Flight, super speed, x-ray vision, and invisibility: the abilities people once believed possible only through magic have become realities through centuries of research and development. First cars, then planes, then MRI scans, and now, invisibility cloaks make their way out of storybooks and into the scientific world.
Early Invisibility Cloaks
It all started in 1998, when several military researchers came up with the idea of a camera projection system that could render objects invisible. Military financing ran out, but the researchers claimed to have developed a system that could create the illusion of invisibility at a distance of 50 yards under very specific conditions (1).
Several years later, Susumu Tachi of the University of Tokyo followed through with the idea and created a fully functional invisibility cloak. His device uses a camera to capture the image on the other side of the cloak and a projector to project the background onto the front of the cloak. Although the concept sounds simple enough, the integration of all of the cloak’s parts must be very finely tuned in order to deliver the impressive effect of invisibility.
First, a camera captures the image behind the cloak and sends it to a computer that processes and modifies the video. Then, the video is sent to a projector that sends the image not directly onto the cloak but onto a special mirror called a combiner. The combiner reflects the image onto the cloak and also allows the light rays to bounce off the cloak, through itself, and to the observer who is standing behind the combiner (2).
The garment itself is made of thousands of small, highly reflective beads, which reflect light back in the exact same direction as the source, unlike specular reflection or diffuse reflection (2). Specular reflection reflects light off a surface at the same angle from which it is shone, and diffuse reflection scatters light after it hits a surface. The bead’s unique reflection allows for an observer to see a very bright reflection if he stands at the light source and only at the light source (2). However, if the observer looks at the cloak from a different angle, the cloak loses its invisibility effect and the observer only sees a large, raincoat-like garment.
Tachi suggests several uses for the cloak, such as for backing up cars and seeing through bones, organs, hands, or instruments during surgery (1). However, some have suggested that such a device poses a threat to privacy if walls can be turned on and off with a switch.
A diagram of the 3-D fishnet metamaterial developed by UC Berkeley researchers. The material is made of small circuits that can bend light backwards.
Metamaterials
Although TIME magazine named Tachi’s invisibility cloak one of the coolest inventions of 2003 (3), the future of true invisibility lies in the research and development of metamaterials — materials that can be manipulated to interact with light and other electromagnetic fields.
An object is visible when it absorbs some of the light shone onto it and reflects the rest of it back to our eyes. If light could be directed around an object so that it neither absorbs nor reflects the light, then there would be no way for us to visually perceive the object. If light bends around the object just as water flows around an obstruction, then we would see everything except the object and it would seem as if nothing were there. Thus, the basis for metamaterial development lies directly in its ability to interact with light and other electromagnetic waves to achieve invisibility, similar to how stealth aircraft achieve invisibility by manipulating electromagnetic waves to avoid radar detection.
One might think metamaterials need a special chemical composition to achieve such properties, but John Pendry of the University of the Imperial College of London discovered that the internal microstructure of a material also affects its properties (4). He is often credited as the discoverer of metamaterials and founder of the theory behind it. The very same chemicals can have different properties if arranged in a different structure. In the realm of metamaterials, the main property being studied and manipulated is the refractive index, or in other words, the property of bending light.
The refractive index measures the degree to which light changes direction when it passes through an object of a different density. Materials in nature all have a positive index of refraction because light normally bends inward when it enters a denser medium, but metamaterials have the ability to achieve a negative refractive index and refract light outward in a denser medium, allowing for complete manipulation of light (5).
In 2006, David R. Smith of Duke University created the first functioning invisibility cloak based on this theory (6). The cloak covers a copper cylinder and shields it from microwave detection. The structural array of metamaterials must be smaller than the electromagnetic wavelength being used, which is why the visual spectrum, which has shorter wavelengths, is much harder to manipulate. Microwaves have wavelengths of 1-300 mm while visible light has wavelengths of 380-750 nm (7).
However, it did not take long for scientists to take up the challenge and progress towards shorter wavelengths. Xiang Zhang of the University of California, Berkeley, one of the forefront researchers of metamaterials and invisibility, achieved a negative index of refraction in the near-infrared light range with a wavelength of 1500 nm. In an article published in Nature, Zhang explained how he used alternating layers of silver and magnesium fluoride and designed the material into a fishnet pattern with holes several nanometers in size (8). When the researchers passed infrared light through the material, they measured a negative refractive index.
In July 2009, Zhang created another “carpet cloak” that shields light in the optical frequency between the wavelengths of 1,400-1,800 nm, extremely close to visual light (9). He was able to hide the bump of an object’s outline with his cloak, improved by using silicon metamaterials instead of metal. The functionality of the cloak depends on the density and positioning of the holes in the cloak. Air has a lower optical density than silicon, so where the holes are more dense, the optical density is reduced (9). Since the wavelengths being used on the cloak are larger than the nanoscale holes, light interacts with the material as a whole, not as silicon and air.
Zhang states that the use of silicon is a “big step forward” and should be cheap to manufacture and “upwardly scalable,” since silicon absorbs much less light and does not leave a dark spot on the cloak as metals do (10).
Scientists are still working to achieve negative refractive indexes for visible light, but many are very optimistic about its progress and future. Smith predicts that a fully functioning invisibility cloak will be developed by the end of the year (11).
Other Uses for Invisibility
Total invisibility typically suggests many military uses, such as hiding tanks or other weaponry from enemy sight. Metamaterials can also be applied to Tachi’s goals of surgical or automotive applications as well, but even greater potential lies in this technology than that which readily comes to mind.
For example, Zhang suggests applying his new carpet cloak to processor chip stencil production. Tiny flaws on the stencil could produce defective chips, but Zhang’s carpet cloak makes it possible to hide the flaws on the stencils instead of making new ones, which could save millions of dollars (12).
Metamaterials manipulate electromagnetic waves, but recent research in cloaking suggests applying the concepts for invisibility cloaking to make buildings invisible to seismic waves. Theoretically, the seismic waves would bend around the buildings just as the microwaves bend around Smith’s cloaked copper cylinder, and the buildings would essentially be impervious to earthquakes (13).
With recent developments by Willie J. Padilla and Nathan Landy of Boston College, scientists can now make a seemingly straight beam of light travel along a guided path following a set of directions (14). With this development, the possibilities of manipulating light are endless. Perhaps instead of making mass and densities disappear, metamaterials can be designed to make mass and densities appear where there actually is nothing.
Complications and Conclusion
So far, all of the cloaking techniques only apply to observation in two dimensions, and even in the two-dimensional realm, current invisibility cloaks need further engineering to interact with the shorter wavelengths of visible light. There is still much work to be done in order to make something truly invisible.
However, given the amount of progress and work accomplished just within the past year, the imminent integration of invisibility into our everyday lives might not be far off, and perhaps it would not be too hasty to consider the implications of such technology. Who would get to use it? How useful or dangerous could this technology be? And what kind of illusions could we then be susceptible to?
References
1. J. Brooke, Tokyo Journal; Behold, the Invisible Man, if Not Seeing Is Believing
(27 March 2003). Available at http://www.nytimes.com/2003/03/27/world/tokyo-journal-behold-the-invisible-man-if-not-seeing-is-believing.html (17 August 2009).
2. H, William. How Invisibility Cloaks Work (20 July 2005). Available at http://www.howstuffworks.com/invisibility-cloak.htm (17 August 2009).
3. The Invisible Man (2003). Available at http://www.time.com/time/2003/inventions/invinvisible.html (17 August 2009).
4. S. Markey, Invisibility Cloaks Possible, Study Says (25 May 2006). Available at http://news.nationalgeographic.com/news/2006/05/060525-invisibile.html (17 August 2009).
5. J. Pendry, Metamaterials. Available at http://www.cmth.ph.ic.ac.uk/photonics/Newphotonics/metamaterials.html (17 August 2009).
6. A. Cho, Science (20 October 2006). 314 (5798), 403.
7. Visible Spectrum (15 August 2009). Available at http://en.wikipedia.org/wiki/Visible_spectrum (17 August 2009).
8. X. Zhang et al., Nature 455, 376 (2008).
9. X. Zhang et al., Nature Materials 8, 568 (2009).
10. L. Yarris, Blurring the Line Between Magic and Science: Berkeley Researchers Create an “Invisibility Cloak” (2009). Available at http://newscenter.lbl.gov/feature-stories/2009/05/01/invisibility-cloak/ (17 August 2009).
11. T. Veiru, Invisibility Cloaks Could Be Developed Within Six Months (17 January 2009). Available at http://news.softpedia.com/news/Invisibility-Cloaks-Could-Be-Developed-Within-Six-Months-102209.shtml (13 August 2009)
12. V. Gill, Invisibility cloak edges closer (31 April 2009). Available at http://news.bbc.co.uk/2/hi/science/nature/8025886.stm (17 August 2009).
13. M. Farhat, S. Guenneau, S. Enoch, Phys. Rev. Lett. 103, 024301 (2009).
14. E. Hayward, The guiding of light: A new metamaterial device steers beams along complex pathways (31 July 2009). Available at http://www.eurekalert.org/pub_releases/2009-07/bc-tgo073109.php (17 August 2009).