Metamaterials

Not quite Harry Potter's invisibility cloak, but not far from the cloaking devices envisioned by Star Trek, metamaterials are in the news this week.

The first thing to understand about invisibility, is the kind of invisibility you are talking about. When we're thinking of Harry Potter's cloak, we're thinking of invisibility to light waves. If we go the sf route, we could talk about being invisible to electromagnetic waves, sound waves, microwaves, radar waves, infrared waves, ect.

Light wave invisibility, the kind of invisibility that would fool the human eye, is theoretically possible. How does it work? Metamaterials have a negative refraction index. In laymen's terms, this means they can form a shell around the object to be hidden, then act like light wave crossing guards, ushering the light waves away from the hidden object.

Before you get too excited, keep in mind that whatever form of radiation is blocked from "seeing" the hidden object, also prevents the hidden object from "seeing" out! If you're using your metamaterial shell to hide your "Ancestral Recall" MtG card from your brother, no problem. But if you plan to wrap a metamaterial shell around yourself to sneak out of the house, think again. Your mom couldn't see you, but you couldn't see where you were going. You'd be falling down the staircase and stepping on the family dog- unless you had a reliable navigation system in place, using sonar waves, for instance, to make up for your inability to see light waves.

One more caveat- this more important in military use than for sneaking out of the house (unless Mom has x-ray vision or super hearing)- to be truly, completely invisible means to be undetectable. That means the metamaterial wrapper must be able to hide what's inside from all the types of radiation (electromagnetic, sonar, radar, ect.) that could possibly detect it. To quote David Schurig from Duke University:

"If [you] want to cover the whole visible spectrum, that would [be] a tall order."