Black holes revealed

  When we look up at the Milky Way, it is always the beautiful planets and stars that catch our eye. But in fact, the most amazing and simple celestial bodies lurking in the dark areas of the stars are perfect. They are called “black holes”.

A star collapses into a black hole (schematic)

How stars stay stable
general theory

  The general theory is that a black hole is an infinitely dense block in space (note: the block or point in cosmology is not our perception of the size of the body in our daily life, but an object with a very large scale), entering the black hole No matter, not even light, can escape. Black holes with masses several times the mass of the sun are likely to form in stars 20 to 30 times the mass of the sun. Huge stars burn brightly in extreme heat, emitting blue light, but the brightest stars are also the shortest-lived.
  Stars are giant gaseous worlds that collapse under their own gravity. Fusion reactions that take place inside a star’s core release a huge amount of light, which creates an outward radiation pressure that prevents the star from collapsing. But eventually the collapse will prevail. Massive stars may burn through their nuclear fuel in just a few million years before collapsing under their own gravity.
  Due to the enormous amount of collapsed matter, the huge and dense neutrons produced will continue to collapse under the enormous gravitational force. If the star is 20 times the mass of the sun or a little more, the star will eventually disappear, leaving behind a “ghost” that is a black hole. But not all stars undergo this transition. Less massive stars like the sun turn into dwarfs after the fusion reaction stops, and then gradually burn to ashes.
  In this light, the cause of black holes is simple: enough matter is compressed into a small enough volume. This raises the possibility that almost all massive stars in the early universe would eventually become black holes.

  A river is a good analogy to the nearest neighbors around a black hole. If you swim in the calm waters of the river, then you are safe. In the same way, as long as you are far enough away from the black hole, the black hole can’t help you. But at the location of the black hole, because the matter has collapsed to a very small volume, it has become very dense, and there is no entity, but just an infinitely small point in space. Of course, the “point” here is also a space.
  Take the river water as an analogy. When the river flows to the edge of the waterfall, the flow of water accelerates, and whoever jumps into the current here will only be buried in the waterfall. And very close to the black hole, the space structure is drawn like a waterfall towards the center of the black hole. Whether it’s a star, a planet, or light, once it’s dragged into a black hole, there’s no turning back.

Black holes are like rivers of space and time (schematic diagram)

  But the sphere of influence of black hole gravity is not infinite. One might think that a black hole would devour all the celestial bodies around it. However, this understanding is wrong, because black holes only swallow celestial objects within a certain distance. So, what does the inside of a black hole look like? We can’t see. The boundary between a black hole and the visible universe is called the “event horizon.” Once you cross the event horizon from the visible universe, there is no turning back.
  Beginning with Einstein, scientists no longer see the structure of the universe as static, but instead believe that massive objects distort the space around them and affect time. Hence the term “space-time”, which combines time and space. Einstein’s excellence lay in his recognition of the close connection between time and space: matter not only distorts space, but also changes the course of time. Specifically, the denser the matter, the slower time passes.
  In the area around the black hole, warped space-time stretches out light waves, distorting colors. Looking at a black hole from a great distance, the event horizon is where time stops. If you get closer and closer to the black hole, a person who is far enough away from the black hole will see your skin redder and redder, and your time will slow down compared to his time. Eventually you cross the event horizon and disappear without a trace.

Sagittarius A

|Sagittarius A (Imagination)
Sagittarius A

  Black holes are so weird that even science fiction writers can’t imagine them. But black holes are not fantasy things, they actually exist. The vast majority of black holes are small, less than 32 kilometers in diameter, and they usually make waves in space. But if we look at the center of the Milky Way and see through the clouds of gas and dust that encase the galaxy’s core, something entirely different emerges.
  For more than 20 years, scientists have discovered that the stars at the center of the Milky Way do not orbit any visible objects at all. The center of the region where these stars are located is dark, as if there is an incomparably huge void there, and these stars seem to be orbiting this void – what scientists call “Sagittarius A”. How massive is this void enough to allow a swarm of stars to orbit it? Scientists speculate that the void is a black hole with a mass 4 million times that of the sun.
  In fact, the entire Milky Way surrounds Sagittarius A. Scientists naturally ask: Where did this black hole come from? Why does it grow so big?
  In 1999, the Chandra X-ray Space Telescope (hereinafter referred to as “Chandra”) was launched from the NASA space shuttle. Although it has been in service for more than 20 years, Chandra remains the most powerful astronomical device for scientists to observe high-energy cosmic rays.

  Chandra’s highest orbit is 134,000 kilometers above the Earth’s surface. During this orbit, Chandra scans space with eight high-precision mirrors to detect X-rays emanating from extremely hot regions of the universe. Chandra has been searching for exploding stars and galaxy clusters for nearly 14 years since liftoff. On September 14, 2013, it looked toward Sagittarius A, observing a large cloud of hot gas, but unexpectedly recorded a brief burst of X-rays from the center of the seemingly empty galaxy.

Chandra telescope

  Scientists were immediately alert: there must be a reason for this outbreak! However, they can’t see why. Scientists initially speculated that an asteroid 26,000 light-years away was torn apart and burned in flames hundreds of times brighter than the sun, and the resulting X-ray burst was recorded by Chandra. But the intensity of the explosion made scientists even more convinced that it was the black hole. That is to say, there must be a large black hole at the center of the Milky Way, and Sagittarius A is this black hole. The black hole was just eating “snacks” — possibly an asteroid, which caused a strong but brief burst of X-rays.
super black hole

  Not only can Chandra be used to observe the Milky Way, it can also be used to observe other galaxies. Astronomers have discovered through observations that there are black holes at the center of every large galaxy. The masses of these black holes are very large, millions or even billions of times the mass of the sun. The question arose: How did these black holes form? Why are they so big?
  Black holes take millions to billions of years to form. Some scientists speculate that the largest and oldest black holes didn’t start with stars. In the earliest days of the universe, gas may have collapsed directly to form massive black holes. While not ruling out the possibility, most scientists believe that Sagittarius A formed from the death of a star.
  Whatever the cause of Sagittarius A, one thing is clear: it’s getting bigger. When Sagittarius A first formed, nearby asteroids, stars and huge gas clouds that strayed into its sphere of influence were its “food”. As the food intake increases, the black hole grows in size. But just by eating, a black hole with an initial mass only a few dozen times that of the sun can never have supermassive. So, how does Sagittarius A achieve accelerated growth? On September 14, 2015, scientists found a clue—the aftermath of the collision of two black holes.

LIGO detects gravitational waves

Two black holes collide (schematic diagram)

  It is not difficult to understand that the scale and power of the collision of two black holes are extremely large, and the ripples caused by the collision of the space-time structure spread outward at the speed of light. On Earth, scientists detected these ripples with LIGO. LIGO is an instrument that works by firing a laser beam that bounces off a mirror when it hits a mirror. When gravitational waves pass by Earth, the return time of the laser beam changes. Although the change is small, LIGO, which uses sophisticated technology, can detect it.
  Many scientists now believe that black hole collisions like this are a key factor in how supermassive black holes form. When another black hole wanders towards Sagittarius A, the two will orbit each other, losing energy in the process, and eventually the two will merge into one. For Sagittarius A, this merger occurred billions of years ago, when Sagittarius A was born.
Fermi bubble

  As Sagittarius A’s mass and influence has proliferated, so has its surroundings. The swarms of stars and gas clouds surrounding this black hole continued to grow, gradually evolving into the Milky Way, with Sagittarius A occupying the center of the Milky Way. So, when Sagittarius A became a supermassive black hole, it also truly came of age, able to dominate the evolution of the entire galaxy. Young galaxies have massive swirling clouds of dust and gas at their centers, so black holes have no shortage of food. As Sagittarius A’s gluttonous feast goes on, its gravitational pull rips matter apart, releasing protons, electrons, and twisting magnetic field lines.
  Some of the matter close to the event horizon did not cross the boundary and was swallowed by the black hole. Around the black hole, this super-hot matter is flung along the black hole’s two magnetic poles (two ultra-high-energy jets) and into space. The farthest they reach is hundreds of thousands of light-years from the black hole itself.
  Only recently have scientists begun to appreciate the enormous influence of Sagittarius A on the Milky Way, and the role these ultra-high-energy jets might play. This starts with the Fermi Space Telescope (hereinafter referred to as “Fermi”).
  Launched on June 11, 2008, Fermi was built to detect gamma rays, the most energetic radiation in the universe. Fermi is almost 100 times more sensitive than previous gamma-ray telescopes, so it can see things that were previously invisible. Fermi circles the Earth every 96 minutes, and scientists have used it to create maps of the universe and discover an invisible landscape—the most energetic regions of galaxies, black holes. Fermi set his “eyes” on the black hole of the Milky Way, and scientists have more details in their imagination of what the black hole looks like. Although it is impossible to see directly inside the black hole, Fermi has brought scientists a huge unexpected surprise.
  There are two huge bubbles (called “Fermi bubbles”) on the disk of the Milky Way (the galactic disk), each of which is 25,000 light-years in diameter, which together account for half the diameter of the Milky Way. If you could see gamma rays, the Fermi bubble would be the largest object you’ll ever see in the sky. The two Fermi bubbles are like super-giant dumbbells suspended in the center of the Sagittarius A black hole. If Sagittarius A ever had a huge explosion, it would have left its mark in the Milky Way. And Fermi bubbles are such traces. As a result, scientists have finally found some clues that the Milky Way had a more active and explosive past.

  When Sagittarius A eats, it releases extremely high amounts of superheated material. The energy released per second by a supermassive black hole is equivalent to the sum of the energy of a trillion times a trillion atomic bombs. If someone happens to be in the “fire” of the ultra-high-energy jet of a black hole, he will vaporize and disappear in an instant. If a planet is hit by a jet, the planet’s atmosphere will immediately disappear.
  In the far outer reaches of the Milky Way, the frenzied outburst from Sagittarius A may have an amazing effect: the super-hot gas emitted by the supermassive black hole has a “calm” effect on the galaxy where the black hole is located.
  To form stars, there must be very cold and dense gas, because stars form from the collapse of matter. Because the black hole jets heat up the gas, the gas cannot collapse to form stars. In this sense, black holes determine how often stars and planets form. In the final analysis, black holes determine the existence of the earth, and our existence is due to black holes.
  After spending billions of years devouring the gas, dust, stars and planets around them, black holes have nothing to eat. As a result, Sagittarius A has now entered a dormant state. While it may seem quiet today, the discovery of giant bubbles through Fermi has revealed its violent past.
Hawking equation

  As we learn more about black holes, the image of black holes is no longer an evil, rampant eater, but a creator and shaper of the universe. But for Sagittarius A, there are still too many unsolved mysteries. One of the most interesting ones is: what happens inside and on the event horizon of supermassive black holes?
  Black holes are also the focus of two conflicting physical theories, one of which is the theory of gravity and the other is quantum mechanics. Quantum mechanics is concerned with the behavior of ultra-small matter (such as how electrons and atomic nuclei fit together), and black holes are where gravity meets quantum mechanics. When scientists calculated the macroscopic and microscopic dimensions of black holes, they found that the results were contradictory, that is, there was a lack of a unified theory to describe the two.

Laboratory simulation of Hawking radiation

Hawking explosion (imaginary)

  How to resolve this contradiction? Scientists began to investigate the situation at the center of Sagittarius A. They studied dozens of stars in the orbits of black holes, some of which are only billions of kilometers away from the event horizon (don’t think billions of kilometers are far, far away, on a cosmic scale, billions of kilometers are only equivalent to the diameter of a hair). Getting so close to a black hole could have catastrophic consequences. Some of these stars may be orbited by planets, and these planets may also be buried in the belly of a black hole.

physicist hawking

  If you fall into a black hole, you must have passed the event horizon before that, but you can’t actually see anything, so you don’t even know you’ve passed the event horizon. If you could stand on the surface of a planet near a black hole, you would see a distorted universe where time and space are distorted. You will find that everything happens super fast. But eventually, the tidal and gravitational forces become so strong that the surrounding space and everything in it is stretched. Your body ends up being pulled into a long, ultra-thin “noodle” less than the diameter of an atom. That is, once in a black hole, even planets and stars are broken down to the point that they are smaller than atoms. And at the center of the black hole, or at the end of the black hole, the singularity, all travel stops abruptly.
  The so-called singularity is when an object is compressed to an inexhaustible level, at which point the object seems to exist but still exists. After trillions of years, all the stars and planets around Sagittarius A will disappear, but the black hole continues to wander the universe.
  If nothing that enters the black hole’s sphere of influence can escape, is the story over? Probably not, because scientists now believe that even black holes like Sagittarius A can die.
  In 1975, physicist Hawking published an astonishing paper in which he said that black holes are not absolutely and completely dark. Hawking believed that black holes also emit light, but only very dimly. He also said that black holes also have corresponding temperatures. Hawking gave a very reasonable, very scientific equation, linking many aspects of physics. There is gravity in this equation, there is black hole mass, there is the speed of light, there are constants related to atomic physics, and of course temperature.
  Hawking’s equation perfectly resolves the discrepancy between gravitational and quantum-mechanical calculations for black holes. In simple terms, what this formula means is: as long as an object has temperature, it will emit light, it will emit radiation, and it will lose energy in the process. For a black hole as massive as Sagittarius A, despite the enormous time scale between birth and death—trillions by trillions of years—they eventually vanish and disappear. By then, the Milky Way will be plunged into perpetual darkness.
black hole fantasies

  You might ask: If in the very, very distant future even supermassive black holes will fall apart, what’s the point? In fact, the discovery of “Hawking radiation” brings up some esoteric physics puzzles.

Black Hole Crossing (Sci-Fi Figure)

Black holes may make time travel (especially travel back in time) dreams come true (schematic diagram)

  If Hawking’s paper is burnt down, as it burns and radiates, does it leave the universe forever? Perhaps, we can collect all the ashes, find all the photons and reconstruct them, so that not only the paper, but the entire contents of the paper, including the equations, can be completely reconstructed.
  Of course, this is just scientific madness. So, does the “reconstruction law” also apply to black holes? For all objects that fall into black holes, what happens to all the information they originally contained? As the black hole eventually evaporates, will the black hole really be left behind? All that information disappeared?

  During the evaporation and disappearance of Sagittarius A, if information escapes, its implications are very far-reaching. Scientists now believe that all celestial bodies (whether stars, planets or asteroids) that fall into the black hole may continue to exist after the death of the black hole, and their information such as the orientation of each particle is completely preserved, so theoretically, if it can be found. With this information, we can use them to imagine the original appearance of these celestial bodies. It’s like going back to the past.
  In this way, all the memories of those celestial bodies that fell into Sagittarius A were turned into part of this black hole and were not lost. Now, even when such information is available, scientists cannot read it. We can only fantasize: someday we will be able to read the information contained in the ashes.
  But the question arises again: what object can escape the clutches of a black hole? How to escape? After all, the definition of a black hole means that nothing can escape a black hole. However, a close reading of “Hawking radiation” seems to show that quantum mechanics does indeed connect the inside and the outside of things. Unfortunately, scientists still don’t know exactly how it’s connected.
  Black holes force scientists to think in new and truly mind-bending ways. The scientific community is still debating the question of what would happen if we threw things into a black hole. There’s a fantasy that objects that fall into a black hole end up being sent to another dimension, perhaps another branch of the multiverse.
  Some scientists imagine that black holes might really be just some type of quantum sphere. Other scientists imagine that all the information falling into the black hole will be holographically encoded on the surface of the black hole, but the encoding method is unknown. Whether these conjectures are right or wrong, their significance extends beyond the black hole itself. Why do you say that? Quantum theories of gravity are essential in describing the mechanisms inside black holes, but existing theories of quantum gravity are elusive.
  Perhaps, in the next decade or 100, there will be exciting advances in the theory, even reminding us that Einstein’s theory of gravity is not a gravitational theory.
  Deciphering the mysteries of black holes may provide us with the best opportunity to complete a complete picture of nature. For the past hundred years, the scientific community has been baffled by the inability to accomplish such a picture. Therefore, the most important thing is not to find the answers, but to understand the whole universe by understanding the most amazing celestial bodies – black holes.
  Scientists still have a long way to go to fully understand black holes. But they have begun to unravel the veil of black holes: black holes are not just cosmic monsters, but cosmic shapers.
The latest observations of Sagittarius A

  Einstein’s general theory of relativity still holds.
  According to related media reports in December 2021, scientists have observed the region close to the black hole at the center of the Milky Way, Sagittarius A, with the highest clarity ever observed. These observations show that Einstein’s general theory of relativity about how gravity affects spacetime remains accurate.
  One way to test general relativity is to observe the motion of stars near supermassive black holes. Near supermassive black holes, gravitational effects are more extreme than anywhere else in the universe. Recently, a team used the Very Large Telescope interferometer in Chile to observe Sagittarius A. Using a combination of observations from four telescopes (a technique called interferometry), they made observations about 20 times more accurate than previous observations with a single telescope, so they were able to see objects that were farther and fainter , and higher resolution.
  The latest observations show the stars orbiting the black hole very close to Sagittarius A. By measuring the velocities of these stars, the team calculated that the black hole is about 4.3 million times the mass of the sun. This is the most accurate calculation of the mass of Sagittarius A to date. The team found that the motions of these stars all seemed to match the predictions of general relativity.
  The team also discovered a previously unknown star orbiting Sagittarius A, which may help test other theories than general relativity. The star, known as S300, is older and dimmer than other stars in its region, but its brightness changes allow scientists to measure its motion with greater accuracy. The discovery of a previously unknown star, S300, has led scientists to speculate that there may be other low-brightness stars closer to the black hole. These stars are more helpful for testing general relativity. Scientists are exploring ways to discover these stars.

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