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This 'invisibility cloak' could conceal satellites — or hide your kids

Paul and James Howell demonstrate how mirrors can be used to create a simple invisibility cloak.



Scientists have been slaving over the development of exotic materials that can serve as invisibility cloaks for small objects in specific wavelengths, but father-and-son researchers are showing off a type of invisibility that's done with simple sets of mirrors, lenses or tanks of water.

In a paper submitted to the American Journal of Physics, University of Rochester physicist John Howell and his 14-year-old son, J. Benjamin Howell, say such cloaking devices can conceal high-flying satellites. Or Harry Potter. For real.

MIT Technology Review's Physics ArXiv Blog calls the approach "head-slappingly simple." There are caveats, however. These methods are basically funhouse-mirror tricks, which rely on precise placement of the apparatus to make objects disappear when seen from a specific vantage point. Howell & Howell admit as much. "Invisibility with mirrors has been done and are YouTube hits," they say. "The point we wish to emphasize is not the novelty but the ease of scaling to nearly arbitrary size."


You can watch an example in videos above and below, in which one boy's body appears to be missing below the head, even when another boy walks around behind him. The trick is that two sets of mirrors have been placed in such a way as to reflect the scene behind the "cloaking region" in which the seemingly disembodied kid is standing. (The actors in the movie are all John Howell's sons — Paul and James above, Benjamin and Isaac below.)

The cloaking effect isn't perfect. A wider-angle view would show you the retro-reflecting mirror, standing off to the side. Another major drawback is the fact that the effect is uni-directional: If you move around to a different position, the effect is spoiled.

Wanna see it again? Benjamin and Isaac Howell show how the mirror-based cloaking device works.

John Howell / Univ. of Rochester

This is the setup for a "cloaking device" that can hide a child inside the angle created by the mirrors at right.

The uni-directional drawback also applies to two other methods laid out by the Howells. In one, L-shaped tanks of water are used to refract light around an object in the middle. In another, Fresnel lenses reroute light around the "invisible" object. The effect is way cool when you see it from the correct angle. These images show how the setups can hide a toy copter in front of a toy truck:

John Howell / Univ. of Rochester

L-shaped water tanks have been arranged to show the top of a toy helicopter, while making the bottom of the helicopter "invisible." The bottom half of a toy truck sitting behind the helicopter remains visible, however.

John Howell / Univ. of Rochester

This shows how water tanks are arranged to hide the copter in front while revealing the truck behind it.

John Howell / University of Rochester

An arrangement of Fresnel lenses can also conceal the helicopter in the foreground while showing the toy truck in the background - as long as you view the scene from the correct angle.

Andrea Alu, a researcher at the University of Texas at Austin who is working on more exotic cloaking devices, says he's familiar with these effects. "Overall, I believe that this is interesting engineering work, as a smart composition of mirrors or materials may realize an optical trick that hides an object for certain observers," he told NBC News in an email. "I don’t necessarily find the concept scientifically too exciting. It is more of an optical engineering challenge."

Alu stressed that his efforts are focused on metamaterials that would suppress the overall scattering of light from an object, independent of the illumination or the observer's position. "Still, this and other solutions may have some merit in specific applications, in which we know the illumination position and the object does not move," he said.

Mirrors, lenses and water tanks might make for a cool set of invisibility art installations, for example, or for some more serious cloaking devices. "The devices may have value, for example, in cloaking satellites in mid- to high-Earth orbit, or for any low field-of-view cloaking," the Howells wrote.

John Howell provided a more down-to-Earth example: A police car could theoretically be hidden within a cloaking device, some distance from the side of the highway. The setup could be arranged so that the patrol officer is "invisible" while your speed is being clocked. The apparatus would become visible as you came closer, of course. But by then, "it'd be too late," Howell told NBC News.

In the future, these funhouse tricks just might lead to optical invisibility cloaks that would keep Harry Potter (or the Highway Patrol) hidden from observers looking around from any vantage point. "A cloak with spherical symmetry (much like retro-reflecting spheres achieve multi-directional reflection) may achieve this end," the Howells suggest.

For other approaches to seeming invisibility, check out these videos:

Mercedes-Benz put a camera and LED displays on an "invisible" car to highlight its hydrogen fuel-cell technology. For more about the project, check out http://livetothehype.tumblr.com/

This Halloween costume uses two iPads to make it look as if you have a huge hole in your torso.

Researchers in Japan show off a retro-reflective invisibility cloak.

More about invisibility:


Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.