Could a black hole be passable, and a wormhole built by a highly evolved alien intelligence, as shown in the movie "Interstellar"? What do a neutron star and a black hole have in common? How were neutron stars discovered and could there be another large rocky planet on the outskirts of the solar system? The famous astrophysicist Sergey Popov told us about this and many other things.
Sergey Borisovich Popov - Russian astrophysicist, popularizer of science, doctor of physical and mathematical sciences, leading employee of the State Astronomical Institute named after P. K. Sternberg (GAISH MSU). The circle of scientific interests is black holes and neutron stars. He pays a lot of attention to the popularization of science, writes popular science articles and gives public lectures.
Sergei Borisovich, how were neutron stars discovered?
- They were quite dramatic. First of all, I must say that they were predicted in the 1930s. But no one rushed to look for them, since the prediction said that they should be small hot objects that are difficult to detect, with the technology of that time at their disposal. After that, neutron stars were discovered using the first X-ray observations, but did not recognize. What I mean? These sources did not carry any special marker that would make it possible to very quickly determine what these objects were. As a result, they were officially discovered by accident - like radio pulsars. People simply studied the flickering of sources on cosmic plasma, and one of the sources showed a very periodic signal - a repetition of pulses with very high accuracy.
Further. In 1967, the observations were made by then-unknown graduate student Jocelyn Bell. At first, no one believed her, because some terrestrial sources can really easily get into the radio range. But a clear daily periodicity in the change in the position of the sources showed that it was still an astronomical object, and then it was decided that an artificial extraterrestrial signal was observed. In such a situation, this is a fairly logical assumption, especially since then this topic was very popular.
However, in a short time, several more such sources with different periods were discovered. It became clear that this was not an artificial signal, but simply a new class of astronomical objects. They thought they were either rotating neutron stars or pulsating white dwarfs. Only these objects can give such an accurate short period - a split second.
But there are two simple predictions here. In order to understand how the vibrations of a white dwarf should behave, take a ball that jumps well enough, throw it and watch how the vibrations slowly fade. You will see that the vibration frequency (as the ball jumps lower and lower) will increase. If this is the rotation of a neutron star, then it will slow down. Spin the ball on the table and you will see that it rotates more and more slowly. If pulsars are explained by neutron stars, then the period should increase. Observations have shown that the period is indeed increasing. It was then that it became clear that we were in front of neutron stars.
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Further, unfortunately, followed, as is believed, the biggest of the obvious mistakes of the Nobel Committee: Jocelyn Bell did not receive the Nobel Prize, but was received by the project manager Anthony Hewish, formally not for the discovery of radio pulsars, but for his great contribution to the development of radio astronomy, although, in In principle, everyone understood that this great contribution was associated, first of all, with the discovery of radio pulsars. This is how neutron stars were discovered.
The WISE telescope picture shows a neutron star moving at an incredibly high speed - about 4.8 million km per hour. Due to its record speed, it received the nickname "Space Cannonball"
Photo: NASA
What do a neutron star and a black hole have in common?
- The fact is that neutron stars are a very well-defined class of objects that arise at the late stages of stellar evolution and in real astrophysical conditions have a mass of about one to three times the mass of the Sun. Black holes are a very wide class of objects, in the sense that they, again, can be well defined as certain regions of space, events inside of which cannot affect what happens outside, that is, regions separated from us by the event horizon … But such areas can have both very different sizes and very different origins. Therefore, there can be giant, supermassive black holes with a mass of tens of billions of solar masses, and, of course, they already become completely different from neutron stars. Or there may be primordial black holeswhich arose at the dawn of the universe. They can have a mass like an asteroid, and a size like a proton, for example. But one of the most famous types of black holes, like neutron stars, is the final product of stellar evolution. Massive stars, as a rule, after a supernova explosion turn into either neutron stars or black holes. And only between these two types of objects can you see the connection you are talking about. Indeed, we can take a neutron star, slowly add matter to it, and over time, when its mass exceeds the critical one, it will collapse into a black hole. But, nevertheless, these are still quite different classes of objects.is the final product of stellar evolution. Massive stars, as a rule, after a supernova explosion turn into either neutron stars or black holes. And only between these two types of objects can you see the connection you are talking about. Indeed, we can take a neutron star, slowly add matter to it, and over time, when its mass exceeds the critical one, it will collapse into a black hole. But, nevertheless, these are still quite different classes of objects.is the final product of stellar evolution. Massive stars, as a rule, after a supernova explosion turn into either neutron stars or black holes. And only between these two types of objects can you see the connection you are talking about. Indeed, we can take a neutron star, slowly add matter to it, and over time, when its mass exceeds the critical one, it will collapse into a black hole. But, nevertheless, these are still quite different classes of objects.when its mass exceeds a critical one, it will collapse into a black hole. But, nevertheless, these are still quite different classes of objects.when its mass exceeds a critical one, it will collapse into a black hole. But, nevertheless, these are still quite different classes of objects.
Recently, by the way, scientists have found out how much the ancient black hole, which was discovered in 2013, "weighs". The light from it went to us for 12.9 billion years, that is, it appeared at the dawn of the universe. Its mass is, as it turned out, 12 billion solar masses. Usually, ancient black holes do not have a very large mass, since their formation takes time and, in fact, matter. What hypotheses exist to explain such a mass in such an ancient black hole?
- There is already a significant quantitative difference. Imagine a non-poor person who pays high taxes, which means that he is supposed to have a Porsche Cayenne, and he drives a McLaren F1. In principle, both are expensive cars, the difference between them is quantitative, but it is somehow striking. The point is that black holes located at such a distance from us should have a mass not of 10-12 billion solar masses, but, say, 100 million. From an everyday point of view, these numbers do not differ so much, but when we take specific models of the growth of the masses of black holes, it turns out that over the 700 million years that a black hole had, it is rather difficult to grow to 12 billion solar masses - this is strange. And the most natural explanation at the moment is that the first black hole that began to grow and grew into such awas slightly more massive than the others. That is, we reduce it to the question of where supermassive black holes come from at all.
The most obvious history of a black hole begins with the first stars. The first stars form tens of millions of years after the Big Bang - they are fairly massive stars. At the end of their lives, they turn into black holes, which should typically have a mass of about 200 times the mass of the Sun. After that, they can merge with each other, absorb matter, slowly grow, and so by our time it is possible to grow up to 10 billion solar masses if you get to the center of a large galaxy. To grow to 10 billion solar masses, not in 13 billion years, but in 700 million, you must initially be heavier than 200 solar masses. And everything will work out if such a black hole is initially only ten times heavier. The problem is where to get at the very beginning black holes with a mass of several thousand solar masses. The most undeniable answer to this question now isthat these black holes were formed not from the first stars, but immediately as a result of the collapse of a rather large gas cloud. Then you can initially get a black hole with a mass of several thousand solar masses. Then everything goes as usual, but it is clear that if you initially start with more capital, in the end you will also have more capital.
How, in your opinion, can one solve the well-known dilemma that black holes seem to exist almost one hundred percent, but no one has discovered them so far?
- Indeed, black holes are a very unusual object that is very difficult to observe. From an astrophysical point of view, we see many different sources that behave in the same way as a black hole should behave, which in principle, of course, is not the final argument in favor of their existence. From the point of view of physics, there are many all sorts of interesting processes directly related to the existence of the horizon, which is fantastically difficult to observe in real astrophysics. And strictly speaking, we don't know how to observe it, we can do it only if we are very lucky. On the one hand, there are physical predictions which effects would more or less unequivocally indicate that we are dealing with black holes, on the other hand, we cannot directly verify this, although we can verify all sorts of other things. And black holes are the best interpretation for some observations.
What can we do in the near future and what do we hope for? The most reliable thing that should happen in the next few years is the registration of a gravitational wave signal from a merger of black holes. Black holes are formed, including from massive stars, massive stars like to form binary systems, which means that there must be systems from double black holes, which will gradually merge, which will lead to a burst of gravitational radiation. Registration of such events should begin as early as next year, and, of course, the interaction of bodies with the horizon and solid surfaces should look different. Interestingly, formally this will not be completely final proof of the existence of black holes. But I would say that any normal jury would accept such evidence as sufficient.
It would also be great to see a flash from Hawking's vaporization of black holes. Physicists have a lot of controversy about this, and there are models where there is no Hawking radiation, but, nevertheless, so far this is a kind of standard prediction. One might hope to see the last flash when the black hole evaporates, but this is partly probably a matter of luck. The last time, if I’m not mistaken, we observed a flare quite recently, which they tried to explain in a similar way, and some continue to insist on such an explanation, although it is not generally accepted - this is the so-called Fast Radio Burst. Most likely, this outburst is due to other reasons, however, it would be great to see the "last breath" of the black hole.
At the same time, we just hope that the further we go, the better we will know and understand the properties of known sources. And the development of telescopes, large interferometers will make it possible to see the so-called shadow of a black hole. This is roughly what is shown in the movie "Interstellar". There is a special project called the Event Horizon Telescope - a combination of a dozen ground-based telescopes that have three main goals - the three largest black holes in the sky: a black hole in the center of the M87 galaxy, a black hole in the center of our Galaxy and a black hole in the Andromeda Nebula. Scientists hope to see the structure of radiation right near these objects, and this will also be a good argument in favor of the fact that we are dealing with black holes. (An article appeared after the interview - you can see it in my reviews on this page. This article discusses new arguments in favor of the existence of a horizon for a black hole in M87).
At the same time, all sorts of theoretical disputes will always remain on the topic of the fact that real holes with horizons are not formed, if we are in the appropriate frame of reference, everything will evaporate before the hole appears, etc. But this, by and large, is not affects astrophysical observations, and there will probably remain the possibility of constructing terribly exotic alternative models, where people will come up with sophisticated configurations that are slightly larger than a black hole. And then, from an observational point of view, it will also be quite difficult to refute them, and the arguments against these alternatives will also be rather indirect, although more convincing than the arguments in favor of such models.
Shot from the movie "Interstellar"
If we have already mentioned the movie "Interstellar", then I will ask: can a black hole still be passable or is it too unlikely?
- I would advise everyone to read the book The science of Interstellar, where the famous American physicist and astronomer Kip Thorne answers all these questions in a more or less popular form, who understands them much better than I do. In principle, wormholes still remain a kind of exotic from a variety of points of view. We're not entirely sure if we can apply general relativity to extreme conditions like the center of a black hole: it simply hasn't been tested for such conditions. The theory of relativity is not the final theory of gravity, and it will be replaced by a theory that in its limiting cases will be reduced to the general theory of relativity, but in all extreme situations will give different predictions. That is, the discussion of wormholes, as well as the discussion of the depths of black holes,based on the extrapolation of the equations of general relativity to areas where they are not tested. But I immediately want to defend the theory of relativity, since it itself is not speculation. In principle, wormholes can be associated with black holes, just an assumption about this is a kind of speculation based on a good theory, but untested for objective reasons.
It would certainly be strange if any black hole were the entrance to a wormhole. Still, black holes should be a completely natural object, and wormholes may not occur in nature by themselves. There is nothing wrong with that, for example, iPhones are also not found in nature - people create them themselves. Therefore, it is not for nothing that Interstellar discusses a more powerful civilization that created this wormhole. And this, in a sense, seems less fantastic than the existence of wormholes by themselves, and even in large numbers. Of course, I'm talking about large wormholes, and not about any structures that exist at the level of quantum foam.
Shot from the movie "Interstellar"
What do you think about the existence of a large rocky planet on the outskirts of the solar system? And why, if it exists, has not it been discovered yet, because it would seem that it is so close in comparison with the already discovered exoplanets?
- If I look out the window, I see some objects rather far from me, but I don't see the rolled up cufflink under the sofa, because it's dark there. Roughly the same in the case of the discovery of another planet in the solar system. We see a planet near another star thanks to the light of this star - or rather, on the contrary: the shadow from this planet on the star. We do not see the planet itself - we see that something dark has passed through the disk of the star. And here we would need to see exactly the planet itself. But this is difficult to do, because, firstly, it is far from us, and secondly, it is far from the Sun, therefore it reflects little light. Of course, if you know exactly where this planet is at the moment and point a large telescope at this place, it can be detected. But this is not done simply because large telescopes cannot view the entire sky. And such a planet at a distance of 200 AU. e.would go unnoticed. But so far such luck has not happened, so it is impossible to see it now, if it exists.
There are arguments that such planets on the outskirts of stellar systems can exist. The most serious arguments relate to the behavior of small bodies in the solar system. The study of the orbits of some comets shows that they are quite anomalous, and, of course, this anomaly can be explained in different ways. But the simplest way, requiring just one guess, is to say that at a distance of 100-200 AU. That is, there must be a planet with a mass of the order of the mass of the Earth. This would then explain the anomalies in the orbits of comets. As soon as such an idea arises, a reasonable question arises in astrophysics: where does it come from. That is, if it is a natural object, we must imagine the natural way of its appearance.
There are two versions of how this could have happened. The first seems to be more realistic. It consists in the following: when the planets were just forming, a lot of all kinds of objects of very different masses flew in the solar system. Slowly they collided with each other, absorbed by larger bodies or thrown out - some outside, some inside. Jupiter, being a massive planet, is capable of throwing out rather heavy objects with its gravity. He is able to throw a planet with the mass of the Earth somewhere at 150 AU. e. And if it was thrown there - then it will not be able to change its orbit so as to return back. Modeling has shown that all this is quite real.
The second version seems a little more exotic. If the cloud from which the solar system was formed was massive enough, then, in principle, it is possible to collect the planet so far. This requires, firstly, a lot of matter, and secondly, a lot of time - about 1 billion years. The solar system is over 4 billion, so there was time.
Have any new types of exoplanets been discovered recently (except for super-lands, hot and cold gas giants, water worlds)?
- Over the past months, I have not heard about the discovery of any fundamentally new types of exoplanets. There are some discoveries, but they are not so revolutionary, because the Kepler telescope, as you know, is no longer in the active phase of its work, and experts are working with its archives. Ground-based instruments are discovering planets, but this is not such a large flow, so if we can single out something, say, from the discoveries of last year, then, probably, in my subjective opinion, this is the discovery of a planet that was discovered in a rather exotic way - using gravitational lensing. That, in turn, excludes its detailed study later. The planet is in a fairly close binary system, but its orbit is quite large. That is, the distance from star to star is 15 AU. e., and from planet to star - 1 a. e. Therefore, in my opinion,this is a rather interesting and unusual system. Unfortunately, as I said, it will probably be very difficult to study it in detail.
The fact is that quite often in some popular science films or programs one hears, for example, about "diamond" planets, consisting mainly of carbon. What can be said about this and how to relate to such speculation?
- This is both speculation and not speculation at the same time, this is a kind of exaggeration. There may be planets with very high carbon content. And if they are massive enough, then carbon will be in a compressed form, respectively, part of it will be in a state similar to diamond. This, however, does not mean that flying up to such a planet you will see a large round diamond. The planet will look much more prosaic - outside and there should be far from a diamond. Here, our imagination is simply easily bought into some kind of exaggeration, based, in principle, on something real.
I want to say that in the laboratory we cannot even reproduce the interior of the earth. We do not know at all what is happening in the center of Jupiter. But this does not mean that there is a large bar with drinks in the center of Jupiter - we choose from a limited selection of more boring options. If we know so poorly even our Earth and the planets of the Solar System, then what can we say about distant exoplanets. Most often, we only know about them the mass, radius and that's it. Therefore, it is not surprising that when we are faced with a large variety of exoplanets, a huge number of hypotheses naturally arise about what they might look like and what they consist of. But, unfortunately, any new specific data on exoplanets are still limited.
PLAnetary Transits and Oscillations of stars (PLATO) is a space telescope planned by the European Space Agency, which, using a group of photometers, will detect and characterize exoplanets of all types and sizes in yellow and orange dwarf systems like our Sun. The telescope is slated to launch in 2017 or 2018. It will have a much larger field of view than Kepler (which has a field of view of 100 square degrees), allowing it to study a wider sample of stars.
What is known about the satellites of exoplanets?
- Nothing. It's hard enough to open them. It is believed that next-generation telescopes such as James Webb will be able to do this, and in different ways. So, Webb will be able to see the satellites if they are hot enough. PLATO will be able to detect satellites in the same way that Kepler discovered planets - by transits, by passing along the disk. Therefore, only large satellites will be available to him, like our Moon or the Galilean satellites of Jupiter. In anticipation of this, while there is a lot of discussion, there are many interesting ideas and data waiting.
Olga Fadeeva
The interview was published in Naked Science (# 19, May-June 2015).
