To explain their technique, Smith and Schurig invoke the example of a mirage on a hot summer road. When light rays from the sky hit the hot, thin air just above the surface of the asphalt, they bend. Although light moves through a vacuum at a constant speed, it slows down when traveling through any transparent medium, like water or glass. Light travels faster in the hot, thin air close to the road than it does in the cold, dense air above, and that difference in speed is what causes it to shift direction as it crosses the boundary between the two. Rays once headed from the sky to the ground are redirected to your eye, making the road shimmer like water. In effect, the mirage is cloaking the (now invisible) road behind an image of the blue sky.
To similarly cloak something from electromagnetic radiation, Smith and Schurig must bend the incoming beam around the object in a tightly controlled manner. They managed to do so using a class of recently created "metamaterials" that possess an ability, not found in nature, to bend light at extreme angles (a property known as negative index of refraction). The team's metamaterials consist of thin, rigid sheets of fiberglass insulator stamped with neat rows of conducting metal shapes like loops, coils, or tiny rectangles. The metal circuitry is designed to direct incoming electromagnetic radiation—in this case, microwaves—so it moves in a specific way.
All electromagnetic radiation has two intertwined components: a magnetic field and an electric field. As Schurig explains, these can be redirected when they interact with a material. "Materials are made of atoms, and these atoms respond to electromagnetic waves by acting like a little tiny magnet," he says. Electrons begin moving in circles in response to the magnetic field, as well as back and forth in reaction to the electric field—and the moving charges produce fields of their own. The challenge for the Duke team was to find the right shapes and dimensions for the metal circuitry on the metamaterials so they could precisely dictate how the electrons move around, which in turn controls how the incoming radiation is bent.
To demonstrate their system in action, Smith and Schurig walk into their lab, a room lit with fluorescent bulbs and littered with wires, pliers, plugs, pulleys, flashlights, foam cladding, microscopes, computer terminals, and a lone bicycle. The object to be cloaked is just a small copper cylinder filled with black foam: 5 centimeters (2 inches) in diameter and 1 centimeter (0.4 inch) tall. For the experiment it is sandwiched between two horizontal aluminum plates, the bottom one 3 feet square and the top one 4 feet square. Leading in from the front of the apparatus is a wire that feeds microwaves toward the cylinder as it sits in the center of the bottom plate. Around it, Smith and Schurig have arranged concentric rings of metamaterials, with the empty spaces between the rings forming narrow channels. Having carefully varied the properties of the circuits on those surrounding rings, they can now bend microwaves to flow around the cylinder like water flowing around a pebble in a stream. This makes the object undetectable to an instrument downstream that measures microwaves.
