Even people who don’t especially get thrilled with science or astronomy often find black holes to be fascinating. Part of the reason is that black holes are not only a demonstration of one of the greatest of the extremes to be found in all of creation, they are at the borderline of theory and observable science. One facet of the theory is that all black holes must spin, though nature generally doesn’t function in absolutes.
To understand why black holes must spin, it is necessary to understand some of the simple basics about black holes.
What is a black hole?
Put very simply, a black hole is an object that is so dense and that has such a tremendous gravitation that not even light can escape it. This definition is part of the fascination because in order to observe an object, including those beyond the earth, we must have light to be able to see it. This means that since light cannot escape the gravitational pull of a black hole, we can’t see the black hole.
We can, however, see the accretion disk that forms around a black hole. This is a dense disk of gas and dust that is being pulled into the black hole because of the gravity. The accretion disk circles the black hole and spins so fast that it produces radiation that we can detect. This is, in fact, the importance of whether or not black holes spin, because if the black hole wasn’t spinning, the accretion disk wouldn’t spin, either, and we wouldn’t be able to detect it.
Black hole formation
Black holes are formed by massive stellar explosions. A star the size of our sun goes nova when it reaches the end of its ‘life’. That is, it swells up, cools and then collapses in on itself. It continues to collapse until the pressure of the gasses and radiation pushing outward balance with the force of gravity. At the point of equilibrium, the star will be densely packed. A teaspoon full of this material would weigh many tons. The sun is a small star, but it is so much bigger than the earth that over a million earths would fit inside of it. Yet a star of that size would become just a few times bigger in diameter than the earth, once it collapses, and will be a white dwarf star, gradually cooling down.
A star that is much bigger than the sun has a greater gravity, so it becomes even denser when it collapses. This kind of star blasts a large amount of gas and debris into space when it explodes, before collapsing. The gravity pulls so strongly that the atoms are forced together. About 90 percent of an atom is empty space, with negatively charged electrons circling the positively charged protons in the nucleus. As the large star collapses, though, it doesn’t reach equilibrium until the space inside the atom is gone and the electrons and protons are forced together. This creates particles that have no charge, called neutrons.
The neutron star is so dense that a teaspoonful would weigh millions of tons. If our sun collapsed this much (which it can’t due to its mass), the entire sun would be about the size of the moon. Scientists have detected numerous neutron stars because they spin very rapidly, sending out massive pulses of radiation as they do. They can spin around a hundred times in a second or even faster.
If the star is truly massive and much larger than the sun, the collapse causes such a strong gravitational pull that not even at the stage where neutrons are pressed against one another will stop it from collapsing more. It becomes a black hole. The super nova occurs with stars that are 10 or more times larger than our sun, and we know of stars that are far larger than this. One star with the designation of R136a1 is thought to be 315 times the mass of our sun. A better known star called Betelgeuse is roughly a thousand times bigger than the sun in diameter.
Spinning black holes
The reason for the rapid spin of neutron stars is that all known stars spin. Our sun rotates once every 30 days or so, as compared to the earth, which takes a little more than 24 hours. If a star didn’t spin, the gasses would probably fly off in all directions and the star would dissolve.
As a star collapses, the speed of rotation…its spin…increases. This is the same phenomenon we see when an ice skater spins with their arms outstretched and then tuck their arms in, causing them to rotate faster.
For this reason, a neutron star that has much more mass than our sun but is the size of a small city rotates over a hundred times per second, only slowing down after a great deal of time.
Now, consider a star that is over 10 times more massive than our sun and which collapses to a point that is smaller than a city. Though we can’t actually see it when it compresses that much, the spin has to be enormous, and all black holes must spin. The spin of accretion disks confirm that the black hole is spinning, but it can’t be any other way because all stars spin.
Thus, the conclusion is that all black holes spin. If they didn’t, they would be something other than a black hole.