Ah, but a man's reach should exceed his grasp,

Or what's a heaven for?

— Robert Browning, Andrea del Sarto, 1855

o arrive at the astronomical observatory of the twenty-first century, you must journey through America's Old South—Baton Rouge, Louisiana, to be precise. Coming off a plane, visitors are first greeted by the pungent smells wafting from a nearby petroleum refinery, a major state industry. Continuing along Interstate 10, which partly follows the winding path of the Mississippi River on its way to New Orleans and the Gulf of Mexico, billboards hawk snuff, Killer Joe's seafood, and Louisiana mud painting. East of the state capital in plantation country, the land turns flat. The roads are arrow-straight, like the unswerving grids on a sheet of graph paper. The regularity is relieved only by sinuous chains of trees, green rivers of foliage laden with Spanish moss, that occasionally meander through the farmland.

Louisiana has nearly 14 million acres of forests. Longbed trucks piled high with cut lumber are a familiar sight on the highways. Many of these trucks originate about 25 miles east of Baton Rouge, in the

parish of Livingston, where a vast pine reserve resides. A few miles past a deserted feed store, north along Highway 63, a modest sign announces the presence of the Laser Interferometer Gravitational-Wave Observatory, operated by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT). Those in the know simply call it LIGO (pronounced lie-go). Nothing can be seen from the highway though. You must first turn onto an asphalt road and slowly drive past young-growth forests, some sections newly cut. Longhorn cattle lazily graze along the roadway. After a mile's ride the observatory at last comes into view. It resembles a multistory warehouse, silver-gray in color, with blue and white trim. Observatories are not a common sight in Louisiana, but establishing new technologies is a state tradition. In 1811 Edward Livingston, who would later become a U.S. senator and the parish's namesake, helped Robert Fulton establish the first commercial steamboat operation on the Mississippi River.

The vast room where LIGO's key instruments are mounted soars upward for more than 30 feet. The cross-shaped hall resembles an aircraft hangar or perhaps the transept and nave of a modern-day basilica. On two ends of the cross, at right angles to each other, are large round ports. Attached to each of these two openings is a long tube, which extends out into the countryside for 2½ miles. Each four feet wide, the two tubes resemble oil pipelines and are roomy enough for a crablike walk should the need arise. To accommodate the pipes, the pine forest has been leveled. A huge, raw L has been carved into the woody terrain to make room for these lengthy metal arms. One long swathe stretches to the southeast, the other to the southwest. Alongside each arm a roadway has been constructed about 8 feet above the Louisiana floodplain. The dirt for the roadway was dug up right on site, creating two water-filled canals, each parallel to a tube. An alligator, fed donuts by the construction crew, even adopted one of the borrow pits as his home. The pipes cannot be seen directly. Concrete covers, 6 inches thick, protect them from the wind and rain, as well as any stray bullet that might pass by during hunting season. A hit could be devastating, for the pipes are as empty of air as the vacuum of space.

“Here's our 2½-mile-long hole in the atmosphere,” says Mark Coles, sweeping his hand outward with pride. A big and friendly man,

Coles recently moved from Caltech to direct the Livingston observatory. He took to the Cajun lifestyle with ease, even coming up with a bit of French for the observatory's slogan, displayed on T-shirts sold in the reception area: Laissez les bonnes ondes rouler! (Let the good waves roll.) The signals they seek are waves of gravitational radiation—gravity waves in common parlance.*

Electromagnetic waves, be they visible light, radio, or infrared waves, are released by individual atoms and electrons and generally reveal a celestial object's physical condition—how hot it is, how old it is, or what it is made of. Gravity waves will convey different information. They will tell us about the overall motions of massive celestial objects. They are literally quakes in space-time that will emanate from the most violent events the universe has to offer—a once blazing sun burning out and going supernova, the dizzying spins of neutron stars, the cagey dance of two black holes pirouetting around one another, approaching closer and closer until they merge. Gravity waves will tell us how large amounts of matter move, twirl, and collide throughout the universe. Eventually, this new method of examining the cosmos may even record the remnant rumble of the first nanosecond of creation, what remains of the awesome space-time jolt of the Big Bang itself. At the dedication of the Livingston observatory, Rita Colwell, director of the National Science Foundation, noted that they were “breaking a bottle of champagne on the bow of a figurative galleon that will take us back farther in time than we have ever been.” So compelling is this information that scientists have been pushing the envelope of technology to detect these subtle tremors.

Inside LIGO's main hall the ambiance is almost reverential, akin to the response one might feel inside a darkened telescope dome, the mirror aimed for the star-dappled heavens. But this astronomical venture

is vastly different. Here there are no windows to spy on the universe. A gravitational wave observatory is firmly planted on cosmos firma. It is glued to space-time, awaiting its faint rumbles, vibrations first predicted by Einstein more than 80 years ago.

“The worship of Einstein, it's the only reason we're here, if you want to know the truth,” says Rainer Weiss of MIT. “There was this incredible genius in our midst, in our own lifetime. The average person knows that he was an important guy. If you go to Congress and tell them you're going to try to show that Heisenberg 's uncertainty principle is not quite right, you run into blank stares. But if you say you're measuring something that's proving or disproving Einstein's theory, then all sorts of doors open. There's a mystique.”

Albert Einstein indeed stands like a giant amid the pantheon of scientific figures of the twentieth century. His ideas unleashed a revolution whose changes are still being felt into the new century. What he did was to take our commonplace notions of space and time and completely alter their definitions. The evolution of physics in many ways traces our changing conceptions of space and time. And each adjustment in our worldview—our cosmic frame of reference—brought with it an accompanying change in the physics of handling that new framework. It is the start of every physicist 's musings: an attempt to track an object's motion in space and time. These intangibles are given names, such as “miles” and “seconds,” and ever since Sir Isaac Newton analyzed that falling apple, scientists believed they finally understood the true meanings of those words. But they didn't. Einstein stepped in to shatter that confidence. With his general theory of relativity, he showed us that matter, space, and time are not separate entities but rather eternally linked, producing the force known as gravity. That is why Einstein initiated a revolution. He taught us that space and time are not mere definitions useful for measurements. Instead, they are joined together as an object known as space-time, whose geometric shape is determined by the matter around it. According to general relativity, massive bodies, such as stars, dimple the space-time around them (much the way a bowling ball sitting on a trampoline would create a depression). Planets and comets are then attracted to the star simply because they are following the curved space-time highway carved out by the star.

Nature as we like to think of it, which flows so nicely according to Newton's laws, is actually a special case, where energies and velocities are low. Many who fall under the spell of Einstein feel like Alice in Wonderland: the universe begins to look “curiouser and curiouser.” With his more universal law, Einstein introduced us to a world that is counterintuitive, a place where lengths can contract, time can speed up or slow down, and matter can disappear in a wink down a space-time well. He made space and time palpable.

To the practiced eye, Einstein's equations stand as the quintessence of mathematical beauty. When it was first introduced in 1915, general relativity was hailed as a momentous conceptual achievement. But for a long time it was thought to have little practical importance. While the theory was embraced —and Einstein was recognized for his genius—the evidence for it was largely aesthetic because the tests possible in the early twentieth century were few: a tiny shift observed in the planet Mercury's orbit and evidence of starlight bending around the Sun due to the indentation of space around the orb's enormous mass. There was a perfectly good reason that experimental tests lagged behind theorists' contemplation of relativity's effects. In either describing the motion of a ball falling to the ground or sending a spacecraft to the Moon, Newton 's laws of gravity are still adequate. General relativity is far more subtle, best observed when gravitational fields attain monstrous strengths, making its effects more prominent. But in Einstein's day the universe was deemed more tame than that. After the first few tests were completed—elevating Einstein to the pinnacle of celebrity —general relativity became largely a theoretical curiosity, admired by all but exiled into the hinterland of physics. “General relativists, ” says Clifford Will, a relativist himself, “had the reputation of residing in intellectual ivory towers, confining themselves to abstruse calculations of formidable complexity.”

But like some theoretical Rip Van Winkle, general relativity gradually revived after decades of neglect, especially as astronomers came to discover a host of intriguing celestial objects, such as pulsars, quasars, and black holes, that could be understood only through the physics of general relativity. Neutron stars, gravitational lenses, inflationary universes—all must be studied with Einstein's vision in mind. At the same

time, advancing technology offered physicists new and ingenious means to test relativity's quirky and subtle effects with unprecedented accuracy—and not just in the laboratory. Using planetary probes, radio telescopes, and spaceborne clocks as their tools, investigators have checked Einstein's hypotheses with uncanny degrees of precision. The entire solar system now serves as general relativity's laboratory. In the words of general relativity experts Charles Misner, Kip Thorne, and John Wheeler, from their book Gravitation, a veritable bible for workers in the field, “General relativity is no longer a theorist's Paradise and an experimentalist's Hell.” For nearly a century the theory has held up to every experimental test, which is not only a triumph for Einstein—a celebration of his accomplishment—but also a further example of nature's ability to inspire awe and fascination in the way its rules follow such a precise mathematical blueprint. Furthermore, there are practical reasons why the theory is no longer relegated to the ivory tower. General relativity now has a real impact on our everyday lives. To properly operate the satellites of the Global Positioning System, used regularly by hikers, mariners, and soldiers to keep track of their locations, requires corrections of Einsteinian precision, revisions unaccounted for by Newton's cruder take on gravity. And astronomers who electronically link radio telescopes on several continents—creating a telescope as big as the world for their observations —also demand such accuracy.

And yet the story of general relativity remains incomplete. A secret still resides within general relativity, a major prediction that awaits direct confirmation: gravity waves. To understand this phenomenon, imagine one of the most violent events the universe has to offer—a supernova, the spectacular explosion of a star. More than 160,000 years ago, at a time when woolly mammoths were walking the Asian plains, a brilliant blue-white star known as Sanduleak -69° 202 exploded in the Large Magellanic Cloud, a prominent celestial landmark in the southern sky. Not until the winter of 1987 did a wave of particles and electromagnetic radiation shot from that dying star reach the shores of Earth. And when it arrived, an arsenal of observatories around the world focused their instruments on the flickers of light and energy that represented the star's long-ago death throes. It was the first time as-

tronomers were able to observe a supernova in our local galactic neighborhood since the invention of the telescope.

But Einstein's theory suggests that, when Sanduleak -69° 202 blew, it also sent out waves of gravitational energy, a spacequake surging through the cosmos at the speed of light. A fraction of a second before the detonation, the core of the star had been suddenly compressed into a compact ball some 10 miles wide, an incredibly dense mass in which a thimbleful of matter weighs up to 500 million tons (roughly the combined weight of all of humanity). This is the birth of what astronomers call a neutron star. Jolted by such a colossal stellar collapse, space itself was likely shaken—and shaken hard. The resulting ripples would have rushed from the dying star as if a giant cosmic pebble had been dropped into a space-time pond. These gravitational waves, though growing ever weaker as they spread from the stellar explosion, would have squeezed and stretched the very fabric of space-time itself. Upon reaching Earth they would have passed right through, compressing and expanding, ever so minutely, the planet and all the mountains, buildings, and people in their wake.

As shown by the example of the Sanduleak star, gravity waves are generated whenever space is fiercely disturbed. These waves are not really traveling through space, say in the manner a light wave propagates; rather, they are an agitation of space itself, an effect that can serve as a powerful probe. Light beams are continually absorbed by cosmic debris—stars, gas clouds, and microscopic dust particles—as they roam the universe. Gravity waves, on the other hand, travel right through such obstacles freely, since they interact with matter so weakly. Thus, the gravity wave sky is expected to be vastly different than the one currently viewed by astronomers. More than offering an additional window on space, gravity waves will provide a radically new perception. In addition, they will at last offer proof of Einstein's momentous mental achievement —their existence will demonstrate, in a firm and vivid way, that space-time is indeed a physical entity in its own right.

A few pioneering investigators in both the United States and Europe claimed to have detected the Sanduleak star's faint space-time rumble; other scientists are certain these claims are wrong. But the next time a gravitational wave rolls by, researchers are determined not

to be caught off guard. Hence, the construction of LIGO. LIGO itself consists of two observatories, one in Louisiana and a near-twin in Washington state, which operate in concert with each another. But they are not alone. Similar instruments of varying size are being built in Italy, Germany, Australia, and Japan as well. These groups around the world are turning on the most sophisticated gravitational wave detectors to date, in hope of snaring their elusive cosmic prize. Almost no one doubts that gravitational waves exist, for there is already powerful evidence that such waves are real. Two tiny neutron stars—supernova remnants—in our galaxy have been observed rapidly orbiting each other. They are drawing closer and closer together. The rate of their orbital decay—about 1 yard per year—is just the change expected if this binary pair is losing energy in the form of gravitational waves. Direct reception of a wave, though, would offer the ultimate proof and provide astronomers with the most radical new tool in four centuries with which to explore the heavens.

In the early 1600s Galileo Galilei, then a savvy professor of mathematics at the University of Padua in Italy, pointed a newfangled instrument called a telescope at the nighttime sky and revealed a universe with more richness and complexity than previous observers ever dared to contemplate. The ancients had said that the heavens were perfect and unvarying, but Galileo discovered spots on the Sun and jagged mountains and craters on the Moon. With bigger telescopes came bigger revelations. We came to see that the Milky Way was not alone but one of many other “island universes” inhabiting space. More than that, all of these galaxies were rushing outward, caught up in the expansion of space-time. And as astronomers were able to extend their eyesight beyond the visible light spectrum and detect additional electromagnetic “colors, ” such as radio waves, infrared waves, and x rays, the heavens underwent a complete renovation. Long pictured as a rather tranquil abode, filled with well-behaved stars and elegant spiraling galaxies, the cosmos was transformed into a realm of extraordinary vigor and violence. Arrays of radio telescopes, aimed toward the edge of the visible universe, observed young luminous galaxies called quasars spewing the energy of a trillion suns out of a space no larger than our solar system. Focusing closer in, within our own stellar neigh-

borhood, these same radio telescopes watched how neutron stars—city-sized balls of pure nuclear matter, the collapsed remnants of massive stars—spin dozens of times each second. Meanwhile, x-ray telescopes discovered huge amounts of x-ray-emitting gas, unobservable with optical telescopes, hovering around large clusters of galaxies. The invisible became visible.

The twenty-first century will certainly see the sky remade once again. It will happen as soon as astronomers detect gravity waves. These ripples in space-time will be seen neither with the eye nor as an image on an electronic display, not in the same way that visible light waves, radio waves, or x rays are distinguished. Each gravity wave that passes by Earth will, in a way, be felt—perceived, perhaps, as a delicate vibration, a vibrant boom, or even a low-key cosmic rumble. And when that happens, astronomy will never be the same again. It's as if in studying the sky we've been watching a silent movie—pictures only. Since their frequency happens to fall into the audio range, gravity waves will at last be adding sound to our cosmic senses, turning the silent universe into a “talkie,” one in which we might “hear” the thunder of colliding black holes or the whoosh of a collapsing star. Firm discovery of these waves will at last complete the final movement of Einstein's unfinished symphony.

A gravitational wave telescope essentially acts like a geological seismometer but a seismometer that is placed on the fabric of space-time to register its temblors. The oldest detectors are designed as car-sized, cylindrical metal bars, capable of “ringing” like bells whenever a sizable gravity wave passes through them. The newest instruments, such as LIGO, involve a set of suspended weights that will appear to sway as the peaks and troughs of the traversing gravitational wave alternately squeeze and stretch the space between the masses (although these movements will be exquisitely tiny, the displacement being thousands of times smaller than the width of an atomic nucleus). Together, these various observatories will act as surveyor's stakes in pinpointing the source on the sky; by carefully monitoring the differing times that a gravity wave arrives at the detectors set around the globe, astronomers will be able to determine where the wave originated. The gravity wave signal could be regular or erratic, unceasing or sporadic. We would

discern, in essence, a cosmic symphony of beats. And gravitational wave astronomers will translate these syncopated rhythms—the whines, the bursts, the random roars—into a new map of the heavens, a clandestine cosmos currently impossible to see.

This entire endeavor began very modestly in the 1960s, as one man 's quixotic quest. At the University of Maryland, physicist Joseph Weber cleverly devised the first scheme to trap a gravitational wave and reported a detection in 1969. Inspired by Weber's insight, others quickly joined the campaign. Gravitational wave detectors were erected around the world. In the end, Weber's sighting was never confirmed beyond dispute. Indeed, many contend his evidence has been refuted. But that didn't deter the newcomers to gravitational wave physics in continuing the search. They were energized by the technological challenges of the problem. Weber triggered a movement whose momentum has never diminished to this day. A host of specialists—in optics, lasers, materials science, general relativity, and vacuum technology —have now come together to produce the most complex instruments ever devised for an astronomical investigation. With no guarantee that a signal will be detected, critics have fiercely argued that this attempt is being made too soon. Many in the astronomy and physics communities waged a strong campaign against the endeavor, declaring that the money would be better spent on surer scientific quests. But the potential of the science—not to mention strong politicking —overrode those concerns. As a result, gravitational wave researchers are not just carrying out an experiment, they are founding a field. The questions they are asking stretch back to Aristotle, and answers may at last be within their reach. Einstein's Unfinished Symphony will show how these efforts are the culmination of a centuries-long pursuit—nothing less than the ambition to unravel the enigma of space and time.