The enigmatic nature of black holes has long captured human imagination. Thanks to a series of photographs, our understanding of them has gradually become more concrete.
In April 2019, the first-ever image of a black hole was captured by the Event Horizon Telescope (EHT), revealing the supermassive black hole at the center of the M87 galaxy, stunning the world. Three years later, another image, this time of the black hole at the center of our own Milky Way galaxy, was released by the EHT, providing visual confirmation of Einstein's theory of general relativity from the perspective of intense gravitational fields. In less than a year after that, an international research team captured the first-ever "panoramic view" of the black hole in M87, freezing the black hole's shadow, accretion disk, and jets in a single image.
These three iconic images showcase the journey of the Shanghai Astronomical Observatory's black hole imaging research team, from active participation to occupying a prominent position at the forefront of global black hole research. "Astronomy is undoubtedly romantic, but researchers often toil in obscurity for long periods," describes Shen Zhiqiang, Director of the Shanghai Astronomical Observatory, reflecting on the team's stargazing endeavors.
Breaking the Research "Ceiling"
In 1997, Dr. Shen Zhiqiang completed his Ph.D., drawn by the seemingly infinite allure of understanding black holes. "Back then, we had to wait 20 months for just 5 hours of observation time."
Over the next five years, he and his colleagues conducted over 20 high-resolution Very Long Baseline Interferometry (VLBI) observations, with each step representing the latest advancements in the field. In 2005, Dr. Shen Zhiqiang led an international astronomical research team in publishing the world's first high-resolution image of the Milky Way's central black hole at a wavelength of 3.5 millimeters in the journal Nature.
"I returned to China with the aim of pushing domestic black hole imaging research to the forefront of international standards." Driven by his mentor's aspirations, Lu Rusen, Dr. Shen Zhiqiang brought years of accumulated research experience back to China and joined the Radio Astronomy Science and Technology Laboratory at the Shanghai Astronomical Observatory in 2018.
"Science thrives on debate, and existing research 'ceilings' need to be shattered, with a focus on projecting the 'voice of China'." With determination, Lu Rusen concentrated on advancing cutting-edge 1-millimeter waveband imaging research at the Shanghai Astronomical Observatory, building on existing research foundations.
In 2023, he led an international team in completing imaging research on the M87 black hole and its surrounding environment, capturing the first-ever images of the black hole in the new wavelength band, achieving a breakthrough in capturing a "panoramic view" of the "central engine" of an active galactic nucleus. The research findings, published in the journal Nature, garnered widespread global attention.
"The Cost of Computing"
To observe a black hole from Earth, telescopes operating in the submillimeter waveband are essential, requiring dry weather conditions and thin atmospheres. Consequently, the dormant volcanic summit in Hawaii became a celestial observation sanctuary.
Jiang Wu, an associate researcher at the Radio Astronomy Science and Technology Research Laboratory of the Shanghai Astronomical Observatory, had the opportunity to participate in the Event Horizon Telescope observations on-site. "Apart from experiencing altitude-induced shortness of breath and occasional headaches, what struck me the most was the enthusiastic dedication of everyone despite observing in the middle of the night," recalls Jiang Wu. Sometimes, on the way back to the base in the early morning, he would witness the glowing lava of other volcanoes, deeply moved by the forces of nature.
Zhao Shanshan, a young researcher in her 90s at the High-Resolution Radio Astronomy Astrophysics Research Group of the Shanghai Astronomical Observatory, is responsible for comparing model image libraries with observational data to select the optimal model.
The environment around black holes is extremely complex, resulting in significant uncertainties in physical parameters, requiring substantial human and computational resources behind each model image. A single numerical simulation may take weeks to months to compute on a supercomputer, and any parameter changes necessitate recalculations. "From this perspective, the theoretical interpretation of black hole images is indeed 'costly'," says Zhao Shanshan.
Capturing a "Movie" of Black Holes
"Previously, I thought that scientific research involved sitting in front of a computer, pushing formulas and writing code. I never imagined I could also be involved in building telescopes." In order to use China's own equipment to photograph black holes as soon as possible, Zhao Shanshan accompanied the team on a survey trip to Tibet to find suitable observing sites.
"To gain a strategic advantage in capturing a 'movie' of black holes in the next phase of research and to have more international academic influence, we must have our own observation equipment." Lu Rusen is aware that there is still a long way to go in the future. To this end, the Shanghai Astronomical Observatory has promoted the construction of the Chinese millimeter wave VLBI experimental system, actively advocating for the construction of submillimeter wave telescopes and arrays in western regions, and developing related observational facilities.
Jiang Wu is also working hard to advance the construction of submillimeter wave telescopes. "Multi-frequency simultaneous reception technology is the future trend, as it can detect weaker signals, offering the possibility of observing more black hole images."
"The public is very interested in our research findings and always asks many questions. At these times, I keenly feel that exploring the unknown is a human instinct," says Zhao Shanshan. As pioneers in the exploration of black holes, sharing the latest scientific research results with the public is both a responsibility and an obligation, and she also finds joy in the exchanges.
