Don't Forget Your Invisibility Cloak

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In the world of fiction, making something invisible is a useful device for propelling the story line.  However, in the real world of physical objects, it is a near impossibility.  Any material that reflects or absorbs light will be visible to the human eye.  But what happens if the light never reaches the eye?  This creates the illusion that something is not there.  An illusion of invisibility is somewhat different from something actually being invisible, and yet the end result is the same.  There are many efforts under way to prevent electromagnetic radiation from bouncing off of objects.  Light is a form of electromagnetic radiation that allows us to see the world around us.  Radar is another form.  It allows us to keep track of airplanes and missiles that we can't see. 

Now, researchers at the University of Rochester, University College London, Helsinki University of Technology, and the University of Washington have published a paper in the journal SIAM Review1 that shows how to get light to bend around objects so that they never reflect the light in the first place.  This approach involves metamaterials that are designed to channel light in unprecedented ways — not so much because of what the materials are made of but because of their structure.  In order for them to be effective, they, too, must not reflect the light.  Otherwise, it would simply be like throwing a blanket over a chair and saying that it was an invisibility cloak because you couldn't see the chair. 

Designing these exotic metamaterials takes some challenging mathematics.  The differential equations that are used in manipulating electromagnetic waves are tricky — and the required qualities of the materials are even trickier.  If you take the classic equations for electromagnetic radiation, developed by James Clerk Maxwell in the 1800s, and plug in the parameters needed to make the materials perform as desired, you wind up with so-called singularities; the equations give nonsensical results, such as a material that has no boundaries. 

However, the team of researchers found a mathematical way to avoid that problem and to generate what they term a "wormhole" that creates an invisible tunnel in space between two points, through which light can pass undetected. 

While these metamaterials are still at the theoretical stage, other techniques for cloaking are already in the works.

A group from the Birck Nanotechnology Center at Purdue University recently published a report in Nature2 showing another approach to metamaterials.  They created a film with holes in it about 100 nanometers wide; then they layered silver and aluminum oxide on it...

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