Invisibility becomes reality?
As far back as Plato people imagined the power of invisibility. Now the fantasy is being made real. Philip Ball reports.
A mysterious visitor appears in the sleepy English village of Iping with bandages around his face, dark glasses, a false nose and wig. To their horror, the villagers soon discover that the man underneath all these layers is totally invisible.
He is a medical student who discovered the secret of invisibility by performing experiments on himself – and does not know how to reverse the process.
As Griffin, the antihero of H. G. Wells’ 1897 novel The Invisible Man, grows more desperate, he terrorises the villagers. The situation goes from bad to worse, as he descends into a homicidal fury and plans world domination. “This is day one of year one of the new epoch,” he declares “the epoch of the invisible man”.
In fiction, power corrupts, but invisibility corrupts absolutely. The trope goes back to Plato. In one of his myths, a magical ring that confers invisibility tempts the shepherd Gyges to kill the king of Lydia, seduce his queen, and set himself up as the kingdom’s new ruler. Despite the philospher’s warning, our fascination with invisibility has not waned. Now science is starting to make it possible.
Over the past decade researchers have developed cloaks, shields and other devices that seem to make fish, cats and even people vanish. Spinoff technologies could shield objects from sonar, radar and our sense of touch. They might even protect buildings from earthquake shock waves.
But if we are really embarking on the epoch of the invisible man, we might want to listen to what Plato and H. G. Wells were telling us.
Devices seem to make fish, cats and even people vanish.
As a predator approaches, a flatfish lies still near the sea bottom, its skin almost indistinguishable from the sea floor. These creatures are among a select bunch, including octopuses and squid, that use adaptive camouflage to achieve a kind of invisibility. When a flatfish sees a mottled, brown sea floor, cells in its visual cortex signal other brain cells. These in turn signal skin cells to alter their pigmentation or reflective properties to blend in.
A flatfish-like strategy is behind one of the more ambitious attempts at invisibility – a planned 450-metre skyscraper called Tower Infinity in Seoul, South Korea, that will seemingly disappear on cue. A US-based architecture firm plans to cover the tower with flat LED screens. Like the flatfish’s eyes, video cameras mounted up and down the tower’s exterior will record the scene around it. The screens will display what the cameras “see”, making the tower fade out of sight – at least when seen from the right distance.
The flatfish technique is also being used to hide people. In a much-viewed YouTube demonstration, a man wearing an “invisibility” cloak stands in front of the camera on a busy Japanese street. Passing pedestrians, trucks and buses are visible right through him. The hooded cloak is thickly encrusted with tiny reflective glass beads that allow it to function as a projection screen. It was created by Japanese electronic engineer Susumu Tachi, now at the University of Tokyo.
Here’s how it’s done. Tachi and his colleagues rig up a video camera behind the cloaked figure that records the scene without the figure. The camera relays the signal to a projector in front of the cloak that casts the scene on to the cloak’s coating, creating an image as bright as the day-lit scene around it – and a passable illusion of invisibility.
The technology might have some useful applications, Tachi says. For example, adorning a solid wall with a rectangle of the glass bead carpet, which Tachi calls “retro-reflectum”, could simulate a window, giving residents of protected old buildings a view of sorts without knocking out any walls. And coating a car or truck interior with retro-reflectum and mounting cameras on the outside could give the driver 360-degree visibility.
But the technique is cumbersome. Creating the background image on the cloak requires video cameras and projectors, and you only see it properly if you’re standing in the right spot. So Tachi’s technology could never let anyone walk unseen among us.
An invisibility cloak that uses a mobile version of the Tower Infinity approach would have a better chance. Such a cloak would have miniaturised video cameras and flexible, lightweight LEDs woven into its fabric. As the person moved around, the cloak’s video cameras would record his surroundings, and the LEDs in the fabric would project an image.
Of course the flatfish achieves this feat rather more economically.
Some researchers are getting closer to Harry Potter’s invisibility cloak by doing away with cameras and projection screens altogether. They are relying on materials that can bend light in ways the normal laws of optics would seem to prohibit.
The idea is to erase an object by diverting light rays around it and then restoring them to their original path. To an observer on the far side it looks as though the rays have travelled in a straight line through empty space. In 2006, physicist John Pendry of Imperial College in London and electronic engineer David Smith of Duke University independently reported blueprints for such a device. A few months later, Smith’s team dazzled invisibility aficionados by building a prototype.
These devices employ “optical metamaterials”, electrical components that relay light signals and bend light in unusual ways. When a light ray enters a conventional transparent material, such as glass, it veers (or refracts) somewhat but continues in the same general direction. The degree of bending is called the refractive index. In glass and other ordinary materials it’s a positive number.
The optical metamaterial in Smith’s prototype invisibility shield has a negative refractive index. This means that when a light ray enters, it bends the “wrong” way compared with what we’re used to seeing, creating some bizarre optical effects. The properties that enable this behaviour also allow optical metamaterials, when properly chosen and arranged in space, to steer a light ray around an object in its path.
In the 1990s Pendry charted a course toward such metamaterials. The way he envisioned it, you could design the electrical components from ultrathin metal wires – a few atoms thick – that act as tiny receivers and antennae to guide light around an object. (The rule is that the transmitters need to be of the same dimensions as the wavelength of light they are guiding). The drawback is that working with such thin wires would have been extremely difficult.
Some researchers are getting closer to Harry Potter's invisibility cloak.
So Smith, who was then at the University of California at San Diego, aimed for a more achievable target. Rather than guide light waves, he aimed for much larger microwaves. His team made their transmitters by bending ordinary wire into little loops, several millimetres in diameter. Making a whole array of such loops by hand was painstaking work for Smith’s graduate students, but they managed to assemble them into the world’s first metamaterial with a negative refractive index. Later, he and his co-workers made the process faster and easier by etching arrays of the loops and rings into the copper foil of printed circuit boards.
Smith’s invisibility shield was a breakthrough – but it was still not Harry Potter’s cloak. The shield itself was perfectly visible. And it was not diverting light waves but microwaves.
Making metamaterial objects invisible to the human eye will require microscopically small components – this is quite beyond today’s fabrication methods. But in coming up with designs for invisibility shields, Pendry and, independently, physicist Ulf Leonhardt of the University of St Andrews in Scotland, developed a mathematical theory called “transformation optics”. This describes how to reshape the trajectories of light rays. The theory suggested some easier alternatives for invisibility shields, if you didn’t mind compromising on the cloaking you could achieve.
In 2008 Pendry and his student Jensen Li showed how to create a “carpet cloak”. Imagine placing a box on the floor, then draping a regular carpet over it. You’d see a big hump in the fabric. The carpet cloak interacts with the light in such a way that the carpet appears to lie flat on the floor.
Researchers at the University of California at Berkeley, led by Xiang Zhang, built such a device that works for near-infrared light. They used a simple array of microscopic holes etched into silicon to make a carpet cloak with a tendency to bend light that varies from one part of the structure to another – the largest bends are just above the hump and mask its presence. The Berkeley team has since fashioned a similar cloak from a patterned piece of silicon nitride that works for visible light.
You can also make a similar, though cruder, kind of invisibility carpet from the transparent natural mineral calcite. It has the unusual property of bending light that strikes it from one direction more than light that strikes it from another direction. This property allows a cunning arrangement of specially shaped calcite prisms to bend light rays so that they appear to be bouncing off a flat surface beneath the prisms, thereby hiding a small cavity that contains an object. Although the shield does remain visible, much as an ordinary glass prism does, the objects within it do seem to vanish.
An even cruder, but still effective, invisibility shield relies on a special arrangement of high-quality optical-glass prisms to bend light around the central cavity. Again, the prisms themselves were visible, and the cloaking only works when viewing the device at eye level, rather than from above or below – but it does work, after a fashion. Built in 2013 by researchers at Zhejiang University in Hangzhou, China, the shield made a cat disappear (the feat reminiscent of Alice in Wonderland’s Cheshire cat). When the Chinese feline investigated the cavity, any part of its body that passed inside seemed to vanish.
As they edged closer to invisibility, some scientists realised invisibility was just a start. That’s because transformation optics is actually a general theory of how to manipulate waves, and as such it applies equally to, say, sound waves and even water and seismic waves. The theory has now led to a bevy of bizarre new devices. Transformation optics, Pendry says, “is galloping off in all directions. The fundamental theory is done and dusted, but there seems to be no end of new applications.”
One application takes advantage of fogs. People moving though a fog appear as blurry shapes. But if their raincoats were made of the right kind of fabric, they might entirely disappear. When a light ray enters a fog, it bounces repeatedly off the droplets, making the light appear to come from all directions. Light does the same thing in frosted glass, where it bounces off bumps on the glass. In both cases this makes the outline of an object less distinct, but enough of the light still stays on course to make the object itself visible as a vague shadow.
It’s possible to design a cloak that redirects the diffuse light enough to hide such shadows fully. The trick is to adjust the amount of light scattering in this sort of misty medium, so that the mist looks equally bright everywhere, rendering the object invisible. Physicist Martin Wegener of the Karlsruhe Institute of Technology in Germany and co-workers demonstrated this approach with cylindrical and spherical cloaking shells made from a transparent plastic that contain dispersed small particles that scatter light. The shell and its contents seemed to disappear when placed into a tank of water made misty by adding dilute white paint.
Another application is an acoustic cloak that could, in principle, mask a submarine from sonar. Just as an ocean wave deforms the sea surface as it passes, so too sound waves deform the materials they pass through, squeezing them in some places and stretching them in others. Wegener and his colleagues have created a prototype acoustic cloak that reshapes those deformations. The cloak consists of a sheet of PVC plastic with a specific pattern of holes that are filled with a rubbery polymer. It works by steering sound waves around a central cavity, muffling noise inside it.
Another prototype is an “unfeelability cloak”. Ordinarily, if you push down on an elastic material when it sits over a hard object you would feel the object through the material. Wegener’s unfeelability cloak is a sculpted mesh of soft plastic that reshapes deformations caused by pressing with your fingers, much as the material in his acoustic cloak reshapes the deformations caused by sound waves. In this way the cloak masks the presence of a lumpy object from our sense of touch. The result is more or less the opposite of the fairy-tale mattress in The Princess and the Pea. (You could probably use mechanical metamaterials to make that, too.)
Huge metamaterial-type structures around the base of a building could even redirect seismic waves to cloak the building from earthquakes. Early tests of this idea suggest that it’s feasible. Researchers at the University of Aix Marseilles, in collaboration with a French engineering company, drilled a grid of five-metre deep boreholes in a bed of silty clay near Grenoble, France. They calculated the dimensions of the holes and the grid required to create a geological metamaterial that would reflect the high-frequency seismic waves produced in earthquakes.
Sure enough, when the French researchers shook the ground on one side of the array using mechanical vibrators, the shaking on the other side was reduced considerably — in some places by more than four-fifths — compared to what it would have been without the array. This isn’t exactly seismic invisibility, since the array reflects the waves rather than diverting them, but the principles, based on transformation optics, are the same.
Stage magicians know that invisibility is not just an optical phenomenon
but a mental one.
A cloaking grid that masked damaging lower-frequency seismic waves from earthquakes could be tougher to construct, though, because the holes would have to be correspondingly bigger. They wouldn’t necessarily have to be empty – it might work just to fill them with a softer substance than the surrounding ground.
All this is a long way from Plato’s Ring of Gyges. But perhaps we’ve possessed the trick of invisibility all along. Stage magicians make things vanish all the time with a bit of skilful misdirection. They know that invisibility is not just, or even mainly, an optical phenomenon, but a mental one.
Simply perceiving yourself as invisible has psychological effects. In a recent piece of high-tech deception, neuroscientist Henrik Ehrsson and his team at the Karolinska Institute in Stockholm, Sweden, fitted subjects with a virtual reality headset that made them feel as if they had an invisible body. With the headset, each subject saw a paintbrush that seemed to brush the empty space where his body should have been, while at the same time his real body was touched by a similar paintbrush. This combination of illusory and tactile sensations gave them a strong sense of having an invisible body. So much so, that when they were subjected to an audience of people watching them intently, the subjects reported experiencing far less anxiety about being scrutinised – and their skin’s electrical conductivity confirmed this.
Will such a feeling of invisibility (even if it’s illusory) erode our moral sensibility, as Plato and H. G. Wells predicted? If so, the matter isn’t likely to end well, as Wells’ invisible character Griffin discovered. Despite being unseen, he is eventually cornered by a mob of locals who beat him to death – whereupon his lifeless body reappears, “naked and pitiful on the ground”.
Will we too succumb to the dangerous allure of invisibility? Perhaps. Just witness the nastiness of anonymous internet trolls. The Karolinska team plans to use their virtual-reality set-up to expose their “invisible” participants to dilemmas and see whether their cloak recalibrates their moral compass. Will they, like Griffin, start to run off the rails? If so, we might wonder whether invisibility is desirable after all.