The shape of things to comeNow you see it, now you don’t

Published 20 October 2006

Blue Devils researchers develop a cloak which makes objects invisible to microwave beams; the cloak is based on a new design theory developed by Sir John Pendry of Imperial College London, and these principles may ultimately lead to the production of cloaks which confer invisibility within the visible frequency range

Scientists at Duke University’s Pratt School of Engineering have developed what can only be described as an “invisibility cloak.” The cloak deflects microwave beams so that they flow around a “concealed” object inside with little distortion, making it appear almost as if nothing were there at all. Cloaks which would make objects essentially invisible to microwaves would have a variety of wireless communications or radar applications, and would be of great help to military and antiterror units.

The researchers manufactured the cloak using metamaterials precisely arranged in a series of concentric circles which confer specific electromagnetic properties. Metamaterials are artificial composites that can be made to interact with electromagnetic waves in ways that natural materials cannot reproduce. The cloak represents “one of the most elaborate metamaterial structures yet designed and produced,” the scientists involved in the research said. It also represents the most comprehensive approach to invisibility yet realized, with the potential to hide objects of any size or material property.

There have been earlier efforts to achieve invisibility. We note two approaches: One approach relies on limiting the reflection of electromagnetic waves; the other approach attempted to create cloaks with electromagnetic properties which cancel the electromagnetic properties of the object meant to be hidden. David Smith, Augustine Scholar and professor of electrical and computer engineering at Duke, explains the different approach his team took: “By incorporating complex material properties, our cloak allows a concealed volume, plus the cloak, to appear to have properties similar to free space when viewed externally. The cloak reduces both an object’s reflection and its shadow, either of which would enable its detection.”

The team based its work on the cloak on a new design theory proposed by Sir John Pendry of Imperial College London, in collaboration with the Duke scientists (see the new theory explained in the 23 June 2006 issue of Science, cited below).

The principles behind the new design theory are mathematically too rigorous to be produced here, so we will note only the following three points:

—How it works: David Schurig, a research associate in Duke’s electrical and computer engineering department, explains: “One first imagines a distortion in space similar to what would occur when pushing a pointed object through a piece of cloth, distorting, but not breaking, any threads. In such a space, light or other electromagnetic waves would be confined to the warped ‘threads’ and therefore could not interact with, or ‘see,’ objects placed inside the resulting hole.”

—Why use metamaterials: The properties of natural materials are determined by their chemistry, but the properties of metamaterials depend instead on their physical structure. In the new cloak, for example, that structure consists of copper rings and wires patterned onto sheets of fiberglass composite that are traditionally used in computer circuit boards

—How soon will true invisibility be achieved: There is now a need to develop a three-dimensional cloak and perfect the cloaking effect. Yes, the same principles applied to the new microwave cloak may ultimately lead to the production of cloaks which confer invisibility within the visible frequency range, but achieving this goal is by no means certain. The reason: To make an object literally vanish, a cloak would have simultaneously to interact with all of the wavelengths, or colors, which make up light. That technology would require much more intricate and tiny metamaterial structures, which scientists have yet to devise

In addition to David Smith and David Schurig, the collaborators on the study included Steven Cummer, Jack Mock, and Bryan Justice of Duke; John Pendry of Imperial College London; and Anthony Starr of SensorMetrix in San Diego, California. Pendry’s research is supported by the United Kingdom’s Engineering and Physical Sciences Research Council. The research was funded by the Intelligence Community Postdoctoral Fellowship Program.

-read more in D. Schurig et al., “Metamaterial Electromagnetic Cloak at Microwave Frequencies,” Science (3 October 2006) (sub. req.); see also Adrian Cho, “Voilà! Cloak of Invisibility Unveiled,” Science 314. no. 5798 (20 October 2006): 403 (sub req.); Adrian Cho, “High-Tech Materials Could Render Objects Invisible,” Science 312. no. 5777 (26 May 2006): 1120 (sub. req.); J. B. Pendry et al., “Controlling Electromagnetic Fields,” Science 312. no. 5781 (23 June 2006):1780-82 (sub. req.); and this Technology News Daily report

This cloaking device is practically invisible — invisible, that is, if you see the world in microwaves with a wavelength of 3.5 centimeters

Credit: David Schurig /Science Magazine