The science behind super-thin invisibility cloaks

Shakib Rahman
7 April 2016

Above: Image © Miroslaw Pieprzyk, iStockphoto

This article is part of a series on how science fiction influences modern medicine.

You might think of the power of invisibility as something you only “see” on TV or in the movies. But a team of electrical engineers in California are making it into a real-world superpower. They have managed to cloak objects as large as a baseball!

Wait...is invisibility real? Kind of. By manipulating your perception of how light rays interact with an object, you can see around or through the object. As a result, it becomes invisible to you.

Did you know? From Lord of the Rings to Harry Potter, and Star Trek to Predator, Invisibility has long been a staple of magic and science fiction. It was first dreamt up by H. G. Wells in his book,The Invisible Man. That was over a century before scientists found a way to make objects invisible!

Blind spots

To understand how invisibility works, try to find your blind spot. That’s the part of your eyeball where light receptors are missing. It’s where your eye nerve connects so it can link up to your brain and share the images you see. Because there are no light receptors in this part of your eye, you can’t see anything in that area of your visual field.

To find your blind spot, close your left eye. Now, with your right eye, look at the plus symbols below. By moving your head forward and backwards, you can make the large circle disappear. You can also make the line appear filled in. When the circle disappears, or the line becomes filled in, you have found your blind spot.

Why does the circle disappear and the line get filled in? Your brain fills in the bits of information it doesn’t get from the missing light receptors in your eye. It assumes the white background continues over the dot and that the line is solid all the way across. This optical trick very similar to how an invisibility cloak works.

Invisibility cloaks

In the past, scientists cloaked objects by bending light rays, relying on the chemical properties of the cloaking material. Many layers of material were needed to bend light rays. So many layers, in fact, that the cloak ended up being thicker than the object it was hiding.

The members of the research team in California did things a little differently. They used nanoengineered materials like Teflon, metal, and gold. The amazing part of this research is that the cloak they created is only 80 nanometres thick. That’s less than one ten-thousandth of a millimetre (8x10-5 mm)!

Unlike previous invisibility cloaks, this one relies on the physical properties of the material. Instead of bending light rays, this new cloak scatters light rays away from the object it’s covering. By scattering light, the cloak creates the illusion of a flat surface with nothing in front of or behind it. This prevents you from seeing the object, as if it were invisible.

Did you know? Eighty nanometres is about 1250 times thinner than the average human hair. That means you could fit about 40 invisibility cloaks in the same space as a single strand of a spider's web!

Limitations

These breakthroughs mark a huge leap forward in invisibility and cloaking technology. However, you won’t be able to use these cloaks in your everyday life anytime soon. Currently, they need to be built specifically for the object they are going to cloak. The same cloak can’t be used to hide different objects yet. So if you’re looking to sneak a few snacks into class or pass invisible notes, you’ll have to wait!

Also, there is a limit on how large the object being cloaked can be. Right now, only small objects can be cloaked. It will be a while before you can hide something bigger than a hand or a baseball. You won’t be able to cloak your entire body and make yourself invisible anytime soon!

Finally, the metals used in these cloaks reflect some wavelengths of light instead of scattering them. This makes the object a little darker when it's being cloaked, creating a partial shadow.. Scientists are planning to see if using ceramic instead of metal can solve this problem.

Despite these limitations, cloaking technology still holds a lot of promise. One day soon, it might just be that much easier to skip out of your charms class, sneak into the forest, and visit all those magical creatures!

Learn More!

General information on the science of invisibility and cloaking:

Engineers give invisibility cloaks a slimmer design (2015)
University of California San Diego Jacobs School of Engineering

Extremely Thin Dielectric Metasurface for Carpet Cloaking (2015)
L. Y. Hsu, T. Lepetit, and B. Kante, Progress in Electromagnetics Research

Now you see it, now you don’t—scientists developing real ‘ultrathin’ invisibility cloak with scale-up potential (2015)
S. Liverani, The American Ceramic Society

Invisibility
U. Leonhardt, University of St. Andrews

The Rochester Cloak (2014)
The University of Rochester

Weird science: waves, particles, and the origins of quantum mechanics (2014)
J. Kliever, CurioCity

How do invisibility cloaks work?
IOP Institute of Physics

Scientific articles related to invisibility cloaks:

Electromagnetic detection of a perfect carpet cloak (2015)
X. Shi, F. Gao, X. Lin and B. Zhang, Scientific reports 5

A carpet cloak device for visible light (2011)
M. Gharghi, C. Gladden, T. Zentgraf, Y. Liu, X. Yin, J. Valentine and X. Zhang, Nano Letters 11

Shakib Rahman



 Shakib Rahman is a coordinator with Let's Talk Science at the UofA.  He an avid soccer player and a sports nut in general.  He also has a a passion for science, science literature and TV. In his spare time, he writes science articles, some of which you can read here at CurioCity.



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