It was reported on October 6, Beijing time that the 2020 Nobel Prize in physics was announced: half of the prize was awarded to Roger Penrose for “discovering that general relativity predicted the formation of black holes”; the other half was awarded to Reinhard Genzel and Andrea Ghez for “discovering super dense objects in the center of the Galaxy”. Three physicists shared this year’s Nobel Prize in physics for discovering one of the most bizarre phenomena in the universe, black holes. Three scientists share this year’s Nobel Prize in physics for their work on one of the most peculiar phenomena in the universe, black holes. Roger Penrose invented a clever mathematical method to explore Einstein’s general theory of relativity. His research reveals how general relativity predicts the formation of black holes. These space-time and space monsters capture everything that comes into it. Nothing, even light, can escape a black hole. < p > < p > Reinhard Genzel and Andrea Ghez, led by a group of astronomers, have been studying the central region of the Milky way since the early 1990s. As accuracy improved, they successfully mapped the orbits of the brightest star nearest the center of the galaxy. Both groups found an invisible but heavy object that caused the stars to circle around. < / P > < p > this invisible material is about four million solar masses, but it’s about the size of our entire solar system. What makes stars near the center of the galaxy rotate at such amazing speed? According to the current theory of gravity, there is only one possible explanation: supermassive black holes. Einstein, the father of general relativity, did not think that black holes really exist. But ten years after Einstein’s death, British theorist Roger Penrose proved that black holes can form and describe their characteristics. At the center of the black hole lies a singularity where all known laws of nature no longer apply. In order to prove that the formation of black holes is a stable process, Penrose needs to expand the method used to study relativity, that is, to use new mathematical concepts to solve the problem of this theory. Penrose’s breakthrough paper, published in January 1965, is still regarded as the most important contribution to general relativity since Einstein. < / P > < p > black holes are probably the strangest result of general relativity. When Einstein put forward his theory in November 1915, it overturned all previous concepts of space-time. This theory provides a new basis for understanding gravity. Gravity shapes the universe to the greatest extent. Since then, general relativity has provided the basis for all cosmic studies, and has practical applications in our most commonly used navigation tool, GPS. < p > < p > Einstein’s theory describes how gravity controls everything in the universe. Gravity holds us on earth, and it also controls the orbits of planets around the sun and the sun around the Milky way. Gravity also causes stars to emerge from the interstellar clouds, and eventually the stars collapse and die. Massive matter can bend space and slow down time; it can even cut off and wrap space – forming black holes. < / P > < p > the first theory to describe black holes appeared a few weeks after the publication of general relativity. Although the mathematical equations of the theory are extremely complex, Karl Schwarzschild, a German astrophysicist, still brought Einstein a solution to explain how massive matter bends space-time. Later studies have shown that once a black hole is formed, it will be surrounded by the event horizon, which is like a veil around the matter in the center of the black hole. Black holes are always hidden in their event horizon. The greater the mass, the larger the black hole and its horizon. For matter equivalent to the mass of the sun, the diameter of the event horizon is about three kilometers; for matter equivalent to the mass of the earth, the diameter of the event horizon is only nine millimeters. The concept of “black hole” has found new meanings in many cultural expressions, but for physicists, black hole is the natural end of the evolution of giant stars. In the late 1930s, physicist Robert Oppenheimer first calculated the dramatic collapse of a massive star. Oppenheimer later led the creation of the first Manhattan Project. When giant stars many times the mass of the sun run out of fuel, they first explode into supernovae and then collapse into dense debris so massive that gravitational energy pulls everything inside, even light. As early as the end of the 18th century, British philosopher and mathematician John Michell and French famous scientist Pierre Simon de Laplace put forward the concept of “dark star”. Both believe that celestial bodies can be too dense to be seen, because the speed of light is not enough to escape their gravity. More than a century later, Einstein published his general theory of relativity, in which the solutions of some equations described such dark stars. Until the 1960s, these solutions were regarded as pure theoretical conjectures, describing the perfect circular and symmetric ideal state of stars and their black holes. But nothing in the universe is perfect, and Roger Penrose first succeeded in finding a realistic solution for all collapsing matter. In 1963, with the discovery of quasar, the brightest object in the universe, the question of whether a black hole exists has emerged again. For nearly a decade, astronomers have been puzzled by radio rays from mysterious sources such as Virgo 3c273. Visible radiation eventually revealed the true location of the quasar, 3c273, so far away from earth that these rays were traveling toward Earth for more than a billion years. < / P > < p > these sources are so far away from us that they are as intense as hundreds of galaxies. These objects are called quasars. Astronomers soon discovered more distant quasars that had radiated early in the universe. Where does this incredible radiation come from? There is only one way to get so much energy in a quasar’s finite volume – from material that falls into a giant black hole. Whether a black hole can be formed under real conditions is a problem perplexing Roger Penrose. He later recalled that the answer came in the fall of 1964, when he was walking in London with a colleague. Penrose was a professor of mathematics at Berkeley College. When they stopped talking for a while and crossed a small street, an idea suddenly came into his mind. Later that afternoon, he recalled the idea of what he called a “trapped surface.”. This is the key he has been looking for, and it is also an important mathematical tool for describing black holes. < / P > < p > a capture surface forces all rays to point to a center, whether the surface is bent outward or inward. Using the bound surface, Penrose proved that black holes always hide a singularity, that is, a boundary between time and space. The density of the singularity is infinite, but so far, no theory has been able to explain the most peculiar phenomenon in physics. When Penrose perfected the proof of singularity theorem, the capture surface became a central concept. The topological method he introduced plays an important role in the study of curved universe. Once the matter begins to collapse and forms a capture surface, the collapse will never stop. As the physicist and Nobel laureate Subrahmanyan Chandrasekhar tells, there is no turning back. His story is about dragonflies and their larvae that live under water. When the larva is ready to spread its wings, it promises to its companions that it will come back and tell them about the world on the water. But once the larva really comes out of the water and flies like a dragonfly, it can’t go back. The larvae in the water can never hear the story of the world beyond the water. Similarly, all matter can only pass through the event horizon of a black hole in one direction. Then, time replaces space, all possible paths point to the interior, and the passage of time pushes everything to the inevitable end point, singularity. If you cross the event horizon and fall into a supermassive black hole, you don’t feel anything. But from the outside of the black hole, no one will see you fall into it, and your journey will continue. Within the scope of the laws of physics, it is impossible to peep into the interior of black holes; all secrets of black holes are hidden in their event horizon. < / P > < p > formation of black holes (upper left) black hole cross section when a giant star collapses under its own gravity, it will form a massive black hole, capturing everything that passes through its event horizon. Even light cannot escape a black hole. In the event horizon, time replaces space, and all paths point inward. The flow of time takes everything to the deepest singularity of a black hole – where density is infinite and time stops there. The (lower right) cone represents the path of light forward and backward in time. When matter collapses and forms a black hole, the light cone passing through the event horizon of the black hole will move inward toward the singularity. The external observer will never really see the light reach the event horizon. All they see is the light approaching the event horizon. After that, no one can see. < p > < p > Reinhard Genzel and Andrea Ghez each lead an independent team to explore the central region of our galaxy. Our Milky way is like a disk, 100000 light-years in diameter, with clouds and dust, and hundreds of billions of stars; one of them is our sun. As we look from earth, massive interstellar gas and dust block most of the visible light from the center of our galaxy. For the first time, infrared telescopes and radio technology have allowed astronomers to cross these barriers and observe stars in the center of the Milky way. < p > < p > Genzel and Ghez follow the orbits of stars and present the most convincing evidence so far: an invisible supermassive object is hidden in the center of the galaxy. Black holes are the only possible explanation. < / P > < p > Figure 3: top view of the Milky Way galaxy. Our Milky way is like a disk with a diameter of 100000 light-years. The Milky Way’s spiral arms are made up of clouds and dust, and hundreds of billions of stars; one of them is our sun. < p > < p > for more than 50 years, physicists have suspected that there may be a black hole in the center of our galaxy. Since the discovery of quasars in the early 1960s, physicists have speculated that supermassive black holes may exist inside most large galaxies, including the Milky way. However, no one can explain how galaxies and their black holes formed. One hundred years ago, American astronomer Harlow Shapley first identified the center of the Milky way, pointing to Sagittarius. In later observations, astronomers found that there was a strong radio source, and they put this

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