27 things you need to know about black holes

On April 10th, the first black hole photo in human history was announced. We have too many questions about black holes. In view of this, we have integrated some interpretations of the major authoritative media to answer your questions. The black hole belongs to a very strange celestial body. Even light can't escape its gravitational restraint. Since the light can't escape, it can't be seen outside. Not to mention the naked eye, even a lot of scientific instruments on the tall, can not capture its figure.

1. What is a black hole? How is it formed?
   The formation of black holes and the collapse of stars cannot be related. In ancient China, there was a record. One day, the sky suddenly popped up with a particularly bright star. Later, we realized that the star was collapsed because of its weight, and it was accompanied by strong radiation, and we saw the bright light it emitted.
2. When did the study of black holes begin?
   More than a hundred years ago, Einstein proposed general relativity, combining time and space into a four-dimensional space-time, and proposed that gravity can be regarded as a distortion of time and space. This theory has made a number of important predictions, one of which is that when the mass of an object collapses, it can be concealed within the event horizon—in the “sphere of influence” of the black hole. The gravitation is so strong that even the light can't escape.
   In 1916, the German astronomer Carl Schwarzs obtained a vacuum solution of the Einstein gravitational field equation. This solution shows that if a large amount of matter is concentrated in space, it will produce a strange phenomenon around it, that is, at the particle point. There is an interface around it - once the "vision" enters this interface, even light can't escape. This "unbelievable celestial body" was named "black hole" by American physicist John Archibald Wheeler.
3. How many types of black holes are there?
   Divided by composition, black holes can be divided into two categories. One is a dark energy black hole, and the other is a physical black hole.
   According to the physical properties, according to the physical properties of the black hole itself, angular momentum, charge division, black holes can be divided into four categories.
   (I) Do not rotate uncharged black holes
   (II) Do not rotate the charged black hole
   (III) Rotating unpowered black hole
   (IV) General black hole
   (V) Double Star Black Hole
4. Why is the black hole photo red?
   Black hole photos first appeared, many people are excited and confused at the same time? Say good is a black hole, why is the photo red? In fact, the black hole telescope collects not the daily visible light, but a sub-millimeter wave with a longer wavelength, which itself has no color difference. To put it bluntly, the red color of the photo is the effect of post-processing, and you can also understand it as a kind of "scam".
5. Is the photo of the black hole so obscure?
   A black hole is mysterious, but its photo is the same as the resolution of an optical photo, rooted in resolution. Although our current sub-millimeter telescope has reached a baseline of 10,000 kilometers, the spatial resolution has just reached the size of the black hole interface, so in the limited area observed by scientists, it is equivalent to only a few pixels. In the real world, we can only see a few bright spots on the accretion disk in the photo.
6. Which black hole was taken this time?
   The first announcement of this is the photo of the galaxy M87. Don't worry, the black hole photo in the center of the galaxy is still in data processing.
   Beginning on April 5, 2017, an International Horizon Telescope (EHT) began to observe two special celestial bodies: 25,000 light years from Earth, at the core of the Milky Way, Sagittarius A, and 53 million light years from Earth. M87 Virgo Nebula. The target of observation is not a conventional celestial body, but two suspected supermassive black holes. The black hole is exposed to the human retina like never before.
7. Why did the photo rinse for 2 years?
   The visual interface telescope began to take photos of the black hole in 2017. It was only released in 2019, and it took two years to wash this "high-gray" photo. First of all, the amount of data observed by the telescope is very large. It is naturally very difficult to process these data. Secondly, the black hole is too "naughty". The nearby gas is in an extreme environment, and its movement has a lot of uncertainty. At the same time, Our scientists are very rigorous. At the time of final data processing, scientists are separately processed and verified separately in two different places. This counts down and it takes a lot of time.
8. Why can I take photos of black holes that don't shine?
   The most impressive impression of the black hole is the "battle" but a mysterious existence. It is said to be "baze" because it can engulf any substance in the immediate vicinity.
   If it's just a solitary black hole, we really can't take a black hole photo. But usually there is material around the black hole, forming a disk-like structure called "accumulation plate." The material in the accretion disk rotates around the black hole at high speed, rubbing against each other and emitting a glowing glow, including continuous radiation from radio waves to visible light to the X-ray band, which can be escaped to distant locations and detected by us.
   Therefore, what we photographed is not the black hole itself, but the outline of the black hole outlined by the radiation from the material on its boundary.
9. What is the artifact of the "Event Vision Telescope" for taking photos of black holes?
   In order to observe the physical processes on the edge of the black hole horizon, astronomers used eight millimeter/millimeter-wave radio telescopes distributed around the world. These telescopes form a virtual telescope with a caliber close to the whole earth. This virtual telescope is called For the "Event Vision Telescope."
   Among the eight radio telescopes, the Atacama Large Millimeter Wave Array (ALMA) is the most powerful! With a cost of $1.4 billion, ALMA is one of the most expensive ground-based telescopes available today. If there is no ALMA to join, observing the black hole's horizon is simply an impossible task.
10. Why is it only the “Event Vision Telescope” that can shoot black holes?
    Since the black hole in the center of the galaxy is blocked by thick interstellar dust and gas, the optical band telescope is powerless and can only use the radio band. The millimeter wave is already the lower limit of the wavelength used by the radio telescope, and it has been bordered by infrared rays on the electromagnetic spectrum. In order to be able to observe the behavior of matter in the black hole horizon, the Event Vision Telescope has increased the resolution of the radio telescope to an unprecedented height, to the extent of 10 to 20 micro-angle seconds! This is equivalent to seeing the date of issue on coins 4,000 kilometers away. That is to say, the resolution of the event vision telescope is thousands of times that of the Hubble telescope.
11. Can the "Event Vision Telescope" take pictures of all black holes?
    The Event Horizon Telescope utilizes a technique called Very Long Baseline Interferometry (VLBI). It allows simultaneous observation of a celestial body with multiple astronomical telescopes, simulating the observation of a giant telescope of the size equivalent to the maximum separation distance between telescopes.
    The resolution of very long baseline interferometric observations is unmatched by any other telescope. The most visible observations are observational studies of supermassive black hole candidates because the scale of black holes is very small. At present, there are three most successful VLBA observations, namely the supermassive black hole candidate in the center of the Milky Way, the elliptical galaxy M87 and the Seifert galaxy NGC4258.
12. Why did you choose the black hole in the center of the Milky Way and the center of the M87 galaxy as the research object?
    In the Milky Way, humans have discovered more than 20 stellar-quality black holes, more than 3,400 light-years away from us, but why not choose these relatively close black holes for observation, rather than approaching the Milky Way outside 26,000 light years away. The black hole in the center and the black hole in the center of the M87 galaxy beyond 53 million light years? This is because the quality of these star-rated black holes is too small and the diameter is relatively small. Therefore, from the perspective of the earth, the opening angle is not as large as the super-mass black hole at a long distance.
13. What contributions did Chinese scientists make during the black hole photo shoot?
    Chinese scientists have long been concerned with high-resolution black hole imaging research, and have carried out various aspects of international display before the formation of EHT international cooperation. In this EHT cooperation, Chinese scientists jointly promoted EHT cooperation and participated in the EHT telescope observation time application, and assisted the JCMT telescope to observe and participate in data processing and theoretical analysis of results, making EHT black hole imaging. Positive contribution.
14. The black hole is so “greedy”, can it really hold so many things?
    In 1974, physicist Stephen Hawking proposed the theory of radiation in black holes, namely Hawking radiation. According to this theory, black holes lose some of the mass and energy through Hawking radiation.
    Although Hawking radiation will make black holes "sweat", but the amount consumed is far less than the amount of their greed, so it is impossible to lose weight.
    In fact, all the matter in the universe may end up inside these greedy black holes, and by that time, the universe is really dark.
15. What does the shadow image of the black hole represent?
    The black hole photo shows a bright ring-shaped structure with a black center that looks a bit like a donut. The black part is the "shadow" cast by the black hole, and the bright part is the accretion disk that rotates around the black hole at high speed.
    General relativity predicts that due to the existence of black holes, we will see a shadow formed in the central region due to the black hole horizon, surrounded by a crescent-like halo caused by accretion or jet radiation, due to the black hole The rotation is different from the observer's line of sight, and the size of the aura is about 4.8-5.2 times the Schwarzschild radius (note: the Schwarzschild radius refers to the event horizon radius of the black hole without spin).
16. Does black hole research have any meaning for the development of science?
    Scientists say that the study of black holes is to understand the natural laws in this respect. The important aspect is to understand the relationship between quantum mechanics and gravitational theory and the role of black holes in the evolution of the universe.
    This is not to say that black holes are more important than other phenomena. Only a lot of people are interested in this, and many things we still don't understand, so we conduct research. As for the clarification, whether it is possible to realize the good wishes of human beings such as time and space travel is the application problem.
17. How was the black hole seen?
    The Event Vision Telescope consists of eight radio telescopes on four continents. The yellow lines in the picture are the "baselines" that connect these telescopes, thus forming a telescope of comparable size to Earth.
    They traveled north to Spain, south to the South Pole, and selected two targets for the selected target (two years ago, the vision telescope selected one, the Milky Way center black hole Sgr A*, and the other is the black hole in the center of the galaxy M87). Large nets, recovering massive amounts of data to outline the appearance of black holes.
    In fact, the sub-millimeter band is very different from the visible light we are very familiar with. This band is not directly visible to us. Therefore, taking pictures of black holes using the sub-millimeter band is actually a spatial distribution map of the radiation around the black hole.
    For the optical photos we contact daily, it reflects the distribution of photons of different colors or frequencies in the optical band at different spatial locations. With this in mind, it is easy to understand the principle of the sub-millimeter band "black hole photo studio".
    Although the sub-millimeter band observation is performed at a single frequency, since the radiant intensity of the photons in different regions around the black hole is different, we can obtain a photon intensity distribution map, and then we assume that different intensities correspond to different colors. Get a "pseudo-color map" - the color in the picture is probably the color that scientists set according to their personal preferences.
18. How has the black hole been observed and how is it proven to exist?
    A black hole cannot be directly observed, but its existence and quality can be learned indirectly, and its influence on other things is observed. By presenting the "edge message" of the gamma ray due to high heat before the object is inhaled, the message of the existence of the black hole can be obtained. It is speculated that the existence of black holes can also obtain the position and quality by indirect observation of the trajectory of the star or interstellar air mass.
19. What is the significance of black hole photos for verifying relativity and revealing the evolution of galaxies?
    In addition to helping us directly confirm the existence of black holes, this direct imaging also validated Einstein's general theory of relativity by simulating observation data. During the work of the visual telescope and the subsequent data analysis, the scientists found that the observed black hole shadows and the theory of relativity predicted almost exactly the same, and people could not help but sigh again about the greatness of Einstein.
    Another important point is that scientists can limit the quality of the central black hole by the size of the black hole shadow. This time, an independent measurement of the black hole quality of the M87 center was made. Prior to this, the means of accurately measuring the quality of black holes was very complicated.
    Due to factors such as observation resolution and sensitivity, the current black hole detail analysis is still not perfect. In the future, with the addition of more telescopes, we expect to see more and more details around the black hole, so as to gain a deeper understanding of the gas movement around the black hole, distinguish the generation of jets and the mechanism of the cluster, and improve our understanding of the evolution of the galaxy. understanding.
20. Who first imagined the black hole?
    In the years when the black hole was not able to be seen, scientists calculated the "face" of the black hole.
    As early as the late 1910s, the great mathematician David Hilbert calculated the light bending and gravitational lens effects around the black hole.
    In the 1970s, James Bardeen and Jean-Pierre Luminet and others calculated images of black holes.
    In the late 1990s, HeinoFalcke et al. made detailed calculations for the black hole in the center of the Milky Way and introduced the black hole shadow. They also pointed out that if the black hole shadow is "inlaid" in a hot, optically thin (ie transparent to a certain observation wavelength), it can be "seen" by (sub)millimeter long-shallow baseline interferometry. .
    Since then, people have used the general relativity magnetohydrodynamics numerical simulation to carry out a lot of research on black hole imaging, all predicting the existence of black hole shadows. Thus, imaging the shadow of a black hole provides direct "visual" evidence of the existence of a black hole.
    twenty one. What is the ultimate fate of the black hole?
    The future of black holes has only one direction: macroscopic black holes will continue to engulf any material and energy that can be encountered, and grow larger and larger; however, as they become larger and larger, their average density will become smaller and smaller, that is, Say they will become more and more puffy.
21. What if a human falls into a black hole?
    There is still time and space in the black hole. It is possible that the black hole is really immortal, and it will grow bigger and bigger and become fatter.
    The next step is to open the brain: What happens if the astronauts drive a very strong spacecraft that will not be crushed by gravity into the black hole? Scientists will tell you seriously: First of all, whether the spacecraft is strong is not the key, and whether the astronauts themselves are strong is the most crucial.
    In fact, once you "fall" into the black hole, the astronauts will feel the great stretching force along the direction of the drop. It is estimated that a person is pulled hard in the direction of the head and feet! If the quality of the black hole he entered is relatively small, the astronauts will be "pushed" into noodles. If the black hole he entered is of great quality, he may not feel too much when he starts to go in. But after entering, it will always go to the center of the black hole, until the end of the body,
    As for what will happen next or something horrible or wonderful, we will not know.
22. Is the black hole a pipe?
    The size of the black hole horizon depends on gravity, so the horizon is a sphere wrapped around a black hole. If you have a way to see the horizon from the outside, it will look like a black sphere.
    Some people think of a black hole as a circle or a tube. The "pipe" is an interpretation often used to explain the bending of gravity in space, when the collapsed 3-dimensional space is reduced to a two-dimensional space. The space is imagined as a bed sheet, and the curvature of the space by a massive celestial body is as good as putting a bowling ball on the sheets. But the space is not 2D, it is 3D (if the time is 4D), so this interpretation will make people misunderstand the shape of the black hole horizon.
23. Will the black hole turn?
    The black hole does indeed turn. The star will spin and its core will turn. As the star's core shrinks more and more, its rotation will get faster and faster. It's like a figure skater speeding up his own speed by retracting his open arms. If the mass of the core is not enough to form a black hole, it will form a neutron star with a diameter of only a few kilometers. Hundreds of neutron stars have been discovered, and they rotate very fast, sometimes even up to 100 laps per second.
24. Is the black hole always black?
    Few substances will fall directly into the black hole and disappear. With a slight deviation, they will rotate around the black hole. As the material increases, they gather around the black hole. The high-speed rotation of these materials produces intense friction, which causes the material to be heated to millions of degrees of high temperature, so that the material near the black hole emits extremely bright radiation.
    The black hole turned out to be the brightest object in the universe by phagocytizing matter.
25. Will black holes continue to grow bigger?
    What happens when two black holes collide? They will form a bigger black hole. Similarly, black holes will also grow when they swallow other substances. In the early universe, when a galaxy was forming, the material at the core of the baby galaxy collapsed into a massive black hole. As more and more substances fall into it, black holes will greedily digest them and grow. Eventually it will grow into a supermassive black hole with a mass of millions or even billions of times the sun.
26. Is the density of black holes comparable to air?
    This is really unexpected, but it is.
    An ordinary black hole is usually three times the mass of the sun, and the radius of the horizon is 9 kilometers. At this time, its density is 2 trillion grams per cubic centimeter. But if you double its mass, its density will be reduced to 1/4; the mass will increase by 10 times and the density will be reduced by 100 times. For a supermassive black hole of 1 billion solar masses commonly found in clusters of galaxies, its density is only 0.001 grams per cubic centimeter, the same as the density of air on Earth.