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 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.
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. The theory 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.
:The Source
http://www.theguardian.com
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