This is a view of the Pacific Ocean, where different colors indicate the height of the tsunami in that area. The highest waves, in black, were probably more than eight feet tall. The orange and red areas indicate areas where the wave was closer to eight inches. Credit: NOAA

People in Japan face a catastrophe that has gone from terrible to tragic. The disaster began as an earthquake that launched a powerful ocean wave, called a tsunami, that caused destruction all along the coast. Those natural disasters damaged a cluster of nuclear reactors in Japan, leading to one of the worst nuclear disasters in recent years.

“My homeland has been struck by a tragedy of cataclysmic proportions. This has been one of the greatest natural disasters of modern times, the full extent of which is still becoming clear,” said Yukiya Amano, the director general of the International Atomic Energy Agency, in a press conference on March 14. “The events of the last few days are truly unprecedented.”

On the afternoon of Friday, March 11, an earthquake struck the floor of the Pacific Ocean 80 miles east of northern Japan. The U.S. Geological Survey reported the earthquake had a magnitude of 9.0, making it one of the most powerful in recent history. The magnitude of an earthquake is determined by measuring how strongly the Earth shook. People reported feeling the ground move all across the island country, both during the first quake itself and during hundreds of others that followed. Those later, smaller earthquakes are called aftershocks.

During the earthquake, the sea floor moved. That motion made the water go up and down and caused a tsunami, or a powerful ocean wave, that moved outward from the center of the earthquake site like the ripples that you see when a pebble drops in a pond. These ripples, though, were giant and spread out over the ocean.

Waves are how energy moves across the surface of the ocean, and when these waves struck Japan’s coast, they brought destruction. Within minutes, coastal towns flooded as unknown numbers of people were swept away with cars, boats and even buildings. Countries around the Pacific Ocean received tsunami warnings.

About 180 miles north of Tokyo, Japan’s capital city, two nuclear reactors were seriously damaged by the earthquake and tsunami. Although the reactors were equipped with safety devices intended to shut them down in case of an earthquake, the backup power supplies were also damaged by the quake and the tsunami. As a result, they failed.

The inside machinery of the reactors needs to be kept cool; if it’s not cool enough, a reactor could melt and possibly even destroy the protective metal vessel that encases the nuclear fuel. During a meltdown, radiation could be released into the air, threatening the health and safety of people nearby.

These nuclear reactors, part of a facility called Fukushima Daiichi, generate electricity for millions of people through a process called fission. During fission, a larger atom breaks into two or more smaller atoms. At most of the reactors at Fukushima Daiichi, atoms of an element called uranium-235 are the ones that break apart. Uranium-235 is natural on Earth, but it is unstable — which means it’s always ready to break apart. (The number 235 identifies how many protons and neutrons make up the nucleus, or heart, of a single atom.)

When an atom of uranium-235 fissions, it forms smaller atoms but also releases particles called neutrons. These particles can hit other atoms of uranium-235 and cause them to break apart — and then those newly split atoms release neutrons, which keep the reaction going and going. This is what occurs inside a nuclear reactor.

This reaction happens inside narrow rods that hold fuel pellets, which are immersed in water. As they undergo fission, the atoms inside the fuel rods produce heat. That heat will be transferred to water outside the reactor. As that water turns into steam, it is used to turns turbines that produce electricity.

The image on the left was taken by NASA’s Terra satellite on Feb. 26, 2011. The image on the right, which shows the extent of the flooding, was taken by NASA’s Aqua satellite on March 13, 2011, days after an earthquake and tsunami devastated Japan. The orange-red dot near Sendai is likely a large fire. Credit: NASA

Nuclear fission generates a lot of heat, so the nuclear reactors in which fission occurs need water to keep everything cool. At Fukushima Daiichi, which includes six reactors, water became the problem. According to the Nuclear and Industrial Safety Agency in Japan, the reactors lost power after the earthquake. That wasn’t a problem because backup generators started pumping water to remove heat building up inside the reactors. But when the tsunami struck, it took out those backup generators ― leaving the reactors dangerously hot. Then, another backup system, running on batteries, started. But it couldn’t keep up.

Explosions have seriously damaged other buildings associated with four of the reactors. A big and yet-unanswered question is whether it was the explosions or the excess heat in the reactor’s core that damaged the vessel that contains the fuel.

The core has become damaged. It appears to largely be holding the radioactive gases that have been developing. At times, however, reactor-safety teams have released small amounts of those radioactive gases to reduce the pressure in the vessel that holds the fuel.

Radiation levels around the reactors have become dangerous.

One reason is that old fuel rods from the six reactors at Fukushima Daiichi are stored near the reactors in pools of water. In many cases, those fuel rods are still quite hot. Circulating water is needed to cool these old fuel rods. But when the tsunami knocked out backup cooling to these pools, some lost much if not all of their water for a time. Under these conditions, radiation levels can increase enough to imperil the lives of workers. And on several occasions in the first week following the accident, many workers were sent home for their safety.

Some steam released from the reactors has also left the facility. Radiation from that steam has been detected in the countryside. Levels of radiation were not high, but because they could become high on short notice, people living within 20 kilometers of the reactor facility were told to evacuate. Those living between 20 and 30 kilometers of the plants were advised to stay indoors.

Even tinier amounts of radiation — levels almost too low to measure — have crossed the Pacific Ocean and been recorded in Washington State and California. As of March 21, these levels were too small to pose a risk to people living in the United States.

It will take months for Japanese officials to bring the reactors under control and for the country to dig out from the triple disaster that hit. Stay tuned.

POWER WORDS (adapted from the New Oxford American Dictionary)

Richter scale A way to measure the size of an earthquake, based on the measurements of seismographs. Destructive earthquakes typically have magnitudes of 5.5 or higher. A difference of one in the Richter scale represents an approximate 30-fold difference in magnitude.

seismograph A scientific instrument that measures the strength and duration of an earthquake.

tsunami A long, wide wave in the ocean caused by an earthquake or some other disturbance in the Earth.

fission The division or splitting of an atom into smaller atoms and into the particles that serve as building blocks of atoms.

radioactive Capable of producing ionizing radiation, which is powerful enough to remove electrons from other atoms.

uranium A gray, dense, radioactive element used in nuclear reactors.

The loss of a fiber optic cable in San Jose, Calif., last year highlighted how crucial lasers have become in our lives. Credit: Henrick5000/iStock

On April 11, 2009, vandals sliced through a handful of fiber-optic cables in San Jose, Calif., a high-tech hub in Silicon Valley.

Instantly, cell phones and land-based phone lines stopped ringing. Internet service crashed. Credit card machines froze. Banks locked their doors. Traffic lights blinked in disarray, snarling traffic. For a short while, no one could call 911.

The reason for the communications breakdown is that most of the information we send and receive, from text messages to Google searches, travels through fiber-optic cables. And the messages racing through these cables are encoded by lasers. So when the cables were cut, so were all forms of communication that are delivered by laser beam.

“Cutting off the lasers was equivalent to having a disaster in that part of the world,” says Thomas Baer, an expert in laser science from Stanford University in California. “That’s how much we depend upon lasers for communicating with one another these days.”

Lasers used in telecommunications blink on and off at blindingly fast speeds of some 10^-12 seconds, or one millionth of one millionth of a second. These pulses create digital codes, sort of like Morse code. The messages are then beamed through fiber-optic cables and carried across classrooms, neighborhoods and oceans. Eventually the messages make their way to our cell phones, televisions and computer screens.

You might be most familiar with laser beams from your teacher’s laser pointer and from Star Wars light sabers. Lasers are remarkable because they are the brightest source of light on Earth, they produce the purest form of color possible, and they can be focused down to the tiniest spot possible. These qualities make them useful for a seemingly endless list of applications.

Now, as scientists this year mark the 50th anniversary of the invention of the laser, it’s clear that lasers have touched and transformed nearly every aspect of our lives.

DVDs contain digital messages that are written by lasers, and those messages are decoded by lasers inside of DVD players. A laser at the grocery store checkout line reads the bar code on your box of cereal. Lasers are used to weld and shape metal. For instance, every major automobile part, from air bags to cloth seats, brakes, clutch and engine is manufactured with the help of lasers. Lasers are used in delicate eye surgeries to improve vision, and they can measure the distance from Earth to the Moon to within a couple of inches. About half the gross domestic product (GDP), the total income of the United States, depends on lasers to manufacture key parts or deliver information, says Baer.

But for all of these practical uses, scientists who are exploring the future applications of lasers — from harnessing the power of the sun for carbon-free energy to altering weather patterns — say that the future of lasers is only getting brighter, and more intense. The next generation of lasers is going to be 10 to 100 times more powerful than present-day lasers.

Light Amplification

Like many scientific discoveries, that of the laser resulted from decades of step-by-step progress. In the early 1950s, during World War II, several teams of scientists were racing to make the first laser. The U.S. military hoped to create a “death ray” that could shoot down missiles. Through trial and error and experimentation with different types of materials, Theodore Maiman at the Hughes Research Laboratories in Malibu, Calif., succeeded in building the first laser in 1960 using a powerful flash bulb wrapped around a short ruby rod about as long as your finger. When the flash bulb fired, it excited atoms in the ruby. Mirrors on the ends of the rod reflected light through the ruby crystal. When some of the light leaked through one of the mirrors, it exited as an intense burst of red light. The first laser was born.

“When these guys invented this, they had a certain application in mind,” says David Fritz, an expert in X-ray physics at the SLAC National Accelerator Laboratory, at Stanford University. “But certainly they didn’t imagine what it would evolve into.”

The word “laser” stands for “Light Amplification by Stimulated Emission of Radiation,” or LASER. In other words, a laser is an intense or “amplified” pulse of light. This pulse results when atoms are stimulated, or excited, by light but then fall down into a lower energy level and give off energy. Atoms can remain in an excited state only for about one-millionth of a second. When atoms return to their usual, non-excited states, they produce photons, which are the basic units of light.

Scientists Yannick Petit and Jérôme Kasparian first tried out their laser in a cloud chamber to see whether water-based clouds could form. Credit: Daniel Giry, Saga Photos

Lasers have unique properties that make them so useful. Ordinary light, such as sunlight, consists of many different colors. In contrast, laser light consists of just one pure color. Light travels in wavelengths with peaks and valleys, just like the waves of the ocean. But while the light waves from sunlight or a flashlight or light bulb scatter in different directions, the wavelengths of a laser flow in perfect formation, sort of like the rows in a marching band. Because the waves of laser light move together so precisely, the light beams can be focused into a tiny area, even much smaller than a pinhead. These qualities create the sharp, powerful beam of light that we recognize as a laser.

Lasers for fusion

As long as lasers have existed, scientists have envisioned harnessing their power to create a fusion reaction that could someday produce an almost limitless stream of carbon-free energy. Scientists plan to soon fire up the most powerful lasers in existence to work toward that goal. The National Ignition Facility, part of Lawrence Livermore National Laboratory in Livermore, Calif., houses a 10-story building that spans the length of three football fields. Inside of that building sit 192 of the world’s largest lasers.

When all of the laser beams are activated, they will generate about 2 million joules of energy, which is about the amount of energy contained in a stick of dynamite. That pulse of energy will be delivered to a small pellet of hydrogen ice. The resulting reaction produces “fusion,” a process that occurs when two heavy hydrogen atoms collide, or fuse, into one helium atom. The explosion is very fast and powerful. Imagine the power of an exploding stick of dynamite. Dynamite can blow up mountains. This fusion reaction happens about a million times faster than a stick of dynamite explodes. And while a stick of dynamite is about the size of a large breadstick, the energy of these lasers will be focused, or concentrated, onto a target that is about as wide as a human hair. “The laser in this case acts as a powerful hammer that drives this reaction,” says Baer.

The speed and concentration of this reaction, Baer estimates, will make it about a trillion times more powerful than the explosion of a stick of dynamite.

This fusion reaction is similar to what powers the sun and other stars. It will release a burst of energy so powerful that it’s comparable to making a miniature star on Earth. And scientists believe the fusion reaction could someday be used to produce carbon-free energy.

“No one knows if fusion energy is going to work,” says Baer. “But it is an important area of research, and it allows us to understand new forms of matter in ways we just haven’t been able to access before.”

Lasers for brain research

Lasers are also helping researchers understand what makes the tiniest brains tick. Fruit flies possess brains the size of a grain of salt, yet they can taste and smell, see and walk. These brains can even learn — not unlike the human brain.

Fruit fly brains are complex enough to provide insights into the workings of the human brain, and they are just the right size for scientists to study using lasers. With laser beams precisely pinpointing areas of the flies’ brains, scientists can map individual brain cells and study these cells in action, tracing the flow of information through a fly’s brain.

For instance, it’s possible to learn what happens when flies process information — like when a fly sees and smells a watermelon on a picnic table and makes the decision to land on the watermelon or to pester the picnic guests. And it’s possible to observe how the fly’s brain tells its limbs whether to walk, fly or jump. In other words, scientists can study the patterns of brain waves, or neural activity, inside the flies’ brains.

“We’re looking at this right at the neural level, so we’re reading the actual thoughts of these fruit flies,” says Baer.

Fruit flies are being used as models to study human brain diseases such as Parkinson’s and Alzheimer’s. Scientists hope that learning about the brain circuitry of the flies can help in understanding what causes these diseases and someday to develop cures.

Lasers for rain

Lasers may even play a role in improving weather forecasting and one day in triggering rainfall.

As far back as the 1930s, during a time known as the Dust Bowl when North America was stricken by drought, people have hoped to control weather patterns and create rain.

Current attempts at rainmaking involve “cloud seeding,” using rockets to scatter substances into the atmosphere. These tiny particles provide surfaces, or nuclei, on which water can condense and around which clouds can form. But the process is not very efficient, and there are concerns over the potential toxic effects of these particles in the air, says Jérôme Kasparian, an optical physicist at the University of Geneva in Switzerland.

So Kasparian and his colleagues came up with an alternative. They discovered that lasers can produce charges, or ions, in the atmosphere that act as cloud nuclei. The team recently fired a powerful laser through a cloud chamber the size of a small box. To the researchers’ delight, clouds formed before their very eyes. The clouds were small: just 20 to 30 centimeters in diameter, about the length of two pencils end to end, across the cloud chamber.

“The key point is it works — we shot the laser and saw the clouds forming,” says Kasparian.

To test the experiment outside, Kasparian and his team launched a high-powered laser into the sky. Then they fired a second laser. The laser allowed them to see how much light gets scattered back to the ground by water droplets. When it was really humid out, the scientists were able to trigger formation of clouds in the atmosphere.

The experiment is not yet ready for practical applications, says Kasparian. But soon, the work could be used to improve local weather forecasting, he says. By shooting lasers into the atmosphere and analyzing the size of rain droplets and how quickly droplets are growing, meteorologists could gain a better understanding of the way certain air masses behave. Through “customized” forecasts it could soon be possible to know whether it’s going to rain over a sports stadium during a major event, for instance.

“Having very detailed characterization of the atmosphere can feed this kind of forecast,” says Kasparian.

Lasers for biochemistry

This fall, scientists at the SLAC National Accelerator Lab are doing some of the first experiments on the world’s first X-ray laser, which was unveiled in September 2009. Since the wavelength of X-rays is similar to the distance between atoms, this laser can take snapshots of very small stuff, such as, for example, the bonds between atoms in proteins. Proteins are strings of molecules that fold into complex structures and perform lots of services, such as breaking down the food we eat and using it to build muscles. In an upcoming experiment, researchers plan to use the X-ray lasers to study how proteins change shape as one chemical bond is broken and another is formed.

The story of lasers and its many applications illustrates the value of basic research, says Fritz. When the laser was invented, “no one could have envisioned how much of an impact it would have on society.”

POWER WORDS (from the Yahoo! Kids Dictionary)

laser Any of several devices that emit highly amplified and coherent radiation of one or more discrete frequencies.

physics The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics, and electromagnetism, as well as in modern extensions including atomic and nuclear physics, cryogenics, solid-state physics, particle physics and plasma physics.

cloud chamber A gas-filled device. In it, particles smaller than atoms form chains of droplets on ions formed in the gas. These chains help show that the particles were present. It is also used to infer the presence of neutral particles and to study certain nuclear reactions.

wavelength The distance between one peak or crest of a wave of light, heat, or other energy and the next corresponding peak or crest.

fiber optics The science or technology of light transmission through very fine, flexible glass or plastic fibers. A bundle of optical fibers.

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.”

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: