There is something in black holes that instigates our imagination Maybe it’s the fact that, in order for a black hole to exist, a star has to die Or maybe it’s the fact that whatever goes into one cannot escape. Or the idea that in the heart of a black hole, time and space don't work exactly how we expect them to What I’ve just said sounds poetic, but it is actually literal Whatever is the answer, black holes, and particularly what lies in their center, the so-called singularity, are the most enigmatic phenomena in the universe Title: Singularity, the mystery in the heart of black holes For over 100 years we’ve been living in the Universe defined by Albert Einstein His theory of general relativity made it possible to explain from the beginning of the Universe to the orbits of the planets. What Einstein couldn’t foresee was that, from his revolutionary theory, derived the possibility that our Universe could have astronomical phenomena we now know as black holes They are regions in space where gravity is so powerful that nothing, not even light can escape once they come in.
And despite the fact that the first image of a black hole would only be taken in 2019, the proof of its existence was already there in Einstein's equations. But he refused to accept it because he thought that nature wouldn’t be capable of creating such a thing There had to be some kind of unknown process that prevented its formation Accepting the existence of black holes would also mean recognising that in its interior hides the singularity, where all the laws of nature as we know them stop working. Einstein decisive “no”, which he even published in the most respected mathematics magazine in the world, created an obstacle to the investigation about black holes After all, the father of general relativity was saying that they were impossible But in 1965, physicist Roger Penrose managed to demonstrate mathematically that in Einstein’s Universe singularities are not only possible but inevitable It was because of his findings that Penrose was awarded the Nobel Physics prize in 2020, at 89 years old However, the complete acceptance of black holes by the scientific community doesn’t mean that all its mysteries are solved, much less the ones related to the singularity These concepts are very complex, so I’ll start explaining the formation of black holes through something more common: this wooden table.
You can see a table and feel it’s solid, but its density is just a few grams by cubic centimeter But in the scale of black holes, this material is too dispersed This means that in order to turn this table into a black hole, we would have to compress it until its density became incredibly high The problem is that this material will resist compression. And the more I try to squeeze it, the more I put pressure on it, the bigger will be its resistance We would need a very powerful event in order to generate enough energy around the table to compress it to a volume so low and so dense that creates a black hole One type of event capable of liberating such an amount of energy is a supernova That’s what happens when a massive star, meaning a star that is at least 30 times larger than our Sun, dies and, in a matter or seconds, explodes After that, it collapses in on itself, forming a black hole This process is responsible for the formation of stellar black holes, which are the most common ones According to the most recent calculations, there are between 10 million and a billion of them only in the Milky Way There is also another type of black hole called supermassive, which astronomers believe lie in the center of almost all of the major galaxies, including ours In fact, it was because of one of them that Penrose didn’t win the Nobel prize alone He shared it with other scientists who discovered an object in the center of our galaxy, believed to be one of these supermassive black holes I say “believed to be” because what they know is that in this region there is an invisible object which is so heavy that causes all the stars around it to move at impressive speeds. In order to understand why this is a clue as to why that could be a black hole, we have to think about them as objects with mass and imagine space (which is technically called space-time) as an elastic bed But let's take it slow.
The Earth and the Moon are examples of celestial bodies with mass. The first one, of course, has more mass than the second. Earth’s mass distorts the space-time around it, but it also distorts the one around the Moon This is the reason why the Moon orbits the Earth But the star that forms a black hole can be from dozens to billions of times larger than our Sun.
Like we saw in the wooden table example, after it collapses, the star’s mass will be compressed to a volume infinitely low and dense Because of that, the distortion it will cause around it is so powerful that nothing will be able to escape its influence if it comes close enough. This point of no return is called the event horizon, a kind of boundary around a black hole that separates what’s inside it from what is outside it. When something crosses this limit, it cannot go back.
It gets disconnected from the Universe and we don’t know what is its final destination What we do know is that once inside, everything moves toward the singularity, the center of the black hole. And it’s there that space and time, as we know them, seize to exist This is because, at that point, Einstein’s equations don’t work. In fact, none of the laws of physics that we know so far work.
That is why some say that black holes and, more specifically, singularity, are the biggest unsolved problem in theoretical Physics.
