Call it Harry Potter's invisible sleeve. New calculations show how to make an electromagnetic "wormhole"—a tube that is invisible from the sides, allowing light to shine down the center unseen [see endnote].

The concept is a twist on a spherical cloak of invisibility proposed last year. Such a device would be made of metamaterial, a thicket of metal rings or other shapes that bends light in funny ways. A hollow shell of metamaterial could in principle channel a single frequency of light around its inner space without slowing the light down, rendering that hidey-hole invisible to the outside world at that frequency.

But the invisibility cuts both ways. If light does not enter, then whatever is in the cloak cannot see outside, says mathematician Allan Greenleaf of the University of Rochester.

So Greenleaf turned the cloak inside out. In work submitted to a major physics journal, he and colleagues report that the light-warping trick works for an open tube with flared ends. Viewed straight on, light zipping down the cylinder would be plainly visible. But from the side, the light would appear to come out of nowhere, as though sent on a detour to another dimension and back.

The idea is the same as that of a wormhole linking two distant points in spacetime, hence the nickname. "We're tricking the electromagnetic waves … into thinking that, actually, space has been changed," Greenleaf says.

"It's a very nice twist" on the spherical cloak, says physicist John Pendry of Imperial College London, one of the physicists who first worked out the idea. "We can invent a secret connection between two parts of space, and that is interesting."

Building an invisible tunnel should be as hard—or easy, depending on your level of optimism—as making a spherical cloak, Greenleaf says. A Duke University team demonstrated an imperfect cloak last year that warps microwaves around a disk of concentric copper rings. But researchers are still struggling to build metamaterials that bend visible light.

Greenleaf's group speculates that wormholes could be used to pass metal objects into an MRI scanner or, by making a prickly sphere of them, create a 3-D video display. Of course, by the time invisibility becomes easy to achieve, modern technology will probably be a bit out of date.

Note: Parts of this article have been modified for clarity.