Fantasy fiction is about magic, science fiction is about … well, science. People who believe in one do not always buy into the other, yet the two can merge. As Arthur C. Clarke wrote, a sufficiently advanced technology cannot be distinguished from magic. Besides, some magical visions represent such deep human yearnings that we ardently wish they were real.
These visions often appear in fantasy, myth, and legend, where they are “explained” simply as being magical. They might reappear in the newer genre of science fiction, justified by more or less credible scientific explanations. And sometimes, if we are lucky, there’s a third step where the idea moves out of fiction altogether and becomes real.
If you think this does not truly happen, consider the common human dream of flying. The vision of freely soaring through space has been expressed in the old Greek story about Icarus, who flew with wings made of feathers and wax; in the flying carpet of the Arabian Nights; in Jules Verne’s Around the World in 80 Days, where an aeronaut circles the globe by balloon; and now, we routinely fly at 600 miles per hour. “Flight” has moved from myth and fairy tale to scientific possibility to everyday reality.
But the dream of becoming invisible has seemed only an impossible fantasy for 2,500 years, from Plato’s Republic to The Lord of the Rings and the Harry Potter series. It became a science-fiction theme in tales like H.G. Wells’ The Invisible Man (1897), which described an optical approach to it, and in film and television. Even with limited special effects, the 1933 film version of The Invisible Man entertainingly displays personal invisibility. Later, Memoirs of an Invisible Man (1992) portrays Chevy Chase in a “state of molecular flux” that makes him invisible. In Hollow Man (2000), spectacular computer generated imagery (CGI) shows how a scientist played by Kevin Bacon becomes invisible after his biomolecules are tweaked. And beyond personal invisibility, Star Trek gave the hostile Romulans a “cloaking device,” based on Einstein’s general relativity, which allows them to hide an entire spacecraft.
Now, after roles in fantasy and science fiction, invisibility has become real. One method, stealth technology, has since the 1980s made U.S. warplanes nearly invisible to radar. The aircraft is coated with absorbing material to reduce its overall reflectivity, and more important, is shaped so radar beams reflecting off it are diverted in directions where they will not be detected. In the 1950s, the B-52 Stratofortress bomber was highly visible on radar screens with a huge cross-sectional area of 150 m2. Today, stealth aircraft like the F-22 Raptor fighter have radar cross-sections the size of an insect; and the technology is still developing, such as the stealth helicopter that participated in the recent U.S. raid on Osama bin Laden’s compound in Pakistan.
A second method, invented in 2003 by Susumi Tachi at the University of Tokyo, makes an object apparently disappear. The front of the object is coated with “retroreflective” material, which sends incoming light directly back out. A video camera records what lies behind the object and this image is projected on to the object’s retroreflective front, so that an observer sees that scene superimposed on the object. The result is a remarkable illusion, as shown in videos where crowded street scenes are seemingly visible right through people wearing retroreflective cloaks. This is not yet Harry Potter’s Cloak of Invisibility; the optics must be carefully set up and the method works for only a static observer in the correct position. Still, it has applications in military camouflage and in enhanced visibility from automobiles and aircraft.
The newest and most striking, however, are methods that could make things truly disappear as completely as in any fantasy. We see an object as it interacts with light rays, for instance, by changing their direction. But in principle, a cloak placed around an object could intercept incoming light rays, bend or refract them into itself, and send them along internal paths so that they leave the cloak along continuations of their original trajectories. An observer would see only apparently undisturbed light and so would conclude there is nothing there – much like water in a stream encountering a rock, splitting around it, then smoothly recombining to give no sign downstream that the rock exists.
In 2006, two separate research groups applied breakthroughs in understanding how light interacts with matter to work out the mathematical theory of cloaking. The same year, a group under D.R. Smith at Duke University used this approach to build the first true cloak, an intricate artificial material or “metamaterial.” Its thousands of millimeter-size copper units were carefully shaped to change the electric and magnetic properties of light – an electromagnetic wave – as it passes through, to bend the light along the desired paths. Tested using microwave radiation, the cloak made a copper cylinder nearly completely disappear.
This was huge, exciting news, but to make a Harry Potter cloak, the method needs to be extended to visible light. Because the units in a metamaterial must be smaller than the wavelength of the light, it takes advanced nanotechnology to make a cloak for the tiny visible wavelengths 400 to 750 nanometers, violet to red. Still, scientists are successfully building appropriate structures, and there are other encouraging results. Using the unusual optical behavior of the naturally occurring crystal calcite, researchers created “carpet” cloaks that make a bump appear flat. This hides small objects underneath, and works for red, green, and blue light.
If your dream is to actually wrap a cloak around yourself, that too could be on the horizon. In 2002, Italian scientists proposed that a flexible cloak covered with many small light sensors and emitters could do what the retroflection method does – record what lies in back and project that image in front, effectively making the cloak disappear – but dynamically, in real time, to maintain the effect as the cloak or an observer moves. Though we are not there yet, we have candidates for the necessary devices; quantum dots, tiny bits of semiconductor that behave like artificial atoms. These can both detect and produce light, and can even be embedded in a cloak by spraying them on to cloth via an inkjet printer.
Just five years after that first report of cloaking at Duke, it has been referenced by other scientists more than 800 times, showing how much effort is going into the practical realization of invisibility. It’s easy to imagine the wondrous, centuries-old power of invisibility turning into a new consumer product like InvisiWrapTM , with the advertising hook “What happens under the Wrap, stays under the Wrap!;” scientific magic, or magical science, at the bargain price of $49.95.
Sidney Perkowitz is the Candler Professor of Physics at Emory University and the author of Hollywood Science. This posting is based on his forthcoming book Slow Light: Invisibility, Teleportation, and Other Mysteries of Light. He can be reached at physp@emory.edu and http://www.sidneyperkowitz.net/.
