Scientists are one step closer to creating the long-sought after invisibility cloak. Kinda. They have figured out how to hide — or cloak — an object without it casting a shadow.
What is cloaking?
Cloaking devices work by rerouting light around an object — an optical trick similar to redirecting water around a rock in a stream. This makes it seem like the object isn’t there, even when we look directly at it.
In a normal river, the flowing water bends around the rock, coming out disturbed downstream. The water downstream of a “cloaked” rock would appear to flow the same as the water upstream of the rock. If you were looking at the water downstream of the rock, it would look as though the rock wasn’t there.
In a way, Schittny and his team performed this trick using light waves instead of water. They succeeded at rerouting light around an object in such a way that the light met up on the other side with little distortion.
The object did not cast a shadow, an impressive feat in the cloaking world. Since light is how we see, this means the object was basically made invisible.
How they did it
The researchers began by putting objects in stainless steel containers, or cores, like the cylinder and sphere pictured below. They then painted the core white to reflect and scatter the light.
They coated the white cylinder in the “cloak,” a hard shell made of silicon and microparticles that scatter light. Below, the core on the left is without a cloak, while the core on the right has the cloak coating.
The magnifying glass shows how the scattering particles move light within the cloak.
“Every light particle takes a more or less random walk, bouncing off the scatterers repeatedly,” much like in fog or frosted glass, Schittney said via email.
The cloaked core was then deposited in a tank filled with a water-paint mixture. The paint particles also act as scatterers. The tank is only partially filled in the picture below, it was usually filled to the top during experiments. A liquid-crystal display screen sits behind the tank and is used as the light source.
The water-paint mixture is crucial to the experiment. This is because the “effective speed of light” in the cloak is about 5 times higher than the surrounding liquid.
“This higher speed inside the shell makes up for the geometrical detour that the light has to take on its way around the opaque core,” Schittney said. The result is that the core is concealed and no longer casts a shadow.
You can see the results of the experiment below. The left panel is the experiment with the cylinder core and the right is with the spherical core. Both the top panels are empty tanks containing only the water-paint mixture. The second panels have the uncloaked cores in the tank as well. The third panels contain the cloaked core, also surrounded by the water-paint mixture, which practically disappears.
We’re not there yet
Since we are surrounded by air and not water-paint, it is worth asking whether the cloak works in air. Unfortunately, the answer is, not yet, as you can see in an experiment with the cylinder on the left panel below:
The optical cloak works in the water-paint because the effective speed of light within the core cloak is faster than in the water-paint. Cloaking in a diffuse medium, such as the water-paint, effectively reduces the speed of light as it moves from point A to point B by bouncing it every which way to get there.
Creating a core cloak where the speed of light is faster than the surrounding air would would violate the speed of light’s upper boundary, Schittny said.
So far, all attempts to visibly cloak objects when its surrounded by air have come “with restrictions in wavelength, or colour, and size,” Schittny said. “‘Like this’ it has never been done before, but there have been several other realisations of cloaking in optics and other physical systems,” he said. We have seen cloaks that hide things from sonar and that could cloak the electromagnetic signature of a Humvee.
For an invisibility cloak to work in air, it needs to work for all colours of light for something to be completely disguised. “We call this a perfect or ideal cloak.”
Schittny suggests that one of the next steps for this research could include adding movement into the mix: if the object itself is moving or the light source is not always on.
However, one of the most immediate practical application would be to disguise the unsightly bars that often protect windows. If the bars were cloaked within frosted glass, light would be allowed to enter without ever casting the shadow of the bars.
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