Here's how you could become invisible in the middle ages: grind up an owl's eye with a ball of beetle dung and some olive oil, and rub it all over your body. There's no record that this method was ever tried, so I guess we don't actually know if it works. But it's a relief that modern science and technology now supply some choices, even if none are perfect. Here are five of them.
Is this a case of science achieving what magic couldn't? We should be cautious about that sort of claim. Invisibility has been a coveted power since antiquity, but the stories we tell about it are fables of power and corruption, irresponsibility and voyeurism. If we ever seem likely to make Harry Potter's cloak of invisibility or Frodo Baggins's ring for real, we might want to ask who wants it, and why. Remember that when Christopher Marlowe's Dr Faustus says "charm me, that I may be invisible, to do what I please, unseen of any", he's asking Mephistopheles.
1 | THE REFLECTION CLOAK
The robotics and computer engineer Susumu Tachi of Keio University has beenmaking people vanish into the urban landscapes of Japan. These ghostly figures stand shrouded in a cloak through which you can see buses and pedestrians passing behind. The scene isn't quite crisp, it's a bit off-colour, and the hooded face and folds of the garment slightly give the game away – but the effect is still uncanny.
Tachi is using much the same illusion that can make a person look transparent if they stand in front of an image projected on to a screen. The difference is that Tachi's cloak is not reflecting some random image, but the real scene behind it. A camera placed just behind the cloaked figure records the view and relays it to a projector in front. The cloak itself is made from a material Tachi calls "retro-reflectum": it is covered with tiny light-reflecting beads, so the projected image bouncing back to our eyes is as bright as daylight.
The catch is that the "invisible" person has to stay put, since the camera and projector are fixed in place. And the cloak only works well if you're looking at it face on – from the side or behind, it's not invisible at all.
That's not such a drawback if you're making static surfaces "invisible" – or rather, apparently transparent. A wall painted with retro-reflectum could be turned into a window without having to knock a hole in it – handy for protected buildings, say. And a car interior you can "see through" could help prevent accidents caused by blind spots. Quite how drivers would feel driving what seems like a glass car is another matter.
2 | THE PROJECTION CLOAK
The immobilising demands of Tachi's cloak might be avoided by placing the cameras and the projector on the cloak itself. In other words, rather than casting an image on to a screen-like cloak, the cloak itself would project the image directly to our eyes – like a wrap-around LED television screen. This trick could work from any viewing direction, provided there are cameras pointing that way to record the scene. The idea, then, is a cloak covered with LED display units that, via fisheye lenses, can send light rays in every direction tailored to create exactly the image that a viewer would see from that direction if the cloak and wearer weren't there at all.
In practice this presents a phenomenal engineering challenge. It's not so hard to make the full-colour light emitters and cameras that would cover the cloak like tiny sequins; the real difficulty comes from all the computing needed to turn the information from the array of micro-cameras into instructions for what to project at every angle, especially since this would constantly change as the wearer moves. Italian computer scientists Franco Zambonelli and Marco Mamei have worked out all the technological and computational requirements and estimate that such a cloak, giving a reasonable semblance of invisibility, could be made for €500,000. Other computer experts are sceptical – for one thing, because of parallax effects (the distance sense we get from binocular vision), no camera could record exactly what we would see unless it were situated right where we were standing.
All the same, this vanishing trick is already being planned. The American architectural company GDS Architects has designed a 1,500ft skyscraper called Tower Infinity for the Seoul suburb of Incheon in South Korea that would be covered with banks of cameras and LEDs on the glass facade so that it could project itself into invisibility – albeit only "perfectly" from a few select viewing locations. The artists' impressions show the tower perhaps rather optimistically fading from view in the dusk sky. Construction has been granted approval, so maybe we'll get to see if it works.
3 | PERFECT TRANSPARENCY
In The Invisible Man (1897), HG Wells wanted his anti-hero Griffin to be made invisible by a scientifically plausible method rather than mere "jiggery-pokery magic". A more naive writer would have suggested (and some did) that all you needed was to make a person totally transparent, like glass. True, that doesn't sound easy, but Wells pointed out – with only a little bit of artistic licence – that apart from our blood and the pigment in hair, our body tissues are transparent. (Bone scatters light rather as milk does, but never mind.) Griffin gets rid of this pigmentation with a chemical drug he devises – as he's an albino, he doesn't have far to go anyway. Wells realised that this would make a person blind, because the retina has to absorb light to work at all – but he felt he could ignore that.
The bigger problem is that – as we can see with a glass beaker – transparency alone doesn't guarantee invisibility. One problem is that the smooth glass surface reflects light, although our rougher skin might not. But glass also refracts light: it bends the rays that pass through, distorting the image behind it. This is because light travels more slowly in glass than in air, and so to take the quickest possible route from an object to our eye, the light takes a crooked path to reduce the time it spends in the "slow" substance. The amount by which a material slows light is called its refractive index: air has a refractive index of one, and the index of all ordinary transparent materials, like glass and water, is greater than one.
To eliminate refraction, Wells realised that he had to somehow reduce the refractive index of Griffin's tissues to one. There was no known way to do that, and there still isn't, so Wells had to resort to a bit of magic after all: Griffin uses electrical gadgets to produce a kind of invisible ray, similar to the X-rays discovered only two years earlier, that induces this transformation.
All the same, the principle of matching the refractive index of an object and its surrounding medium is sound, and transparent things really can be hidden that way. Place a glass rod (refractive index of around 1.5) into clear baby oil or benzene, which has essentially the same refractive index, and it seems to disappear entirely.
4 | METAMATERIALS
Some researchers believe the real future of invisibility lies with a new science, transformation optics. This is all about controlling the paths of light rays, and it is analogous to the way that light curves when space itself is curved: something caused by strong gravitational fields, as predicted by Einstein's theory of general relativity.
Transformation optics is so called because it is rather like transforming the co-ordinate grid of space. But it doesn't literally do that. Instead, light rays are passed step by step between tiny receivers and transmitters – the "meta-atoms" of the material – in a way that traces out paths rays could never follow in an ordinary transparent material. Thetheory was developed in the late 1990s by John Pendry at Imperial College London, and later he and Ulf Leonhardt at the University of St Andrews independently figured out how to use it to make metamaterial "invisibility shields". The idea is that light rays are bent smoothly around an object placed in the centre of a metamaterial shell, and recombined on the other side like water flowing around a rock in a stream. To a viewer on the far side, it's as if nothing has happened to the light during its passage: both shield and internal object are invisible.
Electrical engineer David Smith and his team at Duke University in North Carolina worked out how to make a real metamaterial in the late 1990s, using slotted arrays of printed circuit boards on which the metal loops and rings are etched. Because the "meta-atoms" have to be about the same size as the wavelength of the light they are manipulating, this design works for microwaves, not for visible light (which has wavelengths of just a few tenths of a micrometre). In 2006 Smith's group, working with Pendry, unveiled the first microwave invisibility shield: a set of 10 concentric, cylindrical rings of meta-atoms, which could more or less hide an object inside from microwaves.
Shrinking the metamaterial to the size needed for visible light is hard. But Pendry and his student Jensen Li proposed a simpler design in 2008: a "carpet cloak" that sits on a surface and hides an object under a bump. Here the light can be bent simply by arrays of tiny holes, and researchers at the University of California at Berkeley carved such a microscopic structure out of a silicon chip in 2009.
5 | ILLUSION OPTICS AND WAVE CLOAKS
The control of light rays offered by transformation optics can be used to alter appearances beyond making objects invisible. In principle, one can design a metamaterial shield that will twist and bend light so any object inside can be made to look like any other object. It's a fearsome task to figure out what kind of meta-atoms you need, let alone to make them, but the idea is clear enough in theory. Che Ting Chan of the Hong Kong University of Science and Technology and his co-workers have proposed a new field called illusion optics, which enables this more or less infinitely protean shape-shifting.
One simple variant of the idea is to make an object look bigger than it really is. Pendry compares this to the way the scattering of light rays passing through a milk bottle make it seem as though the milk goes all the way to the edge: you can't see the thickness of the glass walls at all. With a metamaterial, you could make the light seem to extend beyond the physical edge of the structure, into empty space. Then you could make a hidden portal, concealed because the metamaterial walls on each side appear to extend across the open space.
The principles behind transformation optics apply to all kinds of wave, not just to light. Researchers have proposed and constructed acoustic versions of invisibility shields and other structures: devices that seem invisible to sound waves as they pass through, so that for example a submarine might be made invisible to sonar. Using large arrays of holes in the ground, perhaps filled with softer material, it might even be possible to create seismic shields that make buildings or even cities invisible to earthquakes. And researchers at the Karlsruhe Institute of Technology in Germany have used the same ideas developed for acoustic cloaking – where the aim is to shield an object from the mechanical vibrations of sound waves – to create an "unfeelability cloak", a delicately structured polymer foam that, when squeezed, deforms in a way that smoothes away any bumps created by objects hidden beneath them.
Perhaps the most remarkable cloak demonstrated so far hides objects from light not just in space but in time: it's a"spacetime cloak". The researchers who devised it, physicists Martin McCall and Paul Kinsler of Imperial College London, illustrate the idea by imagining a thief breaking into a safe that is watched by a security video camera. The act could be hidden by editing out that part of the video, but you'd notice the jump and the missing time. But with spacetime cloaking, it's as if the time before and after the edit is stretched so that the two segments are joined seamlessly with no obvious jump at all. That's not just some trick of editing software: the spacetime itself is deformed this way.
The trick depends on manipulating the speed of light, which is a kind of universal gauge of the rate of time passing. Metamaterials could do it, but a simpler (slightly imperfect) way is to vary the refractive index of the material the light passes through. This can be done in optical fibres: an intense laser "control beam" can manipulate the refractive index of the glass fibre so the light from a signal beam seems to slow down and speed up. In 2011 a team at Cornell University demonstrated the idea, in effect hiding a light beam in a spacetime hole for 15 trillionths of a second.
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