Use gravitational waves. The essence of gravitational waves in simple words

Valentin Nikolayevich Rudenko shares the history of his visit to the city of Kashina (Italy), where he spent a week on the then newly built "gravitational antenna" - an optical Michelson interferometer. On the way to the destination, the taxi driver asks what the installation was built for. “Here people think it's for talking with God,” the driver admits.

- What are gravitational waves?

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

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

- Did Einstein predict gravitational waves?

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

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

- What is the peculiarity of the gravitational interaction?

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

The gravitational force is the weakest we know of today.

- Who was the first to try to catch the gravitational wave?

- The gravitational wave experiment was first conducted by Joseph Weber of the University of Maryland (USA). He created a gravitational detector, which is now housed in the Smithsonian Museum in Washington. In 1968-1972, Joe Weber conducted a series of observations on a pair of spatially separated detectors, trying to isolate cases of "coincidence". The technique of coincidences is borrowed from nuclear physics. The low statistical significance of the gravitational signals received by Weber caused a critical attitude to the results of the experiment: there was no certainty that it was possible to fix gravitational waves. Later, scientists tried to increase the sensitivity of the Weber-type detectors. It took 45 years to develop a detector, the sensitivity of which was adequate for astrophysical predictions.

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

- Why was it not immediately announced that the signal was fixed?

- Gravitational waves were recorded back in September 2015. But even if a coincidence was recorded, it is necessary, before declaring, to prove that it is not accidental. In a signal taken from any antenna, there are always noise spikes (short-term bursts), and one of them can accidentally occur simultaneously with a noise burst on another antenna. It is possible to prove that the coincidence did not happen by chance only with the help of statistical evaluations.

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

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

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

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

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

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

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

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

- What can the discovery give in the future?

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

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

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

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

Interviewed by Anna Utkina

Gravity Waves - Artist's Image

Gravitational waves are perturbations of the space-time metric, breaking away from the source and propagating like waves (the so-called "ripple of space-time").

In general relativity and in most other modern theories of gravity, gravitational waves are generated by the motion of massive bodies with variable acceleration. Gravitational waves propagate freely in space at the speed of light. Due to the relative weakness of gravitational forces (in comparison with others), these waves have a very small magnitude, which is difficult to register.

Polarized gravitational wave

Gravitational waves are predicted by general relativity (GR), and many others. They were first spotted directly in September 2015 by two twin detectors that detected gravitational waves, likely from the merger of the two to form one more massive rotating black hole. Indirect evidence of their existence has been known since the 1970s - general relativity predicts the rates of convergence of close systems coinciding with observations due to the loss of energy due to the radiation 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 the Einstein equations of the wave type, which are a perturbation of the space-time metric moving at the speed of light (in the linear approximation). A manifestation of this disturbance should be, in particular, a periodic change in the distance between two freely falling (that is, not experiencing the influence of any forces) test masses. Amplitude h a gravitational wave is a dimensionless quantity - the relative change in distance. The predicted maximum amplitudes of gravitational waves from astrophysical objects (for example, compact binary systems) and phenomena (explosions, mergers, captures by black holes, etc.) when measured in 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 in different ways. In general relativity, it is equal to the speed of light (in a linear approximation). In other theories of gravity, it can take any value, including infinity. According to the data of the first registration of gravitational waves, their dispersion turned out to be compatible with a massless graviton, and the speed was estimated as equal to the speed of light.

Generating gravitational waves

A system of two neutron stars creates ripples in spacetime

Any matter moving with asymmetric acceleration emits a gravitational wave. For the appearance of a wave of significant amplitude, an extremely large mass of the emitter or / and huge accelerations are required, the amplitude of the gravitational wave is directly proportional to first derivative of acceleration and the mass of the generator, that is, ~. However, if some object is moving with acceleration, then this means that some force acts on it from the side of another object. In turn, this other object experiences the opposite effect (according to Newton's third law), while it turns out that m 1 a 1 = − m 2 a 2 ... It turns out that two objects emit gravitational waves only in pairs, and as a result of interference, they are mutually extinguished almost completely. Therefore, gravitational radiation in the general theory of relativity always has a multipole character of at least quadrupole radiation. In addition, for nonrelativistic emitters in the expression for the radiation intensity there is a small parameter where is the gravitational radius of the emitter, r- its characteristic size, T- a characteristic period of movement, c- the speed of light in a vacuum.

The strongest sources of gravitational waves are:

• colliding (giant masses, very small accelerations),
• gravitational collapse of a binary system of compact objects (colossal accelerations at a rather large mass). As a particular and most interesting case - the merging of neutron stars. For such a system, the gravitational-wave luminosity is close to the maximum possible Planck luminosity in nature.

Gravitational waves emitted by a two-body system

Two bodies moving in circular orbits around a common center of mass

Two gravitationally bound bodies with masses m 1 and m 2 moving nonrelativistically ( v << c) along circular orbits around their common center of mass at a distance r from each other, emit gravitational waves of the following energy, on average over the period:

As a result, the system loses energy, which leads to the convergence of bodies, that is, to a decrease in the distance between them. The speed of convergence of bodies:

For the solar system, for example, the largest gravitational radiation is produced by the subsystem and. The power of this radiation is about 5 kilowatts. Thus, the energy lost by the solar system for gravitational radiation per year is absolutely negligible in comparison with the characteristic kinetic energy of bodies.

Gravitational collapse of the binary system

Any binary star, when its components rotate around a common center of mass, loses energy (as it is assumed - due to the radiation of gravitational waves) and, in the end, merges together. But for ordinary, non-compact, binary stars, this process takes a very long time, much more than the present age. If the binary compact system consists of a pair of neutron stars, black holes, or a combination of both, then the merger can occur in several million years. At first, the objects approach each other, and their orbital period decreases. Then, at the final stage, there is a collision and an asymmetric gravitational collapse. This process lasts a fraction of a second, and during this time energy is released into gravitational radiation, which, according to some estimates, is more than 50% of the mass of the system.

Basic exact solutions of Einstein's equations for gravitational waves

Bondi - Pirani - Robinson body waves

These waves are described by a metric of the form. If we introduce a variable and a function, then from the equations of general relativity we obtain the equation

Takeno metric

has the form, -functions, satisfy the same equation.

Rosen metric

Where are satisfied

Perez metric

Wherein

Einstein - Rosen cylindrical waves

In cylindrical coordinates, such waves have the form and are fulfilled

Registration of gravitational waves

Registration of gravitational waves is rather difficult due to the weakness of the latter (small distortion of the metric). Devices for their registration are gravitational wave detectors. Attempts to detect gravitational waves have been made since the late 1960s. The gravitational waves of the detected amplitude are generated during the collapse of the binary. Similar events occur in the vicinity approximately once a decade.

On the other hand, general relativity predicts the acceleration of mutual rotation of binary stars due to energy loss due to the emission of gravitational waves, and this effect has been reliably recorded in several known systems of binary compact objects (in particular, pulsars with compact companions). In 1993 "for the discovery of a new type of pulsars, which gave new possibilities in the study of gravity" to the discoverers of the first binary pulsar PSR B1913 + 16 Russell Hulse and Joseph Taylor Jr. was awarded the Nobel Prize in Physics. The acceleration of rotation observed in this system completely coincides with the predictions of general relativity for the emission of gravitational waves. The same phenomenon was recorded in several more cases: for the pulsars PSR J0737-3039, PSR J0437-4715, SDSS J065133.338 + 284423.37 (usually abbreviated as J0651) and the binary RX J0806 system. For example, the distance between the two components A and B of the first binary star of two pulsars PSR J0737-3039 decreases by about 2.5 inches (6.35 cm) per day due to energy losses due to gravitational waves, and this happens in accordance with general relativity ... All these data are interpreted as indirect confirmation of the existence of gravitational waves.

According to estimates, the strongest and most frequent sources of gravitational waves for gravitational telescopes and antennas are catastrophes associated with collapses of binary systems in nearby galaxies. It is expected that in the near future, improved gravitational detectors will register several similar events per year, distorting the metric in the vicinity by 10 −21 -10 −23. The first observations of the optical-metric parametric resonance signal, which makes it possible to detect the effect of gravitational waves from periodic sources of the close binary type on the radiation of cosmic masers, were possibly obtained at the Radio Astronomy Observatory of the Russian Academy of Sciences, Pushchino.

Another possibility of detecting the background of gravitational waves filling the Universe is high-precision timing of distant pulsars - analysis of the time of arrival of their pulses, which changes in a characteristic manner under the action of gravitational waves passing through the space between the Earth and the pulsar. According to estimates for 2013, the timing accuracy needs to be raised by about one order of magnitude so that background waves from many sources in our Universe can be detected, and this problem can be solved by the end of the decade.

According to modern concepts, our Universe is filled with relic gravitational waves that appeared in the first moments after. Their registration will provide information about the processes at the beginning of the birth of the Universe. On March 17, 2014 at 20:00 Moscow time at the Harvard-Smithsonian Center for Astrophysics, the American group of researchers working on the BICEP 2 project announced the detection of nonzero tensor perturbations in the early Universe by polarization of the relict radiation, which is also the discovery of these relic gravitational waves ... However, this result was disputed almost immediately, as it turned out that the contribution had not been properly accounted for. One of the authors, J.M. Kovac ( Kovac J. M.), acknowledged that "the participants and science journalists were a little too hasty with the interpretation and coverage of the BICEP2 data."

Experimental confirmation of existence

The first recorded gravitational wave signal. Left data from the detector in Hanford (H1), right - Livingston (L1). Time is counted from 14 September 2015, 09:50:45 UTC. To visualize the signal, it was filtered by a frequency filter with a bandwidth of 35-350 Hertz to suppress large fluctuations outside the high sensitivity range of the detectors; band-notch filters were also used to suppress the noise of the installations themselves. Top row: voltages h in detectors. GW150914 first arrived on L1 and after 6 9 +0 5 −0 4 ms on H1; for visual comparison, data from H1 are shown on the L1 plot in reverse and time-shifted form (to take into account the relative orientation of the detectors). Second row: voltages h from the gravitational wave signal, passed through the same 35-350 Hz bandpass filter. The solid line is the result of numerical relativity for a system with parameters consistent with those found on the basis of studying the GW150914 signal, obtained by two independent codes with a resultant coincidence of 99.9. Thick gray lines - 90% confidence probability areas of the waveform, recovered from these detectors by two different methods. The dark gray line simulates the expected signals from black hole mergers, the light gray line does not use astrophysical models, but represents the signal as a linear combination of sinusoidal Gaussian wavelets. Reconstructions overlap 94%. Third row: Residual errors after extracting the filtered prediction of the numerical relativity signal from the filtered signal of the detectors. Bottom row: Voltage frequency map view showing the increase in the dominant signal frequency over time.

11 February 2016 with LIGO and VIRGO collaborations. The signal from the merger of two black holes with an amplitude at the maximum of about 10 −21 was recorded on September 14, 2015 at 9:51 UTC by two LIGO detectors in Hanford and Livingstone 7 milliseconds apart, in the region of the maximum signal amplitude (0.2 seconds) combined the signal-to-noise ratio was 24: 1. The signal was designated GW150914. The waveform matches the prediction of general relativity for the merger of two black holes of 36 and 29 solar masses; the resulting black hole should have a mass of 62 solar and the rotation parameter a= 0.67. The distance to the source is about 1.3 billion, the energy emitted in tenths of a second in the merger is the equivalent of about 3 solar masses.

History

The history of the term "gravitational wave" itself, the theoretical and experimental search for these waves, as well as their use for the study of phenomena inaccessible to other methods.

• 1900 - Lorentz suggested that gravity "... can propagate at a speed no greater than the speed of light";
• 1905 - Poincaré first coined the term gravitational wave (onde gravifique). Poincaré, on a qualitative level, removed the well-established objections of Laplace and showed that corrections to the generally accepted Newtonian laws of gravitation of order associated with gravitational waves are canceled, thus, the assumption of the existence of gravitational waves does not contradict observations;
• 1916 - Einstein showed that within the framework of general relativity, a mechanical system will transfer energy to gravitational waves and, roughly speaking, any rotation relative to fixed stars must sooner or later stop, although, of course, under normal conditions, energy losses of the order are negligible and practically cannot be measured (in In this work, he still mistakenly believed that a mechanical system that constantly preserves spherical symmetry can emit gravitational waves);
• 1918 - Einstein derived a quadrupole formula, in which the radiation of gravitational waves turns out to be an order effect, thereby correcting an error in his previous work (there was an error in the coefficient, the wave energy is 2 times less);
• 1923 - Eddington - questioned the physical reality of gravitational waves "... spread ... with the speed of thought." In 1934, while preparing the Russian translation of his monograph "Theory of Relativity", Eddington added several chapters, including chapters with two options for calculating energy losses by a rotating rod, but noted that the methods used for approximate calculations of general relativity, in his opinion, are inapplicable to gravitationally bound systems therefore doubts remain;
• 1937 - Einstein, together with Rosen, investigated cylindrical wave solutions of the exact equations of the gravitational field. In the course of these studies, they had doubts that gravitational waves may be an artifact of approximate solutions of the equations of general relativity (there is a known correspondence regarding the review of the article by Einstein and Rosen "Do gravitational waves exist?"). Later he found an error in reasoning, the final version of the article with fundamental edits was published already in the Journal of the Franklin Institute;
• 1957 - Herman Bondi and Richard Feynman proposed a thought experiment "cane with beads" in which they substantiated the existence of the physical consequences of gravitational waves in general relativity;
• 1962 - Vladislav Pustovoit and Mikhail Hertsenstein describe the principles of using interferometers to detect long-wave gravitational waves;
• 1964 - Philip Peters and John Matthew theoretically describe gravitational waves emitted by binary systems;
• 1969 - Joseph Weber, founder of gravitational wave astronomy, reports the detection of gravitational waves using a resonant detector - a mechanical gravitational antenna. These messages give rise to the rapid growth of work in this direction, in particular, Rainier Weiss, one of the founders of the LIGO project, began experiments at that time. To date (2015), no one has managed to obtain reliable confirmation of these events;
• 1978 - Joseph Taylor reported the discovery of gravitational radiation in the binary system of the pulsar PSR B1913 + 16. Research by Joseph Taylor and Russell Hulse earned the 1993 Nobel Prize in Physics. At the beginning of 2015, three post-Keplerian parameters, including a decrease in the period due to the emission of gravitational waves, were measured for at least 8 such systems;
• 2002 - Sergey Kopeikin and Edward Fomalont measured the deflection of light in Jupiter's gravitational field in dynamics using radio wave interferometry with an ultra-long baseline, which for a certain class of hypothetical extensions of general relativity makes it possible to estimate the speed of gravity - the difference from the speed of light should not exceed 20% (this interpretation does not generally accepted);
• 2006 - the international team of Martha Bourgueil (Parks Observatory, Australia) reported significantly more accurate confirmations of general relativity and the correspondence to it of the magnitude of the radiation of gravitational waves in the system of two pulsars PSR J0737-3039A / B;
• 2014 - Astronomers at the Harvard-Smithsonian Center for Astrophysics (BICEP) report the detection of primary gravitational waves when measuring CMB fluctuations. At the moment (2016), the detected fluctuations are considered not to have a relic origin, but are explained by dust emission in the Galaxy;
• 2016 - LIGO international team reported the detection of a gravitational wave propagation event GW150914. For the first time, it was reported about the direct observation of interacting massive bodies in superstrong gravitational fields with ultrahigh relative velocities (< 1,2 × R s , v/c >0.5), which made it possible to check the correctness of general relativity up to several post-Newtonian high-order terms. The measured dispersion of gravitational waves does not contradict the earlier measurements of the dispersion and the upper limit of the mass of the hypothetical graviton (< 1,2 × 10 −22 эВ), если он в некотором гипотетическом расширении ОТО будет существовать.

What are gravitational waves?

Gravitational waves - changes in the gravitational field, propagating like waves. They are emitted by moving masses, but after radiation they break away from them and exist independently of these masses. Mathematically related to the perturbation of the space-time metrics and can be described as "ripples of space-time".

In general relativity and in most other modern theories of gravity, gravitational waves are generated by the motion of massive bodies with variable acceleration. Gravitational waves propagate freely in space at the speed of light. Due to the relative weakness of gravitational forces (in comparison with others), these waves have a very small magnitude, which is difficult to register.

Gravitational waves are predicted by general relativity (GR). They were first detected directly in September 2015 by two LIGO twin detectors, which recorded gravitational waves, likely from the merger of two black holes and the formation of one more massive rotating black hole. Indirect evidence of their existence has been known since the 1970s - general relativity predicts the rates of convergence of close systems of binary stars coinciding with observations due to the loss of energy due to the radiation 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.

If we imagine our space-time as a grid of coordinates, then gravitational waves are disturbances, ripples that will run along the grid when massive bodies (for example, black holes) distort the space around them.

It can be compared to an earthquake. Imagine that you live in a city. It has some kind of markers that create an urban space: houses, trees, and so on. They are motionless. When a major earthquake occurs somewhere near the city, the vibrations reach us - and even motionless houses and trees begin to vibrate. These vibrations are the gravitational waves; and the objects that vibrate are space and time.

Why have scientists been unable to register gravitational waves for so long?

Concrete efforts to detect gravitational waves began in the post-war period with a few naive devices, the sensitivity of which, obviously, could not be enough to detect such fluctuations. Over time, it became clear that detectors for search must be very large-scale - and they must use modern laser technology. It is with the development of modern laser technologies that it became possible to control the geometry, the perturbations of which are the gravitational wave. Powerful advances in technology played a key role in this discovery. No matter how brilliant the scientists were, 30-40 years ago it was technically impossible to do this.

Why is wave detection important to physics?

Gravitational waves were predicted by Albert Einstein in general relativity about a hundred years ago. Throughout the 20th century, there were physicists who questioned this theory, although more and more confirmation appeared. And the presence of gravitational waves is such a critical confirmation of the theory.

In addition, before the registration of gravitational waves, we knew how gravity behaves only through the example of celestial mechanics, the interaction of celestial bodies. But it was clear that the gravitational field has waves and space-time can deform in a similar way. The fact that we had not seen gravitational waves before was a blind spot in modern physics. Now this white spot has been closed, another brick has been laid at the foundation of modern physical theory. This is a fundamental discovery. There has been nothing comparable in recent years.

"Waiting for Waves and Particles" - a documentary about the search for gravitational waves(by Dmitry Zavilgelskiy)

There is also a practical point in the registration of gravitational waves. Probably, after the further development of technology, it will be possible to talk about gravitational astronomy - about observing the traces of the most high-energy events in the Universe. But now it is too early to talk about this, we are talking only about the very fact of registration of waves, and not about clarifying the characteristics of the objects that generate these waves.

Just a few hours ago, the news came, which has long been awaited in the scientific world. A group of scientists from several countries, working as part of the international project LIGO Scientific Collaboration, claim that with the help of several observatories-detectors they managed to record gravitational waves in laboratory conditions.

They are analyzing data from two Laser Interferometer Gravitational-Wave Observatories (LIGO) located in Louisiana and Washington, USA.

As mentioned at the press conference of the LIGO project, gravitational waves were recorded on September 14, 2015, first at one observatory, and then 7 milliseconds later at another.

Based on the analysis of the data obtained, which was carried out by scientists from many countries, including Russia, it was found that the gravitational wave was caused by the collision of two black holes with a mass of 29 and 36 times the mass of the Sun. After that, they merged into one large black hole.

This happened 1.3 billion years ago. The signal came to Earth from the direction of the Magellanic Cloud constellation.

Sergei Popov (astrophysicist of the Sternberg State Astronomical Institute, Moscow State University) explained what gravitational waves are and why it is so important to measure them.

Modern theories of gravity are geometric theories of gravity, more or less everything, starting with the theory of relativity. The geometric properties of space affect the movement of bodies or objects such as a light beam. And vice versa - the distribution of energy (this is the same as mass in space) affects the geometric properties of space. It's very cool, because it's easy to visualize - all this elastic plane lined in a cell has a certain physical meaning under it, although, of course, everything is not so literal.

Physicists use the word "metric". A metric is what describes the geometric properties of a space. And here we have bodies moving with acceleration. The simplest thing is that the cucumber rotates. It is important that it is, for example, not a ball or a flattened disc. It is easy to imagine that when such a cucumber spins on an elastic plane, ripples will run from it. Imagine that you are standing somewhere, and the cucumber will turn with one end to you, then the other. It affects space and time in different ways, a gravitational wave runs.

So, a gravitational wave is a ripple running along the space-time metric.

Beads in space

This is a fundamental property of our basic understanding of how gravity works, and people have wanted to test it for a hundred years. They want to make sure that the effect is there and that it is visible in the laboratory. In nature, this was seen already about three decades ago. How should gravitational waves manifest themselves in everyday life?

The easiest way to illustrate this is as follows: if you throw beads in space so that they lie in a circle, and when the gravitational wave passes perpendicular to their plane, they will begin to turn into an ellipse, compressed in one direction, then in the other. The point is that the space around them will be outraged, and they will feel it.

"G" on Earth

This is about the kind of thing people do, only not in space, but on Earth.

At a distance of four kilometers from each other hang mirrors in the form of the letter "g" [referring to the American observatories LIGO].

Laser beams are running - this is an interferometer, a well-understood thing. Modern technology makes it possible to measure a fantastically small effect. I still do not really believe, I believe, but it just does not fit in my head - the displacement of the mirrors hanging at a distance of four kilometers from each other is less than the size of an atomic nucleus. This is small even compared to the wavelength of this laser. This was the catch: gravity is the weakest interaction, and therefore the displacements are very small.

It took a very long time, people have been trying to do this since the 1970s, they have spent their lives looking for gravitational waves. And now, only technical capabilities make it possible to obtain registration of a gravitational wave in laboratory conditions, that is, here it came, and the mirrors have shifted.

Direction

Within a year, if everything goes well, then three detectors will work in the world. Three detectors are very important, because these things are very bad at determining the direction of the signal. In about the same way as we do by ear, we poorly determine the direction of the source. “Sound from somewhere to the right” - these detectors feel something like that. But if there are three people at a distance from each other, and one hears a sound to the right, another to the left, and the third from behind, then we can very accurately determine the direction of the sound. The more detectors there are, the more they are scattered around the globe, the more accurately we can determine the direction to the source, and then astronomy will begin.

After all, the ultimate task is not only to confirm the general theory of relativity, but also to obtain new astronomical knowledge. Imagine that there is a black hole weighing ten times the mass of the Sun. And it collides with another black hole weighing ten times the mass of the Sun. The collision takes place at the speed of light. Energy breakthrough. This is true. There is a fantastic amount of it. And its not in any way ... It's just ripples of space and time. I would say that detecting the merger of two black holes for a long time will be the most reliable confirmation that black holes are about the kind of black holes we think of.

Let's go over the issues and phenomena that she could reveal.

Do black holes really exist?

The signal expected from the LIGO announcement may have been produced by two merging black holes. Events like these are the most energetic known; the force of the gravitational waves emitted by them can briefly eclipse all the stars of the observable universe in total. Merging black holes are also quite easy to interpret in terms of very pure gravitational waves.

Merging black holes occurs when two black holes spiral around each other, emitting energy in the form of gravitational waves. These waves have a characteristic sound (chirp) that can be used to measure the mass of these two objects. After that, black holes usually merge.

“Imagine two soap bubbles coming so close that they form one bubble. The larger bubble is deformed, ”says Tybalt Damour, a gravitational theorist at the Institute for Advanced Scientific Research near Paris. The final black hole will be perfectly spherical, but must first emit predictable gravitational waves.

One of the most important scientific implications of black hole mergers will be confirmation of the existence of black holes - at least perfectly circular objects made up of pure, empty, curved spacetime, as predicted by general relativity. Another consequence is that the merger proceeds as predicted by the scientists. Astronomers have a lot of indirect confirmation of this phenomenon, but so far these have been observations of stars and superheated gas in the orbit of black holes, and not the black holes themselves.

“The scientific community, myself included, dislikes black holes. We take them for granted, ”says Frans Pretorius, a specialist in general relativity simulations at Princeton University in New Jersey. "But if you think about what an amazing prediction this is, we need some truly amazing proof."

Do gravitational waves move at the speed of light?

When scientists start comparing LIGO observations with those of other telescopes, the first thing they check is whether the signal arrived at the same time. Physicists believe that gravity is transmitted by graviton particles, the gravitational analogue of photons. If, like photons, these particles have no mass, then gravitational waves will move at the speed of light, consistent with the prediction of the speed of gravitational waves in classical relativity. (Their speed can be influenced by the accelerating expansion of the Universe, but this should manifest itself at distances significantly exceeding those covered by LIGO).

It is quite possible, however, that gravitons have a small mass, which means that gravitational waves will move at a speed less than light. So, for example, if LIGO and Virgo detect gravitational waves and find out that the waves arrived on Earth later than associated with a cosmic event of gamma rays, this could have fatal consequences for fundamental physics.

Is spacetime made of cosmic strings?

An even stranger discovery could happen if bursts of gravitational waves are detected emanating from "cosmic strings." These hypothetical space-time curvature defects, which may or may not be related to string theories, should be infinitely thin but stretched over cosmic distances. Scientists predict that cosmic strings, if they exist, could bend accidentally; if the string bends, it will cause a gravitational surge that detectors like the LIGO or Virgo could measure.

Can neutron stars be jagged?

Neutron stars are the remnants of large stars that collapsed under their own weight and became so dense that electrons and protons began to melt into neutrons. Scientists have little understanding of the physics of neutron holes, but gravitational waves could tell a lot about them. For example, the intense gravity on their surface causes neutron stars to become almost perfectly spherical. But some scientists have suggested that they may also have “mountains” - a few millimeters high - that make these dense objects, no more than 10 kilometers in diameter, slightly asymmetrical. Neutron stars usually spin very quickly, so an asymmetric mass distribution will warp spacetime and produce a constant sinusoidal gravitational wave signal, slowing the star's rotation and emitting energy.

Pairs of neutron stars that revolve around each other also produce a constant signal. Like black holes, these stars spiral and ultimately merge with a distinctive sound. But its specificity differs from the specificity of the sound of black holes.

Why do stars explode?

Black holes and neutron stars form when massive stars stop shining and collapse into themselves. Astrophysicists think this process is at the heart of all common types of Type II supernova explosions. Simulations of such supernovae have not yet revealed why they ignite, but listening to the gravitational wave bursts emitted by a real supernova is believed to provide an answer. Depending on what the burst waves look like, how loud they are, how often they occur, and how they correlate with supernovae tracked by electromagnetic telescopes, this data could help rule out a bunch of existing models.

How fast is the universe expanding?

The expanding universe means that distant objects that move away from our galaxy appear redder than they actually are, as the light they emit is stretched out as they move. Cosmologists estimate the rate of expansion of the universe by comparing the redshift of galaxies to how far away they are from us. But this distance is usually estimated from the brightness of Type Ia supernovae, and this technique leaves a lot of uncertainties.

If several gravitational wave detectors around the world detect signals from the merger of the same neutron stars, together they can absolutely accurately estimate the signal's loudness, as well as the distance at which the merger took place. They will also be able to assess the direction, and with it, identify the galaxy in which the event took place. By comparing the redshift of this galaxy with the distance to the merging stars, an independent rate of cosmic expansion can be obtained, possibly more accurate than current methods allow.

Gravitational waves, theoretically predicted by Einstein back in 1917, are still waiting for their discoverer.

In late 1969, University of Maryland physics professor Joseph Weber made a sensational statement. He announced that he had discovered gravitational waves that came to Earth from the depths of space. Until that time, no scientist had made such claims, and the very possibility of detecting such waves was considered far from obvious. However, Weber was known as an authority in his field, and therefore his colleagues took his message very seriously.

However, disappointment soon followed. The amplitudes of the waves allegedly recorded by Weber were millions of times higher than the theoretical value. Weber argued that these waves came from the center of our Galaxy, which was then little known, covered by dust clouds. Astrophysicists suggested that there is a giant black hole hiding there, which devours thousands of stars every year and throws out part of the absorbed energy in the form of gravitational radiation, and astronomers have engaged in a vain search for more obvious traces of this cosmic cannibalism (now it has been proven that a black hole really exists there, but it leads itself is quite decent). Physicists from the USA, USSR, France, Germany, England, and Italy began experiments with detectors of the same type - and achieved nothing.

Scientists still do not know what to attribute the strange readings of Weber's instruments. However, his efforts were not in vain, although gravitational waves have not yet been detected. Several installations for their search have already been built or are under construction, and in ten years such detectors will be launched into space. It is quite possible that in the not so distant future gravitational radiation will become the same observable physical reality as electromagnetic oscillations. Unfortunately, Joseph Weber will no longer know this - he died in September 2000.

What are gravitational waves

It is often said that gravitational waves are perturbations of the gravitational field propagating in space. This definition is correct, but incomplete. According to general relativity, gravity arises from the curvature of the space-time continuum. Gravitational waves are fluctuations of the space-time metric, which manifest themselves as fluctuations of the gravitational field, therefore they are often figuratively called space-time ripples. Gravitational waves were theoretically predicted in 1917 by Albert Einstein. No one doubts their existence, but gravitational waves are still waiting for their discoverer.

Any movement of material bodies, leading to an inhomogeneous change in the force of gravity in the surrounding space, serves as the source of gravitational waves. A body moving at a constant speed does not emit anything, since the nature of its gravitational field does not change. Accelerations are needed to emit gravitational waves, but not any. A cylinder that rotates around its axis of symmetry experiences acceleration, but its gravitational field remains uniform, and gravitational waves do not arise. But if you spin this cylinder around another axis, the field will begin to oscillate, and gravitational waves will run from the cylinder in all directions.

This conclusion applies to any body (or system of bodies) that is asymmetric about the axis of rotation (in such cases, the body is said to have a quadrupole moment). A system of masses, the quadrupole moment of which changes with time, always emits gravitational waves.

Basic properties of gravitational waves

Astrophysicists assume that it is the radiation of gravitational waves, taking away energy, that limits the rotation speed of a massive pulsar while absorbing matter from a nearby star.

Space gravity beacons

The gravitational radiation of terrestrial sources is extremely weak. A steel column weighing 10,000 tons, suspended from the center in the horizontal plane and untwisted around the vertical axis up to 600 rpm, emits a power of about 10 -24 watts. Therefore, the only hope to detect gravitational waves is to find a cosmic source of gravitational radiation.

In this regard, close binary stars are very promising. The reason is simple: the power of the gravitational radiation of such a system grows in inverse proportion to the fifth power of its diameter. It is even better if the trajectories of the stars are strongly elongated, since this increases the rate of change of the quadrupole moment. It is perfectly fine if the binary system consists of neutron stars or black holes. Such systems are similar to gravitational beacons in space - their radiation is periodic.

In space, there are also "impulsive" sources that generate short but extremely powerful gravitational bursts. This happens when a massive star collapses prior to a supernova explosion. However, the deformation of the star must be asymmetric, otherwise radiation will not occur. During the collapse, gravitational waves can carry away with them up to 10% of the total energy of the star! The power of gravitational radiation in this case is about 10 50 W. Even more energy is released during the merger of neutron stars, here the peak power reaches 10 52 W. An excellent source of radiation is the collision of black holes: their masses can exceed the masses of neutron stars by billions of times.

Another source of gravitational waves is cosmological inflation. Immediately after the Big Bang, the Universe began to expand extremely rapidly, and in less than 10 -34 seconds its diameter increased from 10 -33 cm to macroscopic size. This process immeasurably amplified the gravitational waves that existed before it began, and their descendants have survived to this day.

Indirect confirmation

The first proof of the existence of gravitational waves is associated with the work of the American radio astronomer Joseph Taylor and his student Russell Hulse. In 1974, they discovered a pair of orbiting neutron stars (a radio-emitting pulsar with a silent companion). The pulsar rotated around its axis with a stable angular velocity (which is not always the case) and therefore served as an extremely accurate clock. This feature made it possible to measure the masses of both stars and to find out the nature of their orbital motion. It turned out that the orbital period of this binary system (about 3 h 45 min) is annually reduced by 70 μs. This value is in good agreement with the solutions of the equations of general relativity, describing the loss of energy of a stellar pair due to gravitational radiation (however, the collision of these stars will not happen soon, after 300 million years). In 1993, Taylor and Hulse were awarded the Nobel Prize for this discovery.

Gravitational Wave Antennas

How to detect gravitational waves experimentally? Weber used meter-long solid aluminum cylinders with piezoelectric sensors at the ends as detectors. They were carefully insulated from external mechanical influences in a vacuum chamber. Weber installed two of these cylinders in a bunker underneath the University of Maryland golf course, and one in Argonne National Laboratory.

The idea behind the experiment is simple. Space under the influence of gravitational waves contracts and stretches. Due to this, the cylinder vibrates in the longitudinal direction, acting as a gravitational wave antenna, and piezoelectric crystals convert the vibrations into electrical signals. Any passage of cosmic gravitational waves practically simultaneously affects detectors separated by a thousand kilometers, which makes it possible to filter out gravitational impulses from various kinds of noise.

Weber's sensors were able to detect displacements of the ends of the cylinder, equal to only 10 -15 of its length - in this case 10 -13 cm Physical Review Letters... All attempts to repeat these results have been in vain. Weber's data, moreover, contradict the theory, which practically does not allow expecting relative displacements above 10 -18 (and values ​​less than 10 -20 are much more likely). It is possible that Weber got it wrong when statistically processing the results. The first attempt to experimentally detect gravitational radiation ended in failure.

In the future, gravitational-wave antennas have been significantly improved. In 1967, American physicist Bill Fairbank proposed cooling them in liquid helium. This not only made it possible to get rid of most of the thermal noise, but also opened up the possibility of using SQUIDs (superconducting quantum interferometers), the most accurate supersensitive magnetometers. The implementation of this idea turned out to be fraught with many technical difficulties, and Fairbank itself did not live to see it. By the early 1980s, physicists from Stanford University had built a setup with a sensitivity of 10-18, but no waves were detected. Now in a number of countries there are ultra-cryogenic vibration detectors of gravitational waves operating at temperatures only tenths and hundredths of a degree above absolute zero. This is, for example, the AURIGA plant in Padua. The antenna for it is a three-meter cylinder made of an aluminum-magnesium alloy, the diameter of which is 60 cm, and the weight is 2.3 tons. It is suspended in a vacuum chamber cooled to 0.1 K. Its shakes (with a frequency of about 1000 Hz) are transmitted to an auxiliary resonator with a mass of 1 kg, which vibrates with the same frequency, but with a much greater amplitude. These vibrations are recorded by measuring equipment and analyzed using a computer. The sensitivity of the AURIGA complex is about 10 -20 -10 -21.

Interferometers

Another method for detecting gravitational waves is based on the abandonment of massive resonators in favor of light beams. The first to suggest it in 1962 were Soviet physicists Mikhail Hertsenstein and Vladislav Pustovoit, and two years later by Weber. In the early 1970s, an employee of the corporation's research laboratory Hughes aircraft Robert Forward (formerly Weber's graduate student, later a very famous science fiction writer) built the first such detector with quite decent sensitivity. At the same time, professor at the Massachusetts Institute of Technology (MIT) Rainer Weiss performed a very deep theoretical analysis of the possibilities of registering gravitational waves using optical methods.

These methods involve the use of analogs of the device with which 125 years ago the physicist Albert Michelson proved that the speed of light is strictly the same in all directions. In this installation, the Michelson interferometer, a beam of light hits a semitransparent plate and is divided into two mutually perpendicular beams, which are reflected from mirrors located at the same distance from the plate. Then the beams merge again and fall on the screen, where an interference pattern appears (light and dark stripes and lines). If the speed of light depends on its direction, then when you rotate the entire installation, this picture should change, if not, it should remain the same as before.

The gravitational interference detector works in a similar way. The transmitted wave deforms space and changes the length of each arm of the interferometer (the path along which light travels from the divider to the mirror), stretching one arm and squeezing the other. The interference picture changes and it can be registered. But this is not easy: if the expected relative change in the length of the interferometer arms is 10 -20, then with a table-top size of the device (like Michelson's) it turns into oscillations with an amplitude of the order of 10 -18 cm. For comparison: visible light waves are 10 trillion times longer! It is possible to increase the length of the shoulders to several kilometers, but problems will still remain. The laser light source must be both powerful and stable in frequency, the mirrors must be perfectly flat and ideally reflecting, the vacuum in the tubes through which the light travels must be as deep as possible, the mechanical stabilization of the entire system must be truly perfect. In short, an interference gravitational wave detector is expensive and bulky.

Today the largest installation of its kind is the American LIGO complex. (Light Interferometer Gravitational Waves Observatory). It consists of two observatories, one of which is located on the Pacific coast of the United States and the other near the Gulf of Mexico. Measurements are made using three interferometers (two in Washington state, one in Louisiana) with four kilometers of arms. The installation is equipped with mirror light accumulators, which increase its sensitivity. “Since November 2005, all three of our interferometers have been operating normally,” LIGO spokesman Peter Solson, professor of physics at Syracuse University, told Popular Mechanics. - We are constantly exchanging data with other observatories trying to detect gravitational waves with frequencies of tens and hundreds of hertz, which appeared during the most powerful supernova explosions and mergers of neutron stars and black holes. Now in service is the German interferometer GEO 600 (arm length - 600 m), located 25 km from Hanover. The 300-meter Japanese TAMA instrument is currently being upgraded. The three-kilometer Virgo detector near Pisa will join the effort in early 2007, and at frequencies less than 50 Hz it will be able to outperform LIGO. Installations with ultra-cryogenic resonators operate with increasing efficiency, although their sensitivity is still somewhat less than ours. "

Perspectives

So what will happen with the methods of detecting gravitational waves in the near future? Professor Rainer Weiss told about this to Popular Mechanics: “In a few years, more powerful lasers and more advanced detectors will be installed in the observatories of the LIGO complex, which will lead to a 15-fold increase in sensitivity. Now it is 10 -21 (at frequencies of the order of 100 Hz), and after modernization it will exceed 10 -22. The upgraded complex, Advanced LIGO, will increase the depth of penetration into space by 15 times. Professor of Moscow State University Vladimir Braginsky, one of the pioneers in the study of gravitational waves, is actively involved in this project.

In the middle of the next decade, it is planned to launch the space interferometer LISA ( Laser Interferometer Space Antenna) with a shoulder length of 5 million kilometers, it is a joint project of NASA and the European Space Agency. The sensitivity of this observatory will be hundreds of times higher than the capabilities of ground-based instruments. It is primarily designed to search for low-frequency (10 -4 -10 -1 Hz) gravitational waves that cannot be captured on the Earth's surface due to atmospheric and seismic interference. Such waves are emitted by binary star systems, which are quite typical inhabitants of the Cosmos. LISA will also be able to detect gravitational waves generated by the absorption of ordinary stars by black holes. But to detect relict gravitational waves that carry information about the state of matter in the first moments after the Big Bang, more advanced space instruments will most likely be required. Such an installation, Big bang observer, is now being discussed, but it is unlikely that it will be able to create and launch earlier than in 30-40 years. "