“Oh, son of a bitch.

1.3 billion years ago, far, far from the Earth, the Solar system and even our Galaxy, two black holes came extremely close, one with a mass of 29 Suns, and the other with a mass of 36. 20 milliseconds - elusively short for a person - and they merge into one a large black hole, and the excess energy released during the collision causes space-time to ripple from the site of the cosmic catastrophe. On September 14, 2015, at 13:51 Moscow time, this wave reached the Earth and caused the mirrors of gravitational telescopes spaced four kilometers apart near the American cities of Livingston and Hanford to vibrate.

True, it fluctuates just a little, almost imperceptibly: with an amplitude of 10 -19 m (this is as many times smaller than the size of an atom as an orange is smaller than our entire planet). A sophisticated optical design to detect such disturbances, measurements on the verge of the quantum limit of accuracy, decades of theoretical work and several months of careful testing of the results. On February 11, at press conferences in Washington, Moscow, London, Paris and other cities, physicists from the international LIGO collaboration announced: humanity has detected gravitational waves for the first time and this cannot be a mistake. Ahead of us are gravitational telescopes, new physics and, who knows, maybe even new reality.

What it is?

Let's imagine a stretched fabric and several stones of different weights that we will place on it. The heavier the stone, the more it pushes through the fabric - in the same way, massive gravitational objects, according to Einstein’s theory of relativity, push through the fabric of space-time enveloping our world (more precisely, this fabric is our world, but that’s not about that now).

The easiest way to explain the impact of massive objects on space-time is through the example of black holes - they are so compact and heavy that they push space-time to the colossal depths of billions of millions of Mariana Trenches.

Even time in their vicinity begins to flow more slowly, and all the objects that fall into the giant funnel can no longer come out. Stars, dust, light quanta - everything remains trapped forever.

But what will happen if we not only put the stones, but also start rotating them? There will be ripples of folds across the fabric. Likewise, massive gravitational objects moving with variable acceleration generate spreading ripples in space-time around them - the same gravitational waves predicted by Albert Einstein a hundred years ago.

What emits gravitational waves?

Gravitational waves are emitted by any object that has mass and moves with variable acceleration - from a rotating black hole to a braking car and the reader of this text (it’s unlikely that you look at the screen without blinking - and here it is, acceleration). It’s just that gravitational waves from the last two objects cause such modest fluctuations in space that, from the point of view of modern quantum physics, they are simply impossible to register.

Therefore, physicists hoped to find gravitational waves only from massive objects moving with very large differences accelerations. More precisely, from a pair of such objects - simply according to Newton’s second law, if one heavy body moves with a large variable acceleration, then there must be a large force “setting” this movement. The easiest way for this force to appear is from the influence of some massive object nearby. Ideal candidates for such pairs of heavyweights are colliding galaxies and binary systems of black holes or neutron stars “living” together.

Haven't they really tried to find gravitational waves before?

We tried, and more than once. Some of the first experiments to detect gravitational waves were carried out back in the 70s at the Physics Faculty of Moscow State University in a group led by Professor Vladimir Braginsky. Then the device installed in the basement of the building seemed to register a signal, strong and stably repeated every evening. A sensation was brewing. The holiday was ruined by Braginsky himself, who realized that the device was recording seismic noise from the friendly entry of several trams into a nearby depot.

Researchers from the international BICEP collaboration were much less careful than Soviet physicists. Last year, they announced irrefutable traces of gravitational waves in the cosmic microwave background radiation, preserved from the first moments after the Big Bang. But the sensational antiquity turned out to be a mistake: when processing the data, scientists did not take into account the influence of cosmic dust.

Repeated attempts to detect gravitational waves have been made using other gravitational telescopes, including detectors from the LIGO collaboration.

What is LIGO and gravitational telescopes anyway?

LIGO (Laser Interferometer Gravitational-Wave Observatory) is the name of the observatory and at the same time an international collaboration of scientists from 14 countries. Russia is represented in LIGO by two scientific teams: the group of Alexander Sergeev from the Institute of Applied Physics of the Russian Academy of Sciences (Nizhny Novgorod) and the group led by Valery Mitrofanov, a professor at the Faculty of Physics at Moscow State University. The latter, by the way, was headed by the same Vladimir Braginsky until recently.

LIGO as an observatory has a detector and two interferometers: one installed in Livingston (Louisiana, USA), and the other in Hanford (Washington State, USA). Gravitational waves travel at the speed of light, and therefore the signal arrived at them with only a slight delay of 10 milliseconds.

The interferometers themselves are large L-shaped antennas with arms of four km each. Inside they contain high-quality optical circuits (that is, with a low level of extraneous noise), into which laser beams are launched. Under the influence of a gravitational wave, one shoulder should compress, and the other, on the contrary, should stretch. As a result, the laser beams travel slightly different distances along the arms and reach the exit with a small gap between them. Having come out, they come together again and form an interference pattern, according to the characteristics of which it is possible to reconstruct how the antenna arms changed and what was the gravitational wave that caused all this.

The LIGO observatory began its work back in 2002, but then its accuracy was not enough to register gravitational waves. In 2010, LIGO was closed for modernization and started working again only in 2014 (Advanced LIGO). Each design element was literally honed to the limit: for example, the mirrors between which the laser beams run (they are installed at the ends of each arm) were manufactured in a special factory. The European collaboration VIRGO built a similar telescope in parallel with LIGO, but it was not operational in September last year.

What signal did the scientists register?

This is what Valery Mitrofanov says. “At first there was a constant background noise, and suddenly at some point the test masses of the detector, those same mirrors, began to sway with a certain frequency. Then - once, and a break. Moreover, the signal was sent to two detectors at once: first, the gravitational wave approached one, and then, with a slight delay, to the other.”

The frequency of the signal was 150 Hz (it was with this frequency and amplitude of 10 -19 m that the mirrors oscillated, which became closer and then further away from each other), and after processing, its cause was found: the merger of two black holes at a distance of 1.3 billion light years years from Earth. The mass of one of them was equal to 29 solar masses, and the other - 36. The mass of the resulting black hole turned out to be slightly less: the lack of energy of three solar masses was just emitted during the collision in the form of gravitational waves.

The luminosity (that is, the total energy emitted) of this flare was 50 times greater than the luminosity of the entire visible Universe. If it were light and not gravity, observable space would become dazzlingly bright.

Luminosity? Frequency? I'm completely confused

Once again: scientists saw gravitational waves. This is not light (that is electromagnetic waves, or coupled vibrations of magnetic and electric fields propagating in space), and not sound (mechanical vibrations in a solid, liquid or gaseous medium, that is, propagating waves of high/low pressure). It’s just that all these phenomena (light, sound and gravity) can be described by the same equations and terms of wave physics.

Thus, each wave has an oscillation frequency, measured in hertz (Hz). Human hearing is capable of perceiving sounds at a frequency of 20 hertz - 20 kilohertz. The frequency of the incoming gravitational wave was 150 Hz, but this does not mean that it can be heard if you listen very carefully. At a press conference in Washington, scientists even turned on an alarming sound from this collision somewhere unimaginably far away, but it was just a beautiful interpretation of what would have happened if the researchers had registered not a gravitational wave, but exactly the same one in all parameters (frequency, amplitude, form) sound wave.

It's the same with luminosity. It is simply a term for determining the intensity of the radiation flux, used in an unusual but correct context. For example, in the case of light bulbs: the more intensely they emit, the brighter they glow, and the greater their luminosity. For colliding black holes: the greater their mass and the sharper their acceleration, the more powerful gravitational waves they will launch into space. Why, then, did this event of 50 luminosities of the Universe not compress the entire planet Earth into an accordion, but only shake the complexly arranged mirrors with some otherworldly breeze? But because gravitational interaction is much weaker than electromagnetic interaction (which is why it is so difficult to detect) - so much so that we only notice our attraction to the Earth, but, for example, not to a century-old oak tree, no matter how close we come to it.

Could this be a mistake?

Scientists are 100% confident in their findings. At the same time, they have already had false positives before, but outsiders never found out about it, so from the point of view of accuracy they can definitely be trusted.

“Firstly, this is a direct method for recording gravitational waves,” says Valery Mitrofanov. — And secondly, the results coincided with the predictions of theorists. We had a pattern of the gravitational wave signal from the merger of two black holes calculated using quantum physics. The signal was registered only if it fell into this pattern - this is what happened on September 14, and it is thanks to this pattern that we can reconstruct the masses of holes."

By the way, a leak of information about the imminent announcement of results appeared in mid-September. Then many discussed that, among other things, the signal could simply be mixed into the data by the scientists controlling the project to check its readiness. Now all participants in the collaboration unequivocally deny this possibility: the event did not occur during the operational launch of the system, but during a test and engineering one, in which false “injections” are not expected according to the instructions.

Did Russia participate?

Yes. As already mentioned, two laboratories from Moscow and Nizhny Novgorod are participating in the LIGO collaboration from Russia. They developed the design of the telescope (for example, it was Russian physicists who proposed hanging mirrors on quartz threads instead of steel, which reduced extraneous noise in the system) and struggled with quantum effects, distorting the signals of ultra-sensitive antennas.

“We have obtained a quantum device of macroscopic dimensions,” says MSU professor Sergei Vyatchanin. “This is the ultimate achievement of civilization at the moment: LIGO has almost reached the quantum limit of measurements. We were able to detect the displacement of two macroscopic objects weighing several kilograms and separated by several kilometers, with an accuracy predicted by Heisenberg's quantum uncertainty."

One of the initiators of the project, Professor Emeritus of the California Institute of Technology Kip Thorne, especially notes the contribution of our physicists to research. According to him, it was Vladimir Braginsky, a recognized world expert in the field of quantum gravity, who was the first to propose looking for gravitational waves from black holes and was the first to draw attention to the need to take into account quantum effects in measurements.

Let's go upward. First, scientists hope to acquire a third gravitational telescope for their system, which will no longer be located on Earth, but in space. Then, based on the characteristic delays in gravitational wave signals, researchers will be able to reconstruct the exact position of the sources - just as you can now find out your exact position on Earth by exchanging signals with three GPS satellites.

“This is the beginning of a new, gravitational-wave astronomy,” says Valery Mitrofanov. — Ancient people observed the Universe only in visible light. Then X-ray telescopes, radio telescopes, gamma-ray telescopes, neutrino observations appeared, and now we will see the sky in gravitational waves, which, by the way, are not screened by anything.”

“These waves cannot be stopped by any matter, and with them we will be able to understand much more about the Universe than we do now. And there are many mysteries—for example, the mystery of dark matter.”

In addition, a gravitational telescope can scan the entire sky at once: it does not need to be tuned to a specific point in space or to one frequency. Therefore, in the future, many unique astrophysical events will be first recorded by the gravitational telescope - it will be able to determine the exact location of objects, and then other observation tools will be adjusted using this data.

Not without it. Now scientists hope to see relic gravitational waves - the same ones that began to spread throughout the Universe almost immediately after the Big Bang.
“This will allow us to look into the very beginning of time,” says MSU professor Farit Khalili. “Gravitational interaction was the first to stop interacting with matter, and therefore the observation of cosmic microwave background radiation may make it possible to marry gravitational interactions with electromagnetic ones.”

The professor talks about the long-standing dream of physicists - the development of a coherent theory of quantum gravity, within the framework of which both electromagnetic and gravitational interactions are described using unified terms and equations. The maximum task on this path is the “theory of everything” or, as it is also called, the theory of the great unification. It combines all four known physical interactions (in addition to gravitational and electromagnetic interactions, there are also weak and strong interactions that explain the existence of elementary particles).

Einstein's theory of relativity should also become part of such a theory. “We will be able to look into the area where the general theory of relativity ends, since it predicts a singularity in a black hole,” says MSU professor Igor Bilenko. “Perhaps we will see a new physics that includes general relativity as one of its components, one of its special cases.”

Finally, something from this feast may fall to us, ordinary people who do not dream of a grand unified theory. “When Hertz discovered electromagnetic waves, he had no idea that it would lead to power lines, mobile phones and the Internet,” says MSU Associate Professor Sergei Strygin. “Perhaps humanity will someday learn not just to detect gravitational waves, but also to use them for their own purposes.”

What will it be? Transmission of information through time, like in the film “Interstellar”, for which Kip Thorne was the scientific consultant? Time travel? Something incredibly crazy? We can’t predict anything yet—we can only wait and watch.

Valentin Nikolaevich Rudenko shares the story of his visit to the city of Cascina (Italy), where he spent a week on the then just built “gravitational antenna” - the Michelson optical interferometer. On the way to the destination, the taxi driver asks why the installation was built. “People here think it’s for talking to God,” the driver admits.

– What are gravitational waves?

– A gravitational wave is one of the “carriers of astrophysical information.” There are visible channels of astrophysical information; telescopes play a special role in “distant vision”. Astronomers have also mastered low-frequency channels - microwave and infrared, and high-frequency channels - X-ray and gamma. In addition to electromagnetic radiation, we can detect streams of particles from Space. For this purpose, neutrino telescopes are used - large-sized detectors of cosmic neutrinos - particles that weakly interact with matter and are therefore difficult to register. Almost all theoretically predicted and laboratory-studied types of “carriers of astrophysical information” have been reliably mastered in practice. The exception was gravity - the most weak interaction in the microcosm and the most powerful force in the macrocosm.

Gravity is geometry. Gravitational waves are geometric waves, that is, waves that change the geometric characteristics of space when they pass through that space. Roughly speaking, these are waves that deform space. Strain is the relative change in the distance between two points. Gravitational radiation differs from all other types of radiation precisely in that it is geometric.

– Did Einstein predict gravitational waves?

– Formally, it is believed that gravitational waves were predicted by Einstein as one of the consequences of his general theory of relativity, but in fact their existence becomes obvious already in the special theory of relativity.

The theory of relativity suggests that due to gravitational attraction, gravitational collapse is possible, that is, an object being pulled together as a result of collapse, roughly speaking, to a point. Then the gravity is so strong that light cannot even escape from it, so such an object is figuratively called a black hole.

– What is the peculiarity of gravitational interaction?

A feature of gravitational interaction is the principle of equivalence. According to it, the dynamic response of a test body in a gravitational field does not depend on the mass of this body. Simply put, all bodies fall with the same acceleration.

Gravitational interaction is the weakest we know today.

– Who was the first to try to catch a gravitational wave?

– The gravitational wave experiment was first conducted by Joseph Weber from the University of Maryland (USA). He created a gravitational detector, which is now kept in the Smithsonian Museum in Washington. In 1968-1972, Joe Weber conducted a series of observations on a pair of spatially separated detectors, trying to isolate cases of "coincidences". The coincidence technique is borrowed from nuclear physics. The low statistical significance of the gravitational signals obtained by Weber caused a critical attitude towards the results of the experiment: there was no confidence that gravitational waves had been detected. Subsequently, scientists tried to increase the sensitivity of Weber-type detectors. It took 45 years to develop a detector whose sensitivity was adequate to the astrophysical forecast.

During the start of the experiment, many other experiments took place before fixation; impulses were recorded during this period, but their intensity was too low.

– Why was the signal fixation not announced immediately?

– Gravitational waves were recorded back in September 2015. But even if a coincidence was recorded, before announcing it, it is necessary to prove that it is not accidental. The signal taken from any antenna always contains noise bursts (short-term bursts), and one of them can accidentally occur simultaneously with a noise burst on another antenna. It is possible to prove that the coincidence was not accidental only with the help of statistical estimates.

– Why are discoveries in the field of gravitational waves so important?

– The ability to register the relict gravitational background and measure its characteristics, such as density, temperature, etc., allows us to approach the beginning of the universe.

What's attractive is that gravitational radiation is difficult to detect because it interacts very weakly with matter. But, thanks to this same property, it passes without absorption from the objects most distant from us with the most mysterious, from the point of view of matter, properties.

We can say that gravitational radiation passes without distortion. The most ambitious goal is to explore the gravitational radiation that was separated from the primary matter in the Theory Big Bang, which was created at the moment of the creation of the Universe.

– Does the discovery of gravitational waves rule out quantum theory?

The theory of gravity assumes the existence of gravitational collapse, that is, the contraction of massive objects to a point. At the same time, the quantum theory developed by the Copenhagen School suggests that, thanks to the uncertainty principle, it is impossible to simultaneously indicate exactly such parameters as the coordinate, speed and momentum of a body. There is an uncertainty principle here; it is impossible to determine the exact trajectory, because the trajectory is both a coordinate and a speed, etc. It is only possible to determine a certain conditional confidence corridor within the limits of this error, which is associated with the principles of uncertainty. Quantum theory categorically denies the possibility of point objects, but describes them in a statistically probabilistic manner: it does not specifically indicate coordinates, but indicates the probability that it has certain coordinates.

The question of unifying quantum theory and the theory of gravity is one of the fundamental questions of creating a unified field theory.

They continue to work on it now, and the words “quantum gravity” mean a completely advanced area of ​​science, the border of knowledge and ignorance, where all the theorists in the world are now working.

– What can the discovery bring in the future?

Gravitational waves must inevitably form the foundation of modern science as one of the components of our knowledge. They play a significant role in the evolution of the Universe and with the help of these waves the Universe should be studied. Discovery promotes general development science and culture.

If you decide to go beyond the scope of today's science, then it is permissible to imagine gravitational telecommunication lines, jet devices using gravitational radiation, gravitational-wave introscopy devices.

– Do gravitational waves have anything to do with extrasensory perception and telepathy?

Dont Have. The described effects are the effects of the quantum world, the effects of optics.

Interviewed by Anna Utkina

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 in most other modern theories of gravity, gravitational waves are generated by the motion of 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 an important task of 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- its 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 in 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.


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 - general theory of relativity(OTO).

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. Perfect option- 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). 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 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, what longer length shoulder, 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). In two long, several kilometers long, perpendicular to each other vacuum chambers mirrors are hung. 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 a beam that is too powerful will unevenly heat the optical elements, which will have a detrimental effect on the properties of the beam itself. This effect needs to be compensated somehow, and for this, a whole research program on this subject was launched in the 2000s (for a story about this research, see the news Obstacle to highly sensitive gravitational wave detector overcome, “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).

Participates in the race for gravitational waves whole list countries; 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". ABOUT early history the creation of LIGO can be read 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 the last decade of the 20th 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, analyze it responsibly, and only at the most last stages analysis “the cards are revealed” and the members of the collaboration will 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 an eternal and almost unchanged element of physics on the scale of your scientific life. 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, an oscillation of the desired shape first appears weakly, and then increases in amplitude and frequency. Comparison with the results of numerical simulations made it possible to clarify 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, corresponding to a 90% 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 is the main goal 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 (Fig. 10), and by collecting their statistics, we can 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, 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 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 exciting. 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 there was Higgs boson 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 New physics. Thanks to the increased statistics of high-energy neutrinos, neutrino astrophysics takes 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.