Invisibility Using Carbon Nanotubes: Creating a Mirage
October 5, 2011 1 Comment
This is very cool: researchers have achieved pretty convincing invisibility in a completely different way from the antimagnet I discussed earlier, by creating a mirage using carbon nanotubes.
If you’re curious as to how this works – and don’t even pretend like you’re not – I’ll explain the phenomenon, but I have to take a few steps back (or you can skip the next two paragraphs if you’re comfortable with refraction and mirages):
Every medium, like a gas or liquid, has an index of refraction, meaning the speed of light traveling through that medium. It’s called an index of refraction (or refractive index) because when light changes speed – when it hits the border of two media with different refractive indices – it refracts, changing direction slightly. This is why, for example, images underwater, when seen from above water, can look distorted – the light refracts when it hits the air-water surface, so it doesn’t come straight from the visible object to your eye like it would if it were just going through air or water.
(Edit: Here’s a One-Minute-Physics video from New Scientist explaining why light changes speed in different media.)
A refractive index depends on the medium but also the temperature of the medium. Hot air has a lower refractive index than cold air, and this is the cause of mirages, in the common oasis-in-the-desert sense. When light from the sky nears the ground, it refracts, bending away from the highest heat (the ground), meaning it bends up towards your eyes. This means that you see blue light coming from the direction of the ground, and since we’re usually safe to assume that light travels in a straight line (otherwise we really couldn’t trust anything we see), our brain interprets it as something blue on the ground – water.
So how did these University of Dallas researchers use this to create invisibility? Well, carbon nanotubes – one-molecule-thick sheets of carbon rolled into tubes – are apparently very good at transferring heat to the surrounding air. They can be electrically heated, causing the air around them to rapidly heat up – like sunlight heating up sand, which heats up the air above it – and (some) light that approaches them will be refracted away.
Because the nanotubes shed heat so quickly, they can also be turned on and off very quickly – pretty awesome. I don’t know that I’d want to wear a suit of extremely hot carbon, but I’m sure they’ll put it to amazing use somehow; apparently (via New Scientist) they can use this effect for acoustical cloaking, possibly for submarines.