This is the first black hole photo captured by astronomers. It is said that it took two years to print this picture, but there are also some netizens make complaints about this picture, which is like a fuzzy doughnut. What do you think?
What is a black hole?
Since the middle of last century, people have never stopped exploring black holes.
More than 200 years ago, Michelle of England and Laplace of France proposed that a star with enough mass but small volume will produce strong gravity, so that even light can not escape from its surface, so the star is completely 'Black', but this inference was later forgotten.
Shortly after Einstein published his general theory of relativity in 1915, German mathematician Schwarzschild obtained a solution of Einstein's field equation under static spherical symmetry. The solution has singularity at a special radius (later called Schwarzschild radius).
In 1939, American physicist Oppenheimer and others also proved that there is indeed a time space region, and light can not escape from this region and reach the distant observer. The boundary of this region is called the horizon. In the case of static spherical symmetry, the radius of the horizon is the Schwarzschild radius. If the radius of an object is less than the Schwarzschild radius, then the object should be 'Black' and cannot be seen.
Scientists call these special objects, which are extremely attractive and invisible, black holes. Therefore, black hole is also a product predicted by Einstein's general theory of relativity.
How black holes form: it's all caused by gravity
Lu Youjun, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, said: "at present, it is relatively clear that stellar black holes are the remains of stellar collapse, while massive black holes may be formed by the accretion of medium mass black holes produced by other mechanisms. '
All stars are nuclear fusion reactors in which light elements (mainly hydrogen) polymerize into heavy elements. Nuclear fusion provides most of the energy of a star's lifetime. Eventually, however, the nuclear fuel ran out, and the energy generated by the center was no longer able to resist the huge weight of the shell, and gravity began to dominate.
According to relativity, nothing can travel faster than light. If no light can escape, nothing else is more likely: everything will be pulled back by the gravitational field. In this way, there is a collection of events or a region of time and space, from which no light or anything can escape to the distant observer - we call this region black hole, and its boundary is called event horizon.
How to confirm the existence of black hole
Scientists can indirectly feel the existence of black hole through its influence on the surrounding celestial bodies, especially the space-time distortion caused by its huge gravity, just as they can indirectly infer the mass of the earth through the orbit and speed of the moon.
Secondly, as mentioned above, the accretion disk and jet will produce luminescence, accompanied by radiation of other frequency bands.
Finally, the interaction between a black hole and other celestial bodies or another black hole will produce a large number of gravitational waves, which is also a detectable clue.
Why take pictures of black holes
There's nothing like a photo to prove the existence of a black hole.
Scientists have been photographing black holes for as long as two years. Why do scientists insist on photographing black holes? The idea of photographing black holes in the scientific community has existed for at least 30 years. Einstein predicted the existence of black holes in the theory of relativity. Although the scientific community has generally reached a consensus, there are still doubts for the public. How can we better prove that black holes were actually there? Taking a picture with a picture and a truth is certainly the most convincing.
However, this photo is not very difficult to take. Black holes are so far away from us that even the 'high-grade' telescopes still have insufficient resolution. Scientists estimate that at least an astronomical telescope the size of the earth is needed to take pictures of black holes.
The existing single telescope is certainly not enough. What should we do? This time, scientists came up with the method of "breaking the whole into parts". MIT scientists, together with researchers from other research institutions, launched the "event horizon telescope" project, and deployed eight submillimeter radio telescopes around the world to observe black holes at the same time. The black hole 'true face' you see tonight is the result of the concerted efforts of the telescope '8 brothers'.
What kind of black hole is suitable for taking pictures
The shadow of a black hole and the crescent halo around it are very small. If we want to take pictures of a black hole, there is no doubt that we want to find a black hole that looks big enough in diameter.
Because the size of event horizon of black hole is proportional to its mass, which means that the larger the mass is, the larger the event horizon is. Therefore, the supermassive black hole nearby is a perfect candidate for black hole imaging. The center black hole Sgr A * and the center of the neighboring radio galaxy M87 * in the direction of Sagittarius are two best known candidates.
How is the picture of black hole taken?
Although scientists can't see the essence of black holes, they can trace back to the 'horizon' where photons disappear, which is the limit of what we can 'see'.
There are indeed some luminous phenomena around black holes. For example, when a black hole eats the stars around it, it will tear the gas of the stars to its side and form a rotating accretion disk. Black holes sometimes burp, and part of the accreted gas is ejected along the rotational direction to form jets.
Accretion disk and jet will produce bright light due to gas friction, as well as radiation of other frequency bands.
What kind of telescope can image black holes?
To image a black hole, it is necessary to make sure that the telescope is sensitive enough and the resolution details are small enough so that it can be seen clearly. In order to meet these requirements, the best tool is the very long baseline interferometry (VLBI) which appeared in 1967 (it is worth mentioning that the VLBI technology has also been successfully applied to the probe positioning of Chang'e lunar exploration project in China). It is assumed that a baseline with a length of 10000 km can obtain a resolution of about 21 micro arcsec at the wavelength of 1 mm.
Which black hole did you take this time?
Two supermassive black holes. One is the central black hole Sgr A *, the center of M87 Galaxy in Virgo. These two targets, rather than the nearer stellar black holes in the galaxy, were chosen because their horizons are large enough from earth.
For a long time, scientists have found hundreds of billions of stars rotating around the center of the galaxy, and speculated that there is a super massive object there. According to the calculation, the mass of Sgr A * is about 4 million suns, and the horizon radius is about 24 million km. It sounds big enough, but given that the black hole at the center of the Milky Way galaxy is 25000 light-years away, the actual effect is equivalent to observing an orange on the moon on earth, or seeing a small hole on a golf ball in Shanghai in Beijing.
The supermassive black hole at M87 center is 6.6 billion times the mass of the sun, and its horizon is about three times the orbit of Pluto. Of course, due to the distance, the actual effect of M87 central black hole on earth may not be different from that of Sgr A *.
How to take a picture with such a high resolution?
Global telescopes form an array to observe jointly, forming a large telescope with an effective aperture equal to the diameter of the earth. The virtual telescope is called the event horizon telescope (EHT) and consists of eight telescopes. They are: South Pole Telescope; Atacama large millimeter array (ALMA) in Chile; Atacama Pathfinder experiment in Chile; Large Millimeter Telescope in Mexico; submillimeter telescope in Arizona, USA; James Clerk Maxwell Telescope (jcmt) in Hawaii; submillimeter array in Hawaii; and 30 meter millimeter wave telescope of millimeter wave radio astronomy Institute in Spain. They made joint observations of two black hole targets in April 2017.
Since 2018, the Greenland telescope, IRAM noema Observatory in France and Kitt Peak National Observatory in the United States have joined in the follow-up research and calibration work.
More than 60 research institutions around the world participated in the research, including Shanghai Observatory and Yunnan Observatory under the Chinese Academy of Sciences, as well as Huazhong University of science and technology, Nanjing University, Sun Yat sen University, Peking University, University of Chinese Academy of Sciences, Taiwan University and other universities. This is also the reason why Shanghai and Taipei jointly hold press conferences.
Only from a distance: it will turn people into spaghetti
Although people's enthusiasm for black holes is high, they can only be seen from afar, otherwise, the consequences will be very serious. In short, if you get too close to a black hole, you're stretched like spaghetti. This phenomenon has a very creative name 'spaghetti effect'. The reason for this effect is that the force of gravity is different in different parts of the human body.
If you fly to the black hole with your feet down, because your feet are closer to the black hole, it will receive more gravity than its head. What's worse, because the arms are not in the center of the body, they will be stretched slightly different from the direction of the head, and the edges of your body will be pulled into the body. As a result, your body is not only lengthened, but also thinned. So before you (or something else) get to the center of the black hole, you become a spaghetti.