Can a person become invisible? In science-fiction movies, it’s easy. In the 1933 movie “The Invisible Man” and in the Harry Potter movie “The Cloak of Invisibility,” both made it happen with ease.
But why would invisibility be of interest? Certainly, crooks, spies, invading army or counter-terrorism forces would love it. The military could hide their airplanes and ships. No one would know they’re around until it was too late. In the public sector studying animals within their environment without disturbing them would be great. Also, bringing the outside inside by making a wall of a house invisible.
To duplicate that cloak, Dr. Susumu Tachi, from Keio University in Tokyo, using movie cameras, projectors and a special cloak material tried to mimic “The Cloak of Invisibility.” His system takes a movie of the background behind a cloaked object and projects the movie onto the front of the cloak, in effect, looking through the cloaked object. Although it worked, it’s impractical for everyday use.
A more down-to-earth concept was predicted theoretically almost 50 years ago by Russian physicist Victor Veselago. He showed when the electric and magnetic permeability of a material are changed, light bending is possible.
His prediction was confirmed in 2000, when professor David Smith and a group at Duke University created a composite material with Veselago characteristics. They called this composite “Metamaterial.” The material is arranged in repeating patterns, at scales smaller than a specific wavelength of light. It provided the ability to bend light electronically, the effect, invisibility or cloaking. Think of it as a rock deflecting water in a stream. The water flows around the rock and reconnects with the rest of the water as though the rock was never there, ergo, the rock became invisible to the water stream.
That same year, Smith and colleagues demonstrated the use of metamaterial at microwave frequencies. Two years later, a cloaking device was demonstrated on a metal cylinder. The invisibility cloak deflected microwave beams, so they flowed around the cylinder with only minor distortions, making it appear as if nothing was there.
Unfortunately, visible light is difficult to bend, since light in the visible spectrum tends to degrade to nothing after passing through materials just a fraction of a wavelength thick. A step forward was accomplished by a team from King’s College in London and the Valencia Nanophotonics Technology Center in Italy. They overcame the red and near-infrared parts of the spectrum problem using what they call a nanofishnet structure. This was achieved by punching tiny holes in metamaterials and varying the hole size to tune for one section of the visible-light spectrum. The first use of this material would be used in switching fiber-optic networks, and as computer building blocks, since they need only a single light wavelength to work.
But for human invisibility, there is a small problem, and it revolves around how our eye sees objects. Seeing something is done as light is reflected from an object into our eyes. If we were in a completely dark room, no light, we wouldn’t see anything, even with night-vision goggles. Simply stated, to become invisible, light can’t reflect off a person into the viewer’s eyes. Think of it as material absorbing the light and not reflecting it.
Forget for a moment seeing behind the person. Now, if light can’t get through the cloak to the cloaked person’s eyes, they wouldn’t see anything either. So, let’s cut two holes in the cloak material, so they can see. From a viewer’s standpoint, it would appear there were two eyes moving about, mind you just the eyes, five or six feet from the ground. Now that should freak someone out!
A possible solution is to let a little light in and amplify it, so the cloaked person could see.
However, there are non-human issues where that problem doesn’t exist. Professor Nicholas Fang and researchers at the University of Illinois-Urbana-Champaign, have worked on applications of metamaterials in the acoustical area. They developed an “Acoustic Superlens” for high-resolution ultrasound imaging. This technique could be used for non-destructive structural testing of buildings and bridges, evaluating material for deeply embedded flaws, which are invisible to the eye or unable to be detected by optical imaging.
As for the military, they are always trying to make their aircraft and ships invisible. Today for a radar environment, they design the surface of airplanes and ships having sharp edges to reflect away rather than back to the source. They also work with submarines by reducing noise, magnetic signatures and using surfaces with acoustical absorption material to make the subs invisible.
From an acoustical standpoint, submarines can be detected by pinging sounds off their hull using sonar. In 2011, using the metamaterial as an acoustical diverter, Professor Fang and group developed an underwater acoustic cloaking device. This bent the sonar sound around an object, providing stealth capability for submarines.
There is no question the U.S. military is backing the development of camouflage fabrics that could one day make troops invisible. Two groups within the U.S. and Canadian Military and Counter Terrorism units have been engaged in developing that technology.
Remember, if you see two eyes floating in space, well you’ve been had!