How Does an Invisibility Cloak Work?
Admit it. You'd love to own an invisibility cloak. Utter an embarrassing faux pas at a party? Just throw on your magical garment and vanish from the snooty gaze of your fellow partygoers. Want to hear what your boss is really saying about you? Stroll right into his or her office and get the goods.
Such fantastic fashion accessories have become ridiculously standard in the world of science fiction and fantasy. Everyone, from Harry Potter and his bestie Ron Weasley to intergalactic safari hunters, has at least one invisible blouse in their wardrobe, but what about us poor saps in the real world?
Well, muggles, science has some good news for you: Invisibility cloaks are a reality. While the technology is far from perfect and won't provide actual invisibility like a true invisibility cloak, we'll guide you through your invisibility cloak options.
First, let's try this carbon nanotube invisibility cloak on for size and experience the wonders of the mirage effect.
The Mirage Effect: Carbon Nanotubes
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.
Experimentation and Potential
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). 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 thermal camouflage jacket 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.
The Concept of Metamaterials
Next, let's slip into an invisibility cloak made from metamaterials.
Using Metamaterials to Bend Light Waves
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.
Refractivity and Wave Interaction
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?
Metamaterial Fabric and Energy Waves
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.
The Smallest Frontier
Metamaterials, a creation of science, don't occur naturally. In order to create the minute structures required to redirect electromagnetic waves, scientists employ nanotechnology.
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.
Augmented Reality vs. Virtual Reality
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.
Components of Optical Camouflage
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:
a garment made from highly reflective material
a digital video camera
a computer
a projector
a special, half-silvered mirror called a combiner
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