Is it possible to make a cloaking device?
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.