- Gravitational waves - changes gravitational field, propagating like waves. They are emitted by moving masses, but after radiation they are separated from them and exist independently of these masses. Mathematically related to the perturbation of spacetime metrics and can be described as "spacetime ripples".
In general relativity and most others modern theories gravity gravitational waves are generated by movement massive bodies with variable acceleration. Gravitational waves spread freely in space at the speed of light. Due to the relative weakness of gravitational forces (compared to others), these waves have a very small magnitude, which is difficult to register.
Gravitational waves are predicted by the general theory of relativity (GR) and many other theories of gravity. They were first directly discovered in September 2015 by LIGO's twin detectors, which detected gravitational waves likely resulting from the merger of two black holes to form one more massive rotating black hole. black hole. Indirect evidence of their existence has been known since the 1970s - general relativity predicts rates of convergence of close systems that coincide with observations double stars due to the loss of energy due to the emission of gravitational waves. Direct registration of gravitational waves and their use to determine the parameters of astrophysical processes is an important task of modern physics and astronomy.
Within the framework of general relativity, gravitational waves are described by solutions of wave-type Einstein equations, which represent a perturbation of the space-time metric moving at the speed of light (in the linear approximation). The manifestation of this indignation should be, in particular, periodic change distances between two freely falling (that is, not influenced by any forces) test masses. The amplitude h of a gravitational wave is a dimensionless quantity - a relative change in distance. The predicted maximum amplitudes of gravitational waves from astrophysical objects (for example, compact binary systems) and phenomena (supernova explosions, neutron star mergers, star captures by black holes, etc.) when measured in the Solar System are very small (h = 10−18-10 −23). A weak (linear) gravitational wave, according to the general theory of relativity, transfers energy and momentum, moves at the speed of light, is transverse, quadrupole and is described by two independent components located at an angle of 45° to each other (has two directions of polarization).
Different theories predict the speed of propagation of gravitational waves differently. In general relativity, it is equal to the speed of light (in the linear approximation). In other theories of gravity, it can take any value, including infinity. According to the first registration of gravitational waves, their dispersion turned out to be compatible with a massless graviton, and the speed was estimated to be equal to the speed of light.
"Not so long ago strong interest What sparked the scientific community was a series of long-term experiments to directly observe gravitational waves, wrote theoretical physicist Michio Kaku in his 2004 book Einstein's Cosmos. — The LIGO project (“Laser Interferometer for Observing Gravitational Waves”) 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 scientific direction 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 of a new era of gravitational astronomy, which will allow us 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 the weakness of 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) have thought that there would be a 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 the known ones 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 careful “combing 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. Last stages of collapse stellar cores, leading to the formation of compact objects, as well as the last stages of fusion 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 we need. 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. These are 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 fairly 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 in this case the size indicated that a black hole with a mass of more than 60 solar masses was formed from two holes with a mass of 25-30 and 35-40 solar masses. Knowing this data, you can get full energy splash. 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
At the moment, the most important result of LIGO's work is 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 yield a lot of new, extremely necessary 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 nuclear physics and the behavior of matter at ultra-high density 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.
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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 direction 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 driving at high accelerations.
physical characteristics
Fluctuations in the space-time metric manifest themselves as changes in the gravitational field. This phenomenon is otherwise called space-time ripples. The gravitational wave affects the encountered bodies and objects, compressing and stretching them. The magnitude of the deformation is very insignificant - about 10 -21 from the original size. The whole difficulty of detecting this phenomenon was that researchers needed to learn how to measure and record such changes using appropriate equipment. The power of gravitational radiation is also extremely small - for the entire solar system it is several kilowatts.
The speed of propagation of gravitational waves depends slightly on the properties of the conducting medium. The amplitude of oscillations gradually decreases with distance from the source, but never reaches zero. The frequency ranges from several tens to hundreds of hertz. The speed of gravitational waves in the interstellar medium approaches the speed of light.
Circumstantial evidence
The first theoretical confirmation of the existence of gravitational waves was obtained by the American astronomer Joseph Taylor and his assistant Russell Hulse in 1974. Studying the vastness of the Universe using the Arecibo Observatory radio telescope (Puerto Rico), researchers discovered the pulsar PSR B1913+16, which is a binary system of neutron stars rotating around a common center of mass with a constant angular velocity (a rather rare case). Every year the circulation period, originally 3.75 hours, is reduced by 70 ms. This value is fully consistent with the conclusions from the general relativity equations, which predict an increase in the rotation speed of such systems due to the expenditure of energy on the generation of gravitational waves. Subsequently, several double pulsars and white dwarfs with similar behavior were discovered. Radio astronomers D. Taylor and R. Hulse were awarded the Nobel Prize in Physics in 1993 for discovering new possibilities for studying gravitational fields.
Escaping gravitational wave
The first announcement about the detection of gravitational waves came from University of Maryland scientist Joseph Weber (USA) in 1969. For these purposes, he used two gravitational antennas of his own design, separated by a distance of two kilometers. The resonant detector was a well-vibration-insulated solid two-meter aluminum cylinder equipped with sensitive piezoelectric sensors. The amplitude of the oscillations allegedly recorded by Weber turned out to be more than a million times higher than the expected value. Attempts by other scientists to repeat the “success” of the American physicist using similar equipment did not bring positive results. A few years later, Weber’s work in this area was recognized as untenable, but gave impetus to the development of the “gravitational boom”, which attracted many specialists to this area of research. By the way, Joseph Weber himself was sure until the end of his days that he received gravitational waves.
Improving receiving equipment
In the 70s, scientist Bill Fairbank (USA) developed the design of a gravitational wave antenna, cooled using SQUIDS - ultra-sensitive magnetometers. The technologies existing at that time did not allow the inventor to see his product realized in “metal”.
The Auriga gravitational detector at the National Legnara Laboratory (Padua, Italy) is based on 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, cooled almost to absolute zero vacuum chamber. To record and detect shocks, an auxiliary kilogram resonator and a computer-based measuring complex are used. The stated sensitivity of the equipment is 10 -20.
Interferometers
The operation of interference detectors of gravitational waves is based on the same principles on which the Michelson interferometer operates. The laser beam emitted by the source is divided into two streams. After multiple reflections and travels along the arms of the device, the flows are again brought together, and based on the final one it is judged whether any disturbances (for example, a gravitational wave) affected the course of the rays. Similar equipment has been created in many countries:
- GEO 600 (Hannover, Germany). The length of the vacuum tunnels is 600 meters.
- TAMA (Japan) with shoulders of 300 m.
- VIRGO (Pisa, Italy) is a joint French-Italian project launched in 2007 with three kilometers of tunnels.
- LIGO (USA, Pacific Coast), which has been hunting for gravitational waves since 2002.
The latter is worth considering in more detail.
LIGO Advanced
The project was created on the initiative of scientists from the Massachusetts and California Institutes of Technology. It includes two observatories, separated by 3 thousand km, in and Washington (the cities of Livingston and Hanford) with three identical interferometers. The length of perpendicular vacuum tunnels is 4 thousand meters. These are the largest such structures currently in operation. Until 2011, numerous attempts to detect gravitational waves did not bring any results. The significant modernization carried out (Advanced LIGO) increased the sensitivity of the equipment in the range of 300-500 Hz by more than five times, and in the low-frequency region (up to 60 Hz) by almost an order of magnitude, reaching the coveted value of 10 -21. The updated project started in September 2015, and the efforts of more than a thousand collaboration employees were rewarded with the results obtained.
Gravitational waves detected
On September 14, 2015, advanced LIGO detectors, with an interval of 7 ms, recorded gravitational waves reaching our planet from the largest phenomenon 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 big hopes are assigned to 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 relict gravitational radiation, which can tell about the first moments of our world after Big Bang, more sensitive space instruments will be required. Such a project exists ( Big Bang Observer), but its implementation, according to experts, is possible no earlier than in 30-40 years.
Wave your hand and gravitational waves will run throughout the Universe.
S. Popov, M. Prokhorov. Phantom Waves of the Universe
An event has occurred in astrophysics that has been awaited for decades. After half a century of searching, gravitational waves, the vibrations of space-time itself, predicted by Einstein a hundred years ago, have finally been discovered. On September 14, 2015, the upgraded LIGO observatory detected a gravitational wave burst generated by the merger of two black holes with masses of 29 and 36 solar masses in a distant galaxy approximately 1.3 billion light years away. Gravitational-wave astronomy has become a full-fledged branch of physics; she opened up to us new way 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 universal gravity. 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 gravitational interaction so weak that no experimental facilities are not able to notice the emission of gravitational waves from ordinary “emitters”.
To “chase” a powerful wave, you need to greatly distort space-time. Perfect option- two black holes rotating around each other in a close dance, at a distance of about 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 be a noticeable part of initial mass black holes, and this radiation will splash out in a split 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 set orbital motion 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.
Much more promising seemed to be detectors that use not just one resonating object, but track the distance between two unrelated, independently suspended bodies, for example, two mirrors. Due to the vibration of space caused by the gravitational wave, the distance between the mirrors will be either a little larger or a little smaller. Moreover, the greater the length of the arm, the greater the absolute displacement will be caused by a gravitational wave of a given amplitude. These vibrations can be felt by a laser beam running between the mirrors. Such a scheme is capable of detecting oscillations in a wide frequency range, from 10 hertz to 10 kilohertz, and this is exactly the 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).
Participates in the race for gravitational waves whole list countries; Russia has its own installation, at the Baksan Observatory, and, by the way, it is described in the documentary popular science film by Dmitry Zavilgelsky "Waiting for Waves and Particles". The leaders of this race are now two laboratories - the American LIGO project and the Italian Virgo detector. LIGO includes two identical detectors, located in Hanford (Washington State) and Livingston (Louisiana) and separated by 3000 km from each other. Having two settings is important for two reasons. Firstly, the signal will be considered registered only if it is seen by both detectors at the same time. And secondly, by the difference in the arrival of a gravitational wave burst at two installations - and it can reach 10 milliseconds - one can approximately determine from which part of the sky this signal came. True, with two detectors the error will be very large, but when Virgo comes into operation, the accuracy will increase noticeably.
Strictly speaking, the idea of interferometric detection of gravitational waves was first proposed 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 affect the development real projects(cm. historical overview Physics of gravitational wave detection: resonant and interferometric detectors).
The creation of the LIGO gravitational observatory was the initiative of three scientists from the Massachusetts Institute of Technology (MIT) and the California Institute of Technology (Caltech). These are Rainer Weiss, who realized the idea of an interferometric gravitational wave detector, Ronald Drever, who achieved stability of laser light sufficient for detection, and Kip Thorne, the theoretician behind the project, now well known to the general public as a scientific consultant movie "Interstellar". You can read about the early history of LIGO in a recent interview with Rainer Weiss and in the memoirs of John Preskill.
Activities related to the project of interferometric detection of gravitational waves began in the late 1970s, and at first many people also doubted the feasibility of this undertaking. However, after demonstrating a number of prototypes, the current LIGO design was written and approved. It was built throughout 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. An important role in the implementation of the project was played by Soviet and Russian physicists. From the very beginning, the already mentioned group of Vladimir Braginsky from Moscow State University took an active part in the implementation of the LIGO project, and later the Institute of Applied Physics from Nizhny Novgorod also joined the collaboration.
The LIGO observatory began operation in 2002 and until 2010 it hosted six scientific observation sessions. No gravitational wave bursts were reliably detected, and physicists were only able to set upper limits on the frequency of such events. This, however, did not surprise them too much: estimates showed that in that part of the Universe that the detector was then “listening” to, the probability of a sufficiently powerful cataclysm was low: approximately once every few decades.
Finish line
From 2010 to 2015, the LIGO and Virgo collaborations radically modernized the equipment (Virgo, however, is still in the process of preparation). And now the long-awaited target was in direct sight. LIGO - or rather, aLIGO ( Advanced LIGO) - was now ready to catch bursts generated by neutron stars at a distance of 60 megaparsecs, and black holes - at a distance of hundreds of megaparsecs. The volume of the Universe open to gravitational wave listening has increased tenfold compared to previous sessions.
Of course, it is impossible to predict when and where the next gravitational wave boom will occur. But the sensitivity of the updated detectors made it possible to count on several neutron star mergers per year, so the first burst could be expected already during the first four-month observation session. If we talk about the entire aLIGO project, which lasted several years, then the verdict was extremely clear: either bursts will fall one after another, or something in general relativity fundamentally does not work. Both will be big discoveries.
From September 18, 2015 to January 12, 2016, the first aLIGO observation session took place. During all this time, rumors about the registration of gravitational waves circulated on the Internet, but the collaboration remained silent: “we are collecting and analyzing data and are not yet ready to report the results.” An additional intrigue was created by the fact that during the analysis process the members of the collaboration themselves cannot be completely sure that they are seeing a real gravitational wave burst. The fact is that in LIGO, a computer-generated burst is occasionally artificially introduced into the stream of real data. It’s called “blind injection,” and out of the entire group, only three people (!) have access to the system that carries it out at an arbitrary point in time. The team must track this surge, analyze it responsibly, and only at the most last stages analysis “the cards are revealed” and the members of the collaboration will find out whether this was a 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 difficult, unapproachable scientific problem does not give in 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. Of course, you understand that something is going on somewhere active work according to her decision, but the final result for you as an outsider remains unchanged. The problem is perceived as a static background, as a decoration, as an eternal and almost unchanged element of physics on the scale of your scientific life. Like a task that has always been and will be.
And then - they solve it. And suddenly, on a scale of several days, you feel that the physical picture of the world has changed and that now it must be formulated in other terms and ask other questions.
For the people directly working on the search for gravitational waves, this task, of course, did not remain unchanged. They see the goal, they know what needs to be achieved. They, of course, hope that nature will also meet them halfway and throw a powerful splash in some nearby galaxy, but at the same time they understand that, even if nature is not so supportive, it will no longer be able to hide from scientists. The only question is when exactly they will be able to achieve their technical goals. A story about this sensation from a person who has been searching for gravitational waves for several decades can be heard in the film already mentioned "Waiting for Waves and Particles".
Opening
In Fig. 7 shown main result: 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 simply 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 do gravitational waves tell you?
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 direct proof and 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 to turn in these seconds in the right direction and start shooting 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 by S. Rosswog, 2015. Multi-messenger picture of compact binary mergers.
Thirdly, recording the burst that came from the supernova and comparing it with optical observations will finally make it possible to understand in detail what is happening inside, at the very beginning of the collapse. Now physicists still have difficulties with numerical modeling of this process.
Fourthly, physicists involved in the theory of gravity have a coveted “laboratory” for studying the effects of strong gravity. Until now, all the effects of general relativity that we could directly observe related to gravity in weak fields. We could guess what happens in conditions of strong gravity, when distortions of space-time begin to strongly interact with themselves, only from indirect manifestations, through the optical echo of cosmic catastrophes.
Fifth, there is a new opportunity to test exotic theories of gravity. There are already many such theories in modern physics, see, for example, the chapter dedicated to them from the popular book by A. N. Petrov “Gravity”. Some of these theories resemble conventional general relativity in the limit of weak fields, but can be very different when gravity becomes very strong. Others admit the existence of a new type of polarization for gravitational waves and predict a speed slightly different from the speed of light. Finally, there are theories that include additional spatial dimensions. What can be said about them based on gravitational waves is an open question, but it is clear that some information can be profited from here. We also recommend reading the opinion of astrophysicists themselves about what will change with the discovery of gravitational waves, in a selection on Postnauka.
Future plans
The prospects for gravitational wave astronomy are most encouraging. Now only the first, shortest observational session of the aLIGO detector has completed - and already in this short time a clear signal was detected. 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 restore the position of sources on 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 recent years 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 neutrinos high energies. 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.
