The path to the Nobel Prize in Physics for black holes: Commentary on the 2020 Nobel Prize in Physics

The Nobel Prize in Physics was kicked off' on 18:00 October 6th, 2020 (Beijing time). In Stockholm, the capital of Sweden, the Royal Swedish Academy of Sciences announced that it would award the 2020 Nobel Prize in Physics to three scientists who uncover the mysteries of black holes: Roger Penrose (from University of Oxford, UK), Reinhard Genzel (from both Max Planck Institute for Extraterrestrial Physics, Germany, and University of California, Berkeley, CA, USA), and Andrea Ghez (from University of California, Los Angeles, CA, USA). Penrose won the prize for discovering that the formation of black holes was a robust prediction of general relativity, and both Genzel and Ghez won the prize for discovering a supermassive and compact object at the center of our Milky Way galaxy. This is the third time within 4 years that it has been awarded to the field of astrophysics, and it is also the first time that it has been explicitly awarded to the field of black holes. In the search for black holes, mankind has paid a century of efforts. In 1915, Albert Einstein proposed the general relativistic theory. A few months later, in 1916, Karl Schwarzschild derived a vacuum solution to Einstein's field equation and predicted the existence of black holes. In the following decades, the study of black holes almost came to a standstill because of the war. During this time, the exploration work did not make much progress. Even though Schwarzschild had already obtained a black hole solution without rotation, and it is well known that celestial bodies rotate, so before Einstein died in 1955, he never believed that black holes really existed. However, starting from the 1960s, the situation has changed, a series of discoveries and theoretical studies have aroused scientists' interest in the research on black holes. One of the most important work was made by British mathematical physicist Roger Penrose in 1965, and by introducing ingenious mathematical methods, he published his landmark academic paper "Gravitational collapse and singularity of space-time", which proved the singularity can form even if the collapse deviates from the symmetry, hence predicting the existence of black holes theoretically. Compared with Oppenheimer's previous work on spherically symmetric collapse, Penrose's work was further extended to aspheric symmetry. In 1969, British astronomers Donald Lynden-Bell suggested that the Milky Way's center might harbour a supermassive black hole, but there was no evidence because the Milky Way's center region was obscured by interstellar dust and gas, only being detected as a relatively weak radio source then. In the early 1990s, German astronomer Reinhard Genzel and American astronomer Andrea Ghez each led an astronomical team that worked for more than a decade and discovered the existence of the supermassive Sagittarius A* in the center of the Milky Way Galaxy. On September 14th, 2015, the laser interferometer gravitational-wave observatory (LIGO) detected the gravitational wave generated by the merger of two black holes for the first time, which provides direct evidence for the existence of black holes. In terms of black hole imaging, after more than 10 years of continuous efforts, on April 10th, 2019, the Event Horizon Telescope team released the first picture of a black hole. At this point, after a hundred years of continuous efforts, mankind has finally proved the existence of black holes in theory and observation. Although black hole was awarded the Nobel Prize, the research on black holes is far from over, and there are still many problems to be solved in the future. With the multi-messenger era forthcoming, we can expect more discoveries on black holes will be made.