The Physics of Invisibility

Question: How might scientists create invisibility cloaks?

Jim Kakalios: The idea is that, how do we see anything?  We don’t actually have an analog of Superman’s X-ray vision.  Superman presumably emits x-rays from his eyes that pass through an object, bounces off of something that are detected back by Superman. 

But we see things when an external light source, the light bounces off the object and then the reflected light strikes our eye.  If the light is not reflected from the object, but instead bends around the object, then as far as our eyes are concerned, it’s as if the object isn’t there.  Because the only way we know about the presence of the object is that when it reflects light.  And we can see that light by our eyes. 

So, metamaterials has something what’s called a negative index of refraction.  So the index of refraction normally is a positive quantity and it’s responsible for like the mirage effect that you see.  When you put a pencil in a glass of water and look at the interface between the air and water, the pencil will appear broken.  And that’s because the light travels faster in air than it does in water and so the difference between the speed of light in water creates a bending of the light rays.  And so our eye processes that as the pencil appears to be broken because of the difference of the speeds of light in the different medium. 

A negative index in refraction would bend the pencil all the way backwards and still be broken and for an object that was coated in a material that had a negative index of refraction, these metamaterials, the light would bend around it, but not strike it and bounce toward our eyes.  So the material would appear to be invisible from our point of view. 

There’s been progress made if you use microwave, or in some cases even invisible light, but it has to be only one particular wave length.  Whereas, the white light we see has a multitude of wave lengths.  So we’re a long way away from getting materials that could produce invisibility cloaks.  However, the science of sound and it has been experimentally confirmed, and so I am not going to bet against the cleverness of scientists and engineers coming up in the future.  

Is the idea of an invisibility cloak pure science fiction? Don't bet against the cleverness of the scientists and engineers of the future.

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