invisibility

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Invisibility is the state in which an object , substance or radiation is imperceptible to the human or animal eye . Invisibility in the narrower sense is about physical environmental conditions under which a normally visible object can no longer be recognized by humans.

meaning

Invisibility is of relatively great importance in many areas. For predators and prey, i.e. for living beings in both the animal and plant kingdoms, it is an important factor in order to be successful even in terms of species conservation.

For humans, the topic is important in the fields of espionage, the military, astronomy, behavioral science, philosophy and the arts. Instigated by these areas of interest, physics, chemistry and other disciplines deal with it. In general, the understanding of scenarios in which an environmental state is momentarily imperceptible also brings an understanding of where the limits of this perception gap are to be found and how these limits can be circumvented. On the one hand, there is still an interest in bringing about such a state of relative invisibility in a targeted manner and, on the other hand, being able to specifically expose correspondingly invisible things.

Simple examples

The following applies to people and their perception:

  1. Light is invisible when it is not in the eye of a beholder.
  2. Electromagnetic waves with wavelengths other than light are invisible even if they fall into the human eye.
  3. Colorless gases , such as air, are invisible due to their low interaction with electromagnetic waves of light.
  4. Fast moving objects, such as B. pistol bullets are invisible due to their high speed .
  5. Small objects, such as B. microorganisms are invisible due to their small size.
  6. Objects that are far away, such as B. galaxies , are invisible because of their great distance and also because of their low apparent brightness.

Examples 5 and 6 for invisibility are based primarily on the resolution of the human eye of around one millimeter in size at a distance of 3.5 m (angle of one minute of arc ).

Transparent objects that are embedded in liquids with a similar refractive index become invisible. When used in immersion microscopy , the glass surfaces are covered with oil for this purpose in order to reduce the interface reflection. In nature there are many transparent creatures in the water, where they are largely invisible. If you take these creatures out of the water, then they are clearly recognizable because the refractive index of the air differs significantly from that of the water.

Another cause of invisibility is insufficient brightness , such as at night (see also example 6). Even in an environment that is filled with a light-scattering fog or smoke, objects are invisible if they are outside the then reduced field of vision.

Concealment

If this object is behind another object, then it is indeed invisible to the observer, but you can then see the other object in front of it. An intelligent observer who is present in the scenario may therefore want to inspect the areas that are currently hidden from him sooner or later, if the covering object gives him any reason to do so. There is the possibility that it is only covered from one direction, from several or even from all. The invisibility is not brought about by a property of the object, but rather by the properties of the covering object. For example, a tin can covers its contents in all directions. If you paint this can black and place it in front of an equally black background, the can and its contents become invisible. The can was merely tampered with, while the contents were not touched.

Indirect perception

In order to make an object invisible, it is often not enough to prevent light from reaching the observer from this object. Even if this object is completely black and so no light can reach the observer, it still covers part of the background . If this is not also completely black, the object can be perceived through its contour.

If an object has exactly the same color and brightness as its surroundings and background, it is still invisible.

Typical real backgrounds are structured and their properties change over time, for example through the perspective of a moving observer or depending on the time of day and the incidence of light. A military camouflage suit or a camouflage paint tries to represent a certain scenario, e.g. B. forest, desert or stone landscapes to recreate in its appearance. A more or less cursory observer is less likely to be able to separate the contour of the respective carrier from the impression of the surroundings. This probability decreases when the object and background move towards each other and the observer is able to identify independent movement fields from the scenario.

Some representatives of the molluscs have mastered camouflage methods better than humans. Octopuses can simulate the look of the background better than any human camouflage suit. The jelly squid is almost transparent. The gun squid has luminous organs on the underside that can outshine its dark silhouette in front of the water surface.

Technical and physical concepts for invisibility

Invisibility with photonic crystals

Refraction-reflection-refraction

A theoretically conceivable possibility of all- round invisibility could be based on refraction - reflection - refraction . A hollow sphere made of photonic crystals ( metamaterials ) and materials with a negative refractive index (left-handed materials) with the usual exotic refraction and reflection behavior could divert the light coming from all spatial directions in an orderly manner. The hollow sphere made of photonic crystals should have the same optical properties from all spatial directions, and the light rays could cross one another unhindered, because the photons belong to the bosons . Instead of the hollow sphere, a hollow cylinder with a vertical axis could be used for the first experiments , which would then be invisible from all horizontal viewing directions.

Continuously varying index of refraction

In October 2006, scientists succeeded for the first time in producing a magic hat. Microwaves were guided around a ring that consisted of several layers of copper wire and glass fiber foils so that it did not reflect the radiation and appeared partially invisible in the corresponding electromagnetic area. However, light could not yet be deflected. So far, the process only works on one level and not with three-dimensional objects.

In 2010, researchers succeeded in creating a flexible polymer film, a kind of metamaterial that also deflects visible wavelengths around 620 nm.

Invisibility with mirrors

Much easier than the all-round invisibility, the invisibility for only one viewing direction can be credibly realized, at least for a human observer. The light rays from a certain field of view are guided around the object. However, since the distance changes slightly due to the redirection, a perfect identity cannot be achieved. Certain remnants of the mirror edges will also always remain recognizable. The observation point is flexible within certain limits.

Planar mirror arrangement

In this application it is advantageous if the mirrors are mirrored on the front , similar to those of overhead projectors . This avoids annoying multiple reflections on the glass surface . An even number of mirrors is always required to display the image correctly (this rule only applies to plane mirrors).

The problem with being invisible with mirrors is that you have to arrange the mirrors in such a way that they can hardly be seen themselves. When using four plane mirrors like a double periscope, one plane mirror can still be seen from the rear. When using three plane mirrors, the image of the background is upside down.

  • Invisibility device, animation

  • Invisibility with mirrors, principle

  • Optimization of the 4-mirror system

  • Invisibility device, three views

With four plane mirrors you can zigzag the image of the background around the object to be made invisible , creating two invisible zones. The space required by a 4-mirror system is lowest when the two inclined mirrors (shown on the left and right in the drawings) are inclined by 30 degrees to the light rays. The two auxiliary mirrors (shown above and below in the drawings) must always be aligned parallel to the light rays and should also be as thin as possible. Further information can be found in the image description of the optimization of the 4-mirror system.

Parabolic mirror arrangement

With two parabolic cylinder mirrors and a narrow plane mirror, the image of the background can be directed around an object, so that it appears to disappear from at least one direction. Large parabolic cylinder mirrors are relatively expensive, however, and the overall system is highly dependent on the viewing angle. The plane mirror lying in the focal line of the two parabolic cylinder mirrors, and also its two supports, should be as thin as possible.

  • Invisibility with parabolic cylinder mirrors

  • Invisibility with cylinder lenses

  • Invisibility with cylinder lenses

  • Invisibility with Fresnel cylinder lenses

Invisibility with lenses

Since large lenses are required to make them invisible , Fresnel lenses can be used to save weight . A cylinder lens consists of partial surfaces of a cylinder jacket and focuses the light on a focal line. In this focal line there is a narrow plane mirror that reflects the light to a second Fresnel cylinder lens . To avoid two separate invisible zones, two halved Fresnel cylinder lenses are used.

In contrast to other methods of invisibility, this method is relatively space-saving. All components represent the walls of a cuboid , and half of the interior is invisible. Here, too, it is true that only the background of the object can be seen from a certain perspective, but not the object itself.

To the technical realization: The light coming from the rear picture goes through the focal line below the front picture. A small part of the front picture can be seen at the top of the picture. This is a consequence of the spherical (here: cylindrical) aberration . This could be corrected by cropping the lenses.

Invisibility through gravity

Influence of gravity on light

Strong gravitational fields , such as those found near neutron stars and black holes , can deflect the light so strongly that the celestial body causing it becomes invisible. One cannot generate such strong gravitational fields on earth because one would have to compress huge masses to an extremely high density.

If this were to succeed in the distant future, a very dangerous small black hole would have been created due to its strong gravitational effect and its Hawking radiation . The gravitational field of our sun also changes the apparent positions of neighboring stars in a harmless way (see drawing “Influence of gravity on light”). So it could be exaggerated to say that these stars are invisible in their real place. This also applies to other phenomena in astronomy, e.g. B. by aberration and astronomical refraction .

Invisibility with micro camera projectors

Invisibility by reproducing the background image

Transferring the background image to the front of an object makes it invisible. This is very easy with a stationary background and only one position of the observer. For example, in 2016 the photographer JR made the glass pyramid in the inner courtyard of the Louvre seemingly disappear with a trompe-l'œil .

The suggestion was also made that a flat screen with high luminance should be set up in front of the object to be made invisible , and a color video camera behind the object , which transmits the image of the background to the screen. It is clear that in bright sunshine this screen would have big problems with luminance.

With more differentiated background images, there is also the question of the viewing angle and whether a wide-angle or a telephoto lens would be more beneficial for the camera. A high-resolution screen could also be coated with directionally selective micro-ball lenses, but this would result in higher costs. The aim of such a measure is to provide every viewing angle with the appropriate background image. One example that has already been implemented are those stereo images that are generated with micro-cylinder lenses .

Invisibility with camera projectors

Invisibility with micro cameras, which are also micro projectors :

The twelve blue circles represent the micro camera projectors, but in reality a few hundred thousand of them are needed. The black lines on the outside represent the rays of light. The green lines on the inside represent the assignment of the image signals, but by no means a wire mesh that connects the camera projectors.

The micro camera projectors should have plano-convex lenses that are flat outward and curved inward, because outwardly curved lenses would interfere with each other when the light falls.

This technology cannot be implemented at the moment, but let's try to calculate it anyway: The resolution of the human eye is 3 mm at a distance of 10 m, and our system should work at distances greater than 10 m. It follows that the micro camera projectors may be 3 mm in size. The object to be made invisible should initially be a square with an edge length of 90 cm. It follows that 900/3 = 300 camera projectors are required per image line, and a total of 300 x 300 = 90,000 camera projectors must be used. Within the camera projectors, 300 CCD - LED - pixels have a total of 3 mm space, so they can be 3/300 = 0.01 mm in size, but must be three-colored. This means that there must be six areas on an area of ​​0.01 mm × 0.01 mm, three CCD sensors and three LED light areas, each for red, green and blue. This can already be produced without any problems with today's chip technology, such as is used in digital cameras.

So that the light from the LED luminous surfaces cannot reach and disturb the CCD sensors, separating surfaces could be established between them, but this would not be a good method for reasons of space, because of light diffraction, light scattering, and interface reflection. A temporal separation would be better. 1/20 second = 50 ms (milliseconds) (however, this has the disadvantage that the object reproduces its own shadows, which you cannot see if the LEDs are sufficiently intense, when you look through the object at them.) 50 ms = 20 ms CCD active + 5 ms pause + 20 ms LED active + 5 ms pause, and again from the beginning. If, for technical reasons, you do not want to perform a temporal separation, but want to work continuously, then you could arrange the micro cameras and the micro projectors next to each other in separate micro housings. A hemispherical shape would be ideal for the imaging plane of the camera projectors. However, semiconductor chips consist of monocrystals that are only suitable for flat surfaces. As a compromise, one could choose the shape of a cube that is halved parallel to two of its opposite faces.

A big problem is the luminance . Each pixel in charge of a parallel light beam diameter of 3 mm. This means that the luminance of each pixel must be 90,000 times as high as the luminance in the light beam. When facing the sun, the CCD sensors will likely be damaged and the LED light surfaces will not be able to prevent shadows from being cast in the sun.

In the distant future, photonics could solve these problems . Optical fibers already exist today which can actively amplify the incoming light through the laser effect. Then you only have the small problem of how to stuff 8,100,000,000 light guide fibers around one meter long into the interior with a diameter of 90 cm. The correct connection of these optical fibers should be done by microrobots. In principle, the green wiring diagram inside the picture above applies here. With all this, you shouldn't forget that you also need space for the power supply and the passenger, otherwise the device will only make itself invisible.

Invisibility through new metamaterials at the quantum level

The American company HyperStealth Biotechnology Corp. has been working on her Quantum Stealth Technology for several years and secretly developed a Quantum Stealth Suit for the US military. This should allow military special forces not only to operate covertly, but almost invisibly. This is to be made possible by a new material that is currently being researched at the University of Pennsylvania. The idea behind it is to make materials interact with light, much like atoms do. However, this happens on a much smaller level so that the artificial structures are smaller than the light waves themselves. As a result, the optical properties should no longer be as limited as is the case with constitutive materials. Digitizing these metamaterials could be used to reproduce the light exactly on the other side. One advantage of such a metamaterial is that light can not only be directed and reflected by magnifying glasses and mirrors, but can also be stretched, stretched, distorted and manipulated in other ways.

Culture and philosophy

Phenomena that are invisible to the human eye are abstract concepts such as feelings and thoughts , e.g. B. infinity , so things that you do not or only through metaphors a pictorial imagination can do.

In Greek mythology , gods could walk invisibly on the earth, as could spirits . In Germanic legends, the dwarf kings Alberich and Laurin make themselves invisible with the help of an invisible cap .

The philosopher Plato distinguished between the outer surface of things and their abstract nature , the ideas .

The abstract painting tries invisible abstract, z. B. to visualize mental structures; the concept art working with the invisible meanings behind the visual surfaces.

Invisibility in literature and film

Invisibility has always been an attractive topic in sci-fi and fantasy culture. The best known here is HG Wells' novel The Invisible One , which often serves as the basis for other processing of the subject. The Invisible Man was a popular television series starring an invisible hero. Invisible Girl is a member of the superhero group Die Fantastischen Vier .

In the film Mission: Impossible - Phantom Protocol , the protagonists use a high-tech movable wall onto which an image is projected in order to make the space behind the wall invisible to a single person standing in front of it. Behind the partition there is a camera that records the image to be projected. The direction of view of the person in front of the wall is recorded and the projection is adjusted accordingly in order to ensure a spatially correct representation.

Stories about invisible people often also address their moral decline, such as that of the ring bearer in The Lord of the Rings by JRR Tolkien . In 1996 a British children's series called Invisible was released .

See also

Web links

Wiktionary: Invisibility  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Deflected radiation spiegel.de, October 19, 2006, accessed on November 12, 2010
  2. ↑ A step closer to the invisibility cap welt.de, November 5, 2010; Researchers develop stealth technology in visible light handelsblatt.com, November 5, 2010, accessed on November 12, 2010
  3. ↑ Andrea Di Falco, et al .: Flexible metamaterials at visible wavelengths. New Journal of Physics; 12 (2010), 113006, doi : 10.1088 / 1367-2630 / 12/11/113006 .
  4. ↑ Alexander Strahl: The mirror cabinet of the invisibility. Physics teaching: physics in fictional media. Issue 120 Dec. 2010. pp. 43-44 2010, accessed on August 6, 2013
  5. ↑ Cristian Della Giovampaola, Nader Engheta: Digital metamaterials. In: Nature Materials. 13, 2014, pp. 1115-1121, doi : 10.1038 / nmat4082 .