INTRODUCTION

“We eat light, drink it in through our skins.”

-       James Turrell, Light and Space artist

            Galileo was bewildered. Toward the end of his life, a life that witnessed wondrous light none had seen before, the great scientist confessed one failure. Decades had passed since a friend had given him several of the stones Italians called "solar sponges.” Soaking up sunlight, emitting a soft green glow, the stones convinced Galileo that Aristotle had been wrong about light. It was not some warm, ethereal element. Light could be as cold as the moon and as corporeal as water. But what was it? 

            Over the years, Galileo had learned to reflect light, to bend it, to amaze observers with telescopes that spotted ships two hours sail from Venice. Turning his telescope toward the night sky, he had been the first to see the moons of Jupiter and the rings of Saturn. Later he proposed the first experiment to clock the speed of light, bouncing lantern beams across the hilltops of Tuscany. Galileo never conducted his speed test. Other experiments, other trials demanded his attention, yet he continued to wonder about light. Shortly before his death, blind and broken, he admitted how he longed for an answer. Though under house arrest for heresy, he said he would gladly suffer a harsher imprisonment. He would live in a cell with nothing but bread and water if, upon emerging, he could know the truth about light.

            The truth is that, despite three millennia of investigation by humanity's most brilliant detectives, light refuses to surrender all its secrets. As familiar as our own faces, light is the first thing we see at birth, the last before dying. Some, having seen a warm glow as they flirted with death, swear that light will welcome us to another life. "Painting is light," the Italian master Caravaggio noted, and each day light paints a mural that sweeps around the globe, propelling us into the morning. Ever since the Big Bang, light has been stealing the show. And for countless scientists, philosophers, poets, painters, mystics, and anyone who ever stood in awe of a sunrise, light is the show. 

            "If there is magic on this planet," the naturalist Loren Eisley wrote, "it is contained in water.” Light, however, is the magician of the universe. Light makes darkness vanish and worlds reappear. Light opens each day with a blaring overture, then throws its wands to earth and casts diamonds on lakes and oceans. Each night, light’s tricks make the stars seem alive. Seen through telescopes Galileo could never have imagined, light dances across the rings of Saturn, shapes gas clouds into crabs and horse heads, spirals from great galaxies and bursts from newborn stars. As reliable and relentless as time, light will begin tomorrow with another hurrah, then close the show with house lights, low and glimmering. Drawn to it as surely as any moth, we cannot live without it. "When the great night comes, everything takes on a note of deep dejection," psychologist Carl Jung wrote, "and every soul is seized by an inexpressible longing for light."

            But what is light? What meaning have our brilliant detectives found in it? Is it God? Truth? Mere energy? Since the dawn of curiosity, these questions have been at the core of human existence. The struggle for answers has given light a history of its own. In human consciousness, light first appeared in stories of creation, stories spun in the glow of firelight or torch. From Genesis' immortal "Let there be light" to the Icelandic Edda that had God throwing embers into the darkness, light is the primal ingredient of every creation story. Following its mythical creation, Life matured into a mystery that intrigued philosophers from Greece to China. Was light atoms or shimmering eidola? Were we all, as the apostle Paul wrote, “children of light?” Just when each sage had light pinned down, the mystery rose again, posing further questions, fresher metaphors.

            Light was Jesus ("I am the light of the world.") No, it was Allah – “the Light of the heavens and the earth.” No, it was the Buddha, aka Buddha of Boundless Light, Buddha of Unimpeded Light, Buddha of Unopposed Light. .  .  Light was inspiration -- inner light. Light was love (the light in her eyes). Light was sex whose divine coupling, according to Tantric Buddhism, fills sexual organs with radiance. Light was hope, thought, salvation (seeing the light). Dante filled his Paradiso with "the heaven of pure light.” Shakespeare toyed with it:  "Light seeking light doth light of light beguile." The blind poet Milton was obsessed with light:  "Hail, holy light, offspring of Heav'n first-born.”Caravaggio and Rembrandt captured light as a sword cutting through the blackness. Vermeer sent it streaming through windows. Beethoven heard it as French horns. Haydn preferred an orchestra's full blare. Meanwhile, ordinary people spoke of the light of freedom, the light of day, the light of reason, the light of their lives. .  . 

            Yet from the first theories about its origin, light sparked bitter disputes. The earliest philosophers quarreled about light -- was it emitted by the eye or by every object? Holy men debated whether light was God incarnate or merely His messenger. And then there was light's handmaiden -- color. Was it innate in each object or merely perceived by the eye? While some debated, others celebrated in festivals of light ranging from Hanukkah and winter solstice to the Hindu Diwali and the Zoroastrian Nowruz. Meanwhile, without the slightest concern for science or religion, sunrise and sunset circled the planet, rarely disappointing those who paused to admire. 

            Traced from creation to the quantum age, light's trajectory suggests that the miracle has lost some luster. Once we spoke of "the light fantastic," but now we find light cheap and easy, available not just in every home and office but in every palm and pocket. Artificial light once came from precious few sources -- candles, lamps, and torches. But with lighting now a $100 billion industry, beams shoot out of bike helmets, keychains, shower heads, e-readers, smart phones, tablets, and dozens more devices. Light has become our most versatile tool, healing detached retinas, reading bar codes, playing DVDs. Light from liquid crystal displays brings us the Worldwide Web. Light  through fiber optic cables carries the messages that girdle the planet. Thus we have made light as common as breath, as precise as a laptop.

            But there was a time when light waged a heroic battle with darkness. It was a time when night skies were not bleached by urban glare, when candles were not romantic novelties, when light was the source of all warmth and safety. For the vast majority of human history, each sunrise was a celebration; each waxing moon stirred hope of nights less terrifying. And to anyone caught unprepared -- in dark woods, on echoing streets, even at home when lamps flickered and failed -- light was, simply, life. Unlocking its secrets required an uncommon set of keys. Curiosity. Persistence. Mirrors, prisms and lenses. Through the centuries, as civilizations took turns asking and answering, the keys passed from Greece to China to Baghdad, from Medieval France to Italy and back. When the keys came to Isaac Newton, his answers spread to the world, unlocking secrets we are still exploring. 

            Our evolving concepts of light chart the development of human thought, from spiritual to secular, from superstitious to scientific. So long as light was God, divinity incarnate arose each morning. Millennia passed with only a handful of curious men considering light less than holy. Then during the 1600s, the Scientific Revolution gave curiosity the upper hand. Once Kepler saw light as subject to physical laws, once Galileo showed how to gather it, once Newton broke it into color, light was no longer merely God's essence. The rhapsody was finished. Enchantment had met its midnight.

            Some felt this acutely. William Blake raged against the dying of Light, the Mystical. Other Romantics celebrated light in symphonies, paintings, and poems, yet scientists continued to probe. Bouncing beams through dark chambers, measuring first by "candle power," later with lumens and watts and joules, they dimmed divine radiance and opened a new debate about light. Particle or wave? But even after Einstein muddled the question, even as today’s scientists craft miracles in optics labs, light still performs. Tens of thousands gather for the summer solstice at Stonehenge. Festivals of light illuminate Berlin, Chicago, Hong Kong, Ghent, Amsterdam, Lyons. .  .  The anatomy of light -- particle and wave – is a staple of science classrooms, yet no equation has dimmed sun or moon, and no prism rivals the rainbow.

            This book attempts to reconcile the battles between science and humanities, between religion and doubt, between mathematics and metaphor. Like sunlight, which as Thoreau noted "is reflected from the windows of the alms-house as brightly as from the rich man’s abode,” this radiant history illuminates all those in love with light. It focuses equally on the genius of Newton and Dante, the eloquence of equations and scripture, the faith of the Qur’an, the Upanishads, and the Bible. Of light's devout disciples, I ask not “who” but “why.” Why were these believers enamored with light and what did their faith add to human consciousness? Of light's students I ask  not “what” but “how.” How did scientists determine the nature and tame the power of light? And of those who made light their muse, I ask only that they be read and seen as if for the first time.

            Yet the prime mover of this history is neither experiment nor eloquence but awe. The story begins at the approach of dawn. The long night is ending. Daybreak is near. A glimmer touches the eastern horizon. Hail Holy Light, particle, wave, and wonder.