An example of how an object could appear to be invisible through the use of mirrors

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.^[1]

[edit] 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 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.^[2]
• In filmmaking, people, objects, or backgrounds can be made to look invisible on camera through a process known as chroma keying.
• An artificially made meta material that is invisible on the microwave light spectrum.

[edit] Practical efforts

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,^[3][4] 2006 theoretical work predicts that the imperfections need not be serious, and metamaterials may make real-life "cloaking devices" practical.^[5][6] 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 which the water passes around it, but leaves 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.^[7] 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. 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."
• UC Berkeley 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.^[8]
• 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 at Duke University in the US put the idea into practice.^[9][10]

[edit] By magic

Alberich puts on the Tarnhelm and vanishes; illustration by Arthur Rackham to Richard Wagner's Das Rheingold

In fiction, 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.

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.

[edit] 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 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, in 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.) An earlier example can be found in the Traveller role-playing game, in which starships equipped with black globe generators are afforded resistance to physical detection and attack at the cost of being blind. The generators are set to 'flicker' at a pre-set frequency, permitting the ship's sensors to penetrate the globe but at the cost of momentary vulnerability.

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 reasons). 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.

[edit] In mythology

People have attributed invisibility to things that are mythical, things that do not exist and 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.[1] In medieval astronomy, the crystal spheres[2] 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. In religion, gods, goddesses, angels and demons 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, however it has been theorised that a being such as God would exist in dimensions not perceptible to 3-dimensional beings. For example, a 2-dimensional being would only be able to see things in 2 dimensions, length and bredth. Anything outside of these two dimensions, i.e. height, is "invisible" to them. Additionally, Hades, the Greek god of the underworld, possessed a helmet that made the wearer invisible.

[edit] In reality

Some small sea animals (e.g. fish fry) are very transparent, and thus underwater nearly invisible.

[edit] See also

• Qualitative invisibility
• Concealment
• Cloaking device
• Active camouflage
• Invisibility in fiction
• Somebody Else's Problem

[edit] External links

• Light Fantastic: Flirting With Invisibility
• Invisibility in the real world Interesting picture of a test tube's bottom half invisible in cooking oil.
• Info on "stealth suit"
• Metamaterial Technology for Invisibility
• Radar Invisibility
• 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
• Reuters about Ulf Leonhart bending light to flow around an object, making it appear invisible
• Invisibility Cloak One Step Closer - BBC News

[edit] References