The most mysterious area of space exploration is the study of black holes. For decades, scientists have been searching for answers to the question of what we would see and experience inside a black hole. It's a theoretical question, thanks to the powerful force of gravity. Let's find out the details!
A black hole is nothing more than a region of space-time from which nothing, not even light, can escape because of the extremely strong gravity. The black in the name of a black hole refers to the fact that even light cannot hide from gravity. The name is a bit misleading because although they look like holes in the sky at first glance, they are large amounts of matter condensed into a single point.
There are four categories of these amazing space entities: stellar, intermediate, supermassive, and miniature. Most often, black holes are born through a death of a star. As stars reach the ends of their lives, most will inflate, lose mass, and then cool to form white dwarfs. But the largest of them are destined to become either super-dense neutron stars or so-called stellar-mass black holes.
Their origin can be understood using general relativity. The mass shrinks due to gravitational collapse to a point where it is smaller than the critical limit and it bulks into a single point. At this point, factors such as density and space-time curvature become infinite. Due to the strong gravity around the singularity, no matter and no light can escape.
There are several ways to specify the boundaries of a black hole. One is the so-called event horizon, which is defined by the internal light rays, and the other is the apparent horizon. The latter is defined by the fact that the rays emitted from the inside tend to move towards the singularity.
There are many legends about black holes, but their existence is not questioned. They have a sound theoretical basis and have been observed to exist. The first image of a black hole was presented on 10 April 2019 by NASA with an algorithm written by Katie Bouman and a large team behind her.
The supermassive black hole
There are also so-called supermassive black holes, which can weigh billions of times more than our Sun. One such supermassive black hole is Sagittarius A, named after its location in the constellation Sagittarius. It is 26,000 light-years away from Earth.
Will the Sun become a black hole?
Stellar-mass black holes are formed from stars, which might raise the question of whether the Sun will ever become a black hole. The chances of this are very close to zero. Researchers have found that the Sun will never form an iron core large enough to create a black hole. This is supported by the fact that the largest black hole ever observed was four times the mass of the Sun.
Discovering black holes
You might think that black holes are hard to find in the sky. But this is not entirely true. The boundary around a black hole, the event horizon, is swallowed up by infinity. But as the gas and dust get closer and closer to this line, the rotation of the gas and dust creates observable radiation.
Similarly, a black hole may be indicated by a star orbiting at high speed at a particular location, because it can orbit at such speed due to high gravity. This was the method used to discover Cygnus X-1 in the second half of the 1960s.
Black holes can also be detected when two of them are very close together. At a certain point, their event horizons converge and merge, causing a strong shock to their environment. The merger emits a ripple in space-time that can be detected and measured by the Laser Interferometer Gravitational-wave Observatory (LIGO). Researchers later translated the waves from previous cases into sound waves to get the sound of the universe.
Investigating a black hole
The study of black holes is a very complex area. On one hand, they are difficult to spot and require a lot of luck to identify. Even if they are found, they are impossible to approach because of the enormous distance and the enormous gravity. Theoretically, if one were to enter a black hole, death would be certain. On Earth, it is impossible to create such a hostile environment for testing. This of course limits the research possibilities.
The research of black holes goes hand in hand with advances in technology. As the power of quantum computers increases, we can learn more and more about their properties and make more precise calculations about their behaviour. No one knows what happens inside a black hole, but as science advances, we are getting closer and closer to finding the answer.