Invisibility You Can See


By Patricia Rogler

Harry Potter's invisibility cloak may soon become a reality. Scientists are getting closer and closer to making visible objects disappear. Until now, scientists have only been able to make objects invisible at the microscopic level, but two tests recently performed at MIT and the University of Birmingham have, for the first time, made objects invisible at the macroscopic level. The key to these tests is calcite crystals.

Calcite Crystals vs. Metamaterials

In the first invisibility tests scientists performed, they used metamaterials -- artificial materials engineered to have specific properties not found in nature. These metamaterials are expensive and limit scientists to infrared wavelengths or longer, making them impractical for hiding objects greater than 1mm in size. However, with the use of calcite, a common and naturally occurring mineral, scientists have found a way to hide objects over the range of visible wavelengths. Calcite crystals are a naturally anisotropic material, meaning they can bend rays of light in two different directions. They reflect and refract light in such a way as to conceal objects on the other side of them. Using two calcite crystals glued together, known as a "carpet cloak," scientists have made a paper clip and a rolled up piece of paper invisible.

Pros and Cons

Unfortunately, right now the calcite crystals make objects invisible only in one light polarization, not in natural light. Also, although the object appears invisible on all sides, it is still not invisible from all angles. These issues obviously create some real-life application drawbacks. However, scientists are optimistic they can take these tests further with the size of the crystal being the only real limitation. (So far the largest crystals were 23 feet in length.)

The use of crystals has overcome some major hurdles that the use of metamaterials presented -- namely size, expense, bandwidth, loss and image distortion. Invisibility cloaks seemed like a creation of science fiction, but by using calcite crystals, scientists could finally be paving the way toward making them a reality. Who knows what further developments in cloaking devices we could see in the near future.

Classroom Discussion

  • What future applications are there for invisibility cloaks?
  • Are there any drawbacks to creating invisibility cloaks?