On September 3, according to foreign media reports, the United States laser interference gravitational wave observatory (LIGO) and the Virgo gravitational wave observatory (Virgo) in Italy jointly detected a medium mass black hole with a mass of 142 times the sun’s mass, which is the first time scientists have detected a medium mass black hole. It is reported that the black hole is a combination of two black holes with a mass of about 85 times the mass of the sun and 65 times the mass of the sun. The energy lost equal to 8 times the mass of the sun diffuses in the universe in the form of gravitational waves, leading us to discover this “impossible black hole”. < / P > < p > “in recent years, scientists have discovered the double black hole system through gravitational wave detection more than once. The discovery of a medium mass black hole will help astronomers to decipher the origin of supermassive black holes and make the study of black holes more systematic Gou Lijun, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences and a professor at the University of the Chinese Academy of Sciences, said in an interview with science and technology daily. In 1916, Karl Schwarzschild, a German scientist, obtained an exact solution of Einstein’s field equation of general relativity, which laid a solid theoretical foundation for the existence of black holes. As long as any object is compressed to a certain critical radius, it will collapse into a singularity, extremely distort the surrounding space-time, and form a strong gravitational field. In a certain range, even light cannot escape. Today, scientists have discovered many black holes, which can be divided into three categories according to their mass: stellar black holes with masses ranging from several times to 100 times the mass of the sun, supermassive black holes with masses ranging from one million to several billion times the mass of the sun, and medium mass black holes between the two. According to the view of academia, supermassive black holes are evolved from smaller ones. If the evolution process is a continuous process, then theoretically the medium mass black hole should be the intermediate state. < / P > < p > “in 2012, some astronomers observed a super hot gas explosion using a radio telescope in Australia, believing that the explosion was emitted by a medium mass black hole; in 2015, an astronomer claimed to have discovered a medium mass black hole when observing NGC 2276 Unfortunately, many of these findings have not been further confirmed. ” Gou Lijun said that black hole detection needs to wait for the right time, such as when it produces relatively bright electromagnetic radiation or has a large jet. < / P > < p > “at present, the black holes detected by human beings are just the tip of the iceberg of many black holes in the universe. Theoretically, there are hundreds of millions of stellar black holes in the Milky Way alone, but until now only a few dozen have been confirmed Gou Lijun said. Gou Lijun explained that the black hole itself does not produce electromagnetic waves, so it can not be directly detected. At present, we can only detect the black hole through some radiation phenomena around the black hole. For example, when the gas far away from the black hole rubs with the black hole in the process of approaching the black hole, it will produce strong radiation. We can indirectly detect the black hole by using electromagnetic detection. The gravitational wave is also called the ripple of space-time, because the disturbance of space-time will spread out at the speed of light like water wave after being disturbed by gravity. One of the effects of gravitational waves is to make the distance between two points oscillate regularly. < / P > < p > “LIGO can detect gravitational waves by precisely measuring the distance between two points on the earth.” Gou Lijun explained that because the double black hole system usually does not produce any signal other than gravitational wave, gravitational wave is the only weapon to detect double black hole system at present. The gravitational waves in the universe can be detected by ground-based gravitational wave observatory, gravitational wave laser interference antenna in space, pulsar timing array and polarization of cosmic microwave background radiation. Accurate detection of gravitational waves can help reveal the formation and evolution of gravitational wave sources such as black holes and neutron stars. < / P > < p > “both LIGO and Virgo are ground-based laser interferometer gravitational wave detectors.” Gou Lijun said that when the laser is emitted from the center of the detector, it will split into two parts, along two equal length interference arms several kilometers long. After being reflected many times, they will come together again to interfere with each other. The intensity of the interference signal can be observed by the photon detector. When the gravitational wave signal from a wave source in the deep universe crosses space-time and arrives at the earth, the arm length of the two interference arms will change slightly but not the same when passing through the interferometer, resulting in the signal strength after interference will change with time. By measuring this change, the waveform and general direction of the gravitational wave signal can be captured. < / P > < p > “the discovery of medium mass black holes depends largely on the tracking and decoding of gravitational waves. Gravitational wave radiation can be divided into five different bands: high frequency, intermediate frequency, low frequency, very low frequency and extremely low frequency. The wavelength range can be from thousands of kilometers to the cosmic scale. Researchers need to build hypotheses based on a small amount of information deciphered in the detection results. ” Gou Lijun said. < / P > < p > in this study, the gravitational wave signal gw190521 was detected on May 21, 2019, and the signal duration was less than 0.1 seconds. By fitting the gravitational waveform, they calculated that it was produced by the merging of two black holes more than 7 billion years ago. Gou Lijun said that he believed that in the future, the ground-based gravitational wave observatory would detect hundreds of double black holes, double neutron stars and other combined gravitational wave events, which would help us to have a deeper understanding of the formation and evolution of these celestial bodies and systems. For a long time, the medium mass black hole has been regarded as the “seed” of supermassive black hole by the scientific community. < / P > < p > in November 2019, a stellar black hole with 70 times the mass of the sun was discovered by our scientific research team, and a new method was provided to find the black hole by using the advantage of the LAMOST sky survey. This stellar black hole is far beyond the upper mass limit predicted by the theory, which overturns the previous understanding of the formation of stellar black holes. < / P > < p > it is worth noting that the discovery of medium mass black holes is also suspected by some scientists because of their “overweight”, and they believe that one or two of the black holes are not formed by the collapse of stars like most stellar mass black holes. < / P > < p > “according to the existing theory, collapsing stars should not be able to form black holes with a mass of about 65-120 times the mass of the sun, which is known as the” mass gap for instability. ” Gou Lijun said. In this regard, foreign researchers have proposed the possibility of hierarchical coalescence, that is, the merging of two black holes was formed by merging smaller black holes before. < / P > < p > according to the research of LIGO team, medium mass black holes may be formed by merging smaller black holes. If we can find more merging medium mass black holes, we may be able to solve a mystery about the origin of supermassive black holes. The origin of supermassive black holes, most of which exist in the center of galaxies, is still foggy for scientists. According to the existing theory, the gas cloud containing only hydrogen and helium rapidly shrinks to form a supermassive black hole. However, this theory can only explain the origin of some supermassive black holes in the early universe, but it is difficult to explain why the number of supermassive black holes observed is more than expected. It is understood that although astronomers are excited by the discovery of gravitational wave signal gw190521 and the existence of medium mass black holes, astrophysicists do not rule out other possible sources of this discovery. < / P > < p > “there is no doubt that, in terms of research significance, this discovery provides at least one evidence of the coalescence of medium mass black holes. The more we understand the medium mass black hole, the more beneficial it will be for us to analyze the formation of supermassive black hole and even to derive more related theories. Of course, more observational data are needed to achieve this step by step. ” Gou Lijun said. Spontaneous combustion at a Guangzhou Motor vehicle intersection and other traffic lights in Shenzhen