The idea of invisibility cloaking – being able to hide things from view – has been a point of interest to humans for thousands of years. Plato wrote in The Republic about The Ring of Gyges which allowed the wearer to become invisible. Within nature there are many different species of creatures that have evolved survival skills focused on hiding themselves from both their predators and prey. Although man-made camouflage techniques and stealth technology have been around for many years now, mainly developed and used by military forces worldwide, neither of these concepts provides true invisibility in the scientific sense of the term.
The concept of an invisibility cloak has come to prominence from its use throughout science fiction, most notably in the Star Trek TV series and more recently in the books and films of the highly popular Harry Potter franchise, but only in the last few years has scientific understanding and technological developments allowed us to finally realise this idea.
In the simplest sense, we see things around us because light reflects off them into our eyes, with our brains then using these signals to form images. This means it is easy to hide objects, for example consider the ‘cup and balls’ trick or the ‘smoke and mirrors’ analogy. However it is also important to note that all objects give off electromagnetic radiation, and whilst they may be hidden from our sight, they can still be ‘seen’ by using devices such as infrared detectors. Therefore from a scientific perspective the concept of an optical cloak is a device that reflects no light and absorbs no energy, giving the impression that the region of space it occupies is empty.
The properties and behaviour of electromagnetic waves are described by Maxwell’s equations. The theory of invisibility cloaking lies in utilising this set of equations within a field of physics called transformation optics. Metamaterials, artificially structured materials engineered to have particular properties, are used to control the propagation of electromagnetic waves.
Several different types of optical cloaking devices have been theorised and designed; the most well-known is the cylindrical/spherical design.
Through some relatively simple mathematics is it possible to formulate a coordinate transformation; basically creating a two-part region of space that can be used to conceal an object by acting as a single uniform region. This approach is important because it means any arbitrary object can be hidden without having to study the material properties.
This type of optical cloaking device works by bending the incoming electromagnetic waves around a central region that contains the object to be hidden. The idea is that the waves enter and exit the device on the same trajectory, meaning a device would be unable to detect whether the waves had been bent around an object or just travelled through a region of empty space – thereby creating an invisibility cloak that has hidden the object.
Physicists have now managed to successfully demonstrate object cloaking using a cylindrical device, albeit under very specific conditions and parameters. We can see how usually light hitting an object (in this case a cylinder) is scattered away in various directions and the object is easily detectable.
The cloaking device bends the light around the object so it then moves away in the same direction as before the interaction. This means a detector would perceive that the light propagation was unaffected so must have moved through a region of free space, i.e. the presence and effects of the object have been cloaked from detection.
Invisibility cloaks have now been demonstrated, finally turning ideas into reality. However, whilst the theory has been validated and experimentally shown, problems still persist mainly with perfecting the art of bending the light rays and making devices that can operate over a wide range of wavelengths although these issues continue to be worked on by numerous research groups. Until an invisibility cloaking device is created that operates for visible light, which can hide an object from right in front of our eyes, this new science remains quite a niche area of physics. There is continued rapidly growing interest and in the last few years hundreds of scientific papers have been published detailing the theoretical and experimental research of physicists in this field worldwide; so it’s surely only a matter of time before a true invisibility cloak, once residing only in science fiction, finally becomes part of everyday reality.