How would an invisibility cloak work? There are basically three ways you could make something invisible. You could use camouflage to hide it. You could use cameras to project the view behind the object in front of it. Or you could manipulate light so that it bends around the object being hidden.
We have all seen invisibility cloaks in movies, and they are always powered by some kind of magic. Then there are the cloaking devices they have in movies like Star Trek. At the push of a button, an entire spaceship is rendered completely invisible. These days, you can find shields that have the ability to hide the object that is behind them, and they certainly don’t use magic or futuristic technology. Let’s look at the three different methods.
The first method is basic camouflage. Animals have evolved to be fairly well hidden, and soldiers can emulate that. Ghillie suits, which are worn by snipers, aim to make the sniper completely invisible. There are five Ss of camouflage. Shape, Shine, Smell, Shadow, and Sound. Any type of camouflage suit needs to address all of these to make a sniper invisible, and snipers spend a long time sticking grass and plants into their suit to change their shape. However, a ghillie suit is only good so long as the sniper doesn’t move. A true invisibility cloak would work even if the sniper stood up and walked.
The second way you could make an invisibility cloak is to have pieces of material that either change their color to reflect the surroundings or are able to project an image from behind and display it in front. Color-changing materials are available, and this system is called adaptive camouflage. It tries to replicate what some animals, such as octopuses and chameleons, are able to do. There is a material made of tiny cells filled with dye. The dye is black at room temperature, but if you heat it above 47℃, it becomes transparent. With the cells connected in grids to a battery and a processor, it would be possible to make the material change color in specific patterns, or even specific shades. Going from black to white might not seem like a very good camouflage, but this would allow someone wearing the material, or a ship or a tank, to break up its shape. Anybody looking for a ship might pass over an object that was shaded in all the wrong places. It might even become invisible. There are other dyes that can change to different colors, so it might one day be possible to make a suit that can mimic the background. It might not be able to work very quickly, though. Although even chameleons need some time to change color. Another option would be to have microscopic screens that can relay images taken from cameras pointing backwards. There would be cameras pointing in 360 degrees, and the images they took would be projected onto the mini monitors exactly opposite to them. That sounds like a good idea, but if someone moved around the object, the background image would not move in the specific way that our brain would expect, and it would look odd. This is an example of parallax. This is theoretically possible, but not practical.
The third way of making an invisibility cloak is to manipulate light. Light travels in a straight line, unless something acts upon it. When light travels through water, it bends slightly. When light travels past an object of enormous gravity, such as a black hole, it curves. There is nothing with that much gravity on Earth, but like can be made to bend around an object. We see things because everything on Earth absorbs some light and reflects some. The reflected light goes into our eyes and is what we see. To make something invisible, you need to bend the light so it goes around the object and doesn’t reflect back. Just like if you stood in a river, you would get wet, but if there was a way of diverting the water around you on either side, you would stay dry. This sounds impossible, but it can be done with metamaterials. There are different types of metamaterials, but the one we are looking at here is able to deflect light waves. The material is made of lots of tiny metal rings that contain electronic microwave distorters. When they are turned on, they can push specific frequencies of light away from them. They can be adjusted to bend other frequencies of light as well. Light that bends away from an object is said to have a negative refractive index, and if the light is bending away from the object and not hitting it, then there is no light bouncing back to our eyes, and the object is effectively invisible. Presumably, this is the technology that the Starship Enterprise uses. This metamaterial is still very much in the experimentation stage, and there is no way to make anything big enough to hide a person. Plus, it would be incredibly heavy. And it would need the computing power to be able to deflect a range of different wavelengths. And any cloak that can hide you from invisible light could still show up on an infrared camera. But who knows, in the near future, we may have true invisibility cloaks. And this is what I learned today.
Sources
https://physicsworld.com/a/how-to-make-an-object-invisible
https://science.howstuffworks.com/invisibility-cloak.htm
https://serious-science.org/metamaterials_and_invisibility-8424
https://en.wikipedia.org/wiki/Cloaking_device
https://acc.srv.beargryllssurvivalacademy.com/news/the-five-ss-of-effective-camouflage
https://www.bbc.com/future/article/20140915-is-this-the-ultimate-camouflage
Photo by Matheus Bertelli: https://www.pexels.com/photo/person-dressed-as-a-ghost-wearing-a-hat-and-sunglasses-18951425/