Is it possible to make a real invisibility cloak?
We have been reporting on invisibility cloaks since starry-eyed theorists managed to show that it was mathematically possible to design a structure that prevented electromagnetic waves from penetrating a central spherical volume. We were all very impressed until we realized that it only worked for one color of light, and only then if you used materials that didn’t exist. Nevertheless, even an imperfect form of invisibility is quite useful—ask the USAF if they want to give up the stealth bomber—so this seems worth pursuing. It is natural to ask if invisibility can be extended to more than a single color. The answer, provided by a special issue of the New Journal of Physics, is yes, but then again no. The researchers took a long hard look at the theory behind cloaking with passive materials and discovered that there are fundamental issues that limit the range of invisibility cloaks. An invisibility cloak for a single frequency is based on the structured material, which requires that the
