Invisibility 

This was until 2006, when researchers at Duke University in Durham, North Carolina, and Imperial College in London, successfully used metamaterials to render an object invisible to microwave radiation. There are still many hurdles to overcome, however, this is the blue print for how we can render objects completely invisible. 

Nathan Myhrvold, former chief technology officer at Microsoft said of the potential of metamaterials, that they, "will completely change the way we approach optics and nearly every aspect of electronics... Some of these metamaterials can perform feats that would have seemed miraculous a few decades ago." 

What are these metamaterials? They are substances that have optical properties not found in nature. They are created by embedding tiny implants within a substance, that will force electromagnetic waves to bend in unorthodox ways. 

Metamaterials have the ability to manipulate the index of refraction. Refraction is the bending of light as it moves through something transparent. For example, water and glass, distort and bend the path of ordinary light. Light slows down when it enters a dense and transparent medium. The speed of light in a pure vacuum always stays the same. Light travelling through glass or water must pass through trillions of atoms and hence, slows down. The index of refraction is the speed of light divided by the slower speed of light inside the medium. For example, the index of refraction in the vacuum, is 1.00. It is 1.0003 for air, 1.5 for glass and 2.4 for diamond. Typically, the denser the medium, the greater the degree of bending and the greater the index of refraction. 

What if one could control the index of refraction throughout the metamaterial? If you could, in theory, you could have the light pass around an object, rendering the object invisible. To achieve this, the metamaterial must have a negative index of refraction, which optics textbooks say is impossible. 

Plasmonics

The goal of plasmonics is to squeeze light so that one can manipulate objects at the nanoscale. This is especially on the surface of metals. Metal conducts electricity because electrons are loosely bound to metal atoms. They can freely move across the surface of the metal lattice. There are certain conditions where a light beam can collide with a metal surface, where the electrons can vibrate in unison with the original light beam. This creates a wave-like motion of electrons on the metal surface. These are called plasmons. These wave-like motions beat in unison with the original light beam. One can squeeze these plasmons, so that they have the same frequency as the original beam. They would hence carry the same information, however, with a much smaller wavelength. One could, in principle, cram these squeezed waves onto nanowires. 

Nanotechnology

The key to invisibility may be nanotechnology. This is the ability to manipulate atomic-sized structures about a billionth of a meter across. 

Richard Feynman planted the seeds for what would become nanotechnology in his 1959 lecture, There's Plenty of Room at the Bottom. He reasoned that machines could be developed to be smaller and smaller until atomic length scales are reached. These atomic machines would well within the laws of physics, however, would be exceedingly difficult to make. 

As the probe passes over an individual atom, the amount of current flowing through the probe varies. These variations are recorded and by plotting these fluctuations, one is able to obtain beautiful images of the individual atoms making up a lattice. 

The scanning tunneling microscope is made possible by a strange property of quantum mechanics. Usually, electrons do not have enough energy to pass from the probe, to the substance to the underlying surface. However, because of the Uncertainty principle, there is  a small probability that the electrons in the current will tunnel or penetrate through the barrier. This is forbidden by the Newtonian theory, however, a consequence of quantum mechanics nonetheless. 

The probe is also sensitive enough to move individual atoms around and create simple machines. This technology is so advanced now, that atoms can be manipulated via a cursor on a computer screen. Atoms can be moved around any way you want! 

Optical camouflage and holograms

Another method of making someone invisible would be to photograph the scenery behind him and then project that background image onto the persons clothes or on a screen in front of them. From the front, this person appears to be invisible. 

Naoki Kawakami, from the University of Tokyo, calls this process "optical camouflage." His "cloak" is covered with tiny light-reflective beads that act like a movie screen. A video camera photographs what is behind the cloak. This image is fed into a video projector, that lights up the front of the cloak. It thus appears that light has passed through the person. 

There are actually prototypes of this cloak in the lab. If you look at the person wearing the cloak head on, they appear as if they have disappeared, as you see the image behind the person. However, a more realistic optical camouflage would need to create a 3D image. For this, holograms would be required.

Holograms are 3D images created by lasers. A person could be rendered invisible if the background scenery was photographed with a special holographic camera. The holographic image is then projected out through a special holographic screen placed in front of the person. A viewer standing in front of that person, would see the holographic screen, containing the 3D image of the background scenery. You would not see the person! It would appear as if the person had disappeared. In the person's place, you would see a precise 3D image of the background scenery.