Sergey Popov: "Fast radio bursts are a new mystery in space"

Sergey Popov: "Fast radio bursts are a new mystery in space"
Sergey Popov: "Fast radio bursts are a new mystery in space"

What are fast radio bursts and are they related to messages from extraterrestrial civilizations? How does a quark star differ from a neutron star, and why is studying white holes better than classifying dragons? We talked about this with the famous astrophysicist Sergei Popov.

Sergei Popov

Sergey Popov - astrophysicist and popularizer of science, doctor of physical and mathematical sciences, leading researcher at the State Astronomical Institute named after V.I. P.K.Sternberg. Has published over 100 scientific papers. Areas of interest: neutron stars, black holes and exoplanets. Author of popular science books "Stars: Life After Death" (co-authored with Mikhail Prokhorov) and "Superobjects: Stars the Size of a City".

Sergey Borisovich, what are short radio bursts and what is their nature?

Actually, this phenomenon is interesting because it is not clear what it is. In astronomy, in general, bright and mysterious discoveries periodically occur. One of these can be considered the discovery of gamma-ray bursts in the late 1960s. When for almost thirty years people discussed a huge number of hypotheses and could not come to a common opinion. Until new observational data emerged. We are talking about such discoveries that, if the equipment allows, we can observe every day and accumulate quite a lot of statistics, but we cannot make progress in understanding their nature. Therefore, such things are very intriguing and spur the development of science. And fast radio bursts today are just an example of this kind of events.

We can say that theorists converge on some points of view. It seems that fast radio bursts are associated with neutron stars, but on the other hand, if tomorrow it turns out that this is not the case, it will not be a shock, since the set of hypotheses is very large and includes a wide variety of types of objects.

How often do such radio bursts occur?

Almost as often as you breathe. If you round up, then about 10 thousand times a day (and, apparently, there are even more weak radio bursts). Another thing is that they are difficult to observe, because in the radio range we can observe only small parts of the sky each time. So far we do not have such a sensitive radio telescope at a frequency of 1 GHz, which would simultaneously look at half of the sky. But when such a telescope appears, we will register hundreds and thousands of short radio bursts every day.

Are these bursts similar or different in some way?

Again, we don't know. In principle, the very concept of “short radio burst” implies the similarity of different events. On the other hand, we know how different dogs of different breeds are. Take the Yorkshire Terrier and the Great Dane. And if we start to study in detail the details of each burst, we will see that they also differ from each other. Therefore, now it would be easier for everyone if it turned out that several hypotheses are correct at once and the observed diversity of radio bursts is explained by several reasons. But this approach is not very popular with scientists.

On the one hand, this approach is uneconomical: science tries to describe everything in the most laconic way. But on the other hand, observational data from well-studied sources sometimes very seriously testify in favor of one hypothesis for one outburst, and in favor of another hypothesis for another. And therefore, in articles, we increasingly see the phrase that, possibly, there are at least two populations of fast radio bursts of different nature.Moreover, this nature can really be completely different. For example, such bursts can produce single neutron stars, which can quietly flare up many, many times during their life, and the merger of neutron stars is a catastrophic process with a powerful energy release, which can happen only once in the life of a neutron star.

Is it plausible to consider the hypothesis that fast radio bursts are “messengers” of extraterrestrial civilizations?

At the very beginning of the study of fast radio bursts, perhaps someone could have discussed a similar hypothesis. But even the amount of radio bursts works against it. It is very strange to assume that the Universe is studded with some kind of powerful radio transmitters, which, for some unknown reason, transmit short non-periodic signals. Therefore, now, of course, few people think about it. A few years ago, I might have suggested that some science fiction-shooting acquaintance should take something like this as the basis of the plot. Now - hardly. In principle, since we are already talking about little men, some traces of extraterrestrial intelligence will be discovered in the future in this way - by chance, as a result of astronomical observations.

What are the hypotheses that explain the nature of fast radio bursts?

Since we need short signals, it is better to start with sources, firstly, small ones, and, secondly, those about which we know that they emit in the radio frequency range. The first thing that comes to mind in this regard is neutron stars. They are small, so they can give a very short pulsating radio signal, which is what is observed. Therefore, there are a huge number of variants of hypotheses associated with one or another type of neutron stars. All of them cover a huge range of energy release, a huge range of distances to sources.


Historically, the first hypothesis was put forward by astrophysicist Konstantin Postnov and me that short radio bursts are flares of magnetars. Magnetars are neutron stars with very strong magnetic fields. They give off powerful X-ray gamma-ray bursts. And if part of this energy is processed into radio, then we will get exactly what we need. Another version of a very early hypothesis is related to the merger of neutron stars. This is one of the most powerful processes currently taking place in the Universe. We expect to discover a gravitational wave signal from such events right this year. From mergers of two neutron stars, radio emission was predicted, but not short bursts. And when fast radio bursts were discovered, they began to investigate how to make a short burst, and, of course, they found it.

Finally, the third, the main hypothesis associated with neutron stars, is that there may be very young pulsars with very powerful energy release, which sometimes, say, once a day, can produce a very powerful radio burst with a maximum flux of a hundred thousand times greater than the neutron stars we know. Probably, these three hypotheses are the most popular today.

On the other hand, their number has exceeded twenty. Interesting, for example, is the hypothesis that a neutron star can turn into a quark one. This means that a phase transition will occur, and those quarks that are trapped in protons, neutrons and other particles will become free. A substance will appear that does not consist of individual particles, such as protons and neutrons, but of free quarks. This is, in fact, a new, very interesting state of matter. And when one star turns into another, its properties change, this can also create a powerful radio burst.

And what will be the properties of such a star?

Outwardly, it will be very similar to a neutron star, but at the same mass it will be slightly smaller. There will be more density in its center, of course, the entire internal structure will change, it will become simpler. A quark star will rotate like a neutron star, but with a lower amplitude.

On the other hand, there are many variants of hypotheses with black holes.A black hole is also a compact object, so it can also emit short radio bursts. But here it is important that people do not get confused: black holes are larger than neutron stars, they are simply heavier. A typical black hole has a mass of 10 times the mass of the Sun, so the size of its horizon is 30 km. The average neutron star has a mass of 1.5 times the mass of the Sun, and its radius is 10-15 km.

But black holes are called black for a reason: it is difficult to imagine how they can flare up. Although there are also hypotheses about this, they are based on exotic, large by astrophysical standards. The black hole needs to be electrically charged. Most likely, this does not occur in nature. But if there is matter around the black hole, then there may indeed be flares. Or, such a burst could occur when a black hole evaporates. When back in the 1970s, people tried to detect evaporating black holes, they expected to see them in short flashes, including in the radio range. And in 2012, when the data on the second fast radio burst were published, the authors of the article, speaking about its nature, just put forward the hypothesis of the evaporation of black holes.

There are also quite exotic hypotheses of the origin of fast radio bursts. For example, associated with dark matter. One of the candidates for his role are axions - very interesting particles that are actively looking for today. These particles can interact with a magnetic field and turn into photons. Accordingly, if a cloud of such particles flies into the magnetosphere of a neutron star, then some of the particles will turn into photons and, possibly, give a radio flash. That is, on the one hand, we cannot do without a neutron star again, on the other, such exotic particles are the basis. There are hypotheses that associate radio bursts with cosmic strings, with the fall of asteroids on neutron stars, with the merger of white dwarfs.

Science is very conservative in an amicable way. To publish a hypothesis, it needs to be worked out quite well, and, having worked it out, to have some good reason for proposing hypotheses. Explaining some new phenomena by exotic things makes it possible to legitimize the publication of these hypotheses even without a very detailed study. For example, thanks to pop culture, everyone knows not only about string theory, but also about its main competitor - the theory of loop quantum gravity. In this theory, black holes evaporate in a different way than in general relativity.

The last stages of this process are different. Because in the theory of loop quantum gravity, at the last stage, you have not just a small black hole, but a black hole, from which quantum gravitational loops begin to crawl out - it turns into a white hole. Therefore, it became possible to say that if we detect this process (perhaps by directly studying fast radio bursts), then we will be able to determine the correctness of one or another theory of gravity. Most likely, this has nothing to do with real outbursts, but, nevertheless, all this is very interesting and contributes to the development of science.

Since you mentioned white holes, tell us about them

It is easier to identify white holes through black ones. A black hole is a region of space from which no trajectories lead outward, from which no matter, not even light, can escape. With a white hole, everything is exactly the opposite: all trajectories lead only from it. How they can exist in nature is not very clear, except for short events like the final stage of the evaporation of a black hole in the model of loop quantum gravity. By and large, white holes exist as a kind of solution within the framework of general relativity. All attempts to search for them have led nowhere, and if it is a large object, it must be very noticeable, such as the active nucleus of the galaxy. But, again, the search for them was unsuccessful, and there are no realistic models of how to create them in the real world.Therefore, white holes are such white crows, in a sense, "outcasts". And if a person is engaged in black holes, then he is doing something real and interesting, and if white, then this, of course, is better than dealing with the classification of dragons, but many people perceive about the same.

Let's go back to short radio bursts. Are there any other hypotheses of their origin?

There is also a mechanism for the transformation of a neutron star into a black hole. It is also considered as a candidate for the role of a source of fast radio bursts. When a neutron star collapses into a black hole, its entire magnetosphere and particle streams are ejected into space. As a result, we get a short radio signal.

But the problems of many hypotheses are not even that they are very exotic, but that the events they describe do not occur so often in the Universe and do not occur at all at every step. And there are a lot of bursts. Therefore, as we have already said, the population of bursts can be composite. Say, 10% of the bursts give us the merger of neutron stars, 10% - the collapse of a neutron star into a black hole, and another 40% - radio pulsars and magnetars. That is, perhaps this is some kind of very complex "salad" of different ingredients in different proportions.

What does it take to get to the bottom of the truth?

Everyone is waiting for some kind of critical observation and just big statistics on radio bursts. And here all the hopes are for new radio telescopes. One of them has already been created - the UTMOST telescope. Another one on the way is the CHIME telescope. And this year we are expecting a large Chinese telescope with a mirror diameter of 500 meters - Five Hundered Meters Telescope (FAST). Thanks to these tools, we hope, the statistics of fast radio bursts will greatly increase, and perhaps this will allow us to choose one hypothesis. True, if we recall how the hypothesis of gamma-ray bursts was formed, then the number of bursts there did not help, it was necessary to see them in different ranges. So far, short radio bursts are visible only in the radio range. So it would be great to see these flashes in some other way.


How were short radio bursts discovered at all?

It is very difficult to identify them in the radio range, because there is a lot of noise: some technical installations are working on the Earth, lightning strikes, something is constantly happening in the magnetosphere. Therefore, in order to learn to "see" short radio bursts, technical progress was simply required. It reached the required level at the beginning of the 21st century. The first thing that was done was the discovery of a new type of bursts from neutron stars, the so-called rotating radio transients - RRATs. After that, a similar technology for searching for radio bursts was regularly used at the Parkes telescope. And one day, quite by accident, a short radio burst was recorded. Looking where he came from, they did not find anything interesting. Have watched yet and have not seen any repetitions. It became clear that this is a completely new phenomenon.

An article about this appeared in 2007, and the surge itself was registered in 2001. But then only a few were interested in this, including we published our model. But, six years later, in 2013, an article appeared where four bursts were discovered at once. And since July 2013, a stormy history of recording short radio bursts begins. To date, many hundreds of articles are devoted to them, which continue to appear every week, even almost every day. Therefore, I would say that today this is one of the highest publication rates for such a separate topic. This is understandable: fast radio bursts are a new mystery in space. Moreover, all this is a very fresh story taking place before our eyes.

What can be said about the practical benefits of studying short radio bursts?

As always, it is rather difficult to predict something here. For example, in 1963, when quasars were identified (and they were discovered even earlier), hardly anyone would say that the entire orientation system would be based on them in a few decades (without knowing this, in life we ​​are guided by for them).Therefore, why fast radio bursts are needed, we do not know, if only because their nature is unknown.

Here you can draw an analogy with underground nuclear explosions, from which there were very great benefits for science. After all, what is an underground nuclear explosion? This is a very short energy release at a well-known point from which a seismic wave runs across the planet. Thanks to this, you can explore the interior of the Earth.

Short radio bursts, if they occur billions of light years away, shine through the entire universe. Therefore, this is a very powerful tool for studying it; they will be needed to check many fundamental laws. And if they can still be seen in the gamma range, it will be just a gorgeous event. For example, many models predict that the speed of light of different wavelengths in a vacuum is different. And, accordingly, if we have such a short marker in the radio and gamma range, at the two extreme points of the radio magnetic spectrum, then this will be the best way to test these models.

The article was published in Naked Science # 26.

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