Developing InvisibilityBack in 2006, physicist Ulf Leonhardt put forth the idea that you could use exotic "metamaterials" might be able to bend light in such a way as to essentially make an object invisible. This wouldn't be perfect invisibility, but rather the sort of shimmering invisibility that is often shown in films, notably the one used by the alien in the Predator films.
Within just a few months, there had been success using this method to bend microwave radiation around an object. The method contained an overall issue in that the nature of these metamaterials indicated that they'd probably only be able to create objects that were "invisible" to certain specific, limited set of frequencies along the electromagnetic spectrum, which made the whole exercise a lot less fun for those of us hoping for invisibility cloaks. After all, what does it matter to us if something is invisible in microwave wavelengths, because we don't see in that part of the spectrum. Initially, it was completely unclear if the method would ever be transferable into the visible light spectrum, which is the sort of invisibility that we care about, since it's the sort of invisibility that we could see. (Or, in this case, not see, I suppose.)
Progress over the years with these metamaterials would come along every few months, it seemed, with new designs that focused on different segments of the electromagnetic spectrum. Once the initial insight and proof of concept was out there, it seemed that there was no end to the way that the metamaterials could be applied to make small objects invisible.
In August of 2011, just 5 years after the initial proposal for the invisibility machine, these metamaterials are making objects invisible in the visible spectrum, according to two different teams working on the project.
Here are some milestones in the search for invisibility (as reported by About.com Physics, with apologies for any links that have died since the articles were originally written):
- Aug. 1, 2006 - The Invisibility Machine
- Oct. 23, 2006 - Invisibility Achieved
- July 16, 2009 - Does Improved Invisibility Yield Silent Image?
- August 16, 2011 - Visible Spectrum Invisibility, At Last!
Though I haven't reported on each and every advance, it does show that steady work has been performed over the last several years. It seems like very few months there was some sort of report coming out that some group had narrowed in on invisibility in a new band of the electromagnetic spectrum. At this rate, we'll have invisibility cloaks in no time!
How Invisibility Works
Basically, this method works because these exotic metamaterials are designed to have properties which don't normally show up in nature. Specifically, they can be designed so that they have a negative refractive index.
Normally, when a light collides with a material, the angle of the light bends slightly due to the refractive index of the material. This happens, for example, with both glass and water. (Pay attention to your straw in a clear glass of ice water next time you're in a restaurant, and you'll see the effect of the light bending under refraction.) This is depicted in the graphic at the top of this page, when light goes into a "Conventional Material."
Metamaterials designed with a negative refractive index, however, behave very differently. Notice in the graphic that the light beam doesn't just bend a little bit, but instead it flips completely, going down instead of upward. The geometry of the metamaterials actually makes the light's path bend dramatically, and it's this process of bending that allows for the invisibility. The light collides with the front of the object and, instead of reflecting back, it goes around the object and comes out the other side. A person (or computer camera, in the case of more exotic thermal or microwave wavelengths) positioned on the other side of the object would see the light from the other side as if the object wasn't there at all.