Hazing: How to hide in nearly plain sight

 

A new invisibility cloak can hide objects in semi-plain sight — sometimes. Unlike earlier cloaking devices, this one can conceal things from light of any color and coming from any direction. But that flexibility comes at a price: This cloak only works under hazy conditions, such as in fog, in a cloud or when viewed through frosted glass.

“It's a nice demonstration,” Jason Valentine told Science News. A mechanical engineer at Vanderbilt University in Nashville, Tenn., he did not work on the cloak. The new system “sacrifices functionality in one area to gain functionality in another,” he noted. He meant the cloak can work with all kinds of light but not in every situation.

A perfect invisibility cloak may forever be a figment of human imagination. “Employing a cloak in the Harry Potter–type fashion is not doable now and maybe not doable ever,” Valentine said.
Like many ideas, the basic plan behind most invisibility cloaks is easier to describe than to build. People can only see things that emit or reflect light. An invisibility cloak does neither of those things. The cloak guides light around a concealed object, like water moving around a rock in a stream. On the “downstream” side, the light resumes its journey, uninterrupted.

But there's a problem: Light takes more time to move around an object than to move directly past it. As a result, the redirected light can't catch up to the light that moves in a straight line. A person would be able to see that something had changed.

Robert Schittny at the Karlsruhe Institute of Technology in Germany came up with a device that avoids that snag. This physicist and his colleagues thought hiding an object in haze might help. That's because light can't zip straight through foggy or cloudy places. Instead, it moves more like a pinball. It bounces off tiny particles — like water — suspended in the air.

The German scientists hid objects inside a cloak that started with cylindrical and spherical shells made from stainless steel. They painted the shells white and covered them with a coating that contained tiny particles of a hard plastic called melamine (MEL-ah-meen) resin. The scientists then placed the shells in a tank of water mixed with particles of white paint. The paint particles acted as the fog.

When they shined white light through the tank, the shells cloaked themselves by guiding the light around their surfaces. From the other side of the tank, the shells were barely visible. Without the cloak, the shadowy outline of the hidden objects was easy to pick out. The researchers describe their cloaking system in the June 5 issue of Science. 

This technology cloak may not be ready for prime time quite yet. Still, Schittny believes it might one day prove useful. One simple example: It might hide the wires used to reinforce some types of frosted glass.