The only way to see anything is for our eyes to intercept visible radiation—light waves—bouncing off it. That makes the recipe for invisibility obvious: just make the object absorb all the light that falls on it. Last year, for instance, scientists at Duke University manufactured a cloak made out of metamaterials, which are artificial composites that interact with electromagnetic radiation in a certain way. In that case, copper rings and wires on sheets of fiberglass deflected microwaves, causing the cloak to be invisible to any being whose eyes are sensitive to microwaves rather than visible waves. Okay, not exactly what J.K. Rowling had in mind, but definitely a step in the right direction.
Now Muggle physicists have taken one more step toward reproducing Harry Potter’s invisibility cloak, this time with visible light waves. In an upcoming issue of the journal Physical Review Letters, physicists Zhichao Ruan, Min Yan, Curtis W. Neff and Min Qiu of Sweden’s Royal Institute of Technology and China’s Zhejiang University show that a cloak made of metamaterials and shaped into a column could make the Muggle, witch or or wizard inside completely invisible.
The key is that the metamaterials force light to follow a particular path. (That’s also the reason your hair looks darker when wet: the water molecules make light bounce around within the hair, with the result that fewer escape to reach your eye; less light equals darker object.) “When electromagnetic waves pass through the invisibility cloak, the cloak will deflect the waves, guide them around the object, and return them to the original propagation direction without perturbing the exterior field,” the physicists write. Done right, the column (for complicated reasons, a cylindrical shape is better for invisibility than, say, a flat rectangle) would guide light around the inner chamber, preventing it from ever reaching anything—including an eye or camera—beyond. But light scattered from objects behind the cloak might also be guided around it, meaning that when you looked at the cloak you would see whatever was behind it, just as with Harry Potter’s. As the authors say, “a cloak with the ideal material parameters is a perfect invisibility cloak.”
There are a few hurdles to manufacturing what would surely be first on every child’s (and criminal’s) wish list. The most daunting is that if the metamaterials had even the slightest flaw, light would scatter off the cloak, rendering it visible. The cloak would be invisible from the inside, too—that is, you would be unable to see anything outside the cloak.
The metamaterials would be layered, and if the hidden wizard removed one layer he would appear as a thin line, the physicists calculate, and the background would be slightly distorted. As more inner layers were removed, the wizard would become more visible and the background more distorted, until with the final step in this odd strip-tease he would be fully visible again.