How An Invisibility Cloak Could Work Engineering Essay

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Invisibility has been desired by humanity from as far back as Ancient Egypt where an Egyptian deity - Amun-Ra - had the power to disappear and reappear at will. Nowadays we see invisibility as a magic trick or an optical illusion or maybe just something we see in films/TV. Examples include Harry Potter and his famous invisibility cloak or the Star Trek cloaking device which both seem quite fictitious, but a more realistic example is James Bond's Aston-Martin in "Die Another Day". The idea used for this has been developed by an engineering professor at the University of Tokyo, Susumu Tachi, he calls it optical camouflage [1]. Before I get in to that, I have to explain that there are basically two different ways of creating an invisibility cloak. The easiest (and least convincing) way is using the optical camouflage. But using the other way it could be possible to turn science fiction into pure science. The other method involves a far more advanced approach; consider changing the type of material used to create the cloak. These metamaterials are being set as the building blocks of a new age of super lenses, agile antennae and most importantly invisibility cloaks. Using metamaterials offers a more convincing dream of invisibility technology, without the need for various projectors and cameras. It was a Russian physicist Victor Veselago who thought these up in 1967 [1]. The first working demonstration of using metamaterials to create and invisibility cloak took place at Duke University's Pratt School of Engineering, USA in 2006 [4].

Optical Camouflage System [6]The first working test was called Tachi's Cloak [5] , a shiny raincoat that works like a movie screen. A projector projects onto the coat the image that is being captured by a camera on the back of the coat to make you seem transparant. You could even try this at home using a webcam. If you place yourself between the computer screen and the webcam, the camera films what is behind you and you disappear completely. Now turn the screen around facing outwards and hold it up against you towards someone else and you will be invisible. That's pretty cool, isn't it? All you need now is a way for the image on the computer screen to be projected onto your whole body and you would seem to be completely invisible. But what about Harry Potter's cloak where he can just wander around Hogwarts freely and be completely hidden from all angles, or James Bond's Aston-Martin from the "Die Another Day" movie where his invisible car helps him kill the bad guy and save his life? To do this you would have to have a much more sophisticated sysyem than just what Tachi did. But luckily for us at this very moment the US Defence Department are working on this top seceret technology to use it in warfare [5]. If they were to actually dupe anyone with this they would have to be able to reproduce the backgrouond image from all angles at once at any given moment. The only way you could do this would be to use more cameras. To cover all the angles you would probobly need at least six sterioscopic pairs - meaning one facing forward, backward, left, right, upwards and downwards. Then you would need a fancy computer small enough to wear and powerful enough to keep a constantly updated image. Impossible? No, just rather difficult. Most of the technology already exists. Miniture colour cameras could serve as sutible light sensors and for the display, LEDs small enough to produce a 16 bit display are quite common and ought to be enough [5]. The problem now would be with the brightness the cloak produces. It needs to be able to change it's brightness it produces with the different times of day. Then theres the problem that if you walked in front of the sun it would need to be able to reproduce the brightness of it. If you couldn't overcome this would you create a shadow? It's no good being invisible with a shadow. Something else that could limit the cloak could be the response time. You must be able to update the image faster than the human eye can perceive any fickering. It would have to be able to register motion in 'real-time' without blurring, smearing, ghosting or juddering like you get in todays low-end monitors. Therefore you would need the equivilent of millions of superbright LEDs set up as a microarray lattice for it to cut it. I think the biggerst challenge for the maker to overcome in this type of cloak is countering the parralax principle the camera would create when someone far away is looking at you. Astronomers use this principle when trying to work out distances to stars. It just means that the angle from which you view changes with the distance you are away from the object which means if you were a long distance from the cloak your depth perception looking through the cloak may change and it would be clear that something was there. Lastly it would be so easy to detect the object under the cloak with thermal imaging as the person under the cloak would be producing body heat and even if there wasn't a person under there the cloak itself would be producing heat from all the computers it has in it. The other way of creating a cloak of invisibility is using something called metamaterials. Professor David R. Smith of Duke University's Pratt School of Engineering defines a metamaterial as "artificial composites that can be made to interact with electromagnetic waves in ways that natural materials cannot reproduce" [4]. This means that if they are properly constructed it could be possible to divert rays of light around an object and not only that but it could even divert other waves in the electromagnetic spectrum as well. Think of it like a rock in the middle of a fast flowing river, the water gets diverted around it right? But you also get a patch behind the rock where the water hasn't moved back in

Light being refracted around an object by a metamaterial [7]yet. That is the spot where things could be hidden. Anyway, the guys at Duke University did some testing on these metamaterials and managed to come up with a way of deflecting microwave beams around an object. This doesn't really help us with turning things invisible to the human eye but it shows that it is possible to deflect energy waves around an object, which is quite a big first step. They created the cloak by precisely arranging a series of concentric circles of copper with wires patterned onto sheets of fibreglass composite, like you get on circuit boards, to give specific electromagnetic properties [4]. From doing this they managed to reduce both the object's reflection and shadow on their sensors which are the two things you don't want to have appearing if you are trying to be invisible. There are quite a few limitations on this so far. Professor Smith reckons that "To make an object literally vanish before a person's eyes, a cloak would have to simultaneously interact with all of the wavelengths, or colours, that make up light. That technology would require much more intricate and tiny metamaterial structures, which scientists have yet to devise." [3]

Therefore, I can conclude that at present, the optical camouflage style of cloak is looking like the easiest to create and you can even try it in your own home. It could certainly work if all the conditions are met but it will always have the 'ghostly' look about it. I suppose using a holographic display could possibly reduce the computer power needed on the cloak because you don't need to simulate the 3D look if your display can do it by itself. It's easy to say this but todays monitors don't have the resolution to display holograms but it could be a possibility using something like quantum dots which are 1000 times smaller than the standard film grain. Sad thing is that invisibility will remain just out of reach until engineers can find a way around obstacles like this. Still, the technology is possible so it could be on the way soon. On the other hand, we've seen how using metamaterials could create a way of turning invisible but at the moment we are pretty limited with hat we can do. They're not only too small but they can only work in two dimensions at the moment. Also if you were to make them big enough to hide something the size of a human it would weigh more than the average wizard would hope to cope with carrying around. Therefore this technology may be better suited for hiding stationary objects such as buildings or stationary tanks. Even then, we still haven't discovered how to refract all the different wavelengths so it will still be detectable by some means. It definitely has potential but we are quite a long way off. So I've looked at how an invisibility cloak could work in the future but for now, sadly, you will have to put up with the old webcam on your back trick.

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