European researchers pursue bending of light to cloak large objects.
In April 2007, a group of computer scientists from Purdue University debuted a rudimentary “optical cloaking” design that would, if constructed, render objects invisible from a distance. Now, a group of European mathematicians has created a model proving that submarines, airplanes, and other large objects can be “cloaked” even at close range. The breakthrough heralds an important step forward in science’s bold march toward the unseeable future.
“A cloak, such as the one worn by the Harry Potter character, is not yet possible, but it is a good example of what we are trying to move towards,” says Sébastien Guenneau of the University of Liverpool. “Using this new computer model we can prove that light can bend around an object under a cloak and is not diffracted by the object. This happens because the metamaterial that makes up the cloak stretches the metrics of space in a similar way to what heavy planets and stars do for the metrics of space-time in Einstein’s general relativity theory.”
While an invisibility cloak may sound like an exotic, far-off concept, the physics of invisibility play out in nature quite commonly, such as when a desert horizon seems to vanish, or when shallow water distorts the position of objects beneath it. This occurs because rays of light-composed of photons-bend depending on the atomic properties of the objects they are encountering.
The Purdue University engineers’ “cloak” design uses a layered, cylindrical arrangement of nano-needles radiating outward from a spoke, resembling a round hairbrush, to bend light around the object being cloaked. The design is effective only for one wavelength, so the object would only be invisible from far away, and only if it remained perfectly still.
What Guenneau, along with Frédéric Zolla and André Nicolet from the University of Marseille, have proven is that, even with singlewavelength invisibility, objects at close range can disappear.
“Until now, it was not clear whether photons-particles that make up all forms of light-can split and form new waves when the light source is close to the object. If we use ray optic techniques-where light travels in beams-photons break down at close range and the object does not appear invisible. If we study light as it travels in waves, however, invisibility is maintained,” says Guenneau.
The breakthrough will not lead to an actual invisibility cloak for a person any time soon, he says, because metamaterials-the building block of any invisibility device-by their nature only work for one wavelength. “So, no worries, the invisibility cloak is not for tomorrow or the day after, but in the very far future-more than a decade,” Guenneau predicts.
While limited in application, the single-wavelength cloaking technology could still make soldiers invisible to night-vision goggles, which operate at just one wavelength. Vladimir Shalaev of the Purdue team expressed hope that such technology could one day hide objects with a fixed shape, like submarines or airplanes. Guenneau is more interested in nonmilitary uses of cloaking technology. “In terms of civil applications, we can imagine scientists observing animals in the wild without being seen,” he says. -Patrick Tucker
How Does Invisibility Work?
What makes invisibility possible? Specially designed materials “trick” light waves into bending when they want to break.
Normally, light waves come to a halt when they encounter a solid object. Natural materials tend to have a refractive index (molecular composition) greater than one, which means that light neither bends around them nor passes through them and occasionally, as in the case of shin metal, bounces back.
Clear glass has a refractive index that allows light to pass through it relatively unscathed (at least to the naked eye). Blurred glass that’s more dense in some parts than others, like natural crystal, has an atomic structure that bends light rays as the rays pass through. A long expanse of hot, thin air-such as the kind that sits above the surface of the desert-has its own unique refractive index, which causes light rays passing over it-and the particles of light making up the light ray-to bend as though they were passing through warped glass. Depending on your line of sight, the desert horizon seems to disappear or reflect the sky like a pool of water-a mirage.
The images at right from two simulations performed at Purdue University show how objects might be “cloaked” to render them invisible. In the top image, the uncloaked object interrupts the light wave like a branch sticking out of a lake. Because the light neither bends around nor goes through the object, the object is perfectly visible. You see it and not what’s behind it. In the second illustration, the light rays are bending around the object. The object’s molecular composition hasn’t changed, but the viewer sees what’s behind the object rather than the object itself.
For more on the physics of invisibility, go to the Web page of Ulf Leonhardt, professor in theoretical physics at the University of St. Andrews in the United Kingdom: http://www.st-andrews.ac.uk/~ulf/invisibility.html.
Originally published in THE FUTURIST, September-October 2007.