viernes, 12 de febrero de 2016

Gravitational Waves Discovered from Colliding Black Holes

The LIGO experiment has confirmed Albert Einstein’s prediction of ripples in spacetime and promises to open a new era of astrophysics

By Clara Moskowitz on February 11, 2016
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The Laser Interferometer Gravitational-Wave Observatory (LIGO) has discovered ripples in space-time created by merging black holes.
*Credit: Caltech*

About 1.3 billion years ago two black holes swirled closer and closer together until they crashed in a furious bang. Each black hole packed roughly 30 times the mass of our sun into a minute volume, and their head-on impact came as the two were approaching the speed of light. The staggering strength of the merger gave rise to a new black hole and created a gravitational field so strong that it distorted spacetime in waves that spread throughout space with a power about 50 times stronger than that of all the shining stars and galaxies in the observable universe. Such events are, incredibly, thought to be common in space, but this collision was the first of its kind ever detected and its waves the first ever seen. Scientists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced on Thursday at a much-anticipated press conference in Washington, D.C. (one of at least five simultaneous events held in the U.S. and Europe) that the more than half-century search for gravitational waves has finally succeeded.

“This was truly a scientific moonshot, and we did it, we landed on the moon,” LIGO executive director David Reitze said during the announcement.

“There are people who’ve put their entire life into this search, and there are people who died before having a chance to see anything,” says LIGO team member Szabolcs Márka, a physicist at Columbia University. “It’s really a wonderful feeling that you have validated the investment of the tremendous amount of work. And it's not just that you found something, but you gave something to everybody, to the rest of the human race.”

Albert Einstein first predicted gravitational waves in 1916 based on his general theory of relativity, but even he waffled about whether or not they truly exist. Scientists began seeking these ripples in spacetime in the 1960s but none succeeded in measuring their effects on Earth until now. LIGO’s discovery, accepted for publication in Physical Review Letters, not only provides the first direct evidence for gravitational waves but also opens the door to using them to study the powerful cosmic events that create them. “It’s a huge deal,” says Luis Lehner, a physicist at the Perimeter Institute for Theoretical Physics in Ontario who is unaffiliated with the LIGO project. “It has pushed the fundamental theory of gravity forward in a very strong way and gives us an incredible tool to probe very deep questions of the universe.”

“ONE OF THE MOST COMPLEX SYSTEMS EVER BUILT”

More than 1,000 scientists work on the $1-billion LIGO experiment, which is funded by the National Science Foundation. The project uses two detectors, one located in Washington State and the other in Louisiana, to sense the distortions in space that occur when a gravitational wave passes through Earth. Each detector is shaped like a giant L, with legs four kilometers long. Laser light bounces back and forth through the legs, reflecting off mirrors, and amazingly precise atomic clocks measure how long it takes to make the journey. Normally, the two legs are exactly the same length, and so the light takes exactly the same amount of time to traverse each. If a gravitational wave passes through, however, the detector and the ground beneath it will expand and contract infinitesimally in one direction, and the two perpendicular legs will no longer be the same size. One of the lasers will arrive a fraction of a second later than the other.

LIGO must be unbelievably sensitive to measure this change in the length of the legs, which is smaller than one ten-thousandth the diameter of a proton, or less than the size of a soccer ball compared with the span of the Milky Way. “It's one of the most complex systems ever built by mankind,” Márka says. “There are so many knobs to turn, so many things to align, to achieve that [sensitivity].” In fact, the experiment is so delicate that unrelated events such as an airplane flying overhead, wind buffeting the building or tiny seismic shifts in the ground beneath the detector can disturb the lasers in ways that mimic gravitational signals. “If I clap in the control room, you will see a blip,” says Imre Bartos, another member of the LIGO team at Columbia. The researchers carefully weed out such contaminating signals and also take advantage of the fact that the detectors in Washington and Louisiana are highly unlikely to be affected by the same contamination at the same time. “By comparing the two detectors, we can be even more certain that what we are seeing is something that's coming from outside the Earth.”

LIGO began its first run in 2002, and hunted through 2010 without finding any gravitational waves. The scientists then shut down the experiment and upgraded nearly every aspect of the detectors, including boosting the power of the lasers and replacing the mirrors, for a subsequentrun, called Advanced LIGO, that began officially on September 18, 2015. Yet even before then the experiment was up and running: the signal arrived on September 14 at 5:51 A.M. Eastern time, reaching the detector in Louisiana seven milliseconds before it got to the detector in Washington. Advanced LIGO is already about three times more sensitive than the initial LIGO, and is designed to become about 10 times more sensitive than the first iteration in the next few years.

LONG TIME COMING

Before now, the strongest evidence of gravitational waves came indirectly from observations of superdense, spinning neutron stars called pulsars. In 1974 Joseph Taylor, Jr., and Russell Hulse discovered a pulsar circling a neutron star, and later observations showed that the pulsar’s orbit was shrinking. Scientists concluded that the pulsar must be losing energy in the form of gravitational waves—a discovery that won Taylor and Hulse the 1993 Nobel Prize in Physics. Ever since this clue, astronomers have been hoping to detect the waves themselves. “I've certainly been looking forward to this event for a long time,” Taylor says. “There is a long history, and I think projects that take this long to bear fruit require an awful lot of patience. It's about time.”

The discovery is not just proof of gravitational waves, but the strongest confirmation yet for the existence of black holes. “We think black holes exist out there. We have very strong evidence they do but we don’t have direct evidence,” Lehner says. “Everything is indirect. Given that black holes themselves cannot give any signal other than gravitational waves, this is the most direct way to prove that a black hole exists.”

LIGO’s ability to study the characteristics of gravitational waves will allow scientists to study black holes in a whole new way. Researchers would like to know the details of how two black holes collide, and whether a new black hole arises as theory suggests. “We're talking about two objects that do not emit light—they’re completely dark,” says Janna Levin, a theorist at Barnard College at Columbia University who is outside the LIGO collaboration. “In the details of a collision and in terms of the gravitational waves, you could see the formation of a new black hole.” The observatory should also be able to see gravitational waves created by other cataclysmic events, such as exploding supernovae and collisions of two neutron stars.

LIGO and future gravitational wave experiments will also allow physicists to put general relativity to the test. The 100-year-old theory has stood the test of time but it still conflicts with the theory of quantum mechanics that rules over the subatomic realm. “We know general relativity should show cracks at some point, and the way it shows them will guide our theory to one that is more complete,” Lehner says. “This is pushing the theory over six orders of magnitude compared to the previous strongest test,” which came from observations of pulsars.

LIGO is the first of many observatories that will join this new era of gravitational astronomy. A similar project called Virgo will come online this year in Italy, and later this decade the Kamioka Gravitational Wave Detector (KAGRA) in Japan will begin observations. Ground-based telescope projects called pulsar timing arrays aim to study gravitational waves by noting delays in light from pulsars arriving on Earth after traveling through wave-stretched space. And a spacecraft called Lisa Pathfinder launched last December to test technology for a proposed space-based observatory that will be sensitive to longer-wavelength gravitational waves from supermassive black hole collisions.

“Every time you open a new window to the universe we always discover new things,” Lehner says. “It’s like Galileo pointing the first telescope to the sky. Initially he saw some planets and moons, but then as we got radio, UV and x-ray telescopes, we discovered more and more about the universe. We are pretty much at the moment where Galileo was beginning to see the first objects around Earth. It will have such a huge impact on the field.”

Gravitational Waves Discovered at Long Last

LIGO

As gravitational waves sweep past Earth, they alternately stretch and compress the arms of Advanced LIGO’s twin detectors, located in Hanford, Wash. (pictured), and Livingston, La.

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By: Natalie Wolchover

February 11, 2016

Ripples in space-time caused by the violent mergers of black holes have been detected, 100 years after these “gravitational waves” were predicted by Albert Einstein’s theory of general relativity and half a century after physicists set out to look for them.

The landmark discovery was reported today by the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) team, confirming months of rumors that have surrounded the group’s analysis of its first round of data. Astrophysicists say the detection of gravitational waves opens up a new window on the universe, revealing faraway events that can’t be seen by optical telescopes, but whose faint tremors can be felt, even heard, across the cosmos.

“We have detected gravitational waves. We did it!” announced David Reitze, executive director of the 1,000-member team, at a National Science Foundation press conference today in Washington, D.C.

Gravitational waves are perhaps the most elusive prediction of Einstein’s theory, one that he and his contemporaries debated for decades. According to his theory, space and time form a stretchy fabric that bends under heavy objects, and to feel gravity is to fall along the fabric’s curves. But can the “space-time” fabric ripple like the skin of a drum? Einstein flip-flopped, confused as to what his equations implied. But even steadfast believers assumed that, in any case, gravitational waves would be too weak to observe. They cascade outward from certain cataclysmic events, alternately stretching and squeezing space-time as they go. But by the time the waves reach Earth from these remote sources, they typically stretch and squeeze each mile of space by a minuscule fraction of the width of an atomic nucleus.

M. Pössel/Einstein Online

Gravitational waves alternately stretch and squeeze space-time both vertically and horizontally as they propagate.

Perceiving the waves took patience and a delicate touch. Advanced LIGO bounced laser beams back and forth along the four-kilometer arms of two L-shaped detectors — one in Hanford, Wash., the other in Livingston, La. — looking for coincident expansions and contractions of their arms caused by gravitational waves as they passed. Using state-of-the-art stabilizers, vacuums and thousands of sensors, the scientists measured changes in the arms’ lengths as tiny as one thousandth the width of a proton. This sensitivity would have been unimaginable a century ago, and struck many as implausible in 1968, when Rainer Weiss of the Massachusetts Institute of Technology conceived the experiment that became LIGO.

“The great wonder is they did finally pull it off; they managed to detect these little boogers!” said Daniel Kennefick, a theoretical physicist at the University of Arkansas and author of the 2007 book Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves.

The detection ushers in a new era of gravitational-wave astronomy that is expected to deliver a better understanding of the formation, population and galactic role of black holes — super-dense balls of mass that curve space-time so steeply thateven light cannot escape. When black holes spiral toward each other and merge, they emit a “chirp”: space-time ripples that grow higher in pitch and amplitude before abruptly ending. The chirps that LIGO can detect happen to fall in the audible range, although they are far too quiet to be heard by the unaided ear. You can re-create the sound by running your finger along a piano’s keys. “Start from the lowest note on the piano and go to middle C,” Weiss said. “That’s what we hear.”

Audio Player

NSF

Audio: The “chirp” of gravitational waves recorded by the LIGO team.

Physicists are already surprised by the number and strength of the signals detected so far, which imply that there are more black holes out there than expected. “We got lucky, but I was always expecting us to be somewhat lucky,” saidKip Thorne, a theoretical physicist at the California Institute of Technology who founded LIGO with Weiss and Ronald Drever, who is also at Caltech. “This usually happens when a whole new window’s been opened up on the universe.”

C. Henze/ NASA

Video: A simulation of two black holes merging and the resulting emission of gravitational radiation.

Eavesdropping on gravitational waves could reshape our view of the cosmos in other ways, perhaps uncovering unimagined cosmic happenings.

“I liken this to the first time we pointed a telescope at the sky,” saidJanna Levin, a theoretical astrophysicist at Barnard College of Columbia University. “People realized there was something to see out there, but didn’t foresee the huge, incredible range of possibilities that exist in the universe.” Similarly, Levin said, gravitational-wave detections might possibly reveal that “the universe is full of dark stuff that we simply can’t detect in a telescope.”

The story of the first gravitational-wave detection began on a Monday morning in September, and it started with a bang: a signal so loud and clear that Weiss thought, “This is crap. It’s gotta be no good.”

Fever Pitch

That first gravitational wave swept across Advanced LIGO’s detectors — first at Livingston, then at Hanford seven milliseconds later — during a mock run in the early hours of Sept. 14, two days before data collection was officially scheduled to begin.

The detectors were just firing up again after a five-year, $200-million upgrade, which equipped them with new noise-damping mirror suspensions and an active feedback system for canceling out extraneous vibrations in real time. The upgrades gave Advanced LIGO a major sensitivity boost over its predecessor, “initial LIGO,” which from 2002 to 2010 had detected “a good clean zero,” as Weiss put it.

When the big signal arrived in September, scientists in Europe, where it was morning, frantically emailed their American colleagues. As the rest of the team awoke, the news quickly spread. According to Weiss, practically everyone was skeptical — especially when they saw the signal. It was such a textbook chirp that many suspected the data had been hacked.

William Widmer for Quanta Magazine

From left: A four-kilometer arm of the LIGO Livingston Observatory, the control room, and a schematic diagram of the detector’s “optical layout.”

Mistaken claims in the search for gravitational waves have a long history, starting in the late 1960s when Joseph Weber of the University of Maryland thought he observed aluminum bars resonating in response to the waves. Most recently, in 2014, an experiment called BICEP2 reported the detection of primordial gravitational waves — space-time ripples from the Big Bang that would now be stretched and permanently frozen into the geometry of the universe. The BICEP2 team went public with great fanfare before their results were peer-reviewed, and then got burned when their signal turned out to have come from space dust.

When Lawrence Krauss, a cosmologist at Arizona State University, got wind of the Advanced LIGO detection, “the first thought is that it was a blind injection,” he said. During initial LIGO, simulated signals had been secretly inserted into the data streams to test the response, unbeknownst to most of the team. When Krauss heard from an inside source that it wasn’t a blind injection this time, he could hardly contain his excitement.

On Sept. 25, he tweeted to his 200,000 followers: “Rumor of a gravitational wave detection at LIGO detector. Amazing if true. Will post details if it survives.” Then, onJan. 11: “My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!”

LIGO

The first gravitational wave signal was observed seven milliseconds apart on Sept. 14 at Advanced LIGO’s Hanford and Livingston detectors.

The team’s official stance was to keep quiet about their signal until they were dead sure. Thorne, bound by a vow of secrecy, didn’t even tell his wife. “I celebrated in private,” he said. The team’s first step was to go back and analyze in excruciating detail how the signal had propagated through the detectors’ thousands of different measurement channels, and to see whether anything strange had happened at the moment the signal was seen. They found nothing unusual. They also ruled out hackers, who would have had to know more than anyone about the experiment’s thousands of data streams. “Even the team that does the blind injections have not perfected their injections well enough not to leave behind lots of fingerprints,” Thorne said. “And there were no fingerprints.”

Another, weaker chirp showed up in the weeks that followed.

The scientists analyzed these first two signals as even more swept in, and they submitted their paper to Physical Review Letters in January; it appeared online today. Their estimate of the statistical significance of the first, biggest signal is above “5-sigma,” meaning the scientists are 99.9999 percent sure it’s real.

Listening for Gravity

Einstein’s equations of general relativity are so complex that it took 40 years for most physicists to agree that gravitational waves exist and are detectable — even in theory.

Einstein first thought that objects cannot shed energy in the form of gravitational radiation, then changed his mind. He showed in a seminal 1918 paper which ones could: Dumbbell-like systems that rotate about two axes at once, such as binary stars and supernovas popping like firecrackers, can make waves in space-time.

Courtesy of Kip Thorne

Rainer Weiss, a professor of physics at the Massachusetts Institute of Technology, around 1970.

Still, Einstein and his colleagues continued to waffle. Some physicists argued that even if the waves exist, the world will oscillate with them and they cannot be felt. It wasn’t until 1957 that Richard Feynman put that question to rest, with a thought experiment demonstrating that, if gravitational waves exist, they are theoretically detectable. But nobody knew how common those dumbbell-like sources might be in our cosmic neighborhood, or how strong or weak the resulting waves would be. “There was that ultimate question of: Will we ever really detect them?” Kennefick said.

In 1968, “Rai” Weiss was a young professor at MIT who had been roped into teaching a class on general relativity — a theory that he, as an experimentalist, knew little about — when news broke that Joseph Weber had detected gravitational waves. Weber had set up a trio of desk-size aluminum bars in two different U.S. states, and he reported that gravitational waves had set them all ringing.

Weiss’ students asked him to explain gravitational waves and weigh in about the news. Looking into it, he was intimidated by the complex mathematics. “I couldn’t figure out what the hell [Weber] was doing — how the bar interacted with the gravitational wave.” He sat for a long time, asking himself, “What’s the most primitive thing I can think of that will detect gravitational waves?” An idea came to him that he calls the “conceptual basis of LIGO.”

Imagine three objects sitting in space-time — say, mirrors at the corners of a triangle. “Send light from one to the other,” Weiss said. “Look at the time it takes to go from one mass to another, and see if the time has changed.” It turns out, he said, “you can do that quickly. I gave it to [my students] as a problem. Virtually the whole class was able to do that calculation.”

In the next few years, as other researchers tried and failed to replicate the results of Weber’s resonance-bar experiments (what he observed remains unclear, but it wasn’t gravitational waves), Weiss began plotting a much more precise and ambitious experiment: a gravitational-wave interferometer. Laser light would bounce between three mirrors in an L-shaped arrangement, forming two beams. The spacing of the peaks and troughs of the light waves would precisely measure the lengths of the two arms, creating what could be thought of as x and y axes for space-time. When the grid was still, the two light waves would bounce back to the corner and cancel each other out, producing a null signal in a detector. But if a gravitational wave swept across Earth, it would stretch the length of one arm and compress the length of the other (and vice versa in an alternating pattern). The off-alignment of the two light beams would create a signal in the detector, revealing a fleeting tremor in space and time.

Courtesy of the Archives, California Institute of Technology

From left: Kip Thorne, Ron Drever and Robbie Vogt, the first director of the LIGO project, with a 40-meter prototype of the LIGO detectors at the California Institute of Technology in 1990.

Fellow physicists were skeptical at first, but the experiment soon found a champion in Thorne, whose theory group at Caltech studied black holes and other potential gravitational-wave sources and the signals they would produce. Thorne had been inspired by Weber’s experiment and similar efforts by Russian physicists; after speaking with Weiss at a conference in 1975, “I began to believe that gravitational-wave detection would succeed,” Thorne said, “and I wanted Caltech to be involved.” He had Caltech hire the Scottish experimentalist Ronald Drever, who had also been clamoring to build a gravitational-wave interferometer. Thorne, Drever and Weiss eventually began working as a team, each taking on a share of the countless problems that had to be solved to develop a feasible experiment. The trio founded LIGO in 1984, and, after building prototypes and collaborating with a growing team, banked more than $100 million in NSF funding in the early 1990s. Blueprints were drawn up for a pair of giant L-shaped detectors. A decade later, the detectors went online.

In Hanford and Livingston, vacuums run down the center of each detector’s four-kilometer arms, keeping the laser, the beam path and the mirrors as isolated as possible from the planet’s constant trembling. Not taking any chances, LIGO scientists monitor their detectors with thousands of instruments during each data run, measuring everything they can: seismic activity, atmospheric pressure, lightning, the arrival of cosmic rays, vibrations of the equipment, sounds near the laser beam, and so on. They then cleanse their data of these various sources of background noise. Perhaps most importantly, having two detectors allows them to cross-check their data, looking for coincident signals.

Related Articles:

Searching the Sky for the Wobbles of GravityAn October 2015 interview with LIGO spokesperson Gabriela González on the search for gravitational waves.

The Quantum Fabric of Space-TimeImagine the fabric of space-time peeled back layer by layer.

Inside the vacuum, even with isolated and stabilized lasers and mirrors, “strange signals happen all the time,” said Marco Cavaglià, assistant spokesperson for the LIGO collaboration. The scientists must trace these “koi fish,” “ghosts,” “fringy sea monsters” and other rogue vibrational patterns back to their sources so the culprits can be removed. One tough case occurred during the testing phase, said Jessica McIver, a postdoctoral researcher and one of the team’s foremost glitch detectives. It was a string of periodic, single-frequency artifacts that appeared every so often in the data. When she and her colleagues converted the mirror vibrations into an audio file, “you could clearly hear the ring-ring-ring of a telephone,” McIver said. “It turned out to be telemarketers calling the phone inside the laser enclosure.”

The sensitivity of Advanced LIGO’s detectors will continue to improve over the next couple of years, and a third interferometer called Advanced Virgo will come online in Italy. One question the data might help answer is how black holes form. Are they products of implosions of the earliest, massive stars, or do they originate from collisions inside tight clusters of stars? “Those are just two ideas; I bet there will be several more before the dust settles,” Weiss said. As LIGO tallies new statistics in future runs, scientists will be listening for whispers of these black-hole origin stories.

Judging by its shape and size, that first, loudest chirp originated about 1.3 billion light-years away from the location where two black holes, each of roughly 30 solar masses, finally merged after slow-dancing under mutual gravitational attraction for eons. The black holes spiraled toward each other faster and faster as the end drew near, like water in a drain, shedding three suns’ worth of energy to gravitational waves in roughly the blink of an eye. The merger is the most energetic event ever detected.

“It’s as though we had never seen the ocean in a storm,” Thorne said. He has been waiting for a storm in space-time ever since the 1960s. The feeling he experienced when the waves finally rolled in wasn’t excitement, he said, but something else: profound satisfaction.

Einstein's gravitational waves found at last

NATURE | NEWS

Einstein's gravitational waves found at last

LIGO 'hears' space-time ripples produced by black-hole collision.

11 February 2016

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One hundred years after Albert Einstein predicted the existence of gravitational waves, scientists have finally spotted these elusive ripples in space-time.

In a highly anticipated announcement, physicists with the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) revealed on 11 February that their twin detectors have heard the gravitational 'ringing' produced by the collision of two black holes about 400 megaparsecs (1.3 billion light-years) from Earth¹.

“Ladies and gentlemen, we have detected gravitational waves,” David Reitze, the executive director of the LIGO Laboratory, said at a Washington DC press conference. “We did it!”

Gravitational waves: 6 cosmic questions they can tackle

One black hole was about 36 times the mass of the Sun, and the other was about 29 solar masses. As they spiralled inexorably into one another, they merged into a single, more-massive gravitational sink in space-time that weighed 62 solar masses, the LIGO team estimates.

“These amazing observations are the confirmation of a lot of theoretical work, including Einstein's general theory of relativity, which predicts gravitational waves,” says physicist Stephen Hawking of the University of Cambridge, UK. Hawking noted that Einstein himself never believed in black holes.

This is the first black-hole merger that scientists have observed. The violent event temporarily radiated more energy — in the form of gravitational waves — than all the stars in the observable Universe emitted as light in the same amount of time.

Nik Spencer/Nature

When played as an audible sound, the waves make an unmistakeable‘chirp’ — a rapidly rising tone — followed by a ‘ringdown’, the radiation pattern from the merged black hole. The 'loudness' of the recorded signal also provides a rough measure of when the merger occurred: between 600 million and 1.8 billion years ago.

The work will be published in a series of papers in Physical Review Letters¹ and the Astrophysical Journal.

The historic discovery — which physicists say will probably lead shortly to a Nobel prize — opens up the new field of gravitational-wave astronomy, in which scientists will listen to the waves to learn more about the objects that can produce them, including black holes, neutron stars and supernovae.

“This is just the first step in a much larger and more exciting development,” says Ilya Mandel, a theoretical physicist at the University of Birmingham, UK. Gravitational waves will join γ-rays, X-rays and radio waves as "part of the toolkit that we have for understanding the universe", he says.

It is also a long-sought victory for the LIGO experiment, which hadspent a decade searching for the signal in the 2000s before a US$200-million upgrade improved the sensitivity of its twin detectors, one in Livingston, Louisiana, and the other in Hanford, Washington.

Wave of discovery

The discovery itself was made before the upgraded version, Advanced LIGO, had officially begun to take scientific data. At 11:50 a.m. Central European Time on 14 September, during the experiment's first observing run, LIGO physicist Marco Drago at the Max Planck Institute for Gravitational Physics in Hannover, Germany, saw a strange signal on his computer.

Software that analyses data in real time was indicating that both interferometers had seen a wave resembling the chirp of a bird with a rapidly increasing pitch. Within an hour, the news had reached Drago's boss, physicist Bruce Allen. The recording looked too good to be true. “When I first saw it I said, 'Oh, it's an injection, obviously,'” Allen says.

LIGO

The gravitational wave signals detected by the twin LIGO stations.

It was an oscillation that began at 35 cycles per second (hertz) and rapidly increased to 250 hertz. It then became chaotic and rapidly died down; the whole thing was over within one-fourth of a second. Crucially, both detectors saw it at roughly the same time — Livingston first and Hanford 7 milliseconds later. That delay is an indication of how the waves swept through the Earth.

Other gravitational-wave detectors — the Virgo interferometer near Pisa, Italy, and the GEO600 interferometer near Hannover — were not operating at the time and so could not confirm the signal. Had Advanced Virgo been on, it would have probably detected the event as well, says its spokesperson, Fulvio Ricci, a physicist at the University of Rome La Sapienza. LIGO scientists have run a series of careful checks to ensure that the signal is real and means what they think it does.

In the past, a few senior members of the LIGO team have tested the group's ability to validate a potential discovery by secretly inserting ‘blind injections’ of fake gravitational waves into the data stream to test whether the research team can differentiate between real and fake signals. But the September detection happened before blind injections were being made, so it is thought to be a signal from a real astrophysical phenomenon in the Universe.

To pinpoint the source of gravitational waves, researchers have to triangulate a signal spotted by different machines spread around Earth. When both LIGO detectors are operating along with Virgo or GEO600, scientists expect to be better able to locate future gravitational-wave sources. Another interferometer in Japan is under development, and a third LIGO site in India has been proposed. A greater geographic spread of detectors would strengthen confidence in any signals.

Nature Special: Gravitational Waves

Direct detection

Einstein’s general theory of relativity predicts that any cosmic event that disturbs the fabric of space-time with sufficient force should produce gravitational ripples that propagate through the Universe. Earth should be awash with such waves — but by the time they reach us, the disturbances that they produce are minute.

In 1974, physicists Joseph Taylor and Russell Hulse at the University of Massachusetts Amherst indirectly confirmed the existence of gravitational waves by watching radio flashes emitted by a pair of neutron stars whirling around one another; the shifts in the flashes’ timing matched Einstein’s predictions of how gravitational waves would carry energy away from the event. That discovery won them the 1993 Nobel Prize in Physics (see: ‘The hundred-year quest for gravitational waves — in pictures’).

Adapted from Andrew Z. Colvin/CC-BY-SA 3.0

But direct detection of the waves had to await the sensitivity achieved by Advanced LIGO, which can detect stretches and compressions of space-time that are as small as one part in 10²² — comparable to a hair’s-width change in the distance from the Sun to Alpha Centauri, the nearest star to the Solar System.

LIGO’s twin interferometers bounce laser beams between mirrors at the opposite ends of 4-kilometre-long vacuum pipes that are set perpendicularly to each other. A gravitational wave passing through will alter the length of one of the arms, causing the laser beams to shift slightly out of sync.

Paid for by the US National Science Foundation, the machines were designed and built by teams at the California Institute of Technology (Caltech) in Pasadena and the Massachusetts Institute of Technology (MIT) in Cambridge. Caltech’s Kip Thorne and Ronald Drever, along with MIT’s Rainer Weiss, were the original founders.

More than 1,000 scientists now belong to the LIGO collaboration. By studying gravitational waves, this next generation of researchers expects to probe entirely new realms of physics, including strong-field gravity, the very early Universe and how matter behaves at extremely high densities.

Hawking says that he would like to use gravitational waves to test his area theorem: that “the area of the final black hole is greater than the sum of the areas of the internal black holes.” He adds: “This is satisfied by the observations.”

“It’s the very real dawn of a new era,” says Mansi Kasliwal, an astronomer at Caltech.

Nature

doi:10.1038/nature.2016.19361