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

In astrophysics, an event has occurred that has been awaited for decades. After half a century of searching, gravitational waves have finally been discovered, fluctuations in space-time itself, predicted by Einstein a hundred years ago. On September 14, 2015, the updated LIGO observatory detected a gravitational wave burst generated by the merger of two black holes with masses of 29 and 36 solar masses in a distant galaxy at a distance of about 1.3 billion light years. Gravitational-wave astronomy has become a full-fledged branch of physics; it has opened up a new way for us to observe the universe and will allow us to study the effects of strong gravity that were previously inaccessible.

Gravitational waves

Theories of gravity can come up with different. All of them will describe our world equally well, as long as we limit ourselves to one single manifestation of it - Newton's law of universal gravitation. But there are other, more subtle gravitational effects that have been experimentally tested on the scale of the solar system, and they point to one particular theory - general relativity (GR).

General relativity is not just a set of formulas, it is a fundamental view of the essence of gravity. If in ordinary physics space serves only as a background, a receptacle for physical phenomena, then in general relativity it itself becomes a phenomenon, a dynamic quantity that changes in accordance with the laws of general relativity. It is these distortions of space-time against a flat background - or, in the language of geometry, distortions of the space-time metric - that are felt as gravity. In short, general relativity reveals the geometric origin of gravity.

General Relativity has an all-important prediction: gravitational waves. These are distortions of space-time that are able to “break away from the source” and, self-sustaining, fly away. It's gravity in itself, no one's, its own. Albert Einstein finally formulated general relativity in 1915 and realized almost immediately that his equations allowed for the existence of such waves.

As with any honest theory, such a clear prediction of general relativity must be verified experimentally. Any moving body can radiate gravitational waves: planets, a stone thrown upwards, and a wave of a hand. The problem, however, is that the gravitational interaction is so weak that no experimental setups are able to detect the emission of gravitational waves from ordinary "emitters".

To "drive" a powerful wave, you need to distort space-time very strongly. The ideal option is two black holes rotating around each other in a tight dance, at a distance of the order of their gravitational radius (Fig. 2). The distortion of the metric will be so strong that a noticeable part of the energy of this pair will be radiated into gravitational waves. Losing energy, the pair will approach each other, spinning faster 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 radiated energy goes not into light, not into particles, but into vibrations of space. The radiated energy will make up a noticeable part of the initial mass of black holes, and this radiation will splash out in a fraction of a second. Similar fluctuations will generate mergers of neutron stars. A slightly weaker gravitational-wave release of energy also accompanies other processes, such as the collapse of a supernova core.

The gravitational wave burst from the merger of two compact objects has a very specific, well-computed profile, shown in Fig. 3. The oscillation period is given by the orbital motion of two objects around each other. Gravitational waves carry away energy; as a consequence, objects approach each other and spin faster - and this can be seen both in the acceleration of oscillations and in the increase in amplitude. At some point, a merger occurs, the last strong wave is ejected, and then a high-frequency "after-ring" follows ( ringdown) is the jitter of the formed black hole, which “throws off” all non-spherical distortions (this stage is not shown in the picture). Knowing this characteristic profile helps physicists look for the weak signal from such a merger in highly noisy detector data.

Oscillations of the space-time metric - the gravitational-wave echo of a grandiose explosion - will scatter throughout the Universe in all directions from the source. Their amplitude decreases with distance, similar to how the brightness of a point source decreases with distance from it. When a burst from a distant galaxy hits Earth, the fluctuations in the metric will be on the order of 10 −22 or even less. In other words, the distance between physically unrelated objects will periodically increase and decrease by such a relative value.

The order of magnitude of this number is easy to obtain from scaling considerations (see the article by V. M. Lipunov). At the time of the merger of neutron stars or black holes of stellar masses, the distortion of the metrics right next to them is very large - on the order of 0.1, which is why this is strong gravity. Such a severe distortion affects a region of the order of the size of these objects, that is, several kilometers. When moving away from the source, the amplitude of the oscillation falls inversely proportional to the distance. This means that at a distance of 100 Mpc = 3·10 21 km the amplitude of oscillations will fall by 21 orders of magnitude and become about 10 −22 .

Of course, if the merger occurs in our home galaxy, the space-time trembling that has reached the Earth will be much stronger. But such events occur once every few thousand years. Therefore, one should really count only on such a detector that will be able to sense the merger of neutron stars or black holes at a distance of tens to hundreds of megaparsecs, which means that it will cover many thousands and millions of galaxies.

Here it must be added that an indirect indication of the existence of gravitational waves has already been discovered, and even the Nobel Prize in Physics for 1993 was awarded for it. Long-term observations of the pulsar in the binary system PSR B1913+16 have shown that the orbital period decreases exactly at the rate predicted by general relativity, taking into account the energy loss to gravitational radiation. For this reason, practically none of the scientists doubts the reality of gravitational waves; the only question is how to catch them.

Search history

The search for gravitational waves started about half a century ago - and almost immediately turned into a sensation. Joseph Weber of the University of Maryland designed the first resonant detector: a solid two-meter aluminum cylinder with sensitive piezo sensors on the sides and good vibration isolation from extraneous vibrations (Fig. 4). With the passage of a gravitational wave, the cylinder will resonate in time with the distortions of space-time, which should be registered by the sensors. Weber built several such detectors, and in 1969, after analyzing their readings during one of the sessions, he stated in plain text that he had registered the “sound of gravitational waves” in several detectors at once, spaced two kilometers apart from each other (J. Weber, 1969 Evidence for Discovery of Gravitational Radiation). The oscillation amplitude he claimed turned out to be incredibly large, on the order of 10 −16 , that is, a million times larger than the typical expected value. Weber's message was met with great skepticism by the scientific community; besides, other experimental groups, armed with similar detectors, could not catch any such signal in the future.

However, Weber's efforts kick-started this entire area of ​​research and set off the hunt for the waves. Since the 1970s, thanks to the efforts of Vladimir Braginsky and his colleagues from Moscow State University, the USSR has also entered this race (see the absence of gravitational wave signals). An interesting story about those times is in the essay If a girl falls into a hole .... Braginsky, by the way, is one of the classics of the entire theory of quantum optical measurements; he first came up with the concept of the standard quantum measurement limit - a key limitation in optical measurements - and showed how they could in principle be overcome. The Weber resonant circuit was improved, and thanks to the deep cooling of the installation, the noise was drastically reduced (see the list and history of these projects). However, the accuracy of such all-metal detectors was still insufficient for reliable detection of the expected events, and besides, they are tuned to resonate only in a very narrow frequency range around a kilohertz.

Much more promising seemed to be detectors that use not one resonating object, but track the distance between two unrelated, independently suspended bodies, for example, two mirrors. Due to the fluctuation of space caused by the gravitational wave, the distance between the mirrors will be either a little more or a little less. In this case, the longer the arm length, the greater the absolute displacement will be caused by a gravitational wave of a given amplitude. These vibrations can be felt by a laser beam running between the mirrors. Such a scheme is capable of detecting oscillations in a wide frequency range, from 10 hertz to 10 kilohertz, and this is exactly the interval in which merging pairs of neutron stars or stellar-mass black holes will radiate.

The modern implementation of this idea based on the Michelson interferometer is as follows (Fig. 5). Mirrors are suspended in two long, several kilometers long, perpendicular to each other vacuum chambers. At the entrance to the installation, the laser beam is split, goes through both chambers, is reflected from the mirrors, returns back and reunites in a translucent mirror. The quality factor of the optical system is extremely high, so the laser beam does not just pass back and forth once, but lingers in this optical resonator for a long time. In the “calm” state, the lengths are chosen so that the two beams, after recombination, extinguish each other in the direction of the sensor, and then the photodetector is in complete shadow. But as soon as the mirrors move a microscopic distance under the action of gravitational waves, the compensation of the two beams becomes incomplete and the photodetector picks up the light. And the stronger the bias, the brighter the light will be seen by the photosensor.

The words "microscopic displacement" do not even come close to conveying the full subtlety of the effect. The displacement of mirrors by the wavelength of light, that is, microns, is easy to notice even without any tricks. But with a shoulder length of 4 km, this corresponds to space-time oscillations with an amplitude of 10 −10 . It is also not a problem to notice the displacement of mirrors by the diameter of an atom - it is enough to launch a laser beam that will run back and forth thousands of times and get the desired phase incursion. But even this gives a strength of 10 −14 . And we need to go down the displacement scale millions more times, that is, learn how to register the mirror shift not even by one atom, but by thousandths of an atomic nucleus!

On the way to this truly amazing technology, physicists had to overcome many difficulties. Some of them are purely mechanical: you need to hang massive mirrors on a suspension that hangs on another suspension, that one on a third suspension, and so on - and all in order to get rid of extraneous vibration as much as possible. Other problems are also instrumental, but optical. For example, the more powerful the beam circulating in the optical system, the weaker the displacement of the mirrors can be detected by the photosensor. But a beam that is too powerful will unevenly heat the optical elements, which will adversely affect the properties of the beam itself. This effect must somehow be compensated, and for this, a whole research program was launched in this regard in the 2000s (for a story about this study, see the news An obstacle on the way to a highly sensitive gravitational wave detector has been overcome, "Elements", 06/27/2006 ). Finally, there are purely fundamental physical limitations related to the quantum behavior of photons in a resonator and the uncertainty principle. They limit the sensitivity of the sensor to a value called the standard quantum limit. However, physicists have already learned how to overcome it with the help of a cunningly prepared quantum state of laser light (J. Aasi et al., 2013. Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light).

There is a list of countries in the race for gravitational waves; Russia has its own installation, at the Baksan observatory, and, by the way, it is described in a documentary popular science film by Dmitry Zavilgelsky "Waiting for Waves and Particles". The leaders of this race are now two laboratories - the American project LIGO and the Italian Virgo detector. LIGO includes two identical detectors located in Hanford (Washington) and Livingston (Louisiana) and separated from each other by 3000 km. Having two setups is important for two reasons. First, a signal will be considered registered only if it is seen by both detectors at the same time. And secondly, by the difference in the arrival of a gravitational-wave burst at two installations - and it can reach 10 milliseconds - one can approximately determine from which part of the sky this signal came. True, with two detectors the error will be very large, but when Virgo comes into operation, the accuracy will increase markedly.

Strictly speaking, the idea of ​​interferometric detection of gravitational waves was first proposed by Soviet physicists M. E. Gertsenshtein and V. I. Pustovoit back in 1962. Then the laser had just been invented, and Weber started to create his resonant detectors. However, this article was not noticed in the West and, to tell the truth, did not influence the development of real projects (see the historical review Physics of gravitational wave detection: resonant and interferometric detectors).

The creation of the LIGO gravitational observatory was the initiative of three scientists from the Massachusetts Institute of Technology (MIT) and from the California Institute of Technology (Caltech). These are Rainer Weiss, who implemented the idea of ​​an interferometric gravitational wave detector, Ronald Drever, who achieved stability of laser light sufficient to register, and Kip Thorne, the theorist-inspirer of the project, now well known to the general public as a scientific consultant movie Interstellar. The early history of LIGO can be read in a recent interview with Rainer Weiss and in the memoirs of John Preskill.

The activity associated with the project of interferometric detection of gravitational waves began in the late 1970s, and at first the reality of this undertaking was also doubted by many. However, after demonstrating a number of prototypes, the current LIGO project was written and approved. It was built during the entire last decade of the 20th century.

Although the United States gave the initial impetus to the project, the LIGO observatory is a truly international project. 15 countries have invested in it, financially and intellectually, and over a thousand people are members of the collaboration. An important role in the implementation of the project was played by Soviet and Russian physicists. From the very beginning, the already mentioned group of Vladimir Braginsky from Moscow State University took an active part in the implementation of the LIGO project, and later the Institute of Applied Physics from Nizhny Novgorod also joined the collaboration.

The LIGO observatory was launched in 2002 and until 2010 it hosted six scientific observation sessions. No gravitational wave bursts were reliably detected, and physicists were only able to establish upper limits on the frequency of such events. This, however, did not surprise them too much: estimates showed that in that part of the Universe that the detector “listened to” at that time, the probability of a sufficiently powerful cataclysm was small: approximately once every several decades.

finish line

From 2010 to 2015, the LIGO and Virgo collaborations radically modernized the equipment (Virgo, however, is still in preparation). And now the long-awaited goal was in direct line of sight. LIGO - or rather, aLIGO ( Advanced LIGO) - was now ready to catch bursts generated by neutron stars at a distance of 60 megaparsecs, and black holes - hundreds of megaparsecs. The volume of the universe open for gravitational-wave listening has grown tenfold compared to previous sessions.

Of course, it is impossible to predict when and where the next gravitational-wave "bang" will take place. But the sensitivity of the updated detectors made it possible to count on several neutron star mergers per year, so that the first burst could be expected already during the first four-month observation session. If we talk about the entire aLIGO project lasting several years, then the verdict was extremely clear: either bursts will fall one after another, or something in general relativity does not work in principle. Both will be great discoveries.

From September 18, 2015 to January 12, 2016, the first aLIGO observation session took place. During all this time, rumors about the registration of gravitational waves were circulating on the Internet, but the collaboration remained silent: “we are collecting and analyzing data and are not yet ready to report the results.” An additional intrigue was created by the fact that in the process of analysis, the members of the collaboration themselves cannot be completely sure that they see a real gravitational wave surge. The fact is that in LIGO a burst generated on a computer is occasionally artificially introduced into the stream of real data. It is called "blind injection", blind injection, and out of the entire group, only three people (!) Have access to a system that performs it at an arbitrary moment in time. The team must track this surge, responsibly analyze it, and only at the very last stages of the analysis “cards are opened” and members of the collaboration will find out whether this was a real event or a test of vigilance. By the way, in one such case in 2010, it even came to writing an article, but the signal discovered then turned out to be just a “blind stuffing”.

Lyrical digression

To once again feel the solemnity of the moment, I propose to look at this story from the other side, from within science. When a complex, impregnable scientific task does not lend itself to several years, this is a normal working moment. When it does not give in for more than one generation, it is perceived in a completely different way.

As a schoolboy, you read popular science books and learn about this difficult to solve, but terribly interesting scientific riddle. As a student, you study physics, make presentations, and sometimes, appropriately or not, people around you remind you of its existence. Then you yourself do science, work in another area of ​​physics, but you regularly hear about unsuccessful attempts to solve it. Of course, you understand that somewhere active work is being done to solve it, but the final result for you as an outsider remains unchanged. The problem is perceived as a static background, as a decoration, as an element of physics that is eternal and almost unchanged on the scale of your scientific life. As a task that has always been and always will be.

And then - it is solved. And abruptly, on the scale of several days, you feel that the physical picture of the world has changed and that now it needs to be formulated in other terms and ask other questions.

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

Opening

On fig. 7 shows the main result: the profile of the signal recorded by both detectors. It can be seen that against the background of noise, at first, the oscillation of the desired shape appears weakly, and then increases in amplitude and frequency. Comparison with the results of numerical simulations made it possible to find out which objects we observed merging: these were black holes with masses of approximately 36 and 29 solar masses, which merged into a single black hole with a mass of 62 solar masses (the error of all these numbers, corresponding to a 90 percent confidence interval, is 4 solar masses). The authors remark in passing that the resulting black hole is the heaviest stellar-mass black hole ever observed. The difference between the total mass of the two original objects and the final black hole is 3±0.5 solar masses. This gravitational mass defect was completely transformed into the energy of radiated gravitational waves in about 20 milliseconds. Calculations showed that the peak gravitational wave power reached 3.6·10 56 erg/s, or, in terms of mass, approximately 200 solar masses per second.

The statistical significance of the detected signal is 5.1σ. In other words, if we assume that these statistical fluctuations overlapped each other and produced such a surge purely by chance, such an event would have to wait 200 thousand years. This allows us to state with confidence that the detected signal is not a fluctuation.

The time delay between the two detectors was approximately 7 milliseconds. This made it possible to estimate the direction of signal arrival (Fig. 9). Since there are only two detectors, the localization turned out to be very approximate: the area of ​​\u200b\u200bthe celestial sphere that is suitable in terms of parameters is 600 square degrees.

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

What are gravitational waves about?

The discovery of a new phenomenon after decades of searching is not the end, but only the beginning of a new branch of physics. Of course, the registration of gravitational waves from the merger of black two is important in itself. This is a direct proof of the existence of black holes, and the existence of binary black holes, and the reality of gravitational waves, and, speaking in general, proof of the correctness of the geometric approach to gravity, on which general relativity is based. But for physicists, it is no less valuable that gravitational-wave astronomy is becoming a new research tool, making it possible to study what was previously inaccessible.

First, it is a new way to view the Universe and study cosmic cataclysms. There are no obstacles for gravitational waves; they pass through everything in the Universe without any problems. They are self-sufficient: their profile carries information about the process that generated them. Finally, if one grandiose explosion gives rise to both an optical, a neutrino, and a gravitational burst, then you can try to catch all of them, compare them with each other, and sort out previously inaccessible details of what happened there. To be able to catch and compare such different signals from one event is the main goal of all-signal astronomy.

When gravitational wave detectors become even more sensitive, they will be able to register the jitter of space-time not at the very moment of the merger, but a few seconds before it. They will automatically send their warning signal to the general network of observation stations, and astrophysical satellite-telescopes, having calculated the coordinates of the proposed merger, will have time to turn in the right direction in these seconds and start shooting the sky before the start of the optical burst.

Secondly, the gravitational wave burst will allow you to learn new things about neutron stars,. The neutron star merger is, in fact, the latest and most extreme neutron star experiment that nature can put on for us, and we as 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 will be able to better understand the behavior of neutron stars in such exotic conditions. An overview of the current state of affairs in this direction can be found in the recent publication by S. Rosswog, 2015. Multi-messenger picture of compact binary mergers .

Thirdly, registration of a burst that came from a supernova and its comparison with optical observations will finally make it possible to sort out the details of what is going on inside, at the very beginning of the collapse. Now physicists still have difficulties with numerical simulation of this process.

Fourth, physicists involved in the theory of gravity have a coveted "laboratory" for studying the effects of strong gravity. So far, all the effects of general relativity that we have been able to directly observe have been related to gravity in weak fields. About what happens in conditions of strong gravity, when the distortions of space-time begin to strongly interact with themselves, we could guess only by indirect manifestations, through the optical echo of cosmic catastrophes.

Fifth, there is a new opportunity to test exotic theories of gravity. There are already many such theories in modern physics, see, for example, the chapter devoted to them from the popular book by A. N. Petrov "Gravity". Some of these theories resemble ordinary general relativity in the limit of weak fields, but may differ greatly from it when gravity becomes very strong. Others assume the existence of a new type of polarization for gravitational waves and predict a speed slightly different from the speed of light. Finally, there are theories that include additional spatial dimensions. What can be said about them on the basis of gravitational waves is an open question, but it is clear that some information can be profited from here. We also recommend reading the opinion of the astrophysicists themselves about what will change with the discovery of gravitational waves, in the selection on Postnauka.

Future plans

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

The expected schedule of LIGO-Virgo network sessions is shown in fig. 11. The second, six-month, session will begin at the end of this year, the third session will take almost the entire 2018, and at each stage the sensitivity of the detector will increase. Around 2020, aLIGO should reach its planned sensitivity, which will allow the detector to probe the Universe for neutron star mergers that are up to 200 Mpc away from us. For even more energetic black hole merger events, the sensitivity can reach almost a gigaparsec. One way or another, the volume of the Universe available for observation will increase tenfold more compared to the first session.

At the end of this year, the updated Italian laboratory Virgo will also enter the game. It has slightly less sensitivity than LIGO, but it's also quite decent. Due to the triangulation method, a trio of detectors spaced apart in space will make it possible to much better restore the position of sources on the celestial sphere. If now, with two detectors, the localization area reaches hundreds of square degrees, then three detectors will reduce it to tens. In addition, a similar KAGRA gravitational wave antenna is currently being built in Japan, which will begin operation in two to three years, and in India, around 2022, it is planned to launch the LIGO-India detector. As a result, a whole network of gravitational-wave detectors will operate and regularly record signals in a few years (Fig. 13).

Finally, there are plans to take gravitational wave instruments into space, notably the eLISA project. Two months ago, the first trial satellite was launched into orbit, the task of which will be to test technologies. It is still far from the real detection of gravitational waves. But as this constellation of satellites begins collecting data, it will open another window into the universe - through low-frequency gravitational waves. Such an all-wave approach to gravitational waves is the main goal of this field in the long term.

Parallels

The discovery of gravitational waves was already the third case in recent years when physicists finally broke through all the obstacles and got to the previously unknown intricacies of the structure of our world. In 2012, the Higgs boson was discovered - a particle predicted almost half a century ago. In 2013, the IceCube neutrino detector proved the reality of astrophysical neutrinos and began to "look at the universe" in a completely new, previously inaccessible way - through high-energy neutrinos. And now nature has succumbed to man once again: a gravitational-wave “window” has opened up for observing the universe and, at the same time, the effects of strong gravity have become available for direct study.

I must say, nowhere was there any "freebie" from nature. The search was conducted for a very long time, but it did not give in because then, decades ago, the equipment did not reach the result in terms of energy, scale, or sensitivity. It was the steady, purposeful development of technology that led to the goal, a development that was not stopped by either technical difficulties or the negative results of past years.

And in all three cases, the discovery itself was not the end, but, on the contrary, the beginning of a new direction of research, became a new tool for probing our world. The properties of the Higgs boson have become measurable - and in these data, physicists are trying to discern the effects of New Physics. Thanks to the increased statistics of high-energy neutrinos, neutrino astrophysics is taking its first steps. At least the same is now expected from gravitational-wave astronomy, and there is every reason for optimism.

Sources:
1) LIGO Scientific Col. and Virgo Coll. Observation of Gravitational Waves from a Binary Black Hole Merger // Phys. Rev. Lett. Published 11 February 2016.
2) Detection Papers - a list of technical papers accompanying the main discovery paper.
3) E. Berti. Viewpoint: The First Sounds of Merging Black Holes // Physics. 2016. V. 9. N. 17.

Review materials:
1) David Blair et al. Gravitational wave astronomy: the current status // arXiv:1602.02872 .
2) Benjamin P. Abbott and LIGO Scientific Collaboration and Virgo Collaboration. Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO and Advanced Virgo // Living Rev. Relativity. 2016. V. 19. N. 1.
3) O. D. Aguiar. The Past, Present and Future of the Resonant-Mass Gravitational Wave Detectors // Res. Astron. Astrophys. 2011. V. 11. N. 1.
4) The search for gravitational waves - a selection of materials on the journal's website Science in search of gravitational waves.
5) Matthew Pitkin, Stuart Reid, Sheila Rowan, Jim Hough. Gravitational Wave Detection by Interferometry (Ground and Space) // arXiv:1102.3355 .
6) V. B. Braginsky. Gravitational-wave astronomy: new measurement methods // UFN. 2000, vol. 170, pp. 743–752.
7) Peter R. Saulson.

Valentin Nikolaevich Rudenko shares the story of his visit to the city of Kashina (Italy), where he spent a week on the newly built "gravitational antenna" - Michelson's optical interferometer. On the way to the destination, the taxi driver is interested in what the installation was built for. “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, a special role in "far vision" belongs to telescopes. Astronomers have also mastered low-frequency channels - microwave and infrared, and high-frequency - X-ray and gamma. In addition to electromagnetic radiation, we can register particle flows from the Cosmos. For this, neutrino telescopes are used - large-sized detectors of cosmic neutrinos - particles that weakly interact with matter and therefore are difficult to register. Almost all theoretically predicted and laboratory-studied types of "carriers of astrophysical information" are reliably mastered in practice. The exception was gravitation - the weakest 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 as they travel through that space. Roughly speaking, these are waves that deform space. Deformation is the relative change in distance between two points. Gravitational radiation differs from all other types of radiation precisely in that they are 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, the contraction of an object as a result of collapse, roughly speaking, into a point. Then 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 the gravitational interaction?

A feature of the gravitational interaction is the principle of equivalence. According to him, 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.

The gravitational force 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 the gravitational detector, which is now kept in the Smithsonian Museum in Washington. In 1968-1972, Joe Weber made a series of observations with a pair of spaced apart detectors in an attempt to isolate instances of "coincidences". The reception of coincidences is borrowed from nuclear physics. The low statistical significance of the gravitational signals received by Weber caused a critical attitude to the results of the experiment: there was no certainty that gravitational waves could be detected. In the future, 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 prediction.

During the beginning of the experiment before fixation, many other experiments took place, impulses were recorded during this period, but they had too little intensity.

- Why was the fixing of the signal not announced immediately?

– Gravitational waves were recorded back in September 2015. But even if a coincidence was recorded, it is necessary to prove before declaring that it is not accidental. In the signal taken from any antenna, there are always 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 did not happen by chance only with the help of statistical estimates.

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

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

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

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

– 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 into 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 specify exactly such parameters as the position, velocity and momentum of a body at the same time. There is an uncertainty principle here, it is impossible to determine exactly the trajectory, because the trajectory is both a coordinate and a speed, etc. It is possible to determine only a certain conditional confidence corridor within 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 way: it does not specifically indicate the coordinates, but indicates the probability that it has certain coordinates.

The question of the unification of quantum theory and the theory of gravity is one of the fundamental questions of the creation of 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 theorists of the world are now working.

– What can the discovery give in the future?

Gravitational waves must inevitably form the foundation of modern science as one of the components of our knowledge. They are assigned a significant role in the evolution of the Universe and with the help of these waves the Universe should be studied. The discovery contributes to the overall development of science and culture.

If one decides to go beyond the scope of today's science, then it is permissible to imagine telecommunication gravitational communication lines, jet apparatus on gravitational radiation, gravitational-wave introscopy devices.

- Do gravitational waves have any relation to extrasensory perception and telepathy?

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

Interviewed by Anna Utkina

“Recently, a series of long-term experiments to directly observe gravitational waves has sparked strong scientific interest,” theoretical physicist Michio Kaku wrote in his 2004 book Einstein’s Cosmos. - The LIGO (Laser Gravitational Wave Interferometer) project may be the first to "see" gravitational waves, most likely from the collision of two black holes in deep space. LIGO is a physicist's dream come true, the first facility with enough power to measure gravitational waves."

Kaku's prediction came true: on Thursday, a group of international scientists from the LIGO observatory announced the discovery of gravitational waves.

Gravitational waves are fluctuations in space-time that "run away" from massive objects (such as black holes) moving with acceleration. In other words, gravitational waves are a propagating perturbation of space-time, a running deformation of absolute emptiness.

A black hole is a region in space-time whose gravitational attraction is so strong that even objects moving at the speed of light (including light itself) cannot leave it. The boundary separating a black hole from the rest of the world is called the event horizon: everything that happens inside the event horizon is hidden from the eyes of an external observer.

Erin Ryan Photo of the cake posted online by Erin Ryan.

Scientists began to catch gravitational waves half a century ago: it was then that the American physicist Joseph Weber became interested in Einstein's general theory of relativity (GR), took a sabbatical and began to study gravitational waves. Weber invented the first device to detect gravitational waves, and soon claimed to have recorded "the sound of gravitational waves." However, the scientific community denied his message.

However, it was thanks to Joseph Weber that many scientists turned into “wave chasers”. Today Weber is considered the father of the scientific direction of gravitational wave astronomy.

"This is the beginning of a new era of gravitational astronomy"

The LIGO observatory, where scientists recorded gravitational waves, consists of three laser installations in the United States: two are located in Washington state and one in Louisiana. Here is how Michio Kaku describes the operation of laser detectors: “The laser beam is split into two separate beams, which then go perpendicular to each other. Then, reflected from the mirror, they reconnect. If a gravitational wave passes through the interferometer (measuring device), the path lengths of the two laser beams will be perturbed and this will be reflected in their interference pattern. To make sure that the signal registered by the laser installation is not random, the detectors should be placed at different points on the Earth.

Only under the influence of a giant gravitational wave, much larger than our planet, will all detectors work simultaneously.

Now the LIGO collaboration has detected gravitational radiation caused by the merger of a binary system of black holes with masses of 36 and 29 solar masses into an object with a mass of 62 solar masses. “This is the first direct (it is very important that it is direct!) measurement of the action of gravitational waves,” Sergey Vyatchanin, a professor at the Faculty of Physics of Moscow State University, commented to the correspondent of the science department of Gazeta.Ru. - That is, a signal was received from the astrophysical catastrophe of the merger of two black holes. And this signal is identified - this is also very important! It is clear that this is from two black holes. And this is the beginning of a new era of gravitational astronomy, which will allow obtaining information about the Universe not only through optical, X-ray, electromagnetic and neutrino sources, but also through gravitational waves.

We can say that 90 percent of black holes have ceased to be hypothetical objects. Some doubt remains, but still, the signal that is caught fits painfully well with what countless simulations of the merger of two black holes predict in accordance with the general theory of relativity.

This is a strong argument that black holes exist. There is no other explanation for such a signal yet. Therefore, it is assumed that black holes exist.”

"Einstein would be very happy"

Gravitational waves were predicted by Albert Einstein (who, by the way, was skeptical about the existence of black holes) within the framework of his general theory of relativity. In general relativity, time is added to three spatial dimensions, and the world becomes four-dimensional. According to a theory that turned physics on its head, gravity is a consequence of the curvature of space-time under the influence of mass.

Einstein proved that any matter moving with acceleration creates a perturbation of space-time - a gravitational wave. This perturbation is the greater, the higher the acceleration and mass of the object.

Due to the weakness of gravitational forces compared to other fundamental interactions, these waves should have a very small magnitude, which is difficult to register.

When explaining general relativity to the humanities, physicists often ask them to imagine a stretched sheet of rubber on which massive balls are lowered. The balls push through the rubber, and the stretched sheet (which represents space-time) is deformed. According to general relativity, the entire universe is rubber, on which every planet, every star and every galaxy leave dents. Our Earth revolves around the Sun like a small ball rolled around the cone of a funnel formed as a result of the “punching” of space-time by a heavy ball.

HANDOUT/Reuters

The heavy ball is the Sun

It is likely that the discovery of gravitational waves, which is the main confirmation of Einstein's theory, claims the Nobel Prize in physics. “Einstein would be very happy,” said Gabriella Gonzalez, spokesperson for the LIGO collaboration.

According to scientists, it is too early to talk about the practical applicability of the discovery. “Although, did Heinrich Hertz (a German physicist who proved the existence of electromagnetic waves. - Gazeta.Ru) think that there would be a mobile phone? Not! We can’t imagine anything right now,” said Valery Mitrofanov, professor at the Faculty of Physics of Moscow State University. M.V. Lomonosov. - I am guided by the movie "Interstellar". He is criticized, yes, but even a wild man could imagine a magic carpet. And the flying carpet turned into a plane, and that's it. And here it is already necessary to imagine something very complex. In Interstellar, one of the moments is related to the fact that a person can travel from one world to another. If so, do you believe that a person can travel from one world to another, that there can be many universes - anything? I can't answer no. Because a physicist cannot answer such a question with “no”! Only if it contradicts some conservation laws! There are options that do not contradict known physical laws. So, travel around the worlds can be!

Yesterday, the world was shocked by a sensation: scientists have finally discovered gravitational waves, the existence of which Einstein predicted a hundred years ago. This is a breakthrough. The distortion of space-time (this is gravitational waves - now we will explain what's what) was discovered at the LIGO observatory, and one of its founders is - who would you think? - Kip Thorne, author of the book.

We tell why the discovery of gravitational waves is so important, what Mark Zuckerberg said and, of course, we share the story in the first person. Kip Thorne, like no one else, knows how the project works, what makes it unusual and what significance LIGO has for humanity. Yes, yes, everything is so serious.

Discovery of gravitational waves

The scientific world will forever remember the date of February 11, 2016. On this day, the participants of the LIGO project announced: after so many futile attempts, gravitational waves have been found. This is reality. In fact, they were discovered a little earlier: in September 2015, but yesterday the discovery was officially recognized. The Guardian believes that scientists will certainly receive the Nobel Prize in Physics.

The cause of gravitational waves is the collision of two black holes, which happened already ... a billion light years from Earth. Imagine how huge our universe is! Since black holes are very massive bodies, they ripple through space-time, distorting it a bit. So waves appear, similar to those that spread from a stone thrown into the water.

This is how you can imagine gravitational waves coming to the Earth, for example, from a wormhole. Drawing from the book “Interstellar. Science behind the scenes"

The resulting vibrations were converted into sound. Interestingly, the signal from gravitational waves comes at about the same frequency as our speech. So we can hear with our own ears how black holes collide. Hear what gravitational waves sound like.

And you know what? It is more recently that black holes are arranged differently than previously thought. But after all, there was no evidence at all that they existed in principle. And now there is. Black holes really "live" in the Universe.

So, according to scientists, a catastrophe looks like - a merger of black holes, -.

On February 11, a grandiose conference was held, which brought together more than a thousand scientists from 15 countries. Russian scientists were also present. And, of course, not without Kip Thorne. “This discovery is the beginning of an amazing, magnificent quest for people: the search and exploration of the curved side of the Universe - objects and phenomena created from distorted space-time. Collision of black holes and gravitational waves are our first remarkable samples,” said Kip Thorne.

The search for gravitational waves has been one of the main problems of physics. Now they are found. And Einstein's genius is confirmed again.

In October, we interviewed Sergei Popov, a Russian astrophysicist and well-known popularizer of science. He looked into the water! Autumn: “It seems to me that now we are on the verge of new discoveries, which is primarily due to the work of the LIGO and VIRGO gravitational wave detectors (Kip Thorne just made a great contribution to the creation of the LIGO project).” Amazing, right?

Gravitational waves, wave detectors and LIGO

Well, now for some physics. For those who really want to understand what gravitational waves are. Here's an artistic rendering of the tendex lines of two black holes orbiting each other, counterclockwise, and then colliding. Tendex lines generate tidal gravity. Move on. The lines that emanate from the two most distant points on the surfaces of a pair of black holes stretch everything in their path, including the artist's friend who got into the drawing. The lines coming out of the collision area compress everything.

As the holes rotate one around the other, they follow their tendex lines, which are like jets of water from a spinning lawn sprinkler. Pictured from the book Interstellar. The science behind the scenes is a pair of black holes that collide, rotating one around the other counterclockwise, and their tendex lines.

Black holes coalesce into one big hole; it is deformed and rotates counterclockwise, dragging the tendex lines with it. A stationary observer away from the hole will feel vibrations as the tendex lines pass through it: stretching, then squeezing, then stretching - the tendex lines become a gravitational wave. As the waves propagate, the deformation of the black hole gradually decreases, and the waves also weaken.

When these waves reach the Earth, they have the shape shown at the top of the figure below. They stretch in one direction and compress in the other. The stretches and squeezes fluctuate (from red right-left, to blue right-left, to red right-left, etc.) as the waves pass through the detector at the bottom of the figure.

Gravitational waves passing through the LIGO detector.

The detector consists of four large mirrors (40 kilograms, 34 centimeters in diameter) that are attached to the ends of two perpendicular tubes called detector arms. Tendex lines of gravitational waves stretch one shoulder, while compressing the second, and then, on the contrary, compress the first and stretch the second. And so again and again. By periodically changing the length of the arms, the mirrors move relative to each other, and these shifts are tracked using laser beams in a way called interferometry. Hence the name LIGO: Laser Interferometric Gravitational Wave Observatory.

The LIGO control center, from where they send commands to the detector and monitor the received signals. LIGO's gravitational detectors are located in Hanford, Washington and Livingston, Louisiana. Photo from the book “Interstellar. Science behind the scenes"

Now LIGO is an international project involving 900 scientists from different countries, headquartered at the California Institute of Technology.

The twisted side of the universe

Black holes, wormholes, singularities, gravitational anomalies and higher order dimensions are associated with the curvature of space and time. That's why Kip Thorne calls them the "curved side of the universe." Humanity still has very little experimental and observational data from the curved side of the Universe. This is why we give so much attention to gravitational waves: they are made of curved space and provide the most accessible way for us to explore the curved side.

Imagine that you had to see the ocean only when it is calm. You would not know about currents, whirlpools and storm waves. This is reminiscent of our current knowledge of the curvature of space and time.

We know almost nothing about how warped space and warped time behave "in a storm" - when the shape of space fluctuates violently and when the speed of the flow of time fluctuates. This is an unusually alluring frontier of knowledge. Scientist John Wheeler coined the term "geometrodynamics" for these changes.

Of particular interest in the field of geometrodynamics is the collision of two black holes.

Collision of two non-rotating black holes. Model from the book "Interstellar. Science behind the scenes"

The figure above shows the moment when two black holes collide. Just such an event allowed scientists to record gravitational waves. This model is built for non-rotating black holes. Top: orbits and shadows of holes, as seen from our Universe. Middle: curved space and time, viewed from the beam (high-dimensional hyperspace); the arrows show how space is drawn into motion, and the changing colors show how time is bent. Bottom: The shape of the emitted gravitational waves.

Gravitational waves from the Big Bang

Word to Kip Thorne. “In 1975, Leonid Grischuk, my good friend from Russia, made a sensational statement. He said that at the moment of the Big Bang, many gravitational waves arose, and the mechanism for their occurrence (previously unknown) was as follows: quantum fluctuations (random fluctuations - ed.) The gravitational field at the Big Bang were greatly amplified by the initial expansion of the Universe and thus became the original gravitational waves. These waves, if they can be detected, can tell us what was happening at the moment of the birth of our universe.”

If scientists find the original gravitational waves, we will know how the universe began.

People have unraveled far to all the mysteries of the universe. Still ahead.

In subsequent years, as our understanding of the Big Bang improved, it became clear that these initial waves must be strong at wavelengths commensurate with the size of the visible universe, that is, at lengths of billions of light years. Can you imagine how much it is? .. And at wavelengths that LIGO detectors cover (hundreds and thousands of kilometers), the waves are likely to be too weak to recognize them.

Jamie Bock's team built the BICEP2 apparatus, which found a trace of primordial gravitational waves. The North Pole craft is shown here during twilight, which occurs there only twice a year.

BICEP2 apparatus. Image from the book “Interstellar. Science behind the scenes"

It is surrounded by shields that shield the craft from radiation from the surrounding ice sheet. In the upper right corner there is a trace found in the relic radiation - a polarization pattern. Electric field lines are directed along short light strokes.

Trail of the beginning of the universe

In the early 1990s, cosmologists realized that these billions of light-years long gravitational waves must have left a unique imprint on the electromagnetic waves that fill the universe—the so-called cosmic microwave background, or CMB. This marked the beginning of the search for the Holy Grail. After all, if you find this trace and derive from it the properties of the original gravitational waves, you can find out how the Universe was born.

In March 2014, while Kip Thorne was writing this book, the team of Jamie Bok, a Caltech cosmologist whose office is next to Thorne's, finally found this trace in the CMB.

This is an absolutely amazing discovery, but there is one controversial point: the track found by Jamie's team could not be caused by gravitational waves, but something else.

If a trace of gravitational waves from the Big Bang is indeed found, then there has been a cosmological discovery of a level that happens, perhaps, once every half a century. It gives a chance to touch the events that took place a trillionth from a trillionth from a trillionth of a second after the birth of the Universe.

This discovery confirms theories that the expansion of the universe at that moment was extremely fast, in cosmologists' slang - inflationary speed. And heralds the advent of a new era in cosmology.

Gravitational Waves and Interstellar

Yesterday at a conference on the discovery of gravitational waves, Valery Mitrofanov, head of the Moscow collaboration of scientists LIGO, which includes 8 scientists from Moscow State University, noted that the plot of the film Interstellar, although fantastic, is not so far from reality. And all because the scientific consultant was Kip Thorne. Thorne himself expressed the hope that he believes in future manned flights to a black hole. Let them not happen as soon as we would like, and yet today it is much more real than it was before.

The day is not far off when people will leave the limits of our galaxy.

The event shook the minds of millions of people. The notorious Mark Zuckerberg wrote: “The discovery of gravitational waves is the biggest discovery in modern science. Albert Einstein is one of my heroes, which is why I took the discovery so close. A century ago, within the framework of the General Theory of Relativity (GR), he predicted the existence of gravitational waves. But they are so small to be discovered that it has come to look for them at the origins of events such as the Big Bang, star explosions and black hole collisions. When scientists analyze the data obtained, a completely new view of space will open before us. And, perhaps, this will shed light on the origin of the Universe, the birth and development of black holes. It's very inspiring to think about how many lives and efforts have gone into uncovering this mystery of the universe. This breakthrough was made possible thanks to the talent of brilliant scientists and engineers, people of different nationalities, as well as the latest computer technologies that have only recently appeared. Congratulations to all involved. Einstein would be proud of you."

Such is the speech. And this is a man who is simply interested in science. One can imagine what a storm of emotions swept over the scientists who contributed to the discovery. It seems we are witnessing a new era, friends. It's amazing.

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On February 11, 2016, an international group of scientists, including from Russia, at a press conference in Washington announced a discovery that will sooner or later change the development of civilization. It was possible to prove in practice gravitational waves or waves of space-time. Their existence was predicted 100 years ago by Albert Einstein in his.

No one doubts that this discovery will be awarded the Nobel Prize. Scientists are in no hurry to talk about its practical application. But they remind that until quite recently, humanity also did not know exactly what to do with electromagnetic waves, which eventually led to a real scientific and technological revolution.

What are gravitational waves in simple terms

Gravity and universal gravitation are one and the same. Gravitational waves are one of the OTS solutions. They must propagate at the speed of light. It is emitted by any body moving with variable acceleration.

For example, it rotates in its orbit with variable acceleration directed towards the star. And this acceleration is constantly changing. The solar system radiates energy on the order of several kilowatts in gravitational waves. This is a tiny amount, comparable to 3 old color TVs.

Another thing is two pulsars (neutron stars) rotating around each other. They move in very tight orbits. Such a "couple" was discovered by astrophysicists and has been observed for a long time. The objects were ready to fall on each other, which indirectly indicated that pulsars radiate space-time waves, that is, energy in their field.

Gravity is the force of attraction. We are drawn to the ground. And the essence of a gravitational wave is a change in this field, extremely weak when it comes to us. For example, take the level of water in a reservoir. The intensity of the gravitational field is the acceleration of free fall at a particular point. A wave is running across our reservoir, and suddenly the acceleration of free fall changes, just a little bit.

Such experiments began in the 60s of the last century. At that time, they came up with this: they hung a huge aluminum cylinder, cooled to avoid internal thermal fluctuations. And they were waiting for a wave from a collision of, for example, two massive black holes to suddenly reach us. The researchers were enthusiastic and said that the entire globe could be affected by a gravitational wave coming from outer space. The planet will begin to oscillate and these seismic waves (compressional, shear and surface) can be studied.

An important article about the device in plain language, and how the Americans and LIGO stole the idea of ​​the Soviet scientists and built the introferometers that allowed the discovery. Nobody talks about it, everyone is silent!

By the way, gravitational radiation is more interesting from the standpoint of relic radiation, which they try to find by changing the spectrum of electromagnetic radiation. Relic and electromagnetic radiation appeared 700 thousand years after the Big Bang, then in the process of expanding the universe filled with hot gas with traveling shock waves, which later turned into galaxies. In this case, of course, a gigantic, breathtaking number of space-time waves should have been emitted, affecting the wavelength of the cosmic microwave background radiation, which at that time was still optical. Domestic astrophysicist Sazhin writes and regularly publishes articles on this topic.

Misinterpretation of the discovery of gravitational waves

“A mirror hangs, a gravitational wave acts on it, and it starts to oscillate. And even the smallest fluctuations with an amplitude less than the size of an atomic nucleus are noticed by instruments ”- such an incorrect interpretation, for example, is used in the Wikipedia article. Do not be lazy, find an article by Soviet scientists in 1962.

First, the mirror must be massive in order to feel the "ripples". Secondly, it must be cooled to almost absolute zero (Kelvin) to avoid its own thermal fluctuations. Most likely, not only in the 21st century, but in general it will never be possible to detect an elementary particle - the carrier of gravitational waves: