Nov 23 2008
Two physicists may have found an easier way to make things invisible. Cloaking devices, which are a form of advanced stealth technology, are used to render spacecraft invisible in Star Trek. So far all methods for invisibility require exotic materials and would only work at certain colours of light.
An idea, conceived by Professor Ulf Leonhardt of National University of Singapore and the University of St Andrews, United Kingdom, and Associate Professor Tomas Tyc of Masaryk University, Czech Republic, may change all this. Their paper will soon be published in the latest issue of Science Express, an internationally acclaimed journal.
The idea of invisibility has fascinated people for millennia and has been an inspiration or ingredient of myths, novels and films, from the Greek legend of Perseus versus Medusa to H.G. Well's Invisible Man and J.K. Rowling's Harry Potter. In 2006 invisibility turned from a device of fiction into a subject of science. Theoretical ideas for invisibility devices were published by Prof Ulf Leonhardt and by Sir John Pendry's group in Science and a first demonstration of a prototype rapidly followed.
First prototype cloaking device
The prototype, built by Professor David Smith and his group at Duke University, is a cloaking device for microwaves, the electromagnetic waves used in mobile phones, wireless technology and radar. The device is made of a metamaterial that contains cleverly designed metal structures that are smaller than the wavelength, a few centimeters for microwaves. Light is an electromagnetic wave as well, but of much shorter wavelengths around 500 nanometers. There one could use the tools of nanotechnology to miniaturize the cloaking structures for light.
The cloaking device would guide light around any hidden object without disturbing the image of the scenery behind it; light would smoothly flow around the object like water. The material that achieves this feat conjures up a transformation of flat space that makes light rays appear to be curved. However, for flowing around the object, light needs to travel faster than normal and even skip the inner lining of the cloak at infinite speed, which is only possible for single colours of light, but not for all. So for seeing things disappear, everyone has to wear tinted glasses and agree on their colour in advance, which obviously is not of much practical use.
New idea: Creating a curved space for light to bend naturally
To make invisibility practical takes a new theoretical idea, an idea that comes from Einstein's theory of curved space. In curved space, light may naturally go around in circles or curl around an object to be concealed. Ulf Leonhardt and Tomas Tyc worked out an idea for bending the rules of cloaking in curved space that appears in this week's Science Express. As curved space naturally bends light, cloaking is a lot easier there and can occur for all colours of light, whereas in the previous method space is flat and cloaking monochromatic.
Creating a curved space for light is not as outlandish as it sounds - it happens all the time when light is bent, causing optical illusions such as a mirage in the desert. Leonhardt and Tyc's design is partly inspired by the Invisible Woman of the Fantastic Four. In the film she appears to use some mysterious force field to curve space around her. The scientists propose to replace the fictitious field by suitably designed optical metamaterials or perhaps liquid crystals.
About the Authors of the Paper
Prof Ulf Leonhardt has been listed by Scientific American among the 50 top policy, business and research leaders in 2006. Since 2008 he has been holding a Royal Society Wolfson Research Merit Award.
Assoc Prof Tomas Tyc is a theoretical physicist, but he also gives science shows with plenty of experimental demonstrations for the general public. At his university he lectures a course called "Interesting Physics" in front of a packed audience at seven o'clock in the morning.