When Einstein first postulated theories that opened the door to the possibilities regarding the existence of black holes, theorists began to think about what properties such strange objects would or should have. This was long before there was any proof of black holes and it was primarily scientific conjecture, however it was and is supported by solid science. One of the probabilities is that virtually all black holes spin. It is interesting to take a look at why this is predicted by theory.
To put it in a greatly simplified way, stars are huge masses of gas, primarily hydrogen. The bulk and gravitational pressure is so great that friction from collapse creates such intense heat that a nuclear process starts, fusing hydrogen into helium. Equilibrium is reached with the outward force of the hydrogen fusion equals the inward pull of gravitation. The process is far more complex than this, however when most of the hydrogen is converted to helium, there is no longer a balance, and the star begins to collapse, which intensifies internal temperatures. When the temperature and pressure become great enough, helium begins to get converted, which again stabilizes the star. The processes is repeated as heavier elements are produced.
Experts don’t know for certain why it is, but every star they’ve been able to observe in enough detail to tell, spins on its axis. As an example, it is believed that the sun rotates once every 27 days or so, just as the earth does in a little over 24 hours. This has a great deal to do with spinning black holes, as will be seen.
Black hole formation
As has been explained, stars collapse and expand, often several times in their lifetime, depending on what is being converted in the nuclear fires. Still, there comes a point when the nuclear conversion can no longer overcome the force of gravity and the star collapses until it again reaches a balance. At what point this happens is largely dependent on the star’s mass. A star the size of the sun is destined to one day collapse until it becomes a white dwarf star, probably smaller in size than the earth, which is considerably smaller than the sun is now. It is important to note that if the star doesn’t expel any material, it is going to retain its mass and gravitational properties. The matter is still there, just in condensed form.
A larger star than the sun may collapse until protons and electrons are forced together, creating neutrons which are then tightly packed together. These stars are exceptionally dense and while the mass is much greater than the sun, the diameter is far smaller than that of a white dwarf. A star the size of the sun could become the size of a large city, it is believed by many, if it was able to collapse to this point. This is a neutron star.
If a star is much larger than the sun, several times greater in mass, it keeps on collapsing, with the gravity being concentrated in a smaller and smaller area. Finally, the gravity becomes so great that even light cannot escape and a black hole comes into being.
Since all known stars spin, it would stand to reason that white dwarfs, neutron stars and black holes also spin. In fact, it is quite likely that they all spin faster than the stars that they originated from did. This phenomenon can be seen when a spinning ice skater tucks in their arms, reducing the amount of body surface and thus, spinning faster. This is what should happen when a star, retaining its mass and gravity, becomes smaller in diameter. This could be stated mathematically, but it isn’t necessary here.
This means that since the mechanics are understood, it is logical for theory to dictate that black holes spin. Naturally, at this time there isn’t a way to find out if all black holes spin, so it will probably remain a theory for quite some time. The idea is still well founded on current knowledge and observable information.