On April 10, 2019 (March 6, 2019, the lunar calendar), the first black hole photo ever obtained by mankind. At 9 p.m. Beijing time on April 10, 2019, the Black Hole Event Horizon Telescope (EHT) cooperative organization coordinated a joint press conference of six places around the world, announcing that {b} for the first time humans have used a virtual radio telescope with an aperture as large as the Earth to successfully capture the world's first black hole image at the center of the nearby giant elliptical galaxy M87 {/b}(Figure 1). The significance of this image is extraordinary. It provides direct "visual" evidence of the existence of black holes, making it possible to verify Einstein's general theory of relativity under a strong gravitational field and to study in detail the accretion of matter and relativistic jets near the black hole. So why can black holes be imaged? How to image? This article attempts to interpret the whole process of black hole imaging from the perspective of a person who has experienced it. Figure 1. Image of the supermassive black hole (M87 *) at the center of the M87 galaxy (Source: References) The top image is the image on April 11, 2017, and the bottom three images are the images of M87 * on April 5, 6 and 10, 2017. The faint and weak region in the center of the figure is the "black hole shadow"(see below), and the surrounding annular asymmetric structure is caused by the strong gravitational lensing effect and the relativistic beam effect. This asymmetry between top (north) and bottom (south) can determine the spin direction of the black hole. Black holes and general relativity More than a hundred years ago, Einstein proposed general relativity, which combined time and space into a four-dimensional spacetime, and proposed that gravity can be regarded as a distortion of spacetime. This theory makes many important predictions, one of which is that when the mass of an object continues to collapse, it can be hidden within the event horizon-within the "sphere of influence" of this black hole, gravity is so strong that even light cannot escape. The verification of general relativity dates back a century. On May 29, 1919, Arthur Eddington and others experimentally measured the deflection of light near the sun during a total solar eclipse (Figure 2), which kicked off the verification of general relativity in the last century and pushed Einstein to the "altar" of science. Figure 2. Schematic diagram of starlight deflection verifying general relativity (Source: TheIllustrated LondonNews) For a century, general relativity has withstood successive experimental verification, and the existence of black holes has been confirmed by more and more astronomical observations. At present, astronomers generally believe that black holes do exist in the universe, ranging from stellar black holes with masses several times to tens of times the sun, to supermassive black holes with masses as high as millions or even billions of times the sun. Moreover, supermassive black holes exist at the centers of almost all galaxies. However, even today, when LIGO/Virgo detects gravitational waves and authoritatively proves the existence of black holes, humans still have not directly seen the "hole" that can reveal the secrets of time and space under extreme conditions-the "black hole event horizon". This may be caused by the charm of the black hole itself-the density of the black hole is unimaginable! If the Earth is compressed into a black hole, it will be about the size of a glutinous rice ball; while a stellar black hole located 1 kpc (about 3262 light-years) away from the Earth and 10 times the mass of the Sun, the angular diameter of its event horizon is only 0.4 nanoarcseconds. This is about 100 million times smaller than the resolution of the Hubble Telescope. No existing astronomical observation method has such resolution! Why can black holes form images? Since black holes are "black" and not even light can escape, how can we see black holes? In fact, the black hole does not exist in isolation; there is a large amount of gas around it. Due to the strong gravity of the black hole, gas will fall towards the black hole. When these gases are heated to billions of degrees, they emit intense radiation. At the same time, black holes also eject matter and energy in the form of jets and winds. General relativity predicts that we will see a shadow in the central region formed by the black hole horizon (blackholeshadow), surrounded by a ring of radiation caused by accretion or jet jets-it is shaped like a crescent moon, and its size is between 4.8 and 5.2 times the Schwarzschild radius depending on the spin of the black hole and the direction of the observer's line of sight (Note: Schwarzschild radius is the event horizon radius of a black hole without a spin; the horizon radius of a sun-mass black hole is about 3 kilometers). In the years when they were not able to see the true appearance of black holes, scientists learned the "appearance" of black holes through calculations. As early as the late 1910s, the great mathematician David Hilbert calculated the bending of light and the gravitational lensing effect around a black hole. In the 1970s, James Bardeen and Jean-Pierre Luminet and others calculated the image of a black hole (Figure 3, left). In the late 1990s, HeinoFalcke and others made detailed calculations on the black hole at the center of the Milky Way and introduced the term black hole shadow. They also pointed out that if the black hole's shadow is "embedded" in the surrounding bright, optically thin (ie transparent to a certain observation wavelength) hot gas, it can be "seen" by (sub-) millimeter wave very long baseline interferometry. Since then, people have used general relativistic magnetohydrodynamics numerical simulations to conduct a lot of research on black hole imaging, all predicting the existence of black hole shadows (see Figure 3, right). Therefore, imaging the shadow of a black hole provides direct "visual" evidence of the existence of a black hole. Figure 3. Black hole shadow image (left picture is taken from reference, right picture is provided by the author) What kind of black hole is most suitable for imaging? Although the shadow of a black hole can be "seen", not all black holes are eligible for imaging. As mentioned above, black holes are very, very small. For a black hole to be imaged, there is no doubt that the angular diameter must be large enough. Since the size of a black hole's event horizon is proportional to its mass, this means that the greater the mass of the black hole, the larger the event horizon will be and the more suitable it will be for imaging. Therefore, supermassive black holes close to us are perfect candidates for black hole imaging. SgrA *, the central black hole of the Milky Way in the direction of Sagittarius, and M87 *, the central black hole of the nearby radio galaxy M87, are the two best known candidates. The central radio source of the Milky Way, SgrA *, was discovered by Bruce Balick and Robert Brown in 1974 using the National Radio Astronomy Observatory interferometer (for the story of its discovery and naming, see). There is growing evidence that it is a black hole with a mass of about 4 million times the mass of the sun. Since it is about 260,000 light-years away from the Earth, the Schwarzschild radius of the black hole at the center of the Milky Way is about 10 microarcseconds, and the angular diameter of its black hole shadow is correspondingly 47 - 50 microarcseconds, which is equivalent to the angular diameter of an apple on the moon. Size (the angular diameter of the moon is about 0.5 degrees). M87 is a giant elliptical galaxy located in the direction of the constellation Virgo, about 55 million light-years away from Earth. As early as 1918, Heber Curtis noticed a strange collimated beam of light "curious straight ray" connected to the center of the galaxy. In fact, this collimated beam is the jet of M87, emitting from the center and extending for thousands of light-years, becoming the most eye-catching feature of M87. This also makes it the first galaxy to have a jet certified (Figure 4). Like the center of the Milky Way, M87 also has a supermassive black hole at the center (now called M87 * according to the naming convention of the galactic core black hole), with a mass of approximately 6.5 billion times the mass of the sun. Although this black hole is 1500 times more massive than SgrA *, it is also more than 2000 times farther away, so it looks slightly smaller than the silver core black hole-its Schwarzschild radius is about 7.6 microarcseconds, and the size of the black hole's shadow is correspondingly 37 - 40 microarcseconds. Figure 4. Radio jets of M87 at different scales (Source: Reference data) What kind of telescope can image black holes? The goal has been selected, and it is time to "sharpen the sword and go into battle". The ancients said: "If a worker wants to do a good job, he must first sharpen his tool." To image a black hole, the best tool is Very Long Baseline Interferometry (VLBI) technology. VLBI uses widely distributed radio telescopes (distances up to tens of thousands or hundreds of thousands of kilometers) to independently record signals at each station and conduct comprehensive correlation processing of signals in the later stage to obtain a giant (virtual) telescope with a size equivalent to the maximum spacing between stations. This technology can achieve the highest resolution in astronomical research, with a resolution of ¦È~¦Ë/D, where ¦Ë is the observation wavelength and D is the longest baseline length. Assuming observations at a wavelength of 1 millimeter, a baseline of 10,000 kilometers long (approximately the diameter of the Earth) can achieve a resolution of approximately 21 microarcseconds. VLBI uses atomic clocks accurate to only one second every hundreds of millions of years to ensure that the signals collected and recorded by the telescope are synchronized in time and ensure the stability of the signals. Since the realization of VLBI technology in the late 1960s, its performance has been continuously improved with the advancement of technology, and its wavelength coverage has also expanded from the centimeter band to the (sub) millimeter band, which is currently at the forefront of international development. Just as you have to choose the right channel to watch a TV program, being able to make VLBI observations in the right wavelength band is crucial for black hole imaging. The best wavelength band for observing the event horizon of a black hole is around 1 mm, not simply because of its high resolution, but also because of the following important considerations/advantages: the radiation from the gas around the black hole becomes transparent ("optically thin ") in the short millimeter wavelength band. This is crucial for imaging black holes, otherwise no matter how high the resolution is. The radiation of the accreted gas is brightest in this wavelength band. In order to "see" the black hole horizon, the radiation around it must be "bright" enough for the sensitivity of our observation equipment. Radio waves in this band experience little interference from interstellar scattering. This is particularly important for the center of the Milky Way because it is affected by strong interstellar scattering in and above the centimeter band, making it impossible to see the intrinsic structure of radiation around the black hole. In addition, there are many important factors such as station layout and sensitivity improvement that need to be considered. From this, it is not difficult to find that it is not necessary to successfully photograph black holes as long as the resolution of the VLBI array is high enough! EHT and its observations in April 2017 In recent years, 1.3 mm VLBI observations have detected structures on the scale of the black hole event horizon in SgrA * and M87 * respectively, which is very encouraging for black hole imaging. However, due to the limitations of the number and sensitivity of stations, detailed imaging observations have not been possible. With the addition of new, high-sensitivity submillimeter wave stations (especially AtacamaLargeMillimeter/submillimeter Array) to the global 1.3 mm-VLBI array, imaging observations of black holes have become possible. In order to capture the first black hole image, more than 200 scientists from more than a dozen countries (regions) including China have formed the EHT, a major international cooperation program. The technology used for EHT observations is (sub-) millimeter-wave VLBI, which currently works in the 1.3 mm band and is expected to expand to 0.8 mm. By imaging black holes, EHT can verify Einstein's general theory of relativity in extreme environments with strong gravitational fields, and study in detail the accretion of matter and the formation and propagation of jets around the black hole. Echoing the verification of general relativity by Eddington and others 100 years ago, EHT collaborators visited several of the world's tallest and most remote radio observatories in April 2017 to test his general theory of relativity in a way Einstein would never have thought of. Participating in this observation included 8 stations located at 6 locations around the world (Table 1, Figure 5). Table 1. Information on telescopes participating in EHT observations. Among them, the effective apertures of ALMA, LMT, SMA and SPT are only for 2017 Figure 5. The 8 VLBI stations participating in EHT observations in April 2017 (pictures provided by the author) are connected by solid lines to observe M87 (7 stations; due to location limitations, the SPT telescope located in the South Pole cannot observe M87), and the dotted lines are connected by stations observing a calibration source (3C279). In order to increase detection sensitivity, the amount of data recorded by EHT is very large. During observations in April 2017, the data rate at each station reached an astonishing 32 Gbit/s, and eight stations recorded a total of approximately 3500 TB of data during the five-day observation period (equivalent to 3.5 million movies, which will take at least several hundred years to watch it!). EHT uses a dedicated hard disk to record data and then send it back to the data center for processing. There, researchers used supercomputers to correct the time difference between electromagnetic waves arriving at different telescopes, and cross-correlated all data to achieve signal coherence. On this basis, through nearly two years of post-processing and analysis of these data, humans finally captured the first black hole image. Our scientists have long been concerned about high-resolution black hole imaging research, and have carried out various related work with international visibility before the formation of EHT international cooperation. In this EHT cooperation, China scientists jointly promoted EHT cooperation in the early stage and participated in the application for EHT telescope observation time. At the same time, they assisted the JCMT telescope in observations and participated in data processing and theoretical analysis of results, making contributions to EHT black hole imaging. positive contribution. The follow-up is even more exciting. Stay tuned. Since the results obtained by different measurement methods of the mass of the central black hole of M87 (gas dynamics vs. stellar dynamics) are nearly twice as different, it is still a little surprising that M87 * can be imaged. However, smooth imaging of the M87 black hole is by no means the end of the EHT. On the contrary, this exciting result will surely inspire more interest and enthusiasm for black hole research. Currently, the observation data of the 2017 M87 is still being analyzed. Researchers hope to obtain the properties of the magnetic field around the black hole by studying the polarization of radiation, which is crucial to understanding the accretion of matter and the formation of jets around the black hole. The mass of the other best imaging candidate, the black hole at the center of the Milky Way, is more certain. Previous EHT observations have shown that there is a structure around the black hole that is "dark in the middle and bright in the periphery (one side)", with an overall characteristic size of 5 times Schwarzschild radius, consistent with the predictions of general relativity (References and Figure 6). With more observation stations (such as the Northern Extended Millimeter Array, KittPeak Telescope) joining the EHT and the improvement of data quality (sensitivity), we have every reason to believe that EHT will be able to obtain clearer images of the galactic core black hole in the near future. Let us wait and see! Figure 6. Schematic diagram of 1.3 mm VLBI observations on the silver-core black hole (Source: MaxPlanck Society) In 2013, a schematic diagram of 1.3 mm VLBI observations on the silver-core black hole was carried out using 6 VLBI stations located at 4 locations. The in-line diagram gives models of the two most likely radiating structures that are consistent with the observations. Note: In the early days of VLBI development or generally when baseline coverage is less than ideal, simple geometric models (such as Gauss) are often considered to fit the observed (visibility) data. Many early discoveries, such as apparent superluminal motion, were made based on simple geometric models with very limited baselines. Introduction to the author Lu Rusen is a researcher at the Shanghai Astronomical Observatory, China Academy of Sciences. In 2010 and 2011, he received a doctorate in science from the University of Cologne in Germany and the Shanghai Observatory of the China Academy of Sciences respectively. In 2018, he was selected into the 14th batch of "Thousand Talents Project" youth projects, with his research direction in high-resolution radio astrophysics. Zuo Wenwen is an associate researcher at the Shanghai Astronomical Observatory of China Academy of Sciences. She received a doctorate in astrophysics from Peking University in 2014. She is currently engaged in research and scientific communication of high redshift quasars.On April 10, 2019 (March 6, 2019, the lunar calendar), the first black hole photo ever obtained by mankind. At 9 p.m. Beijing time on April 10, 2019, the Black Hole Event Horizon Telescope (EHT) cooperative organization coordinated a joint press conference of six places around the world, announcing that {b} for the first time humans have used a virtual radio telescope with an aperture as large as the Earth to successfully capture the world's first black hole image at the center of the nearby giant elliptical galaxy M87 {/b}(Figure 1). The significance of this image is extraordinary. It provides direct "visual" evidence of the existence of black holes, making it possible to verify Einstein's general theory of relativity under a strong gravitational field and to study in detail the accretion of matter and relativistic jets near the black hole. So why can black holes be imaged? How to image? This article attempts to interpret the whole process of black hole imaging from the perspective of a person who has experienced it. Figure 1. Image of the supermassive black hole (M87 *) at the center of the M87 galaxy (Source: References) The top image is the image on April 11, 2017, and the bottom three images are the images of M87 * on April 5, 6 and 10, 2017. The faint and weak region in the center of the figure is the "black hole shadow"(see below), and the surrounding annular asymmetric structure is caused by the strong gravitational lensing effect and the relativistic beam effect. This asymmetry between top (north) and bottom (south) can determine the spin direction of the black hole. Black holes and general relativity More than a hundred years ago, Einstein proposed general relativity, which combined time and space into a four-dimensional spacetime, and proposed that gravity can be regarded as a distortion of spacetime. This theory makes many important predictions, one of which is that when the mass of an object continues to collapse, it can be hidden within the event horizon-within the "sphere of influence" of this black hole, gravity is so strong that even light cannot escape. The verification of general relativity dates back a century. On May 29, 1919, Arthur Eddington and others experimentally measured the deflection of light near the sun during a total solar eclipse (Figure 2), which kicked off the verification of general relativity in the last century and pushed Einstein to the "altar" of science. Figure 2. Schematic diagram of starlight deflection verifying general relativity (Source: TheIllustrated LondonNews) For a century, general relativity has withstood successive experimental verification, and the existence of black holes has been confirmed by more and more astronomical observations. At present, astronomers generally believe that black holes do exist in the universe, ranging from stellar black holes with masses several times to tens of times the sun, to supermassive black holes with masses as high as millions or even billions of times the sun. Moreover, supermassive black holes exist at the centers of almost all galaxies. However, even today, when LIGO/Virgo detects gravitational waves and authoritatively proves the existence of black holes, humans still have not directly seen the "hole" that can reveal the secrets of time and space under extreme conditions-the "black hole event horizon". This may be caused by the charm of the black hole itself-the density of the black hole is unimaginable! If the Earth is compressed into a black hole, it will be about the size of a glutinous rice ball; while a stellar black hole located 1 kpc (about 3262 light-years) away from the Earth and 10 times the mass of the Sun, the angular diameter of its event horizon is only 0.4 nanoarcseconds. This is about 100 million times smaller than the resolution of the Hubble Telescope. No existing astronomical observation method has such resolution! Why can black holes form images? Since black holes are "black" and not even light can escape, how can we see black holes? In fact, the black hole does not exist in isolation; there is a large amount of gas around it. Due to the strong gravity of the black hole, gas will fall towards the black hole. When these gases are heated to billions of degrees, they emit intense radiation. At the same time, black holes also eject matter and energy in the form of jets and winds. General relativity predicts that we will see a shadow in the central region formed by the black hole horizon (blackholeshadow), surrounded by a ring of radiation caused by accretion or jet jets-it is shaped like a crescent moon, and its size is between 4.8 and 5.2 times the Schwarzschild radius depending on the spin of the black hole and the direction of the observer's line of sight (Note: Schwarzschild radius is the event horizon radius of a black hole without a spin; the horizon radius of a sun-mass black hole is about 3 kilometers). In the years when they were not able to see the true appearance of black holes, scientists learned the "appearance" of black holes through calculations. As early as the late 1910s, the great mathematician David Hilbert calculated the bending of light and the gravitational lensing effect around a black hole. In the 1970s, James Bardeen and Jean-Pierre Luminet and others calculated the image of a black hole (Figure 3, left). In the late 1990s, HeinoFalcke and others made detailed calculations on the black hole at the center of the Milky Way and introduced the term black hole shadow. They also pointed out that if the black hole's shadow is "embedded" in the surrounding bright, optically thin (ie transparent to a certain observation wavelength) hot gas, it can be "seen" by (sub-) millimeter wave very long baseline interferometry. Since then, people have used general relativistic magnetohydrodynamics numerical simulations to conduct a lot of research on black hole imaging, all predicting the existence of black hole shadows (see Figure 3, right). Therefore, imaging the shadow of a black hole provides direct "visual" evidence of the existence of a black hole. Figure 3. Black hole shadow image (left picture is taken from reference, right picture is provided by the author) What kind of black hole is most suitable for imaging? Although the shadow of a black hole can be "seen", not all black holes are eligible for imaging. As mentioned above, black holes are very, very small. For a black hole to be imaged, there is no doubt that the angular diameter must be large enough. Since the size of a black hole's event horizon is proportional to its mass, this means that the greater the mass of the black hole, the larger the event horizon will be and the more suitable it will be for imaging. Therefore, supermassive black holes close to us are perfect candidates for black hole imaging. SgrA *, the central black hole of the Milky Way in the direction of Sagittarius, and M87 *, the central black hole of the nearby radio galaxy M87, are the two best known candidates. The central radio source of the Milky Way, SgrA *, was discovered by Bruce Balick and Robert Brown in 1974 using the National Radio Astronomy Observatory interferometer (for the story of its discovery and naming, see). There is growing evidence that it is a black hole with a mass of about 4 million times the mass of the sun. Since it is about 260,000 light-years away from the Earth, the Schwarzschild radius of the black hole at the center of the Milky Way is about 10 microarcseconds, and the angular diameter of its black hole shadow is correspondingly 47 - 50 microarcseconds, which is equivalent to the angular diameter of an apple on the moon. Size (the angular diameter of the moon is about 0.5 degrees). M87 is a giant elliptical galaxy located in the direction of the constellation Virgo, about 55 million light-years away from Earth. As early as 1918, Heber Curtis noticed a strange collimated beam of light "curious straight ray" connected to the center of the galaxy. In fact, this collimated beam is the jet of M87, emitting from the center and extending for thousands of light-years, becoming the most eye-catching feature of M87. This also makes it the first galaxy to have a jet certified (Figure 4). Like the center of the Milky Way, M87 also has a supermassive black hole at the center (now called M87 * according to the naming convention of the galactic core black hole), with a mass of approximately 6.5 billion times the mass of the sun. Although this black hole is 1500 times more massive than SgrA *, it is also more than 2000 times farther away, so it looks slightly smaller than the silver core black hole-its Schwarzschild radius is about 7.6 microarcseconds, and the size of the black hole's shadow is correspondingly 37 - 40 microarcseconds. Figure 4. Radio jets of M87 at different scales (Source: Reference data) What kind of telescope can image black holes? The goal has been selected, and it is time to "sharpen the sword and go into battle". The ancients said: "If a worker wants to do a good job, he must first sharpen his tool." To image a black hole, the best tool is Very Long Baseline Interferometry (VLBI) technology. VLBI uses widely distributed radio telescopes (distances up to tens of thousands or hundreds of thousands of kilometers) to independently record signals at each station and conduct comprehensive correlation processing of signals in the later stage to obtain a giant (virtual) telescope with a size equivalent to the maximum spacing between stations. This technology can achieve the highest resolution in astronomical research, with a resolution of ¦È~¦Ë/D, where ¦Ë is the observation wavelength and D is the longest baseline length. Assuming observations at a wavelength of 1 millimeter, a baseline of 10,000 kilometers long (approximately the diameter of the Earth) can achieve a resolution of approximately 21 microarcseconds. VLBI uses atomic clocks accurate to only one second every hundreds of millions of years to ensure that the signals collected and recorded by the telescope are synchronized in time and ensure the stability of the signals. Since the realization of VLBI technology in the late 1960s, its performance has been continuously improved with the advancement of technology, and its wavelength coverage has also expanded from the centimeter band to the (sub) millimeter band, which is currently at the forefront of international development. Just as you have to choose the right channel to watch a TV program, being able to make VLBI observations in the right wavelength band is crucial for black hole imaging. The best wavelength band for observing the event horizon of a black hole is around 1 mm, not simply because of its high resolution, but also because of the following important considerations/advantages: the radiation from the gas around the black hole becomes transparent ("optically thin ") in the short millimeter wavelength band. This is crucial for imaging black holes, otherwise no matter how high the resolution is. The radiation of the accreted gas is brightest in this wavelength band. In order to "see" the black hole horizon, the radiation around it must be "bright" enough for the sensitivity of our observation equipment. Radio waves in this band experience little interference from interstellar scattering. This is particularly important for the center of the Milky Way because it is affected by strong interstellar scattering in and above the centimeter band, making it impossible to see the intrinsic structure of radiation around the black hole. In addition, there are many important factors such as station layout and sensitivity improvement that need to be considered. From this, it is not difficult to find that it is not necessary to successfully photograph black holes as long as the resolution of the VLBI array is high enough! EHT and its observations in April 2017 In recent years, 1.3 mm VLBI observations have detected structures on the scale of the black hole event horizon in SgrA * and M87 * respectively, which is very encouraging for black hole imaging. However, due to the limitations of the number and sensitivity of stations, detailed imaging observations have not been possible. With the addition of new, high-sensitivity submillimeter wave stations (especially AtacamaLargeMillimeter/submillimeter Array) to the global 1.3 mm-VLBI array, imaging observations of black holes have become possible. In order to capture the first black hole image, more than 200 scientists from more than a dozen countries (regions) including China have formed the EHT, a major international cooperation program. The technology used for EHT observations is (sub-) millimeter-wave VLBI, which currently works in the 1.3 mm band and is expected to expand to 0.8 mm. By imaging black holes, EHT can verify Einstein's general theory of relativity in extreme environments with strong gravitational fields, and study in detail the accretion of matter and the formation and propagation of jets around the black hole. Echoing the verification of general relativity by Eddington and others 100 years ago, EHT collaborators visited several of the world's tallest and most remote radio observatories in April 2017 to test his general theory of relativity in a way Einstein would never have thought of. Participating in this observation included 8 stations located at 6 locations around the world (Table 1, Figure 5). Table 1. Information on telescopes participating in EHT observations. Among them, the effective apertures of ALMA, LMT, SMA and SPT are only for 2017 Figure 5. The 8 VLBI stations participating in EHT observations in April 2017 (pictures provided by the author) are connected by solid lines to observe M87 (7 stations; due to location limitations, the SPT telescope located in the South Pole cannot observe M87), and the dotted lines are connected by stations observing a calibration source (3C279). In order to increase detection sensitivity, the amount of data recorded by EHT is very large. During observations in April 2017, the data rate at each station reached an astonishing 32 Gbit/s, and eight stations recorded a total of approximately 3500 TB of data during the five-day observation period (equivalent to 3.5 million movies, which will take at least several hundred years to watch it!). EHT uses a dedicated hard disk to record data and then send it back to the data center for processing. There, researchers used supercomputers to correct the time difference between electromagnetic waves arriving at different telescopes, and cross-correlated all data to achieve signal coherence. On this basis, through nearly two years of post-processing and analysis of these data, humans finally captured the first black hole image. Our scientists have long been concerned about high-resolution black hole imaging research, and have carried out various related work with international visibility before the formation of EHT international cooperation. In this EHT cooperation, China scientists jointly promoted EHT cooperation in the early stage and participated in the application for EHT telescope observation time. At the same time, they assisted the JCMT telescope in observations and participated in data processing and theoretical analysis of results, making contributions to EHT black hole imaging. positive contribution. The follow-up is even more exciting. Stay tuned. Since the results obtained by different measurement methods of the mass of the central black hole of M87 (gas dynamics vs. stellar dynamics) are nearly twice as different, it is still a little surprising that M87 * can be imaged. However, smooth imaging of the M87 black hole is by no means the end of the EHT. On the contrary, this exciting result will surely inspire more interest and enthusiasm for black hole research. Currently, the observation data of the 2017 M87 is still being analyzed. Researchers hope to obtain the properties of the magnetic field around the black hole by studying the polarization of radiation, which is crucial to understanding the accretion of matter and the formation of jets around the black hole. The mass of the other best imaging candidate, the black hole at the center of the Milky Way, is more certain. Previous EHT observations have shown that there is a structure around the black hole that is "dark in the middle and bright in the periphery (one side)", with an overall characteristic size of 5 times Schwarzschild radius, consistent with the predictions of general relativity (References and Figure 6). With more observation stations (such as the Northern Extended Millimeter Array, KittPeak Telescope) joining the EHT and the improvement of data quality (sensitivity), we have every reason to believe that EHT will be able to obtain clearer images of the galactic core black hole in the near future. Let us wait and see! Figure 6. Schematic diagram of 1.3 mm VLBI observations on the silver-core black hole (Source: MaxPlanck Society) In 2013, a schematic diagram of 1.3 mm VLBI observations on the silver-core black hole was carried out using 6 VLBI stations located at 4 locations. The in-line diagram gives models of the two most likely radiating structures that are consistent with the observations. Note: In the early days of VLBI development or generally when baseline coverage is less than ideal, simple geometric models (such as Gauss) are often considered to fit the observed (visibility) data. Many early discoveries, such as apparent superluminal motion, were made based on simple geometric models with very limited baselines. Introduction to the author Lu Rusen is a researcher at the Shanghai Astronomical Observatory, China Academy of Sciences. In 2010 and 2011, he received a doctorate in science from the University of Cologne in Germany and the Shanghai Observatory of the China Academy of Sciences respectively. In 2018, he was selected into the 14th batch of "Thousand Talents Project" youth projects, with his research direction in high-resolution radio astrophysics. Zuo Wenwen is an associate researcher at the Shanghai Astronomical Observatory of China Academy of Sciences. She received a doctorate in astrophysics from Peking University in 2014. She is currently engaged in research and scientific communication of high redshift quasars.