When it comes to black holes, people's first impression is: it is a celestial body with extremely high density, so much so that even light cannot escape its gravitational pull. Light can only enter but not exit, which is why what we perceive of it is a pitch-black sphere, like a bottomless hole in the universe. Most of the time, black holes do not appear like this. They are brighter than most stars in the universe, emitting dazzling light. Moreover, intense X-ray and plasma streams are ejected from both ends of the black hole. These light columns, known as astrophysical jets, extend thousands of light-years into space. Scientists discover X-rays from behind a black hole for the first time According to foreign media reports on August 2, a scientific team led by Dan Wilkins, an astrophysicist at Stanford University in the United States, announced that using the European Space Agency's X-ray telescope XMM-Newton and NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), they had observed light from behind a black hole for the first time. The team stated that while observing X-rays emitted by a supermassive black hole at the center of a galaxy 800 million light-years away, they noticed a series of bright X-ray flares. They also observed more "light echoes" that were dimmer than the flares, appeared later, and had different colors. This marked the first time astronomers had observed "echoes" of light reflected from the back of a black hole. It is explained that a black hole is a region in spacetime where gravity is so strong that even light cannot escape. Einstein's theory points out that black holes warp the structure of space around them, and when the warped magnetic field acts as a mirror for the black hole, it should be possible to see light waves emitted from behind the black hole. In an interview with foreign media, Dan Wilkins said: "Any light that enters a black hole cannot escape, so we shouldn't be able to see anything behind the black hole. The reason we can observe this phenomenon is that the black hole is warping space, bending light, and distorting the magnetic field around itself." Einstein's general relativity holds that the essence of gravity is the curvature of spacetime, and mass or energy is the cause of this curvature. In turn, this curved space determines the rules governing the movement of energy and matter. Even if light travels in a straight line, light passing through highly curved regions of spacetime—such as the space around a black hole—will propagate in a curved path. In this case, light travels from behind the black hole to its front. This is not the first time astronomers have discovered gravitational lensing caused by black holes warping light, but it is the first time they have seen "light echoes" from behind a black hole. Dan Wilkins stated that the next goal of the research team is to create a 3D map of the environment around black holes. It is reported that the research team believes this study can help enhance our understanding of black holes and their coronas, and explore how black holes and coronas produce these bright X-ray flares. For example, the famous supermassive black hole M87* emits astrophysical jets that extend 5,000 light-years into space, making it easy for the Hubble Space Telescope to find and locate it. Now, a question arises: we all know that black holes are called "black" because their gravity is extremely strong, so strong that even light cannot escape their gravitational pull. We also know that X-rays are a form of light, so why aren't they swallowed by the black hole but instead ejected to such a great distance? Could this be the legendary Hawking radiation? First, it is certain that the astrophysical jets emitted by black holes have nothing to do with Hawking radiation; they are completely different concepts. We will introduce Hawking radiation in a dedicated article later, as it is also a very interesting topic. Black holes are greedy creatures Black holes are the most powerful "ejectors" in the universe, and at the same time, the biggest gluttons in the universe—they devour everything that comes near. Whether it is a star, a planet, or tiny dust, once it enters the gravitational range of a black hole, it will be pulled into a death waltz, eventually torn to pieces and eaten bit by bit. The process by which a black hole "eats" is subtle and even artistic. It does not pounce and swallow in one bite, but first pulls the star into a dance. Because black holes have extremely high density, their mass is mostly greater than that of ordinary stars. So when a star enters the gravitational field of a black hole, it first orbits the black hole. During this rotation, part of the gas on the star is pulled away from its parent star by the black hole's tidal force and sucked in by the black hole. Most of the gas does not aim directly at the black hole's center of mass when flying toward it. At this point, the gas flow gains angular momentum, rotates rapidly around the black hole, and gradually accumulates toward the center. This is what we commonly call an accretion disk. The formation of a black hole's accretion disk Interstellar matter is mostly composed of elements such as hydrogen and helium, as well as their compounds. When these materials are attracted by the black hole and rotate rapidly around it, due to the conservation of angular momentum, they do not immediately fall into the black hole. As more gas and dust are drawn in by the black hole's gravity, the density of the accretion disk increases, and collisions and friction between gas and particles become more intense, generating extremely high temperatures and powerful energy. At the beginning of the article, it was mentioned that most black holes do not appear "black"; instead, they are among the brightest stars in the universe. Why is this? These bright black holes are called "quasars," and their light and heat are emitted by the hot accretion material in their outer layers. As the black hole devours stars, it draws a large amount of gas and dust around itself. These dense gases and dust collide and rub violently during their extremely fast rotation, converting a large amount of mass into light and heat. At the same time, because this light and heat are pulled by the black hole's strong gravity and cannot escape, they accumulate more and more around the black hole, forming a photon sphere. Meanwhile, the accretion disk of a black hole is not always a thin layer. Because the black hole's gravity is extremely strong, gas at the edges gathers toward the center, causing a region near the event horizon to form a mass of ultra-high-temperature plasma fluid. Particles collide with each other and pile up toward higher latitudes, all the way to the vicinity of the black hole's poles. The friction in the black hole's accretion disk is so intense that near the event horizon, 40% of the remaining mass of the accreted material is converted into electromagnetic radiation, mainly X-ray radiation. It is worth noting that in the core of a star, the conversion efficiency of powerful nuclear fusion is only 0.7%. Relativistic jets On the outer surface of the black hole's event horizon, although a large amount of ultra-high-temperature plasma and X-ray photons generated by the accreted material have not yet been swallowed, they cannot escape either; photons travel along curved paths. At the same time, under the enormous pressure of the surrounding stardust, the material that has not been swallowed gathers toward the black hole's poles, and is finally ejected at nearly the speed of light along the two rotational axes of the accretion disk. The formation and dynamics of astrophysical jets are highly complex phenomena, related to many types of high-energy astronomical sources, and also to the strong magnetic fields generated by the accretion disk during its high-speed rotation. Since the jets are generated outside the range of the black hole's event horizon, the black hole's gravitational pull on them is relatively weak, allowing the jet material to escape the black hole at high speed without being swallowed. In fact, a considerable amount of interstellar matter is thrown into space after being stirred and shattered by the black hole. Black holes do not completely devour stars that approach them; a significant portion of stellar material is converted into gas and radiation streams and ejected into the universe. Summary: The strong gravity of a black hole can indeed attract light. Once photons enter the event horizon of a black hole, they cannot escape, so we cannot see what is inside or what is happening there. Black holes eject X-rays and gaseous matter only when they are devouring nearby objects. Outside the black hole's event horizon, it cannot trap photons, but it still has extremely strong gravity. It tears apart and shatters nearby stars, causing the star's gas to rotate rapidly around itself, forming an accretion body. Due to the extremely high rotation speed, a large amount of material is ionized, forming high-energy radiation streams including X-rays. These radiation streams are squeezed by the surrounding accreting gas and ejected into space from the black hole's poles at nearly the speed of light, forming the astrophysical jets we observe. This is why they can escape the black hole's strong gravitational pull. Source: Guanyuge, China Nuclear Technology Network