by Michio Kaku
Next, scientists will have to create metamaterials that can bend light in three dimensions, not just for flat two-dimensional surfaces. Photolithography has been perfected for making flat silicon wafers, but creating three-dimensional metamaterials will require stacking wafers in a complex fashion.
After that, scientists will have to solve the problem of creating metamaterials that can bend not just one frequency but many. This will be perhaps the most difficult task, since the tiny implants that have been devised so far bend light of only one precise frequency. Scientists may have to create metamaterials based on layers, with each layer bending a specific frequency. The solution to this problem is not clear.
Nevertheless, once an invisibility shield is finally made, it might be a clunky device. Harry Potter’s cloak was made of thin, flexible cloth and rendered anyone draped inside invisible. But for this to be possible the index of refraction inside the cloth would have to be constantly changing in complex ways as it fluttered, which is impractical. More than likely a true invisibility “cloak” would have to be made of a solid cylinder of metamaterials, at least initially. That way the index of refraction could be fixed inside the cylinder. (More advanced versions could eventually incorporate metamaterials that are flexible and can twist and still make light flow within the metamaterials on the correct path. In this way, anyone inside the cloak would have some flexibility of movement.)
Some have pointed out a flaw in the invisibility shield: anyone inside would not be able to look outside without becoming visible. Imagine Harry Potter being totally invisible except for his eyes, which appear to be floating in midair. Any eye holes on the invisibility cloak would be clearly visible from the outside. If Harry Potter were totally invisible, then he would be sitting blindly beneath his invisibility cloak. (One possible solution to this problem might be to insert two tiny glass plates near the location of the eye holes. These glass plates would act as “beam splitters,” splitting off a tiny portion of the light hitting the plates, and then sending the light into the eyes. So most of the light hitting the cloak would flow around it, rendering the person invisible, but a tiny amount of light would be diverted into the eyes.)
As daunting as these difficulties are, scientists and engineers are optimistic that an invisibility shield of some sort can be built in the coming decades.
INVISIBILITY AND NANOTECHNOLOGY
As I mentioned earlier, the key to invisibility may be nanotechnology, that is, the ability to manipulate atomic-sized structures about a billionth of a meter across.
The birth of nanotechnology dates back to a famous 1959 lecture given by Nobel laureate Richard Feynman to the American Physical Society, with the tongue-in-cheek title “There’s Plenty of Room at the Bottom.” In that lecture he speculated on what the smallest machines might look like, consistent with the known laws of physics. He realized that machines could be built smaller and smaller until they hit atomic distances, and then atoms could be used to create other machines. Atomic machines, such as pulleys, levers, and wheels, were well within the laws of physics, he concluded, though they would be exceedingly difficult to make.
Nanotechnology languished for years, because manipulating individual atoms was beyond the technology of the time. But then physicists made a breakthrough in 1981, with the invention of the scanning tunneling microscope, which won the Nobel Prize in Physics for scientists Gerd Binnig and Heinrich Rohrer, working at the IBM lab in Zurich.
Suddenly physicists were able to obtain stunning “pictures” of individual atoms arrayed just as in the chemistry books, something that critics of the atomic theory once considered impossible. Gorgeous photographs of atoms lined up in a crystal or metal were now possible. The chemical formulae used by scientists, with a complex series of atoms wrapped up in a molecule, could be seen with the naked eye. Moreover, the scanning tunneling microscope made possible the manipulation of individual atoms. In fact, the letters “IBM” were spelled out via individual atoms, creating quite a stir in the scientific world. Scientists were no longer blind when manipulating individual atoms, but could actually see and play with them.
The scanning tunneling microscope is deceptively simple. Like a phonograph needle scanning a disk, a sharp probe is passed slowly over the material to be analyzed. (The tip is so sharp that it consists of only a single atom.) A small electrical charge is placed on the probe, and a current flows from the probe, through the material, and to the surface below. As the probe passes over an individual atom, the amount of current flowing through the probe varies, and the variations are recorded. The current rises and falls as the needle passes over an atom, thereby tracing its outline in remarkable detail. After many passes, by plotting the fluctuations in the current flows, one is able to obtain beautiful pictures of the individual atoms making up a lattice.
(The scanning tunneling microscope is made possible by a strange law of quantum physics. Normally electrons do not have enough energy to pass from the probe, through the substance, to the underlying surface. But because of the uncertainty principle, there is a small probability that the electrons in the current will “tunnel” or penetrate through the barrier, even though this is forbidden by Newtonian theory. Thus the current that flows through the probe is sensitive to tiny quantum effects in the material. I will discuss the effects of the quantum theory later in more detail.)
The probe is also sensitive enough to move individual atoms around, to create simple “machines” out of individual atoms. The technology is so advanced now that a cluster of atoms can be displayed on a computer screen and then, by simply moving the cursor of the computer, the atoms can be moved around any way you want. You can manipulate scores of atoms at will as if playing with Lego blocks. Besides spelling out the letters of the alphabet using individual atoms, one can also create atomic toys, such as an abacus made out of individual atoms. The atoms are arrayed on a surface, with vertical slots. Inside these vertical slots one can insert carbon Buckyballs (shaped like a soccer ball, but made of individual carbon atoms). These carbon balls can then be moved up and down each slot, thereby making an atomic abacus.
It is also possible to carve atomic devices using electron beams. For example, scientists at Cornell University have made the world’s smallest guitar, one that is twenty times smaller than a human hair, carved out of crystalline silicon. It has six strings, each one hundred atoms thick, and the strings can be plucked using an atomic force microscope. (This guitar will actually play music, but the frequencies it produces are well above the range of the human ear.)
At present, most of these nanotech “machines” are mere toys. More complicated machines with gears and ball bearings have yet to be created. But many engineers feel confident that the time is coming when we will be able to produce true atomic machines. (Atomic machines are actually found in nature. Cells can swim freely in water because they can wiggle tiny hairs. But when one analyzes the joint between the hair and the cell, one sees that it is actually an atomic machine that allows the hair to move in all directions. So one key to developing nanotechnology is to copy nature, which mastered the art of atomic machines billions of years ago.)
HOLOGRAMS AND INVISIBILITY
Another way to render a person partially invisible is to photograph the scenery behind a person and then project that background image directly onto the person’s clothes or onto a screen in front of him. As seen from the front, it appears as if the person has become transparent, that light has somehow passed right through the person’s body.
Naoki Kawakami, of the Tachi Laboratory at the University of Tokyo, has been hard at work on this process, which is called “optical camouflage.” He says, “It would be used to help pilots see through the floor of the cockpit at a runway below, or for drivers trying to see through a fender to park a car.” Kawakami’s “cloak” is covered with tiny light-reflective beads that act like a movie screen. A video camera photographs what is behind the cloak. Then this image is fed into a video projector that lights up the front of th
e cloak, so it appears as if light has passed through the person.
Prototypes of the optical camouflage cloak actually exist in the lab. If you look directly at a person wearing this screenlike cloak, it appears as if the person has disappeared, because all you see is the image behind the person. But if you move your eyes a bit, the image on the cloak does not change, which tells you that it is a fake. A more realistic optical camouflage would need to create the illusion of a 3-D image. For this, one would need holograms.
A hologram is a 3-D image created by lasers (like the 3-D image of Princess Leia in Star Wars). A person could be rendered invisible if the background scenery was photographed with a special holographic camera and the holographic image was then projected out through a special holographic screen placed in front of the person. A viewer standing in front of that person would see the holographic screen, containing the 3-D image of the background scenery, minus the person. It would appear as if the person had disappeared. In that person’s place would be a precise 3-D image of the background scenery. Even if you moved your eyes, you would not be able to tell that what you were seeing was fake.
These 3-D images are made possible because laser light is “coherent,” that is, all the waves are vibrating in perfect unison. Holograms are produced by making a coherent laser beam split in two pieces. Half of the beam shines on a photographic film. The other half illuminates an object, bounces off, and then shines on the same photographic film. When these two beams interfere on the film, an interference pattern is created that encodes all the information of the original 3-D wave. The film, when developed, doesn’t look like much, just an intricate spiderweb pattern of whirls and lines. But when a laser beam is allowed to shine on this film, an exact 3-D replica of the original object suddenly appears as if by magic.
The technical problems with holographic invisibility are formidable, however. One challenge is to create a holographic camera that is capable of taking at least 30 frames per second. Another problem is storing and processing all the information. Finally, one would need to project this image onto a screen so that the image looks realistic.
INVISIBILITY VIA THE FOURTH DIMENSION
We should also mention that an even more sophisticated way of becoming invisible was mentioned by H. G. Wells in The Invisible Man, and it involved using the power of the fourth dimension. (Later in the book I will discuss in more detail the possible existence of higher dimensions.) Could we perhaps leave our three-dimensional universe and hover over it from the vantage point of a fourth dimension? Like a three-dimensional butterfly hovering over a two-dimensional sheet of paper, we would be invisible to anyone living in the universe below us. One problem with this idea is that higher dimensions have not yet been proven to exist. Moreover, a hypothetical journey to a higher dimension would require energies far beyond anything attainable with our current technology. As a viable way to achieve invisibility, this method is clearly beyond our knowledge and ability today.
Given the enormous strides made so far in achieving invisibility, it clearly qualifies as a Class I impossibility. Within the next few decades, or at least within this century, a form of invisibility may become commonplace.
3: PHASERS AND DEATH STARS
Radio has no future. Heavier-than-air flying machines are impossible. X-rays will prove to be a hoax.
—PHYSICIST LORD KELVIN, 1899
The (atomic) bomb will never go off.
I speak as an expert in explosives.
—ADMIRAL WILLIAM LEAHY
4–3–2–1, fire!
The Death Star is a colossal weapon, the size of an entire moon. Firing point-blank at the helpless planet Alderaan, home world of Princess Leia, the Death Star incinerates it, causing it to erupt in a titanic explosion, sending planetary debris hurtling throughout the solar system. A billion souls scream out in anguish, creating a disturbance in the Force felt throughout the galaxy.
But is the Death Star weapon of the Star Wars saga really possible? Could such a weapon channel a battery of laser cannons to vaporize an entire planet? What about the famous light sabers wielded by Luke Skywalker and Darth Vader that can slice through reinforced steel yet are made of beams of light? Are ray guns, like the phasers in Star Trek, viable weapons for future generations of law enforcement officers and soldiers?
In Star Wars millions of moviegoers were dazzled by these original, stunning special effects, but they fell flat for some critics, who panned them, stating that all this was in good fun, but it was patently impossible. Moon-sized, planet-busting ray guns are outlandish, and so are swords made of solidified light beams, even for a galaxy far, far away, they chanted. George Lucas, the master of special effects, must have gotten carried away this time.
Although this may be difficult to believe, the fact is there is no physical limit to the amount of raw energy that can be crammed onto a light beam. There is no law of physics preventing the creation of a Death Star or light sabers. In fact, planet-busting beams of gamma radiation exist in nature. The titanic burst of radiation from a distant gamma ray burster in deep space creates an explosion second only to the big bang itself. Any planet unfortunate enough to be within the crosshairs of a gamma ray burster will indeed be fried or blown to bits.
BEAM WEAPONS THROUGH HISTORY
The dream of harnessing beams of energy is actually not new but is rooted in ancient mythology and lore. The Greek god Zeus was famous for unleashing lightning bolts on mortals. The Norse god Thor had a magic hammer, Mjolnir, which could fire bolts of lightning, while the Hindu god Indra was known for firing beams of energy from a magic spear.
The concept of using rays as a practical weapon probably began with the work of the great Greek mathematician Archimedes, perhaps the greatest scientist in all of antiquity, who discovered a crude version of calculus two thousand years ago, before Newton and Leibniz. In one legendary battle against the forces of Roman general Marcellus during the Second Punic War in 214 BC, Archimedes helped to defend the kingdom of Syracuse and is believed to have created large batteries of solar reflectors that focused the sun’s rays onto the sails of enemy ships, setting them ablaze. (There is still debate even today among scientists as to whether this was a practical, working beam weapon; various teams of scientists have tried to duplicate this feat with differing results.)
Ray guns burst onto the science fiction scene in 1889 with H. G. Wells’s classic War of the Worlds, in which aliens from Mars devastate entire cities by shooting beams of heat energy from weapons mounted on their tripods. During World War II, the Nazis, always eager to exploit the latest advances in technology to conquer the world, experimented with various forms of ray guns, including a sonic device, based on parabolic mirrors, that could focus intense beams of sound.
Weapons created from focused light beams entered the public imagination with the James Bond movie Goldfinger, the first Hollywood film to feature a laser. (The legendary British spy was strapped onto a metal table as a powerful laser beam slowly advanced, gradually melting the table between his legs and threatening to slice him in half.)
Physicists originally scoffed at the idea of the ray guns featured in Wells’s novel because they violated the laws of optics. According to Maxwell’s equations, the light we see around us rapidly disperses and is incoherent (i.e., it is a jumble of waves of different frequencies and phases). It was once thought that coherent, focused, uniform beams of light, as we find with laser beams, were impossible to create.
THE QUANTUM REVOLUTION
All this changed with the coming of the quantum theory. At the turn of the twentieth century it was clear that although Newton’s laws and Maxwell’s equations were spectacularly successful in explaining the motion of the planets and the behavior of light, they could not explain a whole class of phenomena. They failed miserably to explain why materials conduct electricity, why metals melt at certain temperatures, why gases emit light when heated, why certain substances become superconductors at low temperatures—all of which requires an understand
ing of the internal dynamics of atoms. The time was ripe for a revolution. Two hundred and fifty years of Newtonian physics was about to be overthrown, heralding the birth pangs of a new physics.
In 1900 Max Planck in Germany proposed that energy was not continuous, as Newton thought, but occurred in small, discrete packets, called “quanta.” Then in 1905 Einstein postulated that light consisted of these tiny discrete packets (or quanta), later dubbed “photons.” With this powerful but simple idea Einstein was able to explain the photoelectric effect, why electrons are emitted from metals when you shine a light on them. Today the photoelectric effect and the photon form the basis of TV, lasers, solar cells, and much of modern electronics. (Einstein’s theory of the photon was so revolutionary that even Max Planck, normally a great supporter of Einstein, could not at first believe it. Writing about Einstein, Planck said, “That he may sometimes have missed the target…as for example, in his hypothesis of light quanta, cannot really be held against him.”)
Then in 1913 the Danish physicist Niels Bohr gave us an entirely new picture of the atom, one that resembled a miniature solar system. But unlike in a solar system in outer space, electrons can only move in discrete orbits or shells around the nucleus. When electrons “jumped” from one shell to a smaller shell with less energy, they emitted a photon of energy. When an electron absorbed a photon of a discrete energy, it “jumped” to a larger shell with more energy.
