WILL WE EVER SEE AN INVISIBILITY CLOAK?

Fantastical ideas of invisibility garments go back many years. In medieval Welsh mythology, among the ‘Thirteen treasures of the Island of Britain,’ there is an object called the Mantle of Arthur in Cornwall, which could make its wearer invisible.

In the Wizarding world, the invisibility cloak that Harry Potter inherits is said to be one of the three legendary Deathly Hallows, assumed to be created by Death himself for one of the Peverell brothers in the 13th century. In the legend, the original intention of the cloak was to allow the wearer to go forth without being followed by Death, but generally it was to hide a person from their enemies. Not surprisingly, in the real world the idea of having a garment or device that can render something relatively impossible to see is very attractive; especially to the military.

Camouflage

For years, the military have made use of camouflage clothing to make it harder for enemies to find them. Often camouflage involves wearing colors that match one’s surroundings as well as using shapes and disruptive patterns to break up the perceived outline of the object. It basically messes with the way we perceive what we see rather than making the object completely invisible.

In nature, the ability of an organism to blend into its environment is known as crypsis, and it provides a survival advantage to the organism. For example, due to its color, a polar bear is harder to see in its mostly snowy environment, while a leaf insect has both the color and shape of leaves in the tree that it inhabits. As the adaptive features of these animals do not change with time and place, these animals are using passive camouflage.

On the other hand, chameleons and octopuses use active (also known as adaptive) camouflage as they adapt the color, pattern, texture, or shape of their skin to match whatever surroundings they find themselves in. Octopuses, in particular, can alter all of these features to mimic objects in their surroundings, whereas a chameleon can mainly just change color and pattern.

In the wizarding world, wizards have their own chameleon-like ability. It’s called the Disillusionment Charm and it makes the subject’s body take on the color and texture of the things behind it. Again, with all of the above examples, the target is still visible but disguised in a way that makes it hard for us to recognize what they are.

Adaptive Camouflage

Adaptive camouflage has been investigated by various groups of muggle scientists, for human as well as vehicle applications. In 2003, a professor at the University of Tokyo developed a system named Optical camouflage, which uses what is described as Retro-reflective Projection Technology (RPT). A camera captures the background that’s behind an object and then an optical projector displays this image onto the front of the object in real time. The object is covered in a special retroreflective material which acts as a screen for the projection.

In 2012, the UK series Top Gear fashioned a system for a Ford Transit. They surrounded the van with four walls of flat screen televisions that faced front, back, left, and right. Opposite each wall of televisions, they positioned cameras that relayed live images of the view on the other side of the van. Mercedes-Benz used a similar setup in an advert for their F-CELL Hydrogen fuel technology. Rather than using flat screen TVs for displays, they carpeted one side of the car with arrays of LEDs.

More recently, in the US, a company called Folium Optics has been working on a technology that could be used on combat vehicles. The tech uses an array of hexagonal cells that can switch color to match its surroundings. The cells are also reflective so the system doesn’t require much power while naturally matching the brightness of the ambient lighting conditions.

Despite best efforts a downside with camouflage is that if the concealed object moves, or is viewed from different angles it tends to lose its stealthy advantage. A truly invisible object, such as evoked by the cloak of invisibility, shouldn’t have this problem. So how could something actually become invisible?

The Eye’s the Limit

Invisibility is all in the eye of the beholder, which in this case, is human beings who see things via visible light. The role our eyes play is to make use of light coming from objects in the surroundings. Eyes first evolved underwater, which is a place where only certain light frequencies can penetrate. As such, our eyes became most sensitive to those water penetrating frequencies; specifically, visible light.

Microwave, infrared, and ultraviolet (UV) radiation are absorbed by water molecules meaning they do not penetrate water well, eyes would have no need to evolve sensitivity to those wavelengths of light. Although some insects, like bees, can perceive some frequencies of ultraviolet light.

The reason we see the world at all is because our eyes can absorb the visible light waves coming from objects. The process requires the light to be absorbed in special parts of the eye, particularly in structures called rods and cones, which exist in the retina. We have about 120 million rods and over 6 million cones in our retina. The rods are sensitive to low-level light, while the cones respond to the visible light frequencies associated with colors.

Anything that doesn’t give off visible light is effectively invisible to us, but some animals such as snakes are able to detect infrared radiation. Snakes do this via special pit organs on their faces that allow them to detect infrared light up to one meter away. This ability is why Voldemort’s snake, Nagini, can see Harry and Hermione while they are under the invisibility cloak. This indicates that Potter’s invisibility cloak is only transparent to certain light frequencies and possibly only visible light. Would it be possible to make a visible object invisible to the human eye?

Oh, That Old Trick!

For something to be truly invisible to the naked eye, it has to let light straight through, meaning that the atoms of the object do not noticeably disturb the light waves as they pass through. This is why substances such as water, glass, plastic, and air appear transparent. However, we can still see them. How is that possible?

Other than dirt or smudges on the surface of transparent materials, we can see them based on the way light is affected as it travels through them. For example, as it travels from one transparent substance to another (e.g. air to glass or glass to water) any light that hasn’t been reflected or absorbed can change direction, depending on the angle it approaches the interface between the two substances. This particular bending of light is called refraction.

Refraction happens when the substances don’t have the same refractive index. The refractive index provides an indication of how much the speed and direction of light can be effected as it travels through transparent materials. When light travels through a glass in air its path gets perturbed because air and glass have different refractive indices. Along with reflection and absorption of light, this refraction affects the original light path, revealing the presence of the glass.

Some materials do have roughly the same refractive index though, for example cooking oil and Pyrex. This means that light is not refracted as it travels from one to the other; making it appear like the light has travelled straight through the oil and glass without disturbance. This is the basis of a science ‘magic’ trick that causes Pyrex glass to apparently disappear when it’s placed into a container filled with cooking oil. In essence, it’s the oil that provides the cloak.

This trick only works because the Pyrex glass is already see through and clear, but to make an opaque or tinted object become invisible would require a different technique.

Metamaterials

Scientists have been looking into technologies that can direct light around objects, rather than trying to somehow make the actual object transparent to light. In 2006, physicist John Pendry came up with an idea which was subsequently dubbed the ‘invisibility cloak’. The underlying technology manipulates light by making use of metamaterials.

Metamaterials are specially engineered materials that exhibit properties that are beyond those of naturally occurring materials. For example, they can have negative refraction, which is something not found in nature. The first metamaterials only functioned for longer wavelength radiation such as microwaves and radio waves but researchers have been working to extend this range. For an idea, microwaves have wavelengths between 30cm and 1mm, whereas visible light waves are between 400 and 700 nanometers. A nanometer is a million times smaller than a millimeter.

In 2012, researchers at the University of Texas at Austin successfully cloaked an 18-centimeter tube from certain microwave wavelengths. Their cloak works by suppressing the way light is scattered (reflected in multiple directions) by an object. If the light isn’t remarkably scattered from the object, then we can’t detect any light coming it, rendering the object invisible to those wavelengths of light.

The following year, the same researchers used ultra-thin metascreens to produce what they called a mantle cloak, because it has the advantage of being extremely thin (less than 1 mm thick) and flexible. Co-author of the study, Andrea Alu described it like this. “When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency and invisibility at all angles of observation.” So, could this be used to cloak objects in visible wavelengths?

To make objects invisible to our eyes and not just longer wavelengths, we would have to scale everything else down, including the size of the object being cloaked. This is because the object has to be smaller or comparable in size to the wavelength of the light being used to view it. If the object is much larger, the cloak won’t work. So instead of an 18-centimeter long cylinder, the object could only be about 1 micrometer long, i.e., a thousand times smaller than a millimeter.

In fact, there are fundamental limits to the size of objects and wavelengths that can be used in metamaterial cloaks, although different types of metamaterials such as active metamaterials may provide us with more promising possibilities for the future.

Invisibility Cloaks

Adaptive camouflage is a long way from wearable tech, but it’s showing promise with bigger objects such as vehicles. The work being done is slowly leading to better techniques for hiding objects from view. Bae Systems have already demonstrated an infrared cloaking system for tanks.

Although we will likely never find a way to make opaque objects invisible to visible light, it is possible to bend the light around an object in a way that provides a similar effect. These invisibility cloaks do exist and are made possible by using metamaterials. However, there are limitations with the maximum size of object that can be cloaked from visible frequencies.

At the moment, no such luck on getting an invisibility cloak to throw over your shoulders, but it’s definitely a possibility in particular wavelengths and for objects of limited size.