Chapter 3 2. Invisibility
In Star Trek IV: The Voyage Home, a Klingon battlecruiser is hijacked by the crew of the USS Enterprise.Unlike Federation Starfleet spaceships, Klingon Empire spaceships have a secret "cloaking device" that renders them invisible to light and radar, allowing Klingon ships to approach Federation ships quietly from behind , and then strike suddenly without taking any damage.This concealment device gave the Klingon Empire a strategic advantage over the Federation.
Is such a device really feasible?From the lines of The Hidden Man to the magical invisibility cloak in the Harry Potter series, or the rings in The Lord of the Rings, invisibility has always been one of the wonders of science fiction and fantasy.Yet for at least a century, physicists dismissed the possibility of invisibility cloaks, asserting that they were impossible: They violated the laws of optics and did not conform to the properties of any known matter.
But now the impossible may be possible.An advance in "metamaterials" is powering a major revision of optics textbooks.The prototype of the application of this material has actually been established in the laboratory, and the industrial and military fields are turning the visible into invisible after being aroused by the media.
Invisibility is perhaps one of the oldest concepts in ancient mythology.Since time immemorial, people who have spent chilling nights alone have been terrified by the unseen spirits of the dead, the spirits of the long dead lurking in the darkness.The Greek hero Perseus was able to slay the evil Medusa after being armed with a helmet that made him invisible; army generals have always dreamed of having invisibility equipment, because it can easily break through enemy lines after being invisible, And surprise wins; criminals can use invisibility for clever theft.
Invisibility plays a central role in Plato's theory of ethics and morality.In his philosophical masterpiece (The Republic), Plato recounts in detail the myth of the ring of Gyges.The poor but honest shepherds of Lydia entered a hidden cave and found a corpse in a tomb wearing a golden ring.Gyrges discovers that the ring has the power to make him invisible.Soon the poor shepherd was gripped by the power the ring bestowed on him.After sneaking into the king's palace, Gyges uses his magic powers to seduce the queen and, with her help, kills the king and becomes the next king of Lydia.
The moral Plato wanted to convey was this: No one can resist the temptation to be free to steal and kill.Everyone can be corrupted.Morality is a social construct imposed on people from outside.A man may be able to behave morally in public to maintain his reputation for integrity, but once he has acquired the ability to be invisible, it is imperative to use this ability (some believe this ethics story is JR R Tolkien Inspiration for the Lord of the Rings trilogy, in which a ring that grants the wearer invisibility is also a source of evil).
Invisibility is also a common plot foreshadowing in science fiction.In the 1930s Flash Gordon series, Flash goes invisible to escape Ming the Merciless's firing squad; in the Harry Potter novels and films, Harry dons a A special robe to allow him to roam Hogwarts Castle undetected.
HG Wells brought this myth to life to a great extent in his classic novel The Hidden Man.In the novel, a medical student stumbles upon the power of four dimensions and becomes invisible.Unfortunately, he used this occult ability for personal gain, started a series of crimes, and eventually died in despair while trying to evade the police.
Physicists did not have a definitive understanding of the laws of optics until the work of Scottish physicist James Clerk Maxwell, one of the giants of 19th-century physics.In some ways, Maxwell is the antithesis of Michael Faraday.Faraday had amazing intuition in experiments with no formal training, and Faraday's contemporary Maxwell was a master of advanced mathematics.He specialized in mathematical physics as a student at Cambridge University, where Isaac Newton completed his work two centuries earlier.
Newton invented calculus.Calculus, expressed in the language of "differential equations," describes how things smoothly undergo small changes in time and space.The motion of ocean waves, liquids, gases, and projectiles can all be described in the language of differential equations.Maxwell began his work with a clear goal—to express Faraday's revolutionary findings and his force fields in exact differential equations.
Maxwell started with the discovery that Faraday's electric field could be transformed into a magnetic field and vice versa.He took Faraday's description of force fields and rewritten it in the precise language of differential equations, resulting in one of the most important systems of equations in modern science.They are a set of 8 equations that look very difficult.Every physicist and engineer in the world had to work through these equations when they mastered electromagnetism at graduate level.
Maxwell then posed the decisive question to himself: If magnetic fields could be transformed into electric fields, and vice versa, what would happen if they were transformed into each other forever and ever?Maxwell discovered that these electro-magnetic fields create waves very similar to ocean waves.To his surprise, he calculated the speed of these waves and found that it was the speed of light!After discovering this fact in 1864, he wrote prophetically: "This velocity is so close to the speed of light that it appears that we have good reasons for believing that light itself is an electromagnetic disturbance."
This may be one of the greatest discoveries in human history.For the first time ever, the mysteries of light have finally been revealed.Maxwell suddenly realized that the brilliance of sunrise, the red flame of sunset, the brilliant colors of the rainbow, and the twinkling stars in the sky can all be described by waves that he hastily scribbled on a page.Today we realize that the entire electromagnetic spectrum - from TV antennas, infrared, visible light, ultraviolet, X-rays, microwaves and gamma rays - are nothing but Maxwell waves, vibrating Faraday fields.
Commenting on the importance of Maxwell's equations, Einstein wrote that they were "the most profound and fruitful event experienced by physicists since the time of Newton".
(Tragically, Maxwell, one of the greatest physicists of the 19th century, died untimely at the age of 48 from lung cancer—the same disease that most likely killed his mother at the same age. If he survived Longer, he might be able to discover that his equations lead directly to Einstein's theory of relativity when time and space are allowed to deform. It's really cool to think that if Maxwell had lived longer, the theory of relativity might have been discovered during the American Civil War Surprising.)
Maxwell's optics and atomic theory provided simple explanations for optics and invisibility.In a solid, the atoms are tightly packed, whereas in a liquid or gas the molecules are more loosely packed.Most solids are opaque because light cannot penetrate the dense matrix of atoms in a solid, which acts like a brick wall.In contrast, many liquids and gases are transparent because light passes unhindered through the large spaces between their atoms, which are larger than the wavelength of visible light, for example, water, alcohol, ammonia, acetone, peroxide Hydrogen, gasoline, etc. are transparent, as are gases like oxygen, hydrogen, nitrogen, carbon dioxide, methane, etc.
There are some very important exceptions to this rule.Many crystals are both solid and transparent.But the atoms of crystals are arranged in a precise grid structure, packed in regular rows with regular spaces in between.So there are many ways for light to pass through the crystal grid.So, although crystals are as tightly packed as any solid, light can still pass through them efficiently.
Under certain circumstances, a solid can become transparent if the atoms are arranged randomly.This can be achieved by heating certain materials to high temperatures and then rapidly cooling them.Glass, for example, is a solid that has many liquid properties due to the random arrangement of its atoms.Certain candies are also made transparent by this method.
Clearly, invisibility is a property change that occurs at the atomic level via Maxwell's equations, so it would be extremely difficult—if not impossible—to reproduce using ordinary means.To make Harry Potter invisible, we had to liquefy him, boil him to create steam, crystallize him, reheat him, and then cool him down - all pretty difficult, even for a wizard accomplish.
The military can't make a stealthy plane, so it's already trying the next best thing: developing a plane that's invisible to radar.Stealth technology relies on Maxwell's equations to create a series of tricks.A stealth fighter is completely visible to the naked eye, but its radar image on enemy radar is only the size of a large bird (stealth technology is actually a hodgepodge of blindfolds. By changing the internal materials of the fighter - reducing the metal content and using By replacing plastic and resin, changing the curvature of the fuselage, retuning its exhaust pipes, etc., we can make the radar signals of the enemy hitting the fuselage spread out in all directions, so that they can never return to the enemy's radar On the screen. Even with stealth technology, a fighter jet cannot be completely invisible, it can only refract or disperse radar as far as the technology allows).
Perhaps the most promising new development in invisibility technology, though, is exotic materials called "metamaterials," which may one day actually make objects invisible.Ironically, metamaterials were once considered impossible because they violated the laws of optics.However, in 2006, researchers at Duke University in North Carolina and Imperial College London managed to challenge conventional notions by using metamaterials to make an object invisible to microwave radiation.While there are still many hurdles to overcome, for the first time ever we have a way to make ordinary objects invisible (the Pentagon's Defense Advanced Research Project Agency (DARPA) funded the research).
Nathan Myhrvold, former chief technology officer at Microsoft, said the revolutionary potential of metamaterials "will revolutionize the way we approach optics, and nearly every aspect of electronics... some Metamaterials can do things that would have seemed miraculous just a few decades ago."
What are metamaterials?They are substances with optical properties that do not exist in nature.Metamaterials are created by embedding tiny components into a material that can force electromagnetic waves to bend at unusual angles.At Duke University, scientists embedded tiny circuits in copper rings arranged in flat, concentric circles (somewhat like the rings of an electric stove).The result is a fine mixture of ceramics, Teflon and hybrid fibers, with tiny implants in the copper strips that allow them to bend and direct microwave radiation in specific ways.Imagine a river flowing around a boulder.As the river quickly bypasses the boulder, the boulder is swept downstream.Likewise, metamaterials can constantly alter and bend the paths of microwaves so that they flow around a -- say, cylinder -- essentially making everything inside the cylinder invisible to the microwaves.If the metamaterial could eliminate all reflections and shadows, it could ensure that an object would be completely invisible to this type of radiation.
The scientists successfully demonstrated the principle using a device consisting of 10 fiberglass rings covered with copper electrical elements.A copper ring inside one device was almost completely invisible to microwave radiation, casting only a very small shadow.
At the heart of a metamaterial is its ability to control something called the "index of refraction."Refraction is the deflection of light when it passes through a transparent medium.If you put your hand in water, or look at your hand through the lens of your glasses, you'll notice that the water and glass distort and bend the path of ordinary light.
The reason light bends in glass and water is that light slows down as it enters a dense, transparent medium.The speed of light in a vacuum is always the same, but light traveling through glass or water has to pass through trillions of atoms, so the speed is slow (the speed of light divided by the slowing light in the medium is called the index of refraction. Because Light is slowed down in glass, whose refractive index is always greater than 1.0).For example, the index of refraction in vacuum is 1.0, in air it is 1.0003, in glass it is 1.5, and in diamond it is 2.4.In general, the denser the medium, the greater the degree of curvature and thus the greater the index of refraction.
One of the common examples of the index of refraction is a mirage.If you drive on a hot day and look straight at the horizon, the road may appear to have shimmering lights, creating the illusion of a shimmering lake; in the desert one can sometimes see cities and mountains on the horizon in the distance. The silhouette of mountains, because the hot air rising from roads and deserts is less dense than normal air and thus has a lower index of refraction than the cooler surrounding air, so that light from distant objects is refracted from the road to you in your eyes, creating the illusion that you are looking at something far away.
Usually, the index of refraction is a constant.When a narrow beam of light enters the glass, it is bent and then kept traveling in a straight line.Assuming you can control the index of refraction arbitrarily, then it can change direction continuously at any point in the glass.As light travels through this new material, the light energy is bent and flows in new and different directions, creating serpentine paths that can travel through the entire material.
If the index of refraction inside the metamaterial can be controlled, light can pass around the object, making the object invisible.In order to achieve this, the metamaterial must have a negative index of refraction, which is impossible in all optics textbooks (metamaterials were invented by the Soviet physicist Victor Veselago in 1967) First theorized in a paper and shown to have unusual optical properties such as a negative index of refraction and the reversed Doppler effect. Metamaterials are so weird and anomalous that they have been considered impossible. But in the past few years, metamaterials have indeed been produced in the laboratory, forcing reluctant physicists to rewrite all textbooks on optics).
Metamaterials researchers are constantly harassed by journalists who want to know when invisibility cloaks will hit the market.The answer is: not in the near future.
"Journalists call and they just want you to come up with a number. Months, years. They keep asking, asking, asking, and asking," said David Smith of Duke University. So you end up saying: Well, like 15 years. So they get the headlines, don't they? 15 years for Harry Potter's invisibility cloak." That's why he's refusing to give any detailed timeline now . Harry Potter and Star Trek fans may have to wait.While true invisibility cloaks have become possible within the bounds of the laws of physics, most physicists believe that the remaining insurmountable technical hurdle for this technology will be: extending research to visible light, not just microwaves.
Typically, the internal components to be embedded in a metamaterial must be smaller than the wavelength of the radiation.For example, microwaves have a wavelength of about 3 centimeters, so metamaterials capable of bending microwave paths must have tiny implants smaller than 3 centimeters.But to make an object invisible under green light with a wavelength of 500 nanometers, the metamaterial must have an internal structure that is only about 50 nanometers long, and a nanometer is a unit of length at the atomic level, requiring the use of nanotechnology (1 nanometer is equivalent in length to One-billionth of a meter. 1 nanometer can hold about 5 atoms).This may be the key question we face in our attempts to create true invisibility cloaks.Individual atoms in the metamaterial must be modified to bend the beam into a serpentine shape.
The race goes on.
Since the announcement that metamaterials have been made in the laboratory, the field has been buzzing, with new advances and astonishing breakthroughs emerging every few months.The goal is clear: use nanotechnology to create metamaterials that bend visible light, not just microwaves.Several proposals have been proposed, all very promising.
One option is to use off-the-shelf technology, that is, to borrow existing technology from the semiconductor industry to create new metamaterials.A technique called photolithography is at the heart of computer miniaturization and thus the computer revolution.The technique allows engineers to pack hundreds of millions of tiny transistors onto a silicon chip no bigger than a thumb.
The reason computers' processing power doubles every 18 months (known as "Moore's Law") is that scientists use ultraviolet radiation to "etch" smaller and smaller parts onto silicon chips.The technique is much like stencils are used to produce colored T-shirts (computer engineers start with a thin sheet and then place a very thin outer layer of multiple materials on top of it. The sheet is then covered with a plastic mold that serves as the model .Includes the intricate outlines of the wires, transistors, and computer parts that make up the infrastructure of the circuitry.The flakes are then placed in very short-wavelength ultraviolet rays, which imprint the shapes on the photosensitive wafer.Treated with special gases and acids Once the sheet is removed, the intricate circuitry on the plastic mold is etched into the parts of the sheet that had been exposed to ultraviolet light. This process creates a sheet containing hundreds of millions of microscopic trenches that form the outlines of the transistors).Currently, the smallest features that can be fabricated using this etching method are about 30 nanometers in size (or about 150 atoms in length).
A milestone in the quest for invisibility came when a team of scientists used this silicon chip etching technique to create the first metamaterial that works in the visible range of light.Scientists from Germany and the U.S. Department of Energy announced in early 2007 that, for the first time ever, they had created a metamaterial that could function in red light. The "impossible" was achieved in an unusually short time.
Physicists Costas Soukoulis of Ames Laboratory in Iowa and Stefan Linden of the University of Karlsruhe in Germany ), Martin Wegener, and Gunnar Dolling created a metamaterial with a negative index of refraction of -0.6 in red light at a wavelength of 780 nanometers (previously, metamaterial-bent The world record for this ray is 1400 nanometers, which excludes it from the visible light spectrum and falls into the infrared range).
The scientists used a thin sheet of glass, then coated it with a layer of silver, a layer of magnesium fluoride, and then another layer of silver to form a magnesium "sandwich" just 100 nanometers thick.Then, using conventional etching techniques, a large array of microscopic square holes were created in the "sandwich" to form a fishnet-like lattice structure (the square holes are only 100 nanometers wide, much smaller than the wavelength of red light).They then shot a beam of red light through the material and measured its refractive index: -0.6.
These physicists predict many practical applications of this technology.Metamaterials "may one day lead to the development of superlenses that function in the visible spectrum," Dr Sucules said. Details at much smaller wavelengths.” Immediate applications of this “superlens” would be to photograph miniature objects with unprecedented clarity, such as the inside of a living human cell, or to diagnose a disease in a baby in the womb.Ideally, people would be able to obtain pictures of the building blocks of the DNA molecule without resorting to clumsy X-ray crystallography.
So far, scientists have confirmed the negative refractive index of red light.Their next step will be to use this technique to create a metamaterial that can bend red light completely around an object, rendering the object completely invisible in red light.
Following these paths, further developments may occur in the field of "photonic crystals".The goal of photonics technology is to create chips that use light, rather than electricity, to process information.This involves using nanotechnology to etch tiny components onto the chip so that the index of refraction changes according to each component.Transistors that use light have several advantages over those that use electricity.Photonic crystals, for example, lose much less heat (advanced silicon chips generate enough heat to fry an egg. Therefore, they must be kept cool or they fail. Keeping them cold is expensive).Not surprisingly, the science of photonic crystals is well suited to metamaterials, since both technologies involve manipulating the refractive index of light on the nanometer scale.
Although not yet surpassed, another research group announced in mid-2007 that they had created a metamaterial that bends visible light using a completely different method called "plasmon photonics."Cal Tech's Henri Lezec, Jennifer Dionne and Harry Atwater announced that they have created a Metamaterials with a negative refractive index in the high blue-green visible spectral range.
The purpose of plasmonic photons is to "squeeze" light, allowing us to manipulate objects at the nanoscale, especially on the surface of metals.Metals conduct electricity because electrons are loosely bound to metal atoms so that they can move freely along the metal's structured surface.The current flowing in the wires in your home represents the smooth flow of electrons loosely bound on these metal surfaces.However, under certain conditions, when a beam of light hits a metal surface, the electrons vibrate in unison with the original beam, creating wave-like motions on the metal surface (called plasma) that in turn coincide with the original beam. vibration.What's more, we can "squeeze" these plasmas so that they have the same frequency (and therefore carry the same information) as the original beam, but at a much smaller wavelength.In theory, we could then stuff these squeezed waves into the nanowires.As with photonic crystals, the ultimate goal of plasmonic crystals is to create computer chips that run on light rather than electricity.
The Caltech team fabricated their metamaterial using two layers of silver with an insulating layer of silicon-nickel (just 50 nanometers thick) in between that acts as a "waveguide" that guides the direction of the plasma waves.Laser light enters and exits the instrument through two slits carved into the metamaterial.By analyzing the angle at which the laser light travels through the metamaterial, we can confirm that the light is bent with a negative refractive index.
Advances in metamaterials are likely to accelerate in the future, simply because there is currently a huge need to create transistors that use light beams and non-electric power.Research into invisibility can thus also hitch a ride on ongoing photonic crystals and plasmonic photonics aimed at creating silicon chip replacements.With hundreds of millions of dollars invested in technologies that create alternatives to silicon chips, metamaterials research could benefit from these research efforts.
With breakthroughs being made every few months in this field, it's no surprise that some physicists think some form of practical invisible shield could be produced in the lab within a few decades.For example, scientists are confident that within the next few years they will be able to create metamaterials that can render objects completely invisible at one frequency of visible light, at least in two dimensions.To do this, the tiny nanoimplants must not be fixed in a regular array, but arranged in intricate patterns so that the light beam will bend smoothly around an object.
Next, scientists must create metamaterials that bend light in three dimensions, not just on flat, two-dimensional surfaces.Photolithography is the perfect technique for making flat silicon wafers, but making three-dimensional metamaterials requires building the wafers into complex forms.
Scientists have since had to solve a difficult problem -- creating metamaterials that can bend not just one frequency, but many.This may be the most difficult step, as the tiny implants designed so far can only bend precisely one frequency.Scientists may have to create metamaterials based on layers, with each layer bending a specific frequency.The solution to this problem is not yet clear.
Once the Invisibility Shield is finally made, though, it's likely to be a cumbersome contraption.Harry Potter's invisibility cloak is made of a thin, flexible fabric and can render anyone wearing it invisible.But for this to work, the refractive index inside the cloak would have to change as it flutters, which is impractical.True cloaks are more likely to be constructed from solid cylinders of metamaterials, at least initially.This way the cylinder would have a fixed internal index of refraction (more advanced cloaking shields may eventually incorporate flexible metamaterials that can twist and still keep light passing through the interior on the correct path. By doing this In this way, anyone inside the cloak can move freely).
Someone pointed out a flaw in the invisibility shield: anyone inside it cannot see outside without showing up.Imagine Harry Potter being completely invisible except for the eyes, which appear to float in mid-air.Any holes cut for eyes in the cloak can be clearly seen from the outside.If Harry Potter were completely invisible, he would sit under his cloak with his eyes blacked out (one solution to this problem might be to place two pieces of glass near the eye holes. These two The glass sheet can act as a "light splitter", taking away a small part of the light that hits the glass sheet, and then sending the light into the eyes. In this way, most of the light that reaches the cloak will flow around it, ensuring A person in an invisibility cloak is invisible, but a very small amount of light is diverted into the eyes).
Scientists and engineers are as optimistic as they are about creating some form of stealth shield in the coming decades, as much as they are pessimistic due to these difficulties.
As I mentioned earlier, the key to invisibility may be nanotechnology, that is, the ability to manipulate atomic-sized structures that are a billionth of a meter in diameter.
The birth of nanotechnology can be traced back to a lecture given by Nobel laureate Richard Feynman to the American Physical Society in 1959. space".In that lecture, he predicted what the smallest machines that obey the known laws of physics might take.He realized that machines could be made smaller and smaller until they reached the interatomic distance, and the atoms could then be used to make other machines. "Atomic machines, such as moving pulleys, levers and wheels, are within the domain of the laws of physics," he concludes, even though they would be extremely difficult to manufacture.
Nanotechnology waned for a few years because manipulating individual atoms was beyond the state of the art at the time.But in 1981, with the invention of the scanning tunneling microscope, physicists made a breakthrough, which led to the discovery of Gerd Bining, a scientist at the IBM laboratory in Zurich. and Heinrich Rohrer won the Nobel Prize.
Suddenly, physicists had astonishing "pictures" of individual atoms arranged as they appear in chemistry books, a situation once considered impossible by critics of atomic theory.Brilliant pictures of atoms arranged in crystals and metals are now possible.Scientists often use chemical formulas in which a series of complex atoms are packed into a molecule and can be seen with the naked eye.Furthermore, scanning tunneling microscopy makes it possible to manipulate individual atoms.In fact, the letters "IBM" were spelled out using atoms, creating a stir in the scientific community.Instead of being at a loss when manipulating individual atoms, scientists can actually see and play with them.
The scanning tunneling microscope is surprisingly simple.Like a stylus being swept across a record, a probe (a very sharp needle tip consisting of only one atom) is slowly passed through the material to be analyzed.A small electrical charge is placed on the probe, and a current flows from the probe, through the entire material, to the underlying surface.As the probe passes through individual atoms, the amount of current flowing through the probe varies, and these changes are recorded.The electric current rises and falls as it flows through an atom, thus protruding its outline with extreme finesse.By plotting the fluctuations in the amount of current after many passes, one can get a beautiful picture of the individual atoms that make up a grid structure.
(Scanning tunneling microscopes are made possible by a peculiar law of quantum physics. Normally the electrons don't have enough energy to pass from the probe through the object to the bottom layer. But due to the uncertainty principle, there are gaps in the current flow. The tiny possibility that electrons can "drill" or penetrate barriers. In this way, the current flowing through the probe is sensitive to tiny quantum effects in the material. I'll discuss the implications of quantum theory in more detail later).
The probes are also sensitive enough to move individual atoms, creating simple "machines" made of individual atoms.The technology is so advanced that groups of atoms can now be displayed on a screen, and then just move the computer cursor and the atoms can move in any way you want.You can manipulate large piles of atoms like Lego bricks.In addition to spelling out the letters of the alphabet from individual atoms, we can also make atomic toys, such as an abacus made of individual atoms.Atoms are arranged in planes with longitudinal narrow grooves.These narrow longitudinal grooves can be placed in carbon (shaped like a football, but made of individual carbon atoms).The balls can then be moved up and down in narrow slots, creating an atomic abacus.
It is also possible to sculpt atomic devices using electron beams.For example, scientists at Cornell University have created the world's smallest guitar, 20 times smaller than a human hair, carved out of crystalline silicon.It has six strings, each 100 atoms thick, that can be plucked under an atomic force microscope (the guitar can indeed play music, but it produces a frequency much higher than the human ear can hear hearing range).
For now, these nanotech "machines" are mostly just toys.More complex machinery with gears and ball bearings has not yet been manufactured.But many engineers are confident that the day will come when we will be able to build true atomic machines (atomic machines have already been discovered in nature: cells can swim freely in water because they can wiggle tiny hairs. But when we When we analyze the junction between the hair and the cell, we see that it is in fact an atomic machine that enables the hair to move in all directions. So one of the keys to developing nanotechnology is to imitate nature, which was created billions of years ago mastered the art of atomic mechanics).
Another way to make people partially invisible is to take a picture of the background behind a person and project that background image onto the person's clothing, or onto a screen in front of them.From the front it seemed that the man had become transparent, that the light had somehow passed through his body impartially.
Naoki Kawakami of the Tachi Laboratory at the University of Tokyo has been hard at work on this approach, a process known as "optical camouflage.""This will be used to help pilots see the runway below through the cabin floor, or to help park drivers trying to see on the other side of the guardrail," he said. to the movie screen.A video camera captures an image of the back of the garment, which is then fed to a projector that illuminates the front of the garment so that it appears as if the light is passing through the person.
The prototype of visual camouflage actually exists in the laboratory.If you look directly at a person wearing this screen-like robe, that person will appear to have disappeared, because all you see is what is behind him.But if you move your gaze a little and the image on the robe doesn't change, that lets you know it's an illusion.More realistic visual camouflage requires creating the illusion of 3D images.For this purpose, we need holograms.
全息图是激光制造的3D影像（例如《星球大战》中莱娅公主的3D影像）。如果周围景色被一个特殊的全息照相机拍摄下来，随后全息图像被投射到一个人身前的一整片全息银幕上，那么这个人可以处于隐身状态。站在那人跟前的观看者会看到有着背景景色3D图像的全息银幕，人本身缺省。就算移动视线，你也无法确定自己所见到的是假象。
这些3D图像是由于激光“相干”而成为可能的，即所有的波完全共振。全息图像是通过将一束相干涉的激光分裂成两片而产生的。一半的光束照射在照相胶片上，另一半照射到一个物体上，被弹开，然后反射到同一张照相胶片上。当这两股光束在胶片上产生干涉，一种干涉图形就形成了，并且将原始3D光波的所有信息都编码。胶片上随后会出现错综复杂的蜘蛛网样（看上去不怎么像）回旋和线条。但是随后会有一束激光被投射到这张胶片上，一个原始物体的精确3D复制品突然间就像被施了魔法一样出现了。
然而，全息隐形的技术问题是难以克服的。其挑战之一是制造出1秒钟至少能拍摄30帧画面的全息照相机。另一个问题是储存和处理所有的信息。最后，我们必须把这幅图像投射到一块银幕上，这样图像看起来会显得真实。
我们还必须说说一种更为复杂的隐形方法，威尔斯在《隐形人》中提到了它。这种方法涉及使用第四维度的力量（在本书中我稍后会更加详尽地探讨高维空间存在的可能性）。我们是否可能离开我们的三维宇宙，从四维空间的有利地点在其之上翱翔呢？就像一只三维的蝴蝶在一张二维的纸片上面飞舞一样，我们对于任何生活在我们下方宇宙中的人都是隐形的。这个想法有一个问题：高维空间的存在尚未被证明。而且，去往一个更高维度的假想旅行需要的能量远远超过我们现有科技可实现的水平。作为一种实现隐形的可行方法，这一方法无疑超过了我们当今的知识和能力。
鉴于迄今为止在实现隐形方面的巨大进展，它具备了“一等不可思议”的资格。在未来几十年中，或者至少这个世纪之内，某种形式的隐形将会变得稀松平常。
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