An undergraduate at the University of St. Andrews has overcome a major hurdle in the development of invisibility cloaks — namely, the ability to move while cloaked — by adding an optical device into the cloak’s design that not only remains invisible itself, but also has the ability to slow light. The optical device, known as an "invisible sphere," could open up the possibility for a potential invisibility cloak wearer to move around amongst ever-changing backgrounds of a variety of colors. The research is published in the Institute of Physics and German Physical Society's New Journal of Physics. Under the guidance of professor Ulf Leonhardt, Janos Perczel, a student at the University of St. Andrews, acknowledged the huge potential of the invisible sphere and was able to fine-tune it so that it was a suitable background for cloaking. The usual approach to designing an invisibility cloak is based on using highly specific materials to bend light around the object to be concealed, thereby preventing the light from hitting the object and revealing its presence to the eye of the observer. When the light is bent, it engulfs the object, much as water covering a rock in a riverbed does, and continues on its path, making it seem as if nothing is there. Light, however, can be accelerated only to a speed faster than it would travel in space under certain conditions, and this restricts invisibility cloaks to a limited part of the spectrum — essentially just one color. This limitation would be suitable if somebody were planning to stand still in camouflage; however, the moment that the person starts to move, the scenery will begin to distort, revealing the person under the cloak. By slowing all of the light down with an invisible sphere, it does not need to be accelerated to such high speeds and can therefore work in all parts of the spectrum. "I started to work on the problem of superluminal propagation as professor Leonhardt's summer student with an EPSRC grant. Once the idea was present, I worked for over eight months to overcome the technical barriers and to make the proposal practicable," said Perczel. An Institute of Physics spokesperson said, "This new development opens up further possibilities for the design of a practical invisibility cloak — overcoming the problem of light speed that other advances have struggled to address and, very impressively, this significant advance was achieved by an undergraduate student." For more information, visit: www.iop.org