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Admit it, you really want to have an invisibility cloak. If you say something amazing at a party, you can just wear this magic dress and disappear from the sarcastic eyes of your party companions. If you still want to hear what the boss really thinks of you? Go straight into his or her office and you'll get the answer.

In the world of science fiction and fantasy, this wonderful fashion accessory has become standard. From Harry Potter and his buddy Ron Weasley to Star Trek hunters, each of them has at least one invisibility blouse in their wardrobe, but what about us ordinary people in the real world?

The photo is provided by ©Muggles at the Taiji Lab of the University of Tokyo, and the scientific community has brought good news: the invisibility cloak is already available. Although this technology is far from perfect and will not provide the real invisibility effect like a real invisibility cloak, we will teach you how to choose an invisibility cloak.

Mirage effect in the picture, we can see that after the multi-walled carbon nanotubes (MWCT) changed from inactive to active, these multi-walled carbon nanotubes disappeared from view.

Photo Source: Aliev An et al. 2011 Nanotechnology first, let's try whether this carbon nanotube invisibility cloak fits and experience the wonders of the mirage effect.

Mirage effect: carbon nanotubes you may be most familiar with mirages. Legend has it that a wanderer in the desert glimpses a distant oasis only to find that it is a mirage-there is no magical lake to drink, only a lot of hot sand.

Hot sand is a key cause of the mirage effect (or photothermal deflection) because the high temperature difference between sand and air bends, or refracts, light. Refraction causes light to shoot upward into the viewer's eyes instead of bouncing back from the surface.

In the classic example of a desert mirage, this effect is a "half-acre square pond" on the ground, and you can see the "sky hovering" inside, and your logical (and thirsty) brain will recognize it as a pool of water. You may have seen a similar effect on a hot road, where stagnant water seems to shine in the distance.

Experiments and potential opportunities in 2011, researchers at the Institute of Nanotechnology at the University of Texas at Dallas successfully exploited this effect. They use carbon nanotube sheets, which are wrapped into cylindrical tubular carbon sheets. Each piece is only as thick as a molecule in diameter, but it is as strong as steel because the carbon atoms in each carbon nanotube are very closely bound together. These sheets are also good thermal conductors and are ideal materials for making mirages.

In the experiment, the researchers electrically heated a thin sheet of carbon nanotubes, which transferred heat to the surrounding area (a petri dish containing water). This allows the direction of light to bend at the carbon nanotube, effectively making anything behind the carbon nanotube invisible.

Obviously, you don't want to wear it in many places: an ultra-hot miniature hot camouflage coat that must be soaked in water, but this experiment proves the potential of this material. It is believed that over time, this study will not only achieve the effect of the invisibility cloak, but also other light bending devices-all of which have a convenient switch.

The concept of metamaterials next, let's put on an invisibility cloak made of metamaterials.

Using metamaterials to bend light-wave metamaterials provides a more eye-catching stealth technology without the need for multiple projectors and cameras. Russian physicist Viktor Viserago (Victor Veselago) first proposed the concept of metamaterials in 1967. These tiny man-made structures are smaller than the wavelength of light (they must be so that light waves can be turned) and exhibit negative electromagnetic properties, thus affecting the interaction between objects and electromagnetic fields.

The interaction between refractive index and wave natural materials have positive refractive index, which determines how light interacts with them. The refractive index is partly affected by chemical composition, but the internal structure of the substance plays a more important role. If we change the structure of the material, we can change the way the material refracts the incident light-even from positive refractive index to negative refractive index.

Remember, images are transmitted to us through light. Sound travels to us through sound waves. If you can guide these waves around the object, you can effectively hide the object so that it can not be seen or heard.

Imagine there's a stream here. If you put a tea bag full of red dye into the running water, people downstream will find it because it changes the color, taste and smell of the water. But what if the water can be allowed to flow around the tea bag?

Metamaterial fabrics and energy waves in 2006, David Smith of Duke University created a metamaterial that distorts microwave flow using an earlier theory put forward by the British theoretical physicist John Pendry. The metamaterial fabric developed by Smith consists of concentric rings containing electronic microwave twisters. When activated, they direct microwaves of a specific frequency to the central area of the material.

Obviously, microwaves cannot be seen by humans, but this technology proves that waves can bypass objects. Imagine that a cloak can deflect a third-grader's spit-soaked paper ball from its original direction, bypass the person wearing the cloak, and keep it flying on the other side of the person, just as its trajectory can go straight through the cloak without hindrance.

The metamaterial developed by Smith proves this method. The secret of invisibility is to adapt it to different waves.

The smallest boundary metamaterial is a product of scientific creation that does not exist in nature. Scientists have used nanotechnology to create tiny structures that change the direction of electromagnetic waves.

Metamaterials: this optical image of an invisible tank shows the role of metamaterials developed by the University of Maryland, which guides light waves away from each central circle. The arrow indicates the direction of the light wave.

The image is provided by the Department of Electronic and computer Engineering at the University of Maryland. In 2007, Igor Smolyaninov of the University of Maryland led his team to go further on the invisible road. Smolyaninov has developed a metamaterial that bends visible light around objects, combining the earlier theory of Vladimir Shalev of Purdue University.

Purdue University's cloak is only 10 microns wide and uses concentric gold rings injected with polarized blue light. These rings can guide incoming light waves away from hidden objects, effectively making them invisible. Physicists at Wuhan University applied this principle to the field of sound and proposed an acoustic invisibility cloak that can deflect sound waves around objects.

At present, the metamaterial invisibility cloak still has some limitations. They are not only small, but also limited to a two-dimensional environment-it's hard to make you disappear in a three-dimensional environment.

In addition, the cloak is so heavy that even an adult wizard is reluctant to drag it. As a result, this technology may be more suitable for hiding vehicles such as fixed buildings or tanks.

Optical camouflage: changing reality optical camouflage will not make you invisible to multi-eyed monsters, or even to stray cats and squirrels.

Photo courtesy of ©Taiji Lab, University of Tokyo, are you ready to put on the old optical camouflage? The technology uses augmented reality (AR), a technology pioneered by Ivan Sutherland (Ivan Sutherland) and his students at Harvard and the University of Utah in the 1960s.

The experience of optical camouflage is similar to that of Harry Potter's invisibility cloak, but requires a slightly more complicated arrangement. First of all, the person who wants to be invisible (we call him Harry) wears a dress similar to a hooded raincoat. This dress is made of a special material and will be studied carefully later.

Next, an observer (call him Professor Snape for the time being) stood in a specific position in front of Harry. At that place, Snape did not see Harry in a hooded raincoat, as if looking back through his cloak, making Harry look invisible.

The photo above shows what Snape will see. What if Snape stepped aside and looked at Harry from a slightly different position? It was the wizard in silver that he saw. It may be followed by glaring eyes and detention. Harry is lucky that his cloak in the novel provides him with all-round protection.

Augmented reality and virtual reality augmented reality systems add computer-generated information to the user's senses. For example, imagine you are walking on the streets of a city. When you look at the scenic spots along the way, supplementary information will appear in front of you to enhance and enrich what you see from a normal perspective. Maybe it's the special price of the day in the restaurant, the performance time of the theatre or the bus schedule of the station.

It is important to understand that augmented reality technology is different from virtual reality technology. Virtual reality aims to replace the real world, while augmented reality only tries to supplement the real world with additional and useful content. Think of it as a HUD in daily life.

Part of optical camouflage most augmented reality systems require users to use a special viewing device to observe real-world scenes enhanced with synthetic graphics. The system also includes a powerful computer. Optical camouflage also requires these things, but it also requires several other components. Here's everything you need to make people invisible:

Clothes made of high reflective material: a digital camera, a computer, a projector, a special semi-silver mirror called a combination mirror.

Optical camouflage: the invisibility cloak is made up, with cameras, computers, projectors, combiners and magical reflective raincoats. How does augmented reality turn this strange shopping list into an invisible recipe?

Optical camouflage takes advantage of AR technology to play a role. Understand how it works and find out the components of the invisibility cloak. Antireflective material first of all, take a closer look at this raincoat: it is made of antireflective material. This high-tech fabric is covered with thousands of small beads. When the light hits one of the beads, it bounces back in the direction in which it came.

To understand this unique principle, look at how light is reflected from other types of surfaces. Rough surfaces produce diffuse reflections because incident light is scattered in many different directions. A completely smooth surface, such as a mirror, produces so-called specular reflection-incident light and reflected light form exactly the same angle as the mirror.

Light reflection and reverse reflection in reverse reflection, glass beads are like prisms, bending light by refraction. This causes the reflected light to return along the same path as the incident light. The result: the observer at the light source receives more reflected light, thus seeing a brighter reflection.

Retroreflective materials are actually very common. Traffic signs, road signs and bicycle reflectors all use reverse reflection to make it easier for people who drive at night to see it. Film screens in most modern commercial cinemas also use this material because it produces high brightness in dark conditions.

In optical camouflage, the use of antireflective materials is important because it can be seen from a distance or in strong outdoor light-two necessary conditions for people to create the illusion of invisibility.

As the picture shows, this experience is very similar to walking directly in front of a movie projection screen, except that the background is real. AP Photo / SHIZUO KAMBAYASHI in the rest of the settings, the camera needs to be placed behind the subject to capture the background. The computer captures the image from the camera, calculates the appropriate perspective, and converts the captured image into a picture that should be projected onto the antireflective material.

The projector and combiner then project the modified image onto the clothing through an opening controlled by an aperture.

The diaphragm is made of opaque plates, and the diameter of the central opening can be changed by turning a ring. For optical camouflage to work properly, the opening must be the size of a pinhole. Why is it such a big opening? This ensures a greater depth of field so that the screen (in this case, the cloak) is kept at any distance from the projector.

Finally, the whole system needs a special mirror that reflects the projected image onto the cloak and allows the light bouncing off the cloak to return to the user's eyes. This special mirror is called a splitter or combination mirror-a semi-silver-coated mirror that reflects light (half silver) and transmits light (half transparent).

Integrating computer-generated images if the combined mirror is correctly placed in front of the user, the user can perceive both the computer-enhanced image and the light from the surrounding world.

This is crucial because only when computer-generated images are fully integrated with real-world scenes can invisibility hallucinations look real. Users must see augmented reality through a peephole in the mirror.

Now, put all these components together and see how the invisibility cloak makes people look transparent. The figure shows the typical arrangement of all kinds of devices and devices.

A way to make people look transparent when a person wears a cloak made of retro-reflective material, the order of events is as follows:

Digital cameras capture the scene behind a man in a cloak.

The computer processes the captured image and makes the necessary calculations to adjust the still image or video to make it look realistic when projected.

The projector receives the enhanced image from the computer and projects the image onto the combined mirror through an opening the size of a pinhole.

The silver-plated half mirror is completely reflective, reflecting the projected image onto the person wearing the cloak.

The cloak is like a movie screen, reflecting light directly back to the light source, which is the mirror.

The light reflected by the cloak passes through the transparent part of the mirror and falls on the user's eyes. Keep in mind that the light reflected by the cloak comes from the actual scene image behind the person wearing the cloak.

The person in the cloak looks invisible because the background scene is displayed on the retro-reflective material. At the same time, light from other places can also shine into the user's eyes, giving the impression that there is an invisible person in a seemingly normal world.

Optical camouflage: the word "invisibility cloak" in the real world is often reminiscent of dreamy adventures, magical espionage and otherworldly deception. However, the practical application of optical camouflage is far less exaggerated. Don't fantasize about wandering around in your Lowulun spaceship, but that doesn't mean the technology doesn't have some viable uses.

Although the invisibility cloak is an interesting application of optical camouflage, it may not be the most useful. Applications in aviation and driving, for example, pilots can use this technology to make the cockpit floor transparent when landing an aircraft. In this way, they only need to look down at the floor (which shows the outside of the fuselage) to see the runway and landing gear.

Similarly, drivers do not have to deal with rearview mirrors and blind spots. Instead, they just need to "see through" the whole back of the car. The technique may even be used in the medical field, because surgeons can use optical camouflage to "see through" their hands and devices, giving them a good view of the tissue below.

What is interesting about simultaneous interpretation is that one possible application of this technique actually revolves around improving the visibility of objects. The concept, known as mutual telexistence, is to project the appearance of a remote user onto a robot coated with retro-reflective material.

Suppose a surgeon operates on a patient by remotely controlling a robot. Passing on technology to each other makes human doctors who assist in surgery feel that they are working with a living person rather than a machine.

At present, mutual transmission is still an idea, but scientists are still promoting the development of this technology. For example, casual games have become a reality. Casual games extend the game experience to the real world, whether on city streets or in remote wilderness (such as Pok é mon Go). Players move around the world through a mobile display, while sensors capture information about their environment, including their location. This information provides a different gaming experience depending on where the user is and what the user is doing.

Author: William Harris & Robert Lamb

Translation: sweeping monk

Revision: there are interests in the future

Original link: How Invisibility Cloaks Work | HowStuffWorks

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: William & Robert

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