Try as we might, perfect invisibility cloaks are just impossible, says a new study published in Physical Review A.
A pair of physicists from Germany and New Zealand have made an exposé as to why we cannot have the Harry-Potter invisibility cloak. They show how the ones we can make will only conceal objects from certain observers only.
One of the authors, Jad Halimeh, explains that perfect invisibility cloaks will not make it to the real world, and will thus “not be possible for all observers”.
This is because of the restrictions imposed by special relativity: conspicuous distortions on account of movement will be apparent as the transparency of the objects meant to be hidden will be only partial. In short, invisibility cloaks would not make things completely invisible, but only translucent.
The paper also mentions how single-frequency cloaks will also not be perfect. Previous works have proposed that using only one frequency of light will prevent the formation of the conspicuous deformities; the latter will normally arise because light will have to be diverted around the object when hitting the cloak to continue on the opposite side, and this curved path is longer than the straight path it would have otherwise taken, resulting in a time delay whereby the visible deformations are seen. Halimeh and his colleague explain that these single-frequency cloaks will still be visible when moving at high speeds on account of a relativistic Doppler effect that changes the frequency of incoming light.
They also show how another type of invisibility cloak will not work: the amplitude cloak which enables to conceal light across a range of frequencies. This will only work when both the concealed object and the observer are not moving. If either one moves, the latter will be able to see the cloak.
Well, the Muggle world sucks, right? Though, even in the Harry Potter universe, invisibility cloaks are only rare. So, guess that is a consolation for us.