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How Invisibility Cloaks Work

The Mirage Effect: Carbon Nanotubes
Here we see the multi-walled carbon nanotube (MWCT) switch from inactive to active, vanishing from sight in the process.
Here we see the multi-walled carbon nanotube (MWCT) switch from inactive to active, vanishing from sight in the process.

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.

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