How close can you get to a black hole?

 For humans, the "death fall" of these gases is too far away to be directly observed. However, astronomers have designed a new technology that can detect their last radiation traces and understand the most extreme gravitational environment in the universe.

　　 In this new study, physicists calculated the shortest distance to avoid falling into a black hole by observing specific radiation characteristics. This threshold is called the "innermost stable circular orbit" (ISCO). Using this method, more sensitive X-ray telescopes in the future may be able to truly reveal the mystery of this orbit.

　　 event horizon

　　 Black holes are undoubtedly the most mysterious celestial bodies in the universe. They hide in the darkness and swallow all incoming light. Regardless of size, all black holes have a common feature, which is the event horizon. Cross this line and you will never come back. Once anything passes through the event horizon, even light itself cannot return to the universe. The gravity of the black hole in this area is too strong. However, outside the region, everything is business as usual.

Black holes have a certain mass, some are only several times the mass of the sun, and are distributed in smaller galaxies; some are billions of times the mass of the sun, and they can be called the real "monsters" in the universe. Orbiting a black hole is like orbiting other massive celestial bodies. Gravity is gravity, and orbit is orbit. Supermassive black holes can accumulate matter, such as gas, stars, etc., moving to its vicinity because of their strong gravity. The accreted matter usually has angular momentum and will form a rotating accretion disk or a thicker accretion flow around the black hole, some of which will eventually enter the black hole. Because of the compactness and strong gravitation of black holes, the accretion process of black holes will release a large amount of gravitational energy, which will be converted into kinetic energy of the accreted matter. Part of the kinetic energy will be dissipated into gas internal energy due to the "friction" between the gases or the action of the magnetic field. . The black hole accretion process may be the physical process with the highest energy conversion efficiency in the known universe, and its energy conversion rate is dozens of times the energy conversion rate of thermonuclear fusion.

　　 Therefore, a lot of matter in the universe revolves around black holes. Once these "reckless adventurers" are surrounded by the gravity of the black hole, they begin their journey towards the end of their lives. When matter falls into a black hole, it is often squeezed into a disk called an "accretion disk". This disc keeps spinning, and with the release of heat, friction, magnetic energy and electric energy, the substance in it emits bright light.

Take the most massive black holes as an example. The accretion disks around them emit so much light that they have a new name: active galactic nuclei (AGN), whose brightness can exceed millions of individual galaxies. . In the accretion disk, the masses of matter will rub against each other, suck up their respective rotational energy, and push them continuously towards the event horizon of the black hole. But without these frictional forces, matter can revolve around a black hole forever, just like planets revolving around the sun for billions of years.

　　 The black hole closest to the earth

　　 Astronomers at the European Southern Observatory (ESO) made a remarkable discovery-they found the black hole closest to the earth so far. If the conclusion is true, people in the southern hemisphere can even see the star system where the black hole is located without the aid of observation equipment.

This mysterious black hole is located in the southern part of the Taurus constellation 1000 light-years away from the earth. The black hole cannot be observed. It has a strong gravitational force, so that no object, even light, can escape the gravitational restraint of the black hole. Astronomers initially thought it was a binary star system or two stars orbiting a center of mass celestial body. When they used the MPG/ESO2.2 diameter land-based telescope for in-depth observations, they named the binary star system HR 6819. At the same time, to their surprise, They also observed the third celestial body-black hole.

　　Although astronomers cannot directly observe the black hole, they can infer its existence based on the gravitational interaction between the black hole and the two companion stars. Through months of observation, they were able to map the orbit of the star and infer that another huge, invisible celestial body played an important role in the binary star system.

Observations also showed that one of the binary stars circled the invisible celestial body every 40 days, while the other existed independently in an area further away from the invisible celestial body. They calculated that the invisible object is a black hole of stellar mass, a black hole formed by the collapse of a dying star, about 4 times the mass of the sun.

　　 In addition to the HR 6819 black hole, the nearest black hole to the Earth is located in the constellation of Unicorn 3000 light-years away. However, scientists have analyzed that there may still be potential black holes that are closer, and there may only be millions of black holes in the Milky Way.

　　 So there may be more black holes hidden near us.

　　The disappearance of gas

　　 However, as you get closer and closer to the center of the black hole, you will reach a point where all hope of maintaining stability will be shattered under the action of gravity. At this time you are still outside the black hole and have not yet reached the event horizon, but the gravity has become so extreme that it is impossible to have a stable orbit.

The first human black hole photo published last year allows us to verify the general theory of relativity in a highly gravitational environment like the edge of a black hole. The discovery announced by EHT this time comes from the Messier 87 (M87) black hole. The black hole will cast a silhouette on the "background wall" formed by the radiation of the accretion gas around it. The reason why such a "shadow" is formed is that the black hole will swallow all the light emitted from behind it and directed towards the observer. At the same time, other light emitted from the back of the black hole just past the horizon will brighten the surrounding "shadow" and form a bright area. The powerful gravitational lensing effect will bend light, and even the light emitted by the matter directly behind the black hole can be bent to the periphery of the dark area to contribute a part of "light".

Once you reach this area, you cannot stay on a calm track, but only have two options: you can use rockets or other energy sources to push yourself to a safe place; and if you are a cloud of unlucky gas, then They can only fall freely and fall into the endless darkness.

Of course, Einstein’s general theory of relativity also predicted the existence of the innermost stable circular orbit. However, despite the success of general relativity in predicting and explaining cosmic phenomena, we are also convinced that black holes exist, but scientists still The existence of the innermost stable circular orbit has not been confirmed, and whether it meets the predictions of general relativity. Now, astronomers have found a possible way to verify the existence of the orbit from the gas falling into the black hole.

　　 dancing light

　　 A team of astronomers described how to use the disappearing light to study the innermost stable circular orbit. Their method relies on an astronomical technique called reverberation mapping, which takes advantage of the characteristic of different areas around black holes emitting light in different ways.

　　 When gas flows out of the accretion disk, passes through the innermost stable circular orbit (the innermost part of the accretion disk), and enters the black hole itself, it becomes very hot and emits a lot of high-energy X-rays. X-rays are emitted from the black hole in all directions. We can observe these radiations from the earth, but the details of the accretion disk structure disappear in the X-ray rays (more knowledge of the accretion disk will also help astrophysicists understand the innermost stable circle track).

Similarly, these X-rays can also well illuminate the area outside the accretion disk that is mainly composed of cold gas clusters. The cold gas is excited by X-rays and also starts to emit light. This process is called fluorescence. We can also detect this radiation and distinguish it from the X-rays emitted from the nearest region of the black hole.

It takes a certain amount of time for light to propagate from the innermost stable circular orbit and the outside of the accretion disk to the cold gas mass; if we look closely, we can first see the central area (the innermost stable circular orbit and the innermost part of the accretion disk) ) Flashing, and soon there will be a bright "reverberation" on the outside of the innermost stable circular orbit and around the accretion disk.

　　 The appearance and details of these reflected lights depend on the structure of the accretion disk. Astronomers have previously used reverberation mapping to estimate the mass of the supermassive black hole in the center of an active galaxy. In this latest study, the researchers used complex computer simulations to observe the movement of gas inside the innermost stable circular orbit, that is, how the gas disappears when it finally falls into the black hole event horizon, and how this in turn affects nearby and X-rays emitted from outside air.

　　 Although the current telescopes do not have sufficient sensitivity to measure these gases, the next generation of X-ray telescopes should be able to confirm the existence of the innermost stable circular orbit and verify whether it meets the predictions of general relativity.

　　 Where will the black hole lead(hermes2021.org)?

　　If you could cross a black hole, where would you go? What will be waiting for you? If you can come back safe and sound, what interesting story can you tell?

　　 All these questions can be answered with a simple sentence, that is, "Who knows?" In today's day when science and technology are changing rapidly, the mystery of black holes is still unfathomable. After falling into the event horizon, he actually crossed a barrier. Once someone falls, they can no longer send information back, and they will be torn to pieces by the huge gravity. So in theory, anyone who crosses the event horizon will not go anywhere.

This sounds like a disappointing and painful answer, but it is also expected. Einstein's general theory of relativity links space-time with gravitation and predicts the existence of black holes. Later studies have shown that black holes are produced by the death of stars with sufficient mass. After the star dies, it will leave a small and dense remnant core. Assuming that the mass of this core is about three times that of the sun, gravity will cause it to collapse and become a point called a "singularity" (also known as gravitational force). Singularity or space-time singularity), this is a point with infinitely small volume, infinite density, infinite gravity, and infinite curvature of space-time, which is considered to be the center of a black hole.

　　 The resulting black hole has extremely powerful gravitational force, and even light cannot escape. Therefore, if you find yourself in the event horizon, you are destined to have nowhere to escape. German astronomer Karl Schwarzschild calculated that if the entire mass of a celestial body is compressed to a small "radius of gravitation", then all its matter and energy (including light) will be trapped by gravity. Inside. From the outside, this celestial body is an existence of absolute darkness, that is, a black hole. The event horizon is the space-time boundary around the black hole, and light and matter can only pass through the event horizon inward. According to Messi, tidal forces will shrink your body into a chain of atoms (also known as "spaghettiization") and will eventually be crushed at the singularity. You may want to escape at the other end of the black hole, but this seems completely fantasy.