Participants in the LIGO scientific experiment, in which Russian physicists are also participating, announced that American observatories have detected 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 by 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 they were directly recorded on Earth for the first time. The results of the work were published in the journal Physical Review Letters.

Physicists from Moscow State University at a press conference. Photo by Maxim Abaev.

Diagram of interferometers and their location on a schematic map of the United States. The test mirror masses in the figure are called Test Mass.

Test masses, also known as interferometer mirrors, are 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 is predicted by the general theory of relativity (GTR), developed in 1915-1916 by Albert Einstein, the fundamental physical theory that describes the structure and evolution of our world. General relativity, in essence, is a theory of gravity that establishes its connection with the properties of space-time. Massive bodies produce changes in it, which are commonly called curvature of space-time. If these bodies move with variable acceleration, then propagating changes in space-time arise, which are called gravitational waves.

The problem with registering them is that gravitational waves are very weak, and their detection from any terrestrial source is almost impossible. Over the years, they have not been detected from most space objects either. Hopes remained only for gravitational waves from major cosmic disasters such as supernova explosions, collisions of neutron stars or black holes. These hopes came true. In this work, gravitational waves were discovered precisely from the merger of two black holes.

To detect gravitational waves, a grandiose project called LIGO (Laser Interferometer Gravitational-Wave Observatory) was proposed in 1992. The technology for it has been developed for almost twenty years. And it was implemented by two of the largest research centers in the United States - the California and Massachusetts Institutes of Technology. The overall scientific team, the LIGO collaboration, includes about 1,000 scientists from 16 countries. Russia is represented in it by Moscow State University and Institute of Applied Physics RAS (Nizhny Novgorod)

LIGO includes observatories in the states of Washington and Louisiana, located at a distance of 3000 km, which is an L-shaped Michelson interferometer with two arms 4 km long. The laser beam, passing through a system of mirrors, is divided into two beams, each of which propagates in its own arm. They are reflected from mirrors and come back. Then these two light waves, traveling along different paths, are added together in the detector. Initially, the system is configured so that the waves cancel each other out and nothing hits the detector. Gravitational waves change the distances between test masses, which simultaneously serve as mirrors of the interferometer, which leads to the fact that the sum of the waves is no longer equal to zero and the signal intensity at 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 the detection of gravitational waves. The sensitivity of the devices was not enough (shifts up to 4x10 -18 m were tracked). Then it was decided to stop work in 2010 and modernize 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 detect a shift of a record 10 -19 m. And already during the test run, scientists were waiting for a discovery; they recorded a gravitational burst from the event, which 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 held in Moscow. At it, participants in the experiment, representing the Faculty of Physics of Moscow State University, spoke about their contribution to its implementation. V.B. Braginsky’s group participated in the work from the very beginning of the project. Physicists from Moscow State University ensured the assembly of a complex structure, which consists of interferometer mirrors, which simultaneously serve as test masses.

In addition, their tasks included combating extraneous vibrations (noise) that 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 the sapphire proposed by other researchers. In particular, to reduce thermal noise, it was necessary to ensure that the oscillations of test masses suspended like pendulums did not die out for a very long time. Physicists from Moscow State University have achieved a decay time of 5 years!

The success of the measurements will give rise to a new gravitational-wave astronomy and will allow us to learn a lot of new things 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 materials from the LIGO collaboration press conference.

Gravitational waves - artist's rendering

Gravitational waves are disturbances of the space-time metric that break away from the source and propagate like waves (the so-called “space-time ripples”).

In general relativity and most others modern theories gravity gravitational waves are generated by movement massive bodies with variable acceleration. Gravitational waves propagate freely in space at the speed of light. Due to the relative weakness of gravitational forces (compared to others), these waves have a very small magnitude, which is difficult to register.

Polarized gravitational wave

Gravitational waves are predicted by the general theory of relativity (GR), and many others. They were first directly detected in September 2015 by two twin detectors, which detected gravitational waves, likely resulting from the merger of two and the formation of one more massive rotating black hole. Indirect evidence of their existence has been known since the 1970s - General Relativity predicts the rate of convergence of close systems due to the loss of energy due to the emission of gravitational waves, which coincides with observations. Direct registration of gravitational waves and their use to determine the parameters of astrophysical processes is important task modern physics and astronomy.

Within the framework of general relativity, gravitational waves are described by solutions of wave-type Einstein equations, which represent a perturbation of the space-time metric moving at the speed of light (in the linear approximation). The manifestation of this disturbance should be, in particular, a periodic change in the distance between two freely falling (that is, not influenced by any forces) test masses. Amplitude h gravitational wave is a dimensionless quantity - a relative change in distance. The predicted maximum amplitudes of gravitational waves from astrophysical objects (for example, compact binary systems) and phenomena (explosions, mergers, captures by black holes, etc.) when measured are very small ( h=10 −18 -10 −23). A weak (linear) gravitational wave, according to the general theory of relativity, transfers energy and momentum, moves at the speed of light, is transverse, quadrupole and is described by two independent components located at an angle of 45° to each other (has two directions of polarization).

Different theories predict the speed of propagation of gravitational waves differently. In general relativity, it is equal to the speed of light (in the linear approximation). In other theories of gravity, it can take any value, including infinity. According to the first registration of gravitational waves, their dispersion turned out to be compatible with a massless graviton, and the speed was estimated to be equal to the speed of light.

Generation of gravitational waves

A system of two neutron stars creates ripples in spacetime

A gravitational wave is emitted by any matter moving with asymmetric acceleration. For a wave of significant amplitude to occur, an extremely large mass of the emitter and/or enormous accelerations are required; the amplitude of the gravitational wave is directly proportional first derivative of acceleration and the mass of the generator, that is ~ . However, if an object is moving at an accelerated rate, this means that some force is acting on it from another object. In turn, this other object experiences the opposite effect (according to Newton’s 3rd law), and it turns out that m 1 a 1 = − m 2 a 2 . It turns out that two objects emit gravitational waves only in pairs, and as a result of interference they are mutually canceled out almost completely. Therefore, gravitational radiation in the general theory of relativity always has the multipole character of at least quadrupole radiation. In addition, for non-relativistic emitters in the expression for the radiation intensity there is a small parameter where is the gravitational radius of the emitter, r- his characteristic size, T- characteristic period of movement, c- speed of light in vacuum.

The strongest sources of gravitational waves are:

• colliding (giant masses, very small accelerations),
• gravitational collapse of a binary system of compact objects (colossal accelerations with a fairly large mass). As a special and most interesting case - the merger of neutron stars. In such a system, the gravitational-wave luminosity is close to the maximum Planck luminosity possible in nature.

Gravitational waves emitted by a two-body system

Two bodies moving in circular orbits around a common center of mass

Two gravitationally bound bodies with masses m 1 and m 2, moving non-relativistically ( v << c) in circular orbits around their common center of mass at a distance r from each other, emit gravitational waves of the following energy, on average over the period:

As a result, the system loses energy, which leads to the convergence of bodies, that is, to a decrease in the distance between them. Speed ​​of approach of bodies:

For the Solar System, for example, the greatest gravitational radiation is produced by the and subsystem. The power of this radiation is approximately 5 kilowatts. Thus, the energy lost by the Solar System to gravitational radiation per year is completely negligible compared to the characteristic kinetic energy of bodies.

Gravitational collapse of a binary system

Any double star, when its components rotate around a common center of mass, loses energy (as assumed - due to the emission of gravitational waves) and, in the end, merges together. But for ordinary, non-compact, double stars, this process takes a very long time, much longer than the present age. If a compact binary system consists of a pair of neutron stars, black holes, or a combination of both, then the merger can occur within several million years. First, the objects come closer together, and their period of revolution decreases. Then, at the final stage, a collision and asymmetric gravitational collapse occurs. This process lasts a fraction of a second, and during this time energy is lost into gravitational radiation, which, according to some estimates, amounts to more than 50% of the mass of the system.

Basic exact solutions of Einstein's equations for gravitational waves

Bondi-Pirani-Robinson body waves

These waves are described by a metric of the form . If we introduce a variable and a function, then from the general relativity equations we obtain the equation

Takeno Metric

has the form , -functions satisfy the same equation.

Rosen metric

Where to satisfy

Perez metric

Wherein

Cylindrical Einstein-Rosen waves

In cylindrical coordinates, such waves have the form and are executed

Registration of gravitational waves

Registration of gravitational waves is quite difficult due to the weakness of the latter (small distortion of the metric). The devices for registering them are gravitational wave detectors. Attempts to detect gravitational waves have been made since the late 1960s. Gravitational waves of detectable amplitude are born during the collapse of a binary. Similar events occur in the surrounding area approximately once a decade.

On the other hand, the general theory of relativity predicts the acceleration of the mutual rotation of binary stars due to the loss of energy due to the emission of gravitational waves, and this effect is reliably recorded in several known systems of binary compact objects (in particular, pulsars with compact companions). In 1993, “for the discovery of a new type of pulsar, which provided new opportunities in the study of gravity” to the discoverers of the first double pulsar PSR B1913+16, Russell Hulse and Joseph Taylor Jr. was awarded the Nobel Prize in Physics. The acceleration of rotation observed in this system completely coincides with the predictions of general relativity for the emission of gravitational waves. The same phenomenon was recorded in several other cases: for the pulsars PSR J0737-3039, PSR J0437-4715, SDSS J065133.338+284423.37 (usually abbreviated J0651) and the system of binary RX J0806. For example, the distance between the two components A and B of the first binary star of the two pulsars PSR J0737-3039 decreases by about 2.5 inches (6.35 cm) per day due to energy loss to gravitational waves, and this occurs in agreement with general relativity . All these data are interpreted as indirect confirmation of the existence of gravitational waves.

According to estimates, the strongest and most frequent sources of gravitational waves for gravitational telescopes and antennas are catastrophes associated with the collapse of binary systems in nearby galaxies. It is expected that in the near future several similar events per year will be recorded on improved gravitational detectors, distorting the metric in the vicinity by 10 −21 -10 −23 . The first observations of an optical-metric parametric resonance signal, which makes it possible to detect the effect of gravitational waves from periodic sources such as a close binary on the radiation of cosmic masers, may have been obtained at the radio astronomical observatory of the Russian Academy of Sciences, Pushchino.

Another possibility of detecting the background of gravitational waves filling the Universe is high-precision timing of distant pulsars - analysis of the arrival time of their pulses, which characteristically changes under the influence of gravitational waves passing through the space between the Earth and the pulsar. Estimates for 2013 indicate that timing accuracy needs to be improved by about one order of magnitude to detect background waves from multiple sources in our Universe, a task that could be accomplished before the end of the decade.

According to modern concepts, our Universe is filled with relic gravitational waves that appeared in the first moments after. Their registration will make it possible to obtain information about the processes at the beginning of the birth of the Universe. On March 17, 2014 at 20:00 Moscow time at the Harvard-Smithsonian Center for Astrophysics, an American group of researchers working on the BICEP 2 project announced the detection of non-zero tensor disturbances in the early Universe by the polarization of the cosmic microwave background radiation, which is also the discovery of these relict gravitational waves . However, almost immediately this result was disputed, since, as it turned out, the contribution was not properly taken into account. One of the authors, J. M. Kovats ( Kovac J. M.), admitted that “the participants and science journalists were a bit hasty in interpreting and reporting the data from the BICEP2 experiment.”

Experimental confirmation of existence

The first recorded gravitational wave signal. On the left is data from the detector in Hanford (H1), on the right - in Livingston (L1). Time is counted from September 14, 2015, 09:50:45 UTC. To visualize the signal, it is filtered with a frequency filter with a passband of 35-350 Hertz to suppress large fluctuations outside the high sensitivity range of the detectors; band-stop filters were also used to suppress the noise of the installations themselves. Top row: voltages h in the detectors. GW150914 first arrived at L1 and 6 9 +0 5 −0 4 ms later to H1; For visual comparison, data from H1 are shown in the L1 graph in reversed and time-shifted form (to account for the relative orientation of the detectors). Second row: voltages h from the gravitational wave signal, passed through the same 35-350 Hz bandpass filter. The solid line is the result of numerical relativity for a system with parameters compatible with those found based on the study of the GW150914 signal, obtained by two independent codes with a resulting match of 99.9. The gray thick lines are the 90% confidence regions of the waveform reconstructed from the detector data by two different methods. The dark gray line models the expected signals from the merger of black holes, the light gray line does not use astrophysical models, but represents the signal as a linear combination of sinusoidal-Gaussian wavelets. The reconstructions overlap by 94%. Third row: Residual errors after extracting the filtered prediction of the numerical relativity signal from the filtered signal of the detectors. Bottom row: A representation of the voltage frequency map, showing the increase in the dominant frequency of the signal over time.

February 11, 2016 by the LIGO and VIRGO collaborations. The merger signal of two black holes with an amplitude at maximum of about 10 −21 was recorded on September 14, 2015 at 9:51 UTC by two LIGO detectors in Hanford and Livingston, 7 milliseconds apart, in the region of maximum signal amplitude (0.2 seconds) combined the signal-to-noise ratio was 24:1. The signal was designated GW150914. The shape of the signal matches the prediction of general relativity for the merger of two black holes with masses of 36 and 29 solar masses; the resulting black hole should have a mass of 62 solar and a rotation parameter a= 0.67. The distance to the source is about 1.3 billion, the energy emitted in tenths of a second in the merger is the equivalent of about 3 solar masses.

Story

The history of the term “gravitational wave” itself, the theoretical and experimental search for these waves, as well as their use for studying phenomena inaccessible to other methods.

• 1900 - Lorentz suggested that gravity “...can spread at a speed no greater than the speed of light”;
• 1905 - Poincaré first introduced the term gravitational wave (onde gravifique). Poincaré, on a qualitative level, removed the established objections of Laplace and showed that the corrections associated with gravitational waves to the generally accepted Newtonian laws of gravity of order cancel, thus the assumption of the existence of gravitational waves does not contradict observations;
• 1916 - Einstein showed that, within the framework of general relativity, a mechanical system will transfer energy to gravitational waves and, roughly speaking, any rotation relative to fixed stars must sooner or later stop, although, of course, under normal conditions, energy losses of the order of magnitude are negligible and practically not measurable (in In this work, he also mistakenly believed that a mechanical system that constantly maintains spherical symmetry can emit gravitational waves);
• 1918 - Einstein derived a quadrupole formula in which the emission of gravitational waves turns out to be an effect of order , thereby correcting the error in his previous work (an error remained in the coefficient, the wave energy is 2 times less);
• 1923 - Eddington - questioned the physical reality of gravitational waves "...propagating...at the speed of thought." In 1934, when preparing the Russian translation of his monograph “The Theory of Relativity,” Eddington added several chapters, including chapters with two options for calculating energy losses by a rotating rod, but noted that the methods used for approximate calculations of general relativity, in his opinion, are not applicable to gravitationally bound systems , so doubts remain;
• 1937 - Einstein, together with Rosen, investigated cylindrical wave solutions to the exact equations of the gravitational field. During the course of these studies, they began to doubt that gravitational waves may be an artifact of approximate solutions of the general relativity equations (correspondence regarding a review of the article “Do gravitational waves exist?” by Einstein and Rosen is known). Later, he found an error in his reasoning; the final version of the article with fundamental changes was published in the Journal of the Franklin Institute;
• 1957 - Herman Bondi and Richard Feynman proposed the “beaded cane” thought experiment in which they substantiated the existence of physical consequences of gravitational waves in general relativity;
• 1962 - Vladislav Pustovoit and Mikhail Herzenstein described the principles of using interferometers to detect long-wave gravitational waves;
• 1964 - Philip Peters and John Matthew theoretically described gravitational waves emitted by binary systems;
• 1969 - Joseph Weber, founder of gravitational wave astronomy, reports the detection of gravitational waves using a resonant detector - a mechanical gravitational antenna. These reports give rise to a rapid growth of work in this direction, in particular, Rainier Weiss, one of the founders of the LIGO project, began experiments at that time. To date (2015), no one has been able to obtain reliable confirmation of these events;
• 1978 - Joseph Taylor reported the discovery of gravitational radiation in dual system pulsar PSR B1913+16. The research of Joseph Taylor and Russell Hulse deserves Nobel Prize in physics for 1993. As of early 2015, three post-Keplerian parameters, including period reduction due to gravitational wave emission, had been measured for at least 8 such systems;
• 2002 - Sergey Kopeikin and Edward Fomalont used ultra-long-baseline radio wave interferometry to measure the deflection of light in the gravitational field of Jupiter in dynamics, which for a certain class of hypothetical extensions of general relativity makes it possible to estimate the speed of gravity - the difference from the speed of light should not exceed 20% (this interpretation does not generally accepted);
• 2006 - the international team of Martha Bourgay (Parkes Observatory, Australia) reported significantly more accurate confirmation of general relativity and its correspondence to the magnitude of gravitational wave radiation in the system of two pulsars PSR J0737-3039A/B;
• 2014 - Astronomers at the Harvard-Smithsonian Center for Astrophysics (BICEP) reported the detection of primordial gravitational waves while measuring fluctuations in the cosmic microwave background radiation. At the moment (2016), the detected fluctuations are considered not to be of relict origin, but are explained by the emission of dust in the Galaxy;
• 2016 - international LIGO team reported the detection of the gravitational wave transit event GW150914. For the first time, direct observation of interacting massive bodies in ultra-strong gravitational fields with ultra-high relative velocities (< 1,2 × R s , v/c >0.5), which made it possible to verify the correctness of general relativity with an accuracy of several post-Newtonian terms of high orders. The measured dispersion of gravitational waves does not contradict previously made measurements of the dispersion and upper bound on the mass of a hypothetical graviton (< 1,2 × 10 −22 эВ), если он в некотором гипотетическом расширении ОТО будет существовать.



The official day of discovery (detection) of gravitational waves is February 11, 2016. It was then, at a press conference held in Washington, that the leaders of the LIGO collaboration announced that a team of researchers had managed to record this phenomenon for the first time in human history.

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 his General Theory of Relativity (GTR). 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 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.), until recently it was not possible to practically detect the presence of this type of wave interaction between bodies.

Is gravity an illusion?

In general, in the light of the Theory of Relativity, gravity can hardly be called a force. disturbances or curvatures of the space-time continuum. A good example A stretched piece of fabric can serve as an illustration of this postulate. Under the weight of a massive object placed on such a surface, a depression is formed. Other objects, when moving near this anomaly, will change the trajectory of their movement, as if being “attracted”. And what more weight object (the larger the diameter and depth of curvature), the higher the “force of attraction”. As it moves across the fabric, one can observe the appearance of diverging “ripples”.

Something similar happens in outer 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 enormous accelerations.

physical characteristics

Fluctuations in the space-time metric manifest themselves as changes in the gravitational field. This phenomenon is otherwise called space-time ripples. The gravitational wave affects the encountered bodies and objects, compressing and stretching them. The magnitude of the deformation is very insignificant - about 10 -21 from the original size. The whole difficulty of detecting this phenomenon was that researchers needed to learn how to measure and record such changes using appropriate equipment. The power of gravitational radiation is also extremely small - for the entire solar system it is several kilowatts.

The speed of propagation of gravitational waves depends slightly on the properties of the conducting medium. The amplitude of oscillations gradually decreases with distance from the source, but never reaches zero. The frequency ranges from several tens to hundreds of hertz. The speed of gravitational waves in the interstellar medium approaches the speed of light.

Circumstantial evidence

The first theoretical confirmation of the existence of gravitational waves was obtained by the American astronomer Joseph Taylor and his assistant Russell Hulse in 1974. Studying the vastness of the Universe using the Arecibo Observatory radio telescope (Puerto Rico), 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). Every year the circulation period, originally 3.75 hours, is reduced by 70 ms. This value is fully consistent with the conclusions from the general relativity equations, which predict an increase in the rotation speed of such systems due to the expenditure of energy on 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.

Escaping gravitational wave

The first announcement about the detection of gravitational waves came from 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-vibration-insulated solid two-meter aluminum cylinder equipped with sensitive piezoelectric sensors. The amplitude of the oscillations allegedly recorded by Weber turned out to be more than a million times higher than the expected value. Attempts by other scientists to repeat the “success” of the American physicist using similar equipment 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 the “gravitational boom”, which attracted many specialists to this area of ​​research. By the way, Joseph Weber himself was sure until the end of his days that he received gravitational waves.

Improving receiving equipment

In the 70s, scientist Bill Fairbank (USA) developed the design of a gravitational wave antenna, cooled using SQUIDS - ultra-sensitive magnetometers. The technologies existing at that time did not allow the inventor to see his product realized in “metal”.

The Auriga gravitational detector at the National Legnar Laboratory (Padua, Italy) is designed using this principle. 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 insulated vacuum chamber cooled almost to absolute zero. To record and detect shocks, an auxiliary kilogram resonator and a computer-based measuring complex are used. The stated sensitivity of the equipment is 10 -20.

Interferometers

The operation of interference detectors of gravitational waves is based on the same principles on which the Michelson interferometer operates. The laser beam emitted by the source is divided into two streams. After multiple reflections and travels along the arms of the device, the flows are again brought together, and based on the final one it is judged whether any disturbances (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 the vacuum tunnels is 600 meters.
• TAMA (Japan) with shoulders of 300 m.
• VIRGO (Pisa, Italy) is a joint French-Italian project launched in 2007 with three kilometers of tunnels.
• LIGO (USA, Pacific Coast), which has been hunting for gravitational 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. It includes two observatories, separated by 3 thousand km, in and Washington (the cities of Livingston and Hanford) 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 gravitational 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 the coveted value of 10 -21. The updated project started in September 2015, and the efforts of more than a thousand collaboration employees were rewarded with the results obtained.

Gravitational waves detected

On September 14, 2015, advanced LIGO detectors, with an interval of 7 ms, recorded gravitational waves reaching our planet from the largest event that occurred on the outskirts of the observable Universe - the merger of two large black holes with masses 29 and 36 times greater than 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 consumed in a matter of fractions of a second by emitting gravitational waves. The recorded 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 eliminated. 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 size of the interferometer arms was 10 -19 m - this value is the same number of times smaller than the diameter of an atom, as the atom itself is smaller than an orange.

Future prospects

The 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 disturbances in gravitational fields. Activation of work in this direction can tell a lot of new things about the main stages of the development of the Universe, about processes that are difficult or impossible to observe in traditional ranges. There is no doubt that black holes, whose gravitational waves will be detected in the future, will tell a lot about their nature.

To study the cosmic microwave background radiation, which can tell us 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 no earlier than in 30-40 years.

Wave your hand and gravitational waves will run throughout the Universe.
S. Popov, M. Prokhorov. Phantom Waves of the Universe

An event has occurred in astrophysics that has been awaited for decades. After half a century of searching, gravitational waves, the vibrations of space-time itself, predicted by Einstein a hundred years ago, have finally been discovered. On September 14, 2015, the upgraded 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 approximately 1.3 billion light years away. 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 previously inaccessible effects of strong gravity.

Gravitational waves

You can come up with different theories of gravity. 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 scales solar system, and they point to one particular theory - the general theory of 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 container for physical phenomena, then in GTR it itself becomes a phenomenon, a dynamic quantity that changes in accordance with the laws of GTR. It is these distortions of space-time relative to a smooth 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 a crucial prediction: gravitational waves. These are distortions of space-time that are capable of “breaking away from the source” and, self-sustaining, flying away. This is gravity in itself, no one's, its own. Albert Einstein finally formulated general relativity in 1915 and almost immediately realized that the equations he derived 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 emit gravitational waves: planets, a stone thrown upward, or a wave of a hand. The problem, however, is that the gravitational interaction is so weak that no experimental setup can detect the emission of gravitational waves from ordinary “emitters.”

To “chase” a powerful wave, you need to greatly distort space-time. The ideal option is two black holes rotating around each other in a close dance, at a distance of the order of their gravitational radius (Fig. 2). The distortions of the metric will be so strong that a noticeable part of the energy of this pair will be emitted into gravitational waves. Losing energy, the pair will move closer together, spinning faster and 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 emitted energy goes not into light, not into particles, but into vibrations of space. The emitted 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 oscillations will be generated by 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-calculated profile, shown in Fig. 3. The period of oscillation is determined by the orbital motion of two objects around each other. Gravitational waves carry away energy; as a result, objects come closer together and spin faster - and this is visible both in the acceleration of oscillations and in the increase in amplitude. At some point, a merger occurs, the last strong wave is emitted, and then a high-frequency “after-ring” follows ( ringdown) - the trembling of the resulting 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.

Fluctuations in 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 weakens with distance, similar to how the brightness of a point source decreases with distance from it. When a burst from a distant galaxy reaches Earth, the metric fluctuations will be on the order of 10 −22 or even less. In other words, the distance between objects physically unrelated to each other will periodically increase and decrease by such a relative amount.

The order of magnitude of this number is easy to obtain from scaling considerations (see article by V. M. Lipunov). At the moment of merger of neutron stars or black holes of stellar masses, the distortions of the metric right next to them are very large - on the order of 0.1, which is why gravity is strong. Such a severe distortion affects an area on the order of the size of these objects, that is, several kilometers. As you move away from the source, the amplitude of the oscillation decreases in inverse proportion to the distance. This means that at a distance of 100 Mpc = 3·10 21 km the amplitude of oscillations will drop by 21 orders of magnitude and become about 10 −22.

Of course, if the merger occurs in our home galaxy, the tremors of space-time that reach the Earth will be much stronger. But such events occur once every few thousand years. Therefore, you should really count only on 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 it was even awarded the Nobel Prize in Physics for 1993. Long-term observations of the pulsar in the binary system PSR B1913+16 have shown that the orbital period decreases at exactly the same rate as predicted by general relativity, taking into account energy losses due to gravitational radiation. For this reason, almost none of the scientists doubt 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 from the University of Maryland designed the first resonant detector: a solid two-meter aluminum cylinder with sensitive piezoelectric sensors on the sides and good vibration isolation from extraneous vibrations (Fig. 4). When a gravitational wave passes, the cylinder resonates in time with the distortions of space-time, which is what the sensors should register. Weber built several such detectors, and in 1969, after analyzing their readings during one of the sessions, he directly stated that he had registered the “sound of gravitational waves” in several detectors at once, spaced two kilometers apart (J. Weber, 1969 Evidence for Discovery of Gravitational Radiation). The amplitude of oscillations he declared turned out to be incredibly large, on the order of 10 −16, that is, a million times greater than the typical expected value. Weber's message was met with great skepticism by the scientific community; Moreover, other experimental groups, armed with similar detectors, were unable to subsequently catch a single similar signal.

However, Weber's efforts gave impetus to this entire field of research and launched the hunt for waves. Since the 1970s, through 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). There is an interesting story about those times 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 was the first to come up with the concept of a standard quantum measurement limit - a key limitation in optical measurements - and showed how they could in principle be overcome. Weber's resonant circuit was improved, and thanks to deep cooling of the installation, noise was dramatically reduced (see the list and history of these projects). However, the accuracy of such all-metal detectors was still insufficient to reliably detect expected events, and besides, they were tuned to resonate only at a very narrow frequency range around the kilohertz.

Detectors that used more than one resonating object, but tracked the distance between two unrelated, independently suspended bodies, such as two mirrors, seemed much more promising. Due to the vibration of space caused by the gravitational wave, the distance between the mirrors will be either a little larger or a little smaller. Moreover, the longer the arm is, 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 range of frequencies, from 10 hertz to 10 kilohertz, and this is precisely the range in which merging pairs of neutron stars or stellar-mass black holes will emit.

The modern implementation of this idea based on the Michelson interferometer looks like this (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 is reunited 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 “quiet” state, the lengths are selected so that the two beams, after reuniting, cancel 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 influence of gravitational waves, the compensation of the two beams becomes incomplete and the photodetector catches the light. And the stronger the offset, the brighter the light the photosensor will see.

The words “microscopic displacement” don’t even come close to conveying the 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 an arm length of 4 km, this corresponds to oscillations of space-time with an amplitude of 10 −10. Noticing the displacement of mirrors by the diameter of an atom is also not a problem - it is enough to fire a laser beam, which will run back and forth thousands of times and obtain the desired phase shift. But this also gives a maximum of 10 −14. And we need to go down the displacement scale millions more times, that is, learn to register a 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, which hangs on another suspension, that 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 too much powerful beam will unevenly heat the optical elements, which will adversely affect the properties of the beam itself. This effect must be somehow compensated, and for this in the 2000s, an entire research program was launched on this subject (for a story about this research, see the news Obstacle overcome on the way to a highly sensitive gravitational wave detector, “Elements”, 06.27.2006 ). Finally, there are purely fundamental physical limitations related to the quantum behavior of photons in a cavity and the uncertainty principle. They limit the sensitivity of the sensor to a value called the standard quantum limit. However, physicists, using a cleverly prepared quantum state of laser light, have already learned to overcome it (J. Aasi et al., 2013. Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light).

A whole list of countries are participating in the race for gravitational waves; Russia has its own installation, at the Baksan Observatory, and, by the way, it is described in the documentary popular science film by Dmitry Zavilgelsky "Waiting for Waves and Particles". The leaders of this race are now two laboratories - the American LIGO project and the Italian Virgo detector. LIGO includes two identical detectors, located in Hanford (Washington State) and Livingston (Louisiana) and separated by 3000 km from each other. Having two settings is important for two reasons. Firstly, the 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 noticeably.

Strictly speaking, the idea of ​​interferometric detection of gravitational waves was first proposed by Soviet physicists M.E. Herzenstein and V.I. Pustovoit back in 1962. At that time, the laser had just been invented, and Weber began 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 of 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 the California Institute of Technology (Caltech). These are Rainer Weiss, who realized the idea of ​​​​an interferometric gravitational wave detector, Ronald Drever, who achieved stability of laser light sufficient for detection, and Kip Thorne, the theoretician behind the project, now well known to the general public as a scientific consultant movie "Interstellar". You can read about the early history of LIGO in a recent interview with Rainer Weiss and in the memoirs of John Preskill.

Activities related to the project of interferometric detection of gravitational waves began in the late 1970s, and at first many people also doubted the feasibility of this undertaking. However, after demonstrating a number of prototypes, the current LIGO design was written and approved. It was built throughout last decade XX century.

Although the initial impetus for the project came from the United States, LIGO is a truly international project. 15 countries have invested in it, financially and intellectually, and over a thousand people are members of the collaboration. Soviet and Russian physicists played an important role in the implementation of the project. 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 began operation in 2002 and until 2010 it hosted six scientific observation sessions. No gravitational wave bursts were reliably detected, and physicists were only able to set 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 was then “listening” to, the probability of a sufficiently powerful cataclysm was low: approximately once every few decades.

Finish line

From 2010 to 2015, the LIGO and Virgo collaborations radically modernized the equipment (Virgo, however, is still in the process of preparation). And now the long-awaited target was in direct 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 - at a distance of hundreds of megaparsecs. The volume of the Universe open to gravitational wave listening has increased tenfold compared to previous sessions.

Of course, it is impossible to predict when and where the next gravitational wave boom will occur. But the sensitivity of the updated detectors made it possible to count on several neutron star mergers per year, so the first burst could be expected already during the first four-month observation session. If we talk about the entire aLIGO project, which lasted several years, then the verdict was extremely clear: either bursts will fall one after another, or something in general relativity fundamentally does not work. Both will be big 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 circulated 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 during the analysis process, the collaboration members themselves cannot be completely sure that they are seeing a real gravitational wave burst. The fact is that in LIGO, a computer-generated burst is occasionally artificially introduced into the stream of real data. It’s called “blind injection,” and out of the entire group, only three people (!) have access to the system that carries it out at an arbitrary point in time. The team must track this surge, responsibly analyze it, and only at the very last stages of the analysis “the cards are revealed” and the members of the collaboration find out whether it was real event or a test of vigilance. By the way, in one such case in 2010, it even came to the point of 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 the inside of science. When a complex, inaccessible scientific task remains unanswerable for several years, this is a normal working moment. When it does not yield for more than one generation, it is perceived completely differently.

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, give reports, 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 regularly hear about unsuccessful attempts to solve it. You, of course, understand that somewhere active efforts are being made 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 eternal and almost unchanged on the scale of your scientific life element of physics. Like a task that has always been and will be.

And then - they solve it. And suddenly, on a scale of several days, you feel that the physical picture of the world has changed and that now it must be formulated in other terms and ask other questions.

For the people directly working on the search for gravitational waves, this task, of course, did not remain unchanged. They see the goal, they know what needs to be achieved. They, of course, hope that nature will also meet them halfway and throw a powerful splash in some nearby galaxy, but at the same time they understand that, even if nature is not so supportive, it will no longer be able to hide from scientists. The only question is when exactly they will be able to achieve their technical goals. A story about this sensation from a person who has been searching for gravitational waves for several decades can be heard in the already mentioned film "Waiting for Waves and Particles".

Opening

In Fig. Figure 7 shows the main result: the profile of the signal recorded by both detectors. It can be seen that against the background of noise, the oscillation first appears weakly, and then increases in amplitude and frequency. the desired shape. 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 one black hole with a mass of 62 solar masses (the error in all these numbers corresponds to 90 percent confidence interval, is 4 solar masses). The authors note 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 initial objects and the final black hole is 3 ± 0.5 solar masses. This gravitational mass defect was completely converted into the energy of emitted 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 purely by chance produced such a burst, such an event would have to wait 200 thousand years. This allows us to confidently state 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 region of the celestial sphere suitable in terms of parameters is 600 square degrees.

The LIGO collaboration did not limit itself to merely stating the fact of recording gravitational waves, but also carried out the first analysis of the implications this observation has for astrophysics. In the article Astrophysical implications of the binary black hole merger GW150914, published on the same day in the journal The Astrophysical Journal Letters, the authors estimated the frequency with which such black hole mergers occur. The result was at least one merger per cubic gigaparsec per year, which is consistent with the predictions of the most optimistic models in this regard.

What gravitational waves tell us

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 two blacks is important in itself. This is direct proof of the existence of black holes, and the existence of double black holes, and the reality of gravitational waves, and, generally speaking, 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 gave birth to them. Finally, if one grand explosion generates an optical, neutrino, and gravitational burst, then we can try to catch all of them, compare them with each other, and understand previously inaccessible details of what happened there. To be able to catch and compare such different signals from one event - the main objective all-signal astronomy.

When gravitational wave detectors become even more sensitive, they will be able to detect the shaking of space-time not at the moment of merger, but a few seconds before it. They will automatically send their warning signal to the general network of observation stations, and astrophysical telescope satellites, having calculated the coordinates of the proposed merger, will have time in these seconds to turn in the desired direction and begin photographing the sky before the optical burst begins.

Secondly, the gravitational wave burst will allow us to learn new things about neutron stars. A neutron star merger is, in fact, the latest and most extreme experiment on neutron stars that nature can perform for us, and we, as spectators, will only have to observe the results. The observational consequences of such a merger can be varied (Figure 10), and by collecting their statistics we can better understand the behavior of neutron stars in such exotic environments. Review current state cases in this direction can be found in the recent publication of S. Rosswog, 2015. Multi-messenger picture of compact binary mergers.

Thirdly, recording the burst that came from the supernova and comparing it with optical observations will finally make it possible to understand in detail what is happening inside, at the very beginning of the collapse. Now physicists still have difficulties with numerical modeling of this process.

Fourthly, physicists involved in the theory of gravity have a coveted “laboratory” for studying the effects of strong gravity. Until now, all the effects of general relativity that we could directly observe related to gravity in weak fields. We could guess what happens in conditions of strong gravity, when distortions of space-time begin to strongly interact with themselves, only from indirect manifestations, through the optical echo of cosmic catastrophes.

Fifthly, it appears new opportunity to test exotic theories of gravity. There are already many such theories in modern physics, see, for example, the chapter dedicated to them from the popular book “Gravity” by A. N. Petrov. Some of these theories resemble conventional general relativity in the limit of weak fields, but can be very different when gravity becomes very strong. Others admit 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 based on 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 astrophysicists themselves about what will change with the discovery of gravitational waves, in a selection on Postnauka.

Future plans

The prospects for gravitational wave astronomy are most encouraging. Now only the first, shortest observational session of the aLIGO detector has completed - and already for this a short time a clear signal was received. It would be more accurate to say this: the first signal was caught even before the official start, and the collaboration has not yet reported on all four months of work. Who knows, maybe there are already a few additional spikes there? One way or another, but further, as the sensitivity of detectors increases and the part of the Universe accessible to gravitational-wave observations expands, the number of recorded events will grow like an avalanche.

The expected session schedule for the LIGO-Virgo network is shown in Fig. 11. The second, six-month session will begin at the end of this year, the third session will take almost all of 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 the merger of neutron stars distant from us at distances of up to 200 Mpc. 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 tens of times compared to the first session.

The revamped Italian laboratory Virgo will also come into play later this year. Its sensitivity is slightly less than that of LIGO, but still quite decent. Due to the triangulation method, a trio of detectors spaced apart in space will make it possible to much better reconstruct 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, the LIGO-India detector is planned to be launched. As a result, after a few years, a whole network of gravitational wave detectors will operate and regularly record signals (Fig. 13).

Finally, there are plans to launch gravitational wave instruments into space, in particular the eLISA project. Two months ago, the first test satellite was launched into orbit, the task of which will be to test technologies. Real detection of gravitational waves is still a long way off. But when this group of satellites begins collecting data, it will open another window into the Universe - through low-frequency gravitational waves. This all-wave approach to gravitational waves is a major long-term goal for the field.

Parallels

The discovery of gravitational waves was the third time in history last years a case when physicists finally broke through all the obstacles and got to the previously unknown subtleties 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 for observing the universe and, at the same time, the effects of strong gravity have become available for direct study.

It must be said that there was no “freebie” from nature anywhere here. The search was carried out for a very long time, but it did not yield because then, decades ago, the equipment did not reach the result in terms of energy, scale, or sensitivity. It was the steady, targeted 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 very fact of discovery was not the end, but, on the contrary, the beginning of a new direction of research, it became a new tool for probing our world. The properties of the Higgs boson have become available for measurement - and in this 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 Coll. 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 articles accompanying the main discovery article.
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 magazine’s website Science on the search for 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. T. 170. pp. 743–752.
7) Peter R. Saulson.