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The idea of invisibility seems straight out of science fiction. Harry Potter, Star Trek, Mr. India, Predator and even James Bond showing off invisible humans, spaceships, vehicles and what not!
But the idea of being invisible is not a new one. The idea is as old as 1800’s, when HG Wells in his famous novel ‘The Invisible Man’ wrote about a man who could disappear out of sight.
Scientists have since long observed the NEED to go invisible in nature. Examples include the leopard’s spotted coat and the battledress of a modern soldier – both attempts at staying alive!
Now however, these tricks are useful only as long as the organism is stationary, so they aren’t examples of invisibility in the true sense.. So clearly, there is need for something more – that can make humans invisible while moving – and once again science came to the rescue!
First things first, what is a cloaking device? Cloaking of objects refers to making them disappear. Anything that does that – is called a cloaking device.
Now the object doesn’t really disappear; the vanishing is just an illusion.
And to study how such an illusion is brought about or is even possible, we need to learn how anything is visible to us in the first place.
Basically, light is responsible for everything that the human eye can see. We see objects when light falls on them, bounces off and reaches our eyes. So to make anything invisible, one way is to stop light from interacting with it.
But it’s easier said than done, because light is everywhere and interacts with everything invariably. We can take air out of a space and create vacuum. We can even make a space so soundproof, that its decibel level is negative – but complete absence of light has never been achieved. So how do we go about it? Our scientists have been trying to do that, not by making light go away, but by playing around with it!
This is an interesting concept, because remember, a cloaking device does not mean that you can just cover up the object somehow so that light bouncing from its surface doesn’t reach your eyes; you ALSO have to bend the light around the object so that we can see what’s behind it, but not the object itself.
You can also achieve this by projecting the background on our eyes. One way through which this was achieved by scientists is by making a “Projection Cloak”. It consists of tiny light emitters or LED display units. These LED display units 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 for what to project at every angle, especially since this would constantly change as the wearer moves.
Scientists claim that such a cloak, giving a reasonable semblance of invisibility, could be made for €500,000 (euros). Other computer experts are sceptical – for one thing, because of parallax effects – no camera could record exactly what we would see unless it were situated right where we were standing.
Inspired perhaps by Harry Potter’s invisibility cloak, scientists have been trying to build a cloaking device for some time now, in several ways—some simple and some involving new technologies—to hide objects from view. One successful demonstration for the same was given by a professor and a couple of his graduate students at University of Rochester. The best thing is, it uses inexpensive, readily available materials like lenses in a novel configuration.
Their set-up uses four standard lenses that allows an object to appear invisible even as the viewer moves several degrees away from the optimal viewing positions.
To achieve this level of invisibility and at the same time, allow the background to be seen, we have to carefully coordinate the power of the lens with the distance from one another. This setup has a distinct advantage of being able to work when viewed from a variety of angles, not just straight on from one particular vantage point. The Rochester Cloak can be scaled up as large as the size of the lenses, allowing fairly large objects to be cloaked. And, unlike some other devices, it’s broadband so it works for the whole visible spectrum of light, rather than only for specific frequencies.
However, the set-up has a couple of glitches currently. A simplified path of light rays through Rochester cloak.
But the ‘cloaking’ works from certain angles but not from all angles. So if you were to walk around this device, the cloaked object would become visible. Not only this, it would also be a very bulky set-up, and carrying it along with you to achieve invisibility is just isn’t realistic!
To achieve perfect invisibility, our scientists have had to engineer a completely new product, which could shield something from view by controlling light.
Typically, light moves fastest in vacuum and it becomes slower in gases, followed by liquids and solids. For example, visible light travels through glass about 25 percent slower than it does through vacuum. A material’s fundamental resistance to the transmission of light at a particular frequency is called its ‘index of refraction’ or simply ‘refractive index’. While this number changes with the light’s frequency, it starts at 1 – the index of refraction for a vacuum – and goes up. The higher the index, the slower the light moves, and the more its path bends. This can be seen when looking at a straw in a cup of water and is the basis of how we make lenses for eyeglasses, telescopes and other optical instruments.
Now one path towards making a cloaking device is making a device with negative refractive index. That would mean, for example, that light would bend in the opposite direction when entering the material (like seen in case of straw in water), allowing for new types of lenses to be made. Nothing in nature fits into this category. The properties of such a material – were it to exist – were predicted by Victor Veselago in 1967.
While Veselago could imagine these materials in the late 1960s, he could not conceive of a way to create them. It took an additional 30 years before John Pendry published papers in 1996, 1998 and 1999 describing how to make a composite man-made material with such properties, which he called a metamaterial.
What is this special property that metamaterials possess? We know that when light interacts with an object, it gets reflected and reacher our eyes. But according to physics, if we take an object really really small, light will bend around that object, making us see the background behind the object, and not the object itself.
However, experimentally, such a substance could be created only in 2000.
Uptil now, multiple research groups have made metamaterials that work in the infrared region of light; some are skirting the fringe of the visible portion of the spectrum. For one thing, making a metamaterial work for visible light would require components just 40 nanometers in size, and we are not quite there yet with nanotechnology.
Even so, this approach seems a very practical way forward. We know it is correct theoretically – all that we really want now is to see how well people can build them – which is a challenge, but science is fast catching up.
Yet another way is by achieving near perfect transparency. This idea eliminates the need of a cloaking device altogether.
While this idea seems totally bizzare, a number of animals have evolved to have partially or fully transparent bodies. In fact, even in the human body, a number of components are transparent. In fact, most of our body, while opaque to visible light, is transparent to X-Rays (geddit?! that’s why X-Rays are used to see fractures, because they pass through the rest of the body!)
It might be possible to bring about changes in the human body to make it transparent. This brings us to a very important conundrum in Physics – can an invisible human see? What do you think? Why or why not?