Limitations of Metamaterials and Cloaking

There has been some controversy surrounding some of the scientific concepts associated with metamaterials and cloaking. People have also questioned if an invisibility cloak is really a possibility. Several years ago, some scientists claimed that it was possible to make metamaterials with a negative index of refraction. Initially, many experts claimed that a negative index of refraction was against the laws of physics, but most now accept that it is possible. Even so, it had proven difficult to make negative refraction metamaterials for visible light (Experiments in negative refraction had been done with metamaterials affecting microwave light.) But this year scientists at Germany's Karlsruhe University and the Ames Laboratory in Iowa were able to produce metamaterials with a negative index of refraction for visible light.

However, there's still a lot of work to be done before a working cloak is developed for more than one wavelength of the visible spectrum, much less the sort seen in science-fiction movies. At the moment, making a device that works on all wavelengths of visible light is beyond scientists' capabilities. They also don't yet know if it's even possible to cloak multiple wavelengths simultaneously.

Duke cloaking device
Image courtesy Dr. David R. Smith
The Duke cloaking device only only masks an object from one wavelength of microwave light.

The problem comes from the copper used on metamaterials. The copper has to be smaller than the wavelength of light it's affecting. With microwaves, that's simple, since the microwaves used at Duke were slightly more than 3 centimeters long. That cloaking device's copper loops were about 3 millimeters. But visible light is 400 nanometers to 700 nanometers, thousands of times smaller than microwaves. Copper loops for those metamaterials would have to be about 40 nanometers to 70 nanometers long. Such metamaterials might benefit from future developments in nanotechnology.

While the Duke team's cloaking device clearly has its limitations, the potential for the technology and for metamaterials are tremendous. Dr. Smith has shied away from making grand pronouncements about when a more sophisticated cloaking device could be made, but here are some future possibilities that scientists have proposed:

  • Making a large building invisible so that the park on the other side can be seen
  • Improving the range of wireless devices by allowing waves to bend and flow around obstructing objects
  • Cloaked military vehicles and outposts
  • Eliminating shadows and reflections (from a military plane, for example)
  • Ultra-high capacity storage devices
  • Lenses that have no blurring effect, resulting in ultra-sharp images

If a full invisibility is decades off or simply impossible, one other possibility seems intriguing, and it's not unlike what we've seen in some movies. It may be possible in the future to create some sort of phasing cloaking device, in which each color of the spectrum of visible light is cloaked for a fraction of a second. If accomplished at sufficient speed, an object would likely appear translucent, though not quite invisible. Think of the alien villain in the "Predator" movies, who is barely perceptible when he moves but is otherwise essentially invisible.

Finally, there's one other factor that limits the uses of a cloaking device that scientists say many people don't consider. People inside a cloaked area wouldn't be able to see out because all visible light would be bending around where they are positioned. They'd be invisible, but they'd be blind, too.

For more information about invisibility cloaks and related topics, please check out the links on the next page.