The road to black hole discovery

 　　Physicist Hawking believes that black holes are "stranger than anything science fiction writers can imagine". The reason why black holes are so strange is that there is a singularity of infinite density and infinitesimal volume inside the black hole, which can completely collapse space and time, resulting in consequences that humans cannot understand.

　　In 1783, British scientist Michel proposed that according to Newton's law of gravity, there may be massive celestial bodies in the universe that can even be bound by light forever. Because such objects cannot emit light outward, Michel called them "dark stars." But Michel's "dark star" theory is still based on the law of gravity. Under this theoretical framework, matter is not related to space and time, so "dark stars" and "black holes" are still fundamentally two different things.

　　What really laid the theoretical framework for black holes was the general theory of relativity proposed by Einstein in 1915. As a result, human understanding of the universe has entered a new era...

Deciphering the relationship between matter and space-time

　　In general relativity, matter (energy) and space-time interact with each other, and matter (energy) bends space-time to varying degrees, which is the essence of gravity. But Einstein had trouble calculating the curvature of space-time, and he used 10 equations to calculate how much space-time is approximately curved at different masses/energies (he thought it was impossible to get an exact solution). But in fact, in the field of physics, the simpler the formula for calculating physical quantities, the closer it is to the truth.

　　The introduction of general relativity coincided with the battle between Germany and Austria-Hungary against the Russian Empire on the Eastern Front of Europe. Like many European scientists, 41-year-old German scientist Schwarzschild also applied for the front line. A recognized genius in his hometown, he published two papers on the orbits of binary stars at the age of 16. On the Eastern Front, he was incorporated into the artillery company, responsible for calculating the relevant data of artillery ballistics for the soldiers.

　　Within days of the publication of General Relativity, the text spread around the world, and Schwarzschild received the relevant material. As soon as he saw this bold and elegant new theory, he was immediately fascinated by it. In the days that followed, he used his work breaks to start working on the 10 equations. The sleepless calculation work made him forget the fear hovering over the battlefield on the Eastern Front.

General relativity tells us that space can be bent by matter (or energy)

If the Earth collapsed into a black hole, its diameter would be only 9 mm

The man who calculated the black hole

　　Soon, Schwarzschild discovered that Einstein's equations were too complicated, and that Einstein's ideas were also old-fashioned routines that originated in the 19th century. And Schwarzschild is very familiar with Riemannian geometry for calculating curved spaces, which is a new kind of geometric thinking. After simplifying a series of basic premises, Schwarzschild used Riemannian geometry to reduce Einstein's 10 equations into one equation, and miraculously calculated the only exact solution to this equation.

　　On December 22, 1915, Schwarzschild wrote a paper on the computational process and sent it to Einstein. Einstein wrote back to Schwarzschild shortly after, saying, "I didn't think anyone could figure out a solution to this problem in such a neat way. I like your method very much."

　　Schwarzschild was not satisfied, because he only calculated What about the curvature of space-time on the outside of spherical celestial bodies, but what about the inside? After applying the equation he created, he made an incredible discovery: if a celestial body is compressed into a certain radius by its own gravity, the severely distorted space-time will become a bottomless pit. Any matter that enters this bottomless pit, even light, will Don't try to escape from it.

　　In any case, Schwarzschild could not believe that such a monster celestial body existed in the universe. According to his calculations, if the earth was compressed to only 9 mm in diameter, it would become such a monster (this is only a theoretical value, in fact, this change is only possible for celestial bodies with a mass of at least 20 times the mass of the sun). Schwarzschild sent Einstein his calculations for the monster. On February 13, 1916, Einstein presented this calculation to the Prussian Academy of Sciences. Unfortunately, Schwarzschild died on May 11, 1916 due to illness.

Schwarzschild called 'the man who calculated the existence of black holes'

Mysterious X-ray source in the sky

　　In 1963, an American physicist named the monster object calculated by Schwarzschild as a black hole. But scientists at the time felt that black holes only existed in theory, and even Einstein himself was reluctant to admit the existence of black holes.

　　One day in 1964, at a missile base in New Mexico, a rocket soared into the upper atmosphere. It rotates continuously as it lifts off, and its radioactive detector scans the entire sky and records multiple X-ray sources other than the sun, with the strongest signal in the direction of the constellation Cygnus. This mysterious source of intense X-rays has been named Cygnus X-1. Some scientists speculate that Cygnus X-1 is likely to be a black hole, but some scientists don't think so.

　　Before 1964, scientists had a very shallow understanding of black holes, thinking that it was a hole that allows matter to enter and exit. From 1964 to 1973, after a lot of calculations, scientists concluded that black holes not only rotate, but also cause waves in the surrounding space and time.

　　In 1971, British scientist Mudin came to work at the famous Greenwich Observatory in the United Kingdom. The observatory is housed in the octagonal house of a castle built in the 15th century. In the fall of the same year, Mudin and another astronomer, Webster, discovered that in the region of Cygnus X-1, a blue star was orbiting the void, with what appeared to be a massive, non-luminous mass in the center of its orbit. celestial body. Blue stars are the "big guys" in the family of stars, and the objects they surround must also be massive, but why doesn't this mysterious object emit light?

　　Based on the predicted mass of the blue star and the speed at which it orbits the mysterious object in five or six days, Mudin and Webster calculated the mass of the mysterious object: maybe four solar masses, maybe even six. At that time, the only compact celestial bodies discovered by humans were white dwarfs and neutron stars, but in theory, the masses of these two types of celestial bodies would not be greater than twice the mass of the sun. So, what celestial body is orbiting the blue star? Mudin and Webster speculate that it is a black hole.

　　They believe that the blue star and the black hole form a binary system, and the black hole continuously strips material from the blue star, which is heated to extremely high temperatures by friction as it falls into the black hole, releasing X-rays. They wrote the theory into a paper that they plan to submit to the journal Nature. But their superior, the director of the Royal Observatory, Richard, did not believe in the existence of black holes at all, and was worried that such an article would attract ridicule. However, with the mediation of other senior astronomers, Richard finally agreed to publish the paper.

　　The paper, published in the January 1972 issue of the journal Nature, brought Mudin to fame and a lifetime position at the Greenwich Observatory. Since then, humans have discovered suspected black holes one after another. Humanity has finally shattered the understanding that black holes exist only in theory. Later, after multiple measurements by the Gaia satellite and the Very Long Baseline Interferometry Array, scientists measured that the Cygnus X-1 black hole is more than 7,000 light-years away from the earth, and its mass is about 20 times that of the sun.

Big and small black holes

　　Among all the black holes discovered by human beings, there is a type of black holes that can be described as "alternative". An American astronomer has discovered strange objects called "quasars". Quasars are actually the cores of newborn galaxies. Although they are not as large as the solar system, they release thousands of times the energy of the Milky Way. Scientists generally believe that the essence of a quasar is an active galactic nucleus, and its interior is a supermassive black hole with a mass of millions or even tens of billions of times the mass of the sun.

　　According to their mass, black holes can be divided into three categories:

　　Stellar black holes: black holes formed by the collapse of massive stars under the action of their own gravity.

　　Intermediate-mass black hole: A black hole formed by more massive stars.

　　Supermassive Black Hole: The massive black hole at the center of all known galaxies that is far more massive than the previous two types of black holes.

　　Both stellar black holes and intermediate-mass black holes can evolve from stars, but the formation process of supermassive black holes is still a mystery, because there are no stars in the universe that are massive enough to form supermassive black holes all at once. In this regard, scientists have proposed two possibilities: galactic dust continues to attract, accumulate and form supermassive black holes; stellar black holes or intermediate-mass black holes continue to merge to form supermassive black holes.

　　Entering the 1990s, the liftoff of the Hubble Space Telescope let astronomers know that a supermassive black hole lurks at the center of almost every galaxy, and our own Milky Way is no exception. The supermassive black hole at the center of the Milky Way is named "Sagittarius A*", where "Sagittarius" represents the location of the galactic center black hole, and "*" represents "star". Sagittarius A* is only small in the family of supermassive black holes, with a mass "only" 4.3 million times the mass of the sun.

　　Although astronomers are convinced that Sagittarius A* is a black hole, before 2015, some scientists still called it "some very dense celestial body." It wasn't until 2015, when gravitational detectors first detected gravitational waves from the merger of two black holes, that the existence of black holes became an ironclad fact.

first black hole photo

　　To study black holes, astronomers face two problems: there are indeed quite a few stellar black holes and intermediate-mass black holes in the Milky Way, but they are not only small, but also black (at least from the Earth and indistinguishable from the black background of space); supermassive Although black holes are large and bright (the outer parts of such black holes are very bright), they are located in other galaxies and are too far away to be observed by us. Selected and selected, the black holes with observation conditions are only the central black hole of the M87 galaxy (referred to as M87 black hole), which is closer to the Milky Way, and Sagittarius A* (referred to as the galactic center black hole).

　　In 2012, astronomers from all over the world gathered in Arizona, USA, and proposed a bold black hole observation plan: using eight high-quality radio telescopes located in Antarctica, Chile, Mexico, the United States and Spain to form a virtual diameter equivalent to the diameter of the earth. Telescope, the "Event Horizon Telescope". The data collected by these telescopes can be aggregated to the Haystack Observatory in the United States for processing, and an image of the extremely distant black hole can be obtained.

　　On April 10, 2019, the Event Horizon Telescope project team released the first black hole image, but it is not a galactic center black hole, but the M87 black hole 1,000 times heavier and 1,000 times larger than the galactic center black hole. This image was observed in April 2017. It took scientists two years to process the massive data before they got the final image.

　　The M87 black hole is 55 million light-years away from Earth and has a mass 6.4 billion times that of the sun. It is one of the most massive black holes known, with a black center surrounded by a crescent-shaped halo. To observe this black hole, the difficulty is equivalent to seeing a football on the surface of the moon with a telescope on Earth.

Galactic black hole imaging is more difficult

　　On May 12, 2022, the Event Horizon Telescope project team released the first photo of a galactic black hole. This black hole is 26,000 light-years away from us. The dark area in its center is the black hole including the event horizon. The bright part is located outside the event horizon. They are accelerated by the gravity of the black hole to a great speed, and the temperature can reach up to 10 billion °C.

　　The observations of the galactic center black hole and the central black hole of M87 were both completed in 2017, but the imaging difficulty of the former is much higher than that of the latter, so the release of the former image is 3 years later than the M87 black hole image. The reason why we chose not to deal with the galactic black hole first is that there is a large amount of surrounding matter between the earth and the galactic black hole, which will interfere with the observation. At the same time, the area near the galactic black hole is a dense area of ​​stars, and the light of a large number of stars will also interfere with the observation.

　　At the center of the M87 black hole, a large amount of luminous gas on the black hole accretion disk takes several weeks to orbit the black hole, but the galactic black hole is much smaller than the M87 black hole, and the gas circles the black hole in a few minutes, which means that in the whole night The image of the galactic center black hole is always changing during the observation process. Scientists say it's as hard as taking a picture of a kid running around at night.

　　The eight telescopes of the Event Horizon Telescope are all located on the ground, and they are all disturbed by the atmosphere when they observe the stars, just like we stand in the river in the summer when we watch the fish underwater - their shape is severely distorted by the flowing river water. In order to solve the influence of atmospheric disturbance on the imaging of the galactic center black hole, scientists resorted to adaptive optics technology. Scientists fired a powerful yellow laser from the ground, and the sodium ions in the upper atmosphere were excited, and the luminous point was called a "sodium guide star". Sodium guide stars are also distorted by atmospheric disturbances, but as long as scientists can restore the original shape of the sodium guide stars, they can use the same parameters to restore the original shape of the black hole.

　　The glowing ring structure in the photo lies beyond the event horizon. The bright ring surrounds a dark central region. The intersection of light and dark is the edge of the black hole's event horizon.

　　This image is the first image of a galactic black hole captured by humans. Interestingly, the galactic center black hole and the M87 black hole are very similar in shape, and the similarity between the two confirms a prediction of general relativity: all black holes, regardless of size, are similar in shape.

The weirdness of black holes

　　In the early 1970s, American physicist Wheeler posed a question to his students: "If you pour a cup of tea into a black hole, what would happen?" Wheeler was not concerned with whether the black hole would be scalded by hot tea, Instead, I want to know if the heat and entropy (energy that matter cannot use) in the hot tea will disappear completely after entering the black hole. You know, according to the laws of thermodynamics, heat and entropy do not disappear.

　　In 1971, Hawking proposed the "black hole area theorem", that is, the area of ​​the event horizon of a black hole always increases. Although Hawking himself discovered this feature of black holes, he also did not know the implications. The person who solved this mystery is American physicist Bekenstein. According to his calculations, the entropy of an object after falling into a black hole does not disappear, but is retained by the black hole in the form of an increase in the surface area of ​​the event horizon.

If you pour hot tea into a black hole, where does the heat and entropy of the hot tea go?

Bekenstein explained that the heat and entropy of objects absorbed by a black hole are still retained by the black hole

　　After Bekenstein solved the entropy problem, in 1974 Hawking solved the thermal problem of black holes. According to quantum mechanics, the vacuum is not empty, subatomic particles and their antiparticles keep appearing in pairs, annihilating each other in an instant. If this phenomenon occurs near the event horizon of a black hole, it can have interesting consequences: a pair of particles created by the vacuum may be inside the event horizon and the other outside. Particles within the event horizon will be pulled into the singularity by the black hole's strong gravitational pull, while particles outside the event horizon will escape the black hole with other particles with the same encounter because they have lost their annihilation objects. That is, black holes are constantly emitting particles - a phenomenon known as Hawking radiation. Any object with a temperature above absolute zero will radiate heat energy outward, and black holes will also radiate heat energy outward, so black holes also have temperature, and the heat of objects sucked into black holes will not disappear out of thin air.

The principle of black hole evaporation

What's inside a black hole?

　　At the center of a black hole is a singularity with incredible mass, density and gravity. According to general relativity, a singularity is a region of infinite curvature of space-time, and space-time also disappears completely at the singularity, so all the principles of physics do not apply to the singularity.

　　Black holes don't gobble up objects that come close to them without principle. If the spacecraft passes the black hole far away, the spacecraft's trajectory will only be slightly deflected by the black hole's gravity, just like passing an ordinary star. But if the spacecraft gets too close to the black hole, the spacecraft will inevitably be sucked into the center of the black hole. If the spacecraft enters a stellar black hole, such black holes are often very small (the size of the black hole refers to the radius of the event horizon of the black hole). Down, the spacecraft will be torn into pieces, and then the pieces will continue to be pulled into "noodles" under the strong gravitational difference, spinning into the black hole. But if a supermassive black hole is entered, the gravitational difference between the head and tail of the spacecraft is very small, and the spacecraft can at least remain intact before entering the event horizon. As for what will happen after entering the event horizon, it is unknown.

Black holes are still a very mysterious existence for humans (imagination)

　　Einstein could not be sure of the existence of black holes because at the time scientists believed that only perfectly spherical stars that were perfectly symmetrical from all directions could collapse into singularities, and such stars hardly existed in the universe. However, British physicists Penrose and Hawking found that as long as the mass of the star reaches a certain critical point, whether or not the shape of the star is perfectly symmetrical, a singularity will form. According to Penrose and Hawking's theory, the universe originated from a singularity in the Big Bang, and will one day be squeezed back into a singularity. Although we have seen black holes, human detection of black holes is far from over.