December 19, 2006 3:36pm CST
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. * A recent breakthrough (2006) at Imperial College London has shown that invisibility is possible by using specifically patterened 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 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. * 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) * 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 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.) Invisibility of nonexistent things People have attributed invisibility to things that do not exist in order to explain why they're 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 and brownies have also been described as invisible or able to become invisible. Currently, many entities or phenomena whose existence is disputed, such as ghosts, demons, qi, and auras, are also ascribed invisibility. Real-World Application While an invisibility cloak is an interesting application of optical camouflage, it's probably not the most useful one. Here are some practical ways the technology might be applied: * Pilots landing a plane could use this technology to make cockpit floors transparent. This would enable them to see the runway and the landing gear simply by glancing down. * Doctors performing surgery could use optical camouflage to see through their hands and instruments to the underlying tissue. See Tachi Lab: Optical Camouflage: oc-phantom.mpg to watch a video of how this might work. * Providing a view of the outside in windowless rooms is one of the more fanciful applications of the technology, but one that might improve the psychological well-being of people in such environments. * Drivers backing up cars could benefit one day from optical camouflage. A quick glance backward through a transparent rear hatch or tailgate would make it easy to know when to stop. One of the most promising applications of this technology, however, has less to do with making objects invisible and more about making them visible. The concept is called mutual telexistence: working and perceiving with the feeling that you are in several places at once.