by Adam Frank, NPR.org, (original site)
Hidden in Plain View: the Physics of Cloaking Time, Space, and Experience
You never experience the world as it is. You only experience it in the way light brings it to you.
And light can be taught to lie.
Last week researchers at Cornell University announced they had created a time cloaking device. Using their machine they could hide an event from detection, even if it occurred in plain view of very capable detectors.
This “time cloaking” experiment comes on the heels of a series of results over the last few years of “space cloaking” technologies in which a stationary object could be made invisible to detectors.
Both experiments rely on the complex realization of a simple truth about our experience of the world. We have no “direct” knowledge of the world-in-of-itself but, instead, are forced to rely on signals carried to us from external objects. If the properties of the signals are somehow changed while they are traveling to us then our experience of the world is changed as well.
We all have experienced the simplest example of this at side of the pool. A long, half-immersed pole appears to bend at its point of entry into the water. This illusion occurs because of therefraction — change of direction — of light as it moves from the water to the air. We are skilled enough in the ways of the world — call it folk physics — to know the rod isn’t really bent.
Nature and light can, however, be manipulated in ways that can make illusions impossible to detect. This is the new physics of cloaking.
Both the space and time cloaking devices require the use of what are called metamaterials. These fabricated blocks of matter are made of subunits in which the sum of the parts — rather than inherent atomic properties — determine how it interacts with light.
Light can interact with matter in a number of ways. A light beam can, for example, be partially reflected and partially absorbed. Metamaterials — composed, perhaps, of repeating hoops of different kinds of wire — are carefully designed to manipulate the propagation of incoming light beams. By dissembling and reassembling light beams, metamaterials can be used to cloak objects (or events) in space (or in time).
Imagine a river (idealized) with a large bridge trestle anchored in the middle. The water approaches the bridge in a parallel stream, flows around the trestle, and then reassumes its parallel flow downstream. In essence, this is how metamaterials force light to behave.
By manipulating the speed of light (slowing it down) metamaterials can be designed to force a beam of light to flow around an object such that any “impression” in the light of that object is erased downstream. If you are the observer looking back in the direction from which the light was traveling you would have no way to know the object was there. It would be “cloaked.”
The same process can work with an “event” — something that happens only for a few ticks of time. When the cloaking device is “on,” light beams that would record the event — by interacting with it — flow around it and are reassembled such that the event disappears from view.
These cloaking technologies have some obvious applications. I will let your own devious mind imagine them (shame on you!). There is a problem, of course, with this kind of cloaking. If you are inside a cloaked device you are also in the dark. No light from the outside world can get to you (which is why you are cloaked in the first place. In addition, massive and perhaps insurmountable technological hurdles must be overcome before an iCloak app makes it to your door.
To make cloaking work, the elements of the metamaterial must be smaller than the wavelength of light being manipulated. Most experiments to date have been carried out with microwaves with a wavelength of centimeters, or more. Visible light has wavelengths that are the size of molecules, so good luck building anything useful in that regime soon.
Still, a new result is a new result. In principle at least, our dependence on signals from the outside world leaves us vulnerable to manipulation of those signals. Someday exploiting this vulnerability may be as common as the shimmering reflections off the surface of a pool.