Photo: Light technician on a Las Vegas pyramid

A technician handles one of 39 volatile 7,000-watt xenon lamps atop the Luxor pyramid on the Las Vegas Strip. Touted as the world's brightest light, Sky Beam is strong enough to read a book by—10 miles (16 kilometers) out in space.

Photograph by Joe McNally

Written by Joel Achenbach

Republished from the pages of National Geographic magazine

There has been light from the beginning. There will be light, feebly, at the end. In all its forms—visible and invisible—it saturates the universe. Light is more than a little bit inscrutable. Modern physics has sliced the stuff of nature into ever smaller and more exotic constituents, but light won't reduce. Light is light—pure, but not simple. No one is exactly sure how to describe it. A wave? A particle? Yes, the scientists say. Both. It is a measure of light's importance in our daily lives that we hardly pay any attention to it. Light is almost like air. It's a given. A human would no more linger over the concept of light than a fish would ponder the notion of water.

There are exceptions, certain moments of sudden appreciation when a particular manifestation of light, a transitory glory, appears—a rainbow, a sunset, a pulse of heat lightning in a dark sky, the shimmering surface of the sea at twilight, the dappled light in a forest, the little red dot from a professor's laser pointer. Stained glass in a church, backlit by a bright sky. The flicker of a candle, flooding a room with romance. The flashlight searching for the circuit breakers after a power outage.

Usually, though, we don't see light, we merely see with it. You can't appreciate the beauty of a rose if you ponder that the color red is just the brain's interpretation of a specific wavelength of light with crests that are roughly 700 nanometers apart. A theatrical lighting director told me that she's doing her job best when no one notices the lights at all. Her goal is to create an atmosphere, a mood—not to show off the fancy new dichroic filters that create colors of startling intensity.

As someone whose understanding of light pretty much began and ended with the flipping of switches, I worried that a story about light would be rather ethereal and esoteric. Surely there wouldn't be anything resembling breaking news on the light beat. Wrong!

Try an Internet search under the keyword "photonics." A photon is what you call light when it's behaving like a subatomic particle. Photons, it turns out, are a hot commodity. They are replacing electrons—we know them from grade school as the negatively charged particles that orbit the nuclei of atoms—as the favorite tool of modern industry for transmitting information.

Light is now used for everything from laser eye surgery to telephone technology. The potential military applications of light are straight out of science fiction, and within a decade light may be the preferred weapon for zapping hostile missiles out of the sky. Light could even become the preeminent power source for long-distance space travel. The spaceship would have an ultrathin sail to catch the "wind" of light beamed from an Earth-based laser. In theory such a craft could accelerate to a sizable fraction of the speed of light—without carrying fuel.

The more you look at the topic, the more you realize that ours lives are built around light, that our daily existence is continuously shaped—and made vivid—by that ambiguous stuff that dates from the beginning of time. From our technology to our spiritually, we are creatures of light.

One question won't seem to go away: What is light, exactly? I got a piece of the answer from the world's largest laser, the National Ignition Facility. NIF is under construction at the Lawrence Livermore National Laboratory about an hour east of San Francisco. The laser is actually 192 lasers in collusion—or perhaps one should say collision. The 192 individual beams of light, grouped in bundles of four, will travel the length of a hulking building 700 feet (213 meters) long and 400 feet (122 meters) wide. Entering a switchyard of mirrors, each bundle will ricochet and shoot through one of 48 portals of the target chamber. The chamber is the star attraction of the facility. It's 30 feet (9.1 meters) in diameter, weighing a million pounds. The portals give it a dimpled surface that brings to mind an enormous golf ball from outer space.

Inside the chamber the laser beams will crash into a gold-plated cylinder, slightly smaller than a bite-size Tootsie Roll, with a gas-filled pellet inside. The gases in the pellet, under the pressure of all this light, will compress to the point where they achieve nuclear fusion.

"The goal is to create a miniature star in the laboratory," said Ed Moses, the NIF project manager.

This is, to say the least, an ambitious project, and its 3.4-billion-dollar price tag has not eluded critics. They note that NIF may never actually produce a fusion reaction. Technologically, this is not a slam dunk. No one has ever successfully used light to drive together atomic nuclei. The big laser will let scientists study thermonuclear reactions without detonating a bomb. A long-term goal of NIF is to clear a technological path toward a source of cheap, inexhaustible, pollution-free electricity.

"NIF will produce more power in a one-nanosecond laser pulse than all the power generated in the rest of the world at that moment," said Vaughn Draggoo, the physicist who showed me the target chamber.

How is it, I asked Moses, that light is such a useful source of energy?

"Because you can compress a lot of light's energy in a very small point," he said. Children, he noted, discover this when they play with a magnifying glass on a sunny summer day.

Here we come to one facet of the miracle of light. It has no volume. And photons have no charge, so in the process of being concentrated into a very small space, they don't repulse each other as negatively charged electrons do. (NIF will fit 4 x 1024 photons into the target capsule.) "They don't bother one another" is the way Moses puts it.

How many angels of light can dance on the head of a pin? In theory, an infinite number.

As hard as it is to understand light, the ancients had it that much harder. Lacking scientific instruments, they could probe the nature of light only with their inventive minds. "Light is the activity of what is transparent," was one of Aristotle's rather opaque declarations. This transparency was an essential property of various substances; when activated by sun or fire, it produced light and color.

The fifth-century B.C. philosopher and poet Empedocles had the brilliant intuition that light is a streaming substance emitted by the sun and that we are not conscious of its movement because it moves too fast. But he also subscribed to the notion of the "fire within the eye," comparing the eyes to lanterns. Many Greeks, including Plato and Euclid, shared this belief that the eyes produce some kind of visual ray. It explained the curious fact that sometimes we look in the direction of an object but fail to notice it immediately. The ray, it was surmised, must strike the object directly before it can be seen. Aristotle was among those to point out that if this were true, we'd be able to see in the dark.

A thousand years ago the Arab scientist Alhazen argued that the pain we feel when we look at the sun is evidence that the light is entering the eye and not the other way around. Centuries later Leonardo da Vinci realized that the eye is akin to the camera obscura, pioneered by Alhazen, in which light passes through a pinhole into a darkened room and casts an inverted image of the exterior world onto a wall. Descartes later did a rather dramatic examination of the eyeball of an ox, scraping away the back of the eye and peering through it. He saw that the eye captures an inverted, upside-down image of the world. Why doesn't the world look upside down? Because our minds correct the image. Sight has both a physical and psychological element.

Light soon passed through the laboratory of Isaac Newton and never looked the same again. In the 1660s Newton demonstrated that white light is composed of all the colors of the spectrum. Using a prism, he broke sunlight into a rainbow, then later used a second prism to cohere the colors back into white light.

"Whatever light be," he told the Royal Society in 1675, "I would suppose, it consists of Successive Rays differing from one another in contingent circumstances, as bigness, forms, or Vigour, like as the Sands on the Shoar, the Waves of the Sea, the faces of men, and all other natural things."

Newton believed that light was particulate—"multitudes of unimaginable small and swift Corpuscles of various Sizes, springing from Shining bodies at great distances one after another." Newton was such a giant on the scientific landscape that his rivals had little luck pushing the theory that light is a wave. The wave theory did not begin to rebound until the titans of 19th-century science joined the battle to understand light and overwhelmingly came down on the side of waves. It was James Clerk Maxwell, a Scot, who in the 1860s made one of the most essential breakthroughs. He had been studying electricity and magnetism and realized that they propagated through space at—coincidence—the speed of light. Light, he concluded, is an "electromagnetic" wave.

The particle versus wave debate wound up with a kind of truce, governed by quantum mechanics: Light is produced by changes in the energy level of electrons. Light moves through space as a wave, but when it encounters matter it behaves like a particle. It simply doesn't fit into one of our neat little categories. "Light, indeed, is different from anything else we know," writes Sidney Perkowitz, a physicist at Emory University and the author of Empire of Light. The era of permanent uncertainty began in 1900, when Max Planck's experiments with heat radiation implied that light pounded against matter in discrete chunks—quanta, he called them—like bullets from a machine gun. This seemed contrary to Maxwell's equations, and Planck was reluctant to believe it.

Enter Albert Einstein. It's common knowledge that Einstein, in promulgating the special theory of relativity, destroyed the mechanical, deterministic Newtonian universe. He achieved this theoretical breakthrough by thinking about…yes, light. Einstein did "thought experiments," and in one he asked what would happen if you could ride a beam of light and look at an adjacent beam. Wouldn't the adjacent beam of light appear motionless? Maxwell's equations didn't seem to allow light to slow down or stop when moving through space. Einstein's answer—that light's speed is constant for all observers regardless of their own velocity—obliterated the classical conception of space and time.

The groundwork was laid for Einstein by a famous experiment in 1887 by American scientists Albert Michelson and Edward Morley. The Earth, according to the orthodoxy of the time, moved through a fixed ether that filled space. No one had ever detected this ether, but common sense required its existence. Michelson and Morley set out to detect it by measuring the speed of light when light beams traveled with, and perpendicular to, the motion of the Earth. They expected light to show the effects of the "current" of this ether as Earth hurtled along. It didn't. The speed of light was the same no matter its direction. Scientists, including Michelson and Morley, were aghast and hoped that the results were simply wrong. Einstein accepted them. There is no ether, he said. There's no absolute location in space. There isn't even any absolute time.

I will confess that relativity makes my head spin. A beam of light from the headlamp of a speeding locomotive ought to move faster—says common sense—than the beam from a stationary flashlight. It doesn't. And there's nothing anyone can do about it.

Einstein's relativity presents all manner of head-scratching implications. It reveals that as objects approach the speed of light, time slows down. At the speed of light itself, time stops.

This fact can help us think about the journeys made by starlight and galaxylight and quasarlight across cosmic distances. We use the term light-year to express a unit of distance (about six trillion miles [9.7 trillion kilometers]). But if you were the light itself—if you could be the photon—you'd experience no time. That long journey would be instantaneous.

What we call light is really the same thing—in a different set of wavelengths—as the radiation that we call radio waves or gamma rays or x-rays. But in practice scientists often use the term "light" to mean the portion of the electromagnetic spectrum in the vicinity of visible light. Visible light is unlike any other fundamental element of the universe: It directly, regularly, and dramatically interacts with our senses.

Our eyes each have about 125 million rods and cones—specialized cells so sensitive that some can detect a mere handful of photons. "About one-fifth of your brain does nothing but try to deal with the visual world around you," says Sidney Perkowitz. The position of the eyes, semiprotected in the case of the skull close to the brain, is testament to the importance of visual data.

Light offers high-resolution information across great distances (you can't hear or smell the moons of Jupiter or the Crab Nebula). So much information is carried by visible light that almost everything from a fly to an octopus has a way to capture it—an eye, eyes, or something similar.

It's worth noting that our eyes are designed to detect the kind of light that is radiated in abundance by the particular star—the sun—that gives life to our planet. Visible light is powerful stuff, moving at relatively short wavelengths, which makes it biologically convenient. To see long, stretched-out radio waves, we'd have to have huge eyes, like satellite dishes. Not worth the trouble! Nor would it make sense for our eyes to detect light in the near infrared (though some deep-sea shrimp near hot vents do see this way). We'd be constantly distracted, because any heat-emitting object glows in those wavelengths. "If we were seeing infrared," physicist Charles Townes told me one day, "all of this room would be glowing. The eye itself is infrared—it's warm. We don't want to detect all of that stuff."

There is also darkness in the daytime—shadows. There are many kinds of shadows, more than you probably realize—certainly more than I realized until I consulted the shadow expert. I found him at the end of a long and winding drive through Topanga Canyon, just up the coast from Santa Monica, California.

David Lynch is an astronomer. He's also the co-author of a book called Color and Light in Nature, in which I discovered something about shadows that I'd never thought of before. Lynch points out that a shadow is filled with light reflected from the sky—otherwise it would be completely black. Black is the way shadows on the moon looked to the Apollo astronauts, because the moon has no atmosphere and thus no sky to bounce light into the unlit crannies of the lunar surface. Only the faint glow of earthshine filled the shadowy recesses.

Lynch is a man who, when he looks at a rainbow, sees details that elude most people. He knows, for example, that all rainbows come in pairs, and he always looks for the second rainbow—a faint, parallel rainbow, with the colors in reverse order. The intervening region is darker. That area has a name, wouldn't you know: Alexander's dark band.

We sat on Lynch's deck and drank orange juice squeezed from fruit freshly yanked from trees in his backyard. The view was spectacular, the canyon opening like a great basin, a mountain ridge obscuring the Pacific and running for half a dozen miles (9.7 kilometers) to Santa Monica.

"The reason those mountains over there look a little blue," Lynch continued, "is because there's sky between here and those mountains. It's called airlight."

The sky is blue because the molecules in the air scatter blue light more readily than they scatter red, orange, yellow, and green. We see distant mountains through a mass of blue sky—hence the Blue Ridge and (thanks to poetic license) " purple mountain majesties."

Las Vegas is a multitude of colors—a place that takes light seriously and can't seem to emit enough of it. The Strip is more dazzling by the year. The casinos no longer advertise themselves with mere neon-lit roadside marquees but rather have turned their entire structures into eyeball-popping orgies of illumination. "Now the whole building is a sign," says longtime sign dealer Ken Moultray.

The entirety of the MGM Grand glows emerald. Fiber optics pulse light to the tower of the Stratosphere. The vertical neon stripes adorning the Rio are visible from distant mountains.

The Luxor Resort and Casino is a pyramid and, perversely, remains almost entirely dark at night, a massive black presence dramatically highlighted by the golden glass of the Mandalay Bay Resort next door. Instead of dressing itself in countless little bulbs or immersing itself in floodlights, the Luxor aims its Sky Beam—said to be the brightest light on the planet—straight into space.

I followed John Lichtsteiner, technical manager of rides and attractions at the Luxor, up three metal ladders onto the catwalk at the pyramid's peak to see the 39 xenon lamps, 7,000 watts each that create the Sky Beam. A sign warns that the lights, each about the size of a washing machine, are "extremely volatile" and can explode if jarred or bumped.

Lichtsteiner explained that before a computer turns on the Sky Beam each night, strobe lights flash for 30 seconds. "We don't want to surprise any pilots," he said.

He pressed a button on a digital console to illuminate one of the lamps. Its light was so bright that when I put my notebook into the beam, I had to look away quickly. My pen strokes vanished, and all I could see was a rectangle of blinding white light.

We climbed above the lamps to the very tip of the pyramid. Vegas sprawled for miles in every direction. In the daytime the place is rather washed out, the colors flattened. The sparkle, the glitter, the almost hallucinatory colors of Vegas at night are obliterated in the white light of the desert sun.

"Now there's a lamp," I said, nodding toward that natural beacon in the sky.

Later I went to see a show at the Bellagio. Walking into the theater, I was mesmerized by the red curtain. It looked…really red. It also looked heavy, velvety, like a curtain in a baroque theater.

What I didn't know until later was how much my eyes were being fooled by clever lighting. The lighting designer, Luc Lafortune, uses two different sets of lights—a bright red aimed straight at the curtain and a softer red shining in from the side—to create a sense of depth and heaviness in the fabric. The curtain is actually made of a lightweight fabric that enables the stage director to whisk the whole thing aside in a flash.

Jeanette Farmer, lighting director for the show, showed me around backstage. She keeps track of 2,000 computer-controlled lights and 1,695 dimmers. She can do much of her work at a console—no more climbing ladders to change filters. Little motors in the lamps do all that. She no longer uses just single-gel filters over white lights to create yellow or red or blue. State-of-the-art stage lighting uses dichroic filters, pieces of glass with mineral coatings. These provide a purer, more intense light—the kind that made the curtain look like shimmering red velvet. The old gel filters let in a lot of "noise," while the dichroics select a very narrow slice of the spectrum.

Farmer gives credit where it's due—to Isaac Newton, for realizing that white light is composed of different colors. "In so many ways he laid the groundwork for all of us," she said. "He knew what the deal was."

"This is a very pure, straight beam." Charles Townes was showing me a helium-neon laser in his laboratory at the University of California at Berkeley. He has hundreds of lasers, including some that arrive in the mail as gifts, newfangled lasers smaller than matchboxes, green lasers soothing to the eye—all the various descendants of a gadget he invented in the 1950s. He and his brothers-in-law Arthur Schawlow developed the techniques that led to what was called "light amplification by stimulated emission of radiation." (Obviously the acronym would be easier for everyone to say.)

Light normally spreads out rapidly in all directions; a laser coheres the light in a narrow beam. The key to producing this beam is the basic atomic principle that says that photons—and now we're back to describing light as particles—can be absorbed or emitted by atoms.

When an electron changes from high-energy, or excited, state to a low-energy state, its atom will emit a photon. A laser exploits this process. It starts with a crystal or other medium whose atoms are prone to excitement. These atoms are slammed with light, causing their electrons to do a little dance. When they calm down, they release excess energy as photons. These photons, in turn, incite more electron dancing, which creates more photons—a chain reaction. Its physics, not magic, that causes more light to come out than went in.

The arrival of the laser was heralded by certain newspapers as the start of a new era of military death rays, the killer cousins of the Martian Heart-Ray in The War of the Worlds. But a half century ago Townes and Schawlow weren't actually sure what could be done with their invention—or with its prototype, the maser (which used microwaves instead of visible light). They just knew they'd figured out a nifty way to make light shine strong and straight.

"People used to kid me, 'Lasers are a solution looking for a problem,' " Townes said.

He thinks about that every time he goes to the checkout line at the grocery store, where light is used to scan the price of every product. A laser reads the CD in his CD player. Surveyors used a laser to gauge the precise property lines on Townes's New Hampshire farm. When he makes a long-distance phone call, his words are transmitted by laser light along a fiber-optic cable.

It's hard to overstate the usefulness of a tool that makes light shine straight. Laser beams fired from the Earth have bounced off mirrors left on the moon by Apollo astronauts, allowing scientists to measure the moon's distance—across more than 225,000 miles (362,100 kilometers)—to within half an inch (1.3 centimeters). Laser surgery corrects faulty vision in an increasingly routine procedure.

"When a friend comes to me and says laser surgery saved his eyesight, that's a very emotional thing," Townes says.

"Light is a universal probe," says Michael Hart, a physicist. He was showing me around the National Synchrotron Light Source, in Upton, New York. Built in the early 1980s, it's a sprawling device of comical complexity and is, Hart says, the "most used light source" in the world.

The synchrotron uses magnets to guide electrons around a ring that's about the size of a basketball court. Every time an electron turns a corner, so to speak, it emits a photon. The photons fly away from the ring in what are called beamlines. There are 92 beamlines in operation on two synchrotron rings, and each one has been customized with a dazzling array of mad-scientist gadgetry—dials, gauges, valves, pumps, vacuum chambers, optical sensors, wires, pipes, and lots of slapped-on aluminum foil. The different beamlines are used by researchers from universities, government labs, and places like IBM, Bell Labs, and Exxon.

What do they do with the light? Mostly they look at things—as you'd expect. They look at impurities in materials. They examine the porosity of rocks extracted from the Earth by oil drillers, eight of the beamlines are being used to study the three-dimensional structure of proteins in an effort to decipher some of the secrets of the human body.

For a while one of the beamlines was used for medical diagnostic procedure called coronary angiography. There was one hitch in doing the examinations: Who would want to sit in front of a giant ray gun that looks as though it could burn a hole through the Earth? The researchers constructed an examination room with a blank wall, with only the tiny numb of the beamline apparatus peeking through.

The photons here range from infrared radiations to x-rays—well beyond the range of visible light. Hart marvels that for so much human history we perceived the natural world only with visible light, that slice of the electromagnetic spectrum from red to violet. Making use of light beyond the visible realm has allowed scientists to create a new array of images of the reality around us. "We can see a single layer of atoms on a surface," Hart said.

Like everyone else I talked with who deals with light, Hart seemed almost in awe of the power of light. Technology constantly advances, allowing engineers to create ever brighter beams. The general rule, said Hart, is that brightness has increased a hundredfold every five years.

The telecommunications industry loves light. When you visit Lucent Technologies' Bell Labs in Holmdel, New Jersey, you're greeted with a sign saying "Welcome to Photon Valley." There has arisen something almost like a high-tech cult of light in this industry, built around the belief that human beings will increasingly exploit the almost infinite amount of bandwidth found in a light beam.

Kathy Szelag, a vice president with Lucent's Optical Networking Group, told me, "People like my parents think we're in the Star Warspart of optical networking. We're really in the crude oil part of optical networking. We're just at the beginning." Her colleague Bob Windeler, an optical-fiber researcher, added, "The amount of information you can put on a fiber more than doubles every year." In theory a single fiber could someday transmit every phone call on Earth simultaneously.

The optimism has been tempered of late by business woes among telecommunication companies, but the technology remains impressive. Take, for example, wavelength division multiplexing. Lasers are used to beam different wavelengths of infrared light down a single fiber. Each wavelength is its own data channel—its own pipe. Right now, a fiber can carry dozens of these channels, but that could become thousands or even millions. "It's as close to a miracle as there is," says Dave Bishop, Lucent's vice president of optical research—sounding very much like all the other light-crazy people I'd talked to.

George Gilder, a conservative political theorist who transformed himself into an influential technology guru, has declared in recent years that light will be the medium of a communications revolution. "You can envision a point where everyone in the world could have his own wavelength," says Gilder. "You'd have one wavelength that connected you to the person you wanted to address in Vienna or Tokyo or Tierra del Fuego, and this wavelength could easily accommodate three dimensional images. You could have conversations in which you forget within literally seconds that the person is not present. You see a face, the images saturates your own optical capabilities."

He adds, "I believe that light was made by God for communications." What orthodoxy-busting cosmic information will starlight deliver to our telescopes? Will the rotating disco ball ever make a dance-floor comeback? Above all, you have to wonder: Will we ever fully understand light?

We have spent thousands of years chasing sunbeams, and even if we never quite catch them, we still discover many a marvel in the pursuit. Modern physics, with its paradoxes and uncertainties, emerged from the study of the interaction of matter and light. Modern cosmology, including the stunning revelation that the universe is expanding, came from the scrutiny of faint galactic light. Modern computer engineering may eventually turn to light, crafting devices that, instead of silicon chips, have light beams at their core.

There have been recent headlines about scientists finding ways to make light go faster than the speed of light. This is what science fiction writers and certain overly imaginative folks have dreamed of for decades. If you could make a spaceship that wasn't bound by Einstein's speed limit, you could zip around the universe far more easily.

Lijun Wang, a research scientist at NEC Research Institute in Princeton, New Jersey, managed to create a pulse of light that went faster than the supposed speed limit. "We created an artificial medium of cesium gas in which the speed of a pulse of light exceeds the speed of light in a vacuum," he said. "But this is not at odds with Einstein." Even though light can be manipulated to go faster than light, matter can't. Information can't. There's no possibility of time travel.

I asked Wang why light goes 186,282 miles a second (299,792 kilometers per second) and not some other speed.

"That's just the way nature is," he said.

There are scientists who don't like "why" questions like this. The speed of light just is what it is—that's their belief. Whether light would move at a different velocity in a different universe is something that is currently outside the purview of experimental science. It's even a bit out-there for the theorists.

What's certain is that light is going to remain extremely useful—for industry, science, art, and our daily, mundane comings and goings. Light permeates our reality at every scale of existence. It's an amazing tool, a carrier of beauty, a giver of life.

I can't help but say that it has a very bright future.

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