Now we live in a Universe filled with gravitational waves.
Prior to the historic announcement Thursday morning from the National Science Foundation (NSF) meeting in Washington, there were only rumors that the Laser Interferometric Gravitational Wave Observatory (LIGO) had discovered a key component of Albert Einstein's General Theory of Relativity, but now we know the reality is deeper, than we thought.
With amazing clarity, LIGO was able to "hear" the moment before the merger of the binary system (two black holes rotating around each other) into a single whole, creating such a clear gravitational wave signal in accordance with the theoretical model that did not require discussion. LIGO witnessed the "rebirth" of a powerful black hole, which happened about 1.3 billion years ago.
Gravitational waves have always been and always will be passing through our planet (in fact, passing through us), but only now we know how to find them. Now we have opened our eyes to various cosmic signals, vibrations caused by known energy events, and are witnessing the birth of an entirely new field of astronomy.
The sound of two black holes merging:
“Now we can hear the universe,” said Gabriela Gonzalez, physicist and spokesperson for LIGO, during Thursday's triumphant meeting. “The discovery has ushered in a new era: The field of gravitational astronomy is now a reality.”
Our place in the Universe is changing a lot and this discovery can be fundamental, like the discovery of radio waves and the understanding that the Universe is expanding.
The Theory of Relativity Becomes More Valid
Attempts to explain what gravitational waves are and why they are so important are as complex as the equations that describe them, but finding them not only strengthens Einstein's theories about the nature of spacetime, we now have a tool to probe the part of the universe that was invisible to us. We can now study the cosmic waves created by the most energetic events in the universe, and perhaps use gravitational waves to make new physical discoveries and explore new astronomical phenomena.
"Now we have to prove that we have the technology to go beyond the discovery of gravitational waves, because this opens up many opportunities for us," Lewis Lehner, of the Ontario Institute for Theoretical Physics, said in an interview following Thursday's statement.
Lehner's research focuses on dense objects (such as black holes) that create powerful gravitational waves. Although he is not associated with the LIGO collaboration, Lehner quickly realized the importance of this historic discovery. “There are no better signals,” he said.
The discovery is based on three paths, he reasons. First, we now know that gravitational waves exist, and we know how to detect them. Secondly, the signal detected by the LIGO stations on September 14, 2015 is strong evidence for the existence of a binary system of black holes, and each black hole weighs several tens of solar masses. The signal is exactly what we expected to see as a result of the hard merger of two black holes, one weighing 29 times the Sun, and the other 36 times. Thirdly, and perhaps most importantly, "the possibility of being sent into a black hole" is definitely the strongest evidence for the existence of black holes.
Cosmic intuition
This event was accompanied by luck, like many other scientific discoveries. LIGO is the largest project funded by the National Science Foundation and started in 2002. It turned out that after many years of searching for the elusive signal of gravitational waves, LIGO was not sensitive enough and in 2010 the observatories were frozen while international cooperation was being done to increase their sensitivity. Five years later, in September 2015, "improved LIGO" was born.
At the time, LIGO co-founder and theoretical physics heavyweight Kip Thorne was confident in LIGO's success, telling the BBC: "We're here. We are in the big game. And it is quite clear that we will lift the veil of secrecy.” And he was right, a few days after the reconstruction, a surge of gravitational waves swept over our planet, and LIGO was sensitive enough to detect them.
These black hole mergers are not considered to be anything special; it is estimated that such events occur every 15 minutes somewhere in the universe. But it was this merger that happened at the right place (at a distance of 1.3 billion light years) at the right time (1.3 billion years ago) for the LIGO observatories to pick up its signal. It was a pure signal from the universe, and Einstein predicted it, and his gravitational waves turned out to be real, describing a cosmic event 50 times more powerful than the power of all the stars in the universe combined. This huge explosion of gravitational waves was recorded by LIGO as a high-frequency chirp signal as the black holes spiraled into one.
To confirm the propagation of gravitational waves, LIGO consists of two observation stations, one in Louisiana, the other in Washington. To eliminate false positives, the gravitational wave signal must be detected at both stations. On September 14, the result was obtained first in Louisiana, and after 7 milliseconds in Washington. The signals matched, and with the help of triangulation, physicists were able to find out that they originated in the sky of the Southern Hemisphere.
Gravitational waves: how can they be useful?
So we have confirmation of the black hole merger signal, so what? This is a historic discovery, which is understandable - 100 years ago, Einstein could not even dream of discovering these waves, but it did happen.
The general theory of relativity was one of the most profound scientific and philosophical realizations of the 20th century and forms the basis of the most intellectual research in reality. In astronomy, the applications of general relativity are clear: from the gravitational lens to measuring the expansion of the universe. But the practical application of Einstein's theories is not at all clear, but most of the modern technologies use the lessons from the theory of relativity in some things that are considered simple. For example, take global navigation satellites, they will not be accurate enough unless a simple correction for time dilation (predicted by relativity) is applied.
It is clear that general relativity has applications in the real world, but when Einstein presented his theory in 1916, its application was highly questionable, which seemed obvious. He simply connected the Universe as he saw it, and the general theory of relativity was born. And now another component of the theory of relativity has been proven, but how can gravitational waves be used? Astrophysicists and cosmologists are definitely intrigued.
“After we have collected data from pairs of black holes that will act as beacons scattered throughout the universe,” theoretical physicist Neil Turok, director of the Institute for Theoretical Physics said on Thursday during a video presentation. “We will be able to measure the speed expansion of the universe, or the amount of dark energy with extreme precision, much more precise than we can today.”
“Einstein developed his theory with some clues from nature, but based on a logical sequence. In 100 years, you see very accurate confirmation of his predictions.”
Moreover, the September 14th event has some physics features that still need to be explored. For example, Lehner noticed that from analyzing a gravitational wave signal, one could measure the "spin" or angular momentum of a black hole merging. "If you've been working on a theory for a long time, you should know that a black hole has a very, very special spin," he said.
The formation of gravitational waves during the merger of two black holes:
For some reason, the final rotation of the black hole is slower than expected, indicating that the black holes are colliding at low speed, or they were in such a collision that caused the joint angular momentum to oppose each other. "It's very interesting, why did nature do this?" Lehner said.
This recent puzzle may bring back some basic physics that has been left out, but, more intriguingly, may reveal "new," unusual physics that doesn't fit into general relativity. And this reveals other applications of gravitational waves: since they are created by strong gravitational phenomena, we have the ability to probe this environment from afar, with possible surprises along the way. In addition, we could combine observations of astrophysical phenomena with electromagnetic forces in order to better understand the structure of the universe.
Application?
Naturally, after the huge announcements made from the complex of scientific discoveries, many people outside the scientific community are wondering how they can influence them. The depth of discovery can be lost, which, of course, also applies to gravitational waves. But consider another case, when Wilhelm Roentgen discovered X-rays in 1895, while experimenting with cathode ray tubes, few people know that in just a few years, these electromagnetic waves will become a key component in everyday medicine from diagnosis to treatment. Similarly, with the first experimental creation of radio waves in 1887, Heinrich Hertz confirmed the known electromagnetic equations of James Clerk Maxwell. Only after a while in the 90s of the 20th century, Guglielmo Marconi, who created the radio transmitter and radio receiver, proved their practical application. Also, the Schrödinger equations that describe the complex world of quantum dynamics are now being used in the development of ultrafast quantum computing.
All scientific discoveries are useful, and many ultimately have everyday applications that we take for granted. Currently, the practical application of gravitational waves is limited to astrophysics and cosmology - now we have a window in the "dark universe" that is not visible to electromagnetic radiation. No doubt scientists and engineers will find other uses for these cosmic pulsations than sensing the universe. However, in order to detect these waves, there must be good advances in optical technology in LIGO, in which new technologies will appear over time.
Participants in the LIGO scientific experiment, in which Russian physicists also participate, announced the registration by American observatories of gravitational waves generated by the collision of two black holes.
Gravitational waves were recorded on September 14, 2015, which was announced on February 11, 2016 at a special press conference of LIGO representatives in Washington. It took scientists six months to process and verify the results. This can be considered the official discovery of gravitational waves, since for the first time they were directly registered on Earth. The results of the work are published in the journal Physical Review Letters.
Physicists of Moscow State University at a press conference. Photo by Maxim Abaev.
Scheme of interferometers and their location on a schematic map of the United States. The test mirror masses in the figure are named Test Mass.
Trial masses, they are also interferometer mirrors, made of fused quartz. Photo: www.ligo.caltech.edu
Numerical simulation of gravitational waves from approaching black holes. Figure: Physical Review Letters http://physics.aps.org/articles/v9/17
LIGO Observatory near Livingston, Louisiana. Photo: www.ligo.caltech.edu
Thus, one of the most important problems facing physicists over the past 100 years has been solved. The existence of gravitational waves was predicted by the general theory of relativity (GR) developed in 1915-1916 by Albert Einstein - the fundamental physical theory that describes the structure and evolution of our world. General relativity, in fact, is a theory of gravity, establishing its connection with the properties of space-time. Massive bodies produce changes in it, which are commonly called the curvature of space-time. If these bodies move with variable acceleration, then there are propagating changes in space-time, which are called gravitational waves.
The problem of their registration is that gravitational waves are very weak, and their detection from any terrestrial source is almost impossible. For many years it was not possible to detect them from most space objects. Hopes remained only on gravitational waves from large cosmic catastrophes like supernova explosions, collisions of neutron stars or black holes. These hopes were justified. In this paper, gravitational waves are detected precisely from the merger of two black holes.
To detect gravitational waves in 1992, a grandiose project was proposed, called LIGO (Laser Interferometer Gravitational-Wave Observatory - laser-interferometric gravitational-wave observatory). The technology for it has been developed for almost twenty years. And it was implemented by two of the largest scientific centers in the United States - the California and Massachusetts Institutes of Technology. The common scientific team, the LIGO collaboration, includes about 1,000 scientists from 16 countries. Russia is represented by Moscow State University and the Institute of Applied Physics of the Russian Academy of Sciences (Nizhny Novgorod)
LIGO includes observatories in the states of Washington and Louisiana, located at a distance of 3000 km, which are an L-shaped Michelson interferometer with two arms 4 km long. The laser beam, passing through the system of mirrors, is divided into two beams, each of which propagates in its shoulder. They bounce off the mirrors and come back. Then these two light waves, which have passed through different paths, are added in the detector. Initially, the system is set up so that the waves cancel each other out, and nothing hits the detector. Gravitational waves change the distances between the test masses, which simultaneously serve as interferometer mirrors, which leads to the fact that the sum of the waves is no longer equal to zero and the signal intensity on the photodetector will be proportional to these changes. This signal is used to register a gravitational wave.
The first, initial, stage of measurements took place in 2002-2010 and did not allow detecting gravitational waves. The sensitivity of the devices was not enough (shifts up to 4x10 -18 m were tracked). Then it was decided in 2010 to stop work and upgrade the equipment, increasing the sensitivity by more than 10 times. The improved equipment, which began work in the second half of 2015, was able to notice a shift by a record 10 -19 m. And already on a test run, scientists were waiting for a discovery, they recorded a gravitational surge from an event that, after a long study, was identified as the merger of two black holes with masses at 29 and 36 solar masses.
Simultaneously with Washington, a press conference was also held in Moscow. On it, the participants of the experiment, representing the Faculty of Physics of Moscow State University, spoke about their contribution to its implementation. The group of VB Braginsky participated in the work from the very beginning of the project. The physicists of Moscow State University ensured the assembly of a complex structure, which is represented by interferometer mirrors that simultaneously serve as test masses.
In addition, their tasks included the fight against extraneous fluctuations (noise), which could interfere with the detection of gravitational waves. It was the Moscow State University specialists who proved that the device should be made of fused quartz, which at operating temperatures will make less noise than sapphire offered by other researchers. In particular, in order to reduce thermal noise, it was necessary to ensure that the oscillations of test masses suspended like pendulums did not decay for a very long time. MSU physicists have achieved a decay time of 5 years!
The success of the measurements carried out will give rise to a new gravitational-wave astronomy and will make it possible to learn a lot about the Universe. Perhaps physicists will be able to unravel some of the mysteries of dark matter and the early stages of the development of the Universe, as well as look into areas where general relativity is violated.
Based on the press conference of the LIGO collaboration.
, USA
© REUTERS, Handout Gravitational waves finally discovered
Popular ScienceOscillations in space-time are discovered a century after they were predicted by Einstein. A new era in astronomy begins.
Scientists have been able to detect fluctuations in space-time caused by black hole mergers. 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.
The landmark discovery was reported today by researchers at the LIGO Laser Interferometric Gravitational Wave Observatory. They confirmed the rumors that had been surrounding the analysis of the first set of data they collected for several months. Astrophysicists say the discovery of gravitational waves provides a new way to look at the universe and makes it possible to recognize distant events that cannot be seen in optical telescopes, but you can feel and even hear their faint trembling reaching us through space.
“We have detected gravitational waves. We did it!" David Reitze, executive director of the 1,000-member research team, announced at a press conference in Washington DC at the National Science Foundation today.
Gravitational waves are perhaps the most elusive phenomenon of Einstein's predictions, the scientist discussed this topic with his contemporaries for decades. According to his theory, space and time form a stretching matter that bends under the influence of heavy objects. To feel gravity means to fall into the bends 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 repeatedly changed his point of view. But even the most staunch supporters of his theory believed that gravitational waves were too weak to be observed anyway. They cascade outward after certain cataclysms, and alternately stretch and compress space-time as they move. But by the time these waves reach the Earth, they stretch and compress every kilometer of space by a tiny fraction of the diameter of an atomic nucleus.
© REUTERS, Hangout LIGO observatory detector in Hanford, Washington
To detect these waves, it took patience and caution. The LIGO observatory fired laser beams back and forth along four-kilometer-long, right-angled knees of two detectors, one in Hanford, Washington, and the other in Livingston, Louisiana. This was done in search of matching 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, as little as one-thousandth the size of a proton. Such sensitivity of instruments was unthinkable a hundred years ago. It seemed incredible in 1968, when Rainer Weiss of the Massachusetts Institute of Technology conceived an experiment called LIGO.
“It is a great miracle that in the end they succeeded. They were able to pick up those tiny vibrations!” said University of Arkansas theoretical physicist Daniel Kennefick, who wrote the 2007 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 - those superdense balls of mass that warp space-time so sharply that not even light can escape from it. When black holes approach each other and merge, they generate an impulse signal - space-time fluctuations that increase in amplitude and tone, and then end abruptly. Those signals that the observatory can detect 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 on the lowest note and work your way up to the third octave,” Weiss said. "That's what we hear."
Physicists are already surprised at the number and strength of signals that are recorded at the moment. This means that there are more black holes in the world than previously thought. “We were lucky, but I always counted on this kind of luck,” said Caltech astrophysicist Kip Thorne, who co-created LIGO with Weiss and Ronald Drever, also from Caltech. “It usually happens when a whole new window opens in the universe.”
By listening to gravitational waves, we can form completely different ideas about space, and perhaps discover unimaginable cosmic phenomena.
“I can compare it to the first time we pointed a telescope into the sky,” said theoretical astrophysicist Janna Levin of Columbia University's Barnard College. “People understood that there was something out there, and you can see it, but they could not predict the incredible range of possibilities that exist in the universe.” Similarly, 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 cotton. 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 detectors of the upgraded LIGO—first at Livingston and seven milliseconds later at Hanford—during a simulation run in the early hours of September 14, two days before the official start of data collection.
The detectors were "running in" after the modernization, which lasted five years and cost 200 million dollars. They were equipped with new mirror suspensions for noise reduction 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" between 2002 and 2010, as Weiss put it.
When the powerful signal came in September, scientists in Europe, where it was morning at the time, began to bombard their American colleagues with e-mail messages. When the rest of the group woke up, the news spread very quickly. Virtually everyone was skeptical, Weiss said, especially when they saw the signal. It was a real textbook classic, and so some people thought it was fake.
False claims in the search for gravitational waves have been made many times since the late 1960s, when Joseph Weber of the University of Maryland thought he had detected resonant oscillations in an aluminum cylinder with sensors in response to the waves. In 2014, an experiment called BICEP2 took place, which resulted in the announcement of the discovery of primordial gravitational waves - space-time fluctuations from the Big Bang, which by now have stretched and permanently frozen in the geometry of the universe. Scientists from the BICEP2 group 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 LIGO team's discovery, he initially thought it was a "blind hoax". During the operation of the old observatory, simulated signals were surreptitiously inserted into the data streams to test the response, and most of the staff did not know about it. When Krauss learned from a knowledgeable source that this time it was not a "blind stuffing", he could hardly contain his joyful excitement.
On September 25, he tweeted to his 200,000 followers: “Rumors about the detection of a gravitational wave at the LIGO detector. Astonishing if true. I'll let you know the details if it's not fake. This is followed by an entry from January 11: “Former rumors about LIGO confirmed by independent sources. Follow the news. Perhaps gravitational waves have been discovered!”
The official position of the scientists was this: do not talk about the received signal until there is one hundred percent certainty. Thorne, bound hand and foot by this obligation 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 thousands of measurement channels of various detectors, and to understand if there was something strange at the time the signal was detected. They didn't find anything out of the ordinary. They also ruled out hackers, who should have known best about the thousands of data streams during the course of the experiment. “Even when the team makes blind throws, they are not perfect enough and leave a lot of traces behind them,” Thorn said. “But there were no traces.”
In the weeks that followed, they heard another, weaker signal.
Scientists analyzed the first two signals, and they received more and more new ones. In January, they presented their research in the journal Physical Review Letters. This issue is going online today. According to their estimates, the statistical significance of the first, most powerful signal exceeds "5-sigma", which means that the researchers are 99.9999% sure of its authenticity.
listening to gravity
Einstein's equations of general relativity are so complex that it took most physicists 40 years to agree that 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 mind. In his historical work, written in 1918, he showed what kind of objects could do this: dumbbell-shaped systems that simultaneously rotate around two axes, such as binary and supernova stars that explode like firecrackers. They can generate waves in space-time.
© REUTERS, Handout A computer model illustrating the nature of gravitational waves in the solar system
But Einstein and his colleagues continued to waver. Some physicists have argued that even if waves exist, the world will oscillate with them, and it will be impossible to feel them. It wasn't until 1957 that Richard Feynman closed the 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-shaped systems were in outer space, and how strong or weak the resulting waves were. “Ultimately, the question was: will we ever find them?” Kennefick said.
In 1968, Rainer Weiss was a young professor at MIT 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 resonant detectors out of aluminum and placed them in various 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. Studying the details, he was struck by the complexity of the mathematical calculations. “I couldn't figure out what the hell Weber was doing, how the sensors interacted with the gravitational wave. I sat for a long time and asked myself: “What is the most primitive thing I can think of that detects 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. “Look how long it takes to go from one mass to another, and see if the time has changed.” It turns out, the scientist noted, this can be done quickly. “I entrusted this to my students as a scientific assignment. Literally the whole group was able to make these calculations.”
In the years that followed, when other researchers tried to replicate the results of Weber's resonant detector experiment but continually failed (it's not clear what he observed, but they weren't gravitational waves), Weiss began to prepare a much more accurate and ambitious experiment: the gravitational wave interferometer. The laser beam is reflected from three mirrors installed in the shape of the letter "L" and forms two beams. The interval of peaks and dips of light waves precisely indicates the length of the bends of the letter "G", which create the x and y axes of space-time. 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 alternately). 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 the experiment soon found support in Thorne, whose Caltech group of theorists was investigating black holes and other potential sources of gravitational waves, as well as the signals they generated. Thorne was inspired by the Weber experiment and similar efforts by Russian scientists. After speaking at a conference with Weiss in 1975, "I began to believe that the detection of gravitational waves would be successful," Thorn said. "And I wanted Caltech to be a part of that too." He arranged with the institute to hire the Scottish experimenter Ronald Driver, who also claimed to build a gravitational wave interferometer. Over time, Thorne, Driver, and Weiss began to work as a team, each solving their share of countless problems in preparation for a practical experiment. The trio formed LIGO in 1984, and when prototypes were built and collaboration began as part of an ever-growing team, they received $100 million in funding from the National Science Foundation in the early 1990s. Drawings were drawn up for the construction of a pair of giant L-shaped detectors. A decade later, the detectors started working.
At Hunford and Livingston, at the center of each of the four-kilometer knees of the detectors, there is a vacuum, thanks to which the laser, its beam and mirrors are maximally isolated from the constant oscillations of the planet. To be on the safe side, LIGO scientists monitor their detectors as they operate with thousands of instruments, measuring everything they can: seismic activity, barometric pressure, lightning, cosmic rays, equipment vibration, sounds around the laser beam, and so on. They then filter their data for these extraneous background noises. 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 matching signals.
Context
Gravitational waves: completed what Einstein started in Bern
SwissInfo 13.02.2016How black holes die
Medium 10/19/2014Inside the vacuum created, even with lasers and mirrors completely isolated and stabilized, “weird things happen all the time,” says Marco Cavaglià, deputy spokesman for the LIGO project. Scientists must track these "goldfish", "ghosts", "strange sea monsters" and other extraneous vibrational phenomena, finding out their source in order to eliminate it. One difficult case occurred during the test phase, said LIGO researcher Jessica McIver, who studies such extraneous signals and interference. A series of periodic single-frequency noise often appeared among the data. When she and her colleagues converted the vibrations of the mirrors into audio files, "the ringing of the phone became distinctly audible," McIver said. "It turned out that it was the communications advertisers who were making phone calls inside the laser room."
In the next two years, scientists will continue to improve the sensitivity of the detectors of the upgraded Laser Interferometric Gravitational-Wave Observatory LIGO. And in Italy, a third interferometer called Advanced Virgo will start operating. One answer that the findings will help give is how black holes form. 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 guesses, I believe there will be more when things calm down,” says Weiss. As LIGO begins accumulating new statistics in the course of its upcoming work, scientists will begin to listen to stories about the origin of black holes whispered to them by space.
Judging by the shape and size, the first, loudest pulse signal occurred 1.3 billion light-years from the place 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. The black holes circled faster and faster, like a whirlpool, gradually approaching. Then there was a merger, and in the blink of an eye they released gravitational waves with an energy comparable to the energy of three Suns. This merger was the most powerful energy phenomenon ever recorded.
"It's like we've never seen the ocean in a storm," Thorn said. He has been waiting for this storm in space-time since the 1960s. The feeling that Thorn experienced at the moment when these waves rolled in cannot be called excitement, he says. It was something else: a feeling of profound satisfaction.
The materials of InoSMI contain only assessments of foreign media and do not reflect the position of the editors of InoSMI.
February 11, 2016 is considered the official day of discovery (detection) of gravitational waves. It was then, at a press conference held in Washington, that the leaders of the LIGO collaboration announced that a team of researchers had succeeded in recording this phenomenon for the first time in the history of mankind.
Prophecies of the great Einstein
The fact that gravitational waves exist was suggested by Albert Einstein at the beginning of the last century (1916) within the framework of the General Theory of Relativity (GR) formulated by him. One can only marvel at the brilliant abilities of the famous physicist, who, with a minimum of real data, was able to draw such far-reaching conclusions. Among the many other predicted physical phenomena that were confirmed in the next century (slowing down the flow of time, changing the direction of electromagnetic radiation in gravitational fields, etc.), it was not possible to practically detect the presence of this type of wave interaction of bodies until recently.
Gravity - an illusion?
In general, in the light of the Theory of Relativity, gravity can hardly be called a force. perturbations or curvature of the space-time continuum. A good example illustrating this postulate is a stretched piece of cloth. Under the weight of a massive object placed on such a surface, a recess is formed. Other objects moving near this anomaly will change the trajectory of their movement, as if "attracted". And the greater the weight of the object (the greater the diameter and depth of the curvature), the higher the "force of attraction". When it moves through the fabric, one can observe the appearance of a divergent "ripple".
Something similar happens in world space. Any rapidly moving massive matter is a source of fluctuations in the density of space and time. A gravitational wave with a significant amplitude is formed by bodies with extremely large masses or when moving with huge accelerations.
physical characteristics
Fluctuations of the space-time metric manifest themselves as changes in the gravitational field. This phenomenon is otherwise called space-time ripples. The gravitational wave acts on the encountered bodies and objects, compressing and stretching them. The deformation values are very small - about 10 -21 of the original size. The whole difficulty of detecting this phenomenon was that the researchers had to learn how to measure and record such changes with the help of appropriate equipment. The power of gravitational radiation is also extremely small - for the entire solar system it is a few kilowatts.
The speed of propagation of gravitational waves slightly depends on the properties of the conducting medium. The oscillation amplitude gradually decreases with distance from the source, but never reaches zero. The frequency lies in the range from several tens to hundreds of hertz. The speed of gravitational waves in the interstellar medium approaches the speed of light.
circumstantial evidence
For the first time, the theoretical confirmation of the existence of gravity waves was obtained by the American astronomer Joseph Taylor and his assistant Russell Hulse in 1974. Studying the expanses of the Universe using the radio telescope of the Arecibo Observatory (Puerto Rico), the researchers discovered the pulsar PSR B1913 + 16, which is a binary system of neutron stars rotating around a common center of mass with a constant angular velocity (a rather rare case). Each year, the revolution period, which was originally 3.75 hours, is reduced by 70 ms. This value is quite consistent with the conclusions from the GR equations predicting an increase in the rotation speed of such systems due to the expenditure of energy for the generation of gravitational waves. Subsequently, several double pulsars and white dwarfs with similar behavior were discovered. Radio astronomers D. Taylor and R. Hulse were awarded the Nobel Prize in Physics in 1993 for discovering new possibilities for studying gravitational fields.
An elusive gravitational wave
The first statement about the detection of gravity waves came from the University of Maryland scientist Joseph Weber (USA) in 1969. For these purposes, he used two gravitational antennas of his own design, separated by a distance of two kilometers. The resonant detector was a well-vibrated one-piece two-meter aluminum cylinder equipped with sensitive piezoelectric sensors. The amplitude of the fluctuations allegedly recorded by Weber turned out to be more than a million times higher than the expected value. Attempts by other scientists using such equipment to repeat the "success" of the American physicist did not bring positive results. A few years later, Weber's work in this area was recognized as untenable, but gave impetus to the development of a "gravitational boom" that attracted many specialists to this area of research. By the way, Joseph Weber himself until the end of his days was sure that he received gravitational waves.
Improvement of receiving equipment
In the 70s, the scientist Bill Fairbank (USA) developed the design of a gravitational wave antenna cooled using SQUIDs - supersensitive magnetometers. The technologies that existed at that time did not allow the inventor to see his product, realized in "metal".
According to this principle, the Auriga gravitational detector was made at the National Legnard Laboratory (Padua, Italy). The design is based on an aluminum-magnesium cylinder, 3 meters long and 0.6 m in diameter. The receiving device weighing 2.3 tons is suspended in an isolated vacuum chamber cooled almost to absolute zero. An auxiliary kilogram resonator and a computer-based measuring complex are used for fixing and detecting tremors. The declared sensitivity of the equipment is 10 -20 .
Interferometers
The functioning of interference detectors of gravitational waves is based on the same principles by which the Michelson interferometer operates. The laser beam emitted by the source is divided into two streams. After multiple reflections and travels along the shoulders of the device, the flows are again brought together, and the final one is used to judge whether any perturbations (for example, a gravitational wave) affected the course of the rays. Similar equipment has been created in many countries:
- GEO 600 (Hannover, Germany). The length of vacuum tunnels is 600 meters.
- TAMA (Japan) with shoulders of 300 m.
- VIRGO (Pisa, Italy) is a joint Franco-Italian project launched in 2007 with 3 km tunnels.
- LIGO (USA, Pacific Coast), hunting gravity waves since 2002.
The latter is worth considering in more detail.
LIGO Advanced
The project was created on the initiative of scientists from the Massachusetts and California Institutes of Technology. Includes two observatories separated by 3 thousand km, in and Washington (Livingston and Hanford cities) with three identical interferometers. The length of perpendicular vacuum tunnels is 4 thousand meters. These are the largest such structures currently in operation. Until 2011, numerous attempts to detect gravity waves did not bring any results. The significant modernization carried out (Advanced LIGO) increased the sensitivity of the equipment in the range of 300-500 Hz by more than five times, and in the low-frequency region (up to 60 Hz) by almost an order of magnitude, reaching such a coveted value of 10 -21 . The updated project started in September 2015, and the efforts of more than a thousand employees of the collaboration were rewarded with the results.
Gravitational Waves Detected
On September 14, 2015, advanced LIGO detectors with an interval of 7 ms recorded gravitational waves that reached our planet from the largest phenomenon that occurred on the outskirts of the observable Universe - the merger of two large black holes with masses 29 and 36 times the mass of the Sun. During the process, which took place more than 1.3 billion years ago, about three solar masses of matter were spent on the radiation of gravity waves in a matter of fractions of a second. The fixed initial frequency of gravitational waves was 35 Hz, and the maximum peak value reached 250 Hz.
The results obtained were repeatedly subjected to comprehensive verification and processing, and alternative interpretations of the data obtained were carefully cut off. Finally, last year the direct registration of the phenomenon predicted by Einstein was announced to the world community.
A fact illustrating the titanic work of researchers: the amplitude of fluctuations in the dimensions of the interferometer arms was 10 -19 m - this value is as much less than the diameter of an atom as it is less than an orange.
Future prospects
This discovery once again confirms that the General Theory of Relativity is not just a set of abstract formulas, but a fundamentally new look at the essence of gravitational waves and gravity in general.
In further research, scientists have high hopes for the ELSA project: the creation of a giant orbital interferometer with arms of about 5 million km, capable of detecting even minor perturbations of gravitational fields. The intensification of work in this direction can tell a lot about the main stages in the development of the Universe, about processes that are difficult or impossible to observe in traditional bands. There is no doubt that black holes, whose gravitational waves will be recorded in the future, will tell a lot about their nature.
To study the relic gravitational radiation, which can tell about the first moments of our world after the Big Bang, more sensitive space instruments will be required. Such a project exists Big Bang Observer), but its implementation, according to experts, is possible not earlier than in 30-40 years.
Wave your hand and gravitational waves will run throughout the universe.
S. Popov, M. Prokhorov. Ghost Waves of the Universe
In astrophysics, an event has occurred that has been awaited for decades. After half a century of searching, gravitational waves have finally been discovered, fluctuations in space-time itself, predicted by Einstein a hundred years ago. On September 14, 2015, the updated LIGO observatory detected a gravitational wave burst generated by the merger of two black holes with masses of 29 and 36 solar masses in a distant galaxy at a distance of about 1.3 billion light years. Gravitational-wave astronomy has become a full-fledged branch of physics; it has opened up a new way for us to observe the universe and will allow us to study the effects of strong gravity that were previously inaccessible.
Gravitational waves
Theories of gravity can come up with different. All of them will describe our world equally well, as long as we limit ourselves to one single manifestation of it - Newton's law of universal gravitation. But there are other, more subtle gravitational effects that have been experimentally tested on the scale of the solar system, and they point to one particular theory - general relativity (GR).
General relativity is not just a set of formulas, it is a fundamental view of the essence of gravity. If in ordinary physics space serves only as a background, a receptacle for physical phenomena, then in general relativity it itself becomes a phenomenon, a dynamic quantity that changes in accordance with the laws of general relativity. It is these distortions of space-time against a flat background - or, in the language of geometry, distortions of the space-time metric - that are felt as gravity. In short, general relativity reveals the geometric origin of gravity.
General Relativity has an all-important prediction: gravitational waves. These are distortions of space-time that are able to “break away from the source” and, self-sustaining, fly away. It's gravity in itself, no one's, its own. Albert Einstein finally formulated general relativity in 1915 and realized almost immediately that his equations allowed for the existence of such waves.
As with any honest theory, such a clear prediction of general relativity must be verified experimentally. Any moving body can radiate gravitational waves: planets, a stone thrown upwards, and a wave of a hand. The problem, however, is that the gravitational interaction is so weak that no experimental setups are able to detect the radiation of gravitational waves from ordinary "emitters".
To "drive" a powerful wave, you need to distort space-time very strongly. The ideal option is two black holes rotating around each other in a tight dance, at a distance of the order of their gravitational radius (Fig. 2). The distortion of the metric will be so strong that a noticeable part of the energy of this pair will be radiated into gravitational waves. Losing energy, the pair will approach each other, spinning faster, distorting the metric more and more and generating even stronger gravitational waves - until, finally, a radical restructuring of the entire gravitational field of this pair occurs and two black holes merge into one.
Such a merger of black holes is an explosion of tremendous power, but only all this radiated energy goes not into light, not into particles, but into vibrations of space. The radiated energy will make up a noticeable part of the initial mass of black holes, and this radiation will splash out in a fraction of a second. Similar fluctuations will generate mergers of neutron stars. A slightly weaker gravitational-wave release of energy also accompanies other processes, such as the collapse of a supernova core.
The gravitational wave burst from the merger of two compact objects has a very specific, well-computed profile, shown in Fig. 3. The oscillation period is given by the orbital motion of two objects around each other. Gravitational waves carry away energy; as a consequence, objects approach each other and spin faster - and this can be seen both in the acceleration of oscillations and in the increase in amplitude. At some point, a merger occurs, the last strong wave is ejected, and then a high-frequency "after-ring" follows ( ringdown) is the jitter of the formed black hole, which “throws off” all non-spherical distortions (this stage is not shown in the picture). Knowing this characteristic profile helps physicists look for the weak signal from such a merger in highly noisy detector data.
Oscillations of the space-time metric - the gravitational-wave echo of a grandiose explosion - will scatter throughout the Universe in all directions from the source. Their amplitude decreases with distance, similar to how the brightness of a point source decreases with distance from it. When a burst from a distant galaxy hits Earth, the fluctuations in the metric will be on the order of 10 −22 or even less. In other words, the distance between physically unrelated objects will periodically increase and decrease by such a relative value.
The order of magnitude of this number is easy to obtain from scaling considerations (see the article by V. M. Lipunov). At the time of the merger of neutron stars or black holes of stellar masses, the distortion of the metrics right next to them is very large - on the order of 0.1, which is why this is strong gravity. Such a severe distortion affects a region of the order of the size of these objects, that is, several kilometers. When moving away from the source, the amplitude of the oscillation falls inversely proportional to the distance. This means that at a distance of 100 Mpc = 3·10 21 km the amplitude of oscillations will fall by 21 orders of magnitude and become about 10 −22 .
Of course, if the merger occurs in our home galaxy, the space-time trembling that has reached the Earth will be much stronger. But such events occur once every few thousand years. Therefore, one should really count only on such a detector that will be able to sense the merger of neutron stars or black holes at a distance of tens to hundreds of megaparsecs, which means that it will cover many thousands and millions of galaxies.
Here it must be added that an indirect indication of the existence of gravitational waves has already been discovered, and even the Nobel Prize in Physics for 1993 was awarded for it. Long-term observations of the pulsar in the binary system PSR B1913+16 have shown that the orbital period decreases exactly at the rate predicted by general relativity, taking into account the energy loss to gravitational radiation. For this reason, practically none of the scientists doubts the reality of gravitational waves; the only question is how to catch them.
Search history
The search for gravitational waves started about half a century ago - and almost immediately turned into a sensation. Joseph Weber of the University of Maryland designed the first resonant detector: a solid two-meter aluminum cylinder with sensitive piezo sensors on the sides and good vibration isolation from extraneous vibrations (Fig. 4). With the passage of a gravitational wave, the cylinder will resonate in time with the distortions of space-time, which should be registered by the sensors. Weber built several such detectors, and in 1969, after analyzing their readings during one of the sessions, he stated in plain text that he had registered “the sound of gravitational waves” in several detectors at once, spaced two kilometers apart from each other (J. Weber, 1969 Evidence for Discovery of Gravitational Radiation). The oscillation amplitude he claimed turned out to be incredibly large, on the order of 10 −16 , that is, a million times larger than the typical expected value. Weber's message was met with great skepticism by the scientific community; besides, other experimental groups, armed with similar detectors, could not catch any such signal in the future.
However, Weber's efforts kick-started this entire area of research and set off the hunt for the waves. Since the 1970s, thanks to the efforts of Vladimir Braginsky and his colleagues from Moscow State University, the USSR has also entered this race (see the absence of gravitational wave signals). An interesting story about those times is in the essay If a girl falls into a hole .... Braginsky, by the way, is one of the classics of the entire theory of quantum optical measurements; he first came up with the concept of the standard quantum measurement limit - a key limitation in optical measurements - and showed how they could in principle be overcome. The Weber resonant circuit was improved, and thanks to the deep cooling of the installation, the noise was drastically reduced (see the list and history of these projects). However, the accuracy of such all-metal detectors was still insufficient for reliable detection of the expected events, and besides, they are tuned to resonate only in a very narrow frequency range around a kilohertz.
Much more promising seemed to be detectors that use not one resonating object, but track the distance between two unrelated, independently suspended bodies, for example, two mirrors. Due to the fluctuation of space caused by the gravitational wave, the distance between the mirrors will be either a little more or a little less. In this case, the longer the arm length, the greater the absolute displacement will be caused by a gravitational wave of a given amplitude. These vibrations can be felt by a laser beam running between the mirrors. Such a scheme is capable of detecting oscillations in a wide frequency range, from 10 hertz to 10 kilohertz, and this is exactly the interval in which merging pairs of neutron stars or stellar-mass black holes will radiate.
The modern implementation of this idea based on the Michelson interferometer is as follows (Fig. 5). Mirrors are suspended in two long, several kilometers long, perpendicular to each other vacuum chambers. At the entrance to the installation, the laser beam is split, goes through both chambers, is reflected from the mirrors, returns back and reunites in a translucent mirror. The quality factor of the optical system is extremely high, so the laser beam does not just pass back and forth once, but lingers in this optical resonator for a long time. In the “calm” state, the lengths are chosen so that the two beams, after recombination, extinguish each other in the direction of the sensor, and then the photodetector is in complete shadow. But as soon as the mirrors move a microscopic distance under the action of gravitational waves, the compensation of the two beams becomes incomplete and the photodetector picks up the light. And the stronger the bias, the brighter the light will be seen by the photosensor.
The words "microscopic displacement" do not even come close to conveying the full subtlety of the effect. The displacement of mirrors by the wavelength of light, that is, microns, is easy to notice even without any tricks. But with a shoulder length of 4 km, this corresponds to space-time oscillations with an amplitude of 10 −10 . It is also not a problem to notice the displacement of mirrors by the diameter of an atom - it is enough to launch a laser beam that will run back and forth thousands of times and get the desired phase incursion. But even this gives a strength of 10 −14 . And we need to go down the displacement scale millions more times, that is, learn how to register the mirror shift not even by one atom, but by thousandths of an atomic nucleus!
On the way to this truly amazing technology, physicists had to overcome many difficulties. Some of them are purely mechanical: you need to hang massive mirrors on a suspension that hangs on another suspension, that one on a third suspension, and so on - and all in order to get rid of extraneous vibration as much as possible. Other problems are also instrumental, but optical. For example, the more powerful the beam circulating in the optical system, the weaker the displacement of the mirrors can be detected by the photosensor. But a beam that is too powerful will unevenly heat the optical elements, which will adversely affect the properties of the beam itself. This effect must somehow be compensated, and for this, a whole research program was launched on this subject in the 2000s (for a story about this study, see the news An obstacle on the way to a highly sensitive gravitational wave detector has been overcome, "Elements", 06/27/2006 ). Finally, there are purely fundamental physical limitations related to the quantum behavior of photons in a resonator and the uncertainty principle. They limit the sensitivity of the sensor to a value called the standard quantum limit. However, physicists have already learned how to overcome it with the help of a cunningly prepared quantum state of laser light (J. Aasi et al., 2013. Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light).
There is a list of countries in the race for gravitational waves; Russia has its own installation, at the Baksan Observatory, and, by the way, it is described in a documentary popular science film by Dmitry Zavilgelsky "Waiting for Waves and Particles". The leaders of this race are now two laboratories - the American project LIGO and the Italian Virgo detector. LIGO includes two identical detectors located in Hanford (Washington) and Livingston (Louisiana) and separated from each other by 3000 km. Having two setups is important for two reasons. First, a signal will be considered registered only if it is seen by both detectors at the same time. And secondly, by the difference in the arrival of a gravitational-wave burst at two installations - and it can reach 10 milliseconds - one can approximately determine from which part of the sky this signal came. True, with two detectors the error will be very large, but when Virgo comes into operation, the accuracy will increase markedly.
Strictly speaking, the idea of interferometric detection of gravitational waves was first proposed by Soviet physicists M. E. Gertsenshtein and V. I. Pustovoit back in 1962. Then the laser had just been invented, and Weber started to create his resonant detectors. However, this article was not noticed in the West and, to tell the truth, did not influence the development of real projects (see the historical review Physics of gravitational wave detection: resonant and interferometric detectors).
The creation of the LIGO gravitational observatory was the initiative of three scientists from the Massachusetts Institute of Technology (MIT) and from the California Institute of Technology (Caltech). These are Rainer Weiss, who implemented the idea of an interferometric gravitational wave detector, Ronald Drever, who achieved stability of laser light sufficient to register, and Kip Thorne, the theorist-inspirer of the project, now well known to the general public as a scientific consultant movie Interstellar. The early history of LIGO can be read in a recent interview with Rainer Weiss and in the memoirs of John Preskill.
The activity associated with the project of interferometric detection of gravitational waves began in the late 1970s, and at first the reality of this undertaking was also doubted by many. However, after demonstrating a number of prototypes, the current LIGO project was written and approved. It was built during the entire last decade of the 20th century.
Although the United States gave the initial impetus to the project, the LIGO observatory is a truly international project. 15 countries have invested in it, financially and intellectually, and over a thousand people are members of the collaboration. An important role in the implementation of the project was played by Soviet and Russian physicists. From the very beginning, the already mentioned group of Vladimir Braginsky from Moscow State University took an active part in the implementation of the LIGO project, and later the Institute of Applied Physics from Nizhny Novgorod also joined the collaboration.
The LIGO observatory was launched in 2002 and until 2010 it hosted six scientific observation sessions. No gravitational wave bursts were reliably detected, and physicists were only able to establish upper limits on the frequency of such events. This, however, did not surprise them too much: estimates showed that in that part of the Universe that the detector “listened to” at that time, the probability of a sufficiently powerful cataclysm was small: approximately once every several decades.
finish line
From 2010 to 2015, the LIGO and Virgo collaborations radically modernized the equipment (Virgo, however, is still in preparation). And now the long-awaited goal was in direct line of sight. LIGO - or rather, aLIGO ( Advanced LIGO) - was now ready to catch bursts generated by neutron stars at a distance of 60 megaparsecs, and black holes - hundreds of megaparsecs. The volume of the universe open for gravitational-wave listening has grown tenfold compared to previous sessions.
Of course, it is impossible to predict when and where the next gravitational-wave "bang" will take place. But the sensitivity of the updated detectors made it possible to count on several neutron star mergers per year, so that the first burst could be expected already during the first four-month observation session. If we talk about the entire aLIGO project lasting several years, then the verdict was extremely clear: either bursts will fall one after another, or something in general relativity does not work in principle. Both will be great discoveries.
From September 18, 2015 to January 12, 2016, the first aLIGO observation session took place. During all this time, rumors about the registration of gravitational waves were circulating on the Internet, but the collaboration remained silent: “we are collecting and analyzing data and are not yet ready to report the results.” An additional intrigue was created by the fact that in the process of analysis, the members of the collaboration themselves cannot be completely sure that they see a real gravitational wave surge. The fact is that in LIGO a burst generated on a computer is occasionally artificially introduced into the stream of real data. It is called "blind injection", blind injection, and out of the entire group, only three people (!) Have access to a system that performs it at an arbitrary moment in time. The team must track this surge, analyze it responsibly, and only at the very last stages of the analysis “cards are opened” and members of the collaboration will find out whether this was a real event or a test of vigilance. By the way, in one such case in 2010, it even came to writing an article, but the signal discovered then turned out to be just a “blind stuffing”.
Lyrical digression
To once again feel the solemnity of the moment, I propose to look at this story from the other side, from within science. When a complex, impregnable scientific task does not lend itself to several years, this is a normal working moment. When it does not give in for more than one generation, it is perceived in a completely different way.
As a schoolboy, you read popular science books and learn about this difficult to solve, but terribly interesting scientific riddle. As a student, you study physics, make presentations, and sometimes, appropriately or not, people around you remind you of its existence. Then you yourself do science, work in another area of physics, but you regularly hear about unsuccessful attempts to solve it. Of course, you understand that somewhere active work is being done to solve it, but the final result for you as an outsider remains unchanged. The problem is perceived as a static background, as a decoration, as an element of physics that is eternal and almost unchanged on the scale of your scientific life. As a task that has always been and always will be.
And then - it is solved. And abruptly, on the scale of several days, you feel that the physical picture of the world has changed and that now it needs to be formulated in other terms and ask other questions.
For people who are directly working on the search for gravitational waves, this task, of course, has not remained unchanged. They see the goal, they know what needs to be achieved. Of course, they hope that nature will also meet them halfway and throw a powerful burst in some nearby galaxy, but at the same time they understand that even if nature is not so favorable, it can no longer hide from scientists. The only question is when exactly they will be able to achieve their technical goals. A story about this feeling from a person who has been searching for gravitational waves for several decades can be heard in the film already mentioned. "Waiting for Waves and Particles".
Opening
On fig. 7 shows the main result: the profile of the signal recorded by both detectors. It can be seen that against the background of noise, at first, the oscillation of the desired shape appears weakly, and then increases in amplitude and frequency. Comparison with the results of numerical simulations made it possible to find out which objects we observed merging: these were black holes with masses of approximately 36 and 29 solar masses, which merged into a single black hole with a mass of 62 solar masses (the error of all these numbers, corresponding to a 90 percent confidence interval, is 4 solar masses). The authors remark in passing that the resulting black hole is the heaviest stellar-mass black hole ever observed. The difference between the total mass of the two original objects and the final black hole is 3±0.5 solar masses. This gravitational mass defect was completely transformed into the energy of radiated gravitational waves in about 20 milliseconds. Calculations showed that the peak gravitational wave power reached 3.6·10 56 erg/s, or, in terms of mass, approximately 200 solar masses per second.
The statistical significance of the detected signal is 5.1σ. In other words, if we assume that these statistical fluctuations overlapped each other and produced such a surge purely by chance, such an event would have to wait 200 thousand years. This allows us to state with confidence that the detected signal is not a fluctuation.
The time delay between the two detectors was approximately 7 milliseconds. This made it possible to estimate the direction of signal arrival (Fig. 9). Since there are only two detectors, the localization turned out to be very approximate: the area of \u200b\u200bthe celestial sphere that is suitable in terms of parameters is 600 square degrees.
The LIGO collaboration did not limit itself to just stating the fact of registration of gravitational waves, but also carried out the first analysis of what this observation has implications for astrophysics. In the article Astrophysical implications of the binary black hole merger GW150914 published the same day in the journal The Astrophysical Journal Letters, the authors estimated the frequency with which such black hole mergers occur. It turned out at least one merger in a cubic gigaparsec per year, which converges with the predictions of the most optimistic models in this regard.
What are gravitational waves about?
The discovery of a new phenomenon after decades of searching is not the end, but only the beginning of a new branch of physics. Of course, the registration of gravitational waves from the merger of black two is important in itself. This is a direct proof of the existence of black holes, and the existence of double black holes, and the reality of gravitational waves, and, speaking in general, proof of the correctness of the geometric approach to gravity, on which general relativity is based. But for physicists, it is no less valuable that gravitational-wave astronomy is becoming a new research tool, making it possible to study what was previously inaccessible.
First, it is a new way to view the Universe and study cosmic cataclysms. There are no obstacles for gravitational waves; they pass through everything in the Universe without any problems. They are self-sufficient: their profile carries information about the process that generated them. Finally, if one grandiose explosion gives rise to both an optical, a neutrino, and a gravitational burst, then you can try to catch all of them, compare them with each other, and sort out previously inaccessible details of what happened there. To be able to catch and compare such different signals from one event is the main goal of all-signal astronomy.
When gravitational wave detectors become even more sensitive, they will be able to detect the jitter of space-time not at the very moment of the merger, but a few seconds before it. They will automatically send their warning signal to the general network of observation stations, and astrophysical satellite-telescopes, having calculated the coordinates of the proposed merger, will have time to turn in the right direction in these seconds and start shooting the sky before the start of the optical burst.
Secondly, the gravitational wave burst will allow you to learn new things about neutron stars,. The neutron star merger is, in fact, the latest and most extreme neutron star experiment that nature can put on for us, and we as viewers will only have to observe the results. The observational consequences of such a merger can be varied (Fig. 10), and by collecting their statistics, we will be able to better understand the behavior of neutron stars in such exotic conditions. An overview of the current state of affairs in this direction can be found in the recent publication by S. Rosswog, 2015. Multi-messenger picture of compact binary mergers .
Thirdly, registration of a burst that came from a supernova and its comparison with optical observations will finally make it possible to sort out the details of what is going on inside, at the very beginning of the collapse. Now physicists still have difficulties with numerical simulation of this process.
Fourth, physicists involved in the theory of gravity have a coveted "laboratory" for studying the effects of strong gravity. So far, all the effects of general relativity that we have been able to directly observe have been related to gravity in weak fields. About what happens in conditions of strong gravity, when the distortions of space-time begin to strongly interact with themselves, we could guess only by indirect manifestations, through the optical echo of cosmic catastrophes.
Fifth, there is a new opportunity to test exotic theories of gravity. There are already many such theories in modern physics, see, for example, the chapter devoted to them from the popular book by A. N. Petrov "Gravity". Some of these theories resemble conventional general relativity in the limit of weak fields, but may differ greatly from it when gravity becomes very strong. Others assume the existence of a new type of polarization for gravitational waves and predict a speed slightly different from the speed of light. Finally, there are theories that include additional spatial dimensions. What can be said about them on the basis of gravitational waves is an open question, but it is clear that some information can be profited from here. We also recommend reading the opinion of the astrophysicists themselves about what will change with the discovery of gravitational waves, in the selection on Postnauka.
Future plans
The prospects for gravitational wave astronomy are the most encouraging. Only the first, shortest observation session of the aLIGO detector has now ended - and a clear signal has already been caught in this short time. It would be more accurate to say this: the first signal was caught even before the official launch, and the collaboration has not yet reported on all four months of work. Who knows, maybe there are already a few additional bursts? One way or another, but further, as the sensitivity of the detectors increases and the part of the Universe accessible for gravitational-wave observations expands, the number of registered events will grow like an avalanche.
The expected schedule of LIGO-Virgo network sessions is shown in fig. 11. The second, six-month, session will begin at the end of this year, the third session will take almost the entire 2018, and at each stage the sensitivity of the detector will increase. Around 2020, aLIGO should reach its planned sensitivity, which will allow the detector to probe the Universe for neutron star mergers that are up to 200 Mpc away from us. For even more energetic black hole merger events, the sensitivity can reach almost a gigaparsec. One way or another, the volume of the Universe available for observation will increase tenfold more compared to the first session.
At the end of this year, the updated Italian laboratory Virgo will also enter the game. It has slightly less sensitivity than LIGO, but it's also quite decent. Due to the triangulation method, a trio of detectors spaced apart in space will make it possible to much better restore the position of sources on the celestial sphere. If now, with two detectors, the localization area reaches hundreds of square degrees, then three detectors will reduce it to tens. In addition, a similar KAGRA gravitational wave antenna is currently being built in Japan, which will begin operation in two to three years, and in India, around 2022, it is planned to launch the LIGO-India detector. As a result, a whole network of gravitational-wave detectors will operate and regularly record signals in a few years (Fig. 13).
Finally, there are plans to take gravitational wave instruments into space, notably the eLISA project. Two months ago, the first trial satellite was launched into orbit, the task of which will be to test technologies. It is still far from the real detection of gravitational waves. But as this constellation of satellites begins collecting data, it will open another window into the universe - through low-frequency gravitational waves. Such an all-wave approach to gravitational waves is the main goal of this field in the long term.
Parallels
The discovery of gravitational waves was already the third case in recent years when physicists finally broke through all the obstacles and got to the previously unknown intricacies of the structure of our world. In 2012, the Higgs boson was discovered - a particle predicted almost half a century ago. In 2013, the IceCube neutrino detector proved the reality of astrophysical neutrinos and began to "look at the universe" in a completely new, previously inaccessible way - through high-energy neutrinos. And now nature has succumbed to man once again: a gravitational-wave “window” has opened up for observing the universe and, at the same time, the effects of strong gravity have become available for direct study.
I must say, nowhere was there any "freebie" from nature. The search was conducted for a very long time, but it did not give in because then, decades ago, the equipment did not reach the result in terms of energy, scale, or sensitivity. It was the steady, purposeful development of technology that led to the goal, a development that was not stopped by either technical difficulties or the negative results of past years.
And in all three cases, the discovery itself was not the end, but, on the contrary, the beginning of a new direction of research, became a new tool for probing our world. The properties of the Higgs boson have become measurable - and in these data, physicists are trying to discern the effects of New Physics. Thanks to the increased statistics of high-energy neutrinos, neutrino astrophysics is taking its first steps. At least the same is now expected from gravitational-wave astronomy, and there is every reason for optimism.
Sources:
1) LIGO Scientific Col. and Virgo Coll. Observation of Gravitational Waves from a Binary Black Hole Merger // Phys. Rev. Lett. Published 11 February 2016.
2) Detection Papers - a list of technical papers accompanying the main discovery paper.
3) E. Berti. Viewpoint: The First Sounds of Merging Black Holes // Physics. 2016. V. 9. N. 17.
Review materials:
1) David Blair et al. Gravitational wave astronomy: the current status // arXiv:1602.02872 .
2) Benjamin P. Abbott and LIGO Scientific Collaboration and Virgo Collaboration. Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO and Advanced Virgo // Living Rev. Relativity. 2016. V. 19. N. 1.
3) O. D. Aguiar. The Past, Present and Future of the Resonant-Mass Gravitational Wave Detectors // Res. Astron. Astrophys. 2011. V. 11. N. 1.
4) The search for gravitational waves - a selection of materials on the journal's website Science in search of gravitational waves.
5) Matthew Pitkin, Stuart Reid, Sheila Rowan, Jim Hough. Gravitational Wave Detection by Interferometry (Ground and Space) // arXiv:1102.3355 .
6) V. B. Braginsky. Gravitational-wave astronomy: new measurement methods // UFN. 2000, vol. 170, pp. 743–752.
7) Peter R. Saulson.
