Science of the Impossible --- "invisibility" (Class 1 Impossibility)

Science of the Impossible --- "invisibility" (Class 1 Impossibility)

by : Aria Ratmandanu

     




      “Invisibility is also a common plot device in science fiction. In the Flash Gordon series of the 1930s, Flash becomes invisible in order to escape the firing squad of Ming the Merciless. In the Harry Potter novels and movies, Harry dons a special cloak that allows him to roam Hogwarts Castle undetected.

           H. G. Wells put much of this mythology into concrete form with his classic novel The Invisible Man, in which a medical student accidentally discovers the power of the fourth dimension and becomes invisible. Unfortunately, he uses this fantastic power for private gain, starts a wave of petty crimes, and eventually dies desperately trying to evade the police.”

“MAXWELL’S EQUATIONS AND THE SECRET OF LIGHT”

          It was not until the work of Scottish physicist James Clerk Maxwell, one of the giants of nineteenth-century physics, that physicists had a firm understanding of the laws of optics. Maxwell, in some sense, was the opposite of Michael Faraday. Whereas Faraday had superb experimental instincts but no formal training whatsoever, Maxwell, a contemporary of Faraday, was a master of advanced mathematics. He excelled as a student of mathematical physics at Cambridge, where Isaac Newton had done his work two centuries earlier.

         Maxwell began with Faraday’s discovery that electric fields could turn into magnetic fields and vice versa. He took Faraday’s depictions of force fields and rewrote them in the precise language of differential equations, producing one of the most important series of equations in modern science. They are a series of eight fierce-looking differential equations. Every physicist and engineer in the world has to sweat over them when mastering electromagnetism in graduate school.

          Next, Maxwell asked himself the fateful question: if magnetic fields can turn into electric fields and vice versa, what happens if they are constantly turning into each other in a never-ending pattern? Maxwell found that these electric-magnetic fields would create a wave, much like an ocean wave. To his astonishment, he calculated the speed of these waves and found it to be the speed of light! In 1864, upon discovering this fact, he wrote prophetically: “This velocity is so nearly that of light that it seems we have strong reason to conclude that light itself…is an electromagnetic disturbance.”

        “It was perhaps one of the greatest discoveries in human history. For the first time the secret of light was finally revealed. Maxwell suddenly realized that everything from the brilliance of the sunrise, the blaze of the setting sun, the dazzling colors of the rainbow, and the firmament of stars in the heavens could be described by the waves he was scribbling on a sheet of paper. Today we realize that the entire electromagnetic spectrum—from radar to TV, infrared light, visible light, ultraviolet light, X-rays, microwaves, and gamma rays—is nothing but Maxwell waves, which in turn are vibrating Faraday force fields.”

         Tragically, Maxwell, one of the greatest physicists of the nineteenth century, died at the early age of forty-eight of stomach cancer, probably the very same disease that killed his mother at the same age. If he had lived longer, he might have discovered that his equations allowed for distortions of space-time that would lead directly to Einstein’s relativity theory. It is staggering to realize that relativity might possibly have been discovered at the time of the American Civil War had Maxwell lived longer.)

           Maxwell’s theory of light and the atomic theory give simple explanations for optics and invisibility. In a solid, the atoms are tightly packed, while in a liquid or gas the molecules are spaced much farther apart. Most solids are opaque because light rays cannot pass through the dense matrix of atoms in a solid, which act like a brick wall. Many liquids and gases, by contrast, are transparent because light can pass more readily between the large spaces between their atoms, a space that is larger than the wavelength of visible light. For example, water, alcohol, ammonia, acetone, hydrogen peroxide, gasoline, and so forth are all transparent, as are gases such as oxygen, hydrogen, nitrogen, carbon dioxide, methane, and so on.”

          There are some important exceptions to this rule. Many crystals are both solid and transparent. But the atoms of a crystal are arranged in a precise lattice structure, stacked in regular rows, with regular spacing between them. Hence there are many pathways that a light beam may take through a crystalline lattice. Therefore, although a crystal is as tightly packed as any solid, light can still work its way through the crystal.

          Under certain circumstances, a solid object may become transparent if the atoms are arranged randomly. This can be done by heating certain materials to a high temperature and then rapidly cooling them. Glass, for example, is a solid with many properties of a liquid because of the random arrangement of its atoms. Certain candies can become transparent via this method as well. 

        Clearly, invisibility is a property that arises at the atomic level, via Maxwell’s equations, and hence would be exceedingly difficult, if not impossible, to duplicate using ordinary means. To make Harry Potter invisible, one would have to liquefy him, boil him to create steam, crystallize him, heat him again, and then cool him, all of which would be quite difficult to accomplish, even for a wizard.”

METAMATERIALS AND INVISIBILITY

              

 Figure 1. Metamaterial

           But perhaps the most promising new development involving invisibility is an exotic new material called a “metamaterial,” which may one day render objects truly invisible. Ironically, the creation of metamaterials was once thought to be impossible because they violated the laws of optics. But in 2006 researchers at Duke University in Durham, North Carolina, and Imperial College in London successfully defied conventional wisdom and used metamaterials to make an object invisible to microwave radiation. Although there are still many hurdles to overcome, for the first time in history we now have a blueprint to render ordinary objects invisible. (The Pentagon’s Defense Advanced Research Projects Agency [DARPA] funded this research.

               Nathan Myhrvold, former chief technology officer at Microsoft, says the revolutionary potential of metamaterials “will completely change the way we approach optics and nearly every aspect of electronics…Some of these metamaterials can perform feats that would have seemed miraculous a few decades ago. 

             What are these metamaterials? They are substances that have optical properties not found in nature. Metamaterials are created by embedding tiny implants within a substance that force electromagnetic waves to bend in unorthodox ways. At Duke University, scientists embedded tiny electrical circuits within copper bands that are arranged in flat, concentric circles (somewhat resembling the coils of an electric oven). The result was a sophisticated mixture of ceramic, Teflon, fiber composites, and metal components. These tiny implants in the copper make it possible to bend and channel the path of microwave radiation in a specific way. Think about the way a river flows around a boulder. Because the water quickly wraps around the boulder, the presence of the boulder has been washed out downstream. Similarly, metamaterials can continuously alter and bend the path of microwaves so that they flow around a cylinder, for example, essentially making everything inside the cylinder invisible to microwaves. If the metamaterial can “eliminate all reflection and shadows, then it can render an object totally invisible to that form of radiation.

            Scientists successfully demonstrated this principle with a device made of ten fiberglass rings covered with copper elements. A copper ring inside the device was rendered nearly invisible to microwave radiation, casting only a minuscule shadow. At the heart of metamaterials is their ability to manipulate something called the “index of refraction.” Refraction is the bending of light as it moves through transparent media. If you put your hand in water, or look through the lens of your glasses, you notice that water and glass distort and bend the path of ordinary light.”

       If one could control the index of refraction inside a metamaterial so that light passed around an object, then the object would become invisible. To “do this, this metamaterial must have a negative index of refraction, which every optics textbook says is impossible. (Metamaterials were first theorized in a paper by Soviet physicist Victor Veselago in 1967 and were shown to have weird optical properties, such as a negative refractive index and reversed Doppler effect. Metamaterials are so bizarre and preposterous that they were once thought to be impossible to construct. But in the last few years, metamaterials have actually been manufactured in the laboratory, forcing reluctant physicists to rewrite all the textbooks on optics.

             Researchers in metamaterials are constantly pestered by journalists who wish to know when invisibility cloaks will hit the market. The answer is: not anytime soon.”

             David Smith of Duke University says, “Reporters, they call up and they just want you to say a number. Number of months, number of years. They push and push and push and you finally say, well, maybe fifteen years. Then you’ve got your headline, right? Fifteen years till Harry Potter’s cloak.” That’s why he now declines to give any specific timetable. Fans of Harry Potter or Star Trek may have to wait. While a true invisibility cloak is possible within the laws of physics, as most physicists will agree, formidable technical hurdles remain before this technology can be extended to work with visible light rather than just microwave radiation.

             In general, the internal structures implanted inside the metamaterial must be smaller than the wavelength of the radiation. For example, microwaves can have a wavelength of about 3 centimeters, so for a metamaterial to bend the path of microwaves, it must have tiny implants embedded inside it that are smaller than 3 centimeters. But to make an object invisible to green light, with a wavelength of 500 nanometers (nm), the metamaterial must have structures embedded within it that are only about 50 nanometers long—and nanometers are atomic-length scales requiring nanotechnology. (One nanometer is a billionth of a meter in length. Approximately five atoms can fit within a single nanometer.) This is perhaps the key problem we face in our attempts to create a true invisibility cloak. The individual atoms inside a metamaterial would have to be modified to bend a light beam like a snake.

INVISIBILITY VIA PLASMONICS

         Not to be outdone, yet another group announced in mid-2007 that they have created a met“amaterial that bends visible light using an entirely different technology, called “plasmonics.” Physicists Henri Lezec, Jennifer Dionne, and Harry Atwater at the California Institute of Technology announced that they had created a metamaterial that had a negative index for the more difficult blue-green region of the visible spectrum of light.

         The goal of plasmonics is to “squeeze” light so that one can manipulate objects at the nanoscale, especially on the surface of metals. The reason metals conduct electricity is that electrons are loosely bound to metal atoms, so they can freely move along the surface of the metal lattice. The electricity flowing in the wires in your home represents the smooth flow of these loosely bound electrons on the metal surface. But under certain conditions, when a light beam collides with the metal surface, the electrons can vibrate in unison with the original light beam, creating wavelike motions of the electrons on the metal surface (called plasmons), and these wavelike motions beat in unison with the original light beam. More important, one can “squeeze” these plasmons so that they have the same frequency as the original beam (and hence carry the same information) but have a much smaller “wavelength. In principle, one might then cram these squeezed waves onto nanowires. As with photonic crystals, the ultimate goal of plasmonics is to create computer chips that compute using light, rather than electricity.

              The Caltech group built their metamaterial out of two layers of silver, with a silicon-nitrogen insulator in between (with a thickness of only 50 nm), which acted as a “waveguide” that could shepherd the direction of the plasmonic waves. Laser light enters and exits the apparatus via two slits carved into the metamaterial. By analyzing the angles at which the laser light is bent as it passes through the metamaterial, one can then verify that the light is being bent via a negative index.