Behind the first photo of a black hole at the center of the Milky Way: Scientists have spent decades studying

Yuan Feng explained the "doughnut" of the universe to the public in a popular science lecture. Photo courtesy of the interviewee

Yuan Feng, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, has forged a lifetime fate with "Doughnut". On May 16, the Event Horizon Telescope (EHT) project he participated in photographed the first supermassive black hole "Sagittarius A" at the center of the Milky Way. *" first photoshoot - a giant cosmic "doughnut".
This photo is from the center of the Milky Way, 27,000 light-years away. Yuan Feng is one of 16 researchers in mainland China who participated in the work.
Yuan Feng has been dealing with "doughnuts" for 28 years since he studied for a doctorate in astrophysics in 1994. This photo in May is actually the second black hole photo taken by the EHT team. The first black hole photo was released in April 2019. At that time, the EHT organization announced that it had obtained the first photo of the black hole M87. At the center of the photo is an approximately circular dark area guarded by a glowing bright ring resembling a golden "doughnut".
Many people don't understand that since scientists have already taken pictures of black holes as early as 2019, why should they take pictures again in 2022? What do scientists want to prove?
Yuan Feng told a reporter from China Youth Daily and China Youth Daily that, in fact, the photos of "doughnut" black holes seem to be similar, but they all have important values in different hypothetical environments.
"The pictures we took are, to be precise, not the black hole itself, but the accretion flow around the black hole. These accretion flows are attracted by a strong central gravitational force, and this gravitational force comes from the black hole." Yuan Feng said, "Sweet The glittering golden ring on the outer ring of the doughnut is actually the accretion flow. The gas in the accretion flow falls to the black hole under the gravitational force of the black hole. During the falling process, they heat up and emit strong radiation, as seen in the photo. The bright halo is the radiation emitted by these very hot gases.
Research by astrophysicist Narayan and others found that there are two types of accretion currents in the universe, one is a "normal" accretion current, and the other is a "crazy" accretion current. The former has a weak magnetic field, and the latter The magnetic field is strong. In addition to the magnetic field, the two accretion currents are also very different in other aspects, and also determine the formation of black hole jets.
Yuan Feng told reporters that an important value of black hole photo shooting is that it "tested Einstein's general theory of relativity" in the environment of strong gravitational field. General relativity is a theory describing the gravitational interaction between matter, completed by Albert Einstein in 1915 and officially published in 1916. This theory is the first time that the gravitational field is equivalent to the curvature of space-time.
He introduced that scientists also tested general relativity 100 years ago, but they were all tested in a weak gravitational field environment, and in a weak field, there is little difference between the two theories of general relativity and Newtonian mechanics. Only in the environment of strong gravitational field can there be a more obvious difference between the two.
Black holes are just the right strong gravitational fields for testing general relativity.
"The theory of general relativity and quantum mechanics are the most powerful theories in the field of physics at present, and they are the most important foundations of modern physics." Yuan Feng said, taking the satellite navigation system that people use every day as an example, it also needs to rely on general relativity. On the basis of this theory, "navigation" has become a reality. "For example, if you use Newtonian mechanics theory for navigation, the error may be 1-2 kilometers, and for accurate positioning, you must rely on general relativity."
The black hole photos taken by scientists today are consistent with the "black hole model photos" calculated based on general relativity more than 100 years ago, indicating that general relativity has passed this test.
Yuan Feng is not the "photographer" in the EHT project, but one of the theoretical researchers in the project explaining "why taking pictures".
"In the process of the accretion flow being sucked in, many interesting phenomena occur." He introduced that black holes themselves do not emit light. To find a black hole, the most important thing is to find the accretion flow gas around it, "According to our calculations The real black hole is predicted to be smaller than the black part in this 'doughnut' image."
The "calculation" that Yuan Feng said is to solve a complex equation system of black hole accretion flow. The complexity of this equation is far beyond the imagination of ordinary people. This system of equations has been concerned by scientists since the 1960s, and every small step in solving this equation is worth publishing a high-level academic paper. In the 1990s, Narayan, Yuan Feng's postdoctoral advisor at Harvard University, discovered an innovative solution to this equation, which was an important step for people to understand black holes in the universe. He was elected as an academician of the National Academy of Sciences.
"I wrote a fairly good paper at the time and gave a report at an international conference held in South Korea." Yuan Feng told reporters, "The tutor told me later that when I heard my report, I wanted to give it to me. I have a postdoc position left."
After reporting to Harvard University in 2002, the secretary of the Astronomy Department who was in charge of the reception said after seeing Yuan Feng: "I've wanted to meet you for a long time! Hundreds of people have applied for the postdoctoral position of a professor, and I want to know if he keeps this position. What kind of person did you give it to?"
To the surprise of this well-known professor, 10 years later, in 2012, Yuan Feng, his favorite postdoctoral fellow, demonstrated for the first time that there is indeed a strong outflow in the accretion flow through theoretical analysis and large-scale numerical simulation, thus overturning the His famous accretion flow "convection" theory proposed in 1999. It was not until 2014 that Narayang accepted his latest research results in many discussions with Yuan Feng.
In 2005, Yuan Feng, who had postdoctoral work experience at the Max Planck Institute for Radio Astronomy in Germany, Harvard University and Purdue University in the United States, decided to return to China. "I just like the Chinese culture and environment. No matter how good the foreign environment is, it is not designed for the Chinese. There is no sense of belonging and identity there." Yuan Feng said.
This sense of belonging and identity includes popularizing astronomical knowledge and astrophysics knowledge to the Chinese public.
"Only a dozen universities in the country have astronomy departments, while almost every one of the top 100 universities in the United States has an astronomy department. This seemingly useless subject is one of the most important basic subjects. I want more students The seeds of looking up at the stars and the sea are planted in my heart." Yuan Feng said that since my country's rapid development, it is time to strengthen basic research in various disciplines. "Basic research is of great significance to the country's long-term development."
He also hopes that this beautiful "doughnut" photo can become a "bait" for teenage friends to "follow their interests and keep their curiosity", "I hope our children can follow their own when they choose the direction of study and work in the future. Original interest, maintain curiosity about nature.”
The reason for choosing astrophysics is also Yuan Feng's own interest. During his master's degree, he once switched from theoretical physics to quantum chemistry, but in the three years of graduate study, he found that his interest in general relativity increased day by day, "I think general relativity is very beautiful and I like it very much, so I applied for the exam. A PhD in astrophysics from the University of Science and Technology of China, he has been studying astrophysics ever since."