FAQ: the importance of combining gravitational and electromagnetic waves
The previous article about the results obtained from the LIGO / VIRGO experiments on the recognition of gravitational waves, was informational in nature and did not set as its goal pedagogical instructions. Now I will try to answer the questions of my readers and friends on this topic. Some wanted to better present what happened, while others wanted to clarify why this discovery became so important. Therefore, I wrote this article in which I explained what neutron stars and black holes are, and what their merger looks like, and clarified what the importance of this announcement is. Its importance is contained in several points, and it is rather difficult to reduce them to any one. In addition, I give answers to other questions.
For a start, I will make a reservation: I am not an expert on the complex topic of merging neutron stars and the resulting explosions, known as kilon ones. They are much harder to merge black holes. I myself will find out some details. I hope that I managed to avoid mistakes, but in some cases I do not have all the answers.
Each atom consists of a tiny atomic nucleus consisting of neutrons and protons (very similar to each other) and loosely surrounded by electrons. Most of the atom is an empty space, so under extreme conditions it can be crushed - but only if each electron and proton turns into a neutron (remaining in the same place) and a neutrino (going into space). When a giant star runs out of fuel, the pressure of its nuclear furnace drops and it collapses under its weight, creating the very extreme conditions under which matter can be crushed. Thus, the inside of a star with a mass several times the solar one turns into a ball of neutrons several kilometers in diameter, and the number of neutrons in it approaches 1 with 57 zeros.
')
If the star is big enough, but not too big, the neutron ball becomes strong and holds its shape, and the remnants of the star explode outward, breaking up into pieces - this process is called “supernova with collapsing core”. The neutron ball remains in place - we call it a neutron star. It consists of the densest matter, which, in our opinion, can only exist in the Universe - a pure atomic nucleus several kilometers across. This is a very hard surface; if you tried to get inside a neutron star, your feelings would be much worse than if you ran into a closed door at a speed of several hundred km / h.
If the star was very large, then the formed neutron ball may soon (or immediately) collapse under its own weight and create a black hole. In this case, the supernova may or may not appear - the star may simply disappear. BH is very, very different from a neutron star. BH - this is what remains after the irretrievable collapse of matter inside itself, infinitely contracting under the influence of gravity. And if a neutron star has a surface about which the head can be broken, the BH does not have a surface - it has an edge that simply represents the point of no return, called the horizon [of events]. In Einstein's theory, you can go straight through it, like an open door. You will not even notice the moment of transition. (But this is true in Einstein’s theory. However, there are disagreements over whether the combination of Einstein’s theory and quantum physics does not turn this facet into something new and dangerous for those who enter; this is known as a “ firewall contradiction,” but discussing it would lead us too far in the field of theorizing). But once having passed through this door, it will not be possible to return.
BHs can be formed in other ways - but these are not BHs that we can observe using LIGO / VIRGO detectors.
One of the most simple and obvious ways to create gravitational waves is to make two objects orbit around each other. If you lower two fists into the water and twist them around each other, you will get a pattern of waves moving in different directions on the water; This is a very rough analogy of what happens with two objects rotating around each other, although, since objects move in space, the waves do not appear in some kind of water environment. These are the waves of space itself.
To get powerful GW, it is necessary that both objects have a very large mass, and that they rotate at high speed. To achieve high speed you need a very strong gravitational pull; and for this, the objects should be located as close as possible to each other (since, as Isaac Newton knew, the gravity between the two objects increases as the distance between them decreases). But if the objects are large, they cannot approach each other too much; they will collide with each other and merge long before they are able to accelerate enough. Therefore, to obtain a very fast orbital speed, it is necessary to take two relatively small objects with relatively large masses - such as scientists call compact objects. Neutron stars and BH are the most compact objects known to us. Fortunately, they do often move in pairs, and sometimes, quite shortly before the merger, move around each other quickly enough to give out GWs that can detect LIGO and VIRGO.
Stars often move in pairs. Then they are called double stars . They can start life as a couple, forming together in a large gas cloud, or, if they appeared separately, they can form a couple, being in a starred community where nearby stars often fly close to each other. It may seem unexpected, but such a pair can survive the collapse and explosion of each of the stars, leading to the appearance of two black holes, two neutron stars or one BH and one NS, orbiting around each other.
It is not surprising that there are three classes of associations that can be detected: the merger of two BHs, the merger of two NZs, and the merger of NZ with BHs. We observed the first class in 2015 (they announced this in 2016), the second class was announced in 2017, and waiting for the third was only a matter of time. Two objects can rotate around each other for billions of years, very slowly emitting gravitational waves (this effect was observed in the 70s, for which they won the Nobel Prize), and gradually moving closer. And only on the last day of their life does the orbital speed begin to truly increase. And just before the merger, they begin to rotate at a speed of the order of one revolution per second, then ten revolutions per second, then one hundred revolutions per second. Imagine this if you can: objects, some tens of kilometers across, located a few kilometers from each other, having a mass exceeding solar, rotate around each other at a speed of 100 times per second. The stunning phenomenon is a rotating dumbbell, which even the most outstanding minds of the XIX century could not imagine. I do not know a single scientist who would not be in awe of this spectacle. It all sounds like science fiction, but it is not.
It’s not NF if we trust Einstein’s theory of gravity. She predicts that such a rapidly rotating dumbbell of enormous mass, formed by two compact objects, should produce a characteristic pattern of space perturbation - gravitational waves. This drawing is both complex and accurately predicted. In the case of black holes, the predictions cover the period up to the very moment of the merger, as well as after it, including the description of the signals from the larger BH resulting from the merger. In the case of NZ, the moments shortly before the collision, the merger itself and immediately after it turn out to be more complex and we are not sure that we fully understand them, but within a few tens of seconds before the merger, Einstein's theory very precisely speaks about what should be expected. The theory also predicts further events - how these waves will propagate long distances from where they originated, reach the Earth, and how they will manifest themselves in the LIGO / VIRGO network on three gravitational wave detectors. Therefore, there are several predictions about what should be expected on LIGO / VIRGO: this theory is used to predict the existence and properties of BH and NS, detailed characteristics of their mergers, accurate drawings of the resulting gravity waves, and how exactly gravitational waves propagate in space . LIGO / VIRGO found characteristic patterns of these gravity waves. And the fact that these drawings are exactly consistent with Einstein's theory, is the most reliable evidence ever obtained that the theory has no errors when used in these combined contexts.
I note that the proof in some way refers to itself — but this is how scientific knowledge is promoted, as a set of several detailed consistency checks, which are gradually intertwined with each other so much that it is almost impossible to separate them. Scientific reasoning is not deductive, but inductive. We do this not because it is fully logical, but because it works surprisingly well - and the proof is a computer with a screen on which I type this text, and the wired Internet along with wireless connections, and a computer disk that will used to store and transfer text.
The significance of the announcement is difficult to explain because it is made up of a multitude of important results that overlap each other, and not just from one kind of outcome, which can be re-told with a couple of words.
And here is a list of what we learned. None of its elements shakes the foundations of the universe, but each is quite interesting, and together they form an important event in the history of science.
We knew that such mergers should take place, but we were not sure. And since these things are too far away from us and they are too small to be seen with a telescope, the only way to make sure that a merger takes place, and to learn more details about them, was to use gravity waves. In the coming years, we hope to see much more such mergers, in the process of how gravitational astronomy will increase its sensitivity, and we will learn more about them.
The existence of NZ was predicted almost a hundred years ago, and confirmed in 60-70 years. But their exact properties are unknown; we believe that they resemble giant atomic nuclei, but they are so much larger than ordinary atomic nuclei that we cannot be sure that we understand all their internal properties, and there are disputes in the scientific community that cannot be easily resolved - but they may soon cease.
Scientists have already learned two things from a detailed picture of the gravitational waves of the neutron star fusion that occurred. First, we have confirmed that Einstein's theory correctly predicts the main pattern of gravitational waves emanating from NS or BH rotating around each other. But, unlike BH, there are still many more questions about what happens after the merger of the NC. And the question of what happened to our pair after the merger remains open - did the NZ form, the unstable NZ that collapsed in the BH, or did the BH immediately appear?
But we have already learned something important about the internal properties of the NT. Loads from such a fast rotation would tear me and you apart, and can even break the Earth. We know that NZ is much stronger than ordinary stone, but how much stronger? If they were too fragile, they would break up at some point during the observations made at LIGO / VIRGO, and the expected simple drawing of gravity waves would suddenly become much more complicated. But this did not happen, at least until the moment immediately preceding the merger. Therefore, scientists can use this simplicity of the pattern of gravitational waves to derive new data on how solid and durable the NZ is. Subsequent mergers will improve our understanding of the issue. There is no other simple method for obtaining such information.
BH merging should not create a bright light, because, as I mentioned, they look more like open doors on an invisible playing ground than on stones, so they merge quite quietly, without bright and hot collisions. But neutron stars look like big balls of matter, so their collision can generate a huge amount of heat and light of every kind - just as one would naively expect. By “light” I mean not only visible light, but also all kinds of electromagnetic waves of all wavelengths (and, accordingly, of all frequencies). Scientists divide the spectrum of electromagnetic waves into categories. These are radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma radiation - in order of increasing frequency and decreasing wavelength.
Note that these categories and the division between them is completely arbitrary, but it turns out to be useful for various scientific purposes. The only fundamental difference between yellow light, radio wave and gamma radiation is the frequency and wavelength; in all other respects it is the same: a wave of electric and magnetic fields.
So in the case of the merger of two NSs, we expect the appearance of both gravitational and electromagnetic waves of different frequencies arising from different effects due to the collision of two huge neutron balls. But just because we expect them, does not mean that they will be easy to detect. Such mergers are quite rare — perhaps one every several hundred thousand years in such a large galaxy as ours — so those that we discover with LIGO / VIRGO will usually be quite far from us. If the light show is too dim, our telescopes will not be able to see it.
But this show was quite bright. The gamma-ray detectors in space immediately spotted it, confirming the fact that the gravitational waves from the two NZs led to a collision and a merger, which gave rise to very high frequency light. And this in itself was something unique. It was as if a man had watched lightning all his life, but he had never heard thunder; or he watched the waves from hurricanes, but never saw the hurricane itself. What we saw at once two manifestations of a merger opens up a completely new set of perspectives; sometimes one plus one gives more than two.
Over time, in a few hours and days, fusion effects were observed in the visible range, ultraviolet, infrared, X-rays and radio waves. Some came before others, which in itself is a separate story, but each of them added to the treasury of our understanding of the processes of merging.
Over the years we have seen gamma-ray bursts in the sky. Among them was the class of bursts, shorter in duration than the others, usually lasting a couple of seconds. They came from all parts of the sky, which indicated that they were coming from far intergalactic space, presumably from distant galaxies. Among other explanations, the most popular hypothesis of the origin of these bursts was the NS fusion. The only way to confirm this hypothesis was to detect the gravitational waves of this merger. This test is now passed; apparently, the hypothesis is confirmed. This means that for the first time we have both a good explanation of these brief bursts of gamma rays and, based on the frequency of their occurrence, a good estimate of the frequency of the fusion of NC in the Universe.
The first measurement of the distance to the source using both gravitational waves and the redshift of electromagnetic waves, which made it possible to calibrate the scale of distances of the Universe and the speed of its expansion in a new way.
The pattern of changes in gravitational waves resulting from the merger of two BHs or NSs is difficult enough in time to reveal to us a lot of information about the merging objects, including an approximate estimate of their masses and orientation of the rotating pair relative to the Earth. The total strength of the waves, together with the knowledge of their masses, reveals to us the distance of the pair from the Earth. This is not bad in and of itself, but the real benefit comes when we open an object with visible light, or any light with a frequency less than that of gamma rays. In this case, you can determine the galaxy, where these neutron stars are located.
Knowing their home galaxy can do something very important. Looking at the light of the stars, we can determine how quickly the galaxy moves away from us. For distant galaxies, the speed at which they move away from us must be related to the distance to them due to the expansion of the Universe.
The way the Universe is rapidly expanding has recently been measured with very high accuracy, but the problem is that two different methods are used for this measurement, which do not coincide with each other. This discrepancy is one of the most important problems of our understanding of the Universe. Perhaps one of the methods is imperfect, and perhaps - and that would be much more interesting - the Universe does not behave as we think.
Gravitational waves give us a third method: they directly report the distance to the galaxy, and electromagnetic waves directly give us a runaway speed. For distant galaxies, there is no other method for conducting joint measurements of this type. This method is not accurate enough to be useful in the case of a single merger, but after seeing dozens of mergers, the average result will give us new important information about the expansion of the Universe. Combining with other methods can help us solve this important puzzle.
So far, the best test of Einstein's predictions is that the speed of light and gravitational waves coincide: since the gamma rays from the merger and the peak value of gravitational waves arrived with a difference of two seconds from each other, passing 130 million years - that is, traveling about 5 thousand million million seconds - it can be said that the speed of light and the speed of gravitational waves equals the cosmic speed limit with an accuracy of one part per 2 thousand million millions. For such an accurate test, a combination of observations of gravitational waves and gamma rays was required.
It has long been known that we are composed of matter appearing in stars, or star dust. But if you start to deal with the details of this process, riddles appear. It is known that all chemical elements, from hydrogen to iron, form in stars and can be thrown into space in a supernova explosion, float there and there, and eventually form planets, moons and people - but it was not clear how a large some of the heavier elements are iodine, cesium, gold, lead, bismuth, uranium, and so on. Yes, they may occur in supernovae, but this is not so simple; and in the Universe, apparently, there are more atoms of heavy elements than can be explained by supernovae. Many supernovae have happened in the history of the universe, but the efficiency of their production of heavy elements is too low.
Some time ago it was suggested that the merger of neutron stars may be a suitable candidate for the production of these heavy elements. And although such fusions are rare, they can be much more efficient, since the nuclei of heavy elements contain many neutrons, and, unsurprisingly, the collision of two neutron stars will lead to the appearance of many neutrons in fragments of this collision, suitable for making the mentioned nuclei. The key indicator of this process would be the following: if one could detect the fusion of neutron stars using gravitational waves and determine its location using telescopes, then one could study its light and find in it characteristic traces of what is now called the Kilon explosion. ".
Personally, I do not know all the details of kilon. It is fueled by the formation of heavy elements; Most of the nuclei produced are first radioactive — that is, unstable — and then they decay, emitting high-energy particles, including particles of light (photons) falling into the categories of gamma rays and x-rays. The final characteristic glow should have certain characteristics: initially it should be bright, but then abruptly go out in visible light and glow for a long time in infrared. The reasons for this are complex, so for now we’ll omit them. It is important that these characteristics were recorded, which confirmed the occurrence of the desired type of kilon, and, therefore, a huge number of heavy elements were actually created in this neutron star fusion. Therefore, for the first time, we now have a lot of evidence that almost all the heavy chemical elements of our planet and around it were formed during the merger of neutron stars. I repeat that we could not know this if we were not sure that this event was a merger of neutron stars, and such information can only be obtained from observations of gravitational waves.
This is still unknown to us, and perhaps we will not know. Some scientists engaged in the experiment tend to the possibility of BH, while others say it is inaccurate. Not sure what additional information we can get after some time.
BH - not vacuum cleaners; they attract everything through gravity in the same way as the Earth and the Sun do, and do not suck matter in any particular way. Their only difference is that if you fall inside, you will not get out. But just as you can avoid a collision with the Earth or the Sun, you can avoid falling into a black hole if you move fast enough in an orbit or move away before you reach the [horizon] boundary.
The essence of the NZ fusion is that at the time of the fusion the forces acting on them are so great that one or both stars are torn apart. The material ejected as a result at high speeds and in all directions somehow creates a bright hot burst of gamma rays, and as a result, the kilonic one glows because of the newly created atomic nuclei. These details are not clear to me yet, but I know that they are carefully studied both with the help of approximate equations and with the help of computer simulations . However, the accuracy of simulations can be confirmed only through a careful study of the merger - just the one mentioned in the announcement. Apparently, these simulations did a good job. I am sure that they will be improved after comparison with the obtained data.
For a start, I will make a reservation: I am not an expert on the complex topic of merging neutron stars and the resulting explosions, known as kilon ones. They are much harder to merge black holes. I myself will find out some details. I hope that I managed to avoid mistakes, but in some cases I do not have all the answers.
Basic questions about neutron stars, black holes and their fusion
What are neutron stars, black holes, and how are they related?
Each atom consists of a tiny atomic nucleus consisting of neutrons and protons (very similar to each other) and loosely surrounded by electrons. Most of the atom is an empty space, so under extreme conditions it can be crushed - but only if each electron and proton turns into a neutron (remaining in the same place) and a neutrino (going into space). When a giant star runs out of fuel, the pressure of its nuclear furnace drops and it collapses under its weight, creating the very extreme conditions under which matter can be crushed. Thus, the inside of a star with a mass several times the solar one turns into a ball of neutrons several kilometers in diameter, and the number of neutrons in it approaches 1 with 57 zeros.
')
If the star is big enough, but not too big, the neutron ball becomes strong and holds its shape, and the remnants of the star explode outward, breaking up into pieces - this process is called “supernova with collapsing core”. The neutron ball remains in place - we call it a neutron star. It consists of the densest matter, which, in our opinion, can only exist in the Universe - a pure atomic nucleus several kilometers across. This is a very hard surface; if you tried to get inside a neutron star, your feelings would be much worse than if you ran into a closed door at a speed of several hundred km / h.
If the star was very large, then the formed neutron ball may soon (or immediately) collapse under its own weight and create a black hole. In this case, the supernova may or may not appear - the star may simply disappear. BH is very, very different from a neutron star. BH - this is what remains after the irretrievable collapse of matter inside itself, infinitely contracting under the influence of gravity. And if a neutron star has a surface about which the head can be broken, the BH does not have a surface - it has an edge that simply represents the point of no return, called the horizon [of events]. In Einstein's theory, you can go straight through it, like an open door. You will not even notice the moment of transition. (But this is true in Einstein’s theory. However, there are disagreements over whether the combination of Einstein’s theory and quantum physics does not turn this facet into something new and dangerous for those who enter; this is known as a “ firewall contradiction,” but discussing it would lead us too far in the field of theorizing). But once having passed through this door, it will not be possible to return.
BHs can be formed in other ways - but these are not BHs that we can observe using LIGO / VIRGO detectors.
Why are their mergers the best sources of gravitational waves?
One of the most simple and obvious ways to create gravitational waves is to make two objects orbit around each other. If you lower two fists into the water and twist them around each other, you will get a pattern of waves moving in different directions on the water; This is a very rough analogy of what happens with two objects rotating around each other, although, since objects move in space, the waves do not appear in some kind of water environment. These are the waves of space itself.
To get powerful GW, it is necessary that both objects have a very large mass, and that they rotate at high speed. To achieve high speed you need a very strong gravitational pull; and for this, the objects should be located as close as possible to each other (since, as Isaac Newton knew, the gravity between the two objects increases as the distance between them decreases). But if the objects are large, they cannot approach each other too much; they will collide with each other and merge long before they are able to accelerate enough. Therefore, to obtain a very fast orbital speed, it is necessary to take two relatively small objects with relatively large masses - such as scientists call compact objects. Neutron stars and BH are the most compact objects known to us. Fortunately, they do often move in pairs, and sometimes, quite shortly before the merger, move around each other quickly enough to give out GWs that can detect LIGO and VIRGO.
Why do these objects appear in pairs?
Stars often move in pairs. Then they are called double stars . They can start life as a couple, forming together in a large gas cloud, or, if they appeared separately, they can form a couple, being in a starred community where nearby stars often fly close to each other. It may seem unexpected, but such a pair can survive the collapse and explosion of each of the stars, leading to the appearance of two black holes, two neutron stars or one BH and one NS, orbiting around each other.
What happens when these objects merge?
It is not surprising that there are three classes of associations that can be detected: the merger of two BHs, the merger of two NZs, and the merger of NZ with BHs. We observed the first class in 2015 (they announced this in 2016), the second class was announced in 2017, and waiting for the third was only a matter of time. Two objects can rotate around each other for billions of years, very slowly emitting gravitational waves (this effect was observed in the 70s, for which they won the Nobel Prize), and gradually moving closer. And only on the last day of their life does the orbital speed begin to truly increase. And just before the merger, they begin to rotate at a speed of the order of one revolution per second, then ten revolutions per second, then one hundred revolutions per second. Imagine this if you can: objects, some tens of kilometers across, located a few kilometers from each other, having a mass exceeding solar, rotate around each other at a speed of 100 times per second. The stunning phenomenon is a rotating dumbbell, which even the most outstanding minds of the XIX century could not imagine. I do not know a single scientist who would not be in awe of this spectacle. It all sounds like science fiction, but it is not.
How do we know that this is not science fiction?
It’s not NF if we trust Einstein’s theory of gravity. She predicts that such a rapidly rotating dumbbell of enormous mass, formed by two compact objects, should produce a characteristic pattern of space perturbation - gravitational waves. This drawing is both complex and accurately predicted. In the case of black holes, the predictions cover the period up to the very moment of the merger, as well as after it, including the description of the signals from the larger BH resulting from the merger. In the case of NZ, the moments shortly before the collision, the merger itself and immediately after it turn out to be more complex and we are not sure that we fully understand them, but within a few tens of seconds before the merger, Einstein's theory very precisely speaks about what should be expected. The theory also predicts further events - how these waves will propagate long distances from where they originated, reach the Earth, and how they will manifest themselves in the LIGO / VIRGO network on three gravitational wave detectors. Therefore, there are several predictions about what should be expected on LIGO / VIRGO: this theory is used to predict the existence and properties of BH and NS, detailed characteristics of their mergers, accurate drawings of the resulting gravity waves, and how exactly gravitational waves propagate in space . LIGO / VIRGO found characteristic patterns of these gravity waves. And the fact that these drawings are exactly consistent with Einstein's theory, is the most reliable evidence ever obtained that the theory has no errors when used in these combined contexts.
I note that the proof in some way refers to itself — but this is how scientific knowledge is promoted, as a set of several detailed consistency checks, which are gradually intertwined with each other so much that it is almost impossible to separate them. Scientific reasoning is not deductive, but inductive. We do this not because it is fully logical, but because it works surprisingly well - and the proof is a computer with a screen on which I type this text, and the wired Internet along with wireless connections, and a computer disk that will used to store and transfer text.
The significance of the October announcement of the merger of neutron stars
The significance of the announcement is difficult to explain because it is made up of a multitude of important results that overlap each other, and not just from one kind of outcome, which can be re-told with a couple of words.
And here is a list of what we learned. None of its elements shakes the foundations of the universe, but each is quite interesting, and together they form an important event in the history of science.
The first confirmed observation of the merger of two NZ
We knew that such mergers should take place, but we were not sure. And since these things are too far away from us and they are too small to be seen with a telescope, the only way to make sure that a merger takes place, and to learn more details about them, was to use gravity waves. In the coming years, we hope to see much more such mergers, in the process of how gravitational astronomy will increase its sensitivity, and we will learn more about them.
New information on the properties of neutron stars
The existence of NZ was predicted almost a hundred years ago, and confirmed in 60-70 years. But their exact properties are unknown; we believe that they resemble giant atomic nuclei, but they are so much larger than ordinary atomic nuclei that we cannot be sure that we understand all their internal properties, and there are disputes in the scientific community that cannot be easily resolved - but they may soon cease.
Scientists have already learned two things from a detailed picture of the gravitational waves of the neutron star fusion that occurred. First, we have confirmed that Einstein's theory correctly predicts the main pattern of gravitational waves emanating from NS or BH rotating around each other. But, unlike BH, there are still many more questions about what happens after the merger of the NC. And the question of what happened to our pair after the merger remains open - did the NZ form, the unstable NZ that collapsed in the BH, or did the BH immediately appear?
But we have already learned something important about the internal properties of the NT. Loads from such a fast rotation would tear me and you apart, and can even break the Earth. We know that NZ is much stronger than ordinary stone, but how much stronger? If they were too fragile, they would break up at some point during the observations made at LIGO / VIRGO, and the expected simple drawing of gravity waves would suddenly become much more complicated. But this did not happen, at least until the moment immediately preceding the merger. Therefore, scientists can use this simplicity of the pattern of gravitational waves to derive new data on how solid and durable the NZ is. Subsequent mergers will improve our understanding of the issue. There is no other simple method for obtaining such information.
The first observation of an event that produces both the strongest gravitational waves and bright electromagnetic waves
BH merging should not create a bright light, because, as I mentioned, they look more like open doors on an invisible playing ground than on stones, so they merge quite quietly, without bright and hot collisions. But neutron stars look like big balls of matter, so their collision can generate a huge amount of heat and light of every kind - just as one would naively expect. By “light” I mean not only visible light, but also all kinds of electromagnetic waves of all wavelengths (and, accordingly, of all frequencies). Scientists divide the spectrum of electromagnetic waves into categories. These are radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma radiation - in order of increasing frequency and decreasing wavelength.
Note that these categories and the division between them is completely arbitrary, but it turns out to be useful for various scientific purposes. The only fundamental difference between yellow light, radio wave and gamma radiation is the frequency and wavelength; in all other respects it is the same: a wave of electric and magnetic fields.
So in the case of the merger of two NSs, we expect the appearance of both gravitational and electromagnetic waves of different frequencies arising from different effects due to the collision of two huge neutron balls. But just because we expect them, does not mean that they will be easy to detect. Such mergers are quite rare — perhaps one every several hundred thousand years in such a large galaxy as ours — so those that we discover with LIGO / VIRGO will usually be quite far from us. If the light show is too dim, our telescopes will not be able to see it.
But this show was quite bright. The gamma-ray detectors in space immediately spotted it, confirming the fact that the gravitational waves from the two NZs led to a collision and a merger, which gave rise to very high frequency light. And this in itself was something unique. It was as if a man had watched lightning all his life, but he had never heard thunder; or he watched the waves from hurricanes, but never saw the hurricane itself. What we saw at once two manifestations of a merger opens up a completely new set of perspectives; sometimes one plus one gives more than two.
Over time, in a few hours and days, fusion effects were observed in the visible range, ultraviolet, infrared, X-rays and radio waves. Some came before others, which in itself is a separate story, but each of them added to the treasury of our understanding of the processes of merging.
Confirming the best guesses about the sources of short bursts of gamma rays
Over the years we have seen gamma-ray bursts in the sky. Among them was the class of bursts, shorter in duration than the others, usually lasting a couple of seconds. They came from all parts of the sky, which indicated that they were coming from far intergalactic space, presumably from distant galaxies. Among other explanations, the most popular hypothesis of the origin of these bursts was the NS fusion. The only way to confirm this hypothesis was to detect the gravitational waves of this merger. This test is now passed; apparently, the hypothesis is confirmed. This means that for the first time we have both a good explanation of these brief bursts of gamma rays and, based on the frequency of their occurrence, a good estimate of the frequency of the fusion of NC in the Universe.
The first measurement of the distance to the source using both gravitational waves and the redshift of electromagnetic waves, which made it possible to calibrate the scale of distances of the Universe and the speed of its expansion in a new way.
The pattern of changes in gravitational waves resulting from the merger of two BHs or NSs is difficult enough in time to reveal to us a lot of information about the merging objects, including an approximate estimate of their masses and orientation of the rotating pair relative to the Earth. The total strength of the waves, together with the knowledge of their masses, reveals to us the distance of the pair from the Earth. This is not bad in and of itself, but the real benefit comes when we open an object with visible light, or any light with a frequency less than that of gamma rays. In this case, you can determine the galaxy, where these neutron stars are located.
Knowing their home galaxy can do something very important. Looking at the light of the stars, we can determine how quickly the galaxy moves away from us. For distant galaxies, the speed at which they move away from us must be related to the distance to them due to the expansion of the Universe.
The way the Universe is rapidly expanding has recently been measured with very high accuracy, but the problem is that two different methods are used for this measurement, which do not coincide with each other. This discrepancy is one of the most important problems of our understanding of the Universe. Perhaps one of the methods is imperfect, and perhaps - and that would be much more interesting - the Universe does not behave as we think.
Gravitational waves give us a third method: they directly report the distance to the galaxy, and electromagnetic waves directly give us a runaway speed. For distant galaxies, there is no other method for conducting joint measurements of this type. This method is not accurate enough to be useful in the case of a single merger, but after seeing dozens of mergers, the average result will give us new important information about the expansion of the Universe. Combining with other methods can help us solve this important puzzle.
So far, the best test of Einstein's predictions is that the speed of light and gravitational waves coincide: since the gamma rays from the merger and the peak value of gravitational waves arrived with a difference of two seconds from each other, passing 130 million years - that is, traveling about 5 thousand million million seconds - it can be said that the speed of light and the speed of gravitational waves equals the cosmic speed limit with an accuracy of one part per 2 thousand million millions. For such an accurate test, a combination of observations of gravitational waves and gamma rays was required.
Confirmed the effective creation of heavy elements
It has long been known that we are composed of matter appearing in stars, or star dust. But if you start to deal with the details of this process, riddles appear. It is known that all chemical elements, from hydrogen to iron, form in stars and can be thrown into space in a supernova explosion, float there and there, and eventually form planets, moons and people - but it was not clear how a large some of the heavier elements are iodine, cesium, gold, lead, bismuth, uranium, and so on. Yes, they may occur in supernovae, but this is not so simple; and in the Universe, apparently, there are more atoms of heavy elements than can be explained by supernovae. Many supernovae have happened in the history of the universe, but the efficiency of their production of heavy elements is too low.
Some time ago it was suggested that the merger of neutron stars may be a suitable candidate for the production of these heavy elements. And although such fusions are rare, they can be much more efficient, since the nuclei of heavy elements contain many neutrons, and, unsurprisingly, the collision of two neutron stars will lead to the appearance of many neutrons in fragments of this collision, suitable for making the mentioned nuclei. The key indicator of this process would be the following: if one could detect the fusion of neutron stars using gravitational waves and determine its location using telescopes, then one could study its light and find in it characteristic traces of what is now called the Kilon explosion. ".
Personally, I do not know all the details of kilon. It is fueled by the formation of heavy elements; Most of the nuclei produced are first radioactive — that is, unstable — and then they decay, emitting high-energy particles, including particles of light (photons) falling into the categories of gamma rays and x-rays. The final characteristic glow should have certain characteristics: initially it should be bright, but then abruptly go out in visible light and glow for a long time in infrared. The reasons for this are complex, so for now we’ll omit them. It is important that these characteristics were recorded, which confirmed the occurrence of the desired type of kilon, and, therefore, a huge number of heavy elements were actually created in this neutron star fusion. Therefore, for the first time, we now have a lot of evidence that almost all the heavy chemical elements of our planet and around it were formed during the merger of neutron stars. I repeat that we could not know this if we were not sure that this event was a merger of neutron stars, and such information can only be obtained from observations of gravitational waves.
Different issues
Has the new BH, the larger NC, or the unstable rapidly rotating NC, which subsequently collapsed into the BH, resulted from the merger of these two NCs?
This is still unknown to us, and perhaps we will not know. Some scientists engaged in the experiment tend to the possibility of BH, while others say it is inaccurate. Not sure what additional information we can get after some time.
If two NZs formed BHs, where does the kilon come from? Why all this is not sucked in BH?
BH - not vacuum cleaners; they attract everything through gravity in the same way as the Earth and the Sun do, and do not suck matter in any particular way. Their only difference is that if you fall inside, you will not get out. But just as you can avoid a collision with the Earth or the Sun, you can avoid falling into a black hole if you move fast enough in an orbit or move away before you reach the [horizon] boundary.
The essence of the NZ fusion is that at the time of the fusion the forces acting on them are so great that one or both stars are torn apart. The material ejected as a result at high speeds and in all directions somehow creates a bright hot burst of gamma rays, and as a result, the kilonic one glows because of the newly created atomic nuclei. These details are not clear to me yet, but I know that they are carefully studied both with the help of approximate equations and with the help of computer simulations . However, the accuracy of simulations can be confirmed only through a careful study of the merger - just the one mentioned in the announcement. Apparently, these simulations did a good job. I am sure that they will be improved after comparison with the obtained data.
Source: https://habr.com/ru/post/410913/
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