Not your father’s invisibility cloak

Why science is so much cooler than magic

November 18, 2010

As I am sure many of you are aware, the penultimate Harry Potter film opens at midnight tonight. For all of the fans (addicts?) out there, this is one of the last chances to get their rush from something new out of the world of Hogwarts. (Seriously, the UK should just give up the whole austerity thing and, instead, should publish the next J.K. Rowling book. It’s like printing money.) Anyway, in the final book, Harry’s invisibility cloak is revealed as one of the Deathly Hallows from which the book takes its name. In the novels, we never learn what the Cloak of Invisibility is made out of. We do know that the particular cloak that Harry possesses, which he inherited from his father, is possibly the greatest invisibility cloak that had ever been made. While that might sound cool, I think it’s pretty lame. There is no way to improve on it. There is no way to make it better. If one of these were made in the “muggle”-world, you can be sure that science and technology would keep developing to produce better and more advanced invisibility cloaks.

Pssst … Harry … Here in the muggle world, we can make you a better invisibility cloak.

Well, it turns out that real scientists have already one-upped Ms. Rowling by developing the framework necessary to make an invisibility cloak that would put Harry’s to shame. And, this development has happened at a furious pace.

In 2006, David Smith, a physicist at Duke, was an author on two papers describing the theory and the construction of the world’s first cloaking device. (See the excellent profiles from Discovery Magazine’s online site by Josie Glausiusz here and Andres Moseman here.) The first cloaking device was able to divert light around an object so that it could not be observed. In this case, the cloaking only worked with microwave radiation, a specific type of light. While amazing, this type of behavior is something we encounter all the time. There are lots of materials that “bend” light.

Water is a prime example. When you’re looking down at fish in a pond or at a straw in a cup (as shown in the image below), the fish or straw appear to be in a different place than they actually are. This all occurs because of a physical property called the index of refraction. The index of refraction is just a measure of the angle at which a material bends light. Refraction occurs because light travels at different speeds in different substances (Einstein’s theories hold that the speed of light is constant in a given medium, not in all media). For example, light travels at a speed of 186,282 miles per second in a vacuum and 140,062 miles per second in water. (Water has an index of refraction of 1.33, which means that light travels 1.33 times slower in water than in a vacuum) The bigger the change in speed, the larger the perceived light-bending.

Water bends light. The straw appears to go into the water at one angle and then continue through the water at another angle.

Metamaterials, what the cloaking devices are made out of, are designed specifically to drastically alter the speed of light. Some metamaterials have a negative index of refraction, which means that light does not scatter off of the material’s surface and it doesn’t bend slightly, as is the case for water. Instead, light is bent backwards into the material. We are able to see the objects around us because light does bounce off of things. A material with a negative index of refraction would be invisible because all incoming light would be drawn back onto the object. That was the case for the first cloaking device (at least for microwave light).

Since this first device was developed, technological development has moved astonishingly fast. The first cloak only worked in two dimensions and only with microwave radiation. In 2008, researchers were able to cloak a three dimensional object from visible light (here or see the profile by Andrew Moseman.) According to a paper published on-line just this week, scientists have now pushed the boundaries of cloaking even farther. (Again, read Moseman’s profile here – how nice of him to do such a great job compiling all of this research!!)

Using a material that changes its index of refraction (the index of refraction of a vacuum is always 1 and of water is always 1.33 while this new material could have lots of different indices of refraction) Martin McCall has come up with a way to cloak both space and time. A normal cloak completely hides everything. With this new cloaking device an object could be perceived to move a great distance instantaneously. Here’s how it works. The front end of a ray of light hitting this object would be sped up by the metamaterial’s very low initial index of refraction. Before the light completely passes through the metamaterial, it’s index of refraction grows to be a very large value (drastically slowing the speed of light). For someone observing this object, there is a time gap between where the front end of the light beam (moving really fast) and the back end of the light beam (moving really slowly) are observed. It is during this time gap where the object is free to move around, do a cartwheel, pick its nose, rob a bank, all unbeknownst to the person trying to watch it. The difference between this and a regular cloak is that with a normal cloak, a material would disappear. People tend to notice this sort of thing. With the new 4D (4 dimensional – space and time) cloak, the person watching would never notice that the object was missing while it was doing its dastardly deeds! If Harry had one of these, Professor Snape would always be able to keep his eye on an unmoving young Potter, even as Harry ran around on some new adventure.

Score one for science!

At least Harry is still the best at reciting all of the elements in the periodic table:

Well … almost the best …



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5 Responses to Not your father’s invisibility cloak

  1. Erika says:

    Catchy! Your children are already trying to sing along with Dr. Lehrer!

  2. sciencegeist says:

    some metamaterial-backtorealitiness to balance out my (obvious) over-plugging above: from Joerg Heber

  3. Pingback: The Goblin Element | ScienceGeist

  4. Carley says:

    Invisibility cloak = white elephant idea for Christmas! Good to see everyone :)

  5. Bethany says:

    Wow. We just went over refraction in Physics lab. I can kind of grasp this, but it’s still incredible. But the fact that Daniel Radcliffe knows who Tom Lehrer is and can (mostly) sing “The Elements” makes me happy!

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