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Science
by Pat Murphy & Paul Doherty

VANISHING ACT

 

Harry Potter had a magical cloak of invisibility that let him vanish from view.

Obviously fantasy, right? Pure wish fulfillment. Not possible. No way. No how.

Well, hold on just a minute there. That invisibility cloak sounds like a really useful object. So we started wondering how a cloak like that could be made.

Maybe we could consider the Romulan cloaking device in the Star Trek universe, reputed to bend light to render a starship completely invisible. Or we could think about the black spaceship of Hitchhiker's Guide to the Galaxy, which was invisible against the blackness of space (but ridiculously obvious against a glowing nebula). Or maybe we could start by consulting a fish. A peacock flounder, to be exact.

We went with the flounder.

 

A TWO-DIMENSIONAL LIFE

The task of vanishing is a lot easier if you're flat as a flounder. To disappear, a flounder only needs to change the pattern of its skin to match the pattern of the sea floor it's resting on.

You can experiment with this for yourself. Start with two prints of a photograph of a uniform scene, like grass, or sand. Cut a flounder shape out of one print and drop it on the other. Then step back. The flounder shape vanishes. (For more detailed instructions, check out the Exploratorium science snack Disappearing act at https://www.exploratorium.edu/snacks/disappearing-act)

If you try this, you will notice that you have to be really careful about the edges of your cutout flounder. Human eyes are great at detecting changes at edges. If your cut-out flounder doesn't lie flat but curls up even a little bit, it'll be much easier to find. In nature, animals that hide have to be really careful to conceal their edges. Flounders often dig down on the sea floor, hiding their edges under the sand.

Experimenting with a paper flounder is fun, but Paul says it's even more fun to experiment with real flounders. He had an opportunity to observe these tricky fish firsthand when perceptual scientist Vilayanur S. Ramachandran came to visit the Exploratorium, where Paul works.

Ramachandran put flounders in a tank with a checkerboard bottom. Paul says it was interesting to watch the flounders adapt their coloration to try to match the checkerboard. (Pat is concerned about the effect of this experiment on the flounder's state of mind. It sounds like an attempt to make a flounder have a nervous breakdown.) In any case, the fish struggled and ended up with blotchy coloration that you could kind of see as a checkerboard if you squinted. It was far from perfect, but a pretty good job, given the challenge. (For more details on this experiment, see "Hidden in Plain Sight" by Vilaynur S. Ramachandran and Diane Rogers-Ramachandran at www.SciAmMind.com.)

How do flounder manage to duplicate the pattern that's below them? Scientists know that flounders use their eyes to adjust their coloration because if their eyes are covered, the flounders cannot match the background.

A question that comes to mind here is: How do you blindfold a flounder? This, we can report, is a question that Google cannot answer. We do know, however, that some flounders have their eyes on stalks. This gives these fish a good view of their surroundings, more of an overhead view, like you would have from an elevated viewing stand. This would be useful to a flounder trying to match its surroundings.

 

THAT TROUBLESOME THIRD DIMENSION

The flounder, lying flat on the ocean floor, has it much easier than Harry Potter wearing his cloak of invisibility. Alas for Harry Potter's cloak, Harry is not two-dimensional. The cloak that covers him has a shape in three dimensions. This makes vanishing much harder.

Consider a checkerboard. On top of it, put a thin flat photograph showing four squares of the same checkerboard. You could align the four squares so they perfectly match the background and seem to vanish. Move your head from side to side. Look at the checkerboard from a short distance or from far away. Those four added squares will be invisible.

But what happens if you raise the display an inch above the surface? When viewed from a long distance straight above the middle, the added squares still vanish. But if you move your head to the side, you can see the mismatch between the added squares and the background. Busted!

There is a way to vanish, however. Here's how it would work. The material on the bottom of the photograph has to record the light coming up to it from the checkerboard below. It has to record the color, intensity, and direction the light is moving. Every point on the bottom has to record the light coming into it from every point on the actual checkerboard below.

The Lytro Immerge is a camera that's designed to do this to a limited extent. These cameras are used to record the information needed to give Virtual Reality a lifelike feel. But even if the cloak were equipped with this high-tech camera, the problem is only half solved.

The cloak would also need a display that sends light out along the same path that light came in at from below. This means that one pixel on the top of the invisibility cloak has to send white light out in one direction and dark gray light out in a different direction. In normal displays, a pixel is either light or dark, period. (Actually, a pixel is rather like a period.…)

There is one kind of image, however, that changes its appearance when viewed at different angles. That's a holographic image. Suppose you looked closely at a hologram, staring at a single point. If you move your head while staring fixedly at that exact point, you'd see that the color and brightness of the spot can change. That's exactly what the cloak needs to do!

Now we know how to start making our invisibility cloak. Every point on the cloak has to record the incoming light, using Lytro-like sensors. Then the cloak has to figure out where that light would come out of the other side and use holographic technology to make it so.

 

ON THE MOVE

But don't start making plans for your career as a super spy (or wizard) just yet.

Flounders manage to hide in plain sight by remaining very still. (You can see this behavior in action (or lack of action) in a suspense-filled video of a flounder eating lunch. (https://www.youtube.com/watch?v=EPoIXY4VoR0) The eyes of predators and prey are sensitive to motion. When a flounder (or your flounder cut-out, if you've been experimenting) moves, you can see it easily, but once it stops moving, it vanishes once again.

Flounders change their coloration by releasing pigments at the surface of their skin cells. This takes time. It takes a peacock flounder about eight seconds to match the pattern of its background. That time lag makes movement a problem. To match the background while it is in motion, the flounder would have to be able to make its skin play a full color movie. And that's just not happening.

To succeed at invisibility, Harry Potter's cloak needs to do what the flounder can't—keep changing its camouflage pattern while Harry is in motion. The display on the cloak has to make a motion picture hologram, and that's not easy. Holographic movies of pre-recorded scenes have been made, but no one has created a holographic motion picture display screen that gives a dynamically changing scene with high resolution—at least, not yet.

And even if we could accomplish that, we still wouldn't be home free. To make an invisibility cloak, we'd need a display with pixels smaller than a wavelength of light with adjustable color and brightness (and phase). Such a display is called an optical phased array. This will require progress in nanotechnology. Every piece of the cloak the size of a fingernail will need to have a million pixels. To cover Harry completely, the cloak would need a twenty gigapixel display.

 

SO WHEN DO WE GET OUR NEW CLOAK?

Experimenters have made an "invisibility cloak" that matches the background when viewed from just one point of view. A person wore a flat display on one side of his body and a camera on the other. The display showed a pattern that matched what a viewer would see when looking from one known location.

Unfortunately this system is easy to defeat. The viewer simply had to move to one side or up and down and the pattern would no longer match the background, and the hiding person becomes visible. So it's a cloak of invisibility that only works if your nemesis is frozen in place. And so are you. Not very useful.

Of course, now that we know how to make an invisibility cloak, it's time to start figuring out how to defeat it. Paul pointed out that the high-tech holographic invisibility cloak keeps you from seeing the person wearing the cloak, but you could easily reveal the wearer with sonar.

So Pat suggested that Harry's enemies at Hogwarts could replace their trained owls with trained bats. A bat "sees" the world using eyes plus sound. The bat makes clicks that bounce off objects and listens for the echoes. Those echoes let the bat understand the world around it. By listening to echoing clicks, a bat would have no difficulty detecting invisible Harry.

To defeat detection by bats, you'd have to make a cloak that absorbs sounds or bounces them in directions that do not return them to their source. And so the battle between finding and hiding would continue.
 

__________________________________

Paul Doherty works at The Exploratorium, San Francisco's museum of science, art, and human perception—where science and science fiction meet. For more on Paul's work and his latest adventures, visit www.exo.net/~pauld. Paul's latest book is And Then You're Dead, the science behind the most interesting ways to die.

Pat Murphy is a science educator, a science fiction writer, and occasionally a troublemaker. She works at Mystery Science, developing hands-on lessons for elementary school. You can learn more about what she's up to at www.brazenhussies.net/Murphy.
 

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