3 March 2015
The twin 4-kilometer arms of LIGO Livingston embrace a working forest, where logging generates vibrations that the instrument must damp out.
This patch of woodland just north of Livingston, Louisiana, population 1893, isn’t the first place you’d go looking for a breakthrough in physics. Standing on a small overpass that crosses an odd arching tunnel, Joseph Giaime, a physicist at Louisiana State University (LSU), 55 kilometers west in Baton Rouge, gestures toward an expanse of spindly loblolly pine, parts of it freshly reduced to stumps and mud. “It’s a working forest,” he says, “so they come in here to harvest the logs.” On a quiet late fall morning, it seems like only a logger or perhaps a hunter would ever come here.
Yet it is here that physicists may fulfill perhaps the most spectacular prediction of Albert Einstein’s theory of gravity, or general relativity. The tunnel runs east to west for 4 kilometers and meets a similar one running north to south in a nearby warehouselike building. The structures house the Laser Interferometer Gravitational-Wave Observatory (LIGO), an ultrasensitive instrument that may soon detect ripples in space and time set off when neutron stars or black holes merge.
Einstein himself predicted the existence of such gravitational waves nearly a century ago. But only now is the quest to detect them coming to a culmination. The device in Livingston and its twin in Hanford, Washington, ran from 2002 to 2010 and saw nothing. But those Initial LIGO instruments aimed only to prove that the experiment was technologically feasible, physicists say. Now, they’re finishing a $205 million rebuild of the detectors, known as Advanced LIGO, which should make them 10 times more sensitive and, they say, virtually ensure a detection. “It’s as close to a guarantee as one gets in life,” says Peter Saulson, a physicist at Syracuse University in New York, who works on LIGO.
Detecting those ripples would open a new window on the cosmos. But it won’t come easy. Each tunnel contains a pair of mirrors that form an “optical cavity,” within which infrared light bounces back and forth. To look for the stretching of space, physicists will compare the cavities’ lengths. But they’ll have to sense that motion through the din of other vibrations. Glancing at the pavement on the overpass, Giaime says that the ground constantly jiggles by about a millionth of a meter, shaken by seismic waves, the rumble of nearby trains, and other things. LIGO physicists have to shield the mirrors from such vibrations so that they can see the cavities stretch or shorten by distances 10 trillion times smaller—just a billionth the width of an atom.
IN 1915, Einstein explained that gravity arises when mass and energy warp space and time, or spacetime. A year later, he predicted that massive objects undergoing the right kind of oscillating motion should emit ripples in spacetime—gravitational waves that zip along at light speed.
For decades that prediction remained controversial, in part because the mathematics of general relativity is so complicated. Einstein himself at first made a technical error, says Rainer Weiss, a physicist at the Massachusetts Institute of Technology (MIT) in Cambridge. “Einstein had it right,” he says, “but then he [messed] up.” Some theorists argued that the waves were a mathematical artifact and shouldn’t actually exist. In 1936, Einstein himself briefly took that mistaken position.
Rainer Weiss of the Massachusetts Institute of Technology laid out the basic plan for LIGO 43 years ago. © MATT WEBER
Even if the waves were real, detecting them seemed impossible, Weiss says. At a time when scientists knew nothing of the cosmos’s gravitational powerhouses—neutron stars and black holes—the only obvious source of waves was a pair of stars orbiting each other. Calculations showed that they would produce a signal too faint to be detected.
By the 1950s, theorists were speculating about neutron stars and black holes, and they finally agreed that the waves should exist. In 1969, Joseph Weber, a physicist at the University of Maryland, College Park, even claimed to have discovered them. His setup included two massive aluminum cylinders 1.5 meters long and 0.6 meters wide, one of them in Illinois. A gravitational wave would stretch a bar and cause it to vibrate like a tuning fork, and electrical sensors would then detect the stretching. Weber saw signs of waves pinging the bars together. But other experimenters couldn’t reproduce Weber’s published results, and theorists argued that his claimed signals were implausibly strong.
Still, Weber’s efforts triggered the development of LIGO. In 1969, Weiss, a laser expert, had been assigned to teach general relativity. “I knew bugger all about it,” he says. In particular, he couldn’t understand Weber’s method. So he devised his own optical method, identifying the relevant sources of noise. “I worked it out for myself, and I gave it to the students as a homework problem,” he says.
Weiss’s idea, which he published in 1972 in an internal MIT publication, was slow to catch on. “It was obvious to me that this was pie in the sky and it would never work,” recalls Kip Thorne, a theorist at the California Institute of Technology (Caltech) in Pasadena, California. Thorne recorded his skepticism in Gravitation, the massive textbook that he co-wrote and published in 1973. “I had an exercise that said ‘Show that this technology will never work to detect gravitational waves,’ ” Thorne says.
But by 1978 Thorne had warmed to the idea, and he persuaded Caltech to put up $2 million to build a 40-meter prototype interferometer. “It wasn’t a hard sell at all,” Thorne says, “which was a contrast to the situation at MIT.” Weiss says that Thorne played a vital role in winning support for a full-scale detector from the National Science Foundation in 1990. Construction in Livingston and Hanford finally began in 1994.
Now, many physicists say Advanced LIGO is all but a sure winner. On a bright Monday morning in December, researchers at Livingston are embarking on a 10-day stint that will mark their first attempt to run as if making observations. LIGO Livingston has the feel of an outpost. Roughly 30 physicists, engineers, technicians, and operators gather in the large room that serves as the facility’s foyer, auditorium, and—with a table-tennis table in one corner—rec room. “Engineering run 6 began 8 minutes ago,” announces Janeen Romie, an engineer from Caltech. It seems odd that so few people can run such a big rig.
But in principle, LIGO is simple. Within the interferometer’s sewer pipe–like vacuum chamber, at the elbow of the device, a laser beam shines on a beam splitter, which sends half the light down each of the interferometer’s arms. Within each arm, the light builds up as it bounces between the mirrors at either end. Some of the light leaks through the mirrors at the near ends of the arms and shines back on the beam splitter. If the two arms are exactly the same length, the merging waves will overlap and interfere with each other in a way that directs the light back toward the laser.
The ultimate motion sensor
In a LIGO interferometer, light waves leaking out of the two storage arms ordinarily interfere to send light back to the laser. By stretching the two arms by different amounts, a gravitational wave would alter the interference and send light toward a photodetector. G. GRULLÓN/SCIENCE
But if the lengths are slightly different, then the recombining waves will be out of sync and light will emerge from the beam splitter perpendicular to the original beam. From that “dark port” output, physicists can measure any difference in the arms’ lengths to an iota of the light’s wavelength. Because a gravitational wave sweeping across the apparatus would generally stretch one arm more than the other, it would cause light to warble out of the dark port at the frequency at which the wave ripples. That light would be the signal of the gravitational wave.
In practice, LIGO is a monumental challenge in sifting an infinitesimal signal from a mountain of vibrational noise. Sources of gravitational waves should “sing” at frequencies ranging from 10 to 1000 cycles per second, or hertz. But at frequencies of hundreds or thousands of hertz the individual photons in the laser beam produce noise as they jostle the mirrors. To smooth out such noise, researchers crank up the amount of light and deploy massive mirrors. At frequencies of tens of hertz and lower, seismic vibrations dominate, so researchers dangle the mirrors from elaborate suspension systems and actively counteract that motion. Still, a large earthquake anywhere in the world or even the surf pounding the distant coast can knock the interferometer off line.
To boost the Hanford and Livingston detectors’ sensitivity 10-fold, to a ten-billionth of a nanometer, physicists have completely rebuilt the devices. Each of the original 22-kilogram mirrors hung like a pendulum from a single steel fiber; the new 40-kilogram mirrors hang on silica fibers at the end of a four-pendulum chain. Instead of LIGO’s original 10 kilowatts of light power, researchers aim to circulate 750 kilowatts. They will collect 100,000 channels of data to monitor the interferometer. Comparing the new and old LIGO is “like comparing a car to a wheel,” says Frederick Raab, a Caltech physicist who leads the Hanford site.
The new Livingston machine has already doubled Initial LIGO’s sensitivity. “In 6 months they’ve made equivalent progress to what Initial LIGO made in 3 or 4 years,” says Raab, who adds that the Hanford site is about 6 months behind. But Valery Frolov, a Caltech physicist in charge of commissioning the Livingston detector, cautions that machine isn’t running anywhere close to specs. The seismic isolation was supposed to be better, he says, and researchers haven’t been able to keep the interferometer “locked” and running for long periods. As for reaching design sensitivity, “I don’t know whether it will take 1 year or whether it will take 5 years like Initial LIGO did,” he warns.
Still, LIGO researchers plan to make a first observing run this year and hope to reach design sensitivity next year. “We will have detections that we will be able to stand up and defend, if not in 2016, then in 2017 or 2018,” says Gabriela González, a physicist at LSU and spokesperson for the more than 900-member LIGO Science Collaboration.
That forecast is based on the statistics of the stars. LIGO’s prime target is the waves generated by a pair of neutron stars—the cores of exploded stars that weigh more than the sun but measure tens of kilometers across—whirling into each other in a death spiral lasting several minutes. Initial LIGO could sense such a pair up to 50 million light-years way. Given the rarity of neutron-star pairs, that search volume was too small to guarantee seeing one. Advanced LIGO should see 10 times as far and probe 1000 times as much space, enough to contain about 10 sources per year, González says. However, Clifford Will, a theorist at the University of Florida in Gainesville, notes that the number of sources is the most uncertain part of the experiment. “If it’s less than one per year, that’s not going to be too good,” he says.
Enlarging the search
Compared with Initial LIGO, Advanced LIGO will be able to detect gravitational wave sources up to 10 times as far away, probing 1000 times as much space. Such a volume will likely yield multiple sources. ADAPTED FROM NSF BY G. GRULLÓN/SCIENCE
The hunt will be global. As well as combining data from the two LIGO detectors, researchers will share data with their peers working on the VIRGO detector, an interferometer with 3-kilometer arms near Pisa, Italy, that is undergoing upgrades, and on GEO600, one with 600-meter arms near Hannover, Germany.
By comparing data, collaborators can better sift signals from noise and can pinpoint sources on the sky. Japanese researchers are also building a detector, and LIGO leaders hope to add a third detector, in India.
FOR THEORISTS—if not for the rest of the world—seeing gravitational waves for the first time will be something of an anti-climax. “We are so confident that gravitational waves exist that we don’t actually need to see one,” says Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland. That’s because in 1974 American astrophysicists Russell Hulse and Joseph Taylor Jr. found indirect but convincing evidence of the waves. They spotted two pulsars—neutron stars that emit radio signals with clockwork regularity—orbiting each other. From the timing of the radio pulses, Hulse and Taylor could monitor the pulsars’ orbit. They found it is decaying at exactly the rate expected if the pulsars were radiating energy in the form of gravitational waves.
LIGO’s real payoff will come in opening a new frontier in astronomy, says Robert Wald, a gravitational theorist at the University of Chicago in Illinois. “It’s kind of like after being able to see for a while, being able to hear, too,” Wald says. For example, if a black hole tears apart a neutron star, then details of the gravitational waves may reveal the properties of matter in neutron stars.
All told, detecting gravitational waves would merit science’s highest accolade, physicists say. “As soon as they detect a gravitational wave, it’s a Nobel Prize,” Kamionkowski predicts. “It’s such an extraordinary experimental accomplishment.” But the prize can be shared by at most three people, so the question is who should get it.
Weiss is a shoo-in, many say, but he demurs. “I don’t want to deny that there was some innovation [in my work], but it didn’t come out of the blue,” he says. “The lone crazy man working in a box, that just doesn’t hold true.” In 1962 two Russian physicists published a paper on detecting gravitational waves with an interferometer, as Weiss says he learned long after his 1972 work. In the 1970s, Robert Forward of the Hughes Aircraft Company in Malibu, California, ran a small interferometer. Key design elements of LIGO came from Ronald Drever, project director at Caltech from 1979 to 1987, who, Thorne says, “has to be recognized as one of the fathers of the LIGO idea.”
But to make that prize-winning discovery, physicists must get Advanced LIGO up and running. At 8 a.m. on Tuesday morning, LIGO operator Gary Traylor comes off the night shift. “Last night was a total washout,” he says in his soft Southern accent, swiveling in a chair in the brightly lit control room. “There’s a low pressure area moving over the Atlantic that’s causing 20-foot waves to crash into the coast,” Traylor says, and that distant drumming overwhelmed the detector. So in the small hours, LIGO did sense waves. But not the ones everybody is hoping to see.
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