“Recently, a series of long-term direct observation experiments aroused strong interest in the scientific community. gravitational waves“,” wrote theoretical physicist Michio Kaku in the book “Einstein’s Cosmos” in 2004. — The LIGO (Laser Interferometer for Observing Gravitational Waves) 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 oscillations in space-time that "escape" massive objects (such as black holes) that are moving with acceleration. In other words, gravitational waves are a spreading disturbance of space-time, a traveling 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 A photo of a cake posted online by Erin Ryan.

Scientists began catching gravitational waves half a century ago: it was then that the American physicist Joseph Weber became interested in Einstein’s general theory of relativity (GTR), took a sabbatical and began studying gravitational waves. Weber invented the first device to detect gravitational waves, and soon announced that he had recorded “the sound of gravitational waves.” However, the scientific community refuted 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 field 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. This 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 connect again. If a gravitational wave passes through an 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 recorded by the laser installation is not random, detectors should be placed at different points on the Earth.

Only under the influence of a gigantic gravitational wave, much larger than our planet in size, will all detectors operate 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,” Sergei Vyatchanin, a professor at the Faculty of Physics at Moscow State University, commented to the correspondent of the Gazeta.Ru science department. — 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 new era gravitational astronomy, which will make it possible to obtain 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 was caught fits very well with what is predicted by countless simulations of the merger of two black holes in accordance with the general theory of relativity.

This is a strong argument that black holes exist. There is no other explanation for this signal yet. Therefore, it is accepted 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) as part of his general theory of relativity. In GR, time is added to the three spatial dimensions, and the world becomes four-dimensional. According to the theory that turned all 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 disturbance in space-time - a gravitational wave. This disturbance is greater, the higher the acceleration and mass of the object.

Due to weakness gravitational forces Compared to other fundamental interactions, these waves should have a very small magnitude, difficult to register.

When explaining general relativity to humanities scholars, physicists often ask them to imagine a stretched sheet of rubber onto which massive balls are lowered. The balls press 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 leaves dents. Our Earth rotates around the Sun like a small ball, launched to roll around the cone of a funnel formed as a result of “pushing” 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, is eligible for the Nobel Prize in Physics. “Einstein would be very happy,” said Gabriella Gonzalez, a spokeswoman for the LIGO collaboration.

According to scientists, it is too early to talk about the practical applicability of the discovery. “Although, could Heinrich Hertz (German physicist who proved the existence of electromagnetic waves. - Gazeta.Ru) think that there would be mobile phone? No! “We can’t imagine anything now,” said Valery Mitrofanov, professor at the Faculty of Physics at Moscow State University. M.V. Lomonosov. — I focus on the film “Interstellar”. He is criticized, yes, but even a wild man could imagine a magic carpet. And the magic carpet turned into an airplane, and that’s it. And here we need to imagine something very complex. In Interstellar, one of the points is related to the fact that a person can travel from one world to another. If you imagine this way, 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 “no”! Only if it contradicts some conservation laws! There are options that do not contradict known physical laws. So, there can be travel across worlds!”

A hundred years after the theoretical prediction made by Albert Einstein within the framework of the general theory of relativity, scientists were able to confirm the existence of gravitational waves. The era of a fundamentally new method for studying deep space—gravitational wave astronomy—begins.

There are different discoveries. There are random ones, they are common in astronomy. There are not entirely accidental ones, made as a result of a thorough “combing of the area,” such as the discovery of Uranus by William Herschel. There are serendipal ones - when they were looking for one thing and found another: for example, they discovered America. But planned discoveries occupy a special place in science. They are based on a clear theoretical prediction. What is predicted is sought primarily in order to confirm the theory. Such discoveries include the discovery of the Higgs boson at the Large Hadron Collider and the detection of gravitational waves using the laser interferometer gravitational-wave observatory LIGO. But in order to register some phenomenon predicted by the theory, you need to have a pretty good understanding of what exactly and where to look, as well as what tools are needed for this.

Gravitational waves are traditionally called a prediction of the general theory of relativity (GTR), and this is indeed so (although now such waves exist in all models that are alternative to or complementary to GTR). The appearance of waves is caused by the finiteness of the speed of propagation of gravitational interaction (in general relativity this speed is exactly equal to the speed of light). Such waves are disturbances in space-time propagating from a source. For gravitational waves to occur, the source must pulsate or move at an accelerated rate, but in a certain way. Let's say movements with perfect spherical or cylindrical symmetry are not suitable. There are quite a lot of such sources, but often they have a small mass, insufficient to generate a powerful signal. After all, gravity is the weakest of the four fundamental interactions, so it is very difficult to register a gravitational signal. In addition, for registration it is necessary that the signal changes quickly over time, that is, it has a sufficiently high frequency. Otherwise, we will not be able to register it, since the changes will be too slow. This means that the objects must also be compact.

Initially, great enthusiasm was generated by supernova explosions that occur in galaxies like ours every few decades. This means that if we can achieve a sensitivity that allows us to see a signal from a distance of several million light years, we can count on several signals per year. But later it turned out that initial estimates of the power of energy release in the form of gravitational waves during a supernova explosion were too optimistic, and such a weak signal could only be detected if a supernova had broken out in our Galaxy.

Another option for massive compact objects that move quickly are neutron stars or black holes. We can see either the process of their formation, or the process of interaction with each other. The last stages of the collapse of stellar cores, leading to the formation of compact objects, as well as the last stages of the merger of neutron stars and black holes, have a duration of the order of several milliseconds (which corresponds to a frequency of hundreds of hertz) - just what is needed. In this case, a lot of energy is released, including (and sometimes mainly) in the form of gravitational waves, since massive compact bodies make certain rapid movements. These are our ideal sources.

True, supernovae erupt in the Galaxy once every few decades, mergers of neutron stars occur once every couple of tens of thousands of years, and black holes merge with each other even less often. But the signal is much more powerful, and its characteristics can be calculated quite accurately. But now we need to be able to see the signal from a distance of several hundred million light years in order to cover several tens of thousands of galaxies and detect several signals in a year.

Having decided on the sources, we will begin to design the detector. To do this, you need to understand what a gravitational wave does. Without going into detail, we can say that the passage of a gravitational wave causes a tidal force (ordinary lunar or solar tides are a separate phenomenon, and gravitational waves have nothing to do with it). So you can take, for example, a metal cylinder, equip it with sensors and study its vibrations. This is not difficult, which is why such installations began to be made half a century ago (they are also available in Russia; now an improved detector developed by Valentin Rudenko’s team from the SAI MSU is being installed in the Baksan underground laboratory). The problem is that such a device will see the signal without any gravitational waves. There are a lot of noises that are difficult to deal with. It is possible (and has been done!) to install the detector underground, try to isolate it, cool it to low temperatures, but still, in order to exceed the noise level, a very powerful gravitational wave signal would be needed. But powerful signals come rarely.

Therefore, the choice was made in favor of another scheme, which was put forward in 1962 by Vladislav Pustovoit and Mikhail Herzenstein. In an article published in JETP (Journal of Experimental and Theoretical Physics), they proposed using a Michelson interferometer to detect gravitational waves. The laser beam runs between the mirrors in the two arms of the interferometer, and then the beams from different arms are added. By analyzing the result of beam interference, the relative change in arm lengths can be measured. This is very precise measurements, so if you beat the noise, you can achieve fantastic sensitivity.

In the early 1990s, it was decided to build several detectors using this design. The first to go into operation were relatively small installations, GEO600 in Europe and TAMA300 in Japan (the numbers correspond to the length of the arms in meters) to test the technology. But the main players were to be the LIGO installations in the USA and VIRGO in Europe. The size of these instruments is already measured in kilometers, and the final planned sensitivity should allow seeing dozens, if not hundreds of events per year.

Why are multiple devices needed? Primarily for cross-validation, since there are local noises (e.g. seismic). Simultaneous detection of the signal in the northwestern United States and Italy would be excellent evidence of its external origin. But there is a second reason: gravitational wave detectors are very poor at determining the direction to the source. But if there are several detectors spaced apart, it will be possible to indicate the direction quite accurately.

Laser giants

In their original form, the LIGO detectors were built in 2002, and the VIRGO detectors in 2003. According to the plan, this was only the first stage. All installations operated for several years, and in 2010-2011 they were stopped for modifications, in order to then reach the planned high sensitivity. The LIGO detectors were the first to operate in September 2015, VIRGO should join in the second half of 2016, and from this stage the sensitivity allows us to hope for recording at least several events per year.

After LIGO began operating, the expected burst rate was approximately one event per month. Astrophysicists estimated in advance that the first expected events would be black hole mergers. This is due to the fact that black holes are usually ten times heavier than neutron stars, the signal is more powerful, and it is “visible” from great distances, which more than compensates for the lower rate of events per galaxy. Fortunately, we didn't have to wait long. On September 14, 2015, both installations registered an almost identical signal, named GW150914.

With pretty help simple analysis data such as black hole masses, signal strength, and distance to the source can be obtained. The mass and size of black holes are related in a very simple and well-known way, and from the signal frequency one can immediately estimate the size of the energy release region. In this case, the size indicated that from two holes with a mass of 25-30 and 35-40 solar masses, a black hole with a mass of more than 60 solar masses was formed. Knowing these data, one can obtain the total energy of the burst. Almost three solar masses were converted into gravitational radiation. This corresponds to the luminosity of 1023 solar luminosities - approximately the same amount as all the stars in the visible part of the Universe emit during this time (hundredths of a second). And from the known energy and magnitude of the measured signal, the distance is obtained. The large mass of the merged bodies made it possible to register an event that occurred in a distant galaxy: the signal took approximately 1.3 billion years to reach us.

A more detailed analysis makes it possible to clarify the mass ratio of black holes and understand how they rotated around their axis, as well as determine some other parameters. In addition, the signal from two installations makes it possible to approximately determine the direction of the burst. Unfortunately, the accuracy here is not very high yet, but with the commissioning of the updated VIRGO it will increase. And in a few years, the Japanese KAGRA detector will begin to receive signals. Then one of the LIGO detectors (there were originally three, one of the installations was dual) will be assembled in India, and it is expected that many dozens of events will be recorded per year.

The era of new astronomy

On this moment LIGO's most important result is the confirmation of the existence of gravitational waves. In addition, the very first burst made it possible to improve the restrictions on the mass of the graviton (in general relativity it has zero mass), as well as to more strongly limit the difference between the speed of propagation of gravity and the speed of light. But scientists hope that already in 2016 they will be able to obtain a lot of new astrophysical data using LIGO and VIRGO.

First, data from gravitational wave observatories provide a new avenue for studying black holes. If previously it was only possible to observe the flows of matter in the vicinity of these objects, now you can directly “see” the process of merging and “calming” the resulting black hole, how its horizon fluctuates, taking on its final shape (determined by rotation). Probably, until the discovery of Hawking evaporation of black holes (for now this process remains a hypothesis), the study of mergers will provide better direct information about them.

Secondly, observations of neutron star mergers will provide a lot of new, urgently needed information about these objects. For the first time, we will be able to study neutron stars the way physicists study particles: watching them collide to understand how they work inside. The mystery of the structure of the interiors of neutron stars worries both astrophysicists and physicists. Our understanding of nuclear physics and the behavior of matter at ultrahigh densities is incomplete without resolving this issue. It is likely that gravitational wave observations will play a key role here.

It is believed that neutron star mergers are responsible for short cosmological gamma-ray bursts. In rare cases, it will be possible to simultaneously observe an event both in the gamma range and on gravitational wave detectors (the rarity is due to the fact that, firstly, the gamma signal is concentrated into a very narrow beam, and it is not always directed at us, but secondly, we will not register gravitational waves from very distant events). Apparently, it will take several years of observation to be able to see this (although, as usual, you may be lucky and it will happen today). Then, among other things, we will be able to very accurately compare the speed of gravity with the speed of light.

Thus, laser interferometers together will work as a single gravitational-wave telescope, bringing new knowledge to both astrophysicists and physicists. Well, sooner or later a well-deserved Nobel Prize will be awarded for the discovery of the first bursts and their analysis.

2236

Participants in the LIGO scientific experiment, in which Russian physicists are also participating, announced that American observatories have detected gravitational waves generated by the collision of two black holes.

Gravitational waves were recorded on September 14, 2015, which was reported on February 11, 2016 at a special press conference by LIGO representatives in Washington. It took scientists six months to process and verify the results. This can be considered the official discovery of gravitational waves, since they were directly recorded on Earth for the first time. The results of the work were published in the journal Physical Review Letters.

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

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

Test masses, also known as interferometer mirrors, are made of fused quartz. Photo: www.ligo.caltech.edu

Numerical simulation of gravitational waves from approaching black holes. Figure: Physical Review Letters http://physics.aps.org/articles/v9/17

LIGO Observatory near Livingston, Louisiana. Photo: www.ligo.caltech.edu

Thus, one of the most important problems facing physicists over the past 100 years has been solved. The existence of gravitational waves is predicted by the general theory of relativity (GTR), developed in 1915-1916 by Albert Einstein, the fundamental physical theory that describes the structure and evolution of our world. General relativity, in essence, is a theory of gravity that establishes its connection with the properties of space-time. Massive bodies produce changes in it that are commonly called curvature of space-time. If these bodies move with variable acceleration, then propagating changes in space-time arise, which are called gravitational waves.

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

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

LIGO includes observatories in the states of Washington and Louisiana, located at a distance of 3000 km, which is an L-shaped Michelson interferometer with two arms 4 km long. The laser beam, passing through a system of mirrors, is divided into two beams, each of which propagates in its own arm. They are reflected from mirrors and come back. Then these two light waves, traveling along different paths, are added together in the detector. Initially, the system is configured so that the waves cancel each other out and nothing hits the detector. Gravitational waves change the distances between test masses, which simultaneously serve as mirrors of the interferometer, which leads to the fact that the sum of the waves is no longer equal to zero and the signal intensity at the photodetector will be proportional to these changes. This signal is used to register a gravitational wave.

The first, initial, stage of measurements took place in 2002-2010 and did not allow the detection of gravitational waves. The sensitivity of the devices was not enough (shifts up to 4x10 -18 m were tracked). Then it was decided to stop work in 2010 and modernize the equipment, increasing the sensitivity by more than 10 times. The improved equipment, which began work in the second half of 2015, was able to detect a shift of a record 10 -19 m. And already during the test run, scientists were waiting for a discovery; they recorded a gravitational burst from the event, which after a long study was identified as the merger of two black holes with masses at 29 and 36 solar masses.

Simultaneously with Washington, a press conference was held in Moscow. At it, participants in the experiment, representing the Faculty of Physics of Moscow State University, spoke about their contribution to its implementation. V.B. Braginsky’s group participated in the work from the very beginning of the project. Physicists from Moscow State University ensured the assembly of a complex structure, which consists of interferometer mirrors, which simultaneously serve as test masses.

In addition, their tasks included combating extraneous vibrations (noise) that could interfere with the detection of gravitational waves. It was the Moscow State University specialists who proved that the device should be made of fused quartz, which at operating temperatures will make less noise than the sapphire proposed by other researchers. In particular, to reduce thermal noise, it was necessary to ensure that the oscillations of test masses suspended like pendulums did not die out for a very long time. Physicists from Moscow State University have achieved a decay time of 5 years!

The success of the measurements will give rise to a new gravitational-wave astronomy and will allow us to learn a lot of new things about the Universe. Perhaps physicists will be able to unravel some of the mysteries of dark matter and the early stages of the development of the Universe, as well as look into areas where general relativity is violated.

Based on materials from the LIGO collaboration press conference.

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 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 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 study the gravitational radiation that was separated from the primordial matter in the Big Bang Theory, which was created at 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 lie in the foundation 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

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

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

Gravitational waves

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

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

General Relativity has a crucial prediction: gravitational waves. These are distortions of space-time that are capable of “breaking away from the source” and, self-sustaining, flying away. This is gravity in itself, no one's, its own. Albert Einstein finally formulated general relativity in 1915 and almost immediately realized that the equations he derived allowed for the existence of such waves.

As with any honest theory, such a clear prediction of general relativity must be verified experimentally. Any moving body can emit gravitational waves: planets, a stone thrown upward, or a wave of a hand. The problem, however, is that the gravitational interaction is so weak that no experimental setup can detect the emission of gravitational waves from ordinary “emitters.”

To “chase” a powerful wave, you need to greatly distort space-time. The ideal option is two black holes rotating around each other in a close dance, at a distance of the order of their gravitational radius (Fig. 2). The distortions of the metric will be so strong that a noticeable part of the energy of this pair will be emitted into gravitational waves. Losing energy, the pair will move closer together, spinning faster and faster, distorting the metric more and more and generating even stronger gravitational waves - until, finally, a radical restructuring of the entire gravitational field of this pair occurs and two black holes merge into one.

Such a merger of black holes is an explosion of tremendous power, but only all this emitted energy goes not into light, not into particles, but into vibrations of space. The emitted energy will make up a noticeable part of the initial mass of black holes, and this radiation will splash out in a fraction of a second. Similar oscillations will be generated by mergers of neutron stars. A slightly weaker gravitational wave release of energy also accompanies other processes, such as the collapse of a supernova core.

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

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

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

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

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

Search history

The search for gravitational waves started about half a century ago - and almost immediately turned into a sensation. Joseph Weber from the University of Maryland designed the first resonant detector: a solid two-meter aluminum cylinder with sensitive piezoelectric sensors on the sides and good vibration isolation from extraneous vibrations (Fig. 4). When a gravitational wave passes, the cylinder resonates in time with the distortions of space-time, which is what the sensors should register. Weber built several such detectors, and in 1969, after analyzing their readings during one of the sessions, he directly stated that he had registered the “sound of gravitational waves” in several detectors at once, spaced two kilometers apart (J. Weber, 1969 Evidence for Discovery of Gravitational Radiation). The amplitude of oscillations he declared turned out to be incredibly large, on the order of 10 −16, that is, a million times greater than the typical expected value. Weber's message was met with great skepticism by the scientific community; Moreover, other experimental groups, armed with similar detectors, were unable to subsequently catch a single similar signal.

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

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

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

The words “microscopic displacement” don’t even come close to conveying the subtlety of the effect. The displacement of mirrors by the wavelength of light, that is, microns, is easy to notice even without any tricks. But with an arm length of 4 km, this corresponds to oscillations of space-time with an amplitude of 10 −10. Noticing the displacement of mirrors by the diameter of an atom is also not a problem - it is enough to fire a laser beam, which will run back and forth thousands of times and obtain the desired phase shift. But this also gives a maximum of 10 −14. And we need to go down the displacement scale millions more times, that is, learn to register a mirror shift not even by one atom, but by thousandths of an atomic nucleus!

On the way to this truly amazing technology, physicists had to overcome many difficulties. Some of them are purely mechanical: you need to hang massive mirrors on a suspension, which hangs on another suspension, that on a third suspension, and so on - and all in order to get rid of extraneous vibration as much as possible. Other problems are also instrumental, but optical. For example, the more powerful the beam circulating in the optical system, the weaker the displacement of the mirrors can be detected by the photosensor. But 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 must be somehow compensated, and for this in the 2000s, an entire research program was launched on this subject (for a story about this research, see the news Obstacle overcome on the way to a highly sensitive gravitational wave detector, “Elements”, 06/27/2006 ). Finally, there are purely fundamental physical limitations related to the quantum behavior of photons in a cavity and the uncertainty principle. They limit the sensitivity of the sensor to a value called the standard quantum limit. However, physicists, using a cleverly prepared quantum state of laser light, have already learned to overcome it (J. Aasi et al., 2013. Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light).

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

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

The creation of the LIGO gravitational observatory was the initiative of three scientists from the Massachusetts Institute of Technology (MIT) and the California Institute of Technology (Caltech). These are Rainer Weiss, who realized the idea of ​​​​an interferometric gravitational wave detector, Ronald Drever, who achieved stability of laser light sufficient for detection, and Kip Thorne, the theoretician behind the project, now well known to the general public as a scientific consultant movie "Interstellar". 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 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 operating in 2002 and until 2010 it hosted six scientific observation sessions. No gravitational wave bursts were reliably detected, and physicists were only able to set upper limits on the frequency of such events. This, however, did not surprise them too much: estimates showed that in that part of the Universe that the detector was then “listening” to, the probability of a sufficiently powerful cataclysm was low: approximately once every few decades.

Finish line

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

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

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

Lyrical digression

To once again feel the solemnity of the moment, I propose to look at this story from the other side, from the inside of science. When a complex, inaccessible scientific task remains unanswerable for several years, this is a normal working moment. When it does not yield for more than one generation, it is perceived completely differently.

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

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

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

Opening

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

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

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

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

What gravitational waves tell us

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

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

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

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

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

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

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

Future plans

The prospects for gravitational wave astronomy are most 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, the Higgs boson was discovered, a particle predicted almost half a century ago. In 2013, the IceCube neutrino detector proved the reality of astrophysical neutrinos and began to “look at the universe” in a completely new, previously inaccessible way - through high-energy neutrinos. And now nature has succumbed to man once again: a gravitational-wave “window” has opened for observing the universe and, at the same time, the effects of strong gravity have become available for direct study.

It must be said that there was no “freebie” from nature anywhere here. The search was carried out for a very long time, but it did not yield because then, decades ago, the equipment did not reach the result in terms of energy, scale, or sensitivity. It was the steady, targeted development of technology that led to the goal, a development that was not stopped by either technical difficulties or the negative results of past years.

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

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

Review materials:
1) David Blair et al. Gravitational wave astronomy: the current status // arXiv:1602.02872.
2) Benjamin P. Abbott and LIGO Scientific Collaboration and Virgo Collaboration. Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO and Advanced Virgo // Living Rev. Relativity. 2016. V. 19. N. 1.
3) O. D. Aguiar. The Past, Present and Future of the Resonant-Mass Gravitational Wave Detectors // Res. Astron. Astrophys. 2011. V. 11. N. 1.
4) The search for gravitational waves - a selection of materials on the magazine’s website Science on the search for gravitational waves.
5) Matthew Pitkin, Stuart Reid, Sheila Rowan, Jim Hough. Gravitational Wave Detection by Interferometry (Ground and Space) // arXiv:1102.3355.
6) V. B. Braginsky. Gravitational-wave astronomy: new measurement methods // UFN. 2000. T. 170. pp. 743–752.
7) Peter R. Saulson.