Black holes are one of the most fascinating and mysterious bodies in the universe. They are regions in space with such an incredible density that not even light can escape from them. Although black holes may seem very complex bodies, and different between them, it isn’t like this. There are only three properties that distinguish black holes from each other:
- The mass: the quantity of matter that a black hole has absorbed.
- The electric charge: it can be positive or negative. However, it is thought that black holes tend to have a close to zero electric charge.
- The spin (or angular momentum): designs at which speed rotates a black hole.
All black holes of the universe observed until now rotate, and they do it at extremely high speeds. Even so, many different types of black holes have been theoretically studied, including non-rotating black holes, also known as Schwarzschild black holes. But can these objects really exist in our universe?
The formation of black holes and angular momentum
First, it is essential to understand that black holes form after the gravitational collapse of a very massive star. When a star cannot produce energy any more, it can no longer counteract the gravitational force, causing the star to collapse under its own weight, producing an explosion known as a supernova. Depending on the star’s mass, the latter will leave as a vestige a neutron star or a black hole.
In this process, the black hole conserves the angular momentum of the star. Moreover, as the size of the objects diminishes a lot, this implies that the rotation speed has to greatly increase. To visualise this better, imagine an ice skater with the arms stretched out, that is turning on itself. When the ice skater moves its arms closer to his body, and makes itself smaller, the rotation speed increases. The black hole suffers the exact same effect.
In addition, the rotation speed of these mysterious objects is extremely high: approximately 80% of the speed of light. Even if, theoretically, non-rotating black holes could exist, finding one of them in the universe is extremely unlikely. For a black hole not to rotate on its axis, it would have to be formed from a star that didn’t rotate, which is almost impossible.
Finally, in the unlikely case that a static black hole existed, it would only need to absorb a single photon to acquire a little angular momentum, which would make it start rotating.
So, if we see that the existence of Schwarzschild black holes is very unlikely, how come scientists study them? The reason is given by the fact that they have to thoroughly understand the Schwarzschild metric, where black holes are static, before adding complications such as the rotation of these bodies and all the effects it entails. Furthermore, approximations are commonly used in scientific studies. For example, a simple model of the Earth’s gravity treats the planet like a perfect sphere and ignores other effects. From then on the model is improved, adding the characteristics or effects that are considered necessary.
The study of black holes is in constant evolution, and new discoveries are being produced. However, to better understand these bodies we will still have to wait until having a theory of quantum gravity, which will give us the possibility to deeply study what black holes hide inside them, how the universe started…