Hiding in Plain Sight
By DAVID R. SMITH and NATHAN LANDY
Published: November 17, 2012 New York Times Sunday Edition
How do you make something disappear? Easy ? bend all the light around it.
The answer has much to do with the way our sense of sight works. As we look around a room, our eyes detect the light that bounces off nearby people or objects, and our brains interpret the images formed from the patterns of light received. We can even figure out what material something is made of based on the way it reflects and transmits light: metal is opaque and typically very reflective; plastic, which is more dull and often translucent, absorbs some of the light and reflects the rest in all directions. Our brains, then, turn these signals from reflections into breathtakingly complex pictures of the world around us. And it all happens faster than the blink of an eye. Indeed, after every blink of an eye.
Such lightning-fast cognitions are possible partly because the brain makes certain automatic assumptions: it figures that light has traveled in a straight line from the object to our eyes. Remarkably, in that built-in assumption is the recipe for a bit of magic that humans (and mythical humans) have sought, from the time of Plato to the age of Harry Potter: invisibility.
The trick involves the ability to bend and distort light as it travels through space — in other words, to make it do what the brain assumes it won’t. In some ways, it’s the same sleight of hand that the magician uses with the tiger. He uses a mirror angled in such a way that when we think we’re looking into an empty box, we’re actually seeing the reflection from the bottom of the box and assuming it’s the back. Since we don’t expect that the light reaching our eyes has swerved, making a 90-degree turn along the way, our eyes “tell” us the tiger has vanished. (In reality, he’s hiding comfortably in the box.)
Now we’ve found a way to one-up this neat trick with science: changing the trajectory of light without using mirrors. We do it with the science of materials — designing a “cloak” that can make light curve around an object, and then emerge just as if it had passed in a straight line through space. (Think of it like water flowing past a rock in a stream.)
The phenomenon is indeed supernatural. That’s because nature doesn’t appear to offer any materials that can accomplish this feat. The reason is that light has both electric and magnetic components — and to make it swerve around an object, one has to redirect both of these very different components and have them sync up immediately after the detour. That’s impossible to do with metals, fabrics or any other traditional materials.
But research findings over the past decade have shown us how to develop artificially structured “metamaterials” — in which tiny electrical circuits serve as the building blocks in much the same way that atoms and molecules provide the structure of natural substances. By changing the geometry and other parameters of those circuits, we can give these materials properties beyond what nature offers, letting us simultaneously manipulate both the electric and magnetic aspects of light in striking harmony.
This year, with one such metamaterial, we built the world’s first invisibility cloak capable of managing both components of light.
There is a catch, admittedly. Our cloak works only on microwaves, not on visible light. And humans don’t “see” microwaves in the first place, making the idea of invisibility seem, well, a little extraneous.
Still, even if we mortals don’t see them, many essential devices do. Nearly every time you walk through security at an airport, your body is scanned with microwaves. Also, your cellphone, iPad and other devices make a similar kind of virtual eye contact with one another. So, even in the microwave realm, cloaking can potentially be used to remove obstacles from the paths of direct microwave communications (or hide things we don’t want detected).
More important, microwaves are part of the same electromagnetic spectrum as visible light. In principle, if cloaks can be made to work at microwave frequencies, they might one day be made to work at visible wavelengths.
This will be far more difficult: the wavelengths of visible light are more than 10,000 times smaller than those of microwaves, meaning that the corresponding metamaterials would have to be equally reduced in size.
What excites scientists and Harry Potter fans alike, though, is that our microwave cloak proves there’s no theoretical limitation that would prevent someone from building a visible-light cloak.
There are some tricky technological barriers to work out. But in this case, at least, not seeing is believing.
David R. Smith is a professor of electrical and computer engineering at Duke University, where Nathan Landy is a graduate student.