three ways to become invisible
1 | THE REFLECTION CLOAK
The robotics and computer engineer Susumu Tachi of Keio University has been making 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.
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