How Invisibility Cloaks Work

First, let's try this carbon nanotube invisibility cloak on for size and experience the wonders of the mirage effect.

You're probably most familiar with mirages from tales of desert wanderers who glimpse a distant oasis, only to discover it was only a mirage -- no miraculous lake of drinking water, only more hot sand.

The hot sand is key to the mirage effect (or photothermal deflection), as the stiff temperature difference between sand and air bends, or refracts, light rays. The refraction swings the light rays up toward the viewer’s eyes instead of bouncing them off the surface. In the classic example of the desert mirage, this effect causes a "puddle" of sky to appear on the ground, which the logical (and thirsty) brain interprets as a pool of water. You've probably seen similar effects on hot roadway surfaces, with distant stretches of the road appearing to gleam with pooled water.

In 2011, researchers at the University of Texas at Dallas NanoTech Institute managed to capitalize on this effect. They used sheets of carbon nanotubes, sheets of carbon wrapped up into cylindrical tubes [source: Aliev et al.]. Each page is barely as thick as a single molecule, yet is as strong as steel because the carbon atoms in each tube are bonded incredibly tightly. These sheets are also excellent conductors of heat, making them ideal mirage-makers.

In the experiment, the researchers heated the sheets electrically, which transferred the heat to the surrounding area (a petri dish of water). As you can see from the photographs, this caused light to bend away from the carbon nanotube sheet, effectively cloaking anything behind it with invisibility.

Needless to say, there aren’t many places you'd want to wear a tiny, super-heated outfit that has to stay immersed in water, but the experiment demonstrates the potential for such materials. In time, the research may enable not only invisibility cloaks but also other light-bending devices -- all of them with a handy on/off switch.

Next, let's slip into an invisibility cloak made from metamaterials.

Metamaterials offer a more compelling vision of invisibility technology, without the need for multiple projectors and cameras. First conceptualized by Russian physicist Victor Veselago in 1967, these tiny, artificial structures are smaller than the wavelength of light (they have to be to divert them) and exhibit negative electromagnetic properties that affect how an object interacts with electromagnetic fields.

Natural materials all have a positive refractive index, and this dictates how light waves interact with them. Refractivity stems in part from chemical composition, but internal structure plays an even more important role. If we alter the structure of a material on a small enough scale, we can change the way they refract incoming waves -- even forcing a switch from positive to negative refraction.

Remember, images reach us via light waves. Sounds reaches us via sound waves. If you can channel these waves around an object, you can effectively hide it from view or sound. Imagine a small stream. If you stick a teabag full of red dye into the flowing water, its presence would be apparent downstream, thanks to the way it altered the water's hue, taste and smell. But what if you could divert the water around the teabag?

In 2006, Duke University's David Smith took an earlier theory posed by English theoretical physicist John Pendry and used it to create a metamaterial capable of distorting the flow of microwaves. Smith's metamaterial fabric consisted of concentric rings containing electronic microwave distorters. When activated, they steer frequency-specific microwaves around the central portion of the material.

Obviously humans don't see in the microwave spectrum, but the technology demonstrated that energy waves could be routed around an object. Imagine a cloak that can divert a third grader's straw-fired spitball, move it around the wearer and allow it to continue on the other side as if its trajectory had taken it, unopposed, straight through the person in the cloak. Now how much more of a stretch would it be to divert a rock? A bullet?

Smith's metamaterials proved the method. The recipe to invisibility lay in adapting it to different waves.

More on metamaterials next.

In 2007, the University of Maryland's Igor Smolyaninov led his team even farther down the road to invisibility. Incorporating earlier theories proposed by Purdue University's Vladimir Shaleav, Smolyaninov constructed a metamaterial capable of bending visible light around an object.

A mere 10 micrometers wide, the Purdue cloak uses concentric gold rings injected with polarized cyan light. These rings steer incoming light waves away from the hidden object, effectively making it invisible. Chinese physicists at Wuhan University have taken this concept into the audible range, proposing the creation of an acoustic invisibility cloak capable of diverting sound waves around an object.

For the time being, metamaterial invisibility cloaks are somewhat limited. They're not only small; they're limited to two dimensions -- hardly what you'd need to vanish into the scenery of a 3-D war zone. Plus, the resulting cloak would weigh more than even a full-grown wizard could hope to lug around. As a result, the technology might be better suited to applications such as hiding stationary buildings or vehicles, such as a tank.

Ready to slip into some old-school optical camouflage fashions?

This technology takes advantage of something called augmented-reality technology -- a type of technology first pioneered in the 1960s by Ivan Sutherland and his students at Harvard University and the University of Utah.

Optical camouflage delivers a similar experience to Harry Potter's invisibility cloak, but using it requires a slightly complicated arrangement. First, the person who wants to be invisible (let's call him Harry) dons a garment that resembles a hooded raincoat. The garment is made of a special material that we'll examine more closely in a moment.

Next, an observer (let's call him Professor Snape) stands before Harry at a specific location. At that location, instead of seeing Harry wearing a hooded raincoat, Snape sees right through the cloak, making Harry appear to be invisible. The above photograph shows you what Snape would see. And if Snape stepped to the side and viewed Harry from a slightly different location? Why, he'd simply see the boy wizard wearing a silver garment. Scowls and detentions would likely follow. Lucky for Harry, his fictional cloak affords 360-degree protection.

Optical camouflage doesn't work by way of magic. It works by taking advantage of something called augmented-reality technology -- a type of technology first pioneered in the 1960s by Ivan Sutherland and his students at Harvard University and the University of Utah. You can read more about augmented reality in How Augmented Reality Works, but a quick recap will be helpful here.

Augmented-reality systems add computer-generated information to a user's sensory perceptions. Imagine, for example, that you're walking down a city street. As you gaze at sites along the way, additional information appears to enhance and enrich your normal view. Perhaps it's the day's specials at a restaurant or the showtimes at a theater or the bus schedule at the station. What's critical to understand is that augmented reality is not the same as virtual reality. While virtual reality aims to replace the world, augmented reality merely tries to supplement it with additional, helpful content. Think of it as a heads-up display (HUD) for everyday life.

Most augmented-reality systems require a user to look through a special viewing apparatus to see a real-world scene enhanced with synthesized graphics. They also call for a powerful computer. Optical camouflage requires these things as well, but it also necessitates several other components. Here's everything needed to make a person appear invisible:

On the next page, we'll look at each of these components in greater detail.

All right, so you have your video camera, computer, projector, combiner and wondrous reflective raincoat. Just how does augmented-reality technology turn this odd shopping list into a recipe for invisibility?

First, let's take a closer look at the raincoat: It's made from retro-reflective material. This high-tech fabric is covered with thousands and thousands of small beads. When light strikes one of these beads, the light rays bounce back exactly in the same direction from which they came.

To understand why this is unique, look at how light reflects off other types of surfaces. A rough surface creates a diffused reflection because the incident (incoming) light rays scatters in many different directions. A perfectly smooth surface, like that of a mirror, creates what is known as a specular reflection -- a reflection in which incident light rays and reflected light rays form the exact same angle with the mirror surface.

In retro-reflection, the glass beads act like prisms, bending the light rays by refraction. This causes the reflected light rays to travel back along the same path as the incident light rays. The result: An observer situated at the light source receives more of the reflected light and therefore sees a brighter reflection.

Retro-reflective materials are actually quite common. Traffic signs, road markers and bicycle reflectors all take advantage of retro-reflection to be more visible to people driving at night. The movie screens found in most modern commercial theaters also take advantage of this material because it allows for high brilliance under dark conditions. In optical camouflage, the use of retro-reflective material is critical because it can be seen from far away and outside in bright sunlight -- two requirements for the illusion of invisibility.

For the rest of the setup, the video camera needs to be positioned behind the subject to capture the background. The computer takes the captured image from the video camera, calculates the appropriate perspective and transforms the captured image into the picture that will be projected onto the retro-reflective material.

The projector then shines the modified image on the garment, by shining a light beam through an opening controlled by a device called an iris diaphragm. This diaphragm is made of thin, opaque plates, and turning a ring changes the diameter of the central opening. For optical camouflage to work properly, this opening must be the size of a pinhole. Why? This ensures a larger depth of field so that the screen (in this case the cloak) can be located any distance from the projector.

Finally, the overall system requires a special mirror to both reflect the projected image toward the cloak and to let light rays bouncing off the cloak return to the user's eye. This special mirror is called a beam splitter, or a combiner -- a half-silvered mirror that both reflects light (the silvered half) and transmits light (the transparent half).

If properly positioned in front of the user's eye, the combiner allows the user to perceive both the image enhanced by the computer and light from the surrounding world. This is critical because the computer-generated image and the real-world scene must be integrated fully for the illusion of invisibility to seem realistic. The user has to look through a peephole in this mirror to see the augmented reality.

On the next page, we'll look at how this whole system comes together.

Now let's put all of these components together to see how the invisibility cloak appears to make a person transparent. The diagram below shows the typical arrangement of all the various devices and pieces of equipment.

Once a person puts on the cloak made with the retro-reflective material, here's the sequence of events:

The person wearing the cloak appears invisible because the background scene is being displayed onto the retro-reflective material. At the same time, light rays from the rest of the world are allowed to reach the user's eye, making it seem as if an invisible person exists in an otherwise normal-looking world.

The words "invisibility cloak" tends to summon images of fantastic adventure, magical espionage and otherworldly deception. The actual applications for optical camouflage, however, are far less out there. You can forget hiding your Romulan starship or hanging out in the lady wizards' dormitory, but that doesn't mean there aren't a number of viable uses for the technology.

For instance, 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 at the floor (which would display the view from the outside of the fuselage) Similarly, drivers wouldn't have to deal with mirrors and blind spots. Instead, they could just "look through" the entire rear of the vehicle. The technology even boasts potential applications in the medical field, as surgeons could use optical camouflage to see through their hands and instruments for an unobstructed view of the underlying tissue.

Interestingly enough, one possible application of this technology actually revolves around making objects more visible. The concept is called mutual telexistence and essentially involves projecting a remote user's appearance onto a robot coated in retro-reflective material. Say a surgeon were operating on a patient via remote control robotic surgery. Mutual telexistence would provide the human doctors assisting the procedure with the perception that they're working with another human instead of a machine.

Right now, mutual telexistence is science fiction, but scientists continue to push the boundaries of the technology. For example, pervasive gaming is already becoming a reality. Pervasive gaming extends gaming experiences out into the real world, whether on city streets or in remote wilderness. Players with mobile displays move through the world while sensors capture information about their environment, including their location. This information delivers a gaming experience that changes according to where users are and what they are doing.

Don't disappear on us. We have lots more links for you to explore next.