What happens when a black hole passes close to the Earth?

Imagine if there is a celestial body whose gravity is extremely strong, like a devourer in the universe. Whatever comes close to it will be swallowed up mercilessly, and not even light can escape. What would you think of first? That's right, it's a black hole, a cosmic "big devil" that can manipulate time and space at will.

Generally speaking, the scientific community can divide black holes into two types: one is a "stellar black hole" and the other is a "supermassive black hole."

We know that the reason why stars can burn continuously is due to the reaction of internal nuclear fusion. They will fuse step by step during the burning process until the energy is exhausted and they die. After a massive star runs out of energy, it will eventually stop at the iron element. Because when the star fuses to the iron element, it can no longer trigger nuclear fusion to release energy, so the core begins to collapse under the action of its own gravity. At this time, the core of the star will be ruthlessly squeezed, and eventually the entire star will disintegrate, and the strongest supernova explosion in the universe will occur. After the supernova explosion, most of the matter is ejected into interstellar space, but if the remaining mass of its core is greater than 3 times the mass of the sun (Oppenheimer limit), then the core will continue to collapse under the action of its own gravity, and eventually form a black hole, which is what we call a stellar black hole. The mass of a stellar black hole is usually between 3 times and hundreds of times the mass of the sun. For example, the mass of the Cygnus X-1 black hole known in our Milky Way is about 21 times the mass of the sun, and according to scientists' speculation, there may be hundreds of millions of such stellar black holes in our Milky Way.

However, there are more powerful black holes in the universe, that is, "supermassive black holes". Scientists have discovered that supermassive black holes are generally located in the center of galaxies. For example, there is a supermassive black hole in the center of our Milky Way galaxy - Sagittarius A*, which has a mass of about 4 million times the mass of the sun. Its gravitational influence covers the central area of ​​the entire Milky Way, and many stars move under its gravitational control. However, our Milky Way is not the only galaxy with supermassive black holes. When observing other galaxies, astronomers found that there is such a supermassive black hole in the center of most galaxies. For example, the Andromeda Galaxy, which is closest to the earth, has a higher mass of its central black hole, reaching 1.4 billion times the mass of the sun. At present, the scientific community has not yet reached a definite conclusion on the cause of the formation of supermassive black holes, but there are several theoretical hypotheses, such as the "seed black hole hypothesis", "direct collapse black hole theory", "black hole merger hypothesis", etc. We will not explain them too much.

As we mentioned earlier, scientists speculate that there may be hundreds of millions of stellar black holes in our Milky Way alone. So, among so many black holes, will one day one of them come close to our solar system? Suppose a stellar black hole comes close to the solar system, or even close to our Earth, what will happen? Let's use our imagination to imagine.

First, when the black hole is still very far away from the Earth, we may observe some abnormal astronomical phenomena. Because the strong gravity of the black hole will distort the surrounding space-time, the light passing near the black hole will bend, which will cause the starry sky images we see to be distorted. By that time, astronomers may find that the shapes and positions of some distant galaxies will undergo some strange changes, just like observing an object through a distorted lens. This gravitational lens effect will become more obvious as the black hole gradually approaches.

As the black hole gets closer, the solar system where the Earth is located will begin to be subject to the increasing gravitational pull of the black hole. The orbits of the planets in the solar system will gradually deviate, and the Earth is no exception. This will cause the amount of solar radiation received by the Earth to change, and the climate will become extremely unstable. Areas that were originally warm and suitable may quickly become cold, and some areas that were originally cold may become extremely hot and unbearable. This is all possible.

When a black hole gets close enough, the Earth's atmosphere may be gradually stripped away, and the Earth's tidal phenomena will become extremely violent. The gravity of the black hole will create a huge gravitational difference between the Earth facing the black hole and the Earth facing away from the black hole, which is much stronger than the tidal force exerted by the moon and the sun on the Earth. The oceans on Earth will be stretched into huge tidal bulges by the gravity of the black hole, which may trigger a global super tsunami. At the same time, the geological structure inside the Earth will also be severely damaged by the tidal force, triggering large-scale earthquakes and volcanic eruptions.

If the black hole continues to approach, the Earth will be in danger of being torn apart. When the gravitational difference of the black hole reaches a certain level, the Earth will not be able to withstand this huge tidal force, and its lithosphere, mantle and core layers may be torn apart in turn, and the Earth will be pulled towards the black hole like noodles. In the process of falling towards the black hole, these torn materials may form an accretion disk rotating around the black hole, emitting strong electromagnetic radiation, which may also include high-energy X-rays and gamma rays.

Eventually, the Earth will be completely swallowed by the black hole and become part of it. All matter on Earth, including us humans, will be compressed into the singularity of the black hole and disappear from the universe as we know it.

However, the probability of a black hole approaching us is very, very low. Even if it happens, it will be a long, long time in the future. Because the currently known black holes are very far away from the earth. For example, the supermassive black hole at the center of the Milky Way is about 26,000 light-years away from the earth. This distance is too far, and its gravitational effect at the position of the earth can be ignored. So based on current observations and research, there is no black hole within the distance that can threaten the safety of the earth, and the distribution and movement laws of celestial bodies in the universe make it difficult for such a dangerous situation to occur. But this does not mean that we will stop studying and exploring black holes. Research on black holes can not only let us understand the properties of these mysterious celestial bodies themselves, but also help us better understand the structure and evolution of the universe, as well as the position and destiny of the earth in the universe.

Author: Stars Become Light

Reviewer: Liu Yong, researcher at the National Space Science Center of the Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.