|Posted by George Freund on March 11, 2014 at 9:05 AM|
by William Harris and Robert Lamb
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 boy wizards 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. The technology is far from perfect, but if you'll step into our high-tech boutique of vanishing apparel, we'll guide you through your invisibility cloak options.
First up, we'll look at some wonderful carbon nanotube fashions -- fresh from the UTD NanoTech Institute fall 2011 collection. This new technology is inspired by the same natural phenomena responsible for desert mirages. Heated via electrical stimulation, the sharp temperature gradient between the cloak and the surrounding area causes a steep temperature gradient that bends light away from the wearer. The catch: Wearers must love water and be able to fit inside a petri dish.
Or perhaps you'd prefer something made from metamaterials. These tiny structures are smaller than the wavelength of light. If properly constructed, they guide rays of light around an object -- much like a rock diverting water in a stream. For now, however, the technology only works in two dimensions and only comes in the ultrapetite size of 10 micrometers across.
If you're more into retro fashion, there's also the optical camouflage technology developed by scientists at the University of Tokyo. This approach works on the same principles of the blue screen used by TV weather forecasters and Hollywood filmmakers. If you want people to see through you, then why not just film what's behind you and project it onto your body? If you travel with an entourage of videographers, this may be the cloak for you.
Ready to try some of these fashions on for size?
The Mirage Effect: Carbon Nanotubes
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.
Here we see the multi-walled carbon nanotube (MWCT) switch from inactive to active, vanishing from sight in the process.
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.
This optical image shows the University of Maryland metamaterials in action, steering light waves away from the each central circle. The arrows indicate the direction of the light waves.
Metamaterials: Bending Light Waves
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.
Metamaterials: Invisible Tanks
What an enemy might see of a tank using the new technology
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.
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