Uncloaking the realm of invisibility

Is there hope for us muggles?

Anyone who has read or seen the Harry Potter series has dreamt about being invisible. That is without a doubt. The prospect of invisibility has been an important part of fiction stories since about 400BC when Plato described the legend Ring of Gyges, which when angled in a certain manner could grant the wearer invisibility, so it seems surprising that scientists have only in the last 10 or 15 years really tried to make the fantasy a reality. In this time there have been incredible advances in this area of physics; yet, although we are closer than ever to being able to sneak out of the house or skip the café queue at break without getting caught, there is still a long way to go before invisibility cloaks will be perfected, let alone sold in stores.

The most promising potential design of invisibility cloaks features ‘metamaterials’. These are materials that are chemically and structurally designed so that they possess specific properties that cannot usually be found in nature. These properties can be controlled by altering the fabric of the material, meaning that theoretically this technology could be used to solve an infinite number of different problems. The metamaterials currently used in the development of invisibility cloaks consist of metal, often copper, wires and rings arranged in detailed patterns and shapes. This design allows physicists to break the rule that all students are taught when they first study optics: that light travels in straight lines. As the light reaches this structure it interacts with it and can be curved, like water flowing around a rock or traffic being diverted around a ring road, concealing any object placed inside it.

Based on this theory scientists have already designed some prototypes. The first ‘invisibility cloak’ was made in 2006, but only worked for microwaves, which have a relatively large wavelength. All this would really allow you to do is hide an object from radar sensors or stop your food from heating up when you put it in a microwave oven, which is completely pointless! However, it is incredibly hard to create an invisibility cloak that works for visible light, which has wavelengths much smaller than microwaves. In order for the cloak to interact with the electromagnetic radiation, the coils in the metamaterial must be considerably thinner than the wavelength of the radiation. For microwaves with a wavelength around 3cm this means coils around 3mm thick. However, for visible light the wires and coils could measure no more than 10 nanometres (0.00001mm), which sadly is very difficult, if not impossible, to achieve with the machinery we use at present.

Another problem with invisibility cloaks is that, in order for them to work fully, they must be effective for the whole range of wavelengths of visible light, which reaches from 400 to 700 nanometers. In 2013, a team at Stanford built an invisibility cloak that concealed green and blue objects, but if you were to put something that was not comprised solely of these two colours inside then an onlooker would still see a reddish hue. Scientists have not yet managed to tackle this problem. However, once they have, the only thing stopping us from making actual invisibility devices will be our ability to physically build them. With the current advances in nanotechnology this really is a very minor problem and within a few years it could be solved.

So why do people care so much about invisibility cloaks? Most notably, cloaking devices would be incredibly valuable to the military. Any country with the ability to hide their tanks, planes and personnel would have an unbelievable advantage in combat. In fact, the USA’s military formally announced that they wanted invisibility cloaks for all their soldiers in less than 18 months in May last year. While it seems unlikely that this target will be met, when invisibility does find its use on a battlefield it will revolutionise warfare.

On a lighter note, an invisibility cloak would be incredibly fun to own. The possibilities if you owned one would be endless. Of course the ‘invisibility cloaks’ that would be made according to the current designs would be totally solid structures and not at all cloak-like, so they may not be quite as we dreamed. However, we can always seek solace in the fact that, if DeWitt’s ‘many-worlds’ theory (that there could be an infinite number of universes) is correct, there is one where a parallel version of you has their very own perfect invisibility cloak. And another one where you got the Hogwarts letter on your 11th birthday.

Poppy Boyd-Taylor VII

(Article originally written for CaCO3)