When you hear "black hole", I believe many friends will think of the EHT (Event Horizon Telescope) on April 10, 2019, in Washington, the United States, Shanghai and Taipei, China, Santiago, Chile, and Belgium. Brussels, Lyngby, Denmark, and Tokyo, Japan, will hold press conferences at the same time to release the first major result of the "Event Horizon Telescope" in English, Chinese, Spanish, Danish and Japanese - the first black hole image ever obtained by mankind. Photo[1].
If you want to understand what the "singularity of a black hole" is, you must first know what a black hole is and how are black holes produced? The concept of "black hole" first originated from a letter written by John Mitchell, provost and natural philosopher of Cambridge University, to Cavendish in 1783, proposing the concept of dark stars. [2]. In Michel's dark star concept, light exists in the form of particles. Based on Newton's law of gravity, when a planet is large enough and dense enough, its escape velocity is close to the speed of light. At this time, the light particles on the planet cannot escape the constraints of the planet's gravity, and the light emitted by the planet cannot be observed anywhere else in the universe. Michel calls this type of celestial body a "dark star." The concept of "dark stars" has not caused people to think about black holes. This is because the British physicist Thomas Young conducted the famous Young's double slit experiment in 1801 and discovered the interference properties of light, proving that Light exists in the form of waves, not corpuscles as imagined?[3]. In people's minds at the time, the "dark star" conjecture was absurd.
It was not until 1915 that Einstein proposed the general theory of relativity, which changed people's understanding of gravity. In 1916, German astronomer Karl Schwarzschild obtained a vacuum solution by calculating Einstein's gravitational field equation under the condition of spherical symmetry. This solution describes the geometric structure of space-time around a point with mass. Schwarzschild discovered that The particle will distort the surrounding space-time, so that the space within a certain critical radius is isolated from the rest of the universe, that is, there is an interface - the "event horizon". Once entering this interface, even light cannot escape. This critical radius is called the Schwarzschild radius R=2GM/c^2, where G? is the gravitational constant, c? is the speed of light, and M? refers to the black hole mass. It is known from the Schwarzschild radius that the size of the black hole's event horizon radius is positively related to the mass of the black hole [4]. Normally, the nuclear fusion reaction inside a star produces high-temperature gas. The pressure gradient force in the gas can compete with gravity, allowing the star to be in a state of hydrostatic equilibrium. When a star exhausts its nuclear fuel, the temperature decreases and the pressure gradient force decreases. Unable to contend with gravity, the matter rapidly collapses toward the center, and part of the matter is thrown into space. The remaining matter gathers toward the center under its own gravity, forming a dense celestial body. If the original star is massive enough - greater than about 20 times the mass of the sun (1 solar mass = 1.9891×1030 kg), the dense object formed after the explosion is a black hole.
"Black hole" was officially named on December 29, 1967, when the famous American physicist Wheeler gave a lecture at Columbia University titled "Our Universe: the Known and Un-known" First used in a public lecture, "black hole" has gradually become a proper term in physics since then. For a black hole under strict general relativity, the surface of the black hole can be determined by the spherical surface determined by the Schwarzschild radius. Compared with the simple Schwarzschild black hole model (which is not charged and does not rotate), the actual black hole is more complicated. More than three physical quantities are needed to describe a complete black hole: mass, charge and angular momentum. This is what Hawking, B. Carter and others strictly proved in 1973 as the "Black Hole Hairless Theorem" [5] - after the black hole is formed, there are only three conservations left that cannot be converted into electromagnetic radiation. Quantity, all other information ("hair") is lost, the black hole has almost no complex properties of the material from which it was formed, and has no memory of the shape or composition of the precursor material.
How to discover a black hole? The difference between black holes and most celestial bodies in the universe is that black holes cannot be directly observed. Most of the current observations of black holes are through the physical processes around the black hole: accretion and jets. Black hole accretion refers to the process in which gas around a black hole falls toward the black hole under the influence of the black hole's strong gravity. During the falling process of the gas, the gravitational potential energy is converted into internal energy and kinetic energy. Therefore, the temperature of the gas reaching the vicinity of the black hole can reach several million to tens of billions of degrees Celsius. These high-temperature plasmas will emit strong multi-band electromagnetic radiation, and will also produce jets and winds. Moreover, these gases generally carry angular momentum, so they will form a disk-shaped structure - a black hole accretion disk [6]. Black hole jets refer to particles that are accelerated to close to the speed of light on the black hole accretion disk and fly out from the poles of the black hole under the influence of a strong magnetic field to form elongated jets.
Due to the huge mass of the black hole, its rotation is very stable, and the rotation axis will not change over a long time scale. Therefore, the jet of the black hole is always long and straight, extending far into space. Sometimes the jet is The length can reach tens of thousands of light years [7].
By observing these physical processes, we can obtain "edge information" about how other celestial bodies in the universe emit x-rays and gamma rays due to friction caused by the acceleration caused by the black hole's gravity before being sucked into the black hole, or observe stars. Or the orbit of interstellar clouds and gas masses to infer information such as the position and mass of the black hole. The first black hole discovered in history was Cygnus X-1. Its mass is about 8-10 times that of the sun. The fall of surrounding celestial bodies makes it appear unusually bright in the X-ray band. The black hole inferred by observing the orbits of stars is a supermassive black hole lurking in the Sagittarius A* (Sgr A*) region at the center of the Milky Way. In 2009, an international team of astronomers obtained the result based on 16 years of infrared observations. The orbits of 28 of the stars were found to be revolving around an invisible celestial body. Until 2017, the orbits of 40 stars were determined. Based on the orbit analysis of 17 of the stars, the black hole at the center of the Milky Way was calculated with a lower error. The mass is 4.28 million times the mass of the sun [8]. In such a small area, it has more than 4 million times the mass of the sun. It is difficult to find other celestial bodies with such properties. Astronomers believe this evidence points to a supermassive black hole lurking at the center of the Milky Way.
According to the physical characteristics of the black hole itself, black holes can be divided into four categories: non-rotating and uncharged black holes - Schwarzschild black holes, non-rotating charged black holes - R-N black holes, rotating uncharged black holes - Kerr black hole, rotating charged black hole - Kerr-Newman black hole. Black holes can be divided into three categories according to their mass: stellar-mass black holes, intermediate-mass black holes, and supermassive black holes. Stellar-mass black holes are several to hundreds of times the mass of the sun; supermassive black holes are millions of times the mass of the sun; and intermediate-mass black holes are somewhere in between. It is estimated that there are at least hundreds of millions of stellar black holes in a galaxy similar to the Milky Way. However, because most of them lack accretion raw materials, they are difficult to observe. Currently, only more than 20 have been observed in the Milky Way [ 9].
In 1988, physicist Stephen Hawking and Oxford University mathematics professor Roger Penrose*** both won the Wolf Prize in Physics for their proof of the strange phenomenon in the 1970s. The Singularity Theorem - The Singularity Theorem proves that the general theory of relativity is incomplete, because if the general theory of relativity is universally valid, then there must be some singularities in the space-time of the universe. The research center on black holes found that although the mass and horizon of the black hole are limited, these masses are not evenly distributed across the entire horizon, but are gathered at a point in the center of the black hole. This point is an infinite density. , a singularity with infinitely high space-time curvature, infinitely small system, and infinite heat, surrounded by a space with nothing. This area cannot be seen by the naked eye. This point is called the "singularity" of a black hole [10].
In Hawking's book "The Grand Design" [11], the singularity is explained as follows: a singularity is a point in space and time at which the physical quantity becomes infinite and the classical laws of physics fail. The singularity appeared at the Big Bang and was also formed in the black hole. In other words, the universe and time may originate from the singularity or end at the singularity.
? The "Black Hole Singularity" will serve as the starting point for an explosion of knowledge. Here we will show you rigorous and interesting popular science knowledge, explore the mysteries of the world together, and open up the "black boxes" around you.
References:
[1]. The first black hole photo released! Look, look, the face of the black hole! [N]. People's Daily, 2019-04-10.
[2].John Michell. On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in Case Such a Diminution Should be Found to Take Place in any of Them, and Such Other Data Should be Procured from Observations, as Would be Farther Necessary for That Purpose. By the Rev. John Michell, B. D. F. R. S. In a Letter to Henry Cavendish, Esq. F. R. S. and A. S.[J]. Philosophical Transactions of the Royal Society of London,1784,74:35-57.
[3].Liu Qian. Approaching the Black Hole[ J]. Science and Technology Think Tank, 2015(11):78-85.
[4]. Zuo Wenwen. Uncovering the mystery of black holes by approaching astronomy [J]. Physics, 2020, 49 (03):196-199.
[5].Bekenstein. Novel "no-scalar-hair" theorem for black holes.[J]. Physical review. D, Particles and fields,1995,51 (12).
[6]. Lu Jufu. Progress in black hole accretion disk theory [J]. Progress in Astronomy, 2001(03):365-374.
[7]. Yuan Feng. Research progress on black hole jets[J]. Physics, 2015, 44(02):69-76.
[8].Gillessen S, Plewa P M, Eisenhauer F, et al. An Update on Monitoring Stellar Orbits in the Galactic Center[J]. Astrophysical Journal, 2017, 837(1):30.
[9]. Yuan Feng. Seeing a black hole: "Humanity releases the first black hole photo" event Interpretation [J]. Science Bulletin, 2019, 64(20): 2077-2081.
[10]. Zhao Ang. Research on Hawking and Black Holes [N]. Workers Daily, 2018-03-23( 006).
[11]. Hawking, S. ), Mlodinow, et al. A Brief History of Time·Grand Design: Hawking’s 70th Birthday Celebration Limited Edition[M]. Hunan Science and Technology Press , 2011.