Oscillations in space-time were discovered a century after they were predicted by Einstein. A new era in astronomy begins.
Scientists were able to detect fluctuations in space-time caused by the merging of black holes. This happened a hundred years after Albert Einstein predicted these "gravitational waves" in his general theory of relativity, and a hundred years after physicists started looking for them.
This landmark discovery was reported today by researchers at the LIGO Laser Interferometric Gravitational Wave Observatory. They confirmed the rumors that had surrounded the analysis of the first set of data they had collected for months. Astrophysicists say that the discovery of gravitational waves allows us to look at the universe in a new way and makes it possible to recognize distant events that cannot be seen with optical telescopes, but you can feel and even hear their faint tremors reaching us through space.
“We have detected gravitational waves. We did it! announced David Reitze, executive director of the 1,000-member research team, speaking today at a press conference in Washington at the National Science Foundation.
Gravitational waves are perhaps the most elusive phenomenon from Einstein's predictions; the scientist discussed this topic with his contemporaries for decades. According to his theory, space and time form stretching matter, which bends under the influence of heavy objects. Feeling gravity means getting into the curves of this matter. But can this space-time tremble like the skin of a drum? Einstein was confused, he didn't know what his equations meant. And he repeatedly changed his point of view. But even the staunchest supporters of his theory believed that gravitational waves were too weak to be observed anyway. They cascade outward after certain cataclysms, and as they move, alternately stretch and contract space-time. But by the time these waves reach Earth,they stretch and compress every kilometer of space by a tiny fraction of the diameter of an atomic nucleus.
LIGO observatory detector in Hanford, Washington
Photo: REUTERS, Hangout
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It took patience and caution to detect these waves. The LIGO observatory launched laser beams back and forth along four-kilometer right-angled bends of two detectors, one in Hanford, Washington and the other in Livingston, Louisiana. This was done in search of coinciding expansions and contractions of these systems during the passage of gravitational waves. Using state-of-the-art stabilizers, vacuum instruments and thousands of sensors, the scientists measured changes in the length of these systems, amounting to only one thousandth of the size of a proton. Such sensitivity of instruments was unthinkable a hundred years ago. It also seemed incredible in 1968, when Rainer Weiss of the Massachusetts Institute of Technology conceived an experiment called LIGO.
“It's a great miracle that in the end they succeeded. They were able to detect these tiny vibrations! - said the theoretical physicist at the University of Arkansas, Daniel Kennefick, who wrote in 2007 the book Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves.
This discovery marked the beginning of a new era in gravitational wave astronomy. It is hoped that we will have more accurate ideas about the formation, composition and galactic role of black holes - these superdense mass balls that distort space-time so dramatically that even light cannot escape from there. When black holes get close to each other and merge, they generate an impulse signal - space-time oscillations that increase in amplitude and tone, and then abruptly end. The signals that the observatory can record are in the audio range - however, they are too weak to be heard by the naked ear. You can recreate this sound by running your fingers over the piano keys. “Start at the lowest note and work up to the third octave,” Weiss said. "This is what we hear."
Physicists are already amazed at the number and strength of signals that have been recorded at the moment. This means that there are more black holes in the world than previously thought. “We're lucky, but I've always counted on such luck,” said Caltech astrophysicist Kip Thorne, who created LIGO with Weiss and Ronald Drever, who are also from Caltech. "This usually happens when a whole new window opens up in the universe."
Having eavesdropped on gravitational waves, we can form completely different ideas about space, and perhaps we will discover unimaginable cosmic phenomena.
“I can compare this to the moment when we first pointed a telescope up into the sky,” said theoretical astrophysicist Janna Levin of Barnard College, Columbia University. "People realized that there was something there, and you can see it, but they could not predict the incredible set of possibilities that exist in the universe." Likewise, Levin noted, the discovery of gravitational waves could show that the universe is "full of dark matter that we can't just detect with a telescope."
The story of the discovery of the first gravitational wave began on Monday morning in September, and it began with a clap. The signal was so clear and loud that Weiss thought: "No, this is nonsense, nothing will come of it."
Intensity of emotions
This first gravitational wave swept across the upgraded LIGO's detectors - first in Livingston and seven milliseconds later in Hanford - during a simulation run early in the morning of September 14, two days before the official start of data collection.
The detectors were "run-in" after a five-year upgrade that cost $ 200 million. They are equipped with new mirrors for noise cancellation and an active feedback system to suppress extraneous vibrations in real time. The upgrade gave the upgraded observatory a higher level of sensitivity than the old LIGO, which found “absolute and pure zero,” as Weiss put it, between 2002 and 2010.
When the strong signal came in September, scientists in Europe, where it was morning at that moment, began to hastily bombard their American colleagues with e-mails. When the rest of the group woke up, the news spread very quickly. Almost everyone was skeptical about this, Weiss said, especially when they saw the signal. It was a real textbook classic, and so some people thought it was a fake.
Misconceptions in the search for gravitational waves have been repeated many times since the late 1960s, when Joseph Weber of the University of Maryland believed he had found resonant vibrations in an aluminum cylinder with sensors in response to waves. In 2014, an experiment called BICEP2 took place, according to the results of which it was announced that the original gravitational waves were detected - the space-time oscillations from the Big Bang, which have now stretched out and permanently frozen in the geometry of the universe. Scientists from the BICEP2 team announced their discovery with great fanfare, but then their results were independently verified, during which it turned out that they were wrong, and that this signal came from cosmic dust.
When Arizona State University cosmologist Lawrence Krauss heard about the discovery of the LIGO team, he first thought it was a "blind stuff". During the operation of the old observatory, simulated signals were surreptitiously inserted into data streams to check the response, and most of the team did not know about it. When Krauss learned from a knowledgeable source that this time it was not "blind stuffing", he could hardly contain his joyful excitement.
On September 25, he tweeted to his 200,000 Twitter followers: “Rumors of a gravitational wave detected on the LIGO detector. Amazing if true. I'll give you the details, if it's not a linden tree. " This is followed by a January 11 entry: “Earlier rumors about LIGO are confirmed by independent sources. Follow the news. Perhaps gravitational waves are discovered!"
The official position of the scientists was as follows: do not spread about the received signal until there is one hundred percent certainty. Thorne, bound hand and foot by this commitment to secrecy, did not even say anything to his wife. “I celebrated alone,” he said. To begin with, the scientists decided to go back to the very beginning and analyze everything to the smallest detail in order to find out how the signal propagated through the thousands of measurement channels of various detectors, and to understand if there was something strange at the moment the signal was detected. They found nothing out of the ordinary. They also eliminated the hackers who should have known best about the thousands of data streams in the experiment. “Even when the team throws in, they are not perfect enough and leave a lot of footprints in their wake,” Thorne said. "And there were no traces here."
In the weeks that followed, they heard another, weaker signal.
Scientists analyzed the first two signals, and they received more and more. In January, they presented their research papers in Physical Review Letters. This issue is on the Internet today. According to their estimates, the statistical significance of the first, most powerful signal exceeds the "5-sigma", which means that researchers are 99.9999% confident in its authenticity.
Listening to gravity
Einstein's equations of general relativity are so complex that it took most physicists 40 years to agree: yes, gravitational waves exist and can be detected - even theoretically.
At first, Einstein thought that objects could not release energy in the form of gravitational radiation, but then he changed his point of view. In his historical work, written in 1918, he showed what objects can do this: dumbbell-shaped systems that rotate simultaneously around two axes, for example, binaries and supernovae that explode like firecrackers. It is they who can generate waves in space-time.
Computer model illustrating the nature of gravitational waves in the solar system
Photo: REUTERS, Handout
But Einstein and his colleagues continued to hesitate. Some physicists have argued that even if waves exist, the world will vibrate with them, and it will be impossible to feel them. It was only in 1957 that Richard Feynman closed this question by demonstrating in a thought experiment that if gravitational waves exist, they can theoretically be detected. But no one knew how common these dumbbell systems were in outer space, or how strong or weak the resulting waves were. “Ultimately, the question was: can we ever find them?” Kennefick said.
In 1968, Rainer Weiss was a young professor at the Massachusetts Institute of Technology and was assigned to teach a course in general relativity. As an experimenter, he knew little about it, but suddenly there was news of Weber's discovery of gravitational waves. Weber built three desk-sized resonance detectors out of aluminum and placed them in different American states. Now he said that all three detectors recorded "the sound of gravitational waves."
Weiss's students were asked to explain the nature of gravitational waves and express their opinion about the message sounded. Studying the details, he was amazed at the complexity of the mathematical calculations. “I couldn't figure out what the hell Weber was doing, how the sensors interact with the gravitational wave. I sat for a long time and asked myself: "What is the most primitive thing I can think of to detect gravitational waves?" And then an idea came to my mind, which I call the conceptual basis of LIGO."
Imagine three objects in space-time, say, mirrors at the corners of a triangle. “Send a light signal from one to the other,” Weber said. "See how long it takes to move from one mass to another, and check if the time has changed." It turns out, the scientist noted, that this can be done quickly. “I entrusted this to my students as a scientific assignment. Literally the whole group was able to do these calculations."
In subsequent years, when other researchers tried to repeat the results of Weber's experiment with a resonant detector, but constantly failed (it is not clear what he observed, but these were not gravitational waves), Weiss began to prepare a much more accurate and ambitious experiment: the gravitational wave interferometer. The laser beam reflects off three L-shaped mirrors to form two beams. The spacing of the peaks and troughs of light waves accurately indicates the length of the “G” knees that create the X and Y axes of spacetime. When the scale is stationary, the two light waves bounce off the corners and cancel each other out. The signal in the detector is zero. But if a gravitational wave passes through the Earth, it stretches the length of one arm of the letter "G" and compresses the length of the other (and vice versa in turn). The mismatch of the two light beams creates a signal in the detector, showing slight fluctuations in space-time.
At first, fellow physicists were skeptical, but soon the experiment found support in the person of Thorne, whose group of theorists from Caltech investigated black holes and other potential sources of gravitational waves, as well as the signals they generate. Thorne was inspired by Weber's experiment and similar efforts by Russian scientists. After speaking in 1975 at a conference with Weiss, “I began to believe that the detection of gravitational waves would be successful,” Thorne said. "And I wanted Caltech to be involved in this too." He arranged with the institute to hire Scottish experimenter Ronald Driever, who also announced that he would build a gravitational-wave interferometer. Over time, Thorne, Driver and Weiss began to work as one team, each of them solving their own share of countless problems in preparation for a practical experiment. The trio formed LIGO in 1984, and when prototypes were built and a growing team began collaborating, they received $ 100 million in funding from the National Science Foundation in the early 1990s. Blueprints were drawn up for the construction of a pair of giant L-shaped detectors. A decade later, the detectors started working.
In Hanford and Livingston, in the center of each of the four-kilometer bends of the detectors there is a vacuum, thanks to which the laser, its beam and mirrors are maximally isolated from the constant vibrations of the planet. To insure even more, LIGO scientists monitor their detectors during their operation with thousands of instruments, measuring everything they can: seismic activity, atmospheric pressure, lightning, cosmic rays, equipment vibration, sounds in the area of the laser beam, and so on. They then filter out these extraneous background noise from their data. Perhaps the main thing is that they have two detectors, and this allows you to compare the received data, checking them for the presence of coinciding signals.
Inside the vacuum created, even when the lasers and mirrors are completely isolated and stabilized, "strange things happen all the time," says Marco Cavaglià, deputy spokesman for the LIGO project. Scientists must track these "goldfish", "ghosts", "incomprehensible sea monsters" and other extraneous vibrational phenomena, finding out their source in order to eliminate it. One difficult case occurred during the validation phase, said Jessica McIver, a research scientist with the LIGO team, who studies such extraneous signals and interference. A series of periodic single frequency noises often appeared in the data. When she and her colleagues converted the vibrations of the mirrors into audio files, “the phone was ringing distinctly,” McIver said. “It turned outthat it was the advertisers of communications who were calling on the phone inside the laser room."
In the next two years, scientists will continue to improve the sensitivity of the detectors of the modernized Laser Interferometric Gravitational Wave Observatory LIGO. And in Italy, a third interferometer, called Advanced Virgo, will start working. One answer that the data obtained will help give is how black holes are formed. Are they the product of the collapse of the earliest massive stars, or are they the result of collisions within dense star clusters? “These are just two assumptions, I suppose there will be more when everyone calms down,” says Weiss. As LIGO begins to accumulate new statistics in the course of its upcoming work, scientists will begin to listen to stories about the origin of black holes that space will whisper to them.
In shape and size, the first, loudest pulsed signal originated 1.3 billion light-years from where, after an eternity of slow dance, under the influence of mutual gravitational attraction, two black holes, each about 30 times the mass of the sun, finally merged. Black holes circled faster and faster, like a whirlpool, gradually coming closer. Then there was a merger, and in the blink of an eye they released gravitational waves with an energy comparable to that of the three Suns. This fusion became the most powerful energetic phenomenon ever recorded.
“It's like we've never seen the ocean during a storm,” Thorne said. He has been waiting for this storm in space-time since the 1960s. The feeling Thorne experienced as the waves rolled in was not exactly excitement, he says. It was something else: a feeling of deepest satisfaction.