Invisibility cloaks still distant from reality

Credit: Michael Setzer/SciTech Editor Credit: Michael Setzer/SciTech Editor

It is hard to imagine invisibility, popularized by science fiction and fantasy, becoming part of our world — but every day, science comes closer to making invisibility a reality. Although they’re still experimenting, researchers have made significant progress in developing “invisibility cloaks.” Though Harry Potter’s mythical invisibility cloak is likely the first thing that comes to mind, these cloaks unfortunately won’t have the ability to make humans disappear anytime soon. However, if fully developed, their applications could include masking boats, planes, or military tanks — and in the distant future, they could even render events in time invisible. In the past week, research by scientists at the University of Texas at Dallas has put the idea of invisibility devices in the spotlight. But how do these pieces of technology work?

The fundamental idea behind achieving invisibility is to prevent an object from absorbing, refracting, or reflecting light in any way. In other words, it must be able to bend light away from itself. There have been numerous attempts to achieve this effect, such as carbon nanotube cloaks, but the method closest to perfecting the effect uses metamaterials, manmade materials that have properties natural materials lack.

Most attempts at cloaking could bend light around an object, but could not stop light from reflecting off of it. Thus, much like water or glass, the object was nearly invisible, but reflected just enough light to be detectable. One recent metamaterial cloak, developed at Duke University under the leadership of professor David R. Smith, was able to minimize the reflection by altering the shape of the materials in the cloak.

The cloak works by causing light waves to split, bend around it and reconverge once they pass the object. It resembles a square, with an inverted diamond protruding from the center, and divided into four quadrants, consisting of fiberglass etched with copper. The diamond-shaped center is empty, outlined by copper strips. The strips are designed such that each strip meets its mirror image at every interface. This stops the cloak from reflecting any light from its edges.

The cloak can perfectly hide a 3×0.4-inch (7.5×1cm) cylinder from microwave radiation, but not in the visible light spectrum. Unfortunately, the cloak can only mask small, stationary, two-dimensional items; it cannot hide objects from the human eye in all directions. So, how does invisibility at other wavelengths provide any practical use?

In answering this, it is important to recall that certain objects can be invisible to the human eye but still visible at other wavelengths. For example, while humans do not see at the microwave level, cell phones and radars do. Once fully developed, invisibility at the microwave level could make planes, boats, or tanks undetectable by radar, rendering them invisible. Thus, such invisibility has potential applications in the defense industry. Though it will still be at least a decade before invisibility is put to practical use, its potential application has received wide recognition, and research behind it has been funded by organizations such as DARPA and NASA.

On a more theoretical note, scientists at Cornell University took a step further by not only splitting light waves, but altering the speed at which each wave moved. The effect was achieved by shooting out a beam of light, and creating a time lens — a lens which splits light into temporal components — with other beams to split the light into two different parts that move at different speeds. The difference in speeds created a gap in time, which masked whatever event took place during that gap. The effect lasted no longer than a 40th of a second, and within a fiber thinner than human hair. Nonetheless, the experiment in effect “erased” a brief moment of history.

Though we won't be owning our own invisibility cloaks anytime soon, the ability to revolutionize the defense and communications industries, or to perhaps even render time invisible, is on the horizon. Considering that even Harry Potter could not hide time, we can take solace in the fact that though we cannot recreate science fiction, someday, we may just surpass it.