Introduction to The Search for Gravity Waves
At two remote locations in the United States—one set amid marshes and cypress stands near Livingston, Louisiana, and the other on the arid, treeless plains outside of Richland, Washington—teams of scientists in 2001 were tuning up the largest and most sensitive measuring devices ever built. The instruments were components of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which was designed to observe a natural phenomenon called "gravity waves," predicted by the renowned German-born physicist Albert Einstein in 1916.
Much as waves of water can form ripples on the surface of a quiet pond, gravity waves are ripples in the fabric of space and time. Most scientists in 2001 were convinced that gravity waves exist, despite the fact that none had ever been directly detected. In 1993, two Americans, Russell Hulse and Joseph Taylor, were awarded the Nobel Prize in physics for discovering convincing indirect evidence of the presence of gravity waves. For many scientists, this evidence and the challenge of discovering and studying gravity waves were strong enough to lure them to the remote outposts of LIGO.
LIGO (pronounced LIE go) is a joint effort of the Massachusetts Institute of Technology (MIT) and the California Institute of Technology (Caltech). The National Science Foundation (NSF), an independent U.S.-government agency responsible for promoting science and engineering, provided about $300 million to build LIGO, the largest project ever funded by the agency.
However, LIGO was not alone in the hunt for proof of gravity waves. In 2001, several other gravity-wave observatories were in operation, under construction, or in the planning stages. TAMA, an instrument similar to but smaller than LIGO, was operating near Tokyo. Virgo, a joint French-Italian project, was under construction near Pisa, Italy. And a third observatory, called GEO, was to be built near Hanover, Germany.
What Is Gravity?
Gravity is the attractive force that holds our feet to the ground and keeps planets in orbit around stars. Despite the dominant role that gravity plays in human activities on Earth, it is a feeble force compared with the electrical forces that hold atoms together. For example, the gravitational attraction between an electron (a negatively charged particle in an atom) and the nucleus of an atom is 1041 (1 followed by 41 zeros) times weaker than their electrical attraction. The effects of gravity are so significant to humans on Earth because the constant downward tug we feel comes from the combined pull of every atom that makes up the Earth.
The study of gravity dates from the earliest days of science. In the 300's B.C., the Greek philosopher Aristotle taught that heavy objects fall faster than light ones. This assertion was accepted until the early 1600's, when the Italian astronomer and physicist Galileo Galilei showed that gravity accelerates all falling objects equally, regardless of their mass (quantity of matter). Galileo dropped balls of different masses and found that they all reached the ground at nearly the same time.
Building on the work of Galileo, the English physicist and mathematician Sir Isaac Newton made a careful study of gravity and published his universal law of gravitation in 1687. This law states that every object in the universe attracts every other object with a force that depends on the objects' masses and the distance between them. Newton did not attempt to explain how gravity was generated, going only so far as to say that it was a mysterious force transmitted instantly across empty space.
Newton's theory remained a cornerstone of physics until 1915, when Albert Einstein proposed a new theory of gravity. In Einstein's theory, called the theory of general relativity, space itself plays an active role in gravity. Einstein combined the concepts of three-dimensional space and time into an entity he called “space-time.” He did not think of space as nothingness, but rather described it as an actual “fabric” that had geometrical properties and could be warped by objects within it. Einstein theorized that gravitational attraction is the result of a massive object, such as the sun, distorting the space around it. The distortion of space created by the sun's mass causes Earth and the other planets to remain in orbit around the sun. As an aid to visualizing Einstein's concept, imagine placing a bowling ball on a trampoline. The weight of the bowling ball would create a depression in the surface of the trampoline into which smaller objects, such as marbles, would roll. (The planets are prevented from “rolling” into the sun by their orbital motions, which provide a counterbalance to the inward pull of the sun's gravity.)
Unlike Newton, who thought that the effects of gravity were instantaneous, Einstein theorized that gravitational effects move at the speed of light—299,792 kilometers (186,282 miles) per second. In the Newtonian view, if a star could suddenly increase in mass by 10 percent, all other bodies in the universe would instantly be affected by the star's increased gravitational pull. According to Einstein, however, the effect would radiate outward from the star at the speed of light.
Since its publication, Einstein's theory of relativity has been the basis for scientists' understanding of gravity, and it has been confirmed by observations of tiny effects that Newton's theory could not explain. For example, Newton's theory did not account for very slight but detectable changes in the orbit of the planet Mercury around the sun. General relativity explained those tiny changes.
Einstein's theory that the presence of matter warps space gave rise to the idea that violent events in space must cause large distortions in the fabric of space—"spacequakes"—creating ripples that would eventually reach Earth. Such events would include the collisions of very massive objects such as the collapsed remnants of large, burned-out stars. As the objects spiral toward each other, they lose energy in the form of gravity waves, which radiate outward. The final collision releases a titanic burst of gravitational energy.
Scientists think that as a gravity wave passes through a region of space, it simultaneously stretches space in one direction and squeezes it in the perpendicular direction. This kind of wave motion is called “quadrupole radiation.” A person floating in space near the origin of a spacequake would first be stretched and then squeezed as the gravity waves passed by.
Seeking to Detect the Faintest Waves
In 2001, confirming the existence of gravity waves remained one of the most important testable proofs of Einstein's theory. However, detecting gravity waves posed a towering challenge for scientists. Despite the violent events that generate spacequakes, by the time gravity waves reach Earth from far away, they are so faint that they are very difficult to detect. They are similar to the ripples of water that form when a rock falls into the middle of a large lake. The ripples would be large near the site of the impact but would decrease in size as they radiated outward. By the time the ripples reached the shore they would be almost undetectable.
Experts theorize that a gravity wave passing through Earth would have such a tiny effect that it would squeeze and stretch a large object, such as the Golden Gate Bridge, by a distance of only about a thousandth the diameter of a proton (a positively charged particle in an atomic nucleus). Thus, gravitational effects are difficult to measure.
The hunt for gravity waves began in the 1960's when Joseph Weber, a physicist at the University of Maryland, began building a succession of increasingly sensitive detectors, all of which operated on the same principle. Weber suspended a large, cylindrical bar of aluminum on fine wires to isolate it from ground vibrations. He theorized that if gravity waves passed through the bar, they would cause it to vibrate. Because Weber knew such vibrations would be extremely small, he linked the bar to sensitive detectors made of special crystals that created an electric current when compressed by even a tiny amount. Gravity waves passing through the bar would compress the crystals to create a current and signal their presence. Beginning in the late 1960's, Weber claimed to have detected gravity on a daily basis, but other scientists with even more sensitive detectors of similar design could not duplicate his results. Most physicists eventually dismissed Weber's results, calling his bar detectors too crude. Nonetheless, scientists regard him as a pioneering figure in gravity-wave research.
In 1974, Russell Hulse, an astronomy graduate student at the University of Massachusetts, and his instructor, Joseph Taylor, made an accidental discovery that eventually opened a new door in the search for gravity waves. Hulse was searching for astronomical objects called neutron stars using the world's most powerful radio telescope, the Arecibo Observatory in Puerto Rico.
Neutron stars are the smallest and densest stars known to science. A neutron star forms when a large star runs out of fuel and collapses under its own gravity. The star then undergoes a gigantic explosion called a “supernova,” during which the star blows off its outer layers, leaving a dense core consisting mainly of neutrons (electrically neutral particles in an atomic nucleus). Most neutron stars are about 30 kilometers (18 miles) across, and contain more mass than the sun. Neutron stars are so dense that a pinhead-sized piece of one would weigh about 900,000 metric tons (1 million tons) on Earth. Scientists call neutron stars “compact objects” because they are relatively small in size but have tremendous mass.
Hulse was specifically looking for a type of neutron star known as a “pulsar.” Pulsars are rapidly rotating neutron stars that emit a narrow beam of radio waves. As the pulsar spins, the beam of radio waves sweeps through space like the revolving beacon of a lighthouse. If the pulsar and Earth are aligned so that the beam sweeps across Earth on each rotation, a radio telescope can detect the beam as a repeating pulse of radio energy.
Scientists base their theory that pulsars are the remnants of supernovae on the evidence of several pulsars located in the same part of the sky where supernovae had been recorded by early astronomers. In 1968, astronomers discovered a pulsar inside a shell of hot gases named the Crab Nebula that was still expanding from a supernova that Chinese astronomers witnessed in 1054. In January 2001, astronomers reported evidence of another such pulsar, located in the constellation Sagittarius, which was associated with a spectacular supernova reported by Chinese astronomers in 386.
However, the pulsar Hulse discovered in 1974 was behaving strangely. Most pulsars emit pulses of radio waves so regularly that only the most accurate clocks can detect any variation. In contrast, Hulse's pulsar was emitting pulses at a rate that was speeding up and slowing down in a pattern that repeated every 7 hours 45 minutes.
Hulse and Taylor reasoned that such variability could be explained if the pulsar was in orbit around an unseen companion star. Such an orbit would cause the pulses of radio waves it emitted to be compressed and stretched by the “Doppler effect.” This effect, named for Christian Doppler, an Austrian physicist who discovered it in 1842, is a shift in the length of sound, light, or radio waves from a source that is moving toward or away from an observer. For example, to an observer standing on a railway platform, the whistle of an approaching train seems to have a high pitch because the sound waves are being compressed, and thus are increased in frequency, in the train's direction of travel. When the train passes, the sound waves stretch out and decrease in frequency as the train moves away from the observer, causing the sound of the whistle to drop in pitch.
Indirect Evidence of Gravity Waves
As the orbit of Hulse's pulsar carried it toward Earth the radio waves arrived more frequently. When the pulsar moved away from Earth, the pulses decreased in frequency. However, because the period over which the variation occurred was so short, it was unlikely that the pulsar was orbiting a normal star. The members of a typical binary star system—two stars orbiting a common center of gravity—take months or years to complete one orbit because they are very far apart. But Hulse's pulsar completed an orbit in less than eight hours. This indicated that the pulsar was extremely close to its companion. Scientists calculated that the companion must also be a compact object, probably another neutron star. Hulse and Taylor had discovered the first binary pulsar.
Before Hulse and Taylor's discovery, scientists had assumed that a supernova occurring in a double star would destroy the companion star. However, the pulsar Hulse found appeared to be one member of a binary system in which both stars had become supernovae without destroying each other. Taylor realized that because neutron stars are so massive, a pair of them in such close orbit would warp the fabric of space enough to generate large gravity waves. These gravity waves would carry energy away from the stars, causing them to gradually spiral in toward each other. Taylor predicted that over a period of months or years, this change would become measurable. Einstein's general theory of relativity could be used to calculate the rate at which the orbits would shrink.
Hulse graduated and began working in another field, but Taylor continued to observe the star, which became known as the “Hulse-Taylor pulsar.” By 1978, Taylor had gathered enough data to show that the Hulse-Taylor pulsar and its companion are moving toward each other by about 1 meter (3 feet) each year, neatly matching the prediction of Einstein's theory. In 1993, Hulse and Taylor received the Nobel Prize in physics for their discovery.
Despite this evidence in support of gravity waves, the waves generated by the pulsar are far too weak to be detected on Earth. Scientists estimate that in about 200 million years the pulsar and its companion star will collide, generating a burst of very intense gravity waves that would be detectable. Spacequakes produced by collisions between neutron stars and other compact objects, such as black holes—objects in space with such a strong gravitational field that nothing, not even light, can escape from them—are just the types of disturbances that LIGO and other gravity-wave observatories were designed to detect. A collision between two black holes would generate the most powerful gravity waves imaginable. If such a collision were to occur within about 300 million light-years (the distance light travels in one year, approximately 9.5 trillion kilometers [5.9 trillion miles]) of Earth, a volume of space containing thousands of galaxies, LIGO should detect it. Unfortunately for gravity-wave research, scientists do not think such collisions are common. They estimate that LIGO might detect no more than one such event each year. Gravity waves from colliding neutron stars would be observed even less frequently.
The Construction and Operation of LIGO
LIGO is the brainchild of American physicist Rainer Weiss of MIT, who first proposed building such a device in the early 1970's. LIGO was designed not only to be millions of times more sensitive than Joseph Weber's detectors but also to detect gravity waves over a wider range of frequencies.
LIGO is essentially a giant interferometer, a device invented around 1880 by American physicist Albert Michelson. An interferometer uses a pair of light rays, which are composed of tiny waves of energy less than a millionth of a meter in length, to make extremely precise measurements. Light rays can be used for this purpose because when two identical light rays are combined after traveling the same distance over different paths, their waves exactly match. If, however, the lengths of the paths of the light rays have differed by as little as a fraction of a wavelength, the waves will interfere with each other. A well-designed interferometer can compare the lengths of the paths of two light rays to a fraction of a wavelength and determine if one light ray's path has become longer or shorter.
The U.S. Congress approved funding for LIGO in 1991, and construction began in 1996. The main instruments of LIGO are contained within two tubes about 1.2 meters (4 feet) in diameter and 4 kilometers (2.5 miles) long. The two tubes are set at right angles to each other to form a giant L, a shape that takes advantage of the quadrupole nature of gravity waves. Each tube must be pumped out to create an almost perfect vacuum, reducing the number of air molecules that could scatter the light waves and affect measurements.
When LIGO is in operation, light from a laser strikes a beam-splitter located at the intersection of the two legs of the L. The beam-splitter is a partially reflective mirror set at a 45-degree angle to the laser beam. The mirror allows half of the light to pass through to one leg of the L, while the other half is reflected into the other leg. At the end of each leg is a fully reflective mirror that sends the light back to a partially reflective mirror positioned in front of the beam-splitter. This mirror lets a small percentage of the light striking it pass through to the beam-splitter, sending the rest back to the fully reflective mirror. Thus, the light beam in each leg of the L bounces back and forth many times before it returns to the beam-splitter. The farther the light beams travel, the greater is the sensitivity of the instrument's measurements because any differences between the two beams' paths will be more noticeable than if they only traveled a short distance.
At the beam-splitter, the light beams from the two legs are combined. Because the light is carefully tuned as it enters LIGO, the light waves will exactly cancel each other out at the beam-splitter. This cancellation occurs only if both beams have traveled exactly the same distance. However, if the distance differs by even a small fraction of a light-wavelength, the beams will be slightly out of step with each other, enabling some of the light to leak into a light detector. A gravity wave passing through LIGO should produce just that effect by momentarily altering the distances between the mirrors in both legs. Thus, any excess light that spills into the detector would indicate a possible gravity wave.
Identifying True Signals
Instruments as sensitive as LIGO have one mortal enemy: vibrations. A faint earth tremor, highway traffic, or even noisy machinery could jar the instrument enough to produce false signals. This is why the two LIGO sites were built in isolated regions with very little industry, traffic, or Earth movements. Still, LIGO had to be shielded from even the smallest movements. Critical components were mounted on platforms supported by multiple layers of springs, reducing the amount of vibration by about a million times. Despite these measures, scientists predicted that LIGO would experience a false signal about once a minute.
To identify true signals amid all the false ones, LIGO's two observatories were built 3,030 kilometers (1,880 miles) apart. A gravity wave would take about a hundredth of a second to travel that distance. Thus, only signals that appear at both observatories within this time interval will be regarded as potential evidence of gravity waves. Scientists think that LIGO will experience a simultaneous false signal at both facilities only about once every 10 years.
As a final precaution against false alarms, the Washington facility will operate an additional interferometer half the size of the main instrument. If a signal is indeed a gravity wave, this 2-kilometer (1.2-mile) interferometer should record a signal identical to that of the main instruments but half as strong.
On Oct. 20, 2000, the smaller interferometer at the Washington site reached a milestone called “first lock,” in which its laser beams were locked into synchronization. The first-lock test for the interferometer at the Louisiana site was scheduled for late 2001. LIGO project leaders expected to begin full operations of all three instruments by late 2002.
Despite the complexities of detecting gravity waves, LIGO scientists have a good idea of the type of signal to look for. As two compact objects spiral in toward each other, the most intense gravity waves will be emitted just before the objects collide. Scientists call the wave pattern they expect to see just before the collision a “chirp.” Scientists have used computers to simulate these patterns, but they are optimistic that these chirps will not be the only signals LIGO will detect. Over the past 50 years, as scientists have learned to detect and study different forms of electromagnetic radiation—radio waves, infrared and ultraviolet light, and X and gamma rays—the most significant discoveries have been those that were unexpected.
To be useful for astronomy, LIGO must enable researchers to determine where gravity waves originated. The time interval between signals detected at LIGO's two sites will provide a clue to the direction of a gravity wave. If both detectors were tripped simultaneously, that would mean that the wave is moving perpendicular to a line joining the detectors. If the signals arrived a hundredth of a second apart, the gravity wave is moving parallel to that line. And if the signal was detected at a third location—by one of the instruments in Europe or Japan—scientists could use a mathematical technique called triangulation to determine where in the sky the wave originated. In this way, several detectors will operate as a global network.
Many scientists in 2001 were eager to begin searching for gravity waves, but not just to study the collisions of massive objects. Gravity is believed to have played an important role in the moments immediately after the big bang, the violent expansion that most astronomers think gave birth to the universe, and LIGO may shed some light on that event. As of 2001, all of the instruments available to astronomers to study this question depended on the electromagnetic radiation emitted by objects in the observable universe. Since gravity waves are not a form of electromagnetic radiation, astronomers had no way to study the impact gravity waves may have had on the infant universe. Many scientists think that gravity-wave observatories like LIGO could detect the waves that came into existence immediately after the big bang, providing astronomers with an important new tool in studying how the universe developed.
The Future of Gravity-wave Research
Even as LIGO was being readied for operation, scientists were making plans to increase its sensitivity by as much as 10 times with more advanced equipment and measuring techniques. Those upgrades, which will be made only if LIGO proves itself successful, were to begin around 2005. Among the major planned improvements for the observatory—to be renamed LIGO II—were a more powerful laser, even better cushioning against vibrations, and the installation of pure sapphire mirrors to increase the instrument's sensitivity. While LIGO was designed to detect one black-hole collision as far away as 300 million light-years once a year, LIGO II was expected to be able to detect such events up to 6 billion light-years away, perhaps as often as 10 times a day.
Depending on the success of LIGO, gravity-wave astronomy may get an enormous boost from a proposed space-based instrument called the Laser Interferometer Space Antenna (LISA) around 2010. LISA, a $500-million joint project between the U.S. National Aeronautics and Space Administration and the European Space Agency, would be a laser interferometer composed of three satellites. The satellites would be positioned in a triangular configuration in orbit around the sun, about 50 million kilometers (30 million miles) behind Earth. The satellites would be about 5 million kilometers (3 million miles) from one another, making the instrument tens of thousands of times more sensitive than any Earth-based detector.
Scientists expected that by 2010 gravity waves would become yet another tool astronomers could use to learn more about the universe. Instruments like LIGO and its more powerful successors might open up a new window on the universe, helping to unravel some of its deepest mysteries.