Writing | Lu Rusen (Shanghai Observatory, Chinese Academy of Sciences; Max Planck Radio Astronomy Institute, Germany)
Zuo Wenwen (Shanghai Observatory, Chinese Academy of Sciences)
Editor | Jin Zhuangwei
At 9 p.m. Beijing time on April 10, 2019, the Event Horizon Telescope (EHT) co-organized a joint global news conference to announce the first successful capture of the worlds first black hole image at the center of a neighboring giant elliptical galaxy, M87, using a virtual radio telescope the size of the Earth.
The significance of this image is extraordinary. It provides direct visual evidence for the existence of black holes, which makes it possible to verify Einsteins general relativity in a strong gravitational field and to study carefully the matter accretion and relativistic jet near black holes.
So why can black holes be imaged? How to image? This paper attempts to interpret the black hole imaging from the perspective of the experiencer.
Figure 1. M87 Galaxy Center Super Mass Black Hole (M87*) image, the top image for April 11, 2017, and the bottom three images for M87* on April 5, 6 and 10, 2017. The weak region in the center of the image is called black hole shadow (see below), and the ring-shaped asymmetric structure around it is caused by the strong gravitational lens effect and the relativistic beam effect. The spin direction of the black hole can be determined by the asymmetry of the upper (north) and lower (south). (Source: Reference )
Black Hole and General Relativity
More than a hundred years ago, Einstein put forward the theory of general relativity, which combines time and space into a four-dimensional space-time, and proposed that gravity can be regarded as a distortion of space-time. One of the important predictions of this theory is that when the mass of an object collapses, it can be concealed in the event horizon, within the sphere of influence of the black hole, where gravity is so strong that even light cannot escape.
The verification of general relativity can be traced back to a century ago. On May 29, 1919, Arthur Eddington et al. measured the deflection of light near the sun during the total solar eclipse (Fig. 2), which opened the prelude to the verification of general relativity in the last century and put Einstein on the altar of science.
Over the past century, general relativity has withstood continuous experimental verification, and the existence of black holes has been supported by more and more astronomical observations.
At present, astronomers generally believe that black holes do exist in the universe, ranging from star black holes of several to tens of times the mass of the sun to supermassive black holes of millions or even billions of times the mass of the sun. Moreover, supermassive black holes exist in the center of almost all galaxies.
However, even today when LIGO/Virgo detects gravitational waves and thus proves the existence of black holes authoritatively, human beings have not directly seen the hole - the horizon of black hole event - that can reveal the secret of space-time under extreme conditions.
Perhaps thats what makes black holes so fascinating - the density of black holes is incredible! If the earth is compressed into a black hole, it is about the same size as a dumpling ball, and a star black hole 10 times the mass of the sun at a distance of 1 KPC (about 3262 light years) has an angular diameter of only 0.4 nanoseconds at the event horizon. This is about 100 million times smaller than the resolution of the Hubble Telescope. No existing astronomical observation method has such resolution ability!
Why can black holes be imaged?
Since black holes are black and light cannot escape, how can we see them?
In fact, black holes do not exist in isolation. There is a lot of gas around them. Because of the strong gravitational pull of black holes, gas will fall towards them. When these gases are heated to billions of degrees, they emit intense radiation. At the same time, black holes also eject matter and energy outward in the form of jet and wind .
Generalized relativity predicts that there will be a black holes Hadow in the central region, surrounded by a halo of radiation from accretion or jets, which is like a crescent moon, ranging from 4.8 to 5.2 times the Schwarzschild radius, depending on the spin of the black hole and the direction of the observers line of sight. The event horizon radius of a black hole; the horizon radius of a solar mass black hole is about 3 km.
In the years when they failed to see the real face of a black hole, scientists calculated how it looked.
As early as the late decade of last century, David Hilbert, a great mathematician, calculated the light bending and gravitational lens effects around black holes.
In the 1970s, James Bardeen  and Jean-Pierre Luminet  calculated the image of a black hole (Figure 3, left).
In the late 1990s, Heino Falcke et al. made a detailed calculation of the black hole in the center of the galaxy and introduced the black hole shadow theory . They also point out that if the black hole shadow is mosaic in a bright, thin optical (i.e., transparent to a certain observed wavelength) hot gas, it can be seen by (sub) millimeter wave very long baseline interferometry technology.
Since then, a large number of studies have been carried out on black hole imaging using the generalized relativistic magnetohydrodynamic numerical simulation, which predicts the existence of black hole shadow (Fig. 3, right). Therefore, the imaging of black hole shadows provides direct visual evidence for the existence of black holes.
Figure 3. Black hole shadow image (left image taken from reference , right image provided by the author)
What kind of black hole is best for imaging?
Although the shadow of a black hole can be seen, not all black holes meet imaging requirements. As mentioned earlier, black holes are very, very small. There is no doubt that the angular diameter of an imaging black hole must be large enough. Because the size of event horizon of black hole is proportional to its mass, it means that the larger the mass of black hole, the larger the event horizon, and the more suitable for imaging. Therefore, supermassive black holes close to us are perfect candidates for black hole imaging.
The central black hole SgrA* in the direction of Sagittarius and the central black hole M87* in the adjacent radio galaxy M87 are two of the best known candidates.
The galaxys central radio source SgrA*, discovered in 1974 by Bruce Balick and Robert Brown using the interferometer of the National Radio Astronomical Observatory (see  for the story of its discovery and naming). There is increasing evidence that it is a black hole with a mass of about 4 million times the mass of the sun. Because it is about 260,000 light-years away from Earth, the Schwarzschild radius of the Galactic central black hole is about 10 microseconds, and the angular diameter of its shadow is 47-50 microseconds, which is equivalent to the angular diameter of an apple on the moon (the angular diameter of the moon is about 0.5 degrees).
M87 is a giant elliptical galaxy in the direction of Virgo, about 55 million light-years from Earth. As early as 1918, Heber Curtis noticed a peculiar collimated beam, curious traightray, connected to the galaxys center . In fact, this collimated beam is M87 jet, which emits and extends thousands of light years from the center, becoming the most striking feature of M87. This makes it the first galaxy to be certified as a jet (Fig. 4).
Like the center of the galaxy, M87 has a supermassive black hole (now known as M87*, according to its naming convention) with a mass of about 6.5 billion solar masses. Although the black hole is 1500 times larger than SgrA* in mass, it is more than 2,000 times farther away, so it looks slightly smaller than a silver-core black hole - its Schwarzschild radius is about 7.6 microseconds, and the shadow size of the black hole corresponds to 37-40 microseconds.
Figure 4.M87 Radio Jets at Different Scales (Figure Source: Reference )
What telescopes can image black holes?
Target has been selected. Now its time to grind the knife. Ancient cloud: If you want to do good, you must first use sharp tools. To image black holes, the best tool is Very Long Baseline Interferometry (VLBI) technology.
VLBI uses radio telescopes which are widely distributed (up to tens of thousands or hundreds of thousands of kilometers) to obtain a huge (virtual) telescope of the same size as the maximum distance between stations by recording signals independently and processing signals synthetically later. This technique can achieve the highest resolution in astronomical research. Its resolution is theta lambda/D, where lambda is the observed wavelength and D is the longest baseline length. Assuming that a baseline with a length of 10,000 km (about the diameter of the earth) can be observed at 1 mm wavelength, the resolution power of about 21 microseconds can be obtained. VLBI uses atomic clocks that are accurate to 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 implementation of VLBI technology in the late 1960s, its performance has been continuously improved with the progress of technology, and the wavelength coverage has also expanded from centimeter band to (sub) millimeter band, which is currently at the forefront of international development.
Just as TV programs have to choose the right channel, it is important for black hole imaging to be able to make VLBI observations in the right band. The best band for observing the black hole horizon is near 1mm, not only because of its high resolution ability, but also because of the following important considerations/advantages :
The radiation of the gas around the black hole becomes transparent (optical thin) in the short millimeter wave band. This is crucial for black hole imaging, otherwise no matter how high the resolution is, it will not help.
Accumulated gases radiate brightest in this band. In order to see the black hole horizon, the radiation around it must be bright enough relative to the sensitivity of our observation equipment.
Radio waves in this band suffer little interference from interstellar scattering. This is particularly important for the galactic center because it is affected by intense interstellar scattering at centimeter bands and above, which makes it impossible to see the intrinsic structure of radiation around black holes.
In addition, there are many important factors that need to be considered, such as the layout of stations and the improvement of sensitivity.
Therefore, it is not difficult to find that as long as the resolution of VLBI arrays is high enough, black holes can be successfully photographed!
EHT and its observations in April 2017
In recent years, 1.3 mm VLBI observations have detected the structure of black hole event horizon scale in SgrA* and M87* respectively, which is very encouraging for black hole imaging. However, due to the limitation of the number and sensitivity of stations, detailed imaging observation has been unable to carry out.
With the addition of new and highly sensitive sub-millimeter wave stations (especially Atacama Large Millimeter/submillimeter Array, etc.) to the global 1.3 mm-VLBI array, imaging observations of black holes become possible.
To capture the first black hole image, more than 200 scientists from more than a dozen countries (regions), including China, have formed EHT, a major international cooperation program. The technology used by EHT observations is (sub) millimeter wave VLBI, which currently operates at 1.3 mm and is expected to expand to 0.8 mm.
By imaging black holes, EHT can verify Einsteins general theory of relativity in the extreme environment of strong gravitational field, and carefully study the formation and propagation of matter accretion and jet around black holes.
As an echo of Eddington and othersvalidation of general relativity 100 years ago, EHT collaborators went to several of the worlds tallest and most remote radio observatories in April 2017 to test his general relativity in a way that Einstein would never have imagined.
Participants in the observation included eight stations located in six locations around the world (Table 1, Fig. 5).
Table 1. Telescope information participating in EHT observations, in which the effective calibers of ALMA, LMT, SMA and SPT are only for 2017 observations.
Fig. 5. In April 2017, eight VLBI stations participated in EHT observation. Five sites (seven stations) of M87 were connected by solid line. Due to location limitation, the SPT telescope located in Antarctica could not observe M87. The dotted line was connected to a station of observing a calibration source (3C279). (Pictures provided by the author)
In order to increase the detection sensitivity, the amount of data recorded by EHT is very large. In April 2017, the data rate of each station reached an astonishing 32Gbit/s. During the five-day observation period, eight stations recorded about 3500 TB data (equivalent to 3.5 million movies, at least several hundred years to see!).
EHT uses dedicated hard disks to record data and send them back to the data center for processing. There, researchers use supercomputers to correct the time difference between electromagnetic waves arriving at different telescopes, and integrate all the data to achieve signal coherence.
On this basis, after nearly two years of post-processing and analysis of these data, human finally captured the first black hole image.
Scientists in China have long paid attention to the high resolution black hole imaging research, and many related works with international display have been carried out before the formation of EHT international cooperation. In this EHT cooperation, Chinese scientists jointly promoted EHT cooperation in the early stage and participated in the application of EHT telescope observation time. At the same time, they assisted JCMT telescope in observing and participating in data processing and theoretical analysis of results, which made a positive contribution to EHT black hole imaging.
Follow-up more exciting, please look forward to
Because the results of different measurements of the mass of M87 central black hole (gas dynamics vs. stellar dynamics) are nearly two times worse, it is somewhat surprising that M87* can be imaged. However, the smooth imaging of M87 black hole is by no means the end of EHT. On the contrary, this exciting result will certainly stimulate more interest and enthusiasm in black hole research.
At present, the observation data of M87 in 2017 are still being analyzed. Researchers hope to obtain the magnetic properties around black holes by studying the polarization of radiation, which is essential for understanding the matter accretion and jet formation around black holes.
The mass of another best imaging candidate, the central black hole of the Milky Way Galaxy, is more certain. Previous EHT observations have shown that there is a structure of dark in the middle and bright around (one side) around the black hole. Its overall characteristic size is 5 times the Schwarzschild radius, which is consistent with the predictions of general relativity (Ref.  and Figure 6).
As more observatories (such as Northern Extended Millimeter Array, KittPeak Telescope) join EHT and data quality (sensitivity) improves, we have every reason to believe that EHT will be able to obtain clearer images of galactic black holes in the near future.
Lets wait and see!
Figure 6. In 2013, a 1.3 mm VLBI observation sketch for a galactic black hole was carried out using six VLBI stations located at four locations, in which two most probable radiation structures consistent with the observations were modeled by the embedded maps. Note: In the early stage of VLBI development, or when baseline coverage is not ideal, simple geometric models (such as Gauss) are usually considered to fit observed (visibility) data. Many early discoveries, such as apparent superluminal motion , were based on simple geometric models with very limited baselines. (Source: Max Planck Society)
Author brief introduction
Lu Rusen is a researcher at the Shanghai Observatory of the Chinese Academy of Sciences. In 2010 and 2011, he received his PhD in Science from Cologne University in Germany and Shanghai Observatory of Chinese Academy of Sciences, respectively. In 2018, he was selected into the 14th batch of Thousand People Program youth projects. His research direction is high resolution radio astrophysics.
Zuo Wenwen, associate researcher of Shanghai Observatory, Chinese Academy of Sciences, received a doctorate degree in Astrophysics from Peking University in 2014. He is currently engaged in high redshift quasar research and scientific communication.
Thank you: I would like to thank Shen Zhiqiang, a researcher at the Shanghai Observatory, and Professor Mao Shude of Tsinghua University for their suggestions on this paper.
 FirstM87 Event Horizon Telescope ResultsI: The Shadow of the Super Massive BlackHole, The Event Horizon Telescope Collaboration, ApJL, 2019, inpress
 What happened around a black hole? Yuan Feng, 2017, Mr. Sai Astronomy
 Timelike and null geodesic in the Kerrmetric, Bardeen, J.M. 1973, BlackHoles (Les Astres Occlus), 215
 Image of aspherical Blackhole with hinaccretion isk, Luminet, J. -P. 1979, A&A, 75, 228
 Viewing the Shadow of the Black Holate the Galactic Center, Falcke, H., Melia, F. & Agol, E.2000, ApJL, 528, L13
 Intensesub-arcsecondstructure in the Galactic center, Balick, B. & Brown, R.L. 1974, ApJ, 194, 265