It looks easy enough in Harry Potter and Star Trek. But making things invisible actually extraordinarily challenging. Still, scientists continue trying — and they have lately made some large steps toward invisbility.
It’s no problem for Harry Potter. Child’s play for Star Trek’s Starship Enterprise. But as fascinated as humanity has long been about the possibility of becoming invisible, reality has proven a difficult barrier to breach for would-be inventors.
As it turns out, it is extraordinarily difficult to bend electromagnetic radiation — otherwise known as light — around objects or even people. Yet that’s exactly what must happen in order to produce the effect of invisibility. If light could be made to flow around a body the way water does around a rock — instead of being reflected as usually happens — then that object, or person, could be rendered invisible.
So much for the theory. In practice, though, all attempts so far to create a cloak of invisibility have come up against the fact that no naturally occurring materials bend light in this way. They all simply reflect.
Enter meta-materials. These materials are artificial composites which possess exceptional properties due to their exotic structure. Tiny structures on their surface, smaller than the wavelength of the light striking them, can, if arranged correctly, cause light to be diverted.
So far, most meta-material experiments have been conducted with microwave radiation — its longer wavelengths allow for correspondingly large structures on the material’s surface. The process is more difficult with visible light, which has much shorter wavelengths.
But there is also a further, much more fundamental problem — one for which a German and a Czech scientist now claim to have found a theoretical solution. When light is refracted, by water or by a lens for example, objects appear to be somewhere other than where they really are. A classic example is the effect of attempting to catch a fish in an aquarium: the fish looks like it’s in one place, but always turns out to be somewhere slightly different. Since light is refracted at the interface between air and water, the point viewed — in this case the fish — has been displaced in optical space.
Experimental invisibility devices so far have followed a similar principle, only in a more extreme fashion. “They displace the optical points inside themselves to a single point,” explains physicist Ulf Leonhardt. For the observer, it then appears as though the light ray passes directly through the object — in other words, as if the object weren’t there at all.
“It’s a fairly crude method, though,” explains Leonhardt, a German scientist at the National University of Singapore and University of St. Andrews in Great Britain. Since the light has to flow around the cloaking device, it needs to cover a greater distance. To keep up the illusion that no object is there, the light must speed around the edge of the device with infinite velocity, as if the masked object really were not in the way at all. “Of course that creates some problems,” Leonhardt says. “One is that the meta-materials used up to now functioned only for light of certain frequencies.”
Together with Tomás Tyc, a fellow researcher at the University of St. Andrews, Leonhardt has presented an alternative: cloaking devices using sophisticated geometric shapes, which skillfully direct light around objects. “In our calculations we’ve found spaces that are so curved, they become cloaking devices,” Leonhardt told SPIEGEL ONLINE. This method has two advantages, as the two physicists wrote in the most recent issue of the journal Science: “The process is more natural, as with a lens. And materials with less extreme properties can be used.”
But most importantly, Leonhardt and Tyc’s model makes invisibility possible not just in two dimensions as in previous experiments, but in three dimensions — in the form of a sphere for example. And this is the essential prerequisite for the leap to true cloaking devices that would work for real objects — or even for people.
Jensen Li at the University of California in Berkeley, one of the few specialists in the field of cloaking devices, praises Leonhardt’s model. “Performance in a wide range of frequencies is very important, since the human eye sees a spectrum from red to violet,” Li says, pointing to a shortcoming of devices produced thus far.
Still, certain fundamental problems remain, as David Schurig from Duke University in Durham, North Carolina, points out. Two years ago, Schurig was the first to introduce a cloaking device that could bend electromagnetic radiation around an object. “Nothing that remains in our universe can be completely undiscoverable,” Schurig told SPIEGEL ONLINE. “Every cloaking scheme has some vulnerability.”
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Designs used up to now, Schurig explains, including his own, have not been able to deal well with certain types of light. Leonhardt’s design, on the other hand, could solve such problems — by, for example, scattering light as thin glass does. It would still require the speeding up of light, though not to the extent of previous designs. But Schurig agrees that the new idea is “interesting,” and that it could provide a valuable contribution to the field.
Christopher Davis from the University of Maryland is not nearly as enthusiastic. In December 2007, he introduced the first cloaking device that extended into the range of visible light. Like other devices before it, however, Davis’ only works in two dimensions. “Extension of cloaking ideas to three dimensions is not impossible, but appears to be very difficult,” Davis explained to SPIEGEL ONLINE. True invisibility, he says, requires that everything behind a cloaked object should appear as it would if the cloaked object were not present. “This appears to be possible for viewing in one direction and for a narrow range of wavelengths,” Davis says, “but appears to be extraordinarily difficult for all colors and all directions.”
According to Davis, Leonhardt’s work does little to address the practical difficulties of implementing a three-dimensional cloaking device in reality. Davis emphasizes that he doesn’t doubt the “mathematical sophistication” of Leonhardt’s models. “But I do not see their practical relevance. This is not Harry Potter’s invisibility cloak!”
In contrast, Jensen Li is more optimistic, pointing out that the most important aspect for human perception is simply what trajectories the light takes, not whether the light requires a shorter or longer time to cover the distance. The next step would be to find a “clever way” to construct Leonhardt’s structures on a larger scale.
“The new scheme works only in geometrical optics,” the researcher told SPIEGEL ONLINE, “but this could still be a big step towards Harry Potter’s invisibility cloak.”
Via Spiegel Online