Space-time singularities and black holes

  The Nobel Prize in Physics in 2020 takes us to discover another “high light moment” in the exploration of the universe-black holes and the “secrets of the deepest part” of the Milky Way. The groundbreaking discoveries of the three winners provide us with the most exciting so far Convincing evidence proves that there is a supermassive black hole in the center of the Milky Way. Among them, Roger Penrose is a British mathematical physicist and currently an honorary professor at Oxford University. The reason for his award is to use mathematics to rigorously prove that the generation of black holes conforms to the principle of Einstein’s general theory of relativity; Reinhard Gentze Er is currently the director of the Max Planck Institute for Extraterrestrial Physics in Germany, and Andrea Gates is now a professor of astronomy at the University of California, Los Angeles. These two scientists have discovered through continuous tracking and calculations for nearly 30 years A supermassive object in the center of the Milky Way.

The darkest mystery in the center of the Milky Way

  Our Milky Way is a barred spiral galaxy, which contains 400 billion stars, a large number of star clusters, nebulae, and countless interstellar gas and interstellar dust. The position of the solar system is on the edge of the Milky Way. Starting from the earth, it takes at least 26,000 light-years to reach the galactic center of the Milky Way.
  The Galactic Center is located in the three constellations of Sagittarius, Ophiuchus, and Scorpio. It is the central area surrounded by the Milky Way, and it is also the brightest area in the entire Milky Way. There, 10 billion stars shine, dot the starry sky, spanning thousands of light years. The most central point is marked as a strong radio wave source, possibly a supermassive black hole, named Sagittarius A*.

  Since the early 1990s, the German physicist Reinhard Genzel and the American female astronomer Andrea Gates each led a team to observe the Sagittarius A* area in the center of the Milky Way. The two teams unanimously discovered that weird scenes are being staged here all the time: there seems to be an invisible object with a very large mass, like a monster, pulling this cluster of stars, causing them to scurry around at a dizzying speed. One of the stars, called S2, took less than 16 years to complete a full circle around the center of the Milky Way. This cycle is staggeringly short. In contrast, it takes more than 200 million years for our sun to complete a circle around the center of the Milky Way.
  What makes stars rotate at an unimaginable speed around the center of the Milky Way? According to the current theory of gravity, there is only one possible candidate-a supermassive black hole.
  For nearly 30 years, Genzel and Gates have been tracking the center of the Milky Way galaxy in a mess at the center of our galaxy. The Genzel team uses the European Southern Observatory in Chile, and the Gates team uses the Keck Observatory in Hawaii. Both observatories are equipped with the most powerful telescopes in the world today. Seen from the earth, the galactic center is filled with cosmic dust and disturbing starlight. Genzel and Gates developed a series of methods to observe the center of the Milky Way through huge clouds of interstellar gas and dust. At the same time, they have further improved the accuracy and observation limit of the above methods by correcting the imaging distortions caused by the earth’s atmosphere. In the end, with their efforts, the decisive evidence for the existence of a giant black hole in the center of the Milky Way was presented to the world.
  We cannot see the black hole itself, but we can see the trajectories of the stars around the black hole. By calculating the orbits of these stars, the position and mass of the black hole can be deduced. After many measurements by scientists, although the size of the Sagittarius A* area is about the same as the entire solar system, its mass has reached 4.3 million suns.
Study the history of black holes

  A black hole is the most terrifying celestial body in the universe. Its gravitational force is extremely strong and can swallow everything around it, so that all particles, even light, cannot escape its clutches. Such a dark celestial body cannot be observed by humans, and it was impossible to believe its existence for a long time.
  In 1915, Einstein developed the theory of general relativity. According to general relativity, the gravitational force between matter comes from the bending of time and space. Only a few months later, German astronomer Karl Schwarzschild obtained the famous Schwarzschild solution by calculating the gravitational field equation of general relativity. The Schwarzschild solution shows that if a large amount of matter is gathered at a point in time and space, then this mass of matter will distort time and space so severely that light with a speed of 300,000 kilometers per second cannot escape. The prototype of the black hole.
  But the dramatic thing is that Einstein himself did not believe that black holes really exist, and even publicly stated the reasons why black holes cannot exist in a paper published in 1939. In fact, most physicists at the time couldn’t believe that there were such strange celestial bodies in the universe, but as time went on, more and more calculations proved the possibility of black holes.
  After the 1960s, the field of black hole research ushered in its golden age, and a large number of astronomers and physicists devoted themselves to this field. What people know about black holes is basically obtained during this period of time. During this period, there was a very well-known master of relativistic physics-Professor John Wheeler of Princeton University in the United States. He not only did excellent academic research, but also did a lot of work in science communication. After his naming and promotion, the name of the black hole was known to everyone. After Wheeler, Hawking further discovered the so-called Hawking radiation, which changed the previous understanding of black holes in classical general relativity.
Penrose proved the existence of black holes

  Roger Penrose was born on August 8, 1931 in a scientific family in Essex, England. His father was the famous human geneticist Lionel Penrose. Roger Penrose is the second child at home. His elder brother is a famous theoretical physicist, his younger brother is a chess master, and his younger sister is a well-known British medical scientist and geneticist.
  When he was a child, Penrose was not very good at math, and his reaction was so slow that it was unimaginable. Once in class, the teacher asked to complete some mental arithmetic. The students had to calculate quickly, which was too fast for Penrose, who was only 8 years old. Therefore, the teacher transferred him to a worse class. After discovering that Penrose’s test results were so bad, the teacher in that class decided not to limit the test time, as long as he likes to do it, and the test papers are the same. During the activity time after the exam, every classmate walked out of the classroom to play happily, while Penrose continued to answer questions. In the end, Penrose did a good job. As long as you can take it slowly, Penrose can score high.

  After elementary school, Penrose first entered the affiliated secondary school of University College London, and then entered University College London. After graduating from university, he entered the University of Cambridge to study for a doctorate. In 1958, Penrose received his Ph.D. degree from Cambridge University under the guidance of well-known algebraist and geometer John Todd.
  At that time, some scientists have proved the predictions of general relativity for black holes, but those studies all assumed strict spherical symmetry, that is, assumed a condition that could not be established or even may not be physically established, and stars in the real world Although it is close to spherical, it cannot be strictly spherically symmetric. Therefore, these studies are not solid enough as predictions for black holes. There were still a lot of doubts about whether stars could collapse into black holes, or even black holes, at the time-Einstein himself was among the doubters.
  In 1965 and after, Penrose published a series of papers represented by “Gravitational Collapse and Space-Time Singularity”, using topological geometric methods that were novel in the research of general relativity at that time. Under general conditions, it is proved that in the process of the massive celestial body collapsing into a black hole, there must be a point after which all the collapsed matter no longer exists, and all known natural laws also stop there. In geometric terms, this is a geometric singularity. In the eyes of ordinary people, this is the point of destruction, because the closer to this point, the greater the pulling force generated by gravity, and it will eventually be destroyed.
  The existence of singularities has always been a difficult problem in physics. Fortunately, those of us who are outside the black hole don’t have to worry, because we can’t see them, they are always surrounded by the so-called horizon, and we can’t see anything inside the horizon.
The combination of Hawking and Penrose’s swords

  At that time, Hawking, who was also at Cambridge University and already suffering from “gradual frost”, met Penrose and began their journey of collaborative research on cosmology. Penrose is 11 years older than Hawking. He has a good mathematical background. When others are trying their best to guess and solve equations, he introduces a new method. You can see some of the solutions without needing to solve the equations. nature.

Penrose and Hawking together

  From 1965 to 1970, Hawking and Penrose formed a research team on black holes and baby universes (the “early universe”). Together, they extended the proof of the existence of singularities to the more general universe, including the early universe. They put forward the famous “Penrose-Hawking Singularity Theorem”. The theorem has two parts: one part is a physical concept, and the other part is a strict proof of mathematics.
  From 1965 to 1968, Hawking perfected his predecessors’ assumptions about the origin of the universe: the universe may have originated in a big bang, with a space-time singularity at its center-a point with infinite density and infinite volume. In 1970, Hawking and Penrose collaborated to give a rigorous mathematical proof that under the framework of general relativity, there must be singularities in the universe. This means that the universe has a beginning and an end—time was born at the singularity of the Big Bang and ended at the singularity inside the black hole.
  The Singularity Theorem makes the “Big Bang” theory a matter of course. Because the singularity must exist, the universe must have a beginning. This is the great significance of the singularity theorem. After decades of development, only the Big Bang hypothesis can perfectly deal with the singularity problem. The discovery of microwave background radiation, the discovery of gravitational waves, and the research results of quasars (active galactic nuclei) all show that the Big Bang Theory is currently the only hypothesis that meets all observations. And all of this originated from Hawking and Penrose’s argument about the singularity.
  In 2010, Penrose and another scientist analyzed the data observed by the Wilkinson microwave background radiation detector and discovered that there was mysterious radiation before the Big Bang. According to their research report, a total of 12 concentric circles have been discovered, five of which have special significance, corresponding to five large-scale events in the evolution of the universe. Penrose and his collaborators claimed that this was evidence that another universe existed before the Big Bang. They proposed a new universe model in which our universe is part of a larger oscillating universe. Today’s universe has always been Expansion, but this expansion is not permanent. As the black hole swallows all the matter in the universe, another universe will be opened again in the form of a big bang in the distant future.
More black hole types

  Thanks to the rapid development of astronomical observation technology, scientists have discovered a lot of black holes so far. They can be broken down into three categories by mass:
  one is stellar black holes with masses ranging from 3 to Between 100 solar masses. According to theory, there should be hundreds of millions of stellar black holes in the Milky Way, but unfortunately humans have only detected dozens of them so far, and only less than 20 stellar black holes have very accurate mass measurements. .
  The second type is medium-mass black holes with masses between 100 and 1 million solar masses. For medium-mass black holes, there are very few direct observations at present, but theoretical studies have proved that they should exist. Therefore, searching for medium-mass black holes is also a hot topic of current research.
  The third category is supermassive black holes, with masses ranging from 1 million to 10 billion solar masses. Scientists believe that there will be one or several supermassive black holes in the centers of all galaxies, including the Milky Way.
  For a black hole, only three physical quantities are needed to describe it, one is its mass, one is its rotation, and the other is its charge. In the universe, almost all gases exist in a plasma state, and there will be a lot of free charges. If a black hole is charged, it will easily absorb the charged particles around it to achieve a power balance. So in the end there are only two physical quantities, one mass and one rotation. The main task of scientists is to measure these two basic quantities of black holes.
  The supermassive black hole has been discovered, but it actually brings new problems to theorists. For example: How did a black hole with such a huge mass form? The black hole of stellar mass can still be explained by stellar collapse, but millions to hundreds of millions of solar mass stars have not been found in the universe. The theoretical existence of such stars is also unknown.
  Regarding the formation of supermassive black holes, various models have been proposed for decades, such as the merger of small-mass black holes in globular clusters, and the accumulation of gas from surrounding stars by medium-mass black holes. But some of these models rely on too many assumptions, and some are too demanding on the physical environment after careful consideration. In addition, with the progress in the observation of high-redshift galaxies, astronomers have discovered that these supermassive black holes actually existed in the early universe, which also presents new challenges to this type of theory. This is still one of the most important unanswered questions in the study of supermassive black holes.
  Even without considering how supermassive black holes are formed, they themselves are interesting enough: supermassive black holes occupy considerable mass in galaxies and are bound to have an impact on the evolution of galaxies. The accretion of supermassive black holes emits huge amounts of energy to galaxies, which will definitely affect the behavior of gas and stars in galaxies. However, we do not know how these things happened. All of these are extremely interesting questions and attract a new generation of scholars to explore.
There is no end to black hole research

  The three winners of the Nobel Prize in Physics in 2020 have used groundbreaking wisdom to bring us a new method of studying super-massive celestial bodies. Their respective achievements have undoubtedly opened up a new world for mankind. door.
  From the emergence of general relativity in 1915, to the calculation of the earliest black hole solution, to the American physicist Oppenheimer and others proving the possibility of the event horizon, to Penrose and Hawking proving the possibility of the singularity, to The discovery of Cygnus X-1, the American astronomer Seifert’s classification of galaxies and the later astronomers’ insights into such physical mechanisms, until Genzel and Gates discovered supermassive black holes in the Milky Way, we It is challenging mankind’s cognition of black holes, the most peculiar celestial body in the universe, step by step, and constantly raises new questions and challenges.
  After the discovery of Genzel and Gates, the story about black holes continues. On September 14, 2015, the Laser Interferometric Gravitational Wave Observatory (LIGO) located in the United States captured the gravitational waves generated by the merger of two black holes. This event implies that black holes are real. On April 10, 2019, astronomers from many places around the world simultaneously announced the “true content” of black holes. The black hole is located at the center of M87, a giant elliptical galaxy in Virgo, 55 million light-years away from the Earth, and its mass is about 6.5 billion times that of the sun. There is a shadow in its core area, surrounded by a crescent-shaped halo. These two results confirm that black holes are an objective physical entity, not a virtual concept in science fiction.
  This year’s Nobel Prize in Physics is a summary of decades of black hole research work. However, humans have discovered that there are still countless unknowns waiting outside the door, still waiting for physicists to think and explore.