Optics: Make yourself invisible

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Optics: Make yourself invisible
Optics: Make yourself invisible

Make yourself invisible

Just slip under the covers and no longer be visible to anyone - who hasn't wished for that from time to time? What was once reserved for magicians and science fiction bullies, Muggle researchers are now making possible for everyone. If only in theory. So far. Superpowers, reading minds, looking into the future, flying and turning invisible - the list of children's wishes is short but demanding. But what does "childlike" wishes mean here? If we honestly listen to ourselves, the same dreams are still slumbering in the back corners. Wouldn't it be nice if…

Fortunately for some people the back corners are at the front. And that's how wishes come true - even if not always exactly how we imagined them. After all, the steam engine has given mankind superpowers, airplanes have been taking us into the air for over 100 years, neuroscientists are well on the way to peering into the minds of wired contemporaries, and meteorologists are trying to make rainy and sunny forecasts. Only the thing with the invisibility, that still sticks. The invisibility cloak of Harry Potter or the invisibility shields of the Klingons and Romulans we ordinary non-magical people (muggles) have so far had nothing to oppose.

That could change soon. For some years now, there have been things in some laboratories that don't exist at all - at least not in nature. So-called metamaterials have properties with which the physical world can be turned upside down. These are ordinary substances packaged in unusual combinations or with unusual surface structures. They feel normal, smell normal and react chemically just as normal. However, if light or other electromagnetic radiation of a suitable wavelength falls on the metamaterial, normality is over. The alternating magnetic and electric fields of the radiation interact with the electrons, which are particularly distributed due to the structuring. If the light were a rower, it would no longer be a simple paddling trip across a calm lake, but a canoe trip over a wave-riddled white water. It's already being directed down paths that it would never take in natural materials.

Light decides where to go at the border between two substances based on the refractive index. If it comes from an optically thinner medium (with a smaller index) into a thicker one (with a larger index), it bends in the direction of the line of incidence - which we can observe, for example, with a spoon that is half in a glass with water. This effect can be wonderfully manipulated with the help of metamaterials. While nature is content with refractive indices between 1 and about 4, the metamaterials break all limits: they create indices of 10, 20 or even more – and they even reach negative values below. Tell me where you want your light to shine and I'll bend the beam with my miracle substance.

This is exactly where scientists start when they want to create optical camouflage cloaks and shields. Ulf Leonhardt from the Scottish University of St. Andrews [2] and a team led by John Pendry from Imperial College London [1] calculate that metamaterials could theoretically deflect light rays in such a way that they flow around an object and continue behind it just like they do were before him. If we look at an object or person camouflaged in this way, we see everything that is behind it. He himself would be invisible.

Or almost. The practical implementation would still be relatively difficult. On the one hand, the camouflage shield would have to get thicker and thicker towards the inside in order to let in light and then gently and finally very emphatically direct it to the new path. He should also be able to do this with as many wavelengths as possible, so that the disguised sorcerer's apprentice doesn't walk around as a walking black-and-white seer. And besides, there must be no reflections anywhere and there must be delays due to the longer geometric paths for the deflected light.

Theoretically, these requirements cannot be fully managed: absolute invisibility is not possible. But you can get very, very close to that ideal. The camouflaged person could become optically so thin that he would actually have disappeared from our eyes. Mathematically, at least.

The camouflage shields and cloaks have another small disadvantage for the hidden muggle: he would be blind under them! Since the metamaterial directs all light around him, none gets into his eyes either. That's why it's pitch black under the coat. Whether a cloaking device, with which you have to grope your way through the corridors, knocking over valuable vases and rickety armor and in the end even stepping on the toes of the nasty potions teacher, is really the right thing for secret night wanderings in the castle, every little Harry has to decide decide for yourself. That's what happens when muggles have to make do without magic.

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