So how far off is a real invisibility device? Could such a contraption ever be used to cloak an airplane, a tank, or a ship? Smith doesn't want to be snared by such hypothetical questions. "Reporters, they call up and they just want you to say a number," he says. "Number of months, number of years. They push and push and push and then you finally say, well, maybe 15 years. Then you've got your headline, right? 'Fifteen years till Harry Potter's cloak!' So I have to resist giving you a number."
One major problem with masking objects from visible light, says Schurig, is that light is composed of a range of colors, each with its own wavelength. "We don't know how much of that spectrum we could cloak all at once," he explains. "If you could get past these fabrication issues, you could cloak one color of light, and maybe you could cloak some range of visible light. We might be able to make the cloak work for a brief time, for a microsecond at red, a microsecond at green, a microsecond at blue, and you could make it look translucent. But we don't know that you could make something 100 percent invisible to the whole spectrum simultaneously."
Milton sounds a further note of caution. Of the Duke research, he says: "I think it's a brilliant idea. But there's a certain amount of skepticism in the scientific community in so far as the time line. I remember reading claims that you could cloak some factory that would be an eyesore. I think that's a bit far-fetched. You can make some small things invisible, but making larger things invisible will be a longer time in coming."
There are other factors that neither Harry Potter fans nor the series' fabulously wealthy author, J. K. Rowling, seem to have considered. Ulf Leonhardt—the only one of the researchers who admits to enjoying the books—explains that Harry can see through his cloak, which is made of a thin material in which he can walk and climb. "The present scheme assumes you have something very rigid" surrounding the object, Leonhardt says. "It's not a cloak, it's like a suit of armor. If you want to have something flexible, then the material also has to change its properties, like a chameleon. That is also possible in principle, but with present technology we're a long way away from that."
The other, bigger problem is that to see, the eyes must absorb light—which, of course, makes them visible. "If Harry Potter wants to see through his cloak, then his eyes would be visible, because they have to see. And if they have to see, they have to be seen," Leonhardt says. "For example, a fish that camouflages itself by being transparent has eyes that are not transparent, because they have to see. Yet Harry Potter can see through the invisibility cloak. That, I think, is not possible. He would be blind behind it."