For inspiration, you could hit the books: In Greek mythology, the goddess Athena wore an invisibility cap during the Trojan War. The same cap helped the half-god Perseus, who wore it to hide from Medusa, a monster who could turn someone to stone just by looking at them. In the books of J.R.R. Tolkein, Bilbo Baggins found a ring that could make him invisible; he passed it on to his nephew Frodo. And of course, there’s poor Harry Potter, who used his invisibility cloak to spy on classmates and teachers, hide from dragons or avoid certain spells cast on him by his enemies.

Now that you’ve got some ideas, it’s time for the hard part: building the cloak. To do that, you have to abandon science fiction and turn to real science. Two starting questions: How do you use visible materials to build something that’s supposed be invisible? How would you see it?

“If I were doing it, I’d built my invisibility device to have a remote control on/off switch,” says Steven Cummer, an engineer at Duke University in Durham, N.C. “This way I could have all of the pieces ‘off’ when it was being assembled. And if I lost track of it, I would have at least a chance of finding it by turning it off.”

Cummer has thought about this: In October 2006, Cummer was part of a team of scientists from Duke, including David R. Smith and David Schurig, who built the world’s first version of an invisibility cloak. They had been inspired by the work of a British physicist named John Pendry, who in May 2006 showed that an invisibility cloak was possible. And Pendry wasn’t the only one thinking about a disappearing act — at about the same time, a Scottish physicist named Ulf Leonhardt published a paper on building a cloaking device.

Less than half an inch tall and five inches across, this cloaking device was able to steer microwaves around it. The object to be hidden would be placed in the center.

Jack J. Mock, D. Smith Lab/Duke University

It wasn’t easy or perfect, Cummer says. “As often happens in science and research, it didn’t work very well the first time. It took several redesigns before we built something that worked pretty well.”

The device didn’t much resemble a cloak — at least, not one you would wear. It looked more like a set of circular fences nested inside each other, with a place inside for the object to be hidden. But up close, if you had a powerful magnifying glass, you would see tiny metal circles and rods that made intricate patterns all over these fences. These small details are one reason why the cloak works.

The device was small, about 5 inches across (roughly the diameter of a CD). Plus, that first cloak didn’t work like Harry Potter’s — the scientists didn’t actually see anything disappear.

Of course, they hadn’t expected to. That first version of the invisibility cloak didn’t shield objects from visible light. Instead, it hid things from a type of radiation called microwaves.

Moving the microwaves

An invisibility cloak has to deceive anything or anyone who might be watching. In order to understand how something can be invisible, it’s important to understand how we see.

Human beings see only objects that reflect light waves. These waves enter the eye and then are processed by the brain. But if an object doesn’t reflect light, then the waves don’t enter the eye, and the brain doesn’t process. The challenge of building an invisibility cloak is to build something that does not reflect or in any way interrupt waves of light.

Visible light has shorter wavelengths and higher frequencies than microwave radiation. So it will be harder to build a cloak that hides objects from visible light.

NASA

Light is a type of radiation, and all radiation travels in waves. Waves of radiation move through space somewhat as water waves do. And just as waves in the ocean have high points called crests and low points called troughs, radiation waves have crests and troughs. Unlike water waves, however, waves of radiation are made up of electric fields and magnetic fields that move together through space.

Scientists can learn a lot about a wave by measuring two things: its wavelength and its frequency. Wavelength is the distance from one crest to the next, and frequency is the number of waves that pass by a point in one second. Microwaves, for example, are more spread out than visible light — that means they have longer wavelengths and lower frequencies than visible light. (Microwave ovens, for example, heat food with microwaves that are about 13 centimeters, or about 5 inches, long.) In all kinds of radiation, frequency and wavelength are related — the higher the frequency, the shorter the wavelength. Waves of radiation differ by frequency and wavelength, and all the different types of waves together are called the “electromagnetic spectrum.”

At Duke, the engineers aimed microwave radiation at their device and took measurements. After the experiment, they looked at the data. According to their measurements, the device had shuffled the microwave radiation around the cylinder that is in the middle. Then, on the other side of the device, the waves had resumed their course — as though nothing had happened.

In other words, the waves of radiation moved around the device the way water waves move around a rock in the middle of a stream. Those tiny metal circles and rods were designed to change the directions of the electric and magnetic fields of the waves. By changing those fields in just the right way, the cloak could move the waves around itself.

Since that first successful test, in laboratories from North Carolina to California, Spain to Hong Kong, scientists have been racing to find ways to make invisibility a reality.

The Duke scientists made history with microwaves, but now scientists want to go further — and they are.

Last summer, for example, two independent teams of scientists announced they had created cloaks that worked for visible light. Unfortunately, those cloaks are tiny, so tiny that all they can hide are things so small that people already can’t see them. Plus, researchers have only redirected radiation that is at the far-red end of the electromagnetic spectrum — radiation with low frequency and long wavelengths.

Despite these problems, these breakthroughs are an important step forward in the science of disappearance and show that a cloak that can hide things from plain sight may not be far away.

Marvelous metamaterials

Invisibility cloaks would have remained impossible, forever locked in science fiction, had it not been for the development of metamaterials. In Greek, “meta” means beyond, and metamaterials can do things beyond what we see in the natural world — like shuffle light waves around an object, and then bring them back together. If scientists ever manage to build a full-fledged invisibility cloak, it will probably be made of metamaterials.

“We are creating materials that don’t exist in nature, and that have a physical phenomenon that doesn’t exist in nature,” says engineer Dentcho Genov. “That is the most exciting thing.” Genov designs and builds metamaterials — such as those used in cloaking — at Louisiana Tech University in Ruston, Louisiana.

An invisibility cloak will probably not be the first major accomplishment to come from the field of metamaterials. Other applications are just as exciting. In many labs, for example, scientists are working on building a hyperlens.

A lens is a device — usually made of glass — that can change the direction of light waves. Lenses are used in microscopes and cameras to focus light, thus allowing a researcher to see small things or a photographer to capture image of things that are far away.

A hyperlens, however, would be made of metamaterials. And since metamaterials can do things with light that ordinary materials can’t, the hyperlens would be a powerful tool. A hyperlens would allow researchers to see things at the smallest scale imaginable — as small as the wavelength of visible light.

Genov points out that the science of metamaterials is driven by the imagination: If someone can think of an idea for a new behavior for light, then the engineers can find a way to design a device using metamaterials. “We need people who can imagine,” he says.

Science of metamaterials just forming

The idea of invisibility has shown up in books for centuries, but the science of metamaterials is in its first chapter. Scientists are excited at the possibilities. Since 2006, many laboratories have been exploring other kinds of metamaterials that don’t involve just visible light. In fact, scientists are finding that almost any kind of wave may respond to metamaterials.

At the Polytechnic University of Valencia in Spain, José Sánchez-Dehesa is working with acoustics, or the science of sound. Just as an invisibility cloak shuffles waves of light, an “acoustic” cloak would shuffle waves of sound in a way that’s not found in nature. In an orchestra hall, for example, an acoustic cloak could redirect the sound waves — so someone sitting behind a column would hear the same concert as the rest of the audience, without distortion.

Sánchez-Dehesa , an engineer, recently showed that it’s possible to build such an acoustic cloak, though he doubts we’ll see one any time soon. “In principle, it is possible,” he says, but it might be impossible to make one, he adds.

Other scientists are looking into ways to use larger metamaterials as shields around islands or oil rigs as protection from tsunamis. A tsunami is a giant, destructive wave. The metamaterial would redirect the tsunami around the rig or island, and the wave would resume its journey on the other side without causing any harm.

One of the strangest new ideas for metamaterials came from a team that included Genov when he was a researcher at the University of California, Berkeley. There, he worked with Xiang Zhang and other engineers on the idea of “matter cloaking.” Just as an optical cloak could redirect light, a matter cloak would be able to redirect something solid — such as, say, a bullet. Genov says a matter cloak, were it possible to build, would be a perfect bulletproof vest. The bullet, as it approached the vest, would actually split into multiple pieces and move around the person — and then form again on the other side.

Genov says that the story of metamaterials and cloaking devices is just beginning, and that we’ll probably see a lot more strange, new devices in the very near future. Right now, scientists are working around the clock to build as many strange new devices as they can.

“They’re not perfect yet, but we’re in the beginning of the science,” says Genov. “We’re at the tip of the iceberg and the iceberg is very deep.”

The fish in the picture is alive and you’re looking inside its head. Really. It’s not a medical freak. Just a kind of fish with a naturally see-through forehead.

A new species, you might think. But no. The story is odder than that.

Meet one of the fish called barreleyes. This kind lives some 600 meters deep or more (that’s more than a third of a mile) in the Pacific Ocean.

A see-through forehead sounds like something you might remember to mention when describing a new fish. But 70 years ago scientists didn’t say a word about it when they gave the fish its official scientific name (Macropinna microstoma).

Those earlier fish scientists probably didn’t know about the clear forehead. They had to work from fish caught in deep nets and dragged up to the surface. The long trip up didn’t leave the samples in such good shape.

Today a scientist can send cameras and other equipment down to study deep-sea creatures where they live. Since 1993, cameras from the Monterey Bay Aquarium Research Institute in California have met these bizarre barreleyes three times in deep water off the coast.

And researchers managed to catch one and bring it to the surface in much better shape than usual.

What a difference meeting a live fish makes. Old reports had talked about some slime on the front of the fish. Now researchers see that the slime was probably the remains of the clear forehead.

The covering is tough and like the clear canopy on a fighter jet that lets the pilot see what’s happening, says Monterey Bay scientist Bruce Robison. In the fish, the rounded window is full of clear liquid and covers the eyes.

Like fighter pilots, these barreleye fish look out through their clear covering. Check the picture for the pair of fat, green domes like the tops of balls, inside the head. Those are the lenses of the fish’s eyes. (In the picture, the green lenses point up, and the fish is looking overhead.)

Each lens sits on top of a short, wide tube, which is the rest of the eye. That’s where the name barreleye came from, and that’s an odd shape for an eye. We hope this won’t happen, but if your eyeball fell out of your head, it would look like a ball. If it were All Planet Drop An Eyeball Day, there would be lots of balls. Cats, dogs, mice, elephants and lots of other animals have round eyes.

A front view of a type of barreleye shows its two nostril-like spots above the small mouth. The glow on the fish comes from the lights of a nearby camera vehicle. A glimmer of green inside the glowing clear forehead shows where the eye lenses are.

© 2006 MBARI

On barreleyes’ short tubes, the part that catches the image is at the bottom. That arrangement had puzzled scientists because tubular eyes should see only what’s straight in front of them and not much at the sides. That’s not very convenient. It would be a bit like looking at the world through the tube from a roll of toilet paper.

But looking at a living specimen, the researchers realized that an eye tube can move. Barreleyes point it upright to look overhead and then swing the lens downward so the tube points straight ahead, like the barrel of a mini cannon.

When the eyes point forward, the fish looks toward its pouty little mouth. Its lips stick out a bit, as if they would be good for picking morsels of food out of small places.

A siphonophore, a long string of filmy sea creatures, swims through the water (front end to the right) snagging food in its stinging tentacles. Researchers now wonder if the clear-headed barreleye fish steals food from siphonophores. The barreleye’s

© 2001 MBARI

What these barreleyes may do is steal food from creatures called siphonophores (sigh-FAH-nuh-4s), say the Monterey Bay scientists. Siphonophores in the area look like long, skinny, ultrafrizzy scarves made of bits of pale, soft film. Don’t wrap one around your neck though — they sting.

But the scientists think the barreleye might not care about the stings. Its clear forehead protects its eyes. So its mouth could nip off bits of prey that the siphonophores get tangled in their frizz. A clear forehead may actually be like fish goggles.

Power words: (loosely adapted from Yahoo! Kids Dictionary, which is also the American Heritage® Dictionary of the English Language, Fourth Edition)

canopy: a covering

lens: A clear part of the eye that bends the light passing through it so light rays hit the proper place for forming a picture.

siphonophores: sea creatures with clear, filmy bodies and stinging cells that band together in floating colonies. Famous example: the Portuguese man-of-war.

Going Deeper:

Galaxy clusters (white spots) are shown on a map of the cosmic microwave background, or CMB. The clusters appear to move, on average, in one direction (toward the purple spot).

Scientists have a mystery of cosmic proportions on their hands. Recently astronomers noticed something strange. It seems that millions of stars are racing at high speeds toward a single spot in the sky.

Huge collections of stars, gas and dust are called galaxies. Some galaxies congregate into groups of hundreds or thousands, called galaxy clusters. These clusters can be observed by the X-rays they give off.

Scientists are excited about the racing clusters because the cause of their movement can’t be explained by any known means.

The discovery came about when scientists studied a group of 700 racing clusters. These clusters were carefully mapped in the early 1990s using data collected by an orbiting telescope. The telescope recorded X-rays created by electrons located in the hot core of a galaxy cluster.

The researchers then looked at the same 700 clusters on a map of what’s called the cosmic microwave background, or CMB. The CMB is radiation, a form of energy, leftover from the Big Bang. Scientists believe that the Big Bang marks the beginning of the universe, billions of years ago. The CMB provides a picture of how the early universe looked soon after the Big Bang.

By comparing information from the CMB to the map of galaxy clusters, scientists could measure the movement of the clusters. This is possible because a cluster’s movement causes a change in how bright the CMB appears.

As a galaxy cluster moves across the sky, the electrons from its hot core interact with radiation from the CMB. This interaction creates a change in the radiation’s frequency, or how often an event occurs in a certain amount of time. Scientists can then measure the frequencies to detect movement.

As a galaxy cluster moves toward Earth, the radiation frequency goes up. As a cluster moves away from Earth, the frequency goes down. This shift in the frequencies creates an effect similar to the Doppler effect.

The Doppler effect is commonly used to measure the speed of moving objects, such as cars. Scientists can use this method to measure the speed and direction of moving galaxies by looking at changes in the radiation frequencies.

What the scientists found surprised them. Though the frequency shifts were small, the clusters were moving across the sky at a high speed — about 1,000 kilometers per second. Even more surprising, the clusters were all moving in the same direction toward a single point in the sky.

Researchers don’t know what’s pulling this matter across the sky, but they are calling the source “dark flow.”

Whatever it is, scientists say the source likely lies outside the visible universe. That means it can’t be detected by ordinary means, such as telescopes.

One thing is certain. Dark flow has shown that we don’t understand everything we see in the universe and that there are still discoveries to be made.

Going Deeper:

Scientists are developing “smart” lenses that sense where your eyes are looking and automatically focus to help you see more clearly.

Electric signals from microchips in the black boxes attached to these experimental eyeglasses change the focus setting to improve vision.

Guoqiang Li et al./Proceedings of the National Academy of Sciences

The main market for the glasses is adults older than 45—perhaps your parents or grandparents. At this point in life, most people start to get worse at seeing things that are close to them, such as books and computer screens.

When the decline begins, people usually start wearing reading glasses. Or, they get bifocals, which have divided lenses—a top part for seeing far and a bottom part for seeing near. Some kids with vision problems have to wear such glasses, too.

University researchers are working with a company called PixelOptics, in Roanoke, Va., to replace bifocals with electric lenses that can switch quickly from one type of focus to another.

“You don’t have just the bottom half of your eyeglasses” for close vision, says electrical engineer David L. Mathine of the University of Arizona in Tucson. He’s one of the inventors. “You get the whole view,” he says.

Each lens is made from two layers, and each layer is made up of two sheets of glass, with a thin layer of fluid sandwiched between the sheets. The fluid contains a transparent type of material called a liquid crystal, which is made of molecules that are shaped like rods. To change a lens’ focus, scientists apply electricity to the inner surface of one of the glass sheets in each layer.

In response to the electricity, the crystal rods rotate. Their direction determines how quickly light passes through the liquid-crystal layer. The process allows the material to focus light so that a crisp image forms inside the viewer’s eyes.

An image is out of focus when a special, electrically controlled lens is off (left) but clear when the lens is on (right).

Guoqiang Li et al./Proceedings of the National Academy of Sciences

Scientists had made similar, electrically controlled lenses before, but these earlier lenses couldn’t focus well enough or change focus quickly enough to be useful in eyeglasses, the inventors say.

PixelOptics has announced that it also plans to make a version of the glasses that will help people achieve extrasharp vision—even better than normal 20/20 eyesight.—E. Sohn

Going Deeper:

Weiss, Peter. 2006. Switch-a-vision: Electric spectacles could aid aging eyes. Science News 169(April 22):243-244. Available at http://www.sciencenews.org/articles/20060422/fob2.asp .

You can learn more about smart glasses at www.gravitysedge.com/pixeloptics/ (PixelOptics).