Why Do Black Holes Spin?
One of the most notable aspects of our planet – if it is observed from the outside – is that it turns. The spin of the earth defines our days, defining the fundamental rhythm of life on our world.
The moon also turns. So do the planets and everything their moons. The sun also turns, just like the stars. Even galaxies run; The Milky Way turns while the stars are swinging around its center in orbits of several million years.
It therefore seems obvious that, cosmically speaking, All Spins – But this basic fact becomes downright bizarre in the case of black holes that turns the head. It turns out that the spin, it turns out that one of the most important characteristics for these gravitational monsters and has large -scale impacts in the way they feast on matter in the way they can shape the very structure of galaxies.
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The angular moment is the basic concept to understand when reflecting on black rotation holes. It is like the momentum you know about your daily life (what we call the linear momentum) but for a rotating object. It is easier to think in terms of inertia-that is to say how difficult it is to prevent an object from turning. The more quickly something turns faster – and the more massive this something – the more inertia, the more difficult to stop.
The angular moment is a particular characteristic of an object in that it is preserved. In the absence of an external force, something turning alone will continue to turn forever. If you try to slow it or accelerate it, for example, catch it, part of its angular moment will be transferred to you (or you to you) so that the angular moment combined and total between you and does not change.
The angular moment of an object depends on its rotation rate (of course!), Mass and, especially for our discussion, its size. Ice skaters provide a classic example: they throw their arms wide to start a trick, then when they bring their arms closer to their body, their Skyrocket rotation rate vertiginously. This is how the angular moment is preserved; the size decreases, So the rotation rate increase.
The same goes for the stars, which are balanced on the cusid between the explosion of the outside due to their radiation and their collapse inwards because of their internal severity. When a high mass star lacks fuel, this balance is broken and the nucleus collapses, generating a gigantic explosion – a supernova – which explodes the outer layers of the star. While the nucleus shrinks, it also turns. And if its mass is more than three times the sun, the nucleus (which was previously tens of thousands of kilometers wide) will become a black hole barely 10 kilometers in diameter.
This dramatic reduction can increase the rotation of the black hole by a factor of several million Above his quieter stellar offspring so that he turns hundreds of times per second. And because the angular moment is preserved, even if almost the rest of the star is destroyed in the birth of the black hole, the spin glue.
In fact, only three factors can be used to define a black hole: its mass, its angular moment and its electrical load. In reality, this charge will be neutral or very close, therefore in practical terms, the first two factors are essential.
For this reason, we expect most, if not all, black holes are running very quickly.
It is a strange concept because black holes have no physical surface that can turn. But since the angular moment cannot be destroyed, the black holes must Keep when they form.
And this must be true for the black holes born from the stars, as well as the supermassive black holes that we see in the centers of the large galaxies, even if we do not fully understand how these giants are formed. And remarkably, in some cases, we can in fact measure these colossal cosmic towers.
The trick is to realize that, even if the angular moment of a black hole cannot simply disappear, it can certainly grow. The material falling into a black hole adds its angular moment to the system, increasing the rotation of the black hole. There is a theoretical limit at the speed at which a black hole can turn; It is a complicated mathematical concept, but indeed, this limit is when the black hole runs at the speed of light. It is possible, although difficult, to measure the rotation of a black hole by the way in which the light is emitted by the material just before it falls, and, for example, the Galaxy NGC 1365 nearby has a central supermassif black hole which was measured to be turned almost at this limit.
But of course, it becomes stranger. A bizarre aspect of the general theory of relativity of Einstein is that space-time can act as a fabric, a substance in which the masses are integrated. Einstein predicted that as a massive objects that turned, they drag space-time around them in what is called the observation effect of the lens, or more generally “dragging the frame”. The effect is the strongest very close to the horizon of the black hole event, its point of no return, and weakens with the distance. It’s like sticking a hand mixer in a large bowl of honey; The nearby honey will run with the mixer, but is so viscous as a few centimeters, it will barely move.
This relativistic frame dragged deeply affects the materials just outside the black hole. The material near the black hole is dragged with the space surrounding it, accelerating by stealing energy in the rotation of the black hole. This mobile material generates a strong magnetic field, powered by rotation. While the material orbits the black hole, the magnetic field lines are found, creating twin whirlpools like tornadoes. These are so powerful that they can keep the matter from the black hole and accelerate it at almost the speed of light! Astronomers call these “jets” beams and with supermassive black holes, they can last hundreds of thousands of light years.
Astronomers still do not know how supermassive black holes are formed. Do they become enormous from materials from the still forming host galaxy, or many smaller black holes form in the center and merge to create a single huge? The rotation of the resulting black hole can tell us the answer. If it forms from an infallible material disc, the rotation will be close to the limit, but if it has formed from other black holes moving in random directions which merge, their towers can cancel, leaving a last black hole with a lower rotation. It is not enough This simple, of course, but it may in principle be possible to observe young supermassive black holes with something like the James Webb space telescope, to see if the spin can be measured and one of the sustained or updated training methods.
We know the shapes of supermassive black hole and develops with its host galaxy. While the protogalactic gas of a huge cloud collapses and merges into stars, the black hole in its center is already huge and turns quickly. If it forms jets, these plow the question that falls to form the galaxy itself, slamming this material and even reversing its course, making it explode. This can extinguish the formation of stars, limiting the number of stars of the galaxy. In this way, the rotation of a black hole directly affects the size and structure of the galaxy which surrounds it.
No matter how you look at them, the black holes are bizarre. The fact that they exist and that we can understand them is, for me, exciting and deep. We live in a galaxy with a supermassive black hole in its center, and we can owe it our existence. This alone is a sufficient reason to try to understand them.
