Making Invisibility Cloaks
According to the Hitchhiker's Guide to the Galaxy, the easiest way to make a mountain appear invisible is to dismantle it and move it somewhere else. It’s not easy, but now researchers at the University of Maryland reckon they have it nailed. Sort of. Adam Connors explains all.
From Harry Potter to Klingon Birds-of-Prey, we all know what invisibility means. It means being able to hang out in the women’s changing room without getting arrested. But what does it mean from a scientific perspective?
We see an object because electromagnetic waves (light) bounce off it or are absorbed. So in order to render an object invisible, we must divert the waves around it and recombine them at the far side so that they appear to have passed through free space.In 2006, researchers at Duke University, North Carolina accomplished this for microwaves. No, we’re not talking about rendering microwave ovens invisible... We’re talking microwaves. Electromagnetic waves on the same continuum as light but with a much longer wavelength - around 30 centimetres compared with 500 nanometres. The researchers guided microwaves around a hidden object using a far from extraordinary-looking arrangement of concentric rings made from fibreglass.
What’s unique about the Maryland experiment is that it’s the first time the same thing has been done for light at visible frequencies. The key to this is a “metamaterial”. Metamaterial is just a fancy name for a composite - any material made up from two or more constituents - that has unusual properties. Plywood is a good, if somewhat disappointing, example of a composite material, but before you rush out to construct your own invisibility shield from a few sheets of plywood (a trick that works remarkably well for twitcher Bill Oddie) there’s a few things you need to know.
How does it work then?
In both the Carolina experiment and the Maryland experiment the metamaterial used was designed such that it would have a negative refractive index. This means that light passing through it is deflected in the opposite direction to that caused by conventional materials (like this). This property, combined with the geometry of the material means that the waves are swept into the concentric circles in the material and reconstituted on the far side. Thus rendering the shield, and any object at its centre, invisible (you won't believe it till you see it... or don't see it).
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Both the Carolina experiment and the Maryland experiment use geometrically similar lenses, but the thing that makes the Maryland experiment so important is the scale on which it must work. For this technology to be effective the elements of the metamaterial must be of the same order of magnitude as the wavelength of the radiation you are attempting to shield. For microwaves, which have a wavelength of around 30 centimetres, that’s an intricate piece of engineering. But for visible light - that used in the experiment was 500 nanometres - that’s a truly unique piece of nanoscale fabrication.
When I can I get me one of these?
Okay, now for the disappointing part. Before you get visions of shuffling around the women’s changing room inside one of these puppies, your presence betrayed only by the sound of your gleeful titters, there are, as always, a number of limitations.
Crucially, this experiment only works in two dimensions. To make it work in three dimensions, which would be required to shield a real-world object, is a whole other problem - although theoretical models do exist. Also, it works only under very specific conditions and only for a very specific frequency of light - so making it function under white light would be tricky. And of course, it’s inconveniently tiny - the shielded area is only four micrometres, or four thousandths of a millimetre, in diameter.
All these limitations, however, pale into insignificance when compared to the most fundamental design flaw of this contraption. Because light is focused around the central region, once inside you won’t be able to see a damn thing!
Stuff you'd like to make disappear:
- The postal service - Snail mail really is... snail mail
- Veggies - Sprouts are bad for bugs, but not people
- Teens - Lazy youths in sci shocker
- Maths - Fear of maths thwarts the brain
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