The supermassive black hole at the center of an active galaxy is one of the most destructive and mysterious objects in the universe. Their gravitational pull is enormous, "ingesting" large amounts of matter through the accretion disk, while also "spitting" matter thousands of light-years away at speeds close to the speed of light. However, what is the mechanism of energy transfer between the supermassive black hole, the accretion disk, and the jet? This is a puzzle that has puzzled physicists and astronomers for more than a century.
Lu Rusen, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and the leader of the "High-Resolution Radio Astrophysics" research group, and his colleagues are researchers who are exploring these difficult problems.
In addition to successfully capturing the first human photo of a black hole, polarization images and the first image of a black hole at the center of the Milky Way galaxy in 2019, 2021 and 2022 with his collaborators, Lu Rusen, who returned to China in 2018 to join the Shanghai Astronomical Observatory, led an international team to take the first panorama of the M87 black hole in April last year, showing for the first time the connection between the accretion flow and the origin of the jet near the central supermassive black hole, and the related results were published in the journal Nature.
"At that time, there was a lot of accumulation in the field of black hole imaging research in the world, but the domestic foundation was still relatively weak. When I returned to China, I felt that I should be able to do something in this field. In a recent interview with CBN, Lu Rusen said.
In addition to taking pictures of the M87 black hole and the black hole at the center of the Milky Way, Lu Rusen and his colleagues are also planning to take other black hole photos, as well as "color photos of black holes" and "movies" of black holes. "What we see in the picture of a black hole, will the structure around it change over time? Does it change with (observed) frequency? Sometimes, for the sake of introduction, we will also use color to describe it, and this color represents the frequency of observation. This is our main research direction in the future, and this is what we are doing. ”
What is the significance of studying black holes
When we talk about black holes in the universe, the first thing people think is that they are far away from our lives, but in fact they are not.
Previous astronomical observations have provided many indirect evidence for the existence of black holes, and it is well known that at relatively long distances from black holes, many phenomena have to be explained by the effects of general relativity.
Lu Rusen told reporters that the ultimate purpose of taking pictures of black holes is to verify Einstein's general theory of relativity. "Only in these extreme cases can you need a new theory. These new theories are very important for our future lives. For example, Lu Rusen said that the mobile phone navigation we use every day, if the general theory of relativity is not correct, it will not be possible to realize the navigation, but this kind of hidden function in all aspects of daily life is usually not noticed by everyone.
Behind the results of black hole photos, the most important link is "photo washing".
Last year, he and his team's new work, published in Nature, showed for the first time the shadow of the M87 black hole and its surrounding accretion and jets in the same photo, which was actually taken on April 14 and 15, 2018. After the initial processing of the data, they noticed new features in the data that they had never seen before, which was a great motivation for the team members. After a complex process of data processing and mapping, as well as repeated verification and confirmation of the results, the new image was finally presented five years later.
He explained that the use of VLBI (Very Long Baseline Interferometry) to observe black holes is actually quite complicated, and the time to take pictures can be 5 or even 10 hours. But the process of "washing the photo", including the theoretical explanation that follows, takes a lot more time, which is why they often take years to get such a photo.
However, when scientific research progresses to a certain point, it is not easy for the industry to reach a consensus, especially in the field of black holes, which requires international cooperation.
In the above photograph in Nature, for example, 16 radio telescopes are connected to form a telescope with an aperture equivalent to the diameter of the Earth. This international cooperation project led by Chinese scholars has 121 members from 64 research institutions in 17 countries and regions.
"Our work often involves international cooperation, and once it involves the most cutting-edge things in the world, cooperation is not as smooth as ordinary conventional cooperation, and the process is more painful." When the research results are questioned by foreign scholars, Lu Rusen and his collaborators will check the data analysis results over and over again and develop new algorithms to solve scientific research problems. This allows you to understand the perspectives of others, learn from them, and make the results of your research more rigorous. ”
Lu Rusen said that the next goal of research on the M87 black hole is to photograph "colored black holes" with the EHT (Event Horizon Telescope). He explained that a sharper 3.5 mm photo will be taken, combined with a sharper 1.3 mm photo taken by the future EHT, a 0.8 mm photo taken by the next generation EHT, and a shorter wavelength photo taken in the more distant future space VLBI to obtain a "color photo" of the black hole.
Promote the construction of millimeter-wave telescopes
Whether it's lunar exploration or black hole photography, VLBI technology plays an important role.
In the mid-to-late 60s of the 20th century, in order to further improve the ability of radio astronomy observation, radio astronomers took advantage of the development of high-stability atomic frequency standard technology and high-speed magnetic recording technology at that time, and developed VLBI technology characterized by "independent local oscillator" and "magnetic medium recording" on the basis of traditional wired interferometers. In 1967 United States Broten et al. obtained the VLBI interference fringe for the first time, opening up a new field of radio astronomy.
Under the leadership of Academician Ye Shuhua, the Shanghai Astronomical Observatory established the mainland's first 25-meter aperture radio telescope, opening the way for the development of radio interferometry technology in the mainland. At present, the Shanghai Astronomical Observatory has promoted the construction of the mainland's millimeter-wave VLBI experimental system, and at the same time proposed and actively promoted the construction of sub-millimeter-wave telescopes and arrays in the western region and the development of related observation facilities.
Taking color photos and movies of black holes requires different wavelengths of observation.
In the Tianma Telescope Park in Sheshan, Shanghai, there is not only a Tianma radio telescope with an aperture of 65 meters, but also a smaller millimeter-wave telescope.
Lu Rusen told reporters that the shorter the observation wavelength, the more difficult it is to build a radio telescope. Most radio telescopes work in the centimeter band, and the telescope that takes pictures of black holes needs to be in the millimeter wave band, or even the submillimeter wave band, which puts forward very demanding requirements for the manufacturing process and working environment of the telescope. "The newly built telescope will be able to detect weaker signals, so astronomers can hopefully observe more images of black holes, as well as more binary black holes."
He further explained that the shorter the wavelength of the telescope, the higher the frequency, and the better the resolution. The Shanghai Astronomical Observatory also proposed to build a domestic high-frequency VLBI array, and built a millimeter-wave telescope in the Tianma Park as an experimental model. "Now a mainstream development trend in the world is the development of high-frequency VLBI technology, and I hope that there will be forces from all walks of life to pay attention to and support the construction of our millimeter-wave telescope."
(This article is from Yicai)