To arrive at the astronomical observatory of the twenty-first century, you must journey through America’s Old South. After traveling about 40 kilometers east of Baton Rouge, Louisiana, travelers come upon a vast pine reserve in a region known as Livingston. Here, north along Highway 63, a modest sign announces the presence of the Laser Interferometer Gravitational-Wave Observatory operated by the California Institute of Technology and the Massachusetts Institute of Technology. Those in the know simply call it LIGO (pronounced LIE-go). The complex resembles either a tasteful warehouse or a modern art museum inexplicably placed in the middle of nowhere. An exact duplicate, painted in the same hues of cream, blue, and silver gray, can be found on the Hanford Nuclear Reservation situated in the scrub desert of Washington state 3,000 kilometers away. Together, they form an astronomical tool of the 21st century, a detector like no other before it.

The signals these two observatories seek are waves of gravitational radiation, or more simply gravity waves, as they are better known in the popular media. Electromagnetic waves, be they visible light, infrared light, or radio waves, are produced by molecules, atoms, or 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 not convey such information. Instead, they will tell us about the motions of massive celestial objects. "It's both an exciting and overpowering change," says Gary Sanders, formerly LIGO's deputy director. "There's almost a romantic attraction, this chance to look at a whole new window of the universe."

Gravity waves are literally quakes in space-time that emanate from the most violent events the universe has to offer—a once blazing star burning out and going supernova, the dizzying spin of neutron stars, or the cagey dance of two black holes whirling around each other, approaching closer and closer until they merge. Gravity waves will tell scientists 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, the remains of the ultimate space-time jolt of the Big Bang itself.

Inside LIGO's main halls, at both Hanford and Livingston, the ambiance is almost reverential, akin to the response one might feel inside a darkened telescope dome. But this astronomical venture is vastly different. With LIGO, there are no windows to spy on the universe. Instead, two 1.2-meter-wide tubes at right angles to each other extend out into the countryside for four kilometers. Together, these arms form a giant L in the landscape. The tubes resemble oil pipelines, although they can't be seen directly. Fifteen-centimeter-thick concrete covers protect them from the wind and rain. In Louisiana, the concrete has even stopped occasional stray bullets during hunting season. A hit could be devastating because the pipes are as empty of air as the vacuum of space. Indeed, they surround the largest artificial vacuum in the world. That vacuum is vital for detecting space-time’s rumbles, vibrations first predicted by Albert Einstein in 1916.

Nearly a century ago Einstein unleashed a revolution that altered our commonplace notions of space and time. His general theory of relativity showed that matter, space, and time are eternally linked, producing the force known as gravity. Space and time are joined together into an entity whose geometry is determined by the matter around it. According to general relativity, stars and other massive bodies dimple the space-time around them, much the way a bowling ball creates a depression in an elastic mat. Planets and comets are attracted to the star because they follow the curved space-time highway carved out by the stellar orb.

Given this picture, Einstein recognized that just as radio waves are generated when electrons travel up and down an antenna, gravity waves should be produced when masses move about in space-time. To understand this phenomenon, imagine one of the most violent events the universe has to offer—two supermassive black holes crashing into each other in the center of our galaxy. When this happens, space is shaken and shaken hard. Such a colossal collision would send out a spacequake that surges through the cosmos at the speed of light. Such waves, however, would not travel through space, in the manner a light wave propagates. Rather, they would be an agitation of space itself. The waves would alternately compress and extend the fabric of space-time.

 

These waves would be deadly near the crash site. They would stretch a 6-foot man to 12 feet and within a millisecond squeeze him to three, before stretching him out once again. Any planets in the vicinity would be torn asunder. Fortunately, by the time such waves reached Earth, this cosmic tsunami would be reduced to a subatomic flutter. Were such gravity waves to hit this page, they would be so weak that they would squeeze and stretch the sheet's dimensions by a distance thousands of times smaller than the size of a proton.

Almost no one doubts that gravitational waves exist, for there is already powerful evidence they are real. Two neutron stars in our galaxy are rapidly orbiting each other, drawing closer and closer together. The rate of their orbital decay—about three feet per year—is just the change physicists expect if this binary pair is losing orbital energy in the form of gravitational waves. American radio astronomers Joseph Taylor and Russell Hulse won the Nobel Prize in 1993 for this discovery. Direct reception of a wave, though, would offer the ultimate proof and provide astronomers with one of the most radical new tools to explore the heavens in four centuries. This explains the motivation to construct LIGO, as well as similar instruments of varying sizes in Italy (VIRGO), Germany (GEO 600), and Japan (TAMA 300).

This entire endeavor began modestly in the 1960s as one man's quixotic quest. University of Maryland physicist Joseph Weber cleverly surmised that a burst of gravitational energy should set a large cylindrical bar of aluminum vibrating, much like a gong continuing to ring after being struck, though far more weakly. When he reported a detection in 1969, others around the world quickly constructed bar antennae. While most physicists remained highly skeptical of Weber's detections, he initiated a new branch of physics that has never diminished.

The seeds of LIGO, a scheme different from Weber's, can be traced to a classroom exercise three decades ago. To teach the concept of gravity waves during a general relativity course at MIT, physicist Rainer Weiss asked his students to envision three mirrors suspended above the ground, their orientation forming the shape of an L. One mirror would be in the corner, the others at each end. Weiss understood that as a gravity wave travels it does two things: The wave compresses space in one direction, say north/south, while simultaneously expanding it in the perpendicular direction, east/west. Consequently, a gravity wave coming straight down on this L-shaped set-up would squeeze one of the arms so the mirrors would be closer together, while spreading the mirrors in the other arm farther apart. A millisecond later, as the gravity wave continues onward, this effect would reverse, with the compressed arm expanding and the expanding arm contracting. Weiss was rediscovering an idea that Weber and others had thought of earlier, but Weiss carried out a detailed study that envisioned nearly all the crucial pieces of the observatories now coming into operation.

Weiss figured that a laser beam, bouncing back and forth between the mirrors at each end of the L, could track a gravity wave's expand/contract flutters. The light would enter the corner of the L. A beam splitter would split the light into two beams, each directed down an arm. After multiple reflections off the mirrors, the beams could be recombined, at which time they interfere with each other (hence the term "interferometry"). The beams could be initially set so that their waves arrive out of phase. In other words, when added together, the waves from both arms would cancel each other out. When the crest of the light wave in one beam is added to the trough of the light wave in the other beam, the result is darkness, like adding 1 and -1 to get zero. But if a gravity wave causes the arms to expand or contract, the two laser beams would travel slightly different distances. In that case, the recombined beams will be more in phase and would thus produce some light. The gravity wave would be spotted in those light changes.

Weiss's concept caught on because it had a distinct advantage. A bar antenna can be tuned to only one gravity-wave frequency (as if it were a radio that could pick up only one station). A laser interferometer, on the other hand, is like a broad-band radio. It can detect a wide range of frequencies, making it more versatile for astronomy. Weber's protégé Robert Forward operated the first prototype, a small tabletop instrument, in 1972 at the Hughes Research Laboratories in California. Pioneering groups in Scotland and Germany went on to build interferometers with longer arms, making technological breakthroughs that at last allowed laser interferometry to surpass the bars in sensitivity. A turning point came in 1979 when Caltech lured the Scottish interferometry wizard Ronald Drever to its campus to build an instrument with 40-meter arms, the longest in its day. But these were all test beds, not true gravity-wave telescopes. Detecting space-time tremors requires interferometers with arms miles in length. The subatomic expansions and contractions are easier to measure over long distances. The longer the armspan, the greater the effect.

The Hanford and Livingston observatories are more like fraternal than identical twins. The Hanford facility actually houses two interferometers, which operate side by side through the arms. There is a full–length detector of four kilometers, as well as one half as long. The Livingston observatory has only the full-length interferometer. Each observatory, though, follows the same principles first established by Weiss, Drever, and others some three decades ago.

The mirrors, two in each arm, are made of fused silica. Ten inches wide and four inches thick, each 10-kilogram cylindrical disk is polished to a smoothness that does not vary by more than 30-billionths of an inch. "If Earth were that smooth," notes LIGO's GariLynn Billingsley, who monitored the mirrors' production, "then the average mountain would not rise more than an inch." Such smoothness is a must for the light to be reflected over and over again to extreme accuracy. Each mirror is balanced on a single steel wire that is attached to a gallowslike frame. This support, in turn, rests on an isolation platform, not unlike a car's suspension system, to reduce seismic jitters by a millionfold.

Two observatories are needed to rule out local disturbances that might mimic a gravity wave at any one site, such as a passing garbage truck or seismic tremors. A gravity wave will pass through both observatories within 10 milliseconds of each other. In addition, the shape and size of the wave should be identical in both places.

LIGO will be most receptive to frequencies from 100 to 3,000 hertz, which is coincidentally the same frequencies our ears pick up as sound. You could actually listen to the signal, once it is electronically recorded. "It sounds like a hiss," notes LIGO's Albert Lazzarini. "Actually, it's a hiss with warbles in it due to the suspension. It's eerie, in some ways like whale songs. Gravity waves will at last be adding sound to our cosmic senses.

Computers, not ears, though, will be sifting through LIGO's data. The technique will be similar to the way in which military sonar experts search for the distinctive sound of a submarine amid the many noises of the sea. Essentially, as the data stream comes in, it will be compared to a "template," a theoretical prediction of what a gravity-wave signal might look like. Take, for example, the case of two neutron stars spiraling into each other. According to computer simulations, this system can produce many possible wave patterns because the signal depends on both the masses of the neutron stars and their orientation as viewed from Earth. To do a proper search, LIGO will have to continually compare its stream of data against some 20,000 to 30,000 possible signal patterns worked out by theorists.

 

Neutron star collisions may be the bread and butter of LIGO's trade. Once the detectors are sensitive enough to see hundreds of millions of light-years beyond Earth, they may detect a few mergers per year. LIGO will register the binary's final minutes, a sort of whine that rapidly rises in pitch, like the sound of a swiftly approaching ambulance siren, as the two city-sized balls of dense matter spiral into each other.

The biggest prize of all will be two black holes colliding. As the twirling holes are about to meet, spiraling inward faster and faster at speeds close to that of light, computer models predict that the whine will turn into a chirp, a birdlike trill that races up the scales in a matter of seconds. A cymballike crash, a mere millisecond in length, heralds the final collision and merger. The two black holes become one. A ring down, akin to the diminishing tone of a struck gong, follows as the new black hole swirls around like the fearsome tornado in The Wizard of Oz, wobbles a bit, and then settles down. Such a sighting would be the first direct evidence that black holes truly exist.

Other potential signals include the explosive burst of an asymmetric supernova, a murmur from the Big Bang itself, and the steady beat from a rotating neutron star. "But what if the strongest gravitational-wave signal," asks Lazzarini, "is a belch or burp that arrives sporadically? Then what? You have to assure yourself it wasn't just an amplifier problem or a bad wire." Those sorts of signals, the unexpected or irregular, will be the most difficult of all, "but they're also where the biggest surprises and most profound discoveries may lie," adds Lazzarini.

When new optical telescopes come on line, there is usually a celebratory "first light" event, the moment when the instrumentation is turned on and the first picture taken. LIGO's construction was completed in November, 1999, but its initiation was not so dramatic. Because its engineering and optics are so complex, LIGO will require a few years for its initial shakedown and calibration before all three interferometers—the two at Hanford, the other in Louisiana—can work in concert with one another. Then and only then can the search for gravity waves really begin.

LIGO researchers concede that their first detectors may not register a thing. For its critics, that made LIGO technologically unjustifiable and premature. LIGO, however, was built on the belief that scientists and engineers couldn't have found solutions without first building a full-sized facility to carry out the needed tests. At first, the two interferometers will be able to detect a change in space-time as small as a millionth trillionth of a meter. Even then, the observatories will have only a small chance at observing an event. Over the years, upgrades will increase sensitivities more than tenfold, enabling LIGO to "feel" space-time rumbles emanating from a variety of sources hundreds of millions or even billions of light-years distant. An advanced LIGO might register an event a day.

But what keeps LIGO researchers at work when no signal is guaranteed? "People take pleasure in solving the technical challenges" answers physicist Peter Saulson of Syracuse University, "much the way medieval cathedral builders continued working knowing they might not see the finished church. But if there wasn't a fighting chance to see a gravity wave during my career, I wouldn't be in this field. If you do this, you have the right stuff."

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 This article was adapted from Bartusiak’s book Einstein’s Unfinished Symphony, published by the Joseph Henry Press in 2000. Bartusiak is currently a visiting professor in the Graduate Program in Science Writing at the Massachusetts Institute of Technology and the author of three other books: Thursday’s Universe, Through a Universe Darkly, and Archives of the Universe.

Suggestions for further reading:

• Saulson, Peter R. Fundamentals of Interferometric Gravitational Wave Detectors. Singapore: World Scientific, 1994.
• Barish, Barry and Rainer Weiss. "LIGO and the Detection of Gravitational Waves." Physics Today 52 (October, 1999): 44-50.
• Blair, David and Geoff McNamara. Ripples on a Cosmic Sea. Reading, Massachusetts: Helix Books, Addison-Wesley, 1997.
• Weiss, Rainer. "Gravitational Radiation." In More Things in Heaven and Earth: A Celebration of Physics at the Millennium. New York: Springer-Verlag, 1999.
• Collins, Harry. Gravity's Shadow: The Search for Gravitational Waves. Chicago: University of Chicago Press, 2004.