David Kirkpatrick

April 19, 2010

An invisibility cloak flaw

To date all the invisibility cloak tech blogging I’ve done has covered the rapid development of this branch of science. Here’s some virtual ink on the other side of the cloaking coin.

From the second link:

Since then, Baile Zhang and buddies at the Massachusetts Institute of Technology in Cambridge, have been busy looking for the weak point in this idea and now think they’ve found it. Today, they point out that carpet cloaks have a flaw that makes the objects within them detectable.

The problem, they say, is that isotropic cloaks cannot work perfectly. Here’s why. Light can be thought of as a series of wavefronts each with a certain amount of energy. Ordinarily, the direction of energy propagation is at right angles to these wavefronts.

However, in an invisibility cloak, this perpendicular relationship becomes distorted as the light waves are steered. That’s what an anisotropic material does. But an isotropic material cannot do this–the energy always propagates at right angles to the wavefronts. This limitation means that isotropic materials cannot hide objects in the way Pendry suggests.

Zhang and co go on to prove their assertion by tracing a ray that passes through the kind of isotropic carpet cloak that Pendry suggested. What they’ve discovered will shock carpet cloakers all over the world.

According to Zhang and buddies, carpet cloaks don’t hide objects, they merely shift them to one side by an amount that is just a bit less than they are high. Crucially the effect depends on the angle at which you are looking. So when illuminated at an angle of 45 degrees, an object 0.2 units tall appears laterally shifted by 0.15 units.

If Zhang and co are correct, this could be a substantial blow for isotropic carpet cloaking. It means that the carpet cloaking effect has a limited angle of view.

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