“Recently, a series of long-term experiments on the direct observation of gravitational waves has generated strong scientific interest,” wrote theoretical physicist Michio Kaku in his 2004 book Einstein’s Cosmos. — 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- these are oscillations of space-time that “run away” from massive objects (for example, black holes) 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 within their general theory relativity was predicted by Albert Einstein (who, by the way, was skeptical about the existence of black holes). 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!”

We now live in a Universe filled with gravitational waves.

Until Thursday morning's historic announcement from the National Science Foundation (NSF) meeting in Washington, there were only rumors that the Laser Interferometer Gravitational-Wave Observatory (LIGO) had discovered a key component of Albert Einstein's General Theory of Relativity, but now we know the reality is deeper. than we thought.

With amazing clarity, LIGO was able to “hear” the moment before a binary system (two black holes orbiting each other) merged into a single whole, creating a gravitational wave signal so clear in accordance with the theoretical model that it did not require discussion. LIGO witnessed the “rebirth” of a powerful black hole that happened about 1.3 billion years ago.

Gravitational waves have always been and will always be passing through our planet (indeed, passing through us), but only now do we know how to find them. We have now opened our eyes to various cosmic signals, vibrations caused by known energetic events, and are witnessing the birth of a completely new area astronomy.

The sound of two black holes merging:

"We can now hear the Universe," said Gabriela Gonzalez, a physicist and LIGO spokeswoman, during Thursday's triumphant meeting. "The discovery marks the beginning of a new era: The field of gravitational astronomy is now a reality."

Our place in the Universe is changing greatly and this discovery could be as fundamental as the discovery of radio waves and the understanding that the Universe is expanding.

The Theory of Relativity Becomes More Valuable

Trying to explain what gravitational waves are and why they are so important is as complex as the equations that describe them, but their discovery not only strengthens Einstein's theories about the nature of space-time, we now have a tool for probing parts of the universe that were invisible to everyone. us. We can now study cosmic waves created by the most energetic events occurring in the Universe, and perhaps use gravitational waves to make new physical discoveries and explore new astronomical phenomena.

"We now have to prove that we have the technology to go beyond the discovery of gravitational waves, because that opens up a lot of possibilities," Lewis Lehner of the Institute of Theoretical Physics in Ontario said in an interview after Thursday's announcement.

Lehner's research focuses on dense objects (such as black holes) that create powerful gravitational waves. Although not associated with the LIGO collaboration, Lehner quickly realized the importance of this historic discovery. “There are no better signals,” he said.

The discovery is based on three paths, he reasons. First, we now know that gravitational waves exist, and we know how to detect them. Secondly, the signal detected by LIGO stations on September 14, 2015 is strong evidence of the existence of a binary system of black holes, and each black hole weighs several tens of solar masses. The signal is exactly what we expected to see from the violent merger of two black holes, one weighing 29 times the Sun, the other 36 times. Third, and perhaps most important, “the ability to be sent into a black hole” is by far the strongest evidence for the existence of black holes.

Cosmic intuition

This event was accompanied by luck, like many other scientific discoveries. LIGO is the largest project funded by the National Science Foundation, which originally started in 2002. It turned out that after many years of searching for the elusive signal of gravitational waves, LIGO was not sensitive enough and in 2010 the observatories were frozen during the work international cooperation to increase their sensitivity. Five years later, in September 2015, the “improved LIGO” was born.

At the time, LIGO co-founder and theoretical physics heavyweight Kip Thorne was confident of LIGO's success, telling the BBC: "We're here. We got to the field great game. And it is quite clear that we will lift the veil of secrecy.” - And he was right, a few days after the reconstruction, a burst of gravitational waves swept across our planet, and LIGO was sensitive enough to detect them.

These black hole mergers are not considered anything special; It is estimated that such events occur every 15 minutes somewhere in the Universe. But this particular merger occurred in the right place (1.3 billion light-years away) at the right time (1.3 billion years ago) for the LIGO observatories to pick up its signal. It was a pure signal from the Universe, and Einstein predicted it, and its gravitational waves turned out to be real, describing a cosmic event 50 times more powerful than the power of all the stars in the Universe combined. This huge burst of gravitational waves was recorded by LIGO as a high-frequency signal with linear frequency modulation as the black holes spiraled together and merged into one.

To confirm the propagation of gravitational waves, LIGO consists of two observing stations, one in Louisiana, the other in Washington. To eliminate false alarms, the gravitational wave signal must be detected at both stations. On September 14, the result was obtained first in Louisiana, and 7 milliseconds later in Washington. The signals coincided, and with the help of triangulation, physicists were able to find out that they originated in the celestial space of the Southern Hemisphere.

Gravitational waves: how can they be useful?

So we have confirmation of a black hole merger signal, but so what? This historical discovery, which is quite understandable - 100 years ago Einstein could not even dream of discovering these waves, but it still happened.

General relativity was one of the most profound scientific and philosophical insights of the 20th century and forms the basis of some of the most intelligent research in reality. In astronomy, the applications of general relativity are clear: from the gravitational lens to measuring the expansion of the Universe. But it's not at all clear practical use Einstein's theories, but most modern technologies use lessons from the theory of relativity in some things that are considered simple. For example, take global navigation satellites, they will not be accurate enough unless a simple adjustment to time dilation (predicted by relativity) is applied.

It is clear that general relativity has applications in the real world, but when Einstein presented his theory in 1916, its application was highly questionable, which seemed obvious. He simply connected the Universe as he saw it, and thus the general theory of relativity was born. And now another component of the theory of relativity has been proven, but how can gravitational waves be used? Astrophysicists and cosmologists are definitely intrigued.

“Once we have collected data from pairs of black holes that will act as beacons scattered throughout the universe,” theoretical physicist Neil Turok, director of the Institute for Theoretical Physics, said Thursday during a video presentation. “We will be able to measure the speed.” expansion of the Universe, or the amount of dark energy with extreme precision, much more accurately than we can today."

“Einstein developed his theory with some clues from nature, but based on logical consistency. After 100 years, you see very accurate confirmation of his predictions."

Moreover, the September 14 event has some physics features that still need to be explored. For example, Lehner noted that from analyzing the gravitational wave signal, it is possible to measure the "spin" or angular momentum of a merging black hole. "If you've been working on the theory for a long time, you'll know that the black hole has a very, very special spin," he said.

The formation of gravitational waves during the merger of two black holes:

For some reason, the final rotation of the black hole is slower than expected, indicating that the black holes collided at low speed, or they were in such a collision that caused a joint angular momentum opposing each other. "It's very interesting, why did nature do this?" Lehner said.

This recent mystery may bring back some basic physics that had been left out, but, more intriguingly, it may reveal “new,” unusual physics that doesn't fit into general relativity. And this highlights other uses for gravitational waves: since they are created by strong gravitational phenomena, we have the ability to probe this environment from afar, with possible surprises along the way. In addition, we could combine observations of astrophysical phenomena with electromagnetic forces to better understand the structure of the Universe.

Application?

Naturally, after huge announcements made from a set of scientific discoveries, many people outside the scientific community are wondering how they might be affected. The depth of the discovery may be lost, which certainly applies to gravitational waves. But consider another case, when Wilhelm Roentgen discovered X-rays in 1895, during experiments with cathode ray tubes, few people know that only a few years later, these electromagnetic waves will become a key component in everyday medicine from diagnosis to treatment. Likewise, with the first experimental creation of radio waves in 1887, Heinrich Hertz confirmed the famous electromagnetic equations of James Clerk Maxwell. Only after a while in the 90s of the 20th century, Guglielmo Marconi, who created a radio transmitter and radio receiver, proved their practical application. Also, Schrödinger's equations, which describe the complex world of quantum dynamics, are now being used in the development of ultra-fast quantum computing.

All scientific discoveries are useful, and many ultimately have everyday applications that we take for granted. Currently, the practical applications of gravitational waves are limited to astrophysics and cosmology - we now have a window into the “dark universe”, invisible to electromagnetic radiation. Without a doubt, scientists and engineers will find other uses for these cosmic pulsations besides probing the Universe. However, to detect these waves, there must be good progress in optical engineering at LIGO, in which new technologies will appear over time.

Astrophysicists have confirmed the existence of gravitational waves, the existence of which was predicted by Albert Einstein about 100 years ago. They were detected using detectors at the LIGO gravitational wave observatory, which is located in the United States.

For the first time in history, humanity has recorded gravitational waves - vibrations of space-time that came to Earth from the collision of two black holes that occurred far in the Universe. Russian scientists also contributed to this discovery. On Thursday, researchers talk about their discovery around the world - in Washington, London, Paris, Berlin and other cities, including Moscow.

The photo shows a simulation of a black hole collision

At a press conference at the Rambler&Co office, Valery Mitrofanov, head of the Russian part of the LIGO collaboration, announced the discovery of gravitational waves:

“We were honored to participate in this project and present the results to you. I will now tell you the meaning of the discovery in Russian. We have seen beautiful pictures of LIGO detectors in the US. The distance between them is 3000 km. Under the influence of a gravitational wave, one of the detectors shifted, after which we discovered them. At first we saw just noise on the computer, and then the mass of the Hamford detectors began to rock. After calculating the data obtained, we were able to determine that it was the black holes that collided at a distance of 1.3 billion. light years away. The signal was very clear, it came out of the noise very clearly. Many people told us that we were lucky, but nature gave us such a gift. Gravitational waves have been discovered, that’s for sure.”

Astrophysicists have confirmed rumors that they were able to detect gravitational waves using detectors at the LIGO gravitational wave observatory. This discovery will allow humanity to make significant progress in understanding how the Universe works.

The discovery occurred on September 14, 2015 simultaneously with two detectors in Washington and Louisiana. The signal arrived at the detectors as a result of the collision of two black holes. It took scientists so long to verify that it was the gravitational waves that were the product of the collision.

The collision of the holes occurred at a speed of about half the speed of light, which is approximately 150,792,458 m/s.

“Newtonian gravity was described in flat space, and Einstein transferred it to the plane of time and assumed that it bends it. Gravitational interaction is very weak. On Earth, experiments to create gravitational waves are impossible. They were discovered only after the merger of black holes. The detector shifted, just imagine, by 10 to -19 meters. You can't feel it with your hands. Only with the help of very precise instruments. How to do it? The laser beam with which the shift was recorded was unique in nature. LIGO's second generation laser gravity antenna became operational in 2015. The sensitivity makes it possible to detect gravitational disturbances approximately once a month. This is advanced world and American science; there is nothing more accurate in the world. We hope that it will be able to overcome the Standard Quantum Sensitivity Limit,” explained the discovery Sergei Vyatchanin, employee of the Physics Department of Moscow State University and the LIGO collaboration.

The standard quantum limit (SQL) in quantum mechanics is a limitation imposed on the accuracy of a continuous or repeatedly repeated measurement of any quantity described by an operator that does not commute with itself at different instants of time. Predicted in 1967 by V.B. Braginsky, and the term Standard Quantum Limit (SQL) was proposed later by Thorne. The SKP is closely related to the Heisenberg uncertainty relation.

Summing up, Valery Mitrofanov spoke about plans for further research:

“This discovery is the beginning of a new gravitational wave astronomy. Through the channel of gravitational waves we expect to learn more about the Universe. We know the composition of only 5% of matter, the rest is a mystery. Gravity detectors will allow you to see the sky in “gravitational waves.” In the future, we hope to see the beginning of everything, that is, the relic radiation of the Big Bang and understand what exactly happened then.”

Gravitational waves were first proposed by Albert Einstein in 1916, almost exactly 100 years ago. The equation for waves is a consequence of the equations of the theory of relativity and is not derived in the simplest way.

Canadian theoretical physicist Clifford Burgess previously published a letter saying the observatory 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. The collision and asymmetrical gravitational collapse last a fraction of a second, and during this time energy amounting to up to 50 percent of the mass of the system is lost into gravitational radiation - ripples in space-time.

A gravitational wave is a wave of gravity generated in most theories of gravitation by the movement of gravitating bodies with variable acceleration. Due to the relative weakness of gravitational forces (compared to others), these waves should have a very small magnitude, difficult to register. Their existence was predicted about a century ago by Albert Einstein.

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

Prophecies of the great Einstein

The fact that gravitational waves exist was suggested by Albert Einstein at the beginning of the last century (1916) within the framework of his General Theory of Relativity (GTR). One can only marvel at the brilliant abilities of the famous physicist, who, with a minimum of real data, was able to draw such far-reaching conclusions. Among many other predicted physical phenomena that were confirmed in the next century (slowing down the flow of time, changing the direction of electromagnetic radiation in gravitational fields, etc.), until recently it was not possible to practically detect the presence of this type of wave interaction between bodies.

Is gravity an illusion?

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

Something similar happens in outer space. Any rapidly moving massive matter is a source of fluctuations in the density of space and time. A gravitational wave with a significant amplitude is formed by bodies with extremely large masses or when moving with enormous accelerations.

physical characteristics

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

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

Circumstantial evidence

The first theoretical confirmation of the existence of gravitational waves was obtained by the American astronomer Joseph Taylor and his assistant Russell Hulse in 1974. Studying the vastness of the Universe using the Arecibo Observatory radio telescope (Puerto Rico), researchers discovered the pulsar PSR B1913+16, which is dual system neutron stars rotating around a common center of mass with a constant angular velocity (a rather rare case). Every year the circulation period, originally 3.75 hours, is reduced by 70 ms. This value is fully consistent with the conclusions from the general relativity equations, which predict an increase in the rotation speed of such systems due to the expenditure of energy on the generation of gravitational waves. Subsequently, several double pulsars and white dwarfs with similar behavior were discovered. Radio astronomers D. Taylor and R. Hulse were awarded the Nobel Prize in physics for the discovery of new possibilities for studying gravitational fields.

Escaping gravitational wave

The first announcement about the detection of gravitational waves came from University of Maryland scientist Joseph Weber (USA) in 1969. For these purposes, he used two gravitational antennas of his own design, separated by a distance of two kilometers. The resonant detector was a well-vibration-insulated solid two-meter aluminum cylinder equipped with sensitive piezoelectric sensors. The amplitude of the oscillations allegedly recorded by Weber turned out to be more than a million times higher than the expected value. Attempts by other scientists to repeat the “success” of the American physicist using similar equipment did not bring positive results. A few years later, Weber’s work in this area was recognized as untenable, but gave impetus to the development of the “gravitational boom”, which attracted many specialists to this area of ​​research. By the way, Joseph Weber himself was sure until the end of his days that he received gravitational waves.

Improving receiving equipment

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

The Auriga gravitational detector at the National Legnar Laboratory (Padua, Italy) is designed using this principle. The design is based on an aluminum-magnesium cylinder, 3 meters long and 0.6 m in diameter. The receiving device weighing 2.3 tons is suspended in an insulated vacuum chamber cooled almost to absolute zero. To record and detect shocks, an auxiliary kilogram resonator and a computer-based measuring complex are used. The stated sensitivity of the equipment is 10 -20.

Interferometers

The operation of interference detectors of gravitational waves is based on the same principles on which the Michelson interferometer operates. Emitted by a source laser ray is divided into two streams. After multiple reflections and travels along the arms of the device, the flows are again brought together, and based on the final one it is judged whether any disturbances (for example, a gravitational wave) affected the course of the rays. Similar equipment has been created in many countries:

• GEO 600 (Hannover, Germany). The length of the vacuum tunnels is 600 meters.
• TAMA (Japan) with shoulders of 300 m.
• VIRGO (Pisa, Italy) is a joint French-Italian project launched in 2007 with three kilometers of tunnels.
• LIGO (USA, Pacific Coast), which has been hunting for gravitational waves since 2002.

The latter is worth considering in more detail.

LIGO Advanced

The project was created on the initiative of scientists from the Massachusetts and California Institutes of Technology. It includes two observatories, separated by 3 thousand km, in and Washington (the cities of Livingston and Hanford) with three identical interferometers. The length of perpendicular vacuum tunnels is 4 thousand meters. These are the largest such structures currently in operation. Until 2011, numerous attempts to detect gravitational waves did not bring any results. The significant modernization carried out (Advanced LIGO) increased the sensitivity of the equipment in the range of 300-500 Hz by more than five times, and in the low-frequency region (up to 60 Hz) by almost an order of magnitude, reaching the coveted value of 10 -21. The updated project started in September 2015, and the efforts of more than a thousand collaboration employees were rewarded with the results obtained.

Gravitational waves detected

On September 14, 2015, advanced LIGO detectors, with an interval of 7 ms, recorded gravitational waves reaching our planet from the largest event that occurred on the outskirts of the observable Universe - the merger of two large black holes with masses 29 and 36 times greater than the mass of the Sun. During the process, which took place more than 1.3 billion years ago, about three solar masses of matter were consumed in a matter of fractions of a second by emitting gravitational waves. The recorded initial frequency of gravitational waves was 35 Hz, and the maximum peak value reached 250 Hz.

The results obtained were repeatedly subjected to comprehensive verification and processing, and alternative interpretations of the data obtained were carefully eliminated. Finally, last year the direct registration of the phenomenon predicted by Einstein was announced to the world community.

A fact illustrating the titanic work of researchers: the amplitude of fluctuations in the size of the interferometer arms was 10 -19 m - this value is the same number of times smaller than the diameter of an atom, as the atom itself is smaller than an orange.

Future prospects

The discovery once again confirms that the General Theory of Relativity is not just a set of abstract formulas, but a fundamentally new look at the essence of gravitational waves and gravity in general.

In further research, scientists have high hopes for the ELSA project: the creation of a giant orbital interferometer with arms of about 5 million km, capable of detecting even minor disturbances in gravitational fields. Activation of work in this direction can tell a lot of new things about the main stages of the development of the Universe, about processes that are difficult or impossible to observe in traditional ranges. There is no doubt that black holes, whose gravitational waves will be detected in the future, will tell a lot about their nature.

To study the cosmic microwave background radiation, which can tell us about the first moments of our world after the Big Bang, more sensitive space instruments will be required. Such a project exists ( Big Bang Observer), but its implementation, according to experts, is possible no earlier than in 30-40 years.

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.