Among the many tropes found in science fiction and fantasy, few are more popular than the cloaking device. In the real world, scientists have long engaged in research that would at least improve camouflaging technology, conceal aircraft from radar or further our knowledge of how light and electromagnetic waves work. In 2006, a group of scientists from Duke University demonstrated a simplified cloaking device. In October 2006, a research team from Duke, led by Dr. David R. Smith, published a study in the journal "Science" describing a simplified cloaking device. While their device only masked an object from one wavelength of microwave light, it does provide more information that will help us to consider if a real-life cloaking device is possible.
This cloaking device was made from a group of concentric circles with a cylinder in the middle, where an object could be placed. When researchers directed microwave light at the device, the wave split, flowing around the device and rejoining on the other side. Dave Schurig, a researcher on Dr. Smith's team, compared the effect to "river water flowing around a smooth rock" [Source: Duke University]. Anything placed inside the cylinder is cloaked, or effectively invisible to the microwave light.
The device isn't perfect. It creates some distortion and "shadowing of the microwaves" [Source: New York Times]. It also works for only one wavelength of microwave light.
To achieve their cloaking effect, the Duke team used a relatively new class of materials called metamaterials. The properties of metamaterials are based on their structure rather than their chemistry. For the cloaking device, researchers made mosaic-like constructions out of fiberglass sheets stamped with loops of wire, somewhat similar to a circuit board. The arrangement of the copper wires determines the way it interacts with electromagnetic fields. The unique advantage of metamaterials is that they can be used to create objects with electromagnetic characteristics that can't be found in the natural world.
The key to the cloaking device is taking advantage of a concept known as the index of refraction. An object's index of refraction, or refractive index, determines how much light bends when passing through it. Most objects have a uniform index of refraction throughout, so light only bends when it crosses the boundary into the material. This occurs, for example, when light passes from air into water.
If a material's index of refraction is greater than 1, it causes light to bend inward. Here are some refractive indices for common materials:
- Air - 1.0029
- Ice - 1.31
- Water - 1.33
- Glass - 1.52
- Sapphire - 1.77
- Diamond - 2.417
Metamaterials are used to make objects with refractive indices between zero and 1. The Duke team used metamaterials to make their cloaking device have gradually varying refractive indices -- from 1 on the outside of the device, decreasing to zero in the center. The result is that microwave light subtly bends around the device and is able to reform on the other side, albeit with some detectable distortion.
While metamaterials and cloaking are exciting technologies, they have many limitations. Let's go over some of those on the next page.
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.
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.
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More Great Links
- Chang, Kenneth. "Flirting With Invisibility." New York Times. June 12, 2007. http://www.nytimes.com/2007/06/12/science/12invis.html?ex=1182657600&en=278c566bdab95caf&ei=5070
- Glausiusz, Josie. "How to Build an Invisibility Cloak." DISCOVER Magazine. Nov. 20, 2006. http://discovermagazine.com/2006/nov/building-invisibility-cloak
- Smith, David R. "David R. Smith's Metamaterials and Negative Index Page." The Research Group of David R. Smith. Duke University. http://www.ee.duke.edu/~drsmith/neg_ref_home.htm
- "First Demonstration of a Working Invisibility Cloak." Duke University. Oct. 19, 2006. http://www.dukenews.duke.edu/2006/10/cloakdemo.html
- "Index of Refraction." HyperPhysics. Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/tables/indrf.html
- "The Electromagnetic Spectrum." Department of Physics and Astronomy. University of Tennessee. http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html
- "Theoretical Blueprint for Invisibility Cloak Reported." Duke University. May 25, 2006. http://www.dukenews.duke.edu/2006/05/cloaking.html