Is invisibility a possibility?

Alicia M. Surillo

Since a very long time ago, people started to think about invisibility as something amazing and powerful that they would love to achieve someday. Everyone could use invisibility to accomplish something whether is was good or bad. For example, a thief could use invisibility to make impressive robberies while a policeman could use it to catch a criminal without having him run away. In fiction movies nowadays, we see different methods in which characters become invisible such as invisibility cloaks and potions. Is there really a cloak or potion that can make an object invisible? Physicists have dismissed this possibility by stating that an invisibility cloak would violate optics laws and would not follow the properties of matter.  

This physicists' statement was questioned when in 2006, a group of researchers at Duke University in Durham, North Carolina, and Imperial College in London challenged the conventional optics laws and made an object invisible to microwave radiation by using metamaterials. The existence of metamaterials had been referred to as “impossible” by optics text books (since they would violate optics laws). As a result of this discovery, physicists were forced to rewrite optics text books. Metamaterials are substances with optical properties not found in nature, created by embedding very small implants within a substance to force electromagnetic waves to bend in unorthodox ways (the scientists from Duke embedded tiny electrical circuits with copper bands). The electrical implants on the copper forced the path of the radiation to be in a specific way. Michio Kaku's book, “The Physics of the Impossible” , compares this microwave radiation flow with the way a river flows around a rock. It states, “Because the water quickly wraps around the boulder, the presence of the boulder has been washed out downstream. Similarly, metamaterials can continuously alter and bend the path of microwaves so that they flow around a cylinder, for example, essentially making everything inside the cylinder invisible to microwaves.” Scientists put the metamaterial to the test by inserting a copper ring inside the microwave device. Only a tiny shadow of the ring was reflected, which means that the ring was nearly invisible to microwave radiation.

The most impressive property of metamaterials is their ability to change the “index of refraction”. The index of refraction of an object is how light bends as it passes though it, specifically for transparent objects. For example, as light passes through water and glass, we can see that it bends, taking different paths. Refraction happens because of the larger concentration of particles in a solid or liquid (and even gas) from that of empty space, through which light has to find its way. As a generalization, we can say the denser a transparent medium is, the higher its index of refraction will be. Some common index of refraction are: 1.00 for empty space, 1.0003 for air, and 1.5 for glass. The index of refraction is usually a characteristic of the substance, it does not change throughout it. However, if it could be controlled, light would follow different paths through the substance and would end up roaming through it. Scientists believe an object can be invisible by controlling the index of refraction inside a metamaterial in a way that light passed around the object. To achieve this, the metamaterial has to have a negative index of refraction (which is one of the rare characteristics metamaterials have). Again, optics text books stated that it was impossible for an object to have a negative index of refraction, so they had to change that too.

Many journalists have asked metamaterials researchers when are invisibility cloaks coming to sale, to which they have answered, “Not anytime soon”. Although the assembly of metamaterials is a huge step into accomplishing that ability that everyone wishes to have, there is still more research and discoveries to do. But, who knows? We could possibly be closer than what we think to wearing one.

Reference:

Kaku, Michio. Physics of the Impossible. 1st Edition. New York, NY: Anchor Books, 2009.