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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.
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 does not reflect 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.
Invisibility by environment
An object may be classified as "invisible" if it cannot be seen 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)
- Patterned so that its outline is hard to determine.
- In an environment which is too dark or too bright.
- Not in a particular observer's line-of-sight. (Especially when driving)
- Transparent. (air and many other gases)
Invisibility in physics
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 (e.g. a black hole)
- 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.
- So tiny that it cannot be detected by an observer.
Invisibility by technology
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 optical camouflage and has been used in many science fiction works.
- It has been speculated that an object could appear invisible if light were bent around it.
- Invisibility (lower visibility) for radar is called stealth technology.
- In some science fiction stories, a hypothetical "cloaking device" is used to make objects invisible.
- In filmmaking, people, objects, or backgrounds can be made to look invisible on camera through a process known as chroma keying.
- Theoretically, it is possible to make an object invisible, if the object has the same refractive index as the surrounding medium. (e.g. air)
- Recent theoretical work has been reported by the press as showing that metamaterials with a negative index of refraction could guide light around objects to make them "invisible". This characterization, however, neglects the optical dispersion of physically realizable metamaterials, which would refract different colors of light at different angles, and the optical absorption of known, metal-containing negative-index metamaterials, which would make a macroscopic cloaked object dark or opaque, rather than fully transparent. Scientists are working on this problem.
Invisibility by magic
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 mystical creatures can make themselves invisible at will, such as Chinese dragons in some tales, which can shrink so small that humans cannot see.
- 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.
Where magical invisibility is concerned, the issue may arise of whether or not 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.
Simultaneous invisibility and color
Since something that is invisible has no color 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 as presently understood, an 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 any light. 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 appears to be an inevitable drawback which would always offset the advantage of invisibility, even if a practical method of becoming invisible were discovered (unless one's intent was simply to hide and be still, letting the danger pass).
Nevertheless, 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.)
Examples in fiction
The idea of being unseen and hence undetectable has fascinated mankind for generations. This concept of invisibility has been explored in many myths, novels, movies 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 helmet 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.
- Modern fiction
- The Invisible Man (1897) by H. G. Wells is a well-known novel about invisibility, later made into a film and several TV series.
- 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. Such a suit may actually be possible (see External links below).
- 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 wizards like Doctor Strange who have invisibility spells in their possession.
- In the film Mystery Men (1999), the "Invisible Boy" is invisible when nobody is looking at him.
- In one episode of Invader Zim, Zim got an invisibility robot that had the ability to render itself invisible. There were two problems to this however. 1) The robot had to be kept on a constant power cord, restricting the distance it can travel while invisible and 2), only the robot was able to become invisible, not Zim, therefore the robot is useless.
- 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 also causes serious side effects.
- 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 Star Trek universe, some ships of the Klingon Empire and the Romulan Star Empire, as well as the Federation ship USS Defiant, are equipped with pieces of hardware known as cloaking devices that conceal them from most varieties of scans, including visual.
- Cloaking technology is also employed by large battleships in the game Colony Wars.
- In Metal Gear Solid, Otacon wore a stealth camouflage prototype while sneaking around the facility. Ninja also used a similar suit. The Fear used it in Snake Eater as well. It is also an unlockable item that the player can use to thwart his enemies.
- Blizzard Entertainment's strategy games have always had invisibility as a gameplay device. For example, in StarCraft, many units employ cloaking devices, while in Warcraft III, units employ a range of invisibility spells and camouflage at night time and in World of Warcraft some of the characters and classes have abilities which render them essentially invisible.
- In The Incredibles, one of the family members, Violet Parr (nicknamed Vi) has the ability to render herself invisible. The ability is for her body only. A suit is made for her that can also become invisible when she does.
- The short story "They Go Bump" by David Barr Kirtley is about a group of soldiers who wear invisibility suits. The main character has trouble walking (not being able to see his feet) and starts to wonder if his invisible companions are really who they say they are.
- The videogame, Psychonauts features invisibility as one of many psychic powers unlocked and made available to the player throughout the game. The power can be used to escape enemies, and it particularly useful during boss encounters.
- The computer game Zork Grand Inquisitor includes a very useful spell called Igram which turns purple things invisible. The game also features a character called the "six-armed invisible bridge guard" who apparently had Igram cast on him at one point — if the player tries to cast this spell on him, he replies, "How do you think I got like this in the first place?"
- In the 1980's animated series Thundercats, the Thundercat Tygra can turn himself invisible sometimes with the aid of his weapon, a 'Bolowhip', which he generally whipped around himself to turn invisible.
- In the book series Artemis Fowl, fairies can vibrate at a very high frequency, making them invisible to all creatures except rats and two species of monkey.
- In the Harry Potter book series by J.K. Rowling, Harry has a cloak that renders the wearer invisible.
- In the Sci Fi Channel Series The Invisible Man, Darien Fawkes can turn invisible with the help of a gland in his brain that secreets a liquid called Quicksilver that can bend light around an object/person making the user invisible to the naked eye.
- In the Space Patrol episode "Husky becomes invisible" - Professor Zellor discovers that his new star-measuring apparatus can make objects disappear.
- Certain characters in LEGO's Bionicle line can become invisible.
See also
External links
- Invisibility in the real world Interesting picture of a test tube's bottom half invisible in cooking oil.
- Info on "stealth suit"
- Some examples
- CNN article on an "actual" invisibility suit
- Note: the picture was taken by a special camera that displays images behind the wearer of the suit. The suit itself does not render invisibility. In essence, the suit functions as a bluescreen onto which the image behind the suit can be displayed through camera.
- Plasmonic invisibility effect Plasmons are electron configurations on a metallic surface that resonate with a specific frequency or photon wavelength of light. This effect has so far been observed only in gold foil perforated with holes smaller than the photons. The oversize photons pass through the foil anyway (reportedly with amplification), so a quantum-tunneling phenomenon may be associated with the light wave trapping.
- A monochromatic LED flashlight shined on a plasmonicly resonant surface would not allow that specific color of light to scatter back toward the light source. Plasmons act analogously to wave trap circuits familiar to builders of amateur radio equipment, except plasmons measure in Ångstrom-unit sizes.
- Theoretical method of scattering light (Nature.com subscription) that could make objects "invisible" to certain light frequencies. This is the source of the Livescience.com story on plasmons (above).
- Unfortunately, this could not be used to hide anything but a microscopic object from visible light because the object has to be either the same size as the wavelength of light from which it is hidden, or only somewhat larger.
- Brief piece on why visible light is visible
- Says how an invisibility cloak works
- Scientists Aim to Duplicate Harry Potter's Invisibility Cloak
- Invisibility shield
"Invisibility"