A large number of astronomical observation data have confirmed that in the vast universe, there are countless black holes mysteriously hiding in the galaxies. Yesterday, the world's first black hole photo was finally released. Do you know how this black hole photo was taken?
However, human beings have never seen a black hole directly and do not know what it really looks like.
In order to get a glimpse of the black hole, from April 5 to 14, 2017, scientists from more than 30 research institutes around the world launched an ambitious and huge observation plan. They will be distributed in different parts of the world's eight radio telescope arrays to form a virtual telescope network, hoping to use it to capture black hole images.
Finally, scientists succeeded in taking the first 'picture' of the black hole.
At 21:00 Beijing time on April 10, 2019, this photo was released simultaneously in Washington, USA, Shanghai and Taipei, China, Santiago, Chile, Brussels, Belgium, Lingbi, Denmark and Tokyo, Japan. The legendary black hole has finally been unveiled.
How to take the first black hole photo in human history, the reporter will reveal the whole process for you.
Understanding black holes
Theoretically, black hole is a kind of celestial body predicted by Einstein's general relativity. Its super gravity makes light unable to escape its sphere of influence, which is called the radius of the black hole or event horizon.
So, how do black holes form?
Like all things in the universe, stars age and die. When some massive stars run out of fuel for nuclear fusion reaction, their cores will collapse sharply, and all matter will collapse towards a point quickly, and finally collapse into a singularity the size of a soybean, forming a powerful vortex of force field, distorting the surrounding space-time and becoming a black hole.
In the universe, according to mass astronomers, black holes in the universe are divided into three categories: stellar mass black holes (tens to hundreds of times the mass of the sun), supermassive black holes (millions of times the mass of the sun) and medium mass black holes (between the two).
According to the theoretical calculation, there should be tens of millions of stellar black holes in the galaxy. However, because the black hole itself does not emit and reflect electromagnetic waves, the instrument and the naked eye can not directly observe it.
Since we can't see it, how can we know it exists? Astronomers mainly use various indirect evidences.
Shen Zhiqiang, researcher of Shanghai Observatory, Chinese Academy of Sciences: "there are three types of representative evidence. First, the movement of stars and gases reveals the trace of black holes. Black holes have strong gravitation, which will affect the surrounding stars and gases, so we can confirm the existence of black holes by observing the influence. The second is to judge the existence of black hole according to the accretion matter of black hole, that is, the light emitted when eating. The third is to see black holes through the process of their growth. '
So far, through indirect observation, scientists have found and confirmed more than 20 stellar black holes in the galaxy, but there may be tens of millions of stellar black hole candidates.
Shen Zhiqiang said: 'there is a supermassive black hole at the center of every galaxy in the universe. There is one in the center of our galaxy, which is about four million times the mass of the sun. In addition, there are many stellar black holes in the galaxy. '
What is the relationship between these mysterious black holes and the birth and evolution of the universe? What is the relationship between these mysterious black holes and their galaxies? What is the relationship between these mysterious black holes and us? Will they have an impact on our lives;
To answer these questions more accurately and clearly, scientists want to see black holes directly.
Ready for camera
General relativity predicts that although the black hole itself does not emit light, because of the existence of the black hole, the surrounding space-time bends and the gas is attracted to fall. As the gas falls into the black hole, gravitational energy is converted into light and heat, so the gas is heated to billions of degrees. The event horizon looks like a shadow, surrounded by a crescent shaped halo of accretion or jet radiation.
Einstein's general theory of relativity has predicted the existence of this' shadow ', as well as its size and shape.
Scientists expect to capture the black hole's shadow directly this time.
Lu Rusen, a researcher at the Shanghai Observatory of the Chinese Academy of Sciences, said: "imaging the shadow of a black hole will provide direct & lsquo; Visual & nbsp; evidence for the existence of a black hole. '
"We have to make sure that the telescope is sensitive enough and that the details that can be resolved are small enough to be able to see and see clearly," Lu said. '
But to meet all the above conditions, the telescope needs to be the size of the earth.
However, the largest aperture of a single telescope on earth is only 500 meters.
What should we do?
Clever astronomers have come up with a good idea - to unite the strong.
By combining some existing telescopes on the earth, we can form a virtual telescope with an aperture as large as the Earth's. The sensitivity and resolution achieved by this telescope are unprecedented.
As a result, more than 200 scientists around the world have reached the major international cooperation project of event horizon telescope (EHT) and decided to use very long baseline interferometry.
Shen Zhiqiang said: "it is to use multiple telescopes located in different places to make joint observations at the same time, and finally combine the data after correlation analysis. This technology has been quite mature in the radio band. '
Finally, the scientists selected eight submillimeter radio telescopes from many places around the world, including the Antarctic telescope.
"Most of them are single telescopes, such as the jcmt in Hawaii and the Antarctic telescope. There are also telescope arrays, such as the Alma telescope, which is made up of more than 70 small telescopes. '
Select target
While building large virtual telescopes, scientists are also looking for suitable shooting targets.
The silhouette of the black hole and the crescent halo around it are very, very small. In order to take pictures of black holes, it is necessary to find a black hole with enough angle diameter as the target.
After selecting a circle, the scientists decided to target two nearby black holes: Sgr A *, the central black hole of the galaxy in the direction of Sagittarius, and M87 *, the central black hole of radio galaxy M87.
Shen Zhiqiang said: 'since the size of the event horizon of a black hole is directly proportional to its mass, it also means that the larger the mass, the larger the event horizon. These two black holes are super massive, and their event horizons are also the largest on earth, which can be said to be the best imaging candidates at present. '
However, the two selected black holes are the best candidates for imaging, but it is extremely difficult to take a clear picture of them.
The mass of the Sgr A * black hole is about 4 million suns, and the corresponding visual interface is about 24 million kilometers, which is the size of 17 suns. However, the earth is 25000 light-years (about 2.4 billion kilometers) away from Sgr A *.
Shen Zhiqiang said: 'this means that its huge visual interface, in our opinion, is only as small as the tip of a needle, just like we stand on the earth to watch an orange placed on the surface of the moon. '
The mass of M87 central black hole is even greater, reaching 6 billion solar masses.
Although the distance between M87 central black hole and the earth is farther than that between Sgr A * and the earth, due to its huge mass, the event horizon of M87 central black hole may be about the size of Sgr A * or even slightly larger for scientists.
Debug camera
In order to see the details of the event horizons of the two black holes, the event horizon telescope needs to have a high enough spatial resolution.
How high is it?
"It's more than 1000 times higher than the resolution of the Hubble telescope. '
But don't think that as long as the resolution of the virtual telescope array is high enough, the black hole will be photographed successfully.
The actual situation is not so simple! Just like watching TV programs must choose the right channel, for black hole imaging, it is very important to be able to observe in the right wave band.
A series of previous studies have shown that the best band to observe the 'shadow' of the event horizon of a black hole is about 1 mm.
"Because gas radiation is the brightest in this band, and radio waves can travel from the center of the galaxy to the earth without being blocked," Lu said. '
In this case, the resolution of a telescope depends on the distance between telescopes, not on the size of the aperture of a single telescope.
In order to increase the spatial resolution to see smaller areas, the scientists added telescopes in Chile and Antarctica to the telescope array.
Shen Zhiqiang said: 'this setting is to ensure that all eight telescopes can see the two black holes, so as to achieve the highest sensitivity and spatial resolution. '
Official shooting
As far as Spain in the north and as far as the South Pole in the south, eight telescopes will cast a big net on the selected target, retrieve massive data, and outline the shape of the black hole for us.
The observation window period for scientists is very short, only about 10 days a year. For 2017, it's between April 5 and April 14.
In addition to the limitation of observation time, the requirements of weather conditions are also very strict.
Because the water in the atmosphere has a great influence on this observation band, the water will affect the intensity of radio waves, which means that precipitation will interfere with the observation. Shen Tai said that if the visual interface telescope is to observe smoothly, the weather conditions of all the telescopes must be very good. '
According to the requirements, the eight telescopes selected in the plan are all located at high altitude with little rainfall, and the probability of all sunny days is very high.
In addition, all telescopes must be fully synchronized in time for the imaging to be successful.
On April 4, 2017, Beijing time, the event horizon telescope started shooting and turned its eyes to the universe. The final observation ended on April 11 est.
During the observation, each radio telescope collects and records radio wave signals from the vicinity of the target black hole, which are then integrated to obtain an image of the event horizon.
Shen Zhiqiang said: "in order to ensure the stability of the signal, the event surface telescope uses an atomic clock to ensure that the telescope collects and records the signal in time synchronization. '
develop films
It's not easy to take a picture of a black hole, and it takes a long time to develop a picture.
Radio telescopes can't directly 'see' black holes, but they will collect a lot of data about black holes and use the data to describe them to scientists.
After the observation, the data collected by each station will be collected in two data centers (haystack Observatory in Massachusetts and MARPOL radio station in Bonn, Germany). There, the supercomputer replays the data recorded by the hard disk, and after compensating for the time difference between the radio waves arriving at different telescopes, integrates all the data and conducts calibration analysis, thus producing a high-resolution image of the black hole.
After two years of 'development', on April 10, 2019, the first black hole photo in human history was finally published.