For one of the first times ever, researchers have created an honest-to-goodness, real invisibility cloak that completely and passively cloaks real, tangible objects from all wavelengths and types of light. The catch—and there's always a catch with invisibility cloaks, isn't there—is that the cloak only works in environments in which light doesn't propagate ballistically. In other words, it doesn't work in a vacuum, or in air.
Which, truth be told, takes a whole lot of the usefulness out of the invisibility cloak, doesn't it?
But that hasn't stopped us from getting excited about invisibility cloaks that only work with light in the red and blue spectrum, "acoustic invisibility cloaks," and techniques that make objects invisible to microwaves.
The latest invisibility cloak was developed by Robert Schittny and his team at the Karlsruhe Institute of Technology in Germany, and relies on the principle that, if there are other particles to help diffuse light, making a cloak work isn't all that hard.
"Invisibility cloaks which are macroscopic, three-dimensional, broadband, passive, and that work for all directions and polarizations of light are not consistent with the laws of physics," Schittny wrote in Science. "We show that the situation is different for surroundings leading to multiple light scattering according to Fick's diffusion equation."
And what would those be? Clouds, fog, frosted glass, and, somewhat surprisingly, milk.
Those environments have "many randomly distributed scattering centers," which "slows down light propagation with respect to vacuum and scrambles incident polarization," he wrote.
That's the key—in order to make invisibility work, you've got to bend the light around an object so that it appears as if it's not there in the first place. Schittny was able to do that using a hollow stainless steel cylinder, which itself is the "cloak"—you can put an object inside of it, some white paint, and a coating of light-scattering polymers. They popped it into a tank of water with some paint in it (to create those "diffuse conditions"), and used an LCD flatscreen monitor as a light source.
The whole thing looks like this:
Can't see the rod in the painty water, can you?
If no immediate applications are coming to mind, you're not alone. But Schittny says he's already got at least one idea: "We suggest that one could inset metal bars, which are almost as thick as the glass, into a bathroom frosted-glass window to prevent burglary … by adding thin diffusive cloaking shells around the metal bars, the window would again appear as a homogeneously bright milky glass."
OK, then. That's at least a little more aesthetically pleasing than a window with a bunch of bars in front of it, even if it's not the cloaking breakthrough that'd turn science into something that seems like magic.
Others aren't as convinced. John Pendry, one of the fathers of cloaking theory, told Science Magazine that he "can't imagine why you would want to cloak somebody already cloaked in fog."
Fair enough. But would you want to cloak something that's in a pool of milk? If so, now there's hope.