Researchers claim to have developed the first mathematical model for creating invisibility simulations on a computer, but possible real-world applications -- say, a gadget that works like Harry Potter's cloak -- so far seem far-fetched.
In the May 1 issue of Optics Letters, a team of scientists published a paper outlining a "numeric simulation" for cloaking. The software program recreates a breakthrough 2006 experiment, run by David Smith and David Shurig of Duke University and John Pendry of the Imperial College London, showing that it's possible to cloak a simple cylinder from microwave radiation.
Video: The animation illustrates the response of a cluster of molecules as they approach a coated cylinder, which serves as an cloaking device under a new theory by Graeme Milton, a professor of mathematics at the University of Utah. "The size of everything is taken to be small compared with the wavelength. Although this is not necessary for our sort of cloaking, it makes the simulations easier," Milton explains. "The dashed line represents the boundary of the cloaking region, not a physical boundary. When the cluster is inside the cloaking region it only slightly disturbs the field outside the cloaking region: it becomes invisible."
The mathematical model could help scientists frame up new experiments and further push the edge of cloaking technology, experts said. "It's very valuable to have reliable simulation techniques and to check the conditions in the computer before you build them in the laboratory," said Ulf Leonhardt, a professor of theoretical physics at the University of St. Andrew's in Scotland, who was not an author on the simulation paper.
Magicians have used sleight-of-hand techniques for centuries to make objects seem to disappear, and some recent technology demonstrations have more or less achieved the same thing.
In 2003, Susumu Tachi, an engineering professor at the University of Tokyo, demonstrated a crude invisibility cloak using a shiny raincoat that serves as a movie screen, showing imagery from a video camera positioned behind the wearer. From the right angle and under controlled circumstances, it makes a sort of ghost of the wearer.
Such invisibility feats have generally involved more gimmickry than science, and have little to do with the more ambitious recent laboratory efforts that try to interfere with the behavior of light itself on an object.
Smith, Shurig and Pendry demonstrated that by encasing a cylinder in a particular "metamaterial" -- a synthetic substance that can bend light in a way that's found nowhere in nature -- they could render it impervious to microwave radiation. When the microwave light passed over the encased cylinder, it would be as if the cylinder were not there, making it "invisible."
André Nicolet, a researcher at the University of Marseille, and a co-author on the Optics Letters paper, said that while his team's work has helped advance the field of cloaking research, significant hurdles in the field remain. Namely, it seems impossible to have a perfect cloak at all frequencies of visible light, much less all frequencies of light.
"Perfect invisibility is almost impossible," he said.
Gunther Uhlmann, a professor of mathematics at the University of Washington, explained that in order for perfect invisibility to work for all types of light, the light's phase velocity -- its propagation speed through space -- would have to be faster than that of light at all frequencies.
In other words, for a perfect cloak, the light would have to reach the other side of an object along a curved path in the same time it would if the object were not there. And of course, when the object isn't there, the light travels at the speed of light. So in order to maintain the same travel time, that light would have to go faster than the speed of light, which is impossible according to the modern understanding of physics.
"It (would) contradict physics as we know it today," he said.
Further, metamaterials are constructed only for specific frequencies, and it's mathematically impossible to construct them such that they work for all frequencies, explained Graeme Milton, a mathematics professor at the University of Utah.
Despite this setback, Milton has come up with a theory for an alternative cloaking technique, which was published this month in the journal Optics Express.
It outlines a technique called "cloaking by anomalous resonance." Milton compared his team's technique to that of noise-cancelling technology, where one wave lessens the effect of another.
According to his theory, light comes in to the center object (the theory assumes only particles, or small collections of molecules), scatters off the molecules and then interacts with the cloaking device that sits a certain distance away.
The cloaking device can be made of different possible materials, including silver or metamaterials, and can also be constituted in different shapes as well.
Then, he theorizes, the cloaking device kicks into high gear.
"The cloaking (device) will send back (waves), which will cancel the incoming light," he said. "So the (center) molecules no longer feel any electrical field, and so they don't respond."
Milton said his theory has yet to be tested in the laboratory. Still, with so many ideas floating around, various scientists working on cloaking are excited about its future prospects.
"It's imaginative," Leonhardt said. "It's research where the real breakthrough comes from imaginative ideas, and you really need to come up with good visualizations and good pictures, that's what I like about it. I can tell my children what I'm doing -- they understand that invisibility is a cool subject."
