Invisibility Cloaking

Invisibility Cloaking

In the Harry Potter books and movies, Harry's invisibility cloak comes in quite handy. But could Muggles like us ever make one? Maybe sooner than you think.


Making invisibility possible. I'm Bob Hirshon and this is Science Update.

Within several years, invisibility cloaks could transfer from Harry Potter’s world to our own. This according to theoretical physicist John Pendry of Imperial College London. In the journal Science, he and his colleagues propose making such a cloak from high-tech meta-materials. Pendry explains that in a meta-material, the molecules are artificially re-organized to change the material’s physical properties: for example, the way it refracts and scatters light.


So instead of going in a straight line, it goes in a smooth curve around the object, and then, returns to the same trajectory that it had before it hit the cloak.

If this still sounds like fantasy, Pendry points out that meta-materials invisible to radar already exist. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

To understand Pendry's idea, you first have to understand how we see things. Our eyes actually can't detect objects directly; in other words, when you see a tree, nothing about the tree itself is reaching your eyes, the way the bark or a leaf might touch your fingertips. Instead, you see the tree indirectly, by detecting the light that's bouncing off of it—which is why you can't see in the dark.

Here's how the process works: Whenever light hits an object, some of it is absorbed and some of it bounces and scatters. When that scattered light reaches your eye, the light receptors in your retina detect the light pattern, and send it on to your brain in the form of electrical signals. Your brain's instant interpretation of these signals is what creates the experience of seeing.

So what makes a tree look different from a person, or a building, or a tomato? It comes all the way down to the level of atoms. Just as a tennis ball bounces differently off a brick wall than it would off grass, rock, or asphalt, light bounces differently off different objects, depending on the exact structure of the atoms and molecules on the object's surface.

And as you know from experience, the light doesn't always bounce back. Transparent and semi-transparent materials, like glass or water, allow most or all of the light to pass right through it, revealing what's underneath. Many gases, like air, don't reflect any light at all, so they're invisible.

An invisibility cloak would need to be more than transparent. If it were transparent, you could see what's underneath (big deal; that's called Saran wrap). To make something inside it invisible, light waves would have to actually go around the cloak, and then converge again behind it, like water flowing.

If you were looking at this object from the left side, it would appear that the object wasn't there at all—just as if the light were going straight through the air.

So how do you make this happen? The old-fashioned way to make new materials is to change the chemistry of existing materials, by combining them, heating them, or exposing them to other kinds of stress. But the “meta-materials” Pendry describes are different: his team is able to actually rearrange the organization of the molecules in an object. In short, this gives them much more freedom to change the material's physical properties in radical ways.

As the report said, they can already do this with radar waves, which are part of the electromagnetic spectrum, just like visible light waves. However, radar waves have a much longer wavelength than visible light, which makes them easier to manipulate, just as it's easier to kick around a soccer ball than a grain of sand. In order to re-route visible light in the same way, the researchers will have to build meta-materials on a much smaller scale. That kind of technology isn't here yet, but it may be within five to ten years. Which means your children could very well take their own invisibility cloaks to school.

Now try and answer these questions:

  1. How do we see normal objects?
  2. How would a cloaked material redirect light to make the object invisible?
  3. How would an invisibility cloak differ from something transparent, like water or glass?
  4. What are meta-materials? How are they different from other manufactured materials? How does Pendry think they can be used?

You may want to check out the June 23, 2006, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: Deciphering horses' whinnies, how apes plan ahead, the science of Harry Potter's invisibility cloak, getting really mad over little things, and whether alcohol can cure a cold.

For Educators

Dracula's Library, from the Miami Museum of Science, contains exercises on the properties of light waves for students at various age levels.

In National Geographic's The Power of Light, photographer Joe McNally uses visual images to explore the phenomenon of light.

An article on Pendry's proposal appears in the online magazine PhysicsWeb.

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