Stealth Armor Lab, which fields Akula stealth MBT's, Fennek stealth scout cars, and Optic Camo Soldiers. All invisible units remain so until attacking, or in range of milimetric radar carrying units.
- The titular character in HG Wells novel, The Invisible Man, takes a chemical drug that causes his body to become the same refractive index as air. When he tested it on his pet cat, the only thing not affected was those little mirror-like cells in the back of its eyes.
- In the Halo series the Covenant soldiers known as Elites have a Invisibility cloak on their armor.