The ability to turn invisible has been a literary device in science fiction stories for decades. H.G. Wells wrote his “Invisible Man” in 1897. J.K. Rowling has won many accolades for her “Harry Potter” series, in which the title character can do an array of amazing things, including turn himself invisible. Wells’ story is based on science, theorizing that if a man could change his own reflective index to be exactly that of the air around him then he would neither absorb nor reflect light, and thus be invisible. Rowling bases her story in a world of magic, where invisibility is achieved by wearing an enchanted cloak.
But now, science has caught up with science fiction. Invisibility technology can be counted among the other wonders of the modern age.
There are two approaches currently being tested to achieve what only magicians and fiction writers were able to do previously. Both hold great promise even though neither has been perfected as yet. One is the optical camouflage technology developed at the University of Tokyo. The second is the use of metamaterials.
Optical camouflage was developed at the University of Tokyo. It builds on a technology developed much earlier, in the 1960s, at Harvard University, called augmented-reality technology. In augmented-reality technology, computer generated information is added to the sensory perceptions of onlookers. This type of system requires the onlooker to be using special viewing apparatus, not unlike modern 3-D movies.
In optical camouflage, the target person, that is the person to be turned invisible, wears a garment made of highly reflective material called “retro-reflective material,” which is covered with thousands of small beads. These beads are constructed and placed on the fabric in such a way that when light strikes them, the light rays are refracted back in the exact same direction they came from. The observer, situated at the light source, will receive more of the reflected light and thus experience a brighter reflection.
The next step in the process of optical camouflage involves a video camera setup situated behind the target person. The video camera captures the background behind the person and transmits the image to a projector. The image from behind the person is then projected onto the target person wearing the retro-reflective garment, and the garment acts as a screen for the image. Due to a special device called an iris diaphragm that controls the opening the projected image is transmitted through, the target person can be any distance away from the projector and the technique will still work. The target person can even move forward and backward.
The last piece of this system is a special mirror called a combiner. This is a half-silvered mirror that both reflects light and transmits light, allowing the combiner to both reflect the projected image toward the retro-reflective garment and to let the light rays bouncing off the cloak return to the observer. The mirror must be positioned just right, at a point in front of the observer’s eyes to allow the observer to perceive both the naturally reflected light from the world around them and the computer enhanced image (of the background behind the target person) being projected onto the retro-reflective garment. For this whole process to be effective, the observer actually has to look through a peephole in the combiner.
While promising technology, it is easy to see the limitations that still exist for optical camouflage. The system involved in it is complex enough to make it impractical at the present time. Also, if the observer has to stand at a specific point in front of the combiner in order for the target person to appear invisible, then the whole effect falls apart easily whenever the observer takes a half-step to the side, or even rocks back a bit on their heels. Efforts to date have produced an effect that looks very much like seeing an image displayed on a movie screen.
The other direction that research into invisibility has taken is the creation of minute structures called metamaterials. Metamaterials are man-made materials that redirect energy waves such as light. Smaller than the wavelength of light, metamaterials exhibit negative electromagnetic properties that affect how an object they are applied to will interact with electromagnetic fields.
The concept behind metamaterials was first proposed by Russian physicist Victor Veselago in 1967. However, it wasn’t until 2006 that researcher David Smith at Duke University’s Pratt School of Engineering would create a metamaterial capable of distorting the flow of microwaves. Although this sounds complicated, the premise under which metamaterials work is a simple one. We see things because light rays bounce off an object, carrying the image to our eyes. But, if it were possible to bend the light rays around that object, it would be invisible to anyone looking at it.
In 2007, Igor Smolyaninov and a team of researchers at the University of Maryland created the first metamaterial capable of bending visible light around an object. They called it the “Purdue cloak.” Less than five inches across, it used concentric gold rings injected with polarized cyan light and precisely arranged to steer all incoming light waves away from the object to be hidden.
While an impressive accomplishment, metamaterials have limitations that have yet to be overcome. They are expensive to create, for one. Also, any garment made of them would necessarily weigh so much that they would be impractical to use. And, perhaps more so than anything else, they are so far limited to creating invisibility in two dimensions, meaning stationary objects only. Once a person wearing an “invisibility cloak” constructed of metamaterials moved, they would become visible again.
Back in 1903, the author Jack London published a short story entitled “The Shadow and The Flash.” In this story two chemical researchers develop two separate theories on how to create the effect of invisibility. One theorizes that the answer is to create a paint of absolute black that will absorb all light. The second says that making an object transparent so that light rays do not reflect off the object will make the object invisible. Both succeed, in a manner of speaking. The situation between metamaterials and optical camouflage is very similar. Both methods, although widely different in approach, have the potential to succeed in the very near future and make what was once science fiction into science fact.
How Stuff Works
Duke University, news release 2006
“The Shadow and The Flash,” Jack London, 1903