Stars are bodies with an incredible amount of mass. However, they can become even more massive at the end of their lives. It is then when they can go from emitting to absorbing light, becoming suggestive and mysterious entities, black holes. These pose a challenge to the physics we know of and seem to defy many of the “rules” that we have inferred from observing the cosmos. What do we know about them?
What is a black hole?
In 1783, the English geologist and clergyman John Michell sent a letter to the Royal Society describing a hypothetical body so dense that not even light could escape from it. At that time Newton’s theory of gravitation and the concept of escape velocity were well known. Although Michell’s calculations fell a bit short, the concept was correct. Of course, at that time they were not called “black holes”. This name came in the 20th century, proposed by physicist John Archibald Wheeler. A black hole is such a dense body, with so much mass and so little volume, that it attracts everything by its immeasurable gravity. So much so that not even light can escape its attraction. Another way of interpreting it is that it warps the space around it in such a way that the photons that “fall” into the gravitational well it generates cannot get out.
In 1783, the English geologist and clergyman John Michell sent a letter to the Royal Society describing a hypothetical body so dense that not even light could escape from it. Let’s imagine that we are getting closer to the black hole. Its gravity does not “pull harder” than other similar bodies, such as that of stars. However, as we reduce the distance, this force increases overwhelmingly. There is a point called the event horizon. From this point on, it takes a speed greater than that of light to escape the gravitational force. Therefore, it is “impossible” to escape the attraction from this horizon. And what is there beyond? In reality, we know very little.
What the event horizon encloses is called a singularity, because it contains all the mass at a single point, at a theoretical volume of 0. But this is “impossible” since it breaks with what we know about physics. In reality, what lies beyond the event horizon is only the fruit of conjecture. We do not know what happens “inside” the hole, roughly, and to a large extent, it is because we do not understand them well.
There is a point called the event horizon. From this point on, it takes a speed greater than that of light to escape the gravitational force. Therefore, it is “impossible” to escape the attraction from this horizon.
How do you make a black hole?
Our Sun, for example, is a relatively medium star, or small, depending on how you look at it, and it has almost two to the 30-kilogram mass. That is an incredible amount. As we know, the more mass, the more gravity it generates. So our central star exerts a gravitational force capable of keeping the entire solar system rotating around it. Isn’t this force enough to attract the entire mass of the sun itself? Why doesn’t it collapse in on itself if it’s so big? The gigantic reactions that take place inside it, the result of stellar nuclear fusion, produce titanic forces that prevent the sun from “sinking” into itself. But what if there were no such forces, what would happen?
This is what happens at the end of the life of many stars. Stars can die in various ways. Some are extremely violent, explosive, and generate terrifying supernovae. Others simply fade little by little. In several of these cases, especially when the star was very large, the remaining material can fall under its gravity, becoming denser and denser, occupying less volume. In these cases, with the remnants of a supernova that exploded or with a star that cooled down enough, the material becomes too cold and the fusion that exerts an outward force on the star does not occur. So each time gravity gets bigger and bigger, and the “hole” gets smaller and smaller. At one point the black hole appears.
They are black, but not quite
The name of a black hole is quite clear: a dark point, which does not let light out. However, a consideration of quantum effects on a hole’s event horizon led the eminent Stephen Hawking to discover a physical process by which the hole could emit radiation. According to the uncertainty principle of quantum mechanics, there is the possibility that in the event horizon, pairs of particle-antiparticle of short duration are formed. One of the particles would then fall into the hole irreversibly while the other would escape. This process is formed strictly outside the black hole, so it does not contradict the fact that no material particle can leave the interior. However, there is a net energy transfer effect of the black hole around it. This phenomenon is known as Hawking radiation, and its production does not violate any physical principle.
Thanks to technical advances in recent years, we have finally been able to see a hole in the neighboring galaxy, M87, with our own eyes, reaching one of the most impressive scientific milestones of our time. On the other hand, in 2008, Alan Marscher published an article that described how collimated jets of plasma are produced near black holes that start from magnetic fields located near the edge of them. Again, strictly speaking, it does not occur “inside the black hole”, therefore, it complies with the concept that we marked from the beginning.
It also allows another question: observe and detect this type of phenomenon. By absorbing it all, as you might expect, it is almost impossible to “see” black holes. Until recently we knew that they are thereby indirect detections. Thanks to technical advances in recent years, we have finally been able to see a hole in the neighboring galaxy, M87, with our own eyes, reaching one of the most impressive scientific milestones of our time.