Back in 2006 Harry Potter was all the rage in the engineering world. That year a team at Duke University built the first rudimentary device for hiding objects, akin to the boy wizard’s invisibility cloak. But in technology as in the movies, Harry Potter is now old news. Over the past six years, scientists have moved beyond mere invisibility: If they could build cloaks for light waves, then why not design materials to conceal sound and even ocean waves?
A whole suite of invisibility cloaks are now under development, all building on the same basic principle as the first prototype. When we perceive an object, we are actually detecting the disturbances it creates as energy waves bounce off it. The Duke cloak, constructed from a synthetic structure called a metamaterial, prevented those disturbances by bending light waves around the object, allowing them to continue flowing like water in a stream around a rock (concept shown at right). Sure enough, that technology is not limited to light. In the latest designs it is being applied to mask all kinds of other waves, with the potential for zeroing out sound pollution and protecting cities from earthquakes. Meanwhile, scientists continue to pursue the original invisibility concept—work that is sparking a lot of interest in military surveillance circles.
1 VISIBLE-LIGHT CLOAK
The Tech: A group of physicists led by Tolga Ergin and Joachim Fischer at the Karlsruhe Institute of Technology in Germany built a light-bending fabric last year that—for the first time—rendered a cloaked object invisible to the human eye from any viewing angle.
What It's Made of: A rigid synthetic polymer composed of tiny rods spaced about 350 nanometers (billionths of a meter) apart, a gap small enough to manipulate waves of visible light.
How it Works: As a test, researchers laid the cloak over a flat surface with a small bump in the middle. The cloak bent incoming light rays around the bump and bounced them back as if they had struck a flat surface. Observers would never know the bump existed.
Applications: For now, this cloak can hide only small imperfections on flat surfaces. But eventually, scientists hope to scale it up to conceal much larger objects anywhere in space. The U.S. Defense Advanced Research Projects Agency (Darpa) started investing in metamaterials way back in 2001, and while it doesn’t like to reveal specific intentions, the agency would certainly be interested in cloaks that conceal soldiers and military equipment.
2 SOUND CLOAK
The Tech: Last year a Duke University team led by engineer Steven Cummer built a cloak that rendered an object “invisible” to sound waves.
What It's Made of: Stacked sheets of one-millimeter-thick perforated plastic (the actual engineering of these cloaks is difficult but unglamorous). The sheets’ holes and arrangement allow the cloak to manipulate sound waves.
How It Works: It hides an object much like Ergin’s light cloak does. Cummer placed the perforated sheets over a 10-centimeter-long block of wood. The cloak bent sound waves heading toward the block so that they avoided the cloaked area and rebounded as if it were not there. If the block had ears, it would not have heard any sound from outside the cloak.
Applications: Sonic cloaks could steer sound waves around beams and columns in a concert hall to give every seat perfect acoustics, or block the noise pollution from that chatty coworker in a neighboring cubicle. Such cloaks could also conceal submarines from the pulses of enemy sonar, although Cummer considers that a major challenge—he cannot just slap thick layers of plastic onto a military sub.